Global and Planetary Change 28 Ž2001. 93–106
www.elsevier.comrlocatergloplacha
Physical climate signatures in shallow- and deep-water deltas
George Postma)
Sedimentology Group, Earth Science Department, Utrecht UniÕersity, P.O. Box 80021, 3508 TA Utrecht, Netherlands
Received 27 May 1999; received in revised form 27 August 1999; accepted 21 October 1999
Abstract
Physical signatures of climate change in delta successions occur at various scales and frequencies. This paper shows
examples of physical products of high frequency Žriver floods. and orbital-forced climate change as recorded in
coarse-clastic shallow and deep-water delta systems. Changes in frequency of sea level oscillation occurring on a geological
time scale Ži.e. green-house and ice-house periods. may result in a drastic change in delta architecture from shelf-edge to
shelf deltas. q 2001 Elsevier Science B.V. All rights reserved.
Keywords: climate change; river-flood deposits; turbidites; delta; delta architecture; shelf
1. Introduction
Climate variations affect a multitude of variables
that are all important for the final delta product.
Climate can leave its signature on three different
scales: Ž1. Short period catastrophic events that cause
important changes in run off with instantaneous increase in both competence and carrying capacity of
the streams resulting in river floods with important
increases in the total sediment supply and grain size
delivered onto the delta system; Ž2. Milankovitchscale changes in climate Žrun off and changes in
sediment yield. that have a noticeable long-term
effect on supply which may or may not be enhanced
or counteracted by glacio-eustatic sea level changes
and resultant shelf exposure. This long-term effect
causes progradation or retrogradation of the entire
)
Tel.: q31-30-2534155; fax: q31-30-2535030.
E-mail address: [email protected] ŽG. Postma..
delta system and may result in large scale coarsening
and fining upward sequences in turbiditic prodelta
sediments; Ž3. Geological scale changes governed by
the interaction of plate tectonics and Milankovitch
controlled variations in insolation as reflected by
green-house and ice-house periods ŽFig. 1.. These
climate changes on a geological scale should have,
apart from a possible long-term change in sediment
yield, an important bearing on delta architecture
because of important changes in frequency of sea
level oscillation.
Small deltas fed by streams issuing from a relatively small drainage basin will be more sensitive to
climate variations than very large deltas with a vast
drainage basin. Compare for instance the drainage
basin of the Mississippi delta with any fjord delta
ŽPostma, 1990, his Table 1.. In this perspective, the
prodelta environments of the often small, coarsegrained deltas are probably the best recorders of
relative sea level and climate change. Paralic systems
Že.g. delta plain and delta front. are less suitable,
0921-8181r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 8 1 8 1 Ž 0 0 . 0 0 0 6 7 - 9
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G. Postmar Global and Planetary Change 28 (2001) 93–106
Fig. 1. Mean global temperature curve. Relatively cool periods and warm periods are indicated. There appear to be two periods where high
frequency sea level changes occur Žshaded.: Namurian–Westphalien ŽMississippian–Pennsylvanian. and the late Pliocene–Quaternary
period Žbased on Frakes et al., 1992..
because the record of both short and long period
climatic andror sea level changes are partially or
totally obscured by other factors Že.g. Algeo and
Wilkinson, 1988.. In addition, the preservation potential of the prodelta succession is generally high
being beyond the reach of AnormalB delta reworking
processes such as waves, tides and gravity Žslope
instability..
The prodelta realms vary in significance depending on basin relief and sea level. Shallow water
deltas are generally characterised by three physiographic zones: Ž1. the delta plain, where fluvial
processes are dominant; Ž2. the delta front, where
fluvial and basinal processes operate; and Ž3. the
prodelta, where basinal processes dominate ŽPostma,
1998, 1990; Reading, 1995.. In deep water deltas the
delta front is separated from the prodelta by a distinct delta slope. The slope is beyond the direct
influence of waves and variously dominated by basinal processes Žsuspension settling. and gravity-driven
mass transport. In this classification view, those parts
of the basin influenced directly by delta-controlled
sedimentation processes, albeit undoubtedly combined with other processes ŽCoriolis force, tidal currents, contour currents, etc.. belong to the delta
system. This means that even some very large, clastic deep-sea systems extending hundreds of kilometres away from the delta front Že.g. Bengal fan,
Mississippi fan, Indus fan. can be considered a
AprodeltaB, and most certainly so during sea level
lowstands ŽPostma, 1990..
