The depositional evolution of diapir- and fault-bounded rift basins: examples from the Lusitanian Basin of West Iberia

Sedimentary Geology 162 (2003) 273 – 303 www.elsevier.com/locate/sedgeo

The depositional evolution of diapir- and fault-bounded rift basins: examples from the Lusitanian Basin of West Iberia Tiago M. Alves a,*, Giuseppe Manuppella b, Robert L. Gawthorpe a, David W. Hunt a, Jose´ H. Monteiro b a

Basin Studies and Stratigraphic Group, Department of Earth Sciences, The University of Manchester, M13 9PL, Manchester, UK b Departamento de Geologia Marinha, Instituto Geolo´gico e Mineiro (IGM), Estrada da Portela, Zambujal-Alfragide, 2720-866, Alfragide, Portugal Received 27 February 2002; accepted 25 April 2003

Abstract New data on the evolution of rift basins is presented after analysing the Late Jurassic stratigraphy of the Central Lusitanian Basin (west Iberia). Well, outcrop and regional 2D seismic reflection profiles are used to investigate the differences in stratigraphic signature between diapir- and fault-bounded sub-basins. During the Late Jurassic syn-rift phase, surface rupturing in fault-bounded sub-basins resulted in the formation of tectonic scarps from which footwall-derived gravity flows were sourced. In contrast, the diapir-bounded Bombarral-Alcobac¸a sub-basin evolved as a distal bowl-shaped depocentre with an axis located up to 10 km away from its basin margins. Low-gradient marginal slopes developed in the Bombarral-Alcobac¸a sub-basin during the Late Jurassic rifting, while growing salt pillows limited the vertical propagation of basement normal faults. Differences in tectonic evolution, basin physiography and sediment input are the main factors responsible for the distinct sedimentary evolutions recorded in the study area: (1) transverse footwall-derived sediment fans, predominant in faultbounded regions, give place to axial southwards-prograding fluvial to shallow-marine units in the diapir-bounded subbasins; (2) growing salt pillows, absent in the fault-bounded sub-basins, formed barriers to and limited the development of transverse drainage systems. D 2003 Elsevier Science B.V. All rights reserved. Keywords: West Iberia; Late Jurassic; Rift tectonics; Sedimentation

1. Introduction

* Corresponding author. Departamento de Geologia Marinha, Instituto Geolo´gico e Mineiro (IGM), Estrada da Portela, ZambujalAlfragide, 2720-866, Alfragide, Amadora, Portugal. E-mail addresses: [email protected], [email protected] (T.M. Alves), [email protected] (R.L. Gawthorpe), [email protected] (D.W. Hunt), [email protected] (J.H. Monteiro).

The architecture of rift-related sedimentary successions has been comprehensively discussed in the geological literature as sequence stratigraphy began to be used as a tool to predict lateral and vertical stacking patterns in sedimentary basins (Gawthorpe et al., 1994; Nøttvedt et al., 1995; Ravna˚s and Bondevik, 1997; Ravna˚s and Steel, 1998; Dawers

0037-0738/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0037-0738(03)00155-6

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and Underhill, 2000; Gawthorpe and Leeder, 2000). In the past 20 years, depositional models for extensional basins have been developed based on outcrop (Leeder and Gawthorpe, 1987; Bentham et al., 1991; Eliet and Gawthorpe, 1995; Ravna˚s et al., 1997; Gupta et al., 1999; Janecke et al., 1999; Young et al., 2000) and geophysical data (Papatheodorou and Ferentinos, 1993; Soreghan et al., 1999; Wells et al., 1999; Stefatos et al., 2002; Leeder et al., 2002) of tectonically active regions. Most of this latter information focus on basins where evaporite-related crustal detachments and/or intrabasinal salt structures are absent, in spite of the evidence that the location, geometry, the spatial – temporal growth of salt struc-

tures influence the structure, drainage development, relative sea-level changes and subsidence rates of sedimentary basins (Davison et al., 1996; Edgell, 1996; Talbot and Alavi, 1996; Rowan and Weimer, 1998; Weimer et al., 1998; Stewart and Clark, 1999; Lawton et al., 2001). Thus, to investigate the evolution of diapir-bounded rift basins and comparing it with the existent models for half-graben regions is important to fully understand the tectono-sedimentary processes involved in mixed salt/fault-controlled extensional settings. The study area, located between the Nazare´ and Tagus Faults, comprises the central onshore sector of an abandoned Jurassic rift trough (Lusitanian

Fig. 1. (a) Location of the study area and relationship with the structural units of western Iberia. (b) Structure of the Central Lusitanian Basin showing the interpreted seismic and well data sets.

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Basin) formed on the western Iberian margin (Wilson et al., 1989) (Fig. 1). During its principal Late Jurassic rifting episode, various sectors in the Central Lusitanian Basin evolved distinctively as diapiror fault-bounded sub-basins, a character marked on the stratigraphic record by the deposition of a complex set of sedimentary facies. This latter fact has been acknowledged in previous works (Wilson et al., 1989; Ellis et al., 1990; Leinfelder and Wilson, 1998; Manuppella et al., 1999), but not studied in detail using a comprehensive seismic, well and outcrop database. Hence, using an extensive set of seismic and stratigraphic data, this work focuses on the differences in the Late Jurassic (Oxfordian – early Kimmeridgian) stratigraphy between the diapir-bounded Bombarral-Alcobac¸a and the fault-bounded Turcifal and Arruda half-grabens of the Central Lusitanian Basin (Fig. 1b).

2. Data and methodology A grid of 2D vibroseis and dynamite sourced seismic reflection lines, plus data from 17 wells and 14 outcrop locations, were used to compare the seismic stratigraphy and structure of the three studied sub-basins (Figs. 1 and 2). The seismic interpretation followed the methodologies of Mitchum et al. (1977) and Hubbard et al. (1985). The nomenclature of the interpreted seismic units was based on the framework of Leinfelder and Wilson (1998) for the Arruda subbasin in order to tie the Late Jurassic seismic units to the third-order sequences established by the latter authors (Fig. 3). Well and outcrop stratigraphic data were compiled to define the main Oxfordian– early Kimmeridgian depositional associations within the third-order sequences of the Central Lusitanian Basin (Figs. 2 and 3). The outcrop locations were selected according to their proximity to the wells and structural/stratigraphic significance to this study, taking into account the existence of accurate biostratigraphic data on the studied successions. The dating of the outcrop and well lithostratigraphic units was based on ammonite biostratigraphic data (Ruget-Perot, 1961; Mouterde et al., 1973, 1979; Atrops and Marques, 1986, 1988a,b) and on foraminifera associations (Ramalho, 1971, 1981).

