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Sedimentary Geology 185 (2006) 79 – 92
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Architecture and sedimentary facies evolution in a delta stack
controlled by fault growth (Betic Cordillera, southern Spain,
late Tortonian)
Fernando Garcı́a-Garcı́a a,*, Juan Fernández b, César Viseras b, Jesús M. Soria c
a
b
Departamento de Geologı́a, Universidad de Jaen, Campus de las Lagunillas s/n, 23071 Jaen, Spain
Departamento de Estratigrafı́a y Paleontologı́a, Universidad de Granada, Campus de Fuentenueva s/n, 18071 Granada, Spain
c
Departamento de Ciencias de la Tierra, Universidad de Alicante, 03080 Alicante, Spain
Received 27 July 2004; received in revised form 13 October 2005; accepted 31 October 2005
Abstract
Tectonic control is revealed in various ways (synsedimentary deformation structures, facies, architecture) in the coarse-grained
delta systems that developed in the southeastern margin of the Guadix Basin, an intramontane basin in the central sector of the
Betic Cordillera, Spain, during the late Tortonian (Miocene).
Vertical trends in the architecture of the deltaic succession (230 m thick) show changes in the stratal stacking pattern related to
the variation in time in subsidence rates (in accommodation space). A period of high subsidence controlled by a normal growth
fault began at the base of the succession, producing retrogradational units representing Gilbert-type delta systems onlapped by
shallow platform calcarenites. A period of low subsidence followed, controlled by growth of a listric fault producing aggradational
units representing shoal-water deltas capped by red algal biostromes. A non-subsidence period and decrease in accommodation
space at the top of the succession (sediment supply remained constant) produced progradational deltas. The manuscript focuses on
the part of the succession where the delta deposits show the effects of extensional tectonics, a listric growth fault and its rollover, on
delta development. The progressive increase in accommodation space inherent in fault growth controlled the style of delta
sedimentation in the river mouth. Gilbert-type deltas developed during periods of increase in subsidence rates and shoal-water
deltas during periods of decrease in subsidence rates.
Horizontal trends in the architecture of the delta lobes show changes in the stratal stacking pattern affected by differential subsidence
and pre-existing basin-floor topography. Periods of increase in subsidence rates near the fault scarp correlate with accommodation space
kept constant, thinning of the units and shoal-water deltas development over the rollover, away from the fault scarp.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Gilbert-type deltas; Shoal-water deltas; Normal growth fault; Rollover
1. Introduction
Tectonic effects not only influence the location of
coarse-grained deltas in extensional basins, they also
* Corresponding author.
E-mail address: [email protected] (F. Garcı́a-Garcı́a).
0037-0738/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.sedgeo.2005.10.010
have a marked control on the external form, architecture
and internal geometry of such deltas (Gawthorpe and
Colella, 1990).
Fault growth is an important control on drainage
development in modern extensional margins, but those
links are difficult to establish in ancient basins. The
stratigraphy of rift-related deposits rarely enables us to
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make direct links with coeval structural controls, because
faults located adjacent to synrift strata generally do not
preserve evidence of their timing of activity in relation to
depositional or erosional episodes (Gupta et al., 1999).
This study is aimed at demonstrating the influence of
growth faulting and the rollovers directly linked with the
delta development in terms of architecture, type of delta
and processes in a Tortonian coarse-grained delta stack.
The small coarse-grained deltas as here studied respond quickly to tectonic changes (Postma, 1990). A
variety of deltaic systems changing from one to another
type have been described in tectonic contexts (Colella,
1988; Postma and Drinia, 1993). The change in architecture of small-scale deltas controlled by faults, when
sediment supply is kept constant, is related to the locus
of delta deposition close to or away from the fault scarp
and to subsidence rates. The objective of this paper is to
describe the architecture and facies of delta systems
during the different stages of fault growth and to establish the boundary conditions when and where a type
delta (according the Postma’s classification, 1990) is
changing to another type one and back again.
