Architecture and Sedimentary Facies Evolution in a Delta Stack Controlled by Fault Growth (Betic Cordillera, Southern Spain, Late Tortonian)

Sedimentary Geology 185 (2006) 79–92 www.elsevier.com/locate/sedgeo

Architecture and sedimentary facies evolution in a delta stack controlled by fault growth (Betic Cordillera, southern Spain, late Tortonian)

Fernando Garcıá-Garcıá a,*, Juan Fernańdez b,Ce´sar Viseras b, Jesu´s M. Soria c

a Departamento de Geologıá, Universidad de Jaen, Campus de las Lagunillas s/n, 23071 Jaen, Spain b Departamento de Estratigrafıá y Paleontologıá, 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 have a marked control on the external form, architecture and internal geometry of such deltas (Gawthorpe and Tectonic effects not only influence the location of Colella, 1990). coarse-grained deltas in extensional basins, they also Fault growth is an important control on drainage development in modern extensional margins, but those * Corresponding author. links are difficult to establish in ancient basins. The E-mail address: [email protected] (F. Garcı´a-Garcı´a). stratigraphy of rift-related deposits rarely enables us to

0037-0738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2005.10.010 80 F. Garcıá-Garcıá et al. / Sedimentary Geology 185 (2006) 79–92 make direct links with coeval structural controls, because the entire basin (Soria et al., 1998, 1999). Marine faults located adjacent to synrift strata generally do not sediments (allostratigraphic units I–III) formed during preserve evidence of their timing of activity in relation to the late Tortonian. The Guadix Basin and other sedi- depositional or erosional episodes (Gupta et al., 1999). mentary basins of the central sector of the Betic Cor- This study is aimed at demonstrating the influence of dilleras were then subjected to tectonic uplift during growth faulting and the rollovers directly linked with the late Tortonian (Lukowski, 1987; Sanz de Galdeano, delta development in terms of architecture, type of delta 1990; Sanz de Galdeano and Rodrı´guez Fernańdez, and processes in a Tortonian coarse-grained delta stack. 1996). Messinian to Upper Pleistocene sedimentation The small coarse-grained deltas as here studied re- took place in an internal drainage continental basin spond quickly to tectonic changes (Postma, 1990). A before the Guadix Basin became part of an external variety of deltaic systems changing from one to another drainage basin in the late Pleistocene and was subjected type have been described in tectonic contexts (Colella, to strong erosion (Calvache and Viseras, 1997). 1988; Postma and Drinia, 1993). The change in archi- The deltaic succession studied is located in the south- tecture of small-scale deltas controlled by faults, when eastern part of the Guadix Basin (Fig. 1B). In the sector sediment supply is kept constant, is related to the locus examined, the deltaic succession lies unconformably on of delta deposition close to or away from the fault scarp the Triassic basement. The deltaic deposits are uncon- and to subsidence rates. The objective of this paper is to formably overlain by Pliocene alluvial sediments be- describe the architecture and facies of delta systems longing to the later continental fill of the basin. The during the different stages of fault growth and to estab- presence of the calcareous nannoplankton Discoaster lish the boundary conditions when and where a type quinqueramus in the delta deposits, together with plank- delta (according the Postma’s classification, 1990)is tonic foraminifera Neogloboquadrina humerosa and the changing to another type one and back again. absence of Amaurolithus indicates that the studied del- We compare, in terms of processes, the delta stack taic succession is of latest Tortonian age (NBN-12 bio- described in this example with some classical examples zone from Martıń Perez, 1997). On the basis of the of Neogene delta stacking near normal faults, described biostratigraphy of the succession using calcareous nan- from the Gulf of Corinth (Postma and Drinia, 1993), the noplankton, the delta stacking took place in a short time Gulf of California (Dorsey et al., 1995, 1997) and from period (under 1.5 my), coinciding with the late Tortonian the Suez Rift (Gupta et al., 1999; Young et al., 2003). 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–Quaterna- ry basins located in the central sector of the Betic The deltaic succession, about 230 m thick, is made Cordillera (Fig. 1A). The Guadix Basin is an intramon- up of five delta units separated from one another by tane basin formed in the late Tortonian during the unconformities (Figs. 1C and 2). Each delta unit con- neotectonic stage (late Miocene to Quaternary) of the sists of several delta single delta lobes. Two stratal cordillera, after the main tectonic movements that gave pattern trends can be recognized in the delta units— rise to the large structures the Betic Chain (main thrust the first retrogradational to aggradational (units 1 to 3) nappes and extensional detachments, see Platt and Vis- and the second progradational (units 4 and 5) (Fig. 3). sers, 1989; Garcıá Duenãs et al., 1992). The geody- namic context during the neotectonic stage is 3.1. Retrogradational to aggradational units characterized by N–S to NW–SE regional compression, (units 1 to 3) responsible for the creation of a complex network of faults striking NW–SE and NE–SW to NNE–SSW 3.1.1. Unit 1 (Sanz de Galdeano and Vera, 1992). The Guadix The first unit is approximately 70 m thick and con- Basin is a polygonal basin subjected to considerable tains Gilbert-type delta systems retrogradationally subsidence controlled by tectonics (Viseras et al., stacked (Fernańdez and Guerra-Merchań, 1996). The 2005). single lobes are onlapped by platform calcarenites that The sedimentary fill of the Guadix Basin includes distally give way to hemipelagic marls. The vertical Tortonian to Upper Pleistocene sediments. The succes- trend in delta architecture shows clinoform geometry sion is subdivided into six allostratigraphic units sepa- varying from oblique-tangential clinoforms at the base rated by unconformities of tectonic origin, which affect of the unit to sigmoidal higher in the unit. .Garcı F. á-Garcı ae l eietr elg 8 20)79–92 (2006) 185 Geology Sedimentary / al. et á