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3. Geological setting 3.1. Regional stratigraphy Two pre-Oxfordian rift phases have been recognised in west Iberia, with a first phase (Rift 1, Late Triassic) preceding a Sinemurian – Pliensbachian (Rift 2) extensional phase (Stapel et al., 1996; Rasmussen et al., 1998) (Fig. 3). Triassic – lowermost Jurassic deposits comprise red beds (Silves formation) and evaporites (Dagorda formation) accumulated in halfgrabens/grabens (Rasmussen et al., 1998). From the Sinemurian, carbonate units deposited over a westerly tilted ramp (Coimbra, Brenha and Candeeiros formations) predominated in the Lusitanian Basin (Azereˆdo et al., 2002). The development of the Central Lusitanian Basin is associated with the middle Oxfordian –Berriasian Rift 3 (Fig. 3). Rifting and continental breakup in the Tagus Abyssal Plain resulted in significant subsidence south of the Nazare´ fault (Leinfelder and Wilson, 1998; Rasmussen et al., 1998) (Fig. 1a). Distinct sectors of the basin were then filled with mixed continental-marine deposits showing complex facies patterns. Lacustrine and marine carbonates (Cabac¸os and Montejunto formation) deposited during the prerift phase underlay thick (>2000 m in places) units signing a late Oxfordian– early Kimmeridgian episode of siliciclastic influx into the basin (upper bimammatum to lower hypselocyclum zones; Ruget-Perot, 1961; Atrops and Marques, 1986, 1988a; Reis et al., 1996, 2000). Facies complexity was attenuated in the late Kimmeridgian (upper Abadia and Amaral formations) prior to the progradation of fluvial/deltaic systems into the Central Lusitanian Basin during the latest Kimmeridgian –Berriasian (Leinfelder and Wilson, 1998) (Fig. 3). 3.2. Structure of the Central Lusitanian Basin Figs. 4 –6 depict the deep structure of the Central Lusitanian Basin. North to north – northeast- and northwest-trending faults follow the strike of Hercynian structures identified in west Iberia (Wilson et al., 1989) (Fig. 4). The Turcifal and Arruda sub-basins are divided by the nearly 20-km-long, north- to north – northeast-striking Runa Fault (Fig. 4). In contrast, the Turcifal and Arruda half-grabens are separated from

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Fig. 2. Simplified geological map of Central Lusitanian Basin showing the location of the studied outcrop sections.

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Fig. 3. Lithostratigraphy and seismic stratigraphy of the Central Lusitanian Basin.

the Bombarral-Alcobac¸a sub-basin by a 70-km-long northeast- to east-trending structural lineament, the Torres Vedras –Montejunto lineament (Fig. 4). This comprises, between the Arruda and Bombarral-Alcobac¸a sub-basins, an inverted Late Jurassic salt anticline (Fig. 5). Distinct salt anticlines occur on the western and eastern margins of the Bombarral-Alcobac¸a sub-basin (Fig. 1b). Evidence of halokinesis in the Bombarral-Alcobac¸a sub-basin has been recorded in units as old as

Early Jurassic (Montenat and Gue´ry, 1984). Halokinesis climaxed in two subsequent episodes, a first late Oxfordian– Kimmeridgian related to the major phase of extension in the Lusitanian Basin and a second Miocene episode associated with the Alpine compression (Cane´rot et al., 1995; Rasmussen et al., 1998). However, salt structures are absent from the Turcifal and Arruda sub-basins due to the relative scarcity of thick Late Triassic – Hettangian salt at depth (Leinfelder and Wilson, 1998).

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Fig. 4. Isochron data for the Central Lusitanian Basin. (a) Unit A1: The figure shows narrow north – south depocentres with more than 500 ms TWTT of sediments located on the immediate hanging-wall block to the Runa and Praganc¸a faults. (b) Units A2 and A3: Note the relative spreading of the main sub-basin depocentres after the rift initiation phase. Two-way travel-time values in milliseconds.

A particular character of the Lusitanian Basin is the limited salt extrusion recorded in the Mesozoic. Jurassic –Cretaceous halokinesis resulted in the growth of individual salt pillows at depth, structures that were reactivated during the subsequent Cenozoic basin inversion period (Cane´rot et al., 1995). In similarity with the Northern Lusitanian Basin (Alves et al., 2002), basin inversion in the study area has been related to the Miocene tectonism (Wilson et al., 1989).

the third-order depositional sequences in the Arruda sub-basin with the main rift-basin tectonic phases of Prosser (1993). Thus, following the framework of Leinfelder and Wilson (1998), the seismic unit A1 was deposited during the rift initiation phase (Fig. 3). Unit A2 represents the rift climax phase, preceding the immediate post-rift unit A3 and the late post-rift units A4 to A11 (Fig. 3). 4.1. Pre-Late Jurassic units

4. Seismic stratigraphy The studied seismic reflection data comprise regional 2D surveys published in Leinfelder and Wilson (1989) and reinterpreted in this paper. In order to relate the seismic information with the outcrop and well-log data, the Late Jurassic sequences in the Central Lusitanian Basin were correlated with the third-order sequence framework of Leinfelder and Wilson (1998) (Fig. 3). The two latter authors related

Pre-Late Jurassic units in the Central Lusitanian Basin comprise the Late Triassic Silves formation, the Dagorda formation (salt), plus the Lower – Middle Jurassic Candeeiros and Brenha formations (Fig. 3). Internally, the pre-Late Jurassic units show high- to low-amplitude, sub-parallel reflections that underlay bowl- to wedge-shaped packages (Figs. 5 and 6). The thickness of the pre-Late Jurassic units varies from less than 100 to 1000 ms two-way travel time (TWTT).

T.M. Alves et al. / Sedimentary Geology 162 (2003) 273–303 Fig. 5. Interpretation of line AR5-81 showing the Torres Vedras – Montejunto lineament and part of the Arruda and Bombarral-Alcobac¸a sub-basins. See Fig. 1 for location. In this region, the imaged sub-basins were controlled by Late Jurassic halokinesis, related to the growth of the Montejunto anticline. Consequently, seismic units A1 to A3 are lenticular in shape, lacking of typical wedge-shaped geometries recognised in half-graben basins.

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280 T.M. Alves et al. / Sedimentary Geology 162 (2003) 273–303 Fig. 6. Interpretation of line AR4-80, located 5 km south of the Torres Vedras – Montejunto lineament. See Fig. 1 for location. The fault-bounded Arruda sub-basin is a easterly dipping half-graben bounded by the Praganc¸a fault. The Late Jurassic units A1 to A2 are wedge-shaped, showing clear growth onto the Praganc¸a fault.

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The relative thickness of the latest Triassic – Hettangian Dagorda salt in depth controls the structural style of the basin. Where salt is thick, salt anticlines are formed and the overburden (Jurassic) rocks present high-amplitude folding (Fig. 5). Where salt is relative thin, simple half-graben sub-basins are developed (Fig. 6). This relationship between the relative thickness of salt and the structural style of the basin replicates that of the Northern Lusitanian Basin (Alves et al., 2002). 4.2. Late Jurassic units The Late Jurassic seismic units show a lenticular geometry in the salt-rich areas of the Central Lusita-

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nian Basin (Bombarral-Alcobac¸a sub-basin), (Fig. 5). In contrast, the salt-scarce (fault-bounded) Turcifal and Arruda sub-basins present wedge-shaped seismic units thickening towards the main basin-margin faults (Fig. 6). Unit A1, representing the rift initiation phase, comprises moderate- to high-amplitude reflections and shows limited thickening onto the basin-margin faults of the Arruda and Turcifal sub-basins (Fig. 6). Thickening of A1 within the diapir-bounded Bombarral-Alcobac¸a sector is more pronounced in the centre of the sub-basin, but the internal character of the unit is maintained (Fig. 5). Thickness values for A1 vary from < 100 to 500 ms TWTT (Table 1).

