HIGH RESOLUTION CORRELATION BETWEEN HIGH
LATITUDE CLIMATE EVENTS AND ARABIAN SEA
BIOGEOCHEMISTRY OVER THE LAST GLACIAL CYCLE
M.A Altabet1, M.J. Higginson1, ,and D.W. Murray2,
1
School of Marine Science and Technology, U. Massachusetts Dartmouth, 706 S.
Rodney French Blvd., New Bedford, MA 02744-1221 United States.
2
Center for Environmental Studies, Brown University Box 1943, Providence, RI
02912-1943 United States.
It is now well known that suborbital-scale climate events recorded in Greenland icecores also have manifestations in low latitude marine records. In the Arabian Sea, this
linkage has appeared as productivity variations recorded by sediment organic carbon
content. Plausible forcing is likely via the intensity of the southwest monsoon and
resulting upwelling of nutrient-rich water. We have examined high accumulation rate
cores from the Oman Margin for productivity proxies and nitrogen isotopic ratio which
records denitrification intensity within the oxygen minimum zone (OMZ). Denitrification
requires suboxic intermediate waters, which can arise from intense local productivity.
With temporal resolution approaching 50 yr, nitrogen isotopic ratio was found to vary
with remarkable similarity (duration and morphology) to the oxygen isotopic record from
the GISP2 ice-core. Given a general temporal framework known from foraminiferal
isotope measurements, a series of tie points were used to correlate the Arabian Sea
nitrogen isotope and ice-core records. The resulting correspondence is striking and
statistical comparison shows a very high index of similarity. Chronology for the first c.
20 kyr is confirmed by radiocarbon dating. Warm interstadial D-O events are marked by
denitrification as intense as found at present. In contrast, Heinrich Events correspond to
intervals of little or no denitrification. Transition to intense denitrification often occurs
over only a few data points, indicating a response time on the order of a century or less.
Arabian Sea denitrification is evidently very closely coupled to high latitude climate at the
decadal/centennial time-scale and is amongst the best comparisons between ice-core and
marine records. The high quality of the nitrogen isotopic record holds much potential for
refining our understanding of high- and low-latitude linkages. A poorer comparison with
productivity indices may result from their extrinsic nature, influenced by sediment
accumulation rate and possible diagenetic alteration. Changing denitrification intensity
may produce a global feedback through variations in global ocean nitrate inventory and
productivity in non-HNLC regions. Such feedbacks would occur, though, on time scales
similar to the residence time of marine nitrate, about 3 kyr at present. Filtering the
nitrogen isotopic record to examine variability on time scales of 3 kyr and greater
produces a series of oscillations between 20 and 70 kyr that are well correlated with a
recent high-resolution atmospheric CO2 record from Taylor Dome, Antarctica. A possible
mechanism relating high frequency N. Hemisphere climate change to lower frequency
global change is suggested.
REGIONAL TECTONIC AND CLIMATIC EVENTS RECORDED
IN THE STRATIGRAPHY OF THE MALDIVES CARBONATE
PLATFORM
Andrei V. Belopolsky* and André W. Droxler**
*BP America Inc., Houston, Texas, USA
** Rice University, Houston, TX, USA
Interpretation of the extensive 2-D seismic dataset and well data from the Inner Sea
of Maldives, equatorial Indian Ocean, showed that the evolution of this isolated Tertiary
platform was controlled by a combination of tectonic, eustatic and climatic factors. The
volcanic basement of the Maldives is thought to be created by the hot spot activity. The
late Paleocene basement is dissected by two deep and narrow graben that have
northeast-southwest orientation. The origin of the extension that created the graben is
not clear but must be related to the collision between India and Asia. Neritic carbonate
production was established in the early Eocene on top of volcanic basement highs while
graben served as deeper seaways. The movement along the graben faults continued
until the early Oligocene.
Evolution of the carbonate platform was controlled by the variations in
accommodation space and sediment supply. Climate played major role in controlling the
formation of carbonate material. The middle Miocene time in the Maldives is marked by
a pronounced bi-directional progradation of bank margins. Bank margins prograded
from both east and west towards the central trough. It has been shown previously in the
Bahamas that the difference between the leeward and windward margins was related to
dominating wind patterns. The, the leeward margin prograded while windward margin
aggraded. The apparent symmetry of prograding margins in the Maldives allows to
speculate that a seasonal switch in the wind patterns was responsible for the bidirectional progradation. Such switch occurs presently in South East Asia due to the
monsoons. The origin of monsoons is attributed to the elevation of the Tibet Plateau
and the timing of monsoon initiation is not well constrained at this time. If symmetrical
progradation in the Maldives was indeed driven by the monsoons it would place their
initiation in the early Middle Miocene time.
GASEOUS HYDROCARBONS IN SEDIMENTS AND OCEAN
WATERS OF THE NE ARABIAN SEA
Ulrich Berner1, Jürgen Poggenburg1, Ulrich von Rad1, Bodo Harazim1, Hartmut
Schulz 2, Tim Jennerjahn3, Peter Dietrich4, Peter Linke5 and Nico von Mirbach5
1 Federal Institute for Geosciences and Natural Resources, Stilleweg 2, D-30631
Hannover, Germany
2 Institute of Baltic Sea Research, University of Rostock, Seestrasse 15, D-18119
Rostock-Warnemünde, Germany
3 Institute of Biogeochemistry and Marine Chemistry, University of Hamburg,
Grabenstrasse 27, D-20357 Hamburg, Germany
4 Geological Institute, Technical University and Mining Academy of Freiberg, G.Zeuner-Strasse 12, D-09596 Freiberg, Germany
5 GEOMAR, Wischhofstrasse 1-3, D-24148 Kiel, Germany
During cruise SO90 (Sept. 1993) and SO130 (April 1998) of the German research
vessel "SONNE", we detected areas of active gas seeps at the front of the Makran
accretionary complex using TV-sled observations. The carbon and hydrogen isotopes of
methane from sediments and seeps at Makran point to bacterial generation via CO2reduction. Associated higher hydrocarbons suggest a mixture between bacterial and
thermal gases. The cold seeps discharge into the ocean waters and large plumes of
methane were observed in the deep water and may extent horizontally over more than
20km from the Makran accretion into the open ocean. We suggest that these plumes are
related to fluids emanating from the accretionary prism, rather than resemble in situ
generation of methane from decay of organic particles. However, no significant vertical
flux of methane across the salinity unconformity between surface water and "North
Indian Intermediate Water" was detected. Carbon isotope ratios of methane suggest that
methane is generated from microbial methanogenesis in the oxygen-rich surface water of
the Arabian Sea. Low to moderate rates of sea-air flux of methane are observed at sites of
the NE Arabian Sea. We suggest that differences of methane-flux of the different areas is
related to differences in primary biological production of the surface water of the Arabian
Sea. However, the estimates of methane fluxes are highly variable and are largely
dependent on wind speed and observed concentration changes in the surface waters.
THE AGE AND ORIGIN OF THE INDUS FAN
Peter Clift1, Christoph Gaedicke 2, Nobumichi Shimizu1, Jae Il Lee 1, Shahid
Amjad3 and Hans-Ulrich Schlüter2
1 Dept. of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods
Hole, MA 02543, USA
2 Federal Institute for Geosciences and Natural Resources (BGR), Stilleweg-2, D-3095
Hannover, Germany
3 National Institute of Oceanography, St-47, Block-1, Clifton, Karachi, Pakistan. E-mail:
[email protected]
The Indus is one of the world’s largest river systems, with a discharge equivalent to
the Mississippi. It currently rises in western Tibet and flows through the Indus Suture
Zone before cutting orthogonally through the Himalaya on its way to the Arabian Sea.
Although much smaller than the Bengal Fan the Indus Fan is still >9 km thick under the
Pakistani Shelf. Moreover, age control derived from petroleum wells on the Pakistani
Shelf and from ties to the Murray Ridge, whose uplift is considered to be principally
Early Miocene, ~20 Ma, demonstrate that around 1/3 of the total fan volume is Paleogene,
i.e., the fan pre-dates unroofing of the High Himalaya. There is a significant increase in
the rates of sedimentation during the Middle Miocene, manifest as the start of major
channel-levee structures on the upper mid fan. These pre-date the apparent strengthening
of the SW Monsoon at 8.5 Ma, but postdate the rapid exhumation of the High Himalaya
at ~23 Ma. This mis-match between the sedimentation patterns and major tectonic and
climatic events probably reflects the drainage area of the Indus River. Calculations based
on Neogene unroofing rates, and map areas suggest that only ~40% of the sediment
reaching the Arabian Sea now is derived from Indian Plate sources, compared with >85%
in the Bengal Fan. This estimate is corroborated by bulk Nd isotopic data from
Pleistocene Indus Fan mud. Instead much of the sediment comes from the Hindu Kush,
Karakoram and western Tibet. This is because the High Himalaya are less elevated in the
western Himalaya compared to the east and moreover the Indus cuts orthogonally through
these ranges, reducing the potential for them to significantly contribute to the bed load of
the river. This contrasts with the range-parallel course of the Ganges. If the High
Himalayas are not eroded much by the Indus it is not surprising that their tectonic
unroofing did not cause a major flux of sediment into the Arabian Sea. Since the Hindu
Kush, Karakoram and western Tibet are not as strongly affected by Monsoonal rains as
the Himalaya it might be expected that the strengthening of this climatic system during the
Late Miocene would have a minor effect on the erosion record of the fan. Instead the flux
of sediment may reflect uplift of the Karakoram at this time, or even in western Tibet.
The Indus Fan is now constrained to the east of the Murray Ridge that appears to have
been uplifted during the Early Miocene, at the same time as uplift of the Owen Ridge.
Prior to the uplift ~50% of the Indus River sediments were sedimented to the west of the
ridge and are now accreted into the Makran Accretionary Complex. The Indus Fan dates
from at least the middle Eocene. Mid Eocene sandstone recovered from the Murray Ridge
contain K-feldspar grains whose Pb isotopic composition require derivation from
ophiolitic and arc units within the Indus Suture Zone. This means that a major river was
already draining the Indus Suture at this time. Since several of the grains are also clearly
not Indian Plate in origin this requires that India-Asia collision had occurred by that time.
Significant thicknesses of seismically similar deposits underlie those drilled at DSDP Site
224 and ODP Site 731 suggesting that the fan may have initiated somewhat earlier, maybe
synchronous with the start of deltaic sedimentation in the Katawaz Basin of Pakistan
close to the Paleocene-Eocene boundary.
The Indus Molasse Group of Ladakh within the Indus Suture Zone of northern India
comprises sediments than record the start of flow of the Indus River through the suture
zone. The oldest fluvial sediments (Chogdo Fmn.) that onlap both sides of the suture
(i.e., Indian and Asian) show derivation from the south and have a provenance indicating
erosion of the platform and its overlying ophiolites. These sediments are overlain by
uppermost Paleocene Nummulitic Limestone before the end of all marine sedimentation.
The following fluvial sequences of Eocene-Oligocene age (Choksti and Nurla Fmns.)
show a west-directed flow and the appearance of K-feldspar grains whose Pb isotopic
composition is consistent with the influx of material of Tibet. The Nd isotopic
composition of the interbedded shales also indicate a change of flow that suggests the
start of a river draining western Tibet shortly after India-Asia collision in the latest
Paleocene. The river and its associated fan appear to be a direct reaction to the initial uplift
of southern Tibet.
Figure 1. Pb isotope discrimination diagram showing the affinity of detrital Eocene and Pleistocene
K-feldspars on the Indus Fan to values characteristic of units within and north of the Indus Suture
Zone rather than the Indian Plate. Crystalline source fields after work of Gariepy et al. (1985) and
Khan et al. (1997).
References
C LIFT, P. D., S HIMIZU, N., L AYNE , G. D., G AEDICKE , C . , S CHLÜTER, H. U., C LARK, M . K . &
AMJAD, S. 2000a. Fifty five million years of Tibetan evolution recorded in the Indus Fan. Eos, 81,
277–281.
C LIFT, P. D., S HIMIZU, N., L AYNE , G. D., G AEDICKE , C . , S CHLÜTER, H. U., C LARK, M . K . &
AMJAD, S. 2001. Development of the Indus Fan and its significance for the erosional history of the
western Himalaya and Karakoram. Bulletin of the Geological Society of America, in press.
C LIFT, P. D., B LUSZTAJN , J., K ROL, M., C ARTER, A., K IRBY, E . & G REEN, O. 1999. Timing of
collision and patterns of early uplift along the Indus Suture, Ladakh Himalaya, India. Eos abstract,
80, 313.
GARIEPY, C., A LLÈGRE , C. J., & X U, R.H. 1985. The Pb-isotope geochemistry of granitoids from the
Himalaya-Tibet collision zone: implications for crustal evolution. Earth and Planetary Science
Letters, 74, 220–234.
KHAN , M. A., S TERN, R. J., G RIBBLE, R . F . , & W INDLEY, B. F. 1997. Geochemical and isotopic
constraints on subduction polarity, magma sources and palaeogeography of the Kohistan intra-oceanic
arc, northern Pakistan Himalayas. Journal of the Geological Society, 154, 935–946.