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 Fernańdez and Guerra-Merchań, 1996, modified). 81 82 .Garcı F. á-Garcı ae l eietr elg 8 20)79–92 (2006) 185 Geology Sedimentary / al. et á

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. Garcı´a-Garcı´a et al. / Sedimentary Geology 185 (2006) 79–92 83

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 extent of the clinoforms clearly differentiate two con- Unit 2 is approximately 60 m thick and was deposited struction phases in this unit. The first led to extensive nearer to the basin margin than unit 1, from which it is oblique-tangential clinoforms, becoming wedge-shaped separated by an angular unconformity. It contains three distally. The second construction phase led to a small coarsening- and thickening-upward minor sequences, Gilbert-type deltaic lobe with sigmoidal clinoforms, in- representing aggradational shallow-marine deltas that creasing basinward in thickness from 7 to 15 m. distally give way to onlapping platform calcarenites. 4. Facies associations of the delta deposits 3.1.3. Unit 3 The geometry of this unit is wedge shaped, with a Lithologically, the succession consists of deltaic conglom- thickness of nearly 60 m near the basin margin thinning erates and sandstones with clinoforms 5–45 m high that to 30 m basinward about 500 m away from the basin distally give way to platform calcarenites and hemipelagic margin. This unit contains aggradational, shoal-water marls (Fig. 4). Three types of shallow-water and coarse- delta systems capped by red algae biostromes. Some grained deltas, according to Postma’s (1990) delta clas- shoal-water delta lobes change distally to Gilbert-type sification, have been recognized in the deltaic suc- profile lobes (Fig. 3). cession. They are characterized by their delta front dip: classic Gilbert-type deltas (steeply inclined delta front), 3.2. Progradational units (units 4 and 5) shoal-water deltas (gently inclined delta front) and Gil- bert-type profile (intermediate dipping delta front). This is represented by the last two deltaic units, both of which display a progradational stacking pattern. The 4.1. Classic Gilbert-type delta systems depocentres moved toward the centre of the basin. 4.1.1. Description 3.2.1. Unit 4 This facies association is represented by lobes with This unit, about 45 m thick, is represented by pro- well defined clinoforms ranging in height from 5 m to gradational delta lobes overlying an erosional surface 30 m. Coarse-grained deposits show variable grain size suggesting a slide scar. The deltas developed both and can be divided into two types: sandy-conglomer- oblique and oblique-tangential clinoforms. ates and conglomerates. The three geometric elements making up Gilbert-type deltas (topset, foreset and bot- 3.2.2. Unit 5 tomset) are clearly distinguished (Fig. 5A). The deltaic The first deposits of unit 5, overlying the basin marls, clinoforms have slopes ranging from 208 to 308 in the are represented by a bed of outsized clasts (some up to 2 upper parts, the angle decreasing from the brinkpoint to or 3 m in diameter). It could be interpreted as the pro- the subhorizontal layers of the bottomsets. Their geom- gradational infill of a slump scar. The geometry and etry varies from oblique-tangential to sigmoidal. 84 .Garcı F. ´a-Garcı ae l eietr elg 8 20)79–92 (2006) 185 Geology Sedimentary / al. et ´a

