Sedimentary
Geology
ELSEVIER
Sedimentary
Geology
105 ( 1996) 191-202
A coarsening-upward megasequence generated by a Gilbert-type
fan-delta in a tectonically controlled context (Upper Miocene,
Guadix-Baza Basin, Betic Cordillera, southern Spain)
J. Ferniindez a,*, A. Guerra-Merchin
b
”Departamento de Estratigrafia y Paleontologia, Universidad de Granada, 18071 Granada, Spain
”Departamento de Geologia, Universidad de Mdlaga, 29071 Milaga, Spain
Received
22 March 1995; accepted
25 October
1995
Abstract
The 100 m thick succession at Bodunia of a Gilbert-type fan-delta constitutes an Upper Tortonian, coarsening- and
thickening-upward megasequence. It lies unconformably on Triassic materials of the Alpujarride basement or on older
Neogene materials of the basin infill, and is unconformably covered by the continental Pliocene.
The succession was deposited on one margin of an extensional basin, with active synsedimentary tectonics and rising
sea-level. In these circumstances the accommodation space and the sediment supply increased throughout time. This
determined the development of the coarsening and thickening upward megasequence, resulting from the vertical stacking
of four smaller sequences which, in turn, were built up by stacking of Gilbert-type fan-delta units and were bounded by
calcarenite facies deposited on a shallow shelf.
This sequential organisation and internal structure were controlled by the tectonic behaviour of the faults on the basin
margin, in particular large-scale movements with intervals of stability, which respectively created accommodation space for
the deltaic sequences and conditions for development of calcarenite shelf facies.
1. Introduction
There has been a growing interest in the study
of fan-deltas in recent years, and much of what has
been learnt about them is to be found in monographic
issues (Nemec and Steel, 1988; Colella and Prior,
1990; Dabrio et al., 1991).
The location of fan-deltas on basin margins means
that as depositional systems they are particularly sensitive to tectonic movements and changes in climate
or in base level. For this reason they are an especially interesting object of investigation with regard
to understanding
basin evolution.
* Corresponding
author.
0037-0738/96/$15.00
Copyright
SSDI 0037-0738(95)00137-9
In the case of the Betic Neogene basins (Fernandez et al., 1996a), this type of deposit is frequently
found in association with mainly extensional-type
margins, and several papers have been published on
them in recent years. Some of these studies concentrate on the facies and sedimentary
processes
(Postma, 1984; Postma and Roep, 1985; Dabrio and
Polo, 1988; Dabrio, 1990; Femandez et al., 1991),
while others centre on the influence of base level
variations on the internal structure of the resulting
sedimentary body (Bardaji et al., 1990; Femandez et
al., 1993).
The role played by tectonics in the definition
of the general characteristics
and emplacement
of
the fan-delta deposits has been examined from dif-
0 1996 Elsevier Science B.V. All rights reserved.
192
.I. Femdndez,
A. Guerra-Much&
/Sedimentary
ferent points of view by several authors (Leeder
and Gawthorpe, 1987; Colella, 1988; Gawthorpe and
Colella, 1990; Boorsma, 1992). The present paper
aims to concentrate on tectonic control of the origin
and internal geometry of a coarsening and thickening upward megasequence
(100 m thick), resulting
from the successive stacking of Gilbert-type
fandelta units during the Late Tortonian in the GuadixBaza basin (Betic Cordillera).
2. Geological setting
The Guadix-Baza
basin (Fig. 1) is located in the
central sector of the Betic Cordillera, on the contact
between the South Iberian palaeomargin
(External
Zones) and the Alboran block (Internal Zones) (Sanz
de Galdeano, 1990). It became defined at the beginning of the Late Miocene, and its fill presents a Tortonian marine unit and a Plio-Pleistocene
continental
unit separated by an angular-erosional
unconformity
(Fernandez et al., 1996a).