The aim of this paper is to review and examine
physical features in delta systems and complexes that
relate or may relate to climate change on various
time scales. The examples given here mainly concern
prodelta facies of relative small radius and coarsegrained deltas that are demonstrably related to supply variation inferred to be triggered by climate
induced variation in run off.
2. Physical signatures of short-period, catastrophic changes in climate
Extreme river or stream floods are generally the
result of brief periods Žhours–days. of adverse climate conditions, which may characterise all types of
climate, from wet to arid. A brief but heavy rainfall
over a relatively small area surrounded by steep
slopes may cause a flash flood: a sudden flood or
torrent overflowing a stream Žriver. channel carrying
an immense sediment load Že.g. Allen, 1985.. The
stream power and the inertia of these floods are often
sufficient to erode and largely bypass delta plain and
delta front to deposit much of their load in the
prodelta environment.
Examples of recent flash flood deposits are known
from sidescan sonar images of the south coast of the
G. Postmar Global and Planetary Change 28 (2001) 93–106
Corinthian Gulf ŽKatsonopoulou and Soter, 1991;
Soter and Katsonopoulou, 1998.. These images show
distinct lobes of coarse debris in a modern, shallow
water prodelta environment ŽFig. 2.. Between the
Vouraikos and Selinous rivers of the south coast of
the Gulf of Corinth, the lobes are generally elongated, but yet show strong variation in lengthrwidth
ratios. Lobe L2 is completely buried by younger
sediment and only revealed from seismic profiles.
Lobes L1 and L3-5 are about similar in size. Lobe 5
appears to be composite and built by more than one
event. The origin of lobes 3–5 can be related to
recent catastrophic river flooding. Although there is
95
no river outlet at these positions now, a map of the
delta from 1862, drawn by Schmidt Ž1875., cited in
Soter and Katsonopoulou Ž1999. clearly shows river
outlet positions close to the upslope end of these
lobes. The large size of the L2 deposit may point to
an extreme flood or amalgamation of a number of
flood deposits possibly during late Pleistocene sea
level lowstand judged on basis of the thickness of
the sedimentary cover.
The contribution of floods to deposition on the
delta front of coarse-grained, shallow-water deltas is
also suggested by studies on ancient, fossilised examples of similar setting as those of the Corinthian
Fig. 2. Map showing delta front lobes of various size in the near shore zone. Light grey shading represents shelf up to 40 m depth, dark grey
is alluvium. L1 s 30 = 400 m Ž25 000 m2 .; L2 G 165 = 1400 m Ž) 500 000 m2 .; L3 s 30 = 170 m Ž4000 m2 .; L4 s 30 = 250 m Ž5000
m2 .; L5 s 75 = 150 m Žmore than 1 event. Ž7000 m2 .; data from Dr. Steven Soter, published with permission in Postma, 1998.
96
G. Postmar Global and Planetary Change 28 (2001) 93–106
Fig. 3. Two examples of inferred flash-flood Ždebris-flow. deposits in the shallow marine zone of ancient coarse-grained deltas Žfrom Postma, 1998.. ŽA. Detail of the basal part
of a coarse-grained, clast-supported relatively high Žcompared with the example of ŽB.. discharge flash-flood deposit of the upper Miocene Prina Series ŽPostma and Drinia,
1993, Kalamavka, Crete.. IG s inverse grading at the base. Palaeo-flow direction is to the right. Scale is shown by rubber grip of hammer Žh., which is about 15 cm. ŽB.