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. Garcı´a-Garcı´a et al. / Sedimentary Geology 185 (2006) 79–92 85

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. 86 F. Garcı´a-Garcı´a et al. / Sedimentary Geology 185 (2006) 79–92

The topsets have coarsening and thickening-up- The siliciclastic unit consist of upward-coarsening wards sequences, equivalent to those described for sequences, 5 to 15 m thick in the delta plain where the delta plain of shoal-water deltas. The channel four facies can be distinguished from bottom to top: height ranges from 1 to 3 m and the channels fills (1) mottled silts and clays, with root traces and a few are fining-upward (Fig. 5B).The foresets consist of thin layers of lignite, (2) sand containing marine gas- conglomerates with both matrix-supported and clast- tropods and wavy lamination, (3) beds of seaward supported fabrics. They may be normally graded cross-stratified conglomerates 2 to 3 m thick and (4) (Fig. 5C) or inversely graded, and the larger clasts channelized pebble-conglomerates 1,5 m thick with are commonly located in the upper or lower part of sharp erosional base. Occasional layers (10–15 cm) the foresets, depending on whether they are associated of carbonates, with algal lamination, charophytes and with matrix-supported or clast-supported fabrics. Ero- root traces, are locally associated. The delta front sional scars and upward-coarsening small conglomer- inclination ranges from 28 to 58. It consists of coars- ate lobes are locally observed, respectively in the ening and thickening sequences 3 to 4 m thick upper and lower parts of some steeply sloping fore- (Fig. 6B) of normally graded conglomeratic beds 25 sets. The scars are filled with cross-stratified, imbri- to 30 cm thick (Fig. 6C). Rare outsized clasts are cated conglomerates dipping against the slope. Sandy located at the bed tops. Angular pebble-conglomerates layers with ripple cross-lamination and tabular cross- with clast-supported fabrics fill the delta front beds. bedding are more abundant in the topsets and bottom- The prodelta is represented by massive pebbly sand- sets of the prograding bodies. stone alternating with silty layers. Burrows and sym- The bottomsets are represented by horizontal layers metrical ripples occur. Depth ratio (ratio channel in which sand, and local pebbles, alternate with bur- depth/basin depth following Postma, 1990) at the rowed, marly silts. The fossil content (red algae, bryo- river mouth is 0.4 to 0.5. zoa and oysters) is abundant here and well preserved. Tabular beds of red algae represent the carbonate Depth ratio at the river mouth is 0.1 to 0.2. unit capping the siliciclastic deposits. They overlie openwork gravels consisting of very rounded and im- 4.1.2. Interpretation bricated quartzite clasts. Red algae occur both in the The topsets have minor mouth bar-crevasse channel form of rhodoliths (similar to those described by Braga sequences built out in interdistributary bays with char- and Martıń, 1988, in equivalent environments situated acteristics equivalent to those of shoal-water deltas 30 km to the east), and also as massive, branching described bellow. morphologies, in which case they form biostromes. The foresets were built out by gravitational avalanches These algal beds are up to 8 m thick and extend of alternating cohesive and cohesionless debris flows. The laterally for up to 100 m. The siliciclastic content and steep slope may, in some cases, have caused instability of grain size decrease upwards within these beds. the upper part, leading to avalanches of coarse material down the slope, where it was deposited in the topset as 4.2.2. Interpretation lobes showing inverse grading because of basal shearing. For interpretation these deposits are divided in two The scours left in the foresets could have caused hydraulic units, siliciclastic and carbonate. The former have been jumps in later avalanches, forming backsets filling scours interpreted as shoal-water delta (cf. Postma, 1990) interpreted as due to the turbulence developed in the deposits. In the interdistributary bay subenvironment hydraulic jump (Massari, 1996). Postma and Roep of the deltas, the fine sediments represent suspension (1985) described similar processes for Pliocene Gil- settling, which is periodically interrupted by the input bert-type deltas in Southeast Spain. Backsets could be of coarse siliciclastic deposits through distributary created by the development of strong turbulence, which channels feeding distributary channel-mouth bars accompanies the formation of a hydraulic jump and not which prograded seaward. The stromatolite layers rep- controlled by any pre-existing scour. resent temporary interruptions of sediment supply. The facies of the delta front beds suggest deposition from 4.2. Shoal-water delta systems gravitational avalanches recording periods of river floods during which bedload sediment is deposited 4.2.1. Description just beyond the river mouth into the basin and is not For description, of these, deposits have been divided wave reworked. Symmetrical ripples in the shallow into a siliciclastic unit and a carbonate unit overlying prodelta deposits indicate that the depositional environ- the former (Fig. 6A). ment was to a certain extent controlled by wave action. F. Garcıá-Garcıá et al. / Sedimentary Geology 185 (2006) 79–92 87