The marine unit includes
the Morollon
and
Molicias Formations
(Rodriguez-Fernandez,
1982)
which correspond respectively to the Early and Late
Tortonian. Both of these formations were deposited
on a shallow shelf, with fans of layers towards
the centre of the basin, and their sequential organization shows the respective retrogradational
and
progradational
character of the formations as a result
of sea-level behaviour during their deposition. The
transgression
during the earliest Tortonian (Fernandez and Rodriguez-Fernandez,
1991) and the regression of the latest Tortonian, involving a fall in sealevel of approximately
100 m (Vera and RodriguezFernandez, 1988) are particularly well documented.
In the northern area (Forruchu Formation,
Soria,
1993) the aforementioned
marine unit ends with
restricted-shelf
calcarenite facies of latest Tortonian
in age. The latest Tortonian-earliest
Messinian transgressive event, clearly known from other basins to
the east (Guerra-Merchan,
1993; Ott d’Estevou et al.,
1990), did not reach the Guadix-Baza
basin.
The Plio-Pleistocene
unit is made up of alluvial
and lacustrine materials belonging
to the Guadix,
Gorafe-HuClago
and Baza formations (Vera, 1970;
Viseras, 1991; Fernandez et al., 1996b).
The southern border of the basin in the sector
studied here is extensional and is characterised by
Geology 105 (1996) 191-202
normal faults, whose movements created the principal topographic and bathymetric differences, and
also determined the main entry points for sediment
supply to the basin.
The fan-delta deposits studied here are included in
what is informally known as the ‘Bodurria Miocene’
(Vera, 1970; Guerra-Merchan
and Fernatrdez, 1989)
which, together with the Valcabra outcrop (GuerraMerchan et al., 1988; Guerra-Merchan,
1993) make
up a marine depositional sequence of Late Tortonian
age in the southeastern sector of the basin. Nevertheless, Goy et al. (1989) proposed an Early Pliocene
age, for marine sediments underlying the fan-delta
succession. This question and its paleogeographical
implications have been widely analyzed in GuerraMerchan (1993). The Bodurria Miocene materials fill
a small depression which developed on the Alpujarride basement of limestones and Triassic dolomites,
and which is located on the southern border of the
basin (Figs. 1 and 2). Three units can be distinguished
according to the facies: (a) conglomerates
and red
breccias, (b) conglomerates
with oysters and reefs,
and (c) conglomerates
and sands with mega-crossstratification, calcarenites and basin marls (Fig. 2).
The conglomerates
and red breccias form a discontinuous unit with a maximum thickness of around
15 m, and are thought to have been deposited in
small alluvial cones. The conglomerates with oysters
and reefs lie unconformably
on this unit. They vary
in thickness from 10 to 20 m and were deposited
in a shallow marine environment probably including
beaches. Both units were locally preserved following
irregularities of the basement.
The conglomerates
and sands with mega-crossstratification
(6-30 m in scale), calcarenites
and
marls are the best represented unit in the area of
study. The minimum thickness of the unit is 100
m and it lies unconformably
on the Alpujarride
basement or on the more ancient materials of the
Neogene basin fill mentioned above.
A group of continental
materials made up of
conglomerates
and red sands (alluvial fan deposits)
lies on the materials of the Bodurria Miocene and is
separated from them by an unconformity
extending
throughout the entire basin (Fernandez et al., 1996a).
This group belonging to the Guadix Formation has
a subhorizontal
position and Plio-Pleistocene
age
(Viseras, 1991; Fernandez et al., 1996b).
J. Ferndndez, A. Guerra-Merchdn/Sedimentary
Geology 105 (1996) 191-202
Internal
zone
External
zone
193
PLIO
- PLEISTOCENE
Gorafe-Huelago
n
/
;zice-Quaternary
m
Bozo
Fm
Fm.