Parallel-flow section Žflow direction is to the right. through delta front of the upper Pliocene Gador Formation ŽPostma, 1984, 1995; Almeria, SE Spain.. Coarse-grained
mouthbar systems enveloped by burrowed, wave-worked muds ŽM. show a complex internal structure with large scale cross-bedding ŽCB., parallel layering ŽPL. and truncation
surfaces ŽTS. pointing to a multi-storey build up by a number of events. Each event is characterized by strong current and high sediment load inferred to relate to
moderate-discharge Žcompare with the example of Fig. 3A. flashfloods. The planed surfaces with pebble lags ŽWW. point to intermittent wave reworking Žcompare with
wave-cut platform.. The origin of the structure indicated with A?B in the centre of the photograph is not well understood, but may be related to local liquefaction.
G. Postmar Global and Planetary Change 28 (2001) 93–106
Fig. 4. Inferred flash-flood deposit in the distal prodelta segment Žbelow SWB. of a shoal water type delta of the Gador Formation in the Rambla de dos Areos Žnear the village
Pechina., Almeria, SE Spain. The deposit is heterogeneous showing gravelly, cross-bedded facies at the base Ž2., a graded sand unit with hummocky-like Žlow-angle truncations
in various directions. cross-stratification overlying a conspicuous erosive surface and low-angle truncations of sand drapes Ž1. and small-scale cross-bedded ripple sets Ž3.. A
much younger channel unit is truncating the deposit Ž4.. The entire flash-flood unit is enveloped by bioturbated offshore muds, not showing structures indicative for wave
reworking. The overall geological setting of late Pliocene Gador Formation is described by Postma Ž1984..
97
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G. Postmar Global and Planetary Change 28 (2001) 93–106
Gulf Že.g. Larsen and Steel, 1978; Kleinspehn et al.,
1984; Orton, 1988.. Examples of the Prina Series on
Crete ŽPostma and Drinia, 1993, Fig. 3A. show
boulder containing debris-flow like deposits with
inverse grading at the base of the deposit overlying
shallow marine deposits. Lithophaga borings in some
of the transported limestone boulders and cobbles
indicate that original beach material has been incorporated in the flow. Examples from southeastern
Spain are somewhat different being much finer
grained than the Cretan examples ŽFig. 3B.. The
latter show internally heterogeneous, gravelly sand
lobes that are enveloped by shallow marine sand and
mud Žmixture of fine sand, silt and clay. that show
wave-produced structures characteristic of proximal
delta settings Žabove storm wave base ŽSWB.. and
bioturbated, offshore muds of more distal settings
below SWB. The complex internal structure Žcrossbedding and near-horizontally layered gravel and
sand. of the lobes shows evidence for strong currents
capable of displacing significant amounts of pebble-,
cobble- and boulder-sized sediment. The internal
truncation Žerosion. surfaces indicate that the delta
front lobes either may have been built up by several
events, or that the flood velocity Žand related competence. was typically unsteady. In proximal delta
settings, planed surfaces with gravel lags that are
continuous throughout the outcrop are typical for
wave erosion Žca. wave-cut platform erosion; see
further Postma, 1995.. Offshore Ži.e. below SWB.,
the top of the flash flood deposits are not reworked
by waves. Fig. 4 shows a common sequence found in
offshore flash flood deposits of the Gador Formation
ŽSE Spain, see Postma, 1984.. The sequence of the
deposit is characterised by a gravelly base with
abundant traction Žcross-bedding. structures, capped
by a graded sandy part. The sandy part is characterised by hummocky-type Žlow angle truncations.
cross-bedding, cm-thick sand drapes and small scale
and climbing ripple sets. Similar sequences in the fan
delta conglomerates of the Val Borbera in the Tertiary Piedmont Basin ŽMutti et al., 1995., and the
Tertiary of the Tremp-Graus Basin ŽMutti et al.,
1994. have been attributed to catastrophic river
flooding by these authors.
River flood deposits in deep water prodelta environments are typical turbiditic in nature and often
contain abundant organic material such as plant and
tree remains. Without such independent compositional evidence, these turbiditic deposits cannot easily be identified from turbidites resulting from slope
instability. A typical, deep-water prodelta facies
shows extreme variation in sand bed thickness, often
conspicuous rhythmicity in bed thickness, which has
been variously ascribed to seasonality in run off Žca.