Table 1 Summary of the principal features of the stratigraphic units in the Central Lusitanian Basin Seismic units

Age of base

TWTT thickness (ms)

Internal character, geometry and terminations

Lithology

Cretaceous

Berriasian

< 200

A6 – A11

Latest Kimmeridgian (base beckeri/eudoxus zone) Late Kimmeridgian (upper divisum zone)

0 – 500

Wavy, uncontinuous reflections of low to moderate amplitude, occasionaly hummocky. Baselap is visible. Wavy, uncontinuous reflections of moderate to high amplitude. Two to three high-amplitude sub-parallel reflections.

Fluvial/deltaic sandstones and mudstones (channel to interdistributary areas). Fluvial/deltaic mudstones with relatively minor sandstones. Oolitic to bioclastic limestones marking the development of a carbonate shelf. Mudstones and thin-bedded sandstones (distal turbidites) of a southerly prograding slope. Coarse- to fine-grained turbidites of a slope/basin environment. Coarse-grained turbidites and minor mudstones of a slope/ basin environment. Coarse- to fine-grained limestones, predominantly fine-grained in basinal areas, with interbedded mud. Reefal to slope/basin (upper part) and lacustrine (lower part) environments. Coarse- to fine-grained carbonates deposited on reef, shelf and basin environments. Mudstones and evaporites of a restricted lacustrine basin.

A5

< 50

A4

Early Kimmeridgian (lower hypselocyclum zone)

100 – 400

Moderate-amplitude S-tilting clinoforms. Baselap and toplap are recognised.

A3

Oxfordian – Kimmeridgian Boundary (base platynota zone) Late Oxfordian (lower bimammatum zone)

100 – 400

A1

Middle – lower Oxfordian (plicatilis zone)

50 – 500

Moderate- to high-amplitude sub-parallel reflections showing minor baselap/onlap onto the basin-bounding structures. Moderate- to high-amplitude sub-parallel reflections showing growth and onlap onto the main basin-bounding structures. Moderate-amplitude reflections showing limited growth onto the basin-bounding structures. Two reflections of high amplitude mark its base. Baselap is visible.

Lower – Middle Jurassic Salt

Sinemurian

100 – 600

Low- to moderate-amplitude (sub-parallel) reflections.

Late Triassic

50 – 500

Middle – Early? Triassic

50 – 400

High-amplitude, low-frequency reflections with low continuity. Local growth visible in basin-margin anticlines. Moderate-amplitude reflections, uncontinuous, hummocky in places.

A2

Silves formation

0 – 400

Coarse- to fine-grained siliciclastic units deposited in a continental environment.

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The rift climax phase is signed in the three studied sub-basins by onlap of the seismic reflection of unit A2 onto the basin-margin structures (Figs. 5 and 6). The internal character of A2 is similar to unit A1, but its distribution is limited when compared with the latter (Fig. 5). Thickness values for A2 vary from 0 to 400 ms TWTT (Table 1). Unit A3 (immediate post-rift phase) drapes the three sub-basins and is marked by the absence of onlap onto the basin-margin structures and intrabasinal growth (Figs. 5 and 6). Moderate- to low-amplitude reflections are recorded in A3. Thickness values for the unit vary from 100 to 400 ms TWTT (Table 1). Unit A4 comprises sediments attributed to the onset of the late post-rift phase (Fig. 3). Its thickness varies from 100 to 400 ms (Table 1). Internally, the unit shows low- to moderate-amplitude clinoforms downlaping the top of unit A3 (Figs. 5 and 6). Careful analysis of the interpreted seismic data shows part of the clinoforms draping the southern part of the Bombarral-Alcobac¸a sub-basin, highlighting this way the basin-infill trend of unit A3. Wilson et al. (1989) and Leinfelder and Wilson (1989, 1998) associated this unit with the southward progradation of slope deposits over a tectonic quiescent basin. Units A5 and A6 –A11 (late postrift phase) were deposited in the latter evolution stages of the Central Lusitanian Basin representing: (1) the deposition of a oolith-rich carbonate ramp on a quiescent basin during a late Kimmeridgian highstand event (unit A5); (2) The progressive replenishment of the remaining rift-related topography (units A6 – A11) by prograding basin-margin siliciclastic systems during the latter stages of rifting (Wilson et al., 1989; Leinfelder and Wilson, 1998). In agreement to what is recorded in other rift basins (Gupta et al., 1999; Ravna˚s and Steel, 1998; Gawthorpe and Leeder, 2000), the relatively limited distribution of the rift initiation units next to the main basin-margin structures contrasts, on isochron data, with the evidence for widespread filling of the basin during and after the main period of rifting (Fig. 4). 4.3. Cretaceous– Cenozoic units Cretaceous– Cenozoic units are scarce on the interpreted seismic data. Narrow areas with Cretaceous units are visible north of the Torres Vedras – Monte-

junto anticline (Fig. 5). However, it is generally considered the Cretaceous deposition to have been significant over the basin, with Miocene inversion contributing for the uplift and erosion of most of the post-Jurassic units (Wilson et al., 1989; Rasmussen et al., 1998). Up to 500 ms of Cenozoic sediments are recorded on the easternmost part of the survey area, east of the Ota horst (Fig. 6). These deposits are related to the development of a forearc basin (Lower Tagus Basin) southwest of the study area (Rasmussen et al., 1998).

5. Middle Oxfordian – Early Kimmeridgian Facies Associations and depositional models The sedimentological study of the Late Jurassic depositional sequences A1, A2 and A3 at the selected well and outcrop locations enabled the identification of six facies associations, FA1 to FA6:      

FA1: Pedogenic limestones and conglomerates; FA2: Oolitic carbonates with reefal fauna; FA3: Thin-bedded limestones, microbial laminites and evaporites; FA4: Heterolithic sandstones/conglomerates and red clays; FA5: Coarse-grained Tubiphytes limestones; FA6: Turbiditic sandstones and mudstones.

Data on the interpreted facies associations was complemented by works published in Ellis (1984), Fu¨rsich and Werner (1986), Ellwood (1987), Hill (1989), Ellis et al. (1990), Bernardes et al. (1992), Ravna˚s et al. (1997), Manuppella et al. (1999) and Azereˆdo et al. (2002). The proposed environmental interpretations were based on numerous published works, in which Bhattacharya and Walker (1992), Jones and Desrochers (1992), Pratt et al. (1992), Blair and McPherson (1994), Vera and Cisneros (1993), Dreyer et al. (1999), Wrobel and Michalzik (1999), Cronin et al. (2000), Braga et al. (2001) and Seguret et al. (2001) are included. Analysis of the gamma-ray, sonic and neutron records of the interpreted facies associations are included in the following classification in order to correlate, on a basinal scale, the interpreted sedimentological and wireline data (Fig. 7).