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 À5 cm diameter—for scale). (D) Gilbert-type delta profile (basin margin to the right). 88 F. Garcı´a-Garcı´a et al. / Sedimentary Geology 185 (2006) 79–92

The siliciclastic unit is capped by a carbonate unit 5.1. Margin faults represented as red algae biostromes, which colonized the top of the delta deposits during periods of delta aban- The first three units are characterized by retrograda- donment. Wave reworking evidenced by open work tional/aggradational stacking pattern controlled by the quartzite gravels (gravel beaches?). Algal colonization behaviour of the margin faults. Their activity would of the top of the delta lobes and decrease of the silici- have caused the tectonic subsidence necessary for the clastic content from bottom to top of the algal beds could growth of accommodation space to exceed the stacking be related to delta abandonment, accompanied by star- pattern sedimentation rate, thus leading to a retrograda- vation of terrigenous sediment supply. These horizons tional–aggradational stacking pattern. The platform cal- could be also interpreted as transgressive intervals, re- carenites onlapping the single delta lobes represent the cording times of sediment starvation during which the transgressive-abandonment stage subsequent to the underlying delta-top was drowned and submerged, as delta development. Retrogradational stacking of delta has been interpreted elsewhere (Gupta et al., 1999). units has been noted from other extensional basins related to a progressive basin expansion (cf. Postma 4.3. Gilbert-type profile delta systems and Drinia, 1993). A listric growth fault occurred between the second and third deltaic units as can be 4.3.1. Description inferred from the rollover geometry of unit II, which The three geometric elements making up Gilbert- was responsible for the wedge-shaped geometry of the type deltas (topset, foreset and bottomset) can be dis- third unit, synchronous with fault activity. The most tinguished (Fig. 6D). The deltaic clinoforms show sig- reliable tectonic effects in the sedimentation are strati- moidal geometries 10 to 12 m height. graphic variations in tilting described in coarse-grained The topsets consist of channelized pebble-conglom- delta deposits (cf. Gawthorpe and Colella, 1990). erates 2 to 3 m thick with sharp erosional bases. The channel fill is fining-upward. Foreset inclinations range 5.2. Subsidence rates from 108 to 138. Foresets consist of normally graded conglomeratic beds. Angular and bored pebble-con- The vertical trend in the architecture shows changes glomerates with clast-supported fabrics that distally in the stratal stacking pattern related to decrease in give way to sandstones form the delta front beds (2 to subsidence rates and, hence, accommodation space. A 20 cm diameter). These beds alternate with normally period of high subsidence controlled by normal faults graded oysters beds. Bottomsets are absent. began at the base of the succession, producing the Depth ratio at the river mouth is 0.2. retrogradational units 1 and 2, representing Gilbert- type delta systems onlapped by shallow platform cal- 4.3.2. Interpretation carenites. The presence of thick, vertically stacked The facies of foreset beds suggest deposition from Gilbert-type fan-delta successions has been used in gravitational avalanches of cohesionless debris flows tectonically active basins to infer direct tectonic control recording periods of river floods during which bedload and very rapid subsidence and sedimentation rates sediment is deposited just beyond the river mouth into (Dorsey et al., 1995). A period of low subsidence basin. Fossil beds in the delta front represent resedi- followed, controlled by a listric growth fault, producing mented oysters and barnacles coming from the delta aggradational unit 3, representing shoal-water deltas plain and emplaced by catastrophic floods. Absence of capped by red algae biostromes. A non-subsidence bottomsets could be related to high sediment supply period and decrease in accommodation space at the and rapid delta progradation. top of the succession related to uplift of the basin The characteristics of these delta lobes show both margin, with sediment supply remaining constant, pro- shoal-type and Gilbert-type delta features so they are duced progradational deltas higher in the succession interpreted as shoal deltas with Gilbert-type profiles. (units 4 and 5).