We compare, in terms of processes, the delta stack
described in this example with some classical examples
of Neogene delta stacking near normal faults, described
from the Gulf of Corinth (Postma and Drinia, 1993), the
Gulf of California (Dorsey et al., 1995, 1997) and from
the Suez Rift (Gupta et al., 1999; Young et al., 2003).
the entire basin (Soria et al., 1998, 1999). Marine
sediments (allostratigraphic units I–III) formed during
the late Tortonian. The Guadix Basin and other sedimentary basins of the central sector of the Betic Cordilleras were then subjected to tectonic uplift during
late Tortonian (Lukowski, 1987; Sanz de Galdeano,
1990; Sanz de Galdeano and Rodrı́guez Fernández,
1996). Messinian to Upper Pleistocene sedimentation
took place in an internal drainage continental basin
before the Guadix Basin became part of an external
drainage basin in the late Pleistocene and was subjected
to strong erosion (Calvache and Viseras, 1997).
The deltaic succession studied is located in the southeastern part of the Guadix Basin (Fig. 1B). In the sector
examined, the deltaic succession lies unconformably on
the Triassic basement. The deltaic deposits are unconformably overlain by Pliocene alluvial sediments belonging to the later continental fill of the basin. The
presence of the calcareous nannoplankton Discoaster
quinqueramus in the delta deposits, together with planktonic foraminifera Neogloboquadrina humerosa and the
absence of Amaurolithus indicates that the studied deltaic succession is of latest Tortonian age (NBN-12 biozone from Martı́n Perez, 1997). On the basis of the
biostratigraphy of the succession using calcareous nannoplankton, the delta stacking took place in a short time
period (under 1.5 my), coinciding with the late Tortonian
TB3.2 eustatic sea-level highstand in the standard chronostratigraphy of Haq et al. (1988).
2. Regional setting
3. Stratigraphic architecture
The Guadix Basin is one of the Neogene–Quaternary basins located in the central sector of the Betic
Cordillera (Fig. 1A). The Guadix Basin is an intramontane basin formed in the late Tortonian during the
neotectonic stage (late Miocene to Quaternary) of the
cordillera, after the main tectonic movements that gave
rise to the large structures the Betic Chain (main thrust
nappes and extensional detachments, see Platt and Vissers, 1989; Garcı́a Dueñas et al., 1992). The geodynamic context during the neotectonic stage is
characterized by N–S to NW–SE regional compression,
responsible for the creation of a complex network of
faults striking NW–SE and NE–SW to NNE–SSW
(Sanz de Galdeano and Vera, 1992). The Guadix
Basin is a polygonal basin subjected to considerable
subsidence controlled by tectonics (Viseras et al.,
2005).
The sedimentary fill of the Guadix Basin includes
Tortonian to Upper Pleistocene sediments. The succession is subdivided into six allostratigraphic units separated by unconformities of tectonic origin, which affect
The deltaic succession, about 230 m thick, is made
up of five delta units separated from one another by
unconformities (Figs. 1C and 2). Each delta unit consists of several delta single delta lobes. Two stratal
pattern trends can be recognized in the delta units—
the first retrogradational to aggradational (units 1 to 3)
and the second progradational (units 4 and 5) (Fig. 3).
3.1. Retrogradational to aggradational units
(units 1 to 3)
3.1.1. Unit 1
The first unit is approximately 70 m thick and contains Gilbert-type delta systems retrogradationally
stacked (Fernández and Guerra-Merchán, 1996). The
single lobes are onlapped by platform calcarenites that
distally give way to hemipelagic marls. The vertical
trend in delta architecture shows clinoform geometry
varying from oblique-tangential clinoforms at the base
of the unit to sigmoidal higher in the unit.
F. Garcı́a-Garcı́a et al. / Sedimentary Geology 185 (2006) 79–92
Fig. 1. (A) Location of the Guadix Basin (GB) at the north of Sierra Nevada (SN) in the Betic Cordillera. (B) Position of the studied area within the Guadix Basin. (C) Geological map of the study
area (after Fernández and Guerra-Merchán, 1996, modified).
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Fig. 2. Panoramic view and line drawing of the studied cross-section (see Fig. 1C for location) with the differentiation of the five deltaic units (1–5) bounded by unconformities. Locations of the
stratigraphic logs (a–f) of Fig. 4 are shown.