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Fig. 7. Gamma-ray, neutron and sonic records for facies associations 1 to 6 based on the interpreted well data. 283

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5.1. Facies Association 1 (FA1): pedogenic limestones and conglomerates 5.1.1. Facies character Facies Association 1 comprises thick (1 – 5 m) levels of coarse-grained, poorly sorted carbonates. They occur on the northern flank of the Montejunto anticline at Rocha-Forte and its surroundings (Fig. 2). Clasts of light-coloured limestone, black pebbles and hybrid clasts in a grey argillaceous matrix/cement are characteristics of FA1. In thin section, the carbonate layers show wackestone, packstone and floatstone textures with alveolar-septal features, orthic and dishortic nodules, rhizocretions, circumgranular cracking, growth brecciation fabrics and rare calcite needles (Azereˆdo et al., 2002). Iron staining, disseminated pyrite, ferroan dolomite/dedolomite and diffusion ferruginous halos also occur. Within the carbonate beds occur grey marly horizons (0.1 –1 m thick) with black pebbles and lignitic clays. Coarsening- and fining-up gradations are observed. Fine-grained, grey carbonate levels occur towards the top of the succession at Rocha-Forte. They are organised in layers less than 1 m thick, with a distinctive fenestrae structures. Micaceous grey marls and lignitic clays horizons with black pebbles also occur. Fossil fauna include ostracods and charophytes (abundant) with scarcer gastropods, bivalves and porostromates. Ptygmatis nerineids, Pachyrismella megalodontids and the rudist Plesiodiceras mensteri are the most common megafaunas observed. The ostracods comprise freshwater to oligohaline genus (Theriosynoecum, Darwinula spp., Candonidae, Sinuocythere). Charophytes include Porochara raskyae (Ma¨dler), Porochara minima Ma¨dler and Porochara sp. 1 (Azereˆdo et al., 2002). 5.1.2. Environmental interpretation FA1 represents sub-aerial and tidal-influenced environments, probably associated with low-energy shallow lagoons bounded by diapiric structures and/or located on basin-margin structural highs. Black-pebble horizons are a diagnostic feature of subaerial exposure (Vera and Cisneros, 1993). Other features, such as rhizoconcretions, alveolar-septal textures, nodules and iron staining, are indicative of exposure phases. The fenestrae layers are related to sediment

desiccation and low-energy carbonate precipitation in peritidal environments (Bhattacharya and Walker, 1992). 5.1.3. Well-log character Well-log data is not available for FA1. 5.2. Facies Association 2 (FA2): oolitic carbonates with reefal fauna 5.2.1. Facies character Facies Association 2 is composed of bioclastic/ oolitic limestones (Fig. 8a). Massive (>5 m thick) beds with grey oolitic limestones occur at Cabec¸o do Asno-Moledo (Fig. 2). Overall, thick beds (>2 m) are common in the association and alternate, towards the top of FA 2, with siliciclastic horizons ( < 0.20 m thick). These siliciclastic beds are predominantly composed of immature quartz and feldspar grains, occasionally constituting the nucleus of oolite grains. Stromatoporid bioherms (knoll reefs) are locally visible. Ooids ( < 3 cm wide) are common throughout the association. On thin sections, the carbonates of FA2 comprise wackestones intercalated with intra/bioclastic, oolitic packstones/grainstones and coral-rich wackestones/packstones. Fossil fauna comprise corals, echinoids, acteonellids, nerineid gastropods, Solenopora sp., Alveosepta jaccardi Shrodt, Haplophragmoides sp. and Tubiphytes sp. Horizons with Rhyzocorallium are observed in thin ( < 0.2 m) marly horizons. Scarce ammonoids (Perisphinctes, Orthosphictes and Decipia) occur in the association (Manuppella et al., 1999). 5.2.2. Environmental interpretation The association outcrops in the Montejunto (locations G, H and F) and Bolhos (locations 2, 3 and 4, Fig. 3) regions. FA2 is related to current- and waveaffected areas of shallow-marine reefal buildups. Wave-affected oolitic barriers are represented at Cabec¸o do Asno-Moledo by massive oolitic packstones/grainstones (Fig. 2). Knoll reefs and breccia levels represent distal areas of open marine shelfbreak to talus environments. Open marine conditions are marked by a relative increase in the clayey content of the beds, by a relative decrease in the fossil fauna and by the appearance of ammonoids. The siliciclastic horizons may represent the input of non-carbonate

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Fig. 8. Selected photographs showing the outcrop character of the third-order depositional sequences and related facies associations. See Fig. 3 for location. (a) Upper Sequence A1 at Cabec¸o do Asno-Moledo. Carbonate units in this location comprise bioclastic/oolitic limestones with abundant macrofauna (FA2). (b) Sequence A2 at Casal da Ramada-Chora˜o. Thin-layered ( < 1-m-thick) sandy to conglomeratic turbidites and olistoliths (FA6) form the bulk of the rift climax packages at this location. (c) Sequence A3 at Casal da Guarda. The coarse-grained turbidites of A2 are replaced in A3 by thick (>2-m-thick) grey clays alternating with orange silt layers ( < 50-cm-thick). (d) Aspect of Sequence A3 at Consolac¸a˜o-Pai Mogo on the western margin of the Bombarral-Alcobac¸a sub-basin. The shallowing upwards deltaic/fluvial succession at Consolac¸a˜o is composed of metre-thick, parallel-bedded silts and marls interrupted by thinner conglomerates and sands with fossil fauna. They grade into channel-shaped fluvial channels and red marls towards the top of the section, as shown in the figure.

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material from delta/fluvial systems surrounding the reefal buildups.

1971, 1981). Zoophycos occur on the marly limestone levels.

5.2.3. Well-log character Gamma-ray and sonic data for FA2 are depicted in Fig. 7. Funnel-shaped gamma-ray curves with a coarsening-upwards trend (sensu Cant, 1992) characterise the association. This may be due to differences in the clay content of the facies composing FA2. The sonic record is regular with no visible trend.

5.3.2. Environmental interpretation FA3 was deposited in freshwater to deep marine environments. Lacustrine and marsh zones promoted the deposition of algae- and cyanobacteria-rich laminites and ostracod-rich carbonate layers. Desiccation cracks represent drying periods over the latter lakes/ marshes. The lacustrine deposits grade upwards into marly (ammonitic) limestones representing open marine slope and basin (anoxic) environments. Dark mud beds comprise suspension-transported sediment of unchannelled turbiditic flows and hemipelagic settling, i.e. units Tep and Tet of Bouma (1962). Lime wackestones (Td) comprise relatively proximal, moderate energy flow events of an intermediate to upper slope environment.

5.3. Facies Association 3 (FA3): thin-bedded limestones, microbial laminites and evaporites 5.3.1. Facies character Facies Association 3 comprises thin layers ( < 1 m) of dark marly limestones and bituminous foetid shales in places intercalated with microbial laminites and evaporites. Pyrite and organic matter (lignite) are abundant. Lignite levels ( < 2 m) occur in the association. Bed contacts are commonly sharp. The limestones exhibit mudstone to floatstone textures (rarely packstone) and a variety of bioclasts (Azereˆdo et al., 2002). Clasts of microbial material and porostomate tubes also occur, some forming small ooids. Grainstones occur mainly on the western flank of the Bombarral-Alcobac¸a sub-basin at Olho Marinho-Reguengo and Columbeira (Fig. 2). Foetid shales and laminites with siliceous nodes are found at RochaForte and Espinheira underneath a thick succession (>100 m) of dark marly limestones with ammonoids. The fossil content of the association is high. The macrofauna is dominated by small gastropods, thickshelled bivalves and serpulids (Azereˆdo et al., 2002). Ammonites occur mainly in non-evaporitic beds and comprise faunas attributed to the plicatilis, transversarium, bifurcatus and lower bimammatum biozones (Fig. 3) (Ruget-Perot, 1961; Ramalho, 1971). Nonmarine ostracods are abundant and include Sinuocythere pedrogaensis, Sinuocythere candeeirosensis, Galliaecytheridea sp. 1, Klieana spp. and Darwinula spp. Charophytes include the species in FA1 and the predominant Porochara kimmeridgensis (Azereˆdo et al., 2002). The dasyclad Hetroporella lusitanica is also observed. Benthic forminifera (mainly lituolids) are common including Pseudocyclamina parvula Hottinger, A. jaccardi Schro¨dt, Verneulinidae, Epistominidae, Ammobaculites sp. and Reophax sp. (Ramalho,