SEISMIC STRATIGRAPHY OF THE OFFSHORE INDUS BASIN
T. Daley, N. Simons, S. Beswetherick
LASMO Oil Pakistan Ltd
In 1997 Lasmo Oil Pakistan Ltd gained a significant acreage position in the
Offshore Indus Basin with the award of the Indus A and B Blocks offshore south of
Karachi. The hydrocarbon play considered to have the most potential is Miocene shelfdelta sands interbedded with intraformational shale seals and sourced by presumed gasprone offshore equivalents.
As a result of this acreage position, Lasmo gained access to approximately 14,000
km of 2D seismic data across the region. This was enhanced by reprocessing 5000km
of the existing data and acquiring 3660 km of new proprietary data. Only four wells
have tested the preferred play type to date and no core or rock data were available from
these to provide insights into facies or age dating. The Oligo-Miocene succession has
conventionally been grouped stratigraphically as ‘Gaj Formation’, but for a unit in
excess of 4 km, this did not provide a sufficient basis for play fairway analysis.
Seismic sequence analysis has now provided the key to defining the internal
stratigraphy of the previously undifferentiated Gaj Formation and to understanding the
hydrocarbon prospectivity of the region.
Log data from two key wells in the offshore Indus area record the initial in-fill of
the basin by shale dominated basinal or outer shelf sediments, followed by stacked thinbedded sandstone-shale sequences of a shelf-delta nature. The transition between the
two is marked by a zone of progradational sequences but no other workable divisions
are apparent. Regional seismic correlations quickly established the diachronous nature
of the prograding shelf package and these were matched by distinct bands of seismic
progrades. A series of simple palaeogeographies of the prograding shelf margin were
developed showing initial sediment input from the north and rapid progradation towards
the south and west.
The most obvious seismic features are canyons. These can be dated relative to one
another on the basis of their stratigraphic position. Two main phases of canyon
development are apparent. The earlier phase is interpreted to be Lower Miocene in age.
These early canyons originated in the north and became more widespread. Down
cutting at this time rarely exceeded 300 m. A second phase of canyon development
occurred during the Plio-Pleistocene These later canyons dominate much of the shallow
section, often show down cutting of up to 600m and are characterised by at least 8
phases of cut and fill. Seismic progrades occur along the canyon axes and slumps are
observed along their margins. Where drilled, canyons of both ages are shale prone.
This is probably because they are located on the slope in a zone of erosion and sediment
by-pass and were subsequent filled by fine-grained deposits after abandonment. The
two phases of canyoning are considered to relate to co-eval phases of tectonic activity in
the collision zone between the India-Pakistan and Eurasian plates. The size and
prevalence of the canyons probably reflects either the magnitude of inversion and uplift
in the hinterland, or the proximity of the tectonic activity to the palaeo-shelf.
The implications for the prospectivity of the offshore Indus Basin and for the tectonic
evolution of the Kirthir fold-belt and the Indus hinterland will be discussed in the light
of these new observations.
EVOLUTION AND PROPERTIES OF ON- AND OFFSHORE MUD
VOLCANOES OF THE MAKRAN DESERT
G. Delisle1, U. von Rad1, A. Tabrez2, and A. Inam2
1 Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), Stilleweg 2, D-30655
Hannover, Germany
2 National Institute of Oceanography (NIO), St. 47 Clifton Block 1, Karachi, Pakistan
We present results of reconnaissance surveys of two active onshore mud volcano
fields (Chandragup and Jebel-u-Gurab) and of the newly born (March 1999) offshore
mud volcano (Malan Island). The mud volcanoes on land vary in height from a few to
100 m. Malan Island rose in March 1999 for about three months above the water line at
the same location, where in November of 1945 an island had emerged, only to be washed
away by wave action within months. All visited on- and offshore mud volcanoes line up
along one southwest- northeast trending structural lineament (main axis of Dhakanticline).
Isotopic data for gas components of the muds, temperature of the rising muds and
discharge rates of gas and mud were measured. In addition, we have searched the
literature for historic descriptions of the locations in an attempt to establish the longtime
changes in shape or activity of these complexes.
Onshore mud volcanoes: The Chandragup volcano complex is the largest known
structure along the coast of the Makran desert. He has existed at least since 1840 without
experiencing any significant modification in shape or mud discharge activity. Our
repeated temperature measurements in 1998 and 1999 point to slight variations in mud
flow discharge, which is in line with the report by locals of a dependence of mud
discharge on the periodicity of monsoon activity. Gas analyses from the Chandragup
complex as well as from the Jebel-u-Gurab volcano field about 6 km to the west yielded
invariably methane of bacterial origin with only traces of higher hydrocarbon compounds.
Offshore mud volcanoes: We have obtained echo-sounder profiles across the location of
former Malan Island. In place of the former island we have identified a 2 m deep
depression of the sea floor, on which remnants of the former island rest. The composition
of the gas emanating vigorously from the sea floor is identical to the gas of the onshore
mud volcanoes.
We conclude that mud volcanoes on land and in the sea are emplaced during short violent
episodes, followed by long periods of subdued gas and mud discharge activity. Mud
volcanoes in the sea are quickly washed away, while mud volcanoes on land can maintain
low level discharge activity for centuries.
GAS HYDRATES ACTING AS CAP ROCK TO FLUID AND GAS
DISCHARGE IN THE MAKRAN ACCRETIONARY PRISM?
G. Delisle, and U. Berner
Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, D-30655 Hannover,
Germany.
A thick and extensive gas hydrate layer caps the Makran accretionary wedge at water
depths of > 800 m. Its presence allows us to estimate heat flow density values from the
depth of observed bottom simulation reflectors (BSR) along reflection seismic transects
from the abyssal plain to near the coast of the Makran desert. These estimated values and
data from in-situ measurements, available from the literature, were compiled and
compared with the results of numerical modeling of the geothermal effect of the
subduction of oceanic crust under the Makran accretionary prism. A comparison of
model-derived with BSR-derived and measured heat flow values suggest a predominance
of conductive heat transport within the accretionary complex. Little evidence is found
which suggests fluid flow across the gas hydrate layer to modify the observed geothermal
field to any great extent. We further studied the geothermal field associated with the decay
of the gas hydrate layer as the accretionary prism and the capping gas hydrates are
tectonically uplifted out of the gas hydrate stability field into shallower and warmer sea
water. Theoretical considerations suggest a complete disappearance of gas hydrates at a
water depth of about 760 m. Extensive sampling of the water column (13 stations) along
three N-S transects across the accretionary prism has revealed the presence of gas plumes
of bacterial methane. They emerge from the sea floor at water depths of less than 800 m
and drift laterally for several km seawards, before they disintegrate due to dilution and
bacterial consumption of the methane. The gas seeps are thus concentrated in the zone of
shallow waters and outside the stability field of gas hydrates. This suggests that gas
hydrate layers in the Makran accretionary prism act as a very effective cap rock. The flow
of fluids containing high amounts of dissolved gas from the deep parts of the prism
(below the BSR) is redirected to the near coastal regions of the accretion, where gas
hydrates of low permeability do not exist.
CRUSTAL STRUCTURE OF THE DALRYMPLE TROUGH AND
MURRAY RIDGE - IMPLICATIONS FOR THE LOCATION OF
THE OCEAN-CONTINENT TRANSITION WEST OF INDIA.
R. A. Edwards 1, T. A. Minshull2, C. Kopp3, and E. R. Flueh3
1. Bullard Laboratories, Department of Earth Sciences, University of Cambridge,
Madingley Road, Cambridge, CB3 0EZ, UK;
2. School of Ocean and Earth Sciences, Southampton Oceanography Centre, European
Way, Southampton, UK;
3. GEOMAR, Wischhofstrasse 1-3, D-24148, Kiel, Germany.
The Dalrymple Trough and Murray Ridge represent the northernmost extension of the
Indian-Arabian plate boundary in the Arabian Sea. Seismic reflection profiles and swath
bathymetric data show a significant component of present-day extension across the
predominantly strike-slip plate boundary. The 150 km-long Dalrymple Trough is
bounded by normal faults divided into a series of en-echelon segments, the lengths of
which vary between 30-50 km on the northwestern flank and up to 90 km on the
southeastern flank. The Murray Ridge represents a topographic high immediately to the
southeast of the Dalrymple Trough. Along the crest of the ridge rough acoustic basement
is observed at the sea-floor on reflection profiles. Any magnetic anomalies associated
with the ridge are very weak.
Wide-angle seismic data were collected along a series of four lines in the Dalrymple
Trough and Murray Ridge region as part of FS Sonne cruise 123. Lines 5 and 7 cross the
trough and ridge, line 4 is located along the axis of the trough, while line 6 is parallel to,
and northwest of, the trough. Up to 10 OBH/S (Ocean Bottom HydrophonesSeismographs) were deployed along each line. Travel-time picks from the OBH/S have
been modelled using 2-D ray-tracing to produce velocity models of the crustal structure
along the four lines.
Line 6 and the northwestern end of line 5 show a velocity structure typical of oceanic
crust; with a 6 km-thick two-layer crust. However, line 4 along the axis of the trough,
and the southeastern part of line 5 show a very different structure. Here the crust has a
thickness of 12–14†km and velocities indicative of oceanic layer three are absent. The
increased crustal thickness and relatively low lower crustal velocities suggest that the
Dalrymple Trough is underlain by thinned continental crust.
The Dalrymple Trough therefore forms the northwest boundary of a region of thinned
continental crust which may extend beneath much of the Indus Fan and over 400 km from
the continental shelf of India. Oceanic crust lies immediately northwest of the Dalrymple
Trough and the ocean-continent transition occurs over a less than 10 km-wide region of
active oblique extension along the northwestern flank of the trough. The Moho under the
northwestern flank of the Dalrymple Trough has a very steep dip of 45∞, comparable to
the steep Moho dips observed along transform (or sheared) margins.
TECTONIC STYLE OF THE MAKRAN ACCRETIONARY PRISM
Nadine Ellouz
Institut Français du Pétrole (IFP), 1-4 av. de Bois-Préau, 92852 Rueil-Malmaison
Cedex-France
The Makran accretionary prism in Pakistan, has developed in the vicinity of the triple
junction between the Arabian, Eurasian, and Indian plate. This junction fits the connection
between the E-W Makran tectonic accretionary prism, a N-S transform fault system
(Chaman–Ornachnal) and, finally, a WSW-ENE Ridge (Murray Ridge). The western part
of the prism is developed in a frontal convergence setting, accommodating the low-angle
oceanic subduction of Cretaceous oceanic crust. The eastern part of the prism has been
strongly under the control of the left lateral transform motion related to the Northward
movement of India since Paleogene time. Consequently, deformation has adapted to
strain-partitioning processes here. Also, the oblique Murray Ridge, which is a prominent
poly-phased transtensive/transpressive structure linked to the Owen Fracture zone,
introduces a strong heterogeneity in the plunging panel.
All over the prism, the tectonic style as well as the sedimentary filling have registered
strong lateral variations through time. The periodicity, the orientation and the tectonic style
of the structures are guided by the sedimentary thickness variation as well as by the related
depth to decollement levels. Close to Pasni, total sedimentary thickness, deduced from
seismic data is 3.5 twt up to basal detachment. When close to the triple junction, it is more
than 4.5 twt up to the first more shallow regional detachment (depth of the basal
detachment is unknown).
To the west of the prism, deformation propagates along a basal detachment level close
to the top of the oceanic crust (free border), but laterally some oblique ridges (such as
volcanic Little Murray Ridge or Murray Ridge itself) act as rigid blocks. Secondary
decollement levels are inferred, particularly onshore, justifying the imbricate structure type
clearly expressed on satellite images. To the east, offshore, the northern flank of the
Murray Ridge is starting to be incorporated in the prism itself. Onshore, close to the Las
Bela depression, the development of “en échelon” folds, often associated with mud
extrusive processes, gives evidence of deep overpressure regime. These structures seem
to have extended offshore, as expressed on seismic data and by the emergence of the new
Malan island in 1999. The overpressure shales have probably originated from one of the
deep decollement level.
The age of the offshore basal sedimentary layers (inferred from well correlation across
the Murray Ridge, but not yet defined), may introduce uncertainties in the chronology of
the sedimentary distribution and the deformation. These limitations should be raised either
by drilling or by new constraints on dating and analyses of the mud ejected at the surface
(or sea bottom), giving new ideas about nature and age of the deepest part of the
sedimentary pile. However, the oldest sediments incorporated in the inner prism itself are
Late Cretaceous/Paleogene in age. The sedimentary pattern has been driven by the
architecture of the Paleo-Indus input through time, ever since its initiation during
Paleogene. On the offshore part of the prism, the sedimentary nature and thickness varies
from east (channel-levees) to west (distal sands and mudstones), suggesting that
sedimentary transfer (channel pattern) and deposits (sedimentary lobes) have a rough eastwest spatial distribution. The relationship between the compressive episodes, Eocene to
Present along the W Indian paleo-margin, and the Paleogene to Middle Miocene
sedimentary distribution west of the Chaman Fault is the important key point to be
defined, in order to estimate the migration of the Paleo-Indus drainage area. Especially
during Oligocene time, restoration of the W. Indian (Kirthar and Sulaiman Ranges) in
their Eocene and Oligocene position is crucial.