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Fig. 3. Synthetic geological cross-section interpreted from panoramic views (see Figs. 2, 7 and 8A) showing the development of retrogradational
(units 1–3) to progradational (units 4–5) stratal stacking patterns in the succession.
3.1.2. Unit 2
Unit 2 is approximately 60 m thick and was deposited
nearer to the basin margin than unit 1, from which it is
separated by an angular unconformity. It contains three
coarsening- and thickening-upward minor sequences,
representing aggradational shallow-marine deltas that
distally give way to onlapping platform calcarenites.
extent of the clinoforms clearly differentiate two construction phases in this unit. The first led to extensive
oblique-tangential clinoforms, becoming wedge-shaped
distally. The second construction phase led to a small
Gilbert-type deltaic lobe with sigmoidal clinoforms, increasing basinward in thickness from 7 to 15 m.
4. Facies associations of the delta deposits
3.1.3. Unit 3
The geometry of this unit is wedge shaped, with a
thickness of nearly 60 m near the basin margin thinning
to 30 m basinward about 500 m away from the basin
margin. This unit contains aggradational, shoal-water
delta systems capped by red algae biostromes. Some
shoal-water delta lobes change distally to Gilbert-type
profile lobes (Fig. 3).
3.2. Progradational units (units 4 and 5)
This is represented by the last two deltaic units, both
of which display a progradational stacking pattern. The
depocentres moved toward the centre of the basin.
3.2.1. Unit 4
This unit, about 45 m thick, is represented by progradational delta lobes overlying an erosional surface
suggesting a slide scar. The deltas developed both
oblique and oblique-tangential clinoforms.
3.2.2. Unit 5
The first deposits of unit 5, overlying the basin marls,
are represented by a bed of outsized clasts (some up to 2
or 3 m in diameter). It could be interpreted as the progradational infill of a slump scar. The geometry and
Lithologically, the succession consists of deltaic conglomerates and sandstones with clinoforms 5–45 m high that
distally give way to platform calcarenites and hemipelagic
marls (Fig. 4). Three types of shallow-water and coarsegrained deltas, according to Postma’s (1990) delta classification, have been recognized in the deltaic succession. They are characterized by their delta front dip:
classic Gilbert-type deltas (steeply inclined delta front),
shoal-water deltas (gently inclined delta front) and Gilbert-type profile (intermediate dipping delta front).
4.1. Classic Gilbert-type delta systems
4.1.1. Description
This facies association is represented by lobes with
well defined clinoforms ranging in height from 5 m to
30 m. Coarse-grained deposits show variable grain size
and can be divided into two types: sandy-conglomerates and conglomerates. The three geometric elements
making up Gilbert-type deltas (topset, foreset and bottomset) are clearly distinguished (Fig. 5A). The deltaic
clinoforms have slopes ranging from 208 to 308 in the
upper parts, the angle decreasing from the brinkpoint to
the subhorizontal layers of the bottomsets. Their geometry varies from oblique-tangential to sigmoidal.
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Fig. 4. Correlation diagram between several stratigraphic logs of the deltaic succession from south, next to the basin margin (log f) to north (log a) (1–5: deltaic units; T—topset, F—foreset, B—
bottomset).
F. Garcı́a-Garcı́a et al. / Sedimentary Geology 185 (2006) 79–92
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Fig. 5. (A) Photograph and line drawing of the unit 1 showing a classic Gilbert-type delta system (1a–1c) bounded by calcarenites (dotted line)
showing a retrogradational stratal pattern. Unit 4 and the Pliocene alluvial fans overlie shallow platform calcarenites of the units 2–3. (B) Finegrained interdistributary bay/delta plain sediments overlain by a distributary channel. The channel is overlain by foreset beds of the next classic
Gilbert-type delta lobe. (C) Beds of conglomerate with normal grading in a Gilbert-type foreset.
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The topsets have coarsening and thickening-upwards sequences, equivalent to those described for
the delta plain of shoal-water deltas. The channel
height ranges from 1 to 3 m and the channels fills
are fining-upward (Fig. 5B).The foresets consist of
conglomerates with both matrix-supported and clastsupported fabrics. They may be normally graded
(Fig. 5C) or inversely graded, and the larger clasts
are commonly located in the upper or lower part of
the foresets, depending on whether they are associated
with matrix-supported or clast-supported fabrics. Erosional scars and upward-coarsening small conglomerate lobes are locally observed, respectively in the
upper and lower parts of some steeply sloping foresets. The scars are filled with cross-stratified, imbricated conglomerates dipping against the slope. Sandy
layers with ripple cross-lamination and tabular crossbedding are more abundant in the topsets and bottomsets of the prograding bodies.