Mastalerz, 1990. or compensation cycles ŽMutti and
Sonnino, 1981; Richards and McCaffrey, 1999.. The
absence of channels, the tapering of the turbidite
sequence, the thickening and coarsening upward sequences and the lens-shaped geometries of amalgamated beds Žcompensation cycles., all point to deposition in relatively small sand splays or lobes Že.g.
Prior and Bornhold, 1990; Mutti and Normark, 1987.
and are typical for the prodelta of a small-radius, late
Miocene, deep-water delta on Gavdos, which is discussed below.
3. Physical signatures of Milankovitch-scale
changes in climate
Milankovitch cyclicity in prodelta turbidites is
known from the Eastern Mediterranean Basin for the
early Late Miocene ŽPostma et al., 1993. and early
Pliocene ŽWeltje and de Boer, 1993.. The turbidites
occur in successions of alternating hemipelagic Žbioturbated marls. and sapropelic Žlaminitic. sediments,
the latter being a mixture of well-layered siliciclastic
and organic material. Krijgsman et al. Ž1995. and
Hilgen et al. Ž1995. attributed the origin of the
laminite-marl couplets to astronomical forcing, since
the occurrence and thickness of the sapropelic layer
are roughly correlatable with the insolation curve of
Laskar et al. Ž1993.. One couplet would represent
one cycle of precession, clusters of couplets the 100
and 400 ka cycles of eccentricity. The abundance of
preserved organic material in the sapropels can be
ascribed to a combination of anoxic conditions at the
sea bottom on the one hand and high organic production on the other Žsee Rohling, 1994 for a review on
this matter.. Stable isotopes on planktonic
foraminifers show that sapropel formation was contemporaneous with periods of increased continental
run off ŽVan der Zwaan and Gudjonsson, 1986.. The
latter periods have been correlated with an intensified Indian Ocean SW summer monsoonal system
G. Postmar Global and Planetary Change 28 (2001) 93–106
99
Fig. 5. Sedimentary logs of Metochia-B section. Note that turbidite sequences substitute laminite Žsapropel. intervals ŽL7–L13.. Compare
with Figs. 6 and 7.
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G. Postmar Global and Planetary Change 28 (2001) 93–106
Fig. 6. Deep water prodelta succession of the Metochia-B Žearly Late Miocene, Gavdos, Greece; Postma et al., 1993.. The lowermost
laminite sequence that is marked with arrows on the photograph is L5, the highest marked turbidite cycle is L11. The thickness of both
turbidite beds and individual turbidite sequences vary strongly laterally. Note the rapid wedging of turbidite sands in the top of turbidite
cycle L10. Duration of each turbidite cycle is of the order of 3–5 ka Žsee text.. Width of the view is approximately 15 m.
G. Postmar Global and Planetary Change 28 (2001) 93–106
101
Fig. 7. Rhythmically and very thinly bedded turbidites of cycle L7 Žsee detailed log of Fig. 5.. Note the abundant clay layers Ždark grey.
draping cross-bedded sand layers of various thickness. The whitish mm drapes Žarrowed. are diatomaceous laminites, each laminite marking
seasonal algal blooms.
102
G. Postmar Global and Planetary Change 28 (2001) 93–106
influencing the eastern Mediterranean via the Nile
river and by increased continental activity of
Mediterranean depressions as an element of the
westerly Atlantic system ŽPrell and Kutzbach, 1987;
Rohling, 1994.. The intensification occurs when perihelion coincides with the Northern Hemisphere
summer ŽRossignol-Strick, 1985, 1987; Hilgen,
1991.. The sapropels provide, therefore, important
clues with regards to the origin, the timing and the
duration of climate-related turbidite deposition in
this basin.