STRUCTURE AND DYNAMIC EVOLUTION OF THE INDIANARABIAN PLATE BOUNDARY OFF PAKISTAN: NEW
EVIDENCES FROM CORRELATION OF KEY HORIZONS
C. Gaedicke, H.-U. Schlüter, H. Roeser, A. Prexl, H. Meyer and C. Reichert
Federal Institute for Geosciences and Natural Resources (BGR), Hannover, Germany
Multi-channel seismic (MCS) profiles with a total length of about 2900 km were
recorded during RV Sonne cruise 122 in the northern Arabian Sea. The MCS grid covers
parts the Makran accretionary wedge, the Oman Abyssal Plain, the Murray Ridge System
and the northern Indus Fan off Pakistan. Industrial wells from offshore Pakistan enable
us to extrapolate stratigraphic key reflections in our MCS data. Interpretation was done
with Schlumberger's GeoQuest( software system.
The ocean-continent transition is imaged by a sequence of seaward dipping reflectors
(SDR) below the Indus Fan sediments. A sedimentary ridge above the acoustic basement
north of the SDR sequence consists of Mesozoic sediments deposited prior to the
separation of India from Madagascar and Africa. A Paleogene, hemipelagic syn-drift
sequence onlaps the ridge and covers the entire acoustic basement. Rapid uplift of the
high Himalayas, followed by erosion and subsequent transport of terrigenous sediment
into the northern Arabian Sea is documented in the Lower Miocene by the onset of
channel-levee sedimentation of the Indus Fan. Correlation of the channel-levee sequence
into troughs and onto ridges of the Murray Ridge System indicates that the Murray Ridge
was uplifted later on. Phases of rapid subsidence of the Dalrymple Trough and its northeastern prolongation in the post-Middle Miocene are evident by a sequence of up to 3 s
(TWT) thick sediments. The paleo-Indian continental margin is marked by northward
dipping basement reflectors along the northern flank of the Murray Ridge System. We
correlate this sequence with early Palaeocene basalt flows drilled on the shelf off Karachi.
The sedimentary pile of the Oman Abyssal Plain shares the sedimentary history of the
Indus Fan: two major sequences above the acoustic basement exhibit a similar internal
reflection pattern. The lower sequence is correlated with the syn-drift sediments and the
upper sequence exhibit distal channel-levee pattern. Both sequences and the acoustic
basement below are tilted toward the Makran accretionary wedge while they are
outcropping on the northern ridge of the Murray Ridge System. The sedimentary
sequences are separated by the pronounced unconformity M(akran). Unconformity M
unequivocally could be identified from the Straits of Hormuz to offshore eastern
Pakistan. The compilation and re-interpretation of published data with our MCS in
combination with estimated sedimentation rates of extrapolated well results hint to a Late
Miocene to Early Pliocene age for the M-unconformity. It separates the tilted lower
sedimentary unit from the flat lying upper sequence. Tilting of the basement causes a
wedge shape of this upper sequence. The thickness reaches 2.5 s (TWT) at the toe of the
Makran wedge but is about zero along the northwestern flank of the Murray Ridge
System indicating a hiatus on this part of the Murray Ridge System.
We speculate, that the M-unconformity developed in the frame of the Upper Miocene
uplift of the Makran-Zagros belt which in turn is due to changes in the movement vector
of the Arabian Plate. This event effected the entire Arabian Plate and led to an adjustment
and steepening of the subducting plate.
A COMPARATIVE STUDY OF THE GLACIAL-INTERGLACIAL
AND MILLENNIAL-SCALE OSCILLATIONS IN UPWELLING
RECORDS FROM THE EASTERN (SOMALIAN) AND WESTERN
(INDIAN) ARABIAN SEA MARGINS
R.S. Ganeshram1, Geert-Jan Brummer4 , S. E. Calvert2, Gerald Ganssen3
and D. Kroon 1
1
University of Edinburgh, Geology and Geophysics Dept., Edinburgh, UK, EH9 3JW,
[email protected];
2
University of British Columbia, Department of Earth and Ocean Sciences, Vancouver,
Canada V6T 1Z4
3
Free University of Amsterdam, The Netherlands;
4
Netherlands Institute for Sea Research, Texel, The Netherlands.
Recent studies report rapid oscillations in productivity, oxygenation/denitrification of
subsurface waters and aragonite preservation in the northern Arabian Sea during the Late
Quaternary periods (Schulz et al., 1998; Reichard et al., 1998). These oscillations
expressed as changes in organic carbon and aragonite contents, in the presence or absence
of laminations, and fluctuations in ∂15 N values of sediments have occurred on both glacialinterglacial and sub-Milankovich frequencies. Interglacial and warm interstadial (of GISP
2) periods are marked by high organic carbon content and poorly-oxygenated/denitrifying
upper-intermediate waters which are attributed to the strong summer (southwest) monsoon
winds and resulting high upwelling-induced primary production prevalent during these
periods. However, very little is known about the history of the winter (northeast) monsoon
and its phase relationship with the summer monsoon. In this study, we will compare
productivity and denitrification records from the Somalian margin influenced mainly by
the summer monsoon with those from the Indian margin where winter monsoon also
drives upwelling-induced productivity (Ganeshram et al., 2000). These records will be
used to delineate the late Quaternary histories of South Asian monsoons and their forcing
mechanisms.
References
GANESHRAM, R. S., P EDERSEN, T. F., C ALVERT, S. E., MCNEILL, G.W. & FONTUGNE,
M.R., Glacial-interglacial variability in denitrification in the World's Ocean: Causes and
consequences.Paleoceanography,15,361-376, 2000.
REICHARD, G. J ., L. J. LOURENS, & W. J. ZACHARIASSE, Temporal Variability in the
northern Arabian Sea Oxygen Minimum Zone (OMZ) during the last 225,000 years,
Paleoceanography, 13, 607-621, 1998.
SCHULTZ, H., U. VON RAD & H. ERLENKEUSER, Correlations between Arabian Sea and
Greenland climate oscillations of the past 110,000 years, Nature, 393, 54-57, 1998.
MINERALOGY AND PETROGRAPHY OF MODERN COASTAL
SEDIMENTS, GULF OF OMAN AND ARABIAN SEA (OMAN)
Eduardo Garzanti, Giovanni Vezzoli, Sergio Andu, Giovanna Castiglioni &
Daniela Dellí
Era Universite di Milano-Bicocca, Dipartimento di Scienze Geologiche e Geotecnologie,
Piazza della Scienza 4, 20126 Milano (Italy).
Along the continental margins of south-eastern Arabia, for some 2000 km from the
Gulf of Aden proto-oceanic rift to the Persian Gulf foreland basin, composition of
modern widyan, eolian dune, and beach sands allow recognition of 15
mineralogical/petrographic onshore provinces - several subdivided further in subprovinces allowing identification of main provenance types and sediment dispersal paths.
In this vast area with long and complex geological history, tropical arid climate with
negligible chemical weathering and anthropic modifications allow quantitative assessment
of primary detrital signatures in two main contrasting geodynamic settings: the multistage
rifted-margin of the Arabian Sea and the orogenic obducted belt bordering the Gulf of
Oman.
Rifted-margin detritus changes from pure sedimentaclastic to plutoniclastic with
deepening of erosion level within the rift shoulder (“undissected” to “dissected riftshoulder” stages). Finally, older peneplaned rift reliefs allow long-distance transport of
largely polycyclic detritus from the continental interiors (“continental interior
provenance”). In arid climates, due to scarce rainfall and lack of vegetation, winds are
much more efficient sediment-transporting agents than streams, and polycyclic sands are
only moderately enriched in stable quartz and chert grains.
Orogenic provenance is more complex, including detritus from oceanic allochthons
occupying the structurally highest positions of the obduction range, mixed in various
proportions with detritus from underlying sedimentary to metasedimentary nappes.
Where associated with orogenic belts of great topographic relief as the Samail ophiolite of
the Oman mountains, obducted ophiolitic sequences largely consist of serpentinized
mantle harzburgites shedding dominant serpentinite grains and enstatite (“dissected
ophiolite provenance”). Where instead, as in Masirah Island, relief and erosion rates are
modest, detritus is dispersed only locally and mostly derives from rocks of the oceanic
crust (“undissected ophiolite provenance”).
Provenance from accreted continental-margin successions is reflected by sedimentary
and locally metasedimentary detritus, with dominant recrystallized carbonate grains.
Three sub-types are recognized. Stable-platform successions (Musandam peninsula, Jabal
Akhdar dome) chiefly provide limestone and dolostone grains, with little recycled
siliciclastics. Deep-water successions mostly supply limestone and chert with a few
terrigenous grains. Metasedimentary to metavolcanic sequences, including subducted
platform successions (Sifah Province) or pelagic deposits incorporated in the
metamorphic sole of the Samail ophiolite (Diba Province), supply additional
polycrystalline quartz and metapelite, metafelsite and metabasite including blueschist
lithics.
Detailed petrographic and mineralogical information obtained in actualistic provenance
studies provides a suitable basis to understand coastal dynamics, and to assess dispersal
pathways of detritus and entry points of deep-sea sediments. The recognized riftedmargin to orogenic provenance signatures may represent a powerful tool to unraveling the
evolution of ancient thrust belts and associated sedimentary basins.
QUATERNARY CLIMATIC CHANGES
ARABIA AND THE THAR DESERT, INDIA
OVER
SOUTHERN
K. W. Glennie1, A. K. Singh2, N. Lancaster3 & J. T. Teller4
1 Dept. of Geology and Petroleum Geology, University of Aberdeen, Aberdeen ,UK.
2 Earth Science Division, Physical Research Laboratory, Ahmedabad, 380 009, India
3 Desert Research Institute, Reno, Nevada 89512, USA
4 University of Manitoba, Winnipeg, Manitoba R3T2N2, Canada
Expansion and contraction of deserts and their relationship with changes in the
climate, changes in the albedo and changes in the atmospheric circulation patterns, have
long been investigated. Conventionally the expansion episodes have been associated with
glacial (arid) epochs and the contraction episodes with more humid phases. A
synchronous response of different desert systems to climate change has been implicitly
assumed. However, more recent luminescence dating studies indicate that aeolian
accretion occurs over specific time windows. The window of opportunity for aeolian
accretion depends on a variety of factors that regulate the sediment production, supply,
transport and preservation. Consequently, both the local and global factors participate in
aeolian accretion, implying that the event chronology in deserts need not be in phase,
sensu-stricto. The Arabian Desert is influenced by two wind systems, the Shamal and the
SW Monsoon, and the Thar Desert (NW India) by the NE and SW Monsoons. The
contrasting wind regimes consequently provide a good opportunity to examine some of
the key issues in desert paleoclimatology. These range from, the effect of asynchronous
fluctuations in the two wind systems on the style of aeolian activity, limitations imposed
by sea level changes, changes in sediment supply and changes in preservation potentials
in these regions. Both Arabia and Thar, possess excellent sedimentary records that
indicate a large range of depositional facies including aeolian sand and carbonate dunes,
fluvial gravels and sabkhas together with pedogenic and ground-water carbonates. In this
study we collate and present a synthesis of the published literature and our own research
on the interpretation and chronometry of these landforms. Arabia. The present
distribution of dune sands over the southern half of Arabia conforms to the influence of
two different wind systems: 1) The major Shamal system, which was controlled by
winds that blew to the SSE down the Arabian Gulf and then swung to the SW towards
N. Yemen, and 2) The SW Monsoon system, which was responsible for forming the
Wahiba Sands, of much lesser extent, north of the Arabian Sea coast in SE Oman.
Optically stimulated luminescence (OSL) dating of dune sands in the Emirates and Oman
has enabled an event stratigraphy to be constructed. This indicates that during the later
Quaternary, repeated dune-sand transport and deposition in the opposing Shamal and SW
Monsoon dune systems were probably affected by the presence (or absence) of the north
polar ice cap, and the influence it had on atmospheric circulation including monsoon
winds. During the last glaciation, the dry Arabian Gulf area allowed deflation of quartz
sands (together with carbonate grains that had been deposited under marine conditions
during the previous interglacial) and their transport south to the Emirates. Only the quartz
fraction survived the abrasion and moved into the Rub al Khali basin in Saudi Arabia,
some of it spilling southward to the exposed continental shelf of SE Oman. The SW
monsoon was inactive over that same continental shelf. As the late-glacial polar icecaps
declined, the arid Shamal wind system weakened and its southern limit shrank
northward. This enabled the more humid SW monsoon to blow across Arabia's SE
margin, deflating quartz and carbonate grains from the narrow continental shelf and
depositing them in the S-N aligned Wahiba dune system. As discussed below, the SW
Monsoon was also responsible for the initiation and northward shift of dune activity in
the Thar Desert of India by some 300 km. The northern end of the Wahiba Sands seem to
have been truncated early in the Holocene by the flooding of Wadi-Batha, although
marine evidence of that flood may have been dispersed by along-shore currents driven by
the monsoon winds. Thar Desert. The presence of numerous dune-forms within its
limited areal spread perhaps makes Thar an ideal laboratory to understand factors
determining dune morphologies and the role of local versus global factors. The presence
of fossil sands in regions with several times higher precipitation as compared to the
present day limit (for dune activity) of 250 mm/a suggests large amplitude changes in
dune accretion climate in the region. It is now generally accepted that both fluvial and
aeolian processes in the region are largely controlled by winds and precipitation
associated with the SW Monsoon. The near absence of regionally extant fluvial or aeolian
aggradation during the last glacial epoch reflects this. Despite aridity and abundant sand
supply, dune accretion did not occur during the glacial epochs on account of weaker
winds. Luminescence chronometry of closely spaced samples suggests that the window
of opportunity for aeolian aggradation occurred only for a limited duration that peaked at
times of transition from glacial to interglacial epochs and during the time the monsoon
was re-establishing itself in the region. Chronometry of aeolian aggradation also indicates
a spatial variability within Thar, suggesting changes in the monsoon gradients during the
Holocene. Isotopic studies on pedogenic carbonates within aeolian areas indicate a more
humid phase (with normal monsoon-like conditions) during marine isotopic stage 3. This
is seen as a red soil horizon in fluvial deposits. On the Holocene time scale, a largeamplitude change is seen in lake hydrology. Saline lakes had fresh water during the earlymid Holocene followed by a period of desiccation; the lakes and dunes appear to present a
1500 year cyclicity with a phase lag of a few centuries. Published data on the Quaternary
marine sequences of the NW Arabian Sea give general support to the varying influence of
the Shamal & SW Monsoon wind systems. Chronometry of desert sequences indicate
significant differences between records from Oman and Thar as compared to the Arabian
records, and together these permit reconstruction of past changes in circulation patterns in
the region.