The bottomsets are represented by horizontal layers
in which sand, and local pebbles, alternate with burrowed, marly silts. The fossil content (red algae, bryozoa and oysters) is abundant here and well preserved.
Depth ratio at the river mouth is 0.1 to 0.2.
4.1.2. Interpretation
The topsets have minor mouth bar-crevasse channel
sequences built out in interdistributary bays with characteristics equivalent to those of shoal-water deltas
described bellow.
The foresets were built out by gravitational avalanches
of alternating cohesive and cohesionless debris flows. The
steep slope may, in some cases, have caused instability of
the upper part, leading to avalanches of coarse material
down the slope, where it was deposited in the topset as
lobes showing inverse grading because of basal shearing.
The scours left in the foresets could have caused hydraulic
jumps in later avalanches, forming backsets filling scours
interpreted as due to the turbulence developed in the
hydraulic jump (Massari, 1996). Postma and Roep
(1985) described similar processes for Pliocene Gilbert-type deltas in Southeast Spain. Backsets could be
created by the development of strong turbulence, which
accompanies the formation of a hydraulic jump and not
controlled by any pre-existing scour.
4.2. Shoal-water delta systems
4.2.1. Description
For description, of these, deposits have been divided
into a siliciclastic unit and a carbonate unit overlying
the former (Fig. 6A).
The siliciclastic unit consist of upward-coarsening
sequences, 5 to 15 m thick in the delta plain where
four facies can be distinguished from bottom to top:
(1) mottled silts and clays, with root traces and a few
thin layers of lignite, (2) sand containing marine gastropods and wavy lamination, (3) beds of seaward
cross-stratified conglomerates 2 to 3 m thick and (4)
channelized pebble-conglomerates 1,5 m thick with
sharp erosional base. Occasional layers (10–15 cm)
of carbonates, with algal lamination, charophytes and
root traces, are locally associated. The delta front
inclination ranges from 28 to 58. It consists of coarsening and thickening sequences 3 to 4 m thick
(Fig. 6B) of normally graded conglomeratic beds 25
to 30 cm thick (Fig. 6C). Rare outsized clasts are
located at the bed tops. Angular pebble-conglomerates
with clast-supported fabrics fill the delta front beds.
The prodelta is represented by massive pebbly sandstone alternating with silty layers. Burrows and symmetrical ripples occur. Depth ratio (ratio channel
depth/basin depth following Postma, 1990) at the
river mouth is 0.4 to 0.5.
Tabular beds of red algae represent the carbonate
unit capping the siliciclastic deposits. They overlie
openwork gravels consisting of very rounded and imbricated quartzite clasts. Red algae occur both in the
form of rhodoliths (similar to those described by Braga
and Martı́n, 1988, in equivalent environments situated
30 km to the east), and also as massive, branching
morphologies, in which case they form biostromes.
These algal beds are up to 8 m thick and extend
laterally for up to 100 m. The siliciclastic content and
grain size decrease upwards within these beds.
4.2.2. Interpretation
For interpretation these deposits are divided in two
units, siliciclastic and carbonate. The former have been
interpreted as shoal-water delta (cf. Postma, 1990)
deposits. In the interdistributary bay subenvironment
of the deltas, the fine sediments represent suspension
settling, which is periodically interrupted by the input
of coarse siliciclastic deposits through distributary
channels feeding distributary channel-mouth bars
which prograded seaward. The stromatolite layers represent temporary interruptions of sediment supply. The
facies of the delta front beds suggest deposition from
gravitational avalanches recording periods of river
floods during which bedload sediment is deposited
just beyond the river mouth into the basin and is not
wave reworked. Symmetrical ripples in the shallow
prodelta deposits indicate that the depositional environment was to a certain extent controlled by wave action.