The Metochia-B section on Gavdos Žsmall island
south of Crete. shows rhythmic alternation of sapropelic Žnon-bioturbated laminitic. intervals and bioturbated homogenised marl beds deposited in approximately 850 m water depth ŽPostma et al., 1993.. The
sapropelic intervals, with an estimated duration of
2–5 ka ŽTroelstra et al., 1990. become dominated by
sandy turbidites from L7-13 ŽFig. 5.. The turbidite
sequences are characterised by multiple events of
thinly bedded, wedging turbidites, each turbidite sequence being covered by a marl layer ŽFig. 6.. Fig. 7
gives a detail of the L7 turbidite succession, which
contains at least 100 separate turbidite events
recorded as thinly layered fine sandy to silty TŽab.c-e
turbidites ranging in thickness from 0.5 to 3 cm. The
fine-grained intervals have a typical brown to yellow
colour and contain organic plant material, which
contrasts markedly with the blue-grey colours of the
overlying homogeneous, hemipelagic marl interval.
L12 and L13 are very similar to L7 and contain few
thin, up to a few centimeters thick sand layers.
Turbidite sequences L8–L11 display crude thickening and coarsening upward trend. In the centre and
upper part of these sequences, the sandrclay ratio
increases, with turbidite beds becoming thicker and
coarser upwards. The thick sand beds show crude
internal stratification delineated by imbricated clay
pebbles and grain-size variations, which points to
amalgamation of small-volume turbidite events into
thick, composite Žmulti-event. sand beds. The geometry of the thinly bedded turbidites is sheet-like, that
of the thicker amalgamated beds clearly lens-shaped.
These lenses pinch out over a distance of several
tens of meters Že.g. cycle L10 on Fig. 6.. On a larger
scale, the turbidite sequences L8–L11 taper gradually over a distance of a few hundreds of metres.
The strong association between turbidites and
laminites suggest that growth of the prodelta lobes
occurred during precession punctuated periods of
increased precipitation and increased run-off. Tectonic tilting, seismic shock and sediment overloading
due to sea level lowering are believed to be far less
important triggering mechanisms for the L7–L13
turbidites. Weltje and De Boer Ž1993. similarly related their Early Pliocene turbidites exposed on the
island Corfu ŽGreece. to precession punctuated paleoclimatic fluctuations. They noticed a precessionpunctuated change in composition, mainly by the
sediment maturity reflected by the feldspar content.
Where turbidites of the Metochia-B section are found
unrelated to laminites or occur in a hemipelagic
interval Že.g. between L9 and L10., tectonics andror
sea level fluctuations probably played a more active
role in their triggering.
4. Physical signatures of climate changes on a
geological scale
Phase relationships between climate and Žplate.
tectonics may have important consequences for the
frequency of sea level change. In general, during the
warm, ice-free periods, the frequency of eustatic sea
level change will be slow, much depending on plate
tectonics. During the cool periods, the rate of eustatic sea level change will vary as it now depends
on both plate tectonics and the formation of ice caps.
For much of the Cenozoic Ž- 55 Ma, see Fig. 1., for
instance, we can recognise periods with AmesoB
frequency Žcycles of the order of 1 Ma. sea level
Fig. 8. Radial seismic-section of a stretch of the Ebro delta ŽTarranco profile. showing a conspicuous change in delta architecture going
from the Middle and Late Miocene delta progradation Ždrawn in C. to the late Pliocene–Quaternary succession ŽB.. It shows clearly that the
post late Pliocene successions are characterised by a shelf Žcontinuous reflectors. with a typical shelf-delta system as depicted in Fig. 9. Dots
indicate roll-over points Žslope break., arrows show onlap and downlap. Deposition during highstand is shaded light grey, during
transgression dark grey, and is dotted for lowstand. M is the unconformity formed as a result of the Messinian salinity crisis in the
Mediterranean.