EARLY HOLOCENE SUB-CENTENNIAL MONSOON
VARIATIONS IN THE ARABIAN SEA
S.J.A. Jung, G.R. Davies, G.M. Ganssen
(1) Institute of Earth Sciences, Free University Amsterdam, De Boelelaan 1085
1081 HV Amsterdam, The Netherlands. e-mail.:[email protected], tel. 0031-20-4447424
The Asian monsoon system belongs to the most dynamic parts of the Earth climate
system. Strength variations of either the summer (SW-) monsoon or the winter (NE-)
monsoon in the Arabian Sea may not only reflect local climate variations but may have
implications for the global climate system.
Based on Dansgaard-Oeschger type of strength variations of the monsoon off Somalia
occurring over the last 35 ka a ultra-high resolution study of the early Holocene section
(11-6.5 ka) of core 905 off Somalia was used to trace monsoon variations down to
decadal time scales. At a sedimentation rate of up to 30 cm/ka sampling at 0.5 cm resulted
in a time resolution around 20 years. In a multiproxy study O-/C-stable isotope data were
determined on planktonic foraminifera that are today indicative for either of the monsoon
seasons. Radiogenic isotope analysis (87 Sr/86 Sr, 143 Nd/144 Nd and Pb-isotopes) of the
lithic - dust borne sediment fraction are used as a dust provenance tool. Both sets of
parameters clearly show significant early Holocene monsoon variations on multi- down to
sub-centennial frequencies. Moreover, these data indicate a succession coupled SW- and
NE-monsoon variations with periods of decoupled seasonal monsoon variations.
Using the epibenthic foraminifera C. kullenbergi, known as a reliable recorder of
ambient water mass properties, a detailed early Holocene mid-depth water record was
established. Following a 10 ∂18 O-lowering along with Termination I a general further
decrease of 0.2 0 until 8.2 kyr was observed. Unexpectedly, subsequent to the well
known 8.2 kyr cold event, clearly depicted in the ∂18 O-record of the planktonic
foraminifera G. Ruber from the same core, a rapid additional decrease of 0.2-0.3 0
occurred. Even more important, prior and subsequent to the 8.2 kyr event centennial scale
∂18 O-variations were found. Prior to 8.2 kyr their amplitude is 0.2-0.3 0, hence close to
the detection limit, whereas thereafter their amplitudes are 0.5-0.7 0. These changes
possibly reflect variations in the composition of mid-depth water masses resulting from
early Holocene Red Sea Water (RSW) with a significantly higher salinity than today.
Accordingly this RSW may have settled deeper in the water column and bathed site 905.
Around 6.5 kyr BP RSW approached modern day salinity and temperature properties
leading to a lower density. RSW subsequently shoaled not affecting the benthic
foraminifera at site 905 anymore.
PALEOCLIMATE VARIABILITY DOCUMENTED IN
HOLOCENE VARVED SEDIMENTS OFF PAKISTAN.
UPPER
Athar Ali Khan 1, Ulrich von Rad 2 and Andreas Lockge 2
1 National Institute of Oceanography, St-47, Block-1, Clifton, Karachi. Pakistan.
2 Federal Institute for Geosciences and Natural Resources (BGR), Stilleweg-2, D-3095
Hannover, Germany
The northeastern Arabian Sea is characterized by an exceptionally stable mid-water
oxygen minimum zone (OMZ) and a strong seasonal monsoon-controlled variability of
primary productivity. OMZ cores from the slope off Karachi and Ormara (640 - 700m
water depth) have a thick upper Holocene section (0-5000 cal. yr. BP) of continuously
laminated sediments.
Our age model is based on varve stratigraphy (counting about 5600 varve couplets in
both cores) and 36 AMS-14C ages for both cores. Average linear sedimentation rate is
130 cm/ka (1.3 mm/a) off Karachi and 220 cm/ka (2.2 mm/a) off Ormara, where about
40% of the total thickness consists of turbidites and event deposits. We studied the fabric
and composition of the varve-type laminations by photography, X-radiography, thinsections, digital sediment color analysis, and by the digital recording of lamina thickness.
Our main independent proxies for terrigenous input and river runoff are varve thickness
and inorganic geochemical composition (Ti/Al, Zr/Al, K/Al, Sr, Ca), as well as oxygen
isotopic data of planktonic foraminifera.
The hemipelagic sediments contain alternating 0.5-2 mm-thick dark and light colored
laminae. The light-gray laminae consist of well-sorted, terrigenous, unfossiliferous silty
clay. The dark olive gray laminae are relatively thicker and consist of poorly sorted silty
clay, rich in organic carbon coccoliths and foraminifera . Turbidites (> 5 mm thick) were
derived from the continent, mainly by re-suspension of material deposited by episodic
and perennial rivers and consist of medium to dark-gray, homogeneous clayey silt. Thick
turbidites are commonly graded. These are based on the varve chronology the variations
in turbidites frequency and turbidity thickness suggest times a marked decrease in
turbidite frequency and thickness around 500, 3000 and 5200 years BP
Subtle changes of geochemical proxies (normalized to Al) and 18O values make it
possible to differentiate three climatic periods: I: 4700-3,500 yr. BP; II: 3,500-1,200 yr.
BP; III: 1,200 yr. BP - Present). From 4,700-3,500 yr. BP we note a general increase in
varve thickness and element/Al ratios. Similar trends of varve thickness and Si/Al, Ti/Al,
Zr/Al, and Mg/Al ratios suggest that the climate was relatively humid. Increased varve
thickness and higher lithogenic element /Al ratios are probably due to increased
precipitation and enhanced fluvial input. During period II (3,500-1,200 yr. BP) varve
thicknesses decrease up-core, whereas Si/Al, Ti/Al, Zr/Al, Mg/Al ratios are still high.
Increased lithogenic element /Al ratios and reduced varve thicknesses during period II
(3,500-1,200 yr. BP) suggest slightly more arid conditions in the source area. In a
similar way as during period III, the climate became more humid from 1,200 yr. BP on
climate became more humid again as evidenced by the increased varve thickness and
element/al ratios.
Spectral analysis of the varve thickness values documents the presence of cycles around
1490 yr., 460 yr., 288 yr. 180 yr., 103-107 yr., between 94-95 yr., as well as peaks
around 74 yr., 42-43 yr., 30-32 yr., 29-32 yr., and 25-yr., whereas so far no
unambiguous 11-yr. sunspot or ENSO signal was discovered (cf. Poster Abstract by von
Rad and Berger). These cycles are not present during the whole 5000-yr. period, but
change with time, i.e., the power seems to jump from longer to shorter cycles with time.
A SUMMARY OF THE GROUND-TRUTH GEOLOGY OF THE
MAKRAN
G. J. H. McCall
44 Robert Franklin Way, South Cerney. Glos., U.K.,GL7 5UD
E-mail: [email protected]: Fax: 44 1285 862449
The Iranian Makran has been entirely mapped geologically on a scale of 1:250,000,
except for a narrow coastal strip, which exposes the very youngest Cenozoic sediments
of the main Makran accretionary prism. The geology of the Makran is less widely known
than the geology of Oman, facing it across the Oman Sea, because it has been published
only in detail in reports of the Geological Survey. It has little in common with the
Geology of Oman, the only representatives of Oman Geology in Iran being ophiolites at
Neyriz and Kermanshahr, many kilometres to the northwest of the Makran. This
summary of the geology of a region the size of England is based on on-the-ground
mapping, photo-interpretation only being used to connect up traverse lines.
The oldest rocks in the Makran are metamorphic rocks which form the basement to
the Bajgan/Dur-kan microcontinental ìsliverî, a narrow block which extends hundreds of
kilometres from the Bitlis massif in Turkey through the Sanandaj/Sirjan block of the
Zagros to north of Nikshahr in the east of the Makran. Other metamorphic rocks form the
Deyader Complex near Fannuj on the southern margin of the Jaz Murian Depression
(desert): these include blueschists, and are thought to form the tip of the Tabas
microcontinental block, largely exposed to the north of that desert. There is also a small
microcontinental block to the east, the Birk block, which exposes only Cretaceous
platform limestones and peripheral Permian sediments. The Bajgan metamorphics are
overlain by highly deformed and disrupted platform carbonates of early Cretaceous to
Early Paleocene age (Dur-kan Complex), containing tectonic inliers of Carboniferous,
Permian and, rarely, Jurassic age.
Ophiolites occur in two structural positions. South of the Bajgan/Dur-kan bloc, the
Coloured Mélange of the Zagros, a tectonic mélange, continues eastwards immediately
inland of the Bashakerd Fault; this includes two layered ultramafic complexes, one with
chromitites, also radiolarites and limestones of Jurassic to early Paleocene age. Ophiolites
developed north of the microcontinental block form three distinct complexes, two layered
and one with intermediate sheeted dykes. There are small developments of Cretaceous
sediments carrying rudists in the extreme northwest of the inner ophiolite tract. In the
extreme northeast, there is another ophiolite development, the Talkhab mélange. All these
ophiolite developments represent former, essentially Mesozoic, tracts of deep ocean.
The Cenozoic rocks form two immense accretionary prisms. The main Makran prism
includes Eocene-Oligocene and Oligocene-Miocene flysch turbidite sequences, estimated
as individually more than 10,000 m thick.. There is an abrupt passage up, through reeflike Burdigalian limestones, and locally a harzburgite conglomerate development, into
neritic sequences with minor turbidites, extending just into the Pliocene. The Saravan
accretionary prism to the east repeats tectonically three immensely thick flysch turbidite
sequences of Eocene-Oligocene age, but younger sediments are restricted here to meager
developments of Oligocene-Miocene intramontane conglomerates. There is a line of
Oligocene (?) granodiorite bodies within the Saravan accretionary prism.
Intense folding and development of schuppen structure, dislocation and melanging of
the sediments affected the entire region in the late Miocene-early Pliocene. Post-tectonic
uplift was followed by scattered developments of fanglomerates beneath the fault scarps.
The main tectonic deformation in the Neogene has obscured earlier deformations. There is
unconformity beneath Eocene sediments representing a mid-Paleocene disturbance. There
is evidence of a discontinuity in the mid-Eocene and at least local unconformity. The
Pliocene-Pleistocene fanglomerates are unconformable on the folded rocks.
There are discontinuous developments of Eocene-Oligocene neritic sediments
unconformably above the older rocks (Ophiolites, Platform Limestones, Metamorphics)
and to the north of the southern edge of the Jaz Murian Depression, the limit of the
Makran, there is evidence of the survival here of a very shallow sea through the Neogene
and the deposition in it near Jiroft to the west of Oligocene-Miocene reef-like limestones;
and near Karvandar to the east of Pliocene shallow water sediments. .
The offshore evidence indicates that the region is now being subjected to shallowlyinclined subduction on a NNE-wards vector, the movement emanating mainly from the
Sheba Ridge. There is a 150 km wide tract separating the coast from the active front, the
total Cenozoic accretionary prism fronting the Iranian Makran being 500 km wide.
Spreading from the Murray Ridge affects the extreme east of the region and there is
reason to think it may have a northwestwards vector. The Saravan accretionary prism, it
is suggested, fronted onto a gulf, comparable with the gulf now separating Iran and
Oman, and this Saravan Gulf filled up and closed up by the early Oligocene.
Seismological evidence suggests that there is active continental collision continuing along
this suture.