5. Role of fault growth in delta development: 5.3. Geometry architecture and processes This focuses on the part of the succession where the In the delta succession, tectonic control is revealed delta deposits show the effects on delta development of in various ways: synsedimentary deformation struc- extensional tectonic, normal faults and a listric growth tures, facies, architecture and clast provenances. fault and its rollover (Fig. 7). The listric growth fault F. Garcı´a-Garcı´a et al. / Sedimentary Geology 185 (2006) 79–92 89

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 the scarp of the normal listric growth fault and the influential structure in the stratigraphic architecture rollover (Fig. 8D). and in the sedimentation of the deltaic succession (Fig. 8A). The listric fault strike is NW–SE. Unit 2 5.3.2. Vertical trend deposits located on the hanging wall were folded in a Depth ratios at the distributary river mouth of the rollover whose radius is 300 m. Unit 3, synchronous shoal-water deltas persisted through time, maintained with fault activity, shows a typically wedge shaped by slow steady sea-level rise. When the activity of the geometry, increasing in thickness near the fault scarp growth fault ceased, filling by the Gilbert-type deltaic and decreasing away from the scarp. Sedimentary evi- lobes of the local accommodation space related to the dence of the fault activity synchronous with unit 3 is fault was completed and the shoal-water deltas formed seen in the horizontal and vertical trends in deltaic (Fig. 8E). When palaeo-bathymetric differences disap- sedimentation style (Fig. 8B). peared, the development of shoal-water deltas spread over the smoothed sea-floor topography. 5.3.1. Horizontal trend The progressive increase in accommodation space Depth ratio at the river mouth decreased from 0.5 inherent in fault growth controlled the style of delta away from the scarp fault to 0.2 near the scarp fault. sedimentation in the river mouth. Gilbert-type deltas Gently inclined delta fronts (shoal-water deltas) devel- developed during periods of increasing in subsidence oped away from the fault scarp, evolving to steep rates or near the fault scarp and shoal-water deltas inclined delta fronts (Gilbert-type delta profiles) near during periods of decreasing in subsidence rates or the fault scarp. Shoal-water deltaic lobes developed in away the fault scarp. During the periods of increasing the proximal areas and on the rollover top away from in subsidence rates near the fault scarp, basin depth and the fault scarp (Fig. 8C). Gilbert-type delta profiles accommodation space remained more or less constant developed in the local accommodation created between over the rollover. There, away from the fault scarp, 90 F. Garcı´a-Garcı´a et al. / Sedimentary Geology 185 (2006) 79–92