G. Postmar Global and Planetary Change 28 (2001) 93–106
change related to glaciation on one of the hemispheres. For only two geological periods, we observe
103
a high-frequency change Ž20–400 ka. in glacioeustacy, probably related to simultaneous glaciations
104
G. Postmar Global and Planetary Change 28 (2001) 93–106
on the Northern and the Southern Hemisphere ŽFrakes
et al., 1992.. Examination of more than 200
mesoscale sedimentary cycles by Algeo and Wilkinson Ž1988. showed that Phanerozoic cycle periods are randomly distributed, except for the Late
Mississippian to Late Pensylvanian and the late
Pliocene–Quaternary periods. The Mississippianr
Pennsylvanian delta successions show sedimentary
cyclicity related to high frequency sea level changes
clustering around the 413 ŽAlgeo and Wilkinson,
1988. and 21 ka periods ŽKlein, 1991; De Boer,
1991; Maynard and Leeder, 1992.. The late
PliocenerQuaternary periods show similar high frequency sea level changes clustering around 40 and
100 ka. Between 0.9 and 0.4 Ma BP, the low amplitude sea level oscillations of a period of 40 ka
change into high amplitude glacial cycles of a period
of 100 ka Že.g. Frakes et al., 1992..
The enormous difference in frequency of sea level
oscillation has a significant impact on the architecture and preservation of sedimentary systems, and
the sea level sensitive delta and shelf systems in
particular. A conspicuous change in style of delta
progradation is observed in the Ebro delta system
since the late Miocene ŽFig. 8; Tarranco section; e.g.
Danobeita et al., 1990; Field and Gardner, 1990..
The late Miocene slope break separates a delta plain,
identifiable from the numerous discontinuous reflectors and small seismic truncations Ždelta plain channel-levee systems. from a delta slope ŽFig. 8C.. In
the late Pliocene the slope break separates a marine
shelf that is identifiable from its strong and continuous reflectors Žhighstand drape deposits. from a lowstand wedge system that is characterised by slope
instability and strong lowstand progradation ŽFig.
8B.. The variable that changed most going from
Miocene to Present times in the Ebro delta system is
most likely the frequency of sea level oscillation,
which increased significantly in the late Pliocene.
In the Namurian of mid England an Ebro-delta
type situation may have existed. Cores through the
Namurian delta complexes show a number of highfrequency Žfourth or higher order. depositional sequences, which are related to the glacio-eustatic
Fig. 9. Schematic illustration of two different type of delta systems: The model in ŽA. is similar to the Exxon models. In the model of
illustration ŽB. the lowstand delta is detached from the highstand delta, a situation that occurs is most representative for high frequency sea
level oscillations as occur today Žmodified from Carter et al., 1991..
G. Postmar Global and Planetary Change 28 (2001) 93–106
cycles Že.g. Leeder, 1988; Collier et al., 1990;
Collinson et al., 1991; Martinsen, 1993; Church and
Gawthorpe, 1994.. The amplitude of these cycles is
of the order of 60 m based on uncompacted thickness of sequences and valley incisions. Prograding
highstand deposits are minor in volume, since the
frequency of sea level oscillations were too high for
highstand progradation, resulting in a typical shelfdelta system as portrait in Fig. 9B. During periods of
low frequency sea level change, the shelf-edge
delta-type ŽFig. 9A. is expected to dominate, even
under conditions of very low sediment supply, as
long as the sediment flux is larger than the increase
in accommodation space Žcf. Burgess and Hovius,
1998..
5. Conclusions
The prodelta of shelf-edge deltas that are fed by
rivers issuing from relatively small drainage basins
are considered potentially good recorders of abrupt
changes in climate controlled run-off.
Climate changes that leave outcrop detectable
physical signatures occur on at least three different
time scales: Ž1. Catastrophic events related to changes
in run off are preserved as sedimentary beds; Ž2.
Milankovitch-scale changes in climate Žrun off. are
preserved as sedimentary units Žsystem-scale.; Ž3.
Geological scale changes governed by the interaction
of plate tectonics and Milankovitch controlled variations in insolation Že.g. green-house and ice-house
periods. may be preserved as a conspicuous change
in architectural style of the entire delta system, e.g. a
change from shelf-edge delta to shelf delta, as is
exemplified by the Ebro delta.
It is not easy to identify physical signatures of
abrupt climate change without considering other,
independent evidence.
v
v
v
Acknowledgements
The manuscript benefited from critical comments
of P.L. de Boer, J. Lundqvist and an anonymous
reviewer. W. Nemec is thanked for the many fruitful
discussions on the origin of conglomerate beds of the
Gador Formation.
105
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