NEW CONSTRAINTS ON THE AGE AND TECTONIC HISTORY
OF THE LITHOSPHERE AT MURRAY RIDGE AND DALRYMPLE
TROUGH
T. A. Minshull1, R. A. Edwards2, E. Flueh3 and C. Kopp3
1 SOES, Southampton Oceanography Centre, SO14 3ZH, UK
2 Bullard Laboratories, Madingley Road, Cambridge CB3 0EZ, UK
3 Geomar Research Centre, 24148 Kiel, Germany
The Dalrymple Trough forms a 150 km long and 25 km wide SW-NE trending
depression along the India-Arabia plate boundary, with a water depth of about 4 km. It is
bounded to the southwest by the southern part of the Murray Ridge, a bathymetric high
which locally approaches the sea surface. Results from a recent wide-angle seismic
experiment aboard the Sonne suggest that the Murray Ridge and part of Dalrymple
Trough are composed of continental crust, juxtaposed by the oblique-slip plate boundary
against oceanic crust to the NW. This continental fragment is separated from an area of
shallow basement to the SE which is also interpreted as continental crust by a region of
deep (up to 10 km depth) basement overlain by thick Indus Fan sediments. In this region,
data from the Sonne cruise suggest that mantle velocities are reached within 3-4 km of
basement; the velocity structure is consistent with the presence of highly extended
continental crust, or possibly of an ocean-transition zone. Basement fault fabrics to the
SE of Murray Ridge suggest that this region of deep basement was formed by extension
in a NW-SE direction, almost perpendicular to the Paleocene episode of extension which
led to the separation of the Seychelles from India. Plate reconstructions for this part of the
Indian Ocean are broadly consistent are broadly consistent to M0 time, but reconstructed
relative positions of India and Madagascar in the Jurassic vary widely. Some
reconstructions are consistent with the possibility that the lithosphere currently lying
beneath the Murray Ridge became separated from Africa and India during the initial
opening phase of the West Somali Basin in the early Upper Jurassic. Long-wavelength
gravity anomalies, which show a consistent gravity increase from NW to SE across the
Dalrymple Trough, are consistent with such a model, with older, colder lithosphere to the
SE of the Trough than to the NW.
SEDIMENTATION AND TECTONIC HISTORY ALONG THE
WESTERN CONTINENTAL MARGIN OF INDIA
D. Gopala Rao
National Institute of Oceanography, Dona Paula, Goa - 403 004, India.
Sedimentary, tectonic and volcanic records are crucial to know the earth system
changes that have taken place in the geological past. They are interpreted from marine
geophysical investigations of the western continental margin of India and eastern Arabian
Sea in confirmation with litho-stratigraphy of the industry drill wells and DSDP Site 219.
The most important findings are the 1) the regional seismic sequences, H1 to H6
stratigraphy of the late Cretaceous / Paleocene to Recent sediments, seaward migration (860 km) of mid-Miocene paleo-shelf edge, mid-Miocene to present prograded and
Oligocene to mid-Miocene aggraded shelf sedimentation, 2) Neogene turbidites overlie
the Paleogene pelagic sediments in the area seaward of the Laccadive-Laxmi Ridges, 3)
rifted / tilted half graben centred by linear igneous intrusive structures of the shelf margin
basin paralleling the shelf edge, and 4) rift graben carpeted by pre, syn and post-rift
sediments, seaward dipping reflectors and linear igneous intrusives along the western
flank of the Laccadive-Laxmi Ridges.
Whether initiation of seafloor spreading in the Arabian Sea pre -dates Deccan
volcanism on the western margin or volcanism associates with the initiation of spreading
during the Cretaceous-Tertiary boundary is still an unresolved question. Active seafloor
spreading in two periods from 65 (chron 28) to 50 Ma (chron 21) and 30 Ma (chron 11)
to Recent had largely contributed to the evolution of the eastern Arabian Sea. A hiatus in
seafloor spreading from 50 (chron 21) to 30 Ma (chron 11) is known to occur especially
after formation of 48 Ma age oceanic crust and record pause in seafloor spreading. The
industry drill wells of the margin have noted presence of extrusive volcanics around 93
(CH-1-1 and St. Mary Islands etc.), 65-55 (BH-1, K-1-1 etc.) and 42 (MT-3 etc.) Ma.
The abrupt termination of seafloor spreading process at 50 Ma and change in spreading
direction mark post India-Asia collision event. The early volcanism denotes time of
separation of the greater India from Madagascar. The later volcanic events coincide with
the Reunion plume interaction and subsequent active seafloor spreading. The major
faulting along the paleo-shelf edge is proximal to the second stage of evolution of the
eastern Arabian Basin. It is necessary to investigate the ages of volcanism, tectonism and
active seafloor spreading episodes in the Arabian Sea to understand their interrelationships.
In the eastern Arabian Sea more than 2.0 s (TWT) thick turbidites are noted in the
north and they gradually reduce to about 0.3 s to the south, 8∞N. The thick turbidites
reaching the distal parts of the Indus Fan, 8˚N are to be viewed transported for longer
duration. A prominent, continuous doublet of intermediate frequency at around 15˚N
latitude is noticed bifurcating the turbidites. Such a reflector chronology is crucial to
know the past and constrain age of the earliest turbidites reaching the area. The Paleogene
carbonate section of the margin does contain more than 200 m thick Eocene Chert
reflectors, silica layer which is likely to occur as biogenic by-product while the Indian
plate is in temperate latitudes, i.e. in the vicinity of equator during its northward motion.
While an erosional unconformity of the shelf region mark lowered sea-levels occurred in
Oligocene concurrent with global scenario. The steep-scarp associated with the
Oligocene-mid-Miocene shelf edge mark major tectonism / rift related subsidence and
eventually leading to evolution of the sedimentary basin of the shelf margin. Aggraded
late Oligocene / early Miocene sediments imply less supply to the stable shelf. The
Neogene siliciclastic rapid sediment pulsations into the Arabian Sea during the post midMiocene, Pliocene and upper Pleistocene have contributed to the shelf pro-gradation. The
increased siliciclastic sediments input to the margin suggests intensified erosional activity
that may be related to climate, sea-level, and / or tectonic events.
The structural configuration of the shelf margin basin indicates stretched continental
crust intruded by igneous crust. The basins evolutionary processes result in changes in
ocean circulation, material exchange between volcanism, hydrosphere and atmosphere
and decisively effects climates during those periods. The change in deposition of
carbonate to siliciclastics on the shelf, faulting of the paleo-shelf edge, hiatus in seafloor
spreading, and reorientation of spreading centre directions in the eastern Arabian Sea,
volcanism prior to Miocene suggest that early Neogene is most likely the time for
resetting of the changes.
Email: [email protected]
Fax : 00-91-832-223340/229102
THE PERIODICAL BREAKDOWN OF THE ARABIAN SEA
OXYGEN MINIMUM ZONE
Gert Jan Reichart and Willem Jan Zachariasse
The northern Arabian Sea is presently characterized by a pronounced oxygen
minimum zone (OMZ) with oxygen concentrations reaching values as low as 2 (M
between 150 and 1250 m). This intense mid-water OMZ results from high annual organic
particle fluxes and a moderate rate of thermocline ventilation. Sediment studies have
shown that the intensity of the northern Arabian Sea OMZ has fluctuated on Milankovitch
and sub-Milankovitch time scales, either in conjunction with changes in surface water
productivity or by changes in thermocline ventilation. Oscillations in thermocline
ventilation may be connected with changes in the inflow of southern ocean source water
or by changes in the depth of local convective winter mixing.
To quantify the role of convective winter mixing in the periodic breakdown of the
OMZ we reconstructed SST’s and SSS’s for the last 70 kyrs based on alkenone
thermometry and (18O analyses on planktonic and benthic foraminifers. We will show
that for the studied time span thermocline ventilation by local sinking of surface water is a
viable mechanism to explain the periodical breakdown of the OMZ. We postulate that the
necessary increase in surface water density resulted from intensified winter monsoonal
winds.
SOME PRELIMINARY DATA ON THE ALPINE MAGMATISM
AND RELATED MINERALIZATION IN SISTAN-BALUCHESTAN,
MIDDLE EAST
Alexander Romanko, and Eugene Romanko
Institute of the Lithosphere of Marginal Seas, RAS, Moscow, Russia.
E-mail: [email protected]
Sistan and Baluchestan, Middle East and Alpine/Himalayan conjunction, are
characterized by a complex tectonics and interesting mineralization despite the poor data
available. New field and post-field materials were received by a group led by well known
regional geologists: Drs. E. Romanko, A. Hushmandzadeh and M.A.A. Nogole Sadat.
Several specific preliminary results and peculiarities could be noted as: 1) one vast calcalkaline Cretaceous / Paleocene - Quaternary volcanism-plutonism in this region instead
of previous several independent events and two independent calc-alkaline and subalkaline magmas, 2) almost all metallogeny relates to magmatism here, 3) CretaceousPaleocene flysch is not quite sedimentary being really injected by many mineralized
magmatic bodies, 4) Paleogene-Miocene Lar and Assagie fluid-rich sub-alkaline plutons
are more metallogenically important (by Cu, Pb, Au, even Ag) comparing to calc-alkaline
ones; Lar pluton is a deeper than Assagie one by formational, petrographic, geophysical
and metallogenical data (as a school's example); Hormak trachybasalt massif is a
cogenetic possibly to Lar and Assagie plutons mentioned, 5) traces of an eastward
magmatic migration in the Eastern Iran/Western Pakistan sometimes due to regional
subduction, 6) regional Cu-mineralization is a traditional one while other interesting
components are: FeCr2O4, Cu-Ni-Co, Mn, magnesite-huntite (ophiolite-related); Au-MoPb-Cu-poor Zn (Paleogene magmatism-related); Fe, Mn, U (mainly hydrothermal
processes) etc., 7) dominantly southward mass moving in a compressive and Hi-seismic
conditions (more than 5-6 points by Richter's scale).
PLATE TECTONIC EVOLUTION OF THE ARABIAN SEA AND
EASTERN SOMALI BASIN: FROM SIMPLE TO COMPLEX
MODELS
Jean-Yves Royer 1, A. K. Chaubey 2, J. Dyment 1, G. C. Bhattacharya 2, and K.
Srinivas 2
1 Domaines Océaniques, Institut Universitaire Européen de la Mer, Place Copernic,
29280 Plouzané - France ([email protected]; [email protected])
2 Geological Oceanography Division, National Institute of Oceanography, Dona Paula,
Goa
403
004,
India
([email protected],
[email protected],
[email protected])
The opening of the western Indian Ocean resulted from the break-up and dispersal of
the African, Madagascar and Indian continental blocks. Commonly agreed broad
evolutionary model proposes that seafloor spreading initiated in the western Somali Basin
between Africa and a Madagascar-Seychelles-India block in the Early Cretaceous. In a
second stage that started in the mid-Cretaceous, Seychelles and India separated from
Madagascar, leading to the opening of the Mascarene Basin. During the Paleocene,
seafloor spreading progressively stopped in the Mascarene Basin while resuming north of
the Seychelles-Mascarene Plateau, creating the eastern Somali and Arabian basins.
Although this general evolution of the area appears to be fairly understood, the detailed
chronology and tectonic development of all the ocean basins of this area have not yet been
fully unravelled. In this paper, we present a review of the different models that have been
proposed as new data were collected, and the preliminary results of a model that we
derived from a new data compilation in this area.
The first conjugate magnetic anomalies in the Arabian and eastern Somali basins were
observed in the late sixties. From additional shipboard data, McKenzie and Sclater (1971)
identified Paleogene magnetic anomalies 28 to 23. Deep-Sea Drilling Project in the
Arabian Sea helped dating the seafloor and defining the structural trends of the basin
(Whitmarsh 1974). The E-W oriented magnetic lineations 28 to 18 were considered to be
offset by four N-S oriented fracture zones. A two-stage evolution was proposed with fast
spreading stage between chron 28 and 20 followed by a very slow spreading stage until
present-day. The change in spreading rate and direction was supposed to be
contemporaneous with similar changes in the Indian Ocean between chron 18 and 20,
related to the collision of India with Eurasia. Identifications of conjugate 27 to 23
sequences and of additional fracture zones in the eastern Somali Basin further refined the
picture (Norton & Sclater 1979; Schlich 1982). Extensive surveys by Russian vessels, in
the early eighties, lead to a detailed tectonic chart of the Arabian Sea, which suggested an
initiation of spreading at anomaly 29, or possibly 31, and a major change in rate and
direction at about anomaly 11-12, thus much later than in the rest of the Indian Ocean
(Karasik et al. 1986). Various studies suggests that an episode of fast spreading (~ 6
cm/a, chrons 27-23) decreased at chron 18 (40 Ma) when they became ultra-slow (< 0.6
cm/a) as the spreading direction changed by more than 30 degrees. The latest phase of
spreading started around anomaly 7 (~25 Ma) after a smaller change in spreading
direction (~15 deg.), and is continuing at present at about 1.2 cm/a (Chaubey et al., 1993;
Mercuriev et al. 1996).
The fracture zone pattern, mostly inferred from discontinuities in the magnetic
lineations, was still not consistent in the two conjugate basins. Satellite-derived gravity
charts progressively unveiled, with increasing details, the structural grain of the region:
the old fast-spreading basins are fairly smooth with some structural trends oblique to the
magnetic lineations; the young slow-spreading basins display a rough topography and, to
the east, a series of well-defined and closely-spaced fracture zones. Contour maps
derived from the dense magnetic surveys in the eastern Somali and Arabian basins
(Karasik et al. 1986; Mercuriev & Sochevanova 1990) showed incompatibility to most of
the sub-meridional fracture zones inferred in earlier studies. Subsequent detailed
reinterpretation of the Paleogene magnetic lineations revealed evidence of oblique pseudofaults associated with systematic ridge propagation in both the basins (Miles & Roest,
1993; Chaubey et al. 1998; Dyment 1998). Ridge propagation explains the large
spreading asymmetry between the Arabian and Eastern Somali basins. Between chrons
26 to 25, ~65% of the crust formed at the Carlsberg Ridge was accreted to the African
plate, and after a change in the direction of ridge propagation at chron 24r more than 75%
of the crust was accreted to the Indian plate between chrons 24 to 20 (Dyment, 1998).