F. Garcı́a-Garcı́a et al. / Sedimentary Geology 185 (2006) 79–92
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Fig. 6. (A) Shoal-water deltaic lobe capped by a red algae biostrome (basin margin to the right). Location of picture B is shown. (B) Coarseningupward sequence of shoal-water delta front (scale bar is 2 m long). (C) A detail of picture B showing a normally graded bed in the shoal-water delta
front (camera lens cover 5 cm diameter—for scale). (D) Gilbert-type delta profile (basin margin to the right).
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The siliciclastic unit is capped by a carbonate unit
represented as red algae biostromes, which colonized the
top of the delta deposits during periods of delta abandonment. Wave reworking evidenced by open work
quartzite gravels (gravel beaches?). Algal colonization
of the top of the delta lobes and decrease of the siliciclastic content from bottom to top of the algal beds could
be related to delta abandonment, accompanied by starvation of terrigenous sediment supply. These horizons
could be also interpreted as transgressive intervals, recording times of sediment starvation during which the
underlying delta-top was drowned and submerged, as
has been interpreted elsewhere (Gupta et al., 1999).
4.3. Gilbert-type profile delta systems
4.3.1. Description
The three geometric elements making up Gilberttype deltas (topset, foreset and bottomset) can be distinguished (Fig. 6D). The deltaic clinoforms show sigmoidal geometries 10 to 12 m height.
The topsets consist of channelized pebble-conglomerates 2 to 3 m thick with sharp erosional bases. The
channel fill is fining-upward. Foreset inclinations range
from 108 to 138. Foresets consist of normally graded
conglomeratic beds. Angular and bored pebble-conglomerates with clast-supported fabrics that distally
give way to sandstones form the delta front beds (2 to
20 cm diameter). These beds alternate with normally
graded oysters beds. Bottomsets are absent.
Depth ratio at the river mouth is 0.2.
4.3.2. Interpretation
The facies of foreset beds suggest deposition from
gravitational avalanches of cohesionless debris flows
recording periods of river floods during which bedload
sediment is deposited just beyond the river mouth into
basin. Fossil beds in the delta front represent resedimented oysters and barnacles coming from the delta
plain and emplaced by catastrophic floods. Absence of
bottomsets could be related to high sediment supply
and rapid delta progradation.
The characteristics of these delta lobes show both
shoal-type and Gilbert-type delta features so they are
interpreted as shoal deltas with Gilbert-type profiles.
5. Role of fault growth in delta development:
architecture and processes
In the delta succession, tectonic control is revealed
in various ways: synsedimentary deformation structures, facies, architecture and clast provenances.
5.1. Margin faults
The first three units are characterized by retrogradational/aggradational stacking pattern controlled by the
behaviour of the margin faults. Their activity would
have caused the tectonic subsidence necessary for the
growth of accommodation space to exceed the stacking
pattern sedimentation rate, thus leading to a retrogradational–aggradational stacking pattern. The platform calcarenites onlapping the single delta lobes represent the
transgressive-abandonment stage subsequent to the
delta development. Retrogradational stacking of delta
units has been noted from other extensional basins
related to a progressive basin expansion (cf. Postma
and Drinia, 1993). A listric growth fault occurred between the second and third deltaic units as can be
inferred from the rollover geometry of unit II, which
was responsible for the wedge-shaped geometry of the
third unit, synchronous with fault activity. The most
reliable tectonic effects in the sedimentation are stratigraphic variations in tilting described in coarse-grained
delta deposits (cf. Gawthorpe and Colella, 1990).
5.2. Subsidence rates
The vertical trend in the architecture shows changes
in the stratal stacking pattern related to decrease in
subsidence rates and, hence, accommodation space. A
period of high subsidence controlled by normal faults
began at the base of the succession, producing the
retrogradational units 1 and 2, representing Gilberttype delta systems onlapped by shallow platform calcarenites. The presence of thick, vertically stacked
Gilbert-type fan-delta successions has been used in
tectonically active basins to infer direct tectonic control
and very rapid subsidence and sedimentation rates
(Dorsey et al., 1995). A period of low subsidence
followed, controlled by a listric growth fault, producing
aggradational unit 3, representing shoal-water deltas
capped by red algae biostromes. A non-subsidence
period and decrease in accommodation space at the
top of the succession related to uplift of the basin
margin, with sediment supply remaining constant, produced progradational deltas higher in the succession
(units 4 and 5).