5.3.3. Well-log character Well-log data depicted in Fig. 7 show that both the gamma-ray and the sonic-log curves have a serrate shape derived from the alternating shale-limestone levels composing FA3. Shale beds coincide with high gamma-ray peaks up to 70 API. Limestone layers are characterised by low (up to 20 API) gamma-ray values. The sonic-log curve shows gentler irregularities with limestone layers corresponding to peaks of approximately 50 As/ft in internal velocity. 5.4. Facies Association 4 (FA4): heterolithic sandstones/conglomerates and red clays 5.4.1. Facies character FA4 comprises sandstones and conglomerates alternating with coarsening-upwards massive/parallellaminated red marl/clay (Fig. 8d). Tabular to lenticular beds, commonly more than 2 m thick, predominate in the association. Calcrete horizons are common on the top of the fine-grained levels, which can be more than 7 m thick. Fine-grained sediments show intense borrowing (Scoyena assemblage, Hill, 1989). Load casts and water-escape structures are also visible. Towards the centre of the Bombarral-Alcobac¸a subbasin at Cabec¸o do Asno-Moledo and Olho MarinhoReguengo (locations 2 and 3, Fig. 3), FA4 consists of intercalated yellow micaceous silts (>1 m thick) and

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intraclastic limestones. Oolites, peloids and bioclasts (porostromates, echinoderms, bivalves and corals) are visible in the thin-bedded ( < 1 m thick) limestones. Yellow marls and quartz sandstones with similar fauna, Diceratidae and plant debris occur towards the top of the succession. Also in locations 1 and 2 (Fig. 2), 180 m of ooid- and peloid-rich, bioclastic to sandy limestones occur halfway in the facies association (Calca´rios de Reguengo Pequeno, sensu Manuppella et al., 1999). These limestones comprise isolated corals, bivalve debris, Diceratidae, algae (Marinella lugeoni) and abundant porostromates. 5.4.2. Environmental interpretation FA4 is associated with deltaic/fluvial to proximal shelf depositional systems. Prodelta mouth bar deposits, wave- and tide-reworked sandstones alternate with grey parallel-laminated marls with marine fauna. Grey marls were deposited by buoyant sediment plumes in a distal prodelta environment (Fu¨rsich, 1986). Massive red clays and lignite-rich grey marls are associated with suspension-transported material deposited in interdistributary bays of upper deltaic environments. These sediments frequently confine lenticular sandy deposits of meandering fluvial channels (Fig. 8d). Cross-bedded sandstones and conglomerates of deltaic mouth-bar environments are also common. Interbedded ripple to parallel-laminated sandstones and mudstones occurring at Amoreira, 10 km south of Consolac¸a˜o (Fig. 2), represent distal alluvial fan sediments (heterolithic facies, Hill, 1989). They sign the accumulation of sheetflood deposits with a high suspended load (sandstones) alternated with mudstones deposited under lower flow regime conditions. The intercalations of marls and limestones recorded at Cabec¸o do Asno-Moledo and Olho Marinho-Reguengo (locations 2 and 3, Fig. 2) represent an open carbonate shelf influenced by a relatively high siliciclastic input. This is particularly signed by the rhythmic succession of ooid/bioclastic carbonates and marls/sandstones occurring at the latter locations. 5.4.3. Well-log character Irregular serrated curves with no apparent trend are signed on the gamma-ray records for FA4 (Fig. 7). Shale-rich beds are marked by peaks in the gammaray curve. The sonic curve is irregular for the entire association.

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5.5. Facies Association 5 (FA5): coarse-grained Tubiphytes limestones 5.5.1. Facies character The FA5 was studied in detail by Ellis et al. (1990) in the Ota region (Fig. 2). The association is composed of coarse-grained carbonate units including Tubiphytes bindstones, reefal framestones, bindstones, bafflestones and intra/bioclastic packstones/grainstones. Finer-nerineid wackestones/packstones, fenestrae limestones and lime mudstones/wackestones are also represented. They comprise beds up to 2 m thick, rich in fossil fauna. Tubiphytes foraminifera, stromatoporoids, Solenopora and Dasycladacea algae, sponges (including chaetetids), nerineids, bivalves, lithophagic bivalves and encrusting algae occur in the association. 5.5.2. Environmental interpretation FA5 is related to reefal environments. The coarser levels represent external, high-energy zones. Finegrained facies are associated with protected lagoon to peritidal zones. 5.5.3. Well-log character The relative absence of mud in the association is signed on well-log data by relative low values of gamma-ray, approaching 0 API (Fig. 7). Differences in grain-size and fluid-content result in an irregular Neutron curve with values close to the 300 – 350 counts/s. 5.6. Facies Association 6 (FA6): turbiditic sandstones and mudstones 5.6.1. Facies character FA6 comprises siliciclastic beds of diverse grain sizes intercalated with scarcer limestone horizons. Fining-upwards turbiditic sands and marls (Fig. 8b), massive siltstones/sandstones (Fig. 8c) and massive mudstones compose 60 –70% of the association. The remaining 30 – 40% comprises coarse-grained polymictic conglomerates and sandstones intercalated with lime packstones/grainstones. Allochthonous limestone blocks, less than 100 m3 in volume, occur within the siliciclastic beds. These blocks differ from autochthonous limestones (>100 m3 in volume) developed at several locations in the Arruda sub-basin, which

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comprise oolitic/bioclastic limestones alternating with black-pebble horizons (Ellis et al., 1990). Bedding in the marly/silty layers is massive to parallel, with cross bedding developed in coarser sandstones. Mica, pyrite and siderite nodules are abundant. Foraminifera, ammonites and ostracods are frequent in the association. At Montejunto, the ammonite fauna includes the species Sutneria platynota (Reinecke), Ataxioceras stromera (Wegele), Epipeltoceras bimammatum (Quenstedt) and the genus Orthosphinctes (abundant), Sowerbyceras, Aspidoceras and Taramelliceras, amongst the most common (Atrops and Marques, 1986). They occur with bivalves (Eomiodon sp., Corbulids), gastropods (mainly Terebrella, Neritopsis and Promathilda genus) and algae (Marinella sp., Girvanella sp., Cayeuxia sp., Hedstroemia sp.). Trace fossils include Cylindrichnus sp., Chondrites sp., Thalassinoides sp., Rhizocorallium sp., Zoophycus sp. and Theichichus sp. 5.6.2. Environmental interpretations A complex pattern of depositional facies is recorded in FA6. The finning-upwards turbidites are related to deposition on a fault-bounded slope. Sequences Ta, Tbe/Tbde and Tbcde of Bouma (1962) compose the bulk of the sandy deposits, but proximal Ta sequences predominate in the coarser levels. Polymictic conglomerates and sands relate to submarine canyon deposits. Massive marls, siltstones/sandstones and massive mudstones comprise, respectively, lower slope Td and Tet turbidites and distal Tep/Tet turbidites/ hemipelagic fall-out. The oolitic/bioclastic carbonates occurring in the Arruda sub-basin comprise deep-water bioherms deposited on the flanks of a submarine fan system (Wilson et al., 1989; Leinfelder and Wilson, 1998). In contrast, isolated allochthonous limestone blocks are related to the dismantle of carbonate buildup areas in tectonically active zones. In other words, they represent the tectonically induced dismantle of Oxfordian – early Kimmeridgian carbonate buildups during the active phases of rift-related faulting. 5.6.3. Well-log character Well-log data show irregular curves for the gamma-ray, sonic and neutron records (Fig. 7). A serrated gamma-ray curve with a fining-upwards trend characterises the association. Higher gamma-ray values

are related to the finer clayey or marly beds, contrasting with the lower gamma-ray troughs of sandy turbidite layers. Values of gamma-ray vary between 40 and 50 API (Fig. 7). Neutron values follow the gamma-ray curve indicating a higher fluid content (therefore, higher-porosity values) for the sandy layers. In contrast, the sonic-log curves show minor indentations with no apparent trend.

