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Sedimentary Geology 1

Architecture and sedimentary facies evolution in a delta stack

controlled by fault growth (Betic Cordillera, southern Spain,

late Tortonian)

Fernando Garcıa-Garcıa a,*, Juan Fernandez b, Cesar Viseras b, Jesus M. Soria c

a Departamento de Geologıa, Universidad de Jaen, Campus de las Lagunillas s/n, 23071 Jaen, Spainb Departamento de Estratigrafıa y Paleontologıa, 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

0037-0738/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.sedgeo.2005.10.010

* Corresponding author.

E-mail address: [email protected] (F. Garcıa-Garcıa).

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

85 (2006) 79–92

F. Garcıa-Garcıa et al. / Sedimentary Geology 185 (2006) 79–9280

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 re-

spond 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 archi-

tecture 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 estab-

lish 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).

2. Regional setting

The Guadix Basin is one of the Neogene–Quaterna-

ry basins located in the central sector of the Betic

Cordillera (Fig. 1A). The Guadix Basin is an intramon-

tane 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 Vis-

sers, 1989; Garcıa Duenas et al., 1992). The geody-

namic 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 succes-

sion is subdivided into six allostratigraphic units sepa-

rated by unconformities of tectonic origin, which affect

the entire basin (Soria et al., 1998, 1999). Marine

sediments (allostratigraphic units I–III) formed during

the late Tortonian. The Guadix Basin and other sedi-

mentary basins of the central sector of the Betic Cor-

dilleras were then subjected to tectonic uplift during

late Tortonian (Lukowski, 1987; Sanz de Galdeano,

1990; Sanz de Galdeano and Rodrıguez Fernandez,

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 south-

eastern part of the Guadix Basin (Fig. 1B). In the sector

examined, the deltaic succession lies unconformably on

the Triassic basement. The deltaic deposits are uncon-

formably overlain by Pliocene alluvial sediments be-

longing to the later continental fill of the basin. The

presence of the calcareous nannoplankton Discoaster

quinqueramus in the delta deposits, together with plank-

tonic foraminifera Neogloboquadrina humerosa and the

absence of Amaurolithus indicates that the studied del-

taic succession is of latest Tortonian age (NBN-12 bio-

zone from Martın Perez, 1997). On the basis of the

biostratigraphy of the succession using calcareous nan-

noplankton, 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 chron-

ostratigraphy of Haq et al. (1988).

3. Stratigraphic architecture

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 con-

sists 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 con-

tains Gilbert-type delta systems retrogradationally

stacked (Fernandez and Guerra-Merchan, 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.

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 Fernandez and Guerra-Merchan, 1996, modified).

F.Garcıa

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etal./Sedimentary

Geology185(2006)79–92

81

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.

F.Garcıa

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etal./Sedimentary

Geology185(2006)79–92

82

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.

F. Garcıa-Garcıa et al. / Sedimentary Geology 185 (2006) 79–92 83

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.

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 pro-

gradational 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 pro-

gradational infill of a slump scar. The geometry and

extent of the clinoforms clearly differentiate two con-

struction 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, in-

creasing basinward in thickness from 7 to 15 m.

4. Facies associations of the delta deposits

Lithologically, the succession consists of deltaic conglom-

erates 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 coarse-

grained deltas, according to Postma’s (1990) delta clas-

sification, have been recognized in the deltaic suc-

cession. They are characterized by their delta front dip:

classic Gilbert-type deltas (steeply inclined delta front),

shoal-water deltas (gently inclined delta front) and Gil-

bert-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-conglomer-

ates and conglomerates. The three geometric elements

making up Gilbert-type deltas (topset, foreset and bot-

tomset) 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 geom-

etry varies from oblique-tangential to sigmoidal.

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.Garcıa

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etal./Sedimentary

Geology185(2006)79–92

84

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) Fine-

grained 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.



The topsets have coarsening and thickening-up-

wards 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 clast-

supported 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. Ero-

sional scars and upward-coarsening small conglomer-

ate lobes are locally observed, respectively in the

upper and lower parts of some steeply sloping fore-

sets. The scars are filled with cross-stratified, imbri-

cated conglomerates dipping against the slope. Sandy

layers with ripple cross-lamination and tabular cross-

bedding are more abundant in the topsets and bottom-

sets of the prograding bodies.

The bottomsets are represented by horizontal layers

in which sand, and local pebbles, alternate with bur-

rowed, marly silts. The fossil content (red algae, bryo-

zoa 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 char-

acteristics 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 Gil-

bert-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 gas-

tropods 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 coars-

ening 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 sand-

stone alternating with silty layers. Burrows and sym-

metrical 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 im-

bricated quartzite clasts. Red algae occur both in the

form of rhodoliths (similar to those described by Braga

and Martın, 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.


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 rep-

resent 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 environ-

ment was to a certain extent controlled by wave action.

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) Coarsening-

upward 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).



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 aban-

donment. Wave reworking evidenced by open work

quartzite gravels (gravel beaches?). Algal colonization

of the top of the delta lobes and decrease of the silici-

clastic content from bottom to top of the algal beds could

be related to delta abandonment, accompanied by star-

vation of terrigenous sediment supply. These horizons

could be also interpreted as transgressive intervals, re-

cording 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 Gilbert-

type deltas (topset, foreset and bottomset) can be dis-

tinguished (Fig. 6D). The deltaic clinoforms show sig-

moidal geometries 10 to 12 m height.

The topsets consist of channelized pebble-conglom-

erates 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-con-

glomerates 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.


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 resedi-

mented 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 struc-

tures, facies, architecture and clast provenances.

5.1. Margin faults

The first three units are characterized by retrograda-

tional/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 retrograda-

tional–aggradational stacking pattern. The platform cal-

carenites 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 be-

tween 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 strati-

graphic 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 Gilbert-

type delta systems onlapped by shallow platform cal-

carenites. 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, pro-

duced 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

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 evi-

dence 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) devel-

oped 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 disap-

peared, 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,

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.



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 pre-

dicted 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 transi-

tions 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 demon-

strates 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 pro-

gressively 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

sedimentation rates required to maintain the gentle

slope and accommodation space necessary for develop-

ment 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 accom-

modation 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 subsi-

dence or no subsidence, coupled with high sediment

supply as in the transfer zone of the Suez Rift coarse-

grained 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 (Gua-

dix 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 sub-

sidence 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.


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 Educacion y Cien-

cia-FEDER, BTE2001-2872 and Research Group

RNM0163 of the Junta de Andalucıa. 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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