Main faults
Fig. I. Geographical
and geological location. The Guadix-Baza
Basin is located on the contact between the Internal and External Zones
of the Betic Cordilleras. The basin basement consists of the former in the southeastern part and the latter in the northwestern
part.
The tilling includes Neogene-Quaternary
sediments. The area studied is situated at the extreme southeast of the basin, on the margin
associated with the Internal Zones.
3. Stratigraphic framework
The fan-delta deposits constitute a coarsening and
thickening upward megasequence approximately 100
m thick, made up of four smaller sequences of increasing thickness, bounded by shelf calcarenites
(Fig. 3 and Fig. 4). Each of these sequences contains several fan-delta units separated by surfaces
which are erosional towards the basin edge and conformable towards the centre.
Sequence I (Fig. 5). This is made up of three
intervals: a, b and c. Interval ‘a’ is characterised by
steeply inclined (30”) foresets beds approximately
25-30 cm thick, whose corresponding
top- and bottomsets cannot be observed. It ends with an intensely
deformed bed of medium sands a few centimetres
J. Fermdndez, A. Guerra-Merchdn /Sedimentary Geology 105 (1996) 191-202
194
SECTION
ALLUVIAL
FAN
DEPOSITS
sequence
nr
f
5
z
E
t-
2
Modern fluviol
deposits
:
sequence
III
aL
5
sequence
w
A
h
Alluvial fan deposits. Recent Quoternory
L
C
-_
T----r
Im”l-i?
0
Alluvial fan deposits.
7
Plio- Pleistocene
Fan delta deposits : sequences I to W
and bosin facies. Uppe; Tortonion
Reef deposits
coxvl
_.
Coast?1 cong;omerotes.
Loterolly
relaTea ro reers
Alpujarride
substratum
SUBSTRATUM
Fig. 2. Geological
map and stratigraphic
thick, which wedges out towards the upper part of
the foresets in the same way as the basin facies deposited on it. Interval ‘b’ is not as thick as ‘a’ and
the beds of the low-angle foresets are joined to their
respective topsets and bottomsets. The contact with
section of the Bodurria
Miocene.
the lower interval is erosional in proximal areas and
conformable in distal areas. Interval ‘c’ presents similar characteristics to ‘b’ and ends with a calcarenite
level. The three intervals are organised in a thinning
and fining upward sequence.
J. Ferndndez, A. Guerra-Merchdn/Sedimentaly
PLIO
-
Geology 105 (1996) 191-202
PLEISTOCENE
Fig. 3. Internal geometry and sequential organisation of fan-delta deposits. The Gilbert-type fan-delta succession at Bodurria constitutes a
coarsening and thickening upward megasequence consisting of four smaller, fining upward sequences. Sequences I and II are the result of
the stacking of three delta lobes, sequence III consists of a single lobe and sequence IV consists of a single lobe prograding three times
at different directions. In all cases the internal geometry is indicative of the relation between supply and subsidence at each moment of
the stratigraphic succession.
Sequence II (Fig. 5). This sequence is also made
up of three intervals. Intervals ‘a’ and ‘b’ show
characteristics
similar to the corresponding
intervals
in sequence I. Interval ‘c’ is made up of high-angle
foresets wedging out rapidly towards the centre of
the basin, where they come into contact with a gravel
bed (80 cm thick) with coarsely imbricated boulders
(55 cm). This bed wedges out and its grain size
decreases towards land, as a consequence
of the
reworking by coastal processes of a debris flow bed.
The sequence ends with a bed of calcarenites
(50 cm) wedging out towards land, and eventually
disappears where the layers rise in the foresets.
Sequence III (Fig. 5). This is a thicker sequence
than I and II, made up of a single interval of sigmoidal beds which permit a gradual change from the
topset, through the foreset, to the bottomset. It ends
towards the top with a layer of calcarenites approximately 15 m thick which wedges out towards land
in an onlap structure. Cross-bedding caused by sandwaves migrating towards land (N180E) is present
locally.