5.3. Geometry
This focuses on the part of the succession where the
delta deposits show the effects on delta development of
extensional tectonic, normal faults and a listric growth
fault and its rollover (Fig. 7). The listric growth fault
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Fig. 7. Summary table of the main tectonic events occurring throughout the record of the deltaic succession.
which affected the deposits of unit 3 was the most
influential structure in the stratigraphic architecture
and in the sedimentation of the deltaic succession
(Fig. 8A). The listric fault strike is NW–SE. Unit 2
deposits located on the hanging wall were folded in a
rollover whose radius is 300 m. Unit 3, synchronous
with fault activity, shows a typically wedge shaped
geometry, increasing in thickness near the fault scarp
and decreasing away from the scarp. Sedimentary evidence of the fault activity synchronous with unit 3 is
seen in the horizontal and vertical trends in deltaic
sedimentation style (Fig. 8B).
5.3.1. Horizontal trend
Depth ratio at the river mouth decreased from 0.5
away from the scarp fault to 0.2 near the scarp fault.
Gently inclined delta fronts (shoal-water deltas) developed away from the fault scarp, evolving to steep
inclined delta fronts (Gilbert-type delta profiles) near
the fault scarp. Shoal-water deltaic lobes developed in
the proximal areas and on the rollover top away from
the fault scarp (Fig. 8C). Gilbert-type delta profiles
developed in the local accommodation created between
the scarp of the normal listric growth fault and the
rollover (Fig. 8D).
5.3.2. Vertical trend
Depth ratios at the distributary river mouth of the
shoal-water deltas persisted through time, maintained
by slow steady sea-level rise. When the activity of the
growth fault ceased, filling by the Gilbert-type deltaic
lobes of the local accommodation space related to the
fault was completed and the shoal-water deltas formed
(Fig. 8E). When palaeo-bathymetric differences disappeared, the development of shoal-water deltas spread
over the smoothed sea-floor topography.
The progressive increase in accommodation space
inherent in fault growth controlled the style of delta
sedimentation in the river mouth. Gilbert-type deltas
developed during periods of increasing in subsidence
rates or near the fault scarp and shoal-water deltas
during periods of decreasing in subsidence rates or
away the fault scarp. During the periods of increasing
in subsidence rates near the fault scarp, basin depth and
accommodation space remained more or less constant
over the rollover. There, away from the fault scarp,
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Fig. 8. (A) Panoramic view of unit 2, folded as a rollover and overlain by unit 3 forming a divergent fill expanding towards the basin margin (to the
right). (B) Panoramic view of unit 3 with the differentiation of shoal-water deltaic lobes towards the margin (to the right) (C) changing distally to
Gilbert-type delta profile (D). (E) Interpreted cross-section showing the evolution of the deltaic lobes of Unit 3 controlled by a listric growth fault.
Shoal-water deltaic lobes developed on the basement and on the rollover where the accommodation space was reduced and a Gilbert-type delta
profile developed where the listric fault caused local growth in the accommodation space.
F. Garcı́a-Garcı́a et al. / Sedimentary Geology 185 (2006) 79–92
basin infilling occurred by progradation of shoal-water
deltas.
6. Discussion
We have described the architecture and sedimentary
facies of a delta stack deposited during a third-order
highstand sea-level at a tectonically active margin. The
delta stack consists of five delta units controlled by
fourth-order tectonic variations. Two tectonic phases
have been recognized from the sedimentary fill. A
progressive basin expansion controlled by a fault
growth is recorded by the retrogradational stacking of
the first three delta units. Then, after the extensional
phase, the margin became dominated by compression,
recorded by progradational delta units (units 4 and 5).
The vertical trend in bathymetry, suggested by the
characteristics of the base of the studied succession
comprising alluvial fan deposits, corals and the first
three delta units showing the transition from shallow
deltas to platform deposits and hemipelagic marls (see
synthetic column of Fig. 7), are similar to those predicted by theoretical fault-growth models (Schlische,
1991) and to those described in the sedimentary fill
of extensional basins (Postma and Drinia, 1993; Young
et al., 2003). Those models describe the natural transitions from alluvial to deep-marine facies.