6. Sequence stratigraphy and facies distribution in the Central Lusitanian Basin Stratigraphic correlations between the 17 studied wells and the outcrop locations are discussed in this section (Figs. 9 – 13). 6.1. Sequence A1 (plicatilis to lower bimammatum zones) Outcrop and well stratigraphic data sign the onset of marine conditions (Upper Sequence A1) in the Central Lusitanian Basin during the late Oxfordian. In the Bombarral-Alcobac¸a and Arruda sub-basins, this latter event coincides with a transgressive surface at the top of ostracod-rich carbonate deposits of lacustrine environments (Facies Association 3) (Fig. 9). The upper boundary of A1 is coincident with a stratigraphic unconformity attributed to the bimammatum zone (BAG, 1996; Leinfelder and Wilson, 1998), but diachronic on the basin margins. A threefold depositional setting prevailed in the study area before the late Oxfordian rift climax (Figs. 9 and 10). Aggradational, shallow-marine lagoon deposits (Facies Association 1) were deposited on the southeastern flank of the Bombarral-Alcobac¸a subbasin over a rising salt structure episodically exposed (Rocha-Forte area, Fig. 9). The shallow-marine units of FA1 at Rocha-Forte grade basinwards into deep marine carbonates (FA3) within the Bombarral-Alcobac¸a and Arruda sub-basins (Figs. 9 and 10). On the western flank of the Bombarral-Alcobac¸a sub-basin, a series of southeast-prograding shallowwater carbonate buildups also formed over rising salt structures (Bolhos, Vimeiro and Caldas diapirs). These are flanked by oolitic sand barriers and shallow-marine carbonates (Facies Association 2) (Lo-1 and Cp-1 wells, Fig. 9).

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289

Fig. 9. Well-log stratigraphic correlation for Upper Sequence A1, Bombarral-Alcobac¸a sub-basin. Graphic legend for the wells in Fig. 7.

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The fault-bounded Arruda sub-basin presents a distinct tectono-stratigraphic setting. During the rift initiation phase, a by-pass carbonate shelf developed on the eastern margin of the Arruda sub-basin (Fig. 10). Within the basin depocentre, deep marine carbonates thicken towards the AG-2 well, an opposite setting to that in Sequence A2 (Fig. 12). Carbonate deposition is recorded elsewhere in Arruda and Turcifal, with a single exception on the En-1 well (Fig. 13). Here the deep marine carbonates of basinal areas grade into sandy turbidites. In parallel, a basinwide deepening-upwards trend in sequence A1 is interpreted from the well and outcrop data, reflecting a relative sea-level rise at the end of the Oxfordian. The same deepening-upwards trend is not recorded in the Rocha-Forte area (Fig. 9), where lagoonal carbonates continued to accumulate into the early Kimmeridgian (Wilson et al., 1989). We agree with Leinfelder and Wilson (1998) when stating that sequence A1 represents trangressive and highstand depositional systems. Distinctive third-order sequences within the sequence are not identified due to the structural complexity of key outcrop locations and to the incomplete record of well data. It is, however, possible using the interpreted data on A1 to indicate that the Central Lusitanian Basin formed a sediment-balanced basin (sensu Ravna˚s and Steel, 1998) during the Oxfordian rift initiation phase.

parallel, the southeast-prograding siliciclastic deposits (FA4 and FA6) deposited in the centre of the Bombarral-Alcobac¸a sub-basin were locally replaced by the carbonate-rich FA 1 and FA2 over the rising halokinetic structures (Fig. 11). On the fault-bounded southern flank of Montejunto (well Pr-1), a by-pass carbonate shelf may have developed as suggested by the erosion and redeposition of Oxfordian limestones as olistolithic material (Reis et al., 2000). In contrast, fenestrae- and peloid-rich carbonates of FA1 probably continued to form into early Kimmeridgian (Wilson et al., 1989; Ellis et al., 1990) over the Rocha-Forte salt pillow (Fig. 11). On the western margin of the Bombarral-Alcobac¸a sub-basin, southeast-prograding deltaic/fluvial units (FA4) at Consolac¸a˜o and PaiMogo fed the southern part of the salt withdrawal basin during the rift climax phase (Fig. 11). Over the fault-bounded Ota horst (Arruda subbasin), a by-pass carbonate shelf subsisted from the late Oxfordian up to the late Kimmeridgian (Ellis et al., 1990; Leinfelder and Wilson, 1998). On the hanging-wall block of the Arruda sub-basin, turbidites and distal hemipelagic muds intercalated with oolitic limestones mark the deposition of extensive marine siliciclastic units in basinal areas (Fig. 12). Throughout the study area, sequence A2 signs the change from a sediment-balanced (sequence A1) to a sediment-starved basin (sensu Ravna˚s and Steel, 1998).

6.2. Sequence A2 (upper bimammatum to planula zones)

6.3. Sequence A3 (platynota to lower hypselocyclum zones)

Sequence A2 is correlated with the rift climax phase and records the onset of siliciclastic deposition in the Central Lusitanian Basin (Leinfelder and Wilson, 1998). Facies complexity in the three studied sub-basins was increased at this stage in response to salt- and fault-controlled subsidence. In the Bombarral-Alcobac¸a sub-basin, fault-controlled subsidence of the sub-salt basement triggered salt withdrawal from basinal areas into growing salt pillows located on basin-margin footwalls. This is proven on well and outcrop data by the abrupt facies changes recorded on either side of the Caldas diapir (Wilson et al., 1989) and on seismic data by the limited distribution of A2 (restricted to the basin axis) in the Bombarral-Alcobac¸a sub-basin (Fig. 5). In

Sequence A3 marks the onset of axial and transverse progradation of siliciclastic units into basinal areas. In the Bombarral-Alcobac¸a sub-basin, the southeastern progradation of shallowing-upwards units at Consolac¸a˜o-Pai Mogo and Bolhos-Cezareda (location in Fig. 2) denotes the progressive replenishment of the basin depocentre by fluvial/deltaic and shallow-marine deposits (FA4), a trend also confirmed on seismic data (Fig. 5). Carbonate deposition continued over salt-related topographic highs at BolhosVimeiro (Fig. 11). On the eastern margin of the Arruda sub-basin, the by-pass carbonate shelf originated during the rift climax continued its development into the early Kimmeridgian (Ellis et al., 1990). At the same time,

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291

Fig. 10. Well-log stratigraphic correlation for Upper Sequence A1, Arruda sub-basin. Graphic legend for the well stratigraphic data in Fig. 7.