Sequence IV (Fig. 3). This is the thickest sequence of all (30 m) and also has the largest grain
size (80 cm max.) It is made up of a single interval, which in turn is made up of three units
separated by reactivation
surfaces (Fig. 3). These
three units show a proximal to distal increase in the
angle of inclination of the beds, and represent three
phases of progradation
at different directions of a
deltaic lobe.
The area1 distribution
and thickness of the four
sequences described above differed during their development (Fig. 6). Outcrop conditions do not permit
a detailed reconstruction
of the spatial distribution of
each sequence; however, the data on palaeocurrents
obtained from the foresets allow reconstruction
of
the main entry points of materials to the basin and
196
J. Femdtzdrz,
A. Guerra-Merchdn
/Sedimental
Geology
105 (1996)
191-202
Fig. 4. Panoramic view of the type section of the Bodurria Miocene, used as a basis for the diagram in Fig. 3. The Gilbert-type fan-delta
deposits can be recognised in between the coastal reef deposits at the base and the alluvial fan deposits of the Guadix formation at the
top of the succession. The excellent outcrop conditions allow recognition of the different facies and sequences, as well as the internal
geometry of the different units in the succession.
the progradation
directions of deltaic lobes. It can
be said that during the development
of the first
sequence at least three entry points existed, in relation to which small lobes formed, which prograded
only slightly towards the centre of the basin, so
that they probably did not come to coalesce. The
same situation continued during deposition of the
second sequence, although here the lobes which
developed were more extensive, at least during intervals ‘a’ and ‘b’, eventually coalesced. During the
development of the third sequence, sediment supply
to the basin was predominantly
channelled through
two points, whereas during deposition of the fourth
sequence supply to the system took place in a single
point.
The foregoing remarks allow us to visualize the
basin filling in this area as follows:
After the intra-Tortonian
tectonic event, the newly
created relief began to be drained towards the basin.
The drainage network was at first not highly evolved,
so that numerous small streams connected with depressed zones of the basin margin, whose detailed
palaeogeography
is complex. Thus small outlet lobes
developed that did not undergo significant progradation. The evolution of the drainage network gradually
caused the number of sediment entry points to decrease, although the amount of materials supplied
by each of these points increased, thus resulting in
the creation of a smaller number of lobes, which
were, however, more extensive. The homogeniza-
3. Fema’rzdez, A. Guerra-Merch&/Sediwentary
Geology 105 (1996) 191-202
197
SEQUENCE
SEQUENCE
-
111
LI
t
dal;risflow
Fig. 5. Detail of the internal geometry of fan-delta sequences I, II, and III. In sequences I and II can be observed the nature of
the contact between superposed lobes (erosional-conformable in proximal-distal direction), the retrogradational trend in lobe stacking
and the evolution of thickness and inner structure. In sequence III the sigmoidal nature of the beds and their perfect preservation are
noteworthy. The shelf calcarenites show uneven development in the different sequences,but in all cases they increase towards the top.
A11these characteristics are indicative of the relation between supply and subsidence and the tectonic behaviour of the basin margin at all
times, a, h, c = intervals of each sequence; Ch = channel; T = topset; F = foreset; B = bottomset.
tion accompanying the filling of the basin favoured
the establishment of a calcarenite shelf. The entire
process was modified by numerous tectonic pulses.
4. Fades analysis
The fan-delta deposits are made up of clastsupported conglomerates and sands. The foresets
are made up of beds varying in thickness from 3
m in the upper part, where they dip approximately
30”, to 0.5 m in the transition zone to the bottomset. A decrease in maximum grain size towards the
bottomset from cobble to pebble and a larger proportion of sand can also be detected. The conglomerates
are clast-supported in this direction, whereas towards
distal parts they change to sands with pebbles,
Regarding the internal organisation, the foresets
consisting of massive conglomerates show rapid
wedging as a result of the deposit of lobes by gravitational flows. Towards more distal parts these flows
became more fluid and deposition took the form
of successive avalanches causing low-angle crossbedding, in which elementary coarsening-upward or
coarsening-fining-upward sequences can be recognised. These were a consequence first of the shearing
effect on the base and then of the deposit of the fine
fraction during the phase of the waning flow.