The early break-up between the Madagascar, Seychelles and Indian continental blocks
is still a matter of debate. The break-up between Madagascar and India occurred during
the mid-Cretaceous, according to the anomaly 34 (83 Ma) observed in the Mascarene
Basin. Evidence of early spreading (pre-chron 28) between the Seychelles block and
India are found in the Laxmi Basin and its adjacent areas (Bhattacharya et al. 1994; Malod
et al. 1997; Talwani & Reif, 1998). The oldest confidently identified magnetic lineations
in the Arabian and Eastern Somali basins is chron 27, however, chron 28 is considered to
be present in the Arabian Sea immediately south of Laxmi Ridge (Naini and Talwani,
1983; Chaubey et al., 1998).
Because of the complex ridge jumps from the Mascarene Basin to the eastern Somali
and Arabian basins, plate motions of the Indian plate relative to the Madagascar, African
and Arabian plates have mainly been determined through a plate circuit passing by
Antarctica. Reconstructions of the eastern Somali and Arabian basins are also more
difficult due to the numerous propagators. Using a new compilation of magnetic
anomalies in this region, we attempt direct determination of these plate motions.
Bhattacharya, G. C., Chaubey, A. K., Murty, G. P. S., Srinivas, K., Sarma, K. V. L. N. S.,
Subrahmanyam, V. & Krishna, K. S., 1994. Evidence for seafloor spreading in the Laxmi Basin,
northeastern Arabian Sea, Earth Planet. Sci. Lett., 125, 211-220.
Chaubey, A. K., Bhattacharya, G. C., Murty, G. P. S. & Desa, M., 1993. Spreading history of the
Arabian Sea: some new constraints, Mar. Geol., 112, 343-352.
Chaubey, A. K., Bhattacharya, G. C., Murty, G. P. S., Srinivas, K., Ramprasad, T. & Gopala Rao, D.,
1998. Early Tertiary seafloor spreading magnetic anomalies and paleo-propagators in the northern Arabian
Sea, Earth Planet. Sci. Lett., 154, 41-52.
Dyment, J., 1998. Evolution of the Carlsberg ridge between 60 and 45 Ma: ridge propagation, spreading
asymmetry, and the Deccan - Réunion hot spot, J. Geophys. Res., 103, 24067-24084.
Karasik, A. M., Mercuriev, S. A., Mitin, L. I., Sochevanova, N. A. & Yanovsky, V. N., 1986.
Peculiarities in the history of opening of the Arabian Sea from systematic magnetic survey data (in
Russian), Documents of the Academy of Sciences of USSR, 286, 933-938.
Malod, J. A., Droz, L., Mustafa Kemal, B. & Patriat, P., 1997. Early spreading and continental to
oceanic basement transition beneath the Indus deep-sea fan: northeastern Arabian Sea, Mar. Geol., 141,
221-235.
McKenzie, D. P. & Sclater, J. G., 1971. The evolution of the Indian Ocean since the Late Cretaceous,
Geophysical Journal of Royal astronomy Society, 25, 437-528.
Mercuriev, S., Patriat, P. & Sochevanova, N., 1996. Evolution de la dorsale de carlsberg: évidence pour
une phase d'expansion très lete entre 40 et 25 Ma (A18 à A7), Oceanol. Acta, 19, 1-13.
Mercuriev, S. A. & Sochevanova, N. A., 1990. Complex patterns of the magnetic field as a consequence
of an ancient triple junction on the Carlsberg Ridge ? (in Russian), in Electromagnetic induction in the
World Ocean, ed. Jdanov, M., pp. 48-56, USSR Acad. Sci., Moscow.
Miles, P. R. & Roest, W. R., 1993. Earliest seafloor spreading magnetic anomalies in the north Arabian
Sea and the ocean-continent transition, Geophys. J. Int., 115, 1025-1031.
Naini, B.R. & Talwani., M., 1983. Structural framework and the evolutionary history of the continental
margin of western India, in Studies in Continental Margin Geology, eds Watkins, J.S. & Drake, C.L.,
AAPG Memoir, 34, pp. 167-191.
Norton, I. O. & Sclater, J. G., 1979. A model for the evolution of the Indian Ocean and the break-up of
Gondwanaland, J. Geophys. Res., 84, 6803-6830.
Schlich, R., 1982. The Indian Ocean: aseismic ridges, spreading centers and ocean basins, in The Indian
Ocean, eds Nairn, A. E. M. & Stheli, F. G., The Ocean Basins and Margins, 6, pp. 51-147, Plenum
Press, New-York, NY.
Talwani, M. & Reif, C., 1998. Laxmi Ridge-a continental sliver in the Arabian Sea, Mar. Geophys.
Res., 20, 259-271.
Whitmarsh, R. B., 1974. Some aspects of plate tectonics in the Arabian Sea, in Leg XXIII, Initial
Reports of the Deep Sea Drilling Program, eds Whitmarsh, R. B., Weser, O. E., Ross, D. A. et al., 23,
pp. 527-535, U.S. Government Printing Office, Washington DC.
THE
EASTERNMOST
MURRAY
RIDGE-MAKRAN
ACCRETIONARY WEDGE: EVIDENCE FOR A CONTINENTCONTINENT COLLISION?
H. U. Schlüter, Ch. Gaedicke, H. A. Roeser, A. Prexl, Ch. Reichert, and H. Meyer
Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover, Germany
The Murray Ridge extends about 750 km from the northernmost Owen Ridge in the
southwest to the triple junction off Karachi in the northeast, where the Arabian, Indian
and Eurasian Plates interfere. According to marine multi-channel seismic (MCS) data the
entire structural fabric of the Murray Ridge basement is interpreted to be composed of
faulted and tilted blocks of continental origin overlain by a thick volcanic rock
assemblage. This is supported by wide angle and refraction seismic measurements (Flueh
et al., 1997; Minshull et al., 1999) indicating thinned, 14 km to 20 km thick continental
crust below the southern Murray Ridge and the Dalrymple Trough. Below the northern
Murray Ridge the basement is characterized by divergent to subparallel reflections which
dip toward the Little Murray Ridge and the eastward narrowing Oman Abyssal Plain in
the north. This seismic pattern is interpreted as seaward dipping basalt flows, indicating
the continent-ocean transition from the Murray Ridge to the Oman Abyssal Plain.
Continuous positive gravity anomalies associated with ridge-like structures indicate a
continuation of the Murray Ridge to the northeast. There, the Murray Ridge is buried
below thick shelf and slope sediments. Even here the basement shows a seismic pattern
as it is known from basalt flows. This divergent pattern extends from the Murray Ridge
toward the Makran accretionary wedge, and is underlain by reflection elements with low
frequencies which form faulted block structures. Arrangement and shape of the whole
acoustic basement can be best described as downfaulted along listric faults toward the
north with block rotation to the southeast overlain by a thick sequence of north-dipping
basalt flows. This composite crustal type stretches below the easternmost thrusted
Makran wedge, terminating there along the eastern continuation of the Little Murray
Ridge. In consequence, the easternmost Makran accretionary wedge is thrust to the south,
on the volcanic Little Murray Ridge as well as on transitional crust, indicating the onset of
continent-continent collision.
SUBMERGED CONTINENTAL CRUST AND VOLCANISM IN THE
INDUS FAN AND MURRAY RIDGE AREA: IMPLICATIONS
FROM GRAVITY AND MAGNETIC MODELING
B. Schreckenberger, C. Gaedicke, H.-U. Schlüter and H.A. Roeser
Federal Institute for Geosciences and Natural Resources (BGR), Stilleweg 2, 30655
Hannover, Germany
The Murray Ridge/Dalrymple Trough system, that separates the Indus Fan from the
Oman Abyssal Plain, is a key area in the delimitation of plate boundaries in the North
Arabian Sea. We have indications from reflection seismic data, acquired during a
geophysical survey with the German research vessel SONNE, that the Murray Ridge is
underlain by continental crust. On a seismic line about 200 km southeast of the Murray
Ridge we observed reflectors that are inclined to the southeast and that resemble seawarddipping reflector sequences (SDRS) known from volcanic passive continental margins.
We infer that they mark a continental margin that belongs to a piece of submerged
continental crust between this location and the Murray Ridge. North of the Dalrymple
Trough there are also weak indications for northward dipping reflections that may mark
another continental margin, the boundary to the oceanic crust of the Oman Abyssal Plain.
We compiled a magnetic map for the North Arabian Sea from the data of two R/V
Sonne and two H.M.S. Dalrymple cruises and from other available ship track data. The
map shows distinct areas of different magnetic character that can be correlated with the
crustal types deduced from reflection and refraction seismics. Crust that has been inferred
to be of continental origin shows a smooth magnetic field. In the Murray
Ridge/Dalrymple Trough area it is overprinted by anomalies that seem to be caused by
volcanism possibly related to one or more extensional events that led to the development
of the Dalrymple Trough.
From several seismic profiles that cross all relevant structural elements from the
Makran accretionary wedge to the inferred continental margin in the south we constructed
a 500 km long transect for detailed gravity and magnetic model calculations. The gravity
model reveals that the inferred continental crust below the Indus Fan between the SDRS
in the south and the Murray Ridge in the north shows two distinct levels in Moho depth
or crustal thickness that we relate to different amounts of extension. The most distinct
feature that was revealed by the gravity model is a deep crustal root of the Little Murray
Ridge (LMR) in the Oman Abyssal Plain. Obviously, this ridge that seems to be the
source of a linear long-wavelength magnetic anomaly has a continuation below the
Makran accretionary wedge. The nature of the LMR and its relation to the ocean-continent
boundary will be discussed.
Extrapolation of the modeling results using a gravity map from satellite altimetry and
the magnetic map enables us to draw conclusions about the deep crustal structure in areas
where reflection seismic sections reveal the upper crustal structure but where no refraction
seismic measurements are available.
LATE
QUATERNARY
EVOLUTION
OF
MONSOONAL
CONDITIONS IN THE ARABIAN SEA: CONSTRAINTS ON
PALEOPRODUCTIVITY AND RELATED OXYGEN-MINIMUM
CONDITIONS OFFSHORE PAKISTAN DURING THE PAST
200,000 YEARS
Hartmut Schulz*#, Kay-Christian Emeis#, Helmut Erlenkeuser + , and Ulrich von
Rad*
* Bundesanstalt für Geowissenschaften und Rohstoffe, PF 510153, D-30631 Hannover,
Germany, now at: Institut für Ostseeforschung Warnemünde, PF 301161, D-18112
Rostock-Warnemünde, Germany; email: [email protected]
# Institut für Ostseeforschung, PF 301161, D-18112 Rostock-Warnemünde, Germany
+ Leibniz-Labor für Altersbestimmung und Isotopenforschung, Universität Kiel, MaxEyth-Str. 11-18, D-24118 Kiel, Germany
Recent evidence from piston coring in the northeastern Arabian Sea shows that the
intensity of the oxygen-minimum zone and hence, surface water productivity linked to the
monsoonal circulation have been extremely variable during the last glacial/interglacial
cycles, depicting a distinct pattern of fluctuations well correlatable to the record of
temperature fluctuations found in the Greenland ice cores. The possible synchrony
between the two climatic systems is strongly supported by the presence of the Toba
volcanic ash (~70,000 years before present), matching the paleoclimatic records of the
northern Arabian Sea and of Greenland directly and precisely.
New records of planktonic foraminiferal species abundances, of alkenone
paleotemperature, and of magnetic dust proxies investigated for the time interval from
~65,000 to ~75,000 years BP show that the warm North Atlantic interstadials correlate in
very detail to equivalent periods of warm sea surface temperatures (SST), enhanced
surface water productivity and lowered dust flux in the Arabian Sea. We estimate that the
cooling from interglacial to glacial, and from interstadial to stadial periods in the open
Arabian Sea were in the range of 4.5-3∞C and 3-1.5∞, respectively. These SSTamplitudes may have been even more pronounced in the coastal waters, due to stronger
influence of terrestrial temperatures and processes of local upwelling and mixing. Almost
similar to the Greenland record, warming and the establishment of high-productivity
conditions in the Arabian Sea occurred abruptly, within a few centuries or even decades,
followed by a more gradual cooling towards full stadial climate.
New planktonic foraminiferal data from sediment traps and from about 250 sediment
surface counts were used to localize the key areas of the Arabian sea productivity system
and to better constrain the seasonal cycle of fluxes. These data show that two major
processes are important: (1) coastal and open ocean upwelling of fertile subsurface waters
during summer, linked to the strong SW-monsoonal winds (2) deep winter mixing and
related injection of nutrients from greater depth due to surface water cooling by the NEmonsoon. These processes are corroborated by distinct planktonic foraminiferal
assemblages in the sediment, integrating information from a much larger part of the water
column than estimates from remote sensing which are bound to the uppermost surface
waters only. For the Western Indian Ocean, five different assemblages can be mapped off
the (a) East African (b) Somali (c) Oman and south Indian (d) Pakistan and (e) the
northeast Indian coasts. It can be assumed that both, the SW-monsoon and the NE
monsoon have a large impact on the productivity in most parts of the Arabian Sea. Based
on the sediment trap observations, the Oman and northeast Indian assemblages may
represent best the summer SW- and winter NE-monsoonal productivity regime,
respectively, as the Pakistan assemblage may be an ecotone in between. From this model,
a qualitative estimate of the seasonal intensity of the both monsoons in the past can be
given. Together with the climatic implications from the alkenone SSTs, these data will
help to decipher the complex pattern of the relative intensity of the NE and SW monsoons
during the millennial scale events.