The differential response of delta systems to fault
growth is related to the structures linked to the fault
growth, rollovers or the growth of folds. A rollover is a
structural barrier that substantially modifies the delta
progradation transverse to the basin and encourages
delta progradation laterally, parallel to the fault and
the rollover. Individual delta units show convergence
and thinning towards the fold crest, and divergence and
expansion towards the basin margin and the fault scarp.
At the same time, onlap of platform calcarenites onto
the rollover flank gently dipping basinwards demonstrates that the rollover crest represents a structural
barrier. High sedimentation rates in the area between
the steep rollover flank (southern flank) and the fault
scarp in contrast with sediment starvation on the gentle
rollover flank (northern flank). The rollover represents
a passive structure in relation to synsedimentary delta
processes contrasting to fold growth which can progressively rotate and incorporate the deltas into fold
limbs (Gupta et al., 1999).
The delta architecture provides a signature of the
fault activity. During the early stage of fault growth,
retrogradational units representing vertically stacked
Gilbert-type delta systems (delta unit 1) imply direct
tectonic control, with very rapid subsidence and high
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sedimentation rates required to maintain the gentle
slope and accommodation space necessary for development this type of delta, following Dorsey et al. (1995).
During the intermediate stage, aggradational units
representing shoal water-type deltas (delta unit 3)
imply lower rates of subsidence, coupled with lower
rates of sediment supply during red algae biostromes
construction, to maintain the gentle slope and accommodation space necessary for development this type of
delta. Higher rates of sediment supply to shoal deltas
would encourage exposure during their development
but no exposure signatures can be recognized. During
the later stage of fault growth, strongly progradational
deltas (delta units 4 and 5) imply low rates of subsidence or no subsidence, coupled with high sediment
supply as in the transfer zone of the Suez Rift coarsegrained delta described by Young et al. (2000).
7. Conclusions
Field studies of the architecture and sedimentary
evolution of the late Tortonian extensional margin fill
of an intramontane basin in the Betic Cordillera (Guadix Basin) show a delta succession made up of five
delta units. Progressive basin expansion is recorded by
a retrogradational stacking of the first three delta units.
Local variations of subsidence, and variations in subsidence rates related to normal growth faults, controlled
the horizontal and vertical trends, respectively, in the
architecture and facies of the deltas. A variety of delta
types developed, with Gilbert-type deltas at the base
and in the middle of the succession (units 1 and 3)
developed near the fault scarp or during periods of high
subsidence rates, and shoal-water deltas developed
away of the fault scarp or during periods of low
subsidence rates. Shoal-water type deltas change to
Gilbert-type deltas and back again due to variations
in accommodation space controlled by variations in
local subsidence and subsidence rates. Near the fault
scarps the accommodation space progressively
increases and over the rollover the accommodation
space is remains constant.
In the same way that the presence of thick, vertically
stacked Gilbert-type fan-delta successions have been
used in tectonically active basins to infer direct tectonic
control and very rapid subsidence and sedimentation
rates (Dorsey et al., 1995), it may be possible to use the
presence of thick, vertically stacked shoal water-type
delta successions in tectonically active basins to infer
low rates of subsidence with low sediment supply to
maintain the gently slope and accommodation space
necessary for development this type of deltas.
92
F. Garcı́a-Garcı́a et al. / Sedimentary Geology 185 (2006) 79–92
Spatial variability of delta type is linked to the
location near or away from the fault scarp and onto
the rollover, whereas the vertical variability of the delta
type is linked to variation in fault growth rates.
For improvement in understanding the control of
fault growth on basin fill architecture in extensional
margins, modeling studies should take into account
vertical and horizontal variations in delta type.
Acknowledgements
This research was supported by projects CGL
2005-06224 of the Ministerio de Educación y Ciencia-FEDER, BTE2001-2872 and Research Group
RNM0163 of the Junta de Andalucı́a. The paper has
greatly benefited by the constructive comments of Dr.
K.A.W. Crook and two anonymous reviewers, and it
had previously benefited by the comments of Dr. F.
Massari and Dr. P. Haughton.
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