The distal parts of the foresets and bottomsets
rarely contain lenticular bodies of conglomerates
related to slipping mechanisms from higher, very
steeply sloping parts of the foresets (comparable to
those described by Postma and Roep, 1985).
198
_I. Ferndndez,
A. Guerra-Merchdn
/Sedimentary
0
Fig. 6. Interpretative
and the superposition
facies is also shown.
fan-delta lobes (a, b
sandy marls and fine
250
105 (1996)
191-202
’
8
BODURRIA
Geology
,I’
500 m
diagram of the fan-delta deposits and associated facies. The main sediment supply points, developed delta lobes
trends of these lobes are shown for the length of development of each sequence. Distribution of basin and shelf
I = Alpujarride substratum; 2 = Plio-Pleistocene
alluvial fan deposits; 3 = sediment supply points to the basin; 4 =
and c indicate the lobes represented in the corresponding
sequence): 5 = bioclastic calcarenites, shelf deposits: 6 =
sands, basin deposits.
Another characteristic
feature of some coarse
beds of the foreset is the presence of water escape structures similar to those described by Postma
(1984) in the Abrioja fan-delta.
The bottomsets are represented by subhorizontal
beds normally less than 1 m thick, made up of sands
with pebbles and microconglomerates.
They represent the distal parts of the foresets, where the fossil
contents (red algae, corals, bryozoa, lamellibranchia)
and their degree of preservation is higher. The topsets
are formed by layers of clast-supported
conglomerates approximately 3 m thick. They normally present
erosional bases and channel fill fining-upward
sequences.
Other facies associated with the fan-delta deposits
are: “Fine sands and silts with oyster banks and intercalated channelled conglomerates”;
locally these
show horizontal lamination and are yellowish and
J. Fermindet, A. Guerra-Merchdn/Sedimentary
reddish in colour. They are found associated with
topsets and the upper parts of foresets and are interpreted as interdistributary
bay deposits. ‘Sandy
marls’ are found in thin beds (20-30 cm) intercalating the bottomset layers and the distal parts of
foresets and represent the autochthonous
deposit in
the basin. ‘Bioclastic calcarenites’ are better developed in relation to sequences II and III, forming
an onlap structure with the latter. These are bioelastic sandstones containing remains of red algae,
bryozoa, corals, benthic foraminifera
and bivalve
fragments. They are almost always horizontally laminated, rarely cross-bedded due to sandwave migration in a shelf environment.
5. Discussion
The stratigraphic
succession studied here is located to the extreme southeast of the Guadix-Baza
basin, and fills a small, markedly asymmetrical
depression. In a global context of eustatic sea-level fall
(Late Tortonian), the tectonic movements of the border faults created the main topographic and bathymetric differences and thereby also the source areas and depocentres for sedimentation.
The episodic
character of these movements affected the stacking
pattern of the different units and therefore also the
resulting stratigraphic architecture.
Deltaic sedimentation
began in a shallow marine
environment.
Large-scale movements of the border
faults increased the depth of the environment
and
created ideal conditions for the deposit of Gilberttype deltaic lobes.
The coarsening and thickening upward megasequence studied here is made up of four smaller
sequences of increasing thickness towards the top.
Sequence I (Fig. 5) is made up of three intervals
whose thickness decrease towards the top, while the
inclination of the foreset beds decreases in the same
direction, and a certain retrogradational
trend is also
observed. This evolution in the thickness of each
interval and the dipping of the foreset beds indicate decrease in depth. The thickest interval ‘a’ (1
in Fig. 7) is the result of the stacking of several
lobes characterised
by sequences thinning upwards
because of lateral migration. The sandy bed at the
top of each of these elementary sequences represents
the phase of abandonment of the corresponding lobe.