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. Garcıá-Garcıá et al. / Sedimentary Geology 185 (2006) 79–92 91 basin infilling occurred by progradation of shoal-water sedimentation rates required to maintain the gentle deltas. slope and accommodation space necessary for development this type of delta, following Dorsey et al. (1995). 6. Discussion During the intermediate stage, aggradational units representing shoal water-type deltas (delta unit 3) We have described the architecture and sedimentary imply lower rates of subsidence, coupled with lower facies of a delta stack deposited during a third-order rates of sediment supply during red algae biostromes highstand sea-level at a tectonically active margin. The construction, to maintain the gentle slope and accom- delta stack consists of five delta units controlled by modation space necessary for development this type of fourth-order tectonic variations. Two tectonic phases delta. Higher rates of sediment supply to shoal deltas have been recognized from the sedimentary fill. A would encourage exposure during their development progressive basin expansion controlled by a fault but no exposure signatures can be recognized. During growth is recorded by the retrogradational stacking of the later stage of fault growth, strongly progradational the first three delta units. Then, after the extensional deltas (delta units 4 and 5) imply low rates of subsi- phase, the margin became dominated by compression, dence or no subsidence, coupled with high sediment recorded by progradational delta units (units 4 and 5). supply as in the transfer zone of the Suez Rift coarse- The vertical trend in bathymetry, suggested by the grained delta described by Young et al. (2000). characteristics of the base of the studied succession comprising alluvial fan deposits, corals and the first 7. Conclusions three delta units showing the transition from shallow deltas to platform deposits and hemipelagic marls (see Field studies of the architecture and sedimentary synthetic column of Fig. 7), are similar to those pre- evolution of the late Tortonian extensional margin fill dicted by theoretical fault-growth models (Schlische, of an intramontane basin in the Betic Cordillera (Gua- 1991) and to those described in the sedimentary fill dix Basin) show a delta succession made up of five of extensional basins (Postma and Drinia, 1993; Young delta units. Progressive basin expansion is recorded by et al., 2003). Those models describe the natural transi- a retrogradational stacking of the first three delta units. tions from alluvial to deep-marine facies. Local variations of subsidence, and variations in sub- The differential response of delta systems to fault sidence rates related to normal growth faults, controlled growth is related to the structures linked to the fault the horizontal and vertical trends, respectively, in the growth, rollovers or the growth of folds. A rollover is a architecture and facies of the deltas. A variety of delta structural barrier that substantially modifies the delta types developed, with Gilbert-type deltas at the base progradation transverse to the basin and encourages and in the middle of the succession (units 1 and 3) delta progradation laterally, parallel to the fault and developed near the fault scarp or during periods of high the rollover. Individual delta units show convergence subsidence rates, and shoal-water deltas developed and thinning towards the fold crest, and divergence and away of the fault scarp or during periods of low expansion towards the basin margin and the fault scarp. subsidence rates. Shoal-water type deltas change to At the same time, onlap of platform calcarenites onto Gilbert-type deltas and back again due to variations the rollover flank gently dipping basinwards demon- in accommodation space controlled by variations in strates that the rollover crest represents a structural local subsidence and subsidence rates. Near the fault barrier. High sedimentation rates in the area between scarps the accommodation space progressively the steep rollover flank (southern flank) and the fault increases and over the rollover the accommodation scarp in contrast with sediment starvation on the gentle space is remains constant. rollover flank (northern flank). The rollover represents In the same way that the presence of thick, vertically a passive structure in relation to synsedimentary delta stacked Gilbert-type fan-delta successions have been processes contrasting to fold growth which can pro- used in tectonically active basins to infer direct tectonic gressively rotate and incorporate the deltas into fold control and very rapid subsidence and sedimentation limbs (Gupta et al., 1999). rates (Dorsey et al., 1995), it may be possible to use the The delta architecture provides a signature of the presence of thick, vertically stacked shoal water-type fault activity. During the early stage of fault growth, delta successions in tectonically active basins to infer retrogradational units representing vertically stacked low rates of subsidence with low sediment supply to Gilbert-type delta systems (delta unit 1) imply direct maintain the gently slope and accommodation space tectonic control, with very rapid subsidence and high necessary for development this type of deltas. 92 F. Garcıá-Garcıá et al. / Sedimentary Geology 185 (2006) 79–92

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