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GENT

DIAP LE I R -B O UN S tr a ti g rap h

ic U

nco

nfor

m it y

PE DED SLO

Fig. 11. Well-log stratigraphic correlation for Sequences A2 and A3, Bombarral-Alcobac¸a sub-basin. Graphic legend for the well stratigraphic data in Fig. 7.

hemipelagic and turbiditic units were deposited on hanging-wall block of the Arruda and Turcifal subbasins (Figs. 12 and 13). Part of these prograded into the Arruda sub-basin through a submarine canyon system that incised a transfer zone located between the Praganc¸a and Vila-Franca faults (Wilson et al., 1989).

According to Leinfelder and Wilson (1998), sequence A3 comprises a lowstand systems tract depositional system in the Bombarral-Alcobac¸ a and Arruda areas, changing laterally to highstand and trangressive systems at Vila-Franca (Fig. 12). The predominance of massive (fining-upwards) claystones in A3 indicates that the Central Lusitanian Basin

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Fig. 12. Well-log stratigraphic correlation for Sequences A2 and A3, Arruda sub-basin. Graphic legend for the well stratigraphic data in Fig. 7.

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Fig. 13. Well-log stratigraphic correlation for Sequences A2 and A3, Turcifal sub-basin. Graphic legend for the well stratigraphic data in Fig. 7.

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became a sediment-balanced basin (cf. Ravna˚s and Steel, 1998) during the immediate post-rift phase, being gradually replenished by sediment derived from marginal areas of the basin.

7. Discussion The main factors controlling the geometry and lithofacies distribution in marine sequences are: (i) eustatic sea level, (ii) tectonic subsidence or uplift, (iii) sediment supply, (iv) climate and (v) basinmargin physiography (Vail et al., 1977; Jervey, 1988; Posamentier et al., 1988; Galloway, 1989; Van Wagoner et al., 1990; Prosser, 1993; Ravna˚s and Steel, 1998). The interpreted data reveals that the basin physiography, the rate of accommodation space creation and the type and amount of sediment supplied into the basin were the main factors influencing the Late Jurassic deposition in the Central Lusitanian Basin. The relative importance of the three latter factors is discussed in the following sections. 7.1. Tectonic controls on sediment supply The dip and strike variations in segmented fault zones is known to control basin topography and the bathymetric gradients in graben and half-graben structures (Anders and Schlische, 1994; Schlische, 1995; Gawthorpe et al., 1994). In parallel, surface fault propagation (i.e. up-dip propagation of the fault-tip line) is commonly associated with the formation of aggradational alluvial/marine fans sourced from footwall catchments, while whether a depositional system has a ramp or shelf-break physiography alters the degree of fluvial incision and the nature of the lowstand systems tract (Posamentier and Allen, 1993; Gawthorpe and Leeder, 2000). The overall absence of siliciclastic material in sequence A1 suggests a relatively limited supply of coarse-grained sediment into the basin during the rift initiation phase. Instead, the gently warped subbasins formed at this time were gradually filled by fine-grained carbonates of lacustrine to marine environments which, on seismic data, show limited growth onto the main basin-bounding structures (Figs. 5 and 6).

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We interpret the relative absence of coarse-grained sediment in A1 to be associated with the limited development of major sediment sources (eroded fault scarps and uplifted horst blocks) inside the studied sub-basins and/or on their basin margins (Fig. 14a). This setting is directly related to the limited extension recorded in the Central Lusitanian Basin during the rift initiation phase (Leinfelder and Wilson, 1998). As in the Northern Lusitanian Basin (Alves et al., 2002), the presence of a thick detachment layer (underlying evaporites) at depth in the Bombarral-Alcobac¸a sector may also have limited the surface propagation of basement faults at this stage. Exceptions to the latter setting are the footwall-related submarine fans on the hanging-wall block adjacent to the Runa Fault, in the Turcifal sub-basin (Du Chene, 1988), most probably associated with proximal turbidites deposited in a tectonically controlled upper slope environment (Fig. 14a). Two possible source regions may have fed the submarine fans: the footwall adjacent to the Runa Fault (Du Chene, 1988) and/or the area adjacent to the Torres – Vedras lineament (Fig. 14a). Surface propagation of the Praganc¸a and Vila-Franca Faults could have occurred synchronously with the Runa Fault, but this latter assumption cannot be confirmed on well and outcrop data (Figs. 10 and 14a). Widespread surface propagation of sub-salt faults during the rift climax phase (late Oxfordian sequence A2) is signed on outcrop and well data by the deposition of coarse-grained turbidites and carbonate olistoliths (Figs. 12, 13 and 14b). This event marks the through-going phase of fault development (Gupta et al., 1999; Gawthorpe and Leeder, 2000) as late Oxfordian and caused significant changes in the tectono-sedimentary evolution of the Central Lusitanian Basin: (1) from gently warped sub-basins, the Arruda and Turcifal sectors evolved as separate halfgrabens from the late Oxfordian onwards; (2) the carbonate-dominated setting of the rift initiation phase changed abruptly to a siliciclastic-dominated regime in which transverse-derived drainage systems developed (Fig. 14b). Most of the latter transverse drainage systems consisted of canyon- and gully-related submarine fans gradually filling the accommodation space created during the rift climax. During the immediate post-rift phase, the progressive infill of the Central Lusitanian Basin was accentuated in association with a relative decrease in fault-

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Fig. 14. Depositional models for the Central Lusitanian Basin: (a) middle – late Oxfordian rift initiation phase; (b) late Oxfordian rift climax phase.

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related subsidence (Stapel et al., 1996) (Fig. 15a). The relative decrease in basin subsidence contributed to the progradation of axial and transverse drainage systems in the study area which, entering the basin from north, northeast and northwest, began to feed a southeast prograding continental slope (sequence A3) in the southern Bombarral-Alcobac¸a, Turcifal and Arruda sub-basins (Fig. 15a). This setting is similar to that recorded in other rift basins during the post-rift phase (Prosser, 1993; Ravna˚s and Steel, 1998; Gawthorpe and Leeder, 2000), being signed on seismic data by southerly tilted clinoforms and on well/outcrop data by a distinct shallowing-upwards trend from A3 to A4 (Figs. 5 and 12). Such a rapid infill of the studied sub-basins by the prograding deposits of Sequence A3 suggests a relatively high sediment yield during the immediate post-rift phase. Data on active rift basins of the United States (Nevada), North Sea (Oseberg-Brage) and Greece (Gulf of Corinth) relate high values for sediment yield to intrabasin climatic, tectonic and lithological factors (Leeder and Jackson, 1993; Papatheodorou and Ferentinos, 1993; Ravna˚s and Bondevik, 1997). In the case of the Gulf of Corinth, highly uplifted areas on the footwall of active faults, where unconsolidated material covered part of the rocky basement, are preferential areas for the development of important sediment-source areas (Papatheodorou and Ferentinos, 1993). In the Lusitanian Basin, Leinfelder and Wilson’s (1998) suggestion of a brief ( < 1 My) but well-marked rift climax phase in the Arruda subbasin, plus the well data of the Northern Lusitanian Basin (Alves et al., 2002), point to the formation of rift-related topography in regions of previous Late Triassic – Middle Jurassic sedimentation. Therefore, it is plausible to consider that the erosion of friable preLate Jurassic units in the Central Lusitanian Basin contributed to the high sediment yield recorded during the postrift phase. The also relatively friable schistose basement rocks of the Lusitanian Basin and the subtropical climate of the Late Jurassic Lusitanian Basin (Hill, 1989; Leinfelder and Wilson, 1998) are other factors promoting the high rates of sediment input recorded in the study area during the immediate post-rift phase. The development of an oolitic carbonate ramp on the Central Lusitanian Basin at end of the Kimmeridgian (Amaral formation, Fig. 3) indicates that most