Geology 105 (1996) 191-202
199
The steeply inclined foresets were built by turbulent
gravitational
flows, mainly supplied with sediment
accumulated on the topset. The less thick intervals
‘b’ and ‘c’ (2 and 3a in Figs. 7), which are characterised by sigmoidal beds, were formed by the
simultaneous progradation and aggradation of a lobe
in a shallower environment and with a lower degree
of slope than interval ‘a’, whose supply was provided
by stream flows.
The nature of the contact between the successive
intervals (erosional in the proximal parts) reveals
phases of slowing in the tectonic subsidence
and
probable stability of the base level of the streams,
which prevented vertical aggradation
in the high
parts of the foreset and in the topset, and favoured
the development of oblique contacts bounded by bypass surface, that transferred the sediment towards
deeper areas (Colella, 1988; Gawthorpe and Colella,
1990). The change from one interval to the next
is preceded by a tectonic reactivation
as revealed
by the abundant deformation
structures in the last
layers of the previous interval, and the activity of
synsedimentary
faults towards the basin edge and
behind the first foresets (2 in Fig. 7).
Sequence I is therefore the result of the stacking of several deltaic units in a tectonic regime
characterised by an initial large-scale slip event, followed by smaller slip events separated by periods of
calm. The sediment supply/tectonic
subsidence ratio
evolved in such a way that depth and accommodation space decreased with time, and so too did the
thickness of the corresponding intervals and the style
of sedimentation.
Another large-scale movement of the border faults
created enough space for the stacking of another
shallowing sequence (sequence II). Intervals ‘a’ and
‘b’ of this sequence are not substantially
different
to those of sequence I, and an identical mechanism
can be assumed for their generation. Interval ‘c’ (3b
in Fig. 7) represented by a not very extensive lobe
with steeply sloping foresets, can be interpreted in
connection with a period of instability, which caused
an increase in depth on the margin and stimulated the
gravitational avalanche mechanisms, some of which
penetrated to the bottomset.
The phase of stability and decrease in supply at
the end of this sequence brought about the reworking
of the gravels at the top of interval ‘c’, with the
200
.I. Ferndndr~, A. Gurrru-Mrrchdn
/Sedirnenrury
Geology 105 (1996) 191-202
stacking
B
o new
of
lobe
in shallower
condihons
/
abandonment
focies
slowing
in the
subsidence.
stillstand
_--_--and
transference
fine
sediments
deeper
of
towards
areas
Fig. 7. Dynamic behaviour of the fan-delta lobes and possible resulting sedimentary sequence. Lateral migration of the lobe produces
an elementary fining and thinning upward sequence (TI2U-FI/) that ends with fine facies of deltaic abandonment
(1). Deceleration of
subsidence and sea-level stability allow the development of by-pass surfaces, erosional contacts in proximal areas and deposition on
deeper ones (1). In conditions of shallow depth, a minor tectonic pulse lead to stacking of a thin, but rather extensive lobe (2). A new
tectonic pulse lead to stacking of a new lobe (3a) similar to the previous ones, or rather thicker and less extensive ones when depth was
greater (3b).
incipient development of a conglomeratic beach and
deposit of calcarenites towards the centre in shallow
conditions.
Sequence III, made up of a single interval, was deposited in a regime of rather active subsidence, with
increasing depth and supply, which favoured the development of large-scale sigmoidal units related to
the simultaneous
progradation and aggradation of a
Gilbert-type deltaic lobe. During the build-up of this
lobe, shallowing took place and the coast line migrated landwards, as also did the shelf calcarenite
facies. As the terrigenous
supply to the basin decreased, the calcarenite facies moved towards land,
eventually creating an authentic shelf with development of sandwaves migrating towards land. The
physiography
of the basin would at this time have
been that of a shallow shelf connected to land by a
conglomeratic
coast.