Our longest piston cores from the Pakistan Margin clearly show that the scale-scale
events persisted throughout the past 200,000 years, with variable frequencies and
amplitudes. However, little is known about this kind of climatic variability in the geologic
past, for instance beyond the reach of the ice core records. A recent comparison of the
timing of these events from Greenland and Antarctica suggests that there was a strong
interhemispheric asynchrony of the millennial-scale climatic fluctuations, which leads us
to consider that the strong variability of the Arabian monsoon system may be externally
forced by the temperature contrasts between the both hemispheres. Only deep-sea drilling
will considerably extend our knowledge about the millennial-scale climatic variability into
the past, and help to test our present assumptions under changing boundary conditions,
invoking different mechanisms of climatic change, from sub-Milankovitch to tectonic time
scales.
HIGH-RESOLUTION RECORDS OF MONSOONAL RUN-OFF IN
THE EASTERN ARABIAN SEA.
A.D. Singh1, Raja S. Ganeshram2 & Dick Kroon2
1Department Marine Geology and Geophysics, Cochin University of Science and
Technology, Cochin, India.
2Departmenbt of Geology and Geophysics, University of Edinburgh, Edinburgh, UK.
The southwest monsoon is an important source of moisture to India, southern China
and southeast Asia. Monsoonal precipitation is orographically amplified in some localities
resulting in heavy rainfall in some regions. Thus, these areas record sensitively
perturbations in monsoonal systems. One such example is the interception of the
southwest Monsoon by Western Ghats which results in heavy precipitation along the
southwest coast of India (>4000 mm in some locations). Most of the moisture drains
rapidly into the Arabian Sea establishing a seasonal low-salinity lid on the eastern Arabian
Sea. We have constructed two high-resolution records of salinity changes from sediment
cores collected in the eastern Arabian Sea (off Goa; 324 & 840 m water depths) using
∂18 O of planktonic foraminifera Globigerinoides Ruber covering the Holocene and Last
Glacial Maximum. These records show large millennial-scale excursions in ∂18 O
(exceeding 1.5 0 in some intervals) all through the Holocene. We interpret these cyclic
changes to be related to the intensity of monsoonal precipitation and runoff from the
Western Ghats. The isotope salinity records will be compared with other geochemical
proxy records of upwelling (foraminiferal proxies), productivity (organic carbon, opal),
intensity of the OMZ, ∂15 N) and aragonite preservation in these cores.
SEQUENCE STRATIGRAPHY OF THE SOUTHERN KIRTHAR
FOLD BELT AND MIDDLE INDUS BASIN, PAKISTAN
John Smewing1 , John Warburton2 , and Tim Daley3 ,
1 Earth Resources Ltd
2 Lasmo Oil Pakistan Ltd
3 Lasmo Oil Venezuela Ltd
The Jurassic to Recent stratigraphy of the southern Kirthar Fold Belt and Middle
Indus Basin has been divided into 19 depositional sequences and 11 sub-sequences on
the basis of regionally extensive key surfaces. The resulting sequence stratigraphic
template provides a basis for rationalisation of stratigraphic nomenclature and the
potential to extend this scheme more widely on the Indo-Pakistan Plate.
The interval examined traces the evolution of this part of the Indo-Pakistan Plate
from an integral part of Gondwana through its rift-drift phase to final collision with
Eurasia. Many of the key surfaces and system tracts can be convincingly tied to plate
margin and intra-plate tectonic events taking place during this period. Significant
amongst these are the following:
• Erosion of Middle Jurassic limestones prior to rifting of the Indo-Pakistan plate
from Somalia.
• Late Early Cretaceous uplift of the Indian Shield sourcing progradational deltaic
clastics in the Lower Goru and Sembar of the Middle Indus Basin and their equivalent
shelf overspill sands in the Sembar of the western Kirthar Fold Belt.
• Uplift and erosion of at least 1km of pre-Santonian strata in the western Kirthar
Fold Belt as a consequence of the emplacement of Late Cretaceous ophiolites onto the
western margin of the Indo-Pakistan Plate.
• The generation of Campanian and Maastrichtian low-stand fans as a result of early
Deccan thermal uplift.
• The development of a major transgressive surface at the K-T boundary as a result of
emplacement of the Bela ophiolites to the west.
• The sudden influx of northerly derived clastics in the western Kirthar Fold Belt
from the Himalayan mountain front in the late Middle Eocene.
• Differential uplift related to inversion of Mesozoic extensional faults in the Late
Eocene.
SURFACE AND THERMOCLINE PALEOCEANOGRAPHY OF
THE ARABIAN SEA DURING EARLY AND MID HOLOCENE
Michael Staubwasser1,2, Frank Sirocko 3, Pieter. M. Grootes 4, and Helmut
Erlenkeuser 4
1 GeoForschungs Zentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany.
2 University of Oxford, Dept. of Earth Sciences, Parks Road, Oxford, OX1 3PR,
England. [email protected], fax: +44 1865 272072
3 Institut für Geowissenschaften, Universität Mainz, Johann-Joachim-Becher-Weg 21,
55099 Mainz, Germany.
4 Leibniz-Labor für Altersbestimmung und Isotopenforschung, Universität Kiel , MaxEyth-Str. 11-13, 24118 Kiel, Germany .
A high-resolution radiocarbon record of planktonic foraminifera (Globigerinoides
sacculifer, stable oxygen isotope distribution in planktonic foraminifera Globigerinoides
ruber, and a bulk sedimentary Uranium record from two laminated sediment cores SO9063KA and SO90-41KL from the same site on the upper continental margin off Pakistan
allow for a reconstruction of upper ocean hydrography in the Arabian Sea and climate
change within the Indian monsoon system from the Younger Dryas to the mid-Holocene.
The chronology of the cores was derived by least squares fitting of radiocarbon
plateaux observed in the sediment cores to those of the atmospheric radiocarbon.
Reservoir ages for the Arabian Sea surface resulting from this approach are not constant
and range from 765 to 1260 14 C years compared to a pre bomb value of 640 years. High
sedimentary Uranium content indicates low thermocline ventilation during the early
Holocene, which is linked to enhanced upwelling in the western Arabian Sea.
Nevertheless, upwelling alone is not sufficient to sustain surface reservoir ages well
above 1000 years, which at present is found at roughly 1 km water depth A significant
(95 % level) non-stationary centennial scale quasi-oscillation with a period of 227 years
was found in an oxygen isotope record of planktonic foraminifera Globigerinoides ruber
between 10200 cal. years BP and 8400 cal. years BP. It resembles both in frequency and
amplitude variation an early Holocene oscillation observed in the (cosmic ray produced)
atmospheric radiocarbon record, which is generally interpreted as being caused by solar
irradiance variability. Climate events observed elsewhere within the Asian-East African
monsoon system during the early Holocene around 10800 cal. years BP and 8150-8400
cal. years BP are again found in our record. The maximum intensity of the Indian
summer monsoon (SW monsoon) in the Northwest of the Indian subcontinent occurred
between 9300 and 8400 cal. years BP.
RECENT SEDIMENTATION ON THE MAKRAN MARGIN:
TURBIDITY CURRENT-HEMIPELAGIC INTERACTION IN AN
ACTIVE SLOPE-APRON SYSTEM
Dorrik A V Stow1 and Ali R Tabrez2
1 SOES-SOC, University of Southampton, Southampton SO14 3ZH, UK
2 National institute of Oceanography, Karachi, Pakistan
The Makran slope-apron system is a stepped convergent margin across an active
subduction complex. Shallow penetration piston cores have been recovered from the
upper slope region, three mid-slope basins and the abyssal plain. At most sites the upper
5-14 m of cored section is dominated by fine-grained, very thin to thin-bedded turbidites,
averaging 5-10 turbidite events per metre of section. Oxygen isotope stratigraphy yields
mean sedimentation rates of 50-95 cm/ky and a turbidite frequency of one event per 200300 y. The upper slope site has fewer turbidites and a greater proportion of hemipelagic
mud. Stow turbidite sequences are common, with top-cut-out and base-cut-out sequences
most evident. Markov chain analysis of the transition between turbidite divisions
confirms the normal T0-T8 order. In some cases there is an upward gradation into a
hemiturbidite facies. Small-scale vertical variations of turbidite bed thickness can be
interpreted as the result of compensation cycles. The lateral distribution of both turbidites
and hemipelagites is influenced by sediment focusing along pathways between individual
slope basins. At a larger scale, climate, sea-level and tectonic effects have all played an
important role in shaping margin sedimentation.
School of Ocean and Earth Science
Southampton Oceanography Centre
University of Southampton
Waterfront Campus
Southampton SO14 3ZH, UK
Tel: 44 (0)23 80593049*
Fax: 44 (0)23 80593052*
Email: [email protected] also: [email protected]
PALAEOMAGNETIC EXCURSIONS IN THE MAKRAN MARGIN
SEDIMENTS
Ali Rashid Tabrez*, Dorrik A. V. Stow1, Norman Hamilton 1, Maarten A. Prins2
and George Postma3
* National Institute of Oceanography, St-47, Block-1, Clifton, Karachi, Pakistan. E-mail:
[email protected]
1 School of Ocean and Earth Science, Southampton Oceanography Centre, University of
Southampton, Southampton SO 14 3ZH, UK. E-mail: [email protected]
2 Faculty of Earth Sciences, Vreije Universitait Amsterdam, De Boelelaan 1085, 1081
HV Amsterdam, The Netherlands, E-mail: [email protected]
3 Utrecht University, Institute of Earth Sciences, Department of Geology, Budapestlaan 4
P.O. Box 80.021, 3508 TA Utrecht, The Netherlands. E-mail: [email protected]
Shallow piston cores were recovered from N-S slope transect across the Makran
Continental Margin during the joint research programme under Netherlands Indian Ocean
programme (NIOP) in 1992. This was a collaborative programme between
(NIO),Pakistan and the University of Utrecht, Netherlands. Cores from mid-slope basin
of Makran (Water Depth 2482m : 14 m length), and abyssal plain (Water Depth 3274 m :
10.5 m length) reveal the climatic change that perhaps is leading to a more dominant
monsoonal condition in the early Holocene and hence an increased supply of terrigenous
material including magnetic grains.
Short duration palaeomagnetic excursions observed (6000-8000 yr. BP and 26000
yr. BP) noted in both cores reflect true geomagnetics field behaviors on the Makran
margin. These short excursion observed were based on the oxygen isotope stratigraphy
established (Tabrez, 1995). Early Holocene excursion (6000-8000 yr.. BP) in the abyssal
plain core is apparently younger than previously reported excursion events such as the
Gulf of Mexico, event (12500-17000 yr.. BP) Imuruk Lake, Alaska (17000-18000 yr.
BP) and Baffin Bay (around 18000 yr. BP). An older possible reversal (26000 yr. BP)
apparently occurs in the mid-slope core. It is suggested that this could be assigned to a
time prior to the last glacial maximum and may therefore correlate with the Mono Lake
event between 24000 yr. and 29000 yr.. BP, if sedimentation was continuous.
NEOTECTONICS ON THE ARABIAN SEA COASTS
Claudio Vita-Finzi
Department of Geological Sciences, University College, London, UK
The Holocene record on the coasts of the Arabian Sea provides useful information on
the nature and rate of deformation generated by interaction between the Indian, Arabian
and Eurasian plates. Holocene marine terraces show that the southern Makran (AR/EU)
has been subject to the infrequent but vigorous coseismic uplift that characterises other
subduction settings and they indicate landward rotation of the imbricate faults among
which shortening is distributed. The modest deformation recorded on the SE coast of the
Arabian peninsula is consistent with its position parallel to a transform (AR/IN) with the
exception of the southwestern extremity, where there is discontinuous uplift linked to Red
Sea spreading (AF/AR), and on the Straits of Hormuz in the northeast where, in the
absence of subduction, convergence (AR/EU) leads to rapid vertical movements on both
coasts. On the southwestern coast of India there is geomorphological and tide-gauge
evidence for localised uplift which may represent compressional buckling associated with
the Himalayan collision (IN/EU). Advances in bathymetry and geodesy are beginning to
bridge these sequences and thus enhance their value for quantifying plate rheology and
dynamics.