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part of the rifting topography was flattened at the end of the immediate post-rift phase (Leinfelder and Wilson, 1998), precluding a latest Kimmeridgian – Tithonian period of axial/transverse sediment progradation (deltaic/fluvial systems) which completely filled the basin (Fig. 15b). 7.2. Basin physiography as a controlling factor of accommodation and deposition Deposition in the diapir-bounded Bombarral-Alcobac¸a sub-basin was controlled by mixed salt faultcontrolled subsidence (Rasmussen et al., 1998), which generated rates of subsidence above 200 m/ My within the axis of the sub-basin (Stapel et al., 1996). In the Arruda sub-basin, the >2137 m of sediments drilled in sequences A2 and A3 suggest a maximum subsidence rate of 2000 m/My for the hanging-wall area adjacent to the Vila-Franca and Praganc¸a faults (well Ar-1). In contrast, a rate of 240 m/My is recorded in well Sb-1, located on the western flank of the Arruda half-graben (Fig. 16, Wilson et al., 1989, p. 354). These differences in the amount of accommodation space created are responsible for the distinct stacking patterns observed in salt- and fault-controlled areas. In the Arruda and Turcifal sub-basins, rift climax and immediate postrift successions are thicker within along-strike hanging-wall depocentres situated 5 km eastwards of the Praganc¸a, Vila-Franca and Runa faults (Fig. 6). In Bombarral-Alcobac¸a, similar units show higher thickness in the centre of sub-basin (Fig. 5). The development of a bowl-shaped depocentre bounded by growing salt structures and low-gradient basin-margin slopes north of the Torres Vedras –Montejunto lineament (Fig. 5) follows similar settings to those in the North Sea (Williams, 1993; Stewart and Coward, 1995; Stewart and Clark, 1999), Gulf of Mexico (Rowan and Weimer, 1998; Weimer et al., 1998) and Persian Gulf (Edgell, 1996; Talbot and Alavi, 1996). Surface propagation of basement faults was impeded on the margins of the Bombarral-Alcobac¸a sub-basin by growing salt pillows which, in turn, (1) generated preferential areas for carbonate production and (2) controlled the southwestwards progradation of fluvial/deltaic units from hinterland areas (Fig. 16a and b). This setting is particularly marked on the western flank of the Bombarral-Alcobac¸a (Consola-

298 T.M. Alves et al. / Sedimentary Geology 162 (2003) 273–303 Fig. 15. Depositional models for the Central Lusitanian Basin: (a) latest Oxfordian – early Kimmeridgian immediate post-rift phase; (b) late Kimmeridgian late post-rift phase.

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Fig. 16. Transverse sections showing the stacking patterns of depositional facies in Bombarral-Alcobacß a and Arruda/Turcifal in relation to sediment drainage and sub-basin structure: (a) rift initiation phase; (b) rift climax phase; (c) immediate post-rift phase; (d) late post-rift phase.

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c¸a˜o-Bolhos; Hill, 1989; Manuppella et al., 1999) as late Oxfordian– Kimmeridgian transverse deltaic, fluvial and alluvial systems filling synforms located in between growing salt pillows. In the axis of Bombarral-Alcobac¸a, these ‘salt-controlled’ rivers and deltas intersected distinct axial drainage systems derived from north –northeast and constrained by the basinmargin salt structures (Fig. 16). As a result, alluvial/ fluvial units deposited west of the Caldas diapir (Wilson et al., 1989), while shallow marine/deltaic deposits filled the axis of the Bombarral-Alcobac¸a sub-basin. The asymmetric subsidence recorded in the Turcifal and Arruda sub-basins was responsible for the stacking of marine siliciclastic units on the immediate hanging-wall to the basin-bounding faults. Transverse drainage was predominant in these latter sub-basins prior to the late post-rift phase (Fig. 16). Similar structural controls on deposition are best documented in the Northern North Sea (Ravna˚s and Bondevik, 1997; Dawers and Underhill, 2000) and in the Gulf of Corinth (Papatheodorou and Ferentinos, 1993; Leeder et al., 2002; Stefatos et al., 2002). As in the Central Lusitanian Basin, where stacked coarse-grained sediments occupy the immediate hanging-wall area west of the Vila-Franca and Praganc¸a faults (Figs. 14b and 15a), regions in the Oseberg-Brage area of the North Sea show aggradational to backstepping rift climax sedimentary units trapped in proximal sub-basins or adjacent to masterfaults (Ravna˚s and Bondevik, 1997). Large progradational to aggradational fans developed downslope on relay ramps/transfer zones and at the mouth of cross-cutting faults at OsebergBrage, replicating the conditions that led to the formation of the submarine fan drilled at Ar-1 (Figs. 15a and 16). Away from relay ramps, the deposition of smaller talus cones and associated fault-scarp deposits are characteristic features of half-graben sinks of active rift basins, from which the Arruda and Turcifal sub-basins are important examples.

8. Conclusion The results presented in this paper highlight the distinct depositional evolutions of diapir- and faultbounded rift basins. Rift initiation, rift climax and post-rift depositional facies vary in relation to differ-

ences in the subsidence rates, basin-margin physiography and drainage development. Flexural subsidence associated with mild halokinesis predominated in the Central Lusitanian Basin during the rift initiation phase. During the rift climax, the presence of thick Triassic –Hettangian evaporites in depth prevented the propagation of sub-salt faults onto the surface in diapir-bounded areas. Shallow-water carbonate buildups developed over salt-related structural highs and so remained up to the terminal stages of post-rift. The apparent low gradient of the basin-margin slopes in salt-bounded areas resulted in the deposition of predominantly axial to hanging-wall-derived deltaic/shallow-marine deposits with minor turbiditic content. In contrast, transverse footwall-derived submarine canyon fans and olistoliths developed in fault-bounded subbasins. Axial to hanging-wall-derived siliciclastic packages prograded over the diapir- and fault-controlled depocentres during the post-rift phase. This study demonstrates that distinct depositional models should be addressed for diapir- and faultbounded rift basins. Both drainage and basin-margin physiography are substantially altered by the presence of growing salt structures during the active phases of crustal extension. Acknowledgements The authors would like to thank the dissemination of data by the Nu´cleo para a Pesquisa e Prospecc¸a˜o de Petro´leo (NPEP) and the Instituto Geolo´gico e Mineiro (IGM) of Portugal. We thank the comments provided by N. Dawers and an anonymous referee that greatly improved the early versions of this paper. Prolific discussions were also undertaken with the colleagues S. Corfield and M. Young. Finally, the authors acknowledge the data supplied by E.S. Rasmussen, I. Sinclair, and the fieldwork support of A. Bartolomeu. References Alves, T.M., Gawthorpe, R.L., Hunt, D.W., Monteiro, J.H., 2002. Seismic stratigraphy and structure of Northern Lusitanian Basin (west Portugal): Triassic – Jurassic North-Atlantic rifting. Mar. Pet. Geol. 19, 727 – 754.

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