The deposition of sequence IV on this shelf was
preceded by important tectonic activity, as revealed
by both the large-scale movements
of the border
faults, and the fracturing of the calcarenite
shelf
itself (Fig. 3). Accommodation
space was created
in these circumstances,
the reliefs were rejuvenated
and the drainage systems reactivated, thus causing
an important inflow of sediments and allowing the
formation of lobes which prograded and migrated
rather quickly.
The megasequence
described above is the result
of the stacking of four smaller sequences, built up
by the stacking of Gilbert-type
fan-delta units and
bounded by calcarenites deposited in a shallow shelf.
This megasequence reveals a situation of rising sealevel, with the creation of accommodation
space and
increasing
sediment supply towards the top. The
tectonic regime on the basin margin was the most
_I. Ferndndez. A. Guerra-Mercha’n/Sedimentary
significant control factor on the stacking pattern of
the sequences and the different types of sedimentary
units, and therefore also on the internal organisation
of the resulting stratigraphic succession. Large-scale
slips separated by periods of calm created the accommodation
space for the deltaic sequences and the
conditions for development
of the calcarenite shelf
facies which separates them. Smaller tectonic pulses
favoured the stacking of deltaic lobes with differing
internal geometry depending on depth and/or subsidence, separated by oblique contacts (Colella, 1988)
in the proximal areas.
The synsedimentary
activity of the border faults
can clearly be demonstrated by: (a) abundant deformation structures located at the top of the sequences
and/or intervals; (b) tilting of beds in the border
zone, behind the foresets created at the beginning of
each sequence; (c) the retrogradational
character of
the sequences reproduces a retreat of the coastline
related to the activity of border faults in the same direction. All these observations show that the changes
in sea-level during deltaic sedimentation
were controlled tectonically.
6. Conclusions
The Late Tortonian coincides with an important
period of relief uplift, reactivation of drainage systems and development
of fan-deltas in numerous
Neogene basins in the Betic Cordillera. A good example of this is the Gilbert-type fan-delta sequence
which developed in the extreme southeast of the
Guadix-Baza
basin, in the vicinity of Bodurria.
The Bodurria fan-delta succession constitutes a
coarsening and thickening upward megasequence deposited on an extensional-type
basin margin with active synsedimentary
tectonics and rising sea-level resulting from important tectonic subsidence. In these
circumstances
the available accommodation
space
and the sediment supply to the basin increased with
time.
The keys to the interpretation
of the sequential
organization and the internal geometry of the succession studied here lie in the tectonic behaviour of the
border faults: large-scale tectonic movements of the
faults prior to the deposit of each sequence created
the required topographic and bathymetric conditions
for development of Gilbert-type fan-delta lobes.
Geology 105 (1996) 191-202
201
The evolution of the drainage network also had
an effect by modifying the position and number of
sediment entry points to the basin, the quantity of
materials supplied at each point, and consequently
the size of the resulting sedimentary
body and its
internal structure.
The sediment supply/subsidence
ratio determined
the pattern of lobe stacking and their internal geometry. Thus, at the beginning of each sequence, with
a high ratio, the lobes prograded and migrated laterally, whereas towards the top, where the ratio was
low, the lobes prograded and aggraded vertically.
Moreover, the phase of stability produced erosional
contacts, whereas the longer periods of calm, with
scarce sediment supply, allowed the establishment
of a better developed calcarenite shelf towards the
upper part of the succession.
Acknowledgements
This paper has considerably
benefited by the
suggestions
of Drs. C.J. Dabrio, A.D. Miall and
T.B. Roep. Financial aid was provided by Research
Projects PB91-OOSO-C02-01 and PB93-1000 granted
by the Spanish DGICYT and, Research Groups 4085
and 1350 granted by the Junta de Andalucia.
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