Poster Abstract
DECADAL TO MILLENNIAL CYCLICITY IN VARVED ARABIAN
SEA SEDIMENTS: HYPOTHESIS OF TIDAL ORIGIN
Ulrich von Rad 1 and W. H. Berger2
1 Bundesanstalt für Geowissenschaften und Rohstoffe, PF 510153, D-30631 Hannover,
GERMANY ([email protected])
2 Scripps Institution of Oceanography, La Jolla, Calif. 92093-0215, USA
([email protected])
Varved sediments occur in lakes and in the ocean, especially in fjords and restricted,
silled basins (e.g., Cariaco Basin, Santa Barbara Basin, Saanich Inlet), but also in openmarine settings, where the oxygen minimum zone (OMZ) impinges onto the upper
continental slope and hence lack of oxygen prevents burrowing. Year-by-year
information is preserved in such circumstances, potentially over thousands of years. A
number of varved records have been analysed for thickness variability and for other
properties. Although such time series analyses indicate that sedimentation is cyclic, it is
not clear whether the observed cycles have more than local significance and how they are
generated. Already in 1890 the Austrian meteorologist Eduard Brueckner detected climate
cycles with a typical period around 35 years (“Brueckner cycle”). Here we document the
presence, at the margin off Pakistan, of cycles that may have regional or even global
significance. We also propose a possible mechanism, linking many of the cycles found to
interference patterns generated by tidal action.
Using autocorrelation and Fourier decomposition analysis in various combinations,
we studied three, up to 5000-yr long, Late Holocene and Last Glacial Maximum records
of annually dated, varved sediments deposited in the OMZ off Pakistan. Prominent highfrequency cycles were found in the varve thickness around 12.2 yr., 14 yr., 18.7 yr., 23
yr., 29 yr., 37 yr. (“Brueckner cycle”), and around 56 yr. (the sedimentary gray-value
also shows strong variability in the 55-yr. band, which might be of solunar origin) Lowfrequency cycles center around and 95-96 yr., 125 yr. (7 * 17.7 = 123.9), 250 yr., 280
yr. (8 * 35.4 = 283.2), 366 yr. (10 * 35.4 = 354) and 460 yr. (13 * 35.4 = 460.2). The
1470-yr. cycle, well known from the Greenland ice core record, is also present in our
varves. Some cycles of varve thickness match the cyclicity of turbidite frequency (25-27
yr., 30 yr., 43 yr (5*8.85=44.25)., 54 yr. (3*17.7 =53.1), 64 yr., 95 yr., 115 yr., 248
yr.). The match can be explained by the fact that precipitation in the hinterland and river
runoff control varve thickness and turbidite frequency .
We found evidence for cyclicity in the band studied by Brueckner and of a variety of
other cycles that form a staircase arrangement of various harmonics of the lunar tidal
spectra of 4.25 yr., 8.85 yr., 17.7 yr. and 35.4 yr. Similar cycles were found in varves
from the Santa Barbara Basin and in other high-resolution records. In some records, the
cycles emerge especially when analysis is in terms of unusual excursions from
background fluctuations (agitation cycles).
Although up to now mainly solar and ENSO forcing have been proposed to explain
the annual to centennial variability of marine palaeoclimate records, we suggest that there
is evidence for lunar forcing. Lunar tidal activity may be of modest importance as a
determinant of climate change. However, it may be useful in gauging the responsiveness
of the system to weak outside forcing by variable tidal action on ocean mixing, either by
influencing vapour transport to the continent (leading to changes in precipitation and
fluvial input) or by increasing the re-suspension of fine-grained sediment on the shelf
resulting in increased varve thickness or turbidite frequency in the upper slope
environment.
THE KIRTHAR FOLDBELT, PAKISTAN; AN EMERGING
MAJOR PETROLEUM PROVINCE
J. Warburton*, M. Ali*, S. Beswetherick*, J. Craig**, J. Fowler**, R.
Graham**, S. F. Habib , N. Haq*, R. Hedley*, J. Smewing# & J. Watts***
* LASMO Oil Pakistan Ltd
** LASMO plc London(\
# Earth Resources Ltd
*** Late
The Kirthar Fold-belt formed during Late Eocene to Recent times in the Himalayan
collision zone between the Indo-Pakistan and Eurasian Plates. The fold belt is
characterized by broad buckle folds separated by narrow synclines previously
interpreted as the result of thin-skinned tectonics terminating abruptly eastwards in a
major passive backthrust. The current interpretation is of thick-skinned inversion on
pre-existing extensional faults developed on the Indo-Pakistan Plate passive margin.
The Kirthar Fold-belt was established as a major petroleum province in 1997 when
commercial gas reserves were discovered in Late Cretaceous sandstones within the Bhit
Anticline. The depositional environments for source, reservoir and seal play elements in
the fold belt have been evaluated within a revised sequence stratigraphic framework
using regional data extending from the western Kirthar Fold-belt to the Middle Indus
Basin in the east.
Further gas discoveries were made in 1998-99 at Zamzama and Badhra and many
petroleum exploration companies are now actively exploring in the Kirthar Fold-belt for
additional gas reserves. It is estimated that approximately 3 TCF of gas reserves have
been found in the Kirthar Fold-belt to date and that an additional 10 to 15 TCF remain
still to be discovered.
We continue to improve our understanding of the relationships between structural
geology and depositional environments in the Kirthar Fold-belt as new exploration
wells, outcrop and seismic data are evaluated. Delivering new gas reserves will require
rigorous multi-disciplined understanding of the subtle stratigraphic variations caused by
extensional faults that existed prior to compressional deformation. Breakthroughs in
subsurface seismic imaging and 3D modelling and visualization of compressional fold
belt traps are required to improve our capacity to identify, constrain and explore ever
more complex and hidden exploration targets.
A useful analogue for the Kirthar Fold-belt is the Papuan Fold-belt of Papua New
Guinea. Observations from petroleum exploration and production in PNG provide
important clues for generating new and exciting play concepts for the Kirthar Fold-belt.
THE INFLUENCE OF BOTTOM WATER OXYGEN ON CALCITE
PRESERVATION IN THE ARABIAN SEA
Ines Wendler, Karin A.F. Zonneveld, Helmut Willems
FB Geowissenschaften, Universität Bremen, 28334 Bremen, Germany. E-mail:
[email protected]
High productive oceanic areas such as the Arabian Sea are suitable sites for detailed
studies aiming at the reconstruction of paleoceanographic and -climatic conditions.
However, as a consequence of the large amounts of produced organic matter, most
proxies used for such reconstructions (e.g. calcareous microfossils) are affected by
processes of early diagenetic alteration, driven by oxic decay of organic matter. For the
good calibration of a proxy, it is necessary to understand and assess the impact of early
diagenetic processes on its primary signal first. The different oxygen concentrations in the
bottom waters within and below the intense permanent mid-water Oxygen Minimum Zone
(OMZ) in the Arabian Sea provide an excellent opportunity to study these processes in
various diagenetic settings. We use the calcareous cysts of dinoflagellates (unicellular,
primary producers living in the photic zone of the oceans) to examine (1) whether there is
considerable calcite dissolution at the sediment - water interface above the lysocline; (2) if
this is the case, whether dissolution is stronger in the Corg-rich sediments from within
the OMZ or in those from below this zone, where much less organic matter accumulates;
and (3) whether there is species selective preservation.
The distribution of absolute cyst abundances in the studied surface sediments of the
NE Arabian Sea, where the OMZ has its greatest thickness and intensity, reveal a close
relation to the relative bottom water oxygen concentrations: much higher cyst
accumulation rates are found at stations where the OMZ impinges on the continental slope
or on the Murray Ridge than at deeper stations. This indicates better preservation of
calcite within the Corg-rich sediments of the OMZ and strong calcite dissolution at the
sediment - water interface below the oxygen depleted zone but in water depths above the
lysocline. The latter is most probably caused by oxic degradation of organic matter and
associated production of metabolic CO2, which lowers the pH of the pore waters. More
offshore, increasing oxygen exposure times due to decreasing sedimentation rates would
enhance dissolution, as would bioturbation (in which particles pass through gut
environments and become fragmented). Less corrosive pore waters can be expected
within the OMZ, caused by (1) reduced organic matter degradation; and (2) enhanced
sulphate reduction which can increase pore water alkalinity. That there is indication of
different dissolution sensitivity among the individual cyst species can be inferred from the
comparison of nearby samples bracketing the lower boundary of the OMZ. This
comparison shows (1) shifts in the relative abundances; (2) a different degree of decrease
in absolute abundance of the individual species; and (3) differences in the percentage of
cyst fragmentation.
Whereas the primary signals of calcareous microfossils in the NE Arabian Sea seem
to be strongly altered by diagenetic overprinting, a relation between cyst distribution and
the OMZ is not notable in a profile off Somalia (SW Arabian Sea). It is likely that the
strong seasonal upwelling in this region leads to fast sedimentation of particles and to the
preservation of a signal which mainly contains ecological information, except for samples
from water depths greater than 3000 m which are affected by calcite dissolution due to
deep water undersaturation. These results show that the signals preserved in the
sediments in two different parts of the Arabian Sea - although both characterised by high
primary production and the presence of a permanent OMZ - can either represent primary
signals of production in the surface waters or be dominated by early diagenetic alteration.
EVOLUTION OF DEPOSITIONAL ENVIRONMENTS IN THE
MIOCENE TO RECENT INDUS DELTA: EVIDENCE FROM
INTERPRETATION OF 3D SEISMIC, OFFSHORE INDUS BLOCK
C HYDROCARBON CONCESSION.
M. Wilkes1, S. Henderson1, J. Swallow1, Waheed A2 and Z. Zafar2
1 BG group, Middle East Team, 100 Thames Valley Park Drive, Reading, Berks, RG6
1PT, UK
2 BG Pakistan, House 18, Street 18, Sector F-6/2, P.O. Box 2333, Islamabad, Pakistan
The Offshore Indus Block C Hydrocarbon Concession lies at the margin of the
present day shelf break. It is approximately 100 km southeast of Karachi, just to the west
of the Swatch, the modern day canyon incision into the shelf, and 80 km from the mouth
of the Indus river. A 1090 km2 3D seismic survey was acquired in the south of the
concession during 2000. Most of the survey covers shallow water depths 80 to 250m and
the edge of the shelf .
The survey covers a large complex of crosscutting canyons that formed in the
Miocene. Each canyon has a similar geometry to the Swatch, a modern incision into the
shelf, and are stacked together to form an erosional feature up to 40 km wide and 1 km
deep. Analysis of 3D seismic data in cross-section and horizontal slices has identified
geomorphological features that can be related to their depositional environments. Seismic
facies analysis has also been used to build
models of deposition environments both on the shelf and within the canyon fill.
On the basis of these geomorphological features, the sediments in the survey area may be
divided into four “systems” with distinct depositional style.
1. System 1 is comprised of the Miocene host rock into which the canyons have incised.
This contains low sinuosity, shallow channels that are aligned perpendicular to the shelf
break, and crosscut major growth faults.
2. System 2 comprises the canyon fill. Seismic facies analysis of this system has
identified, low-stand channels, channel and levee facies and background pelagic
sediments.
3. System 3 comprises prograding deltas that infill an embayment created by the late
stages of canyon filling.
4. System 4 is comprised of delta and shelf systems in the Pliocene to recent Indus delta.
Analysis of these systems and the differences between them illustrates changes in the
depositional style of the Indus Delta throughout the Miocene to recent within the survey
area.
LATE QUATERNARY SEA-LEVEL
SOUTHERN ARABIAN GULF
HIGHSTANDS
IN
THE
Alun Williams
Oolithica Geoscience Ltd, 489 Union Street, Aberdeen AB11 6DB, Scotland, UK
The Arabian Gulf is a shallow sea with a gentle northwards-sloping floor, connected
to the Arabian Sea at the Straits of Hormuz. This area is presently the site of abundant
carbonate deposition, as was the case during late Quaternary interglacial high-stands.
During glacial low-stands much of the sea floor was exposed, leading to the deposition of
extensive carbonate aeolianites composed of remobilised marine-derived sediment.
Pleistocene sediments are preserved onshore in scattered outcrops in Abu Dhabi, Qatar,
Saudi Arabia and Kuwait. It is proposed that these deposits are largely unaffected by
tectonism, and can be taken as a proxy for late Quaternary sea levels in the region.
Pleistocene deposits in the southern Arabian Gulf belong to the Ghayathi Formation,
the Aradah Formation, and the Fuwayrit Formation. The Ghayathi Formation forms the
most voluminous Pleistocene unit in the region. It consists of predominantly marinederived aeolianites, preserved as lithified paleodunes. Inland, these are overlain in places
by continental sabkha-type deposits of the Aradah Formation. In coastal areas, the
Ghayathi Formation has been truncated by the Fuwayrit Formation, a thin unit of
shallow-marine and aeolianite deposits. Three members have been recognized within the
Fuwayrit Formation, separated by subaerial exposure surfaces. These include two
shallow marine units: the basal Futaisi Member, reaching 1.5m above present sea level,
and the Dabb'iya Member, which reaches 6m above sea level. In Qatar the Dabb'iya
Member is overlain by carbonate aeolianites of the Al Wusayl Member. The Fuwayrit
Formation is interpreted as representing deposition during the peak of the last interglacial,
recording two high-stands within substage 5e. This assumption is thought to be valid as
1) they represent the youngest pre-Holocene marine deposits preserved onshore, and 2)
they are found at an elevation correlative with substage 5e deposits from other parts of the
globe.
In addition, there is widespread evidence for a Holocene sea level higher than present
in the southern Arabian Gulf. This suggests that present sea level was attained
approximately 6000 years B.P., and had reached a peak of 1-2m higher than present by
5800 years B.P. The sea did not return to its present level until after 2300 years B.P..