Jurassic tectono-sedimentary evolution of the Northern Lusitanian Basin (offshore Portugal)

Jurassic tectono-sedimentary evolution of the Northern Lusitanian Basin (offshore Portugal)

Marine and Petroleum Geology 19 (2002) 727–754 www.elsevier.com/locate/marpetgeo Jurassic tectono-sedimentary evolution of the Northern Lusitanian Ba...

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Marine and Petroleum Geology 19 (2002) 727–754 www.elsevier.com/locate/marpetgeo

Jurassic tectono-sedimentary evolution of the Northern Lusitanian Basin (offshore Portugal) Tiago M. Alvesa,*, Robert L. Gawthorpea, David W. Hunta, Jose´ H. Monteirob 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, Alfragide, Apartado 7586, 2720 Alfragide, Portugal Received 5 November 2001; received in revised form 10 June 2002; accepted 11 June 2002

Abstract The Lusitanian Basin is one of a series of rift-related basins developed during the rifting that led to the break-up of Iberia and Canada. The strong influence of halokinesis on the depositional evolution of its northern sector provides an analogue for the development of other evaporite-prone rift basins on Atlantic-type passive margins. In this paper, the pre-Valanginian evolution of the Northern Lusitanian Basin is documented based on regional seismic stratigraphy, isochron and time-structure maps tied with sonic and stratigraphic data from 10 exploration wells. The pre-salt structure of the study area is marked by two sets of faults: (i) a N – S to NE – SW striking set active throughout the Jurassic period and (ii) a NW – SE striking set active during the Late Jurassic. The latter defines five Late Jurassic sub-basins northeast of a basin-margin structure, the Marinha-Grande Fault. The development of half-graben basins in the Northern Lusitanian Basin during the Jurassic was overprinted by halokinesis over the Marinha-Grande Fault and other major marginal basement faults. Halokinesis climaxed during the Oxfordian– Kimmeridgian rifting with the formation of a 10 km long salt ridge over the Marinha-Grande Fault. The ridge limited the southward progradation of fluvial/deltaic units derived from hanging-wall drainage systems and restricted the syn-rift sediments to a 15 km long £ 5 km wide bowl-shaped depocentre located 10 km west of the present coastline. In order to better characterise the Jurassic tectono-sedimentary evolution of the Northern Lusitanian Basin, the interpreted seismic data is compared with physical models for extensional forced folds above active normal faults and with stratigraphic information from well and outcrop. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Western Iberia; Mesozoic; Halokinesis; Accommodation

1. Introduction The occurrence of significant hydrocarbon shows on the western Iberian margin has been related to the tectono-stratigraphic evolution of Mesozoic rift basins and their post-rift successions (Hiscott et al., 1990; Masson & Miles, 1986; Wilson, Hiscott, Willis, & Gradstein, 1989). Oil-source and reservoir units in western Iberia consist of a complex mosaic of facies types deposited in fault- and diapir-bounded sub-basins (Leinfelder & Wilson, 1998), whose development relates to the four Late Triassic – Early Cretaceous rift phases responsible for opening of the North Atlantic * Corresponding author. Present address: Departamento de Geologia Marinha, Instituto Geolo´gico e Mineiro (IGM), Estrada da Portela, Zambujal-Alfragide, 270-866 Amadora, Portugal. E-mail address: [email protected] (T.M. Alves).

Ocean (Rasmussen, Lomholt, Andersen, & Vejbæk, 1998). Since basins tectono-stratigraphically similar to those west of Iberia formed on the European and American margins in response to crustal extension (Driscoll, Hogg, Christie-Blick, & Karner, 1995; Manatschal & Bernoulli, 1999; Sinclair, 1995; Williams, Shannon, & Sinclair, 1999; Wilson, Manatschal, & Wise, 2001), the relationship between tectonic development and the depositional history of the Northern Lusitanian Basin may provide useful pointers for the future hydrocarbon exploration in the North Atlantic. Many papers have described the sedimentary evolution of rift basins (e.g. Gawthorpe, Fraser, & Collier, 1994; Gawthorpe & Leeder, 2000; Janecke, McInstosh, & Good, 1999; Leeder & Gawthorpe, 1987; Leeder, Harris, & Kirkby, 1998). However, much published work relates synrift depositional models to fault-controlled half-graben/graben structures unaffected by the influence of halokinesis on

0264-8172/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 4 - 8 1 7 2 ( 0 2 ) 0 0 0 3 6 - 3

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depositional environments. In contrast, this paper addresses the Jurassic evolution of the diapiric Northern Lusitanian Basin, a segment of a N – S trending rift margin that developed in western Iberia from Late Triassic to Late Jurassic (Fig. 1a). In particular, this work shows how, in the Northern Lusitanian Basin: † the salt structures relate to the underlying fault-bounded basement; † the timing and type of salt movements controlled accommodation space and the location of the main basin depocentres; † the location and relative growth of the halokinetic structures controlled the basin topography and, therefore, influenced facies distribution during Jurassic rifting.

2. Data and methodology Seismic data interpretation, two-way travel-time (TWTT) structural maps, isochron maps, and well stratigraphic data form the basis for the structural, seismic – stratigraphic and basin analysis presented here. The data set used in this study comprises 1100 km of seismic-reflection lines from the Portuguese continental shelf between Aveiro and Nazare´, plus stratigraphic data from 10 key offshore wells (Fig. 1b). Reliable check shot surveys and calibrated sonic logs provided direct well ties with the interpreted seismic sequences, which were correlated with the lithostratigraphic framework established for the Northern Lusitanian Basin by GPEP (1986) and Witt (1977) (Fig. 2). Biostratigraphic dating is based on ammonite data collected onshore (Atrops & Marques, 1986; 1988a,b; Bernardes, 1992; Bernardes, Corrochano, & Reis, 1998; Marques, Olo´ riz, & Rodriguez-Tovar, 1996) and on reviewed foraminifera, dinoflagellate and palynomorph assemblages obtained from offshore wells (BAG, 1996; Reis, Proenc¸a Cunha, Dinis, & Trinca˜o, 2000). In total, 11 seismic sequences and 19 seismic facies have been recognised in the study area following the methodology of Hubbard, Pape, and Roberts (1985) and Mitchum, Vail, and Sangree (1977) (Fig. 2).

3. Geological framework of the Lusitanian Basin 3.1. Structural setting The study area is part of the 150 km long £ 60 km wide Lusitanian Basin, Portugal (Fig. 1a). This basin is divided into three distinct sectors whose boundaries coincide with structural lineaments that formed major transfer zones during Mesozoic rifting (Boillot et al., 1974; Leinfelder & Wilson, 1998; Montenat, Guery, Jamet, & Berthou, 1988; Wilson et al., 1989). The Northern Lusitanian Basin, the

focus of this paper, is bounded by the Aveiro Fault to the north and by the Nazare´ Fault to the south (Fig. 1a). The trends of basement faults in the study area are similar to those recorded in the Central Lusitanian Basin (Wilson et al., 1989; Fig. 1b and c). As in this latter sector, two distinct sets of faults are identified offshore Northern Lusitanian Basin, some of which compose major lineaments 10– 25 km long (Fig. 1b). The primary set includes N- to NNE-trending faults, while the secondary set is NW – SE striking. In the Lusitanian Basin, the movement of basementrelated faults is known to have triggered halokinesis during the distinct phases of Jurassic extension, since salt structures are always associated with faults at depth (Rasmussen et al., 1998; Wilson et al., 1989; Zbyszewski, 1959). Growth of salt pillows over the footwall blocks of main basin-margin faults was accompanied by salt withdrawal in hanging-wall areas, particularly during the Late Jurassic (Wilson et al., 1989). This close relationship between basement normal faulting at depth and shallower salt movement in the saltrich parts of Lusitanian Basin created a mixed thick- and thin-skinned structural style, which in turn controlled deposition and local subsidence rates (Leinfelder & Wilson, 1998; Rasmussen et al., 1998; Reis et al., 2000; Wilson et al., 1989). 3.2. Late Triassic– Jurassic rifting Three rift phases are recorded in the Lusitanian Basin after the Early Triassic (Fig. 2) (Rasmussen et al., 1998; Stapel, Cloething, & Pronk, 1996). Deposition during the first rift phase (Rift 1, Late Triassic) occurred within normal fault-bounded half-grabens as indicated by the variations in thickness and depositional facies recorded on seismic and well data (Rasmussen et al., 1998; Ribeiro et al., 1979; Wilson et al., 1989). In the study area, Late Triassic continental deposits (Silves formation) and evaporites (Dagorda formation, latest Triassic – Hettangian) were deposited during Rift 1 (Fig. 2). Rift 2 (Sinemurian – Pliensbachian) was particularly well developed south of the Nazare´ Fault and was associated with marine deposition (Stapel et al., 1996) (Fig. 2). A NW-dipping carbonate ramp (top Coimbra and Brenha formations) developed after Rift 2 in association with regional thermal subsidence (Toarcian – Late Callovian; Azereˆdo, 1998; Rasmussen et al., 1998). Rift 3 (Late Oxfordian) is related to rifting in the Tagus Abyssal Plain (Reis et al., 2000; Wilson et al., 1989) (Fig. 2). Leinfelder and Wilson (1998) and Reis et al. (2000) relate this phase of rifting to differentiation of the Lusitanian Basin into multiple sub-basins, with the Late Jurassic rift-related units recording two distinct depositional stages. The first (Late Early to Late Oxfordian; Reis et al., 2000) encompasses a period of widespread carbonate deposition (Cabo Mondego and Montejunto formations) in lacustrine to deep marine environments. The second stage, Late

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Fig. 1. (a) Map of the Iberian continental margin showing how major transfer faults divide the Lusitanian Basin into three structurally distinct sectors. The study area is also highlighted. (b) Map of the Northern Lusitanian Basin depicting its sub-salt structure, the seismic grid used and the location of offshore wells. (c) Map of the Central Lusitanian Basin depicting the structure of the basin, the sites of onshore wells drilled up to the present date, and the location of the seismic line represented in Fig. 3c and d.

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Fig. 2. Lithostratigraphy and seismic stratigraphy of the Northern Lusitanian Basin. Lithostratigraphic data taken from GPEP (1986) and Witt (1977).

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Oxfordian to Berriasian in age (Leinfelder & Wilson, 1998), was marked by the influx of siliciclastic sediment, represented in the study area by the shallow marine to fluvial Alcobac¸a formation and by the predominantly fluvial Lourinha˜ formation (Fig. 2). The existence in the study area of a synchronous Late Jurassic extensional phase to that of the onshore Central Lusitanian Basin is corroborated on seismic data by growth of the Oxfordian – Kimmeridgian sequences J30 to J40 onto the Moreia Fault (Fig. 3a and b), a similar character to that observed in the fault-bounded Arruda sub-basin (Fig. 3c and d). Salt-prone regions of both Central and Northern Lusitanian Basin are marked at this time by the development of broad salt withdrawal basins above subsident sub-salt hanging-wall blocks (Fig. 4) (Rasmussen et al., 1998). 3.3. Early Cretaceous rifting The youngest extensional event (Rift 4, Fig. 2) recorded in western Iberia occurred west of the Lusitanian Basin in association with Late Berriasian –Early Valanginian rifting in the area that is now the Iberia Abyssal Plain (Fig. 1a) (Manatschal & Bernoulli, 1998; Wilson, Sawyer, Whitmarsh, Zerong, & Carbonell, 1996; Manatschal et al., 2001; Wilson et al., 2001). This was followed by Late Valanginian –Early Aptian post-rift thermal subsidence (Driscoll et al., 1995; Wilson et al., 2001). Rift 4 is marked in the Central Lusitanian Basin by a second (Berriasian– Early Aptian) phase of siliciclastic influx (Torres Vedras formation, Wilson et al., 1989), but north of the Nazare´ Fault the base of the Torres Vedras formation has been attributed to the Late Aptian (Cunha & Reis, 1995; Rey, 1972; Witt, 1977). This probably reflects a phase of relative uplift on the margins of the present-day continental rise (Manatschal & Bernoulli, 1998; Wilson et al., 1996), and suggests that the Late Berriasian –Early Aptian succession drilled on the deeper margin of Portugal (e.g. Wilson et al., 1996; 2001) does not occur in the study area. In parallel with the apparent uplift of the Northern Lusitanian Basin in the Early Cretaceous, Rasmussen et al. (1998) identified an important preAptian phase of salt pillow growth, interpreted to be synchronous with latest Jurassic– Early Cretaceous inversion (Terrinha et al., 2002). From Late Aptian onwards a passive margin was established in northwest Iberia (Driscoll et al., 1995; Manatschal & Bernoulli, 1998), and deposition ceased in most part of the Central and Southern Lusitanian Basin (Fig. 1a) (Rasmussen et al., 1998; Wilson et al., 1989). In contrast, the Northern Lusitanian Basin records important subsidence during the Late Aptian –Maastrichtian (Fig. 2), mirroring the complex subsidence history recorded on the Iberian passive margin after the Early Cretaceous (Malod & Mauffret, 1990; Manatschal & Bernoulli, 1998; Wilson et al., 2001).

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3.4. Cenozoic tectonics and subsequent basin inversion The Cenozoic evolution of western Iberia was controlled by the Pyrenean and Alpine Orogenies, with a relatively minor Eocene compression event being followed by a more pronounced period of Miocene deformation (Pinheiro, Wilson, Reis, Whitmarsh, & Ribeiro, 1996; Ribeiro, Kullberg, Kullberg, Manuppella, & Phipps, 1990; Wilson et al., 1989). On a regional scale, the degree of inversion recorded in the post-Hercynian cover sequences of reactivated Mesozoic basins varies in relation to the presence of thick Late Triassic – Hettangian evaporites (Ribeiro et al., 1990). In the salt-rich areas of the Lusitanian Basin, the Miocene tectonic style is thinskinned with the latest Triassic – Hettangian salt acting as a detachment horizon within the overlying sedimentary cover (Cane´rot, Rey, Baptista, Manuppella, & Peyberne`s, 1995; Pinheiro et al., 1996; Wilson et al., 1989). Contrasting with this setting, salt-poor areas of western Iberia suffered reactivation of deeper-rooted basement faults (Pinheiro et al., 1996).

4. Seismic stratigraphy This paper focuses on three Late Triassic– Late Jurassic megasequences (J1, J2 and J3, Table 1) that form part of a comprehensive megasequence and seismic sequence analysis of the Northern Lusitanian Basin (Fig. 2). 4.1. J1 megasequence The J1 megasequence is bounded above by a high amplitude reflector showing baselap of the overlying reflections and its lower boundary is coincident with a moderate to low amplitude, irregular reflector (top of the Hercynian basement) capping a chaotic substrate (Fig. 4). Internally, the megasequence is characterised by low to moderate amplitude, discontinuous to chaotic reflections. J1 comprises a single seismic sequence (J10, Late Carnian – Hettangian) filling grabens and half-grabens on seismic data (Figs. 4 and 13). This character associates J1 with the Late Triassic Rift 1 and subsequent latest Triassic – Hettangian post-rift (Rasmussen et al., 1998) (Fig. 2). The megasequence is also equivalent to the lower MS1 of Wilson et al. (1989). 4.1.1. Sequence J10, Late Carnian to Hettangian Sequence J10 is subdivided in four seismic facies showing greater thickness north of the Marinha-Grande Fault (Figs. 1b and 2). Major thickness variations also occur within salt structures in which J10 can reach more than 1300 ms TWTT, while salt withdrawal in depocentre areas is revealed by thinning of seismic facies J10.c and J10.d below a folded overburden (Fig. 4). Seismic facies J10.a and J10.b are characterised by medium to high

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amplitude, sub-parallel reflections, occasionally chaotic (Figs. 4 and 5). High amplitude reflections are restricted to seismic facies J10.b. Seismic facies J10.c and J10.d are composed of chaotic to sub-horizontal, moderate amplitude internal reflections of low continuity (Figs. 4 and 5). Sub-horizontal reflections predominate in seismic facies J10.d. Interbedded red shales, evaporites and limestones (seismic facies J10.a), together with intervals of dolomite, anhydrite, shales and limestones (seismic facies J10.b), comprise the lower part of J10 (Silves formation, Fig. 5). They, respectively, indicate deposition in alluvial/fluvial and evaporitic environments. Massive halite, dolomite and anhydritic dolomite intercalated with mudstone/dolomitic levels in seismic facies J10.c, grade into halite/anhydrite intercalated with thick (. 3 m) intervals of dolomite and lime mudstones of seismic facies J10.d (Dagorda formation) (Fig. 5). This latter unit reflects the development of a confined epicontinental sea. On seismic data, chaotic reflections are generated due to the low acoustic impedance, high attenuation and diffractive character of the halite, anhydrite and shale levels. In contrast, high to moderate amplitude reflections are developed by dolomites/limestones of relatively high acoustic impedance. The Triassic – lowest Jurassic continental deposits forming the base of the sequence do not accurately date the onset of rifting in western Iberia. In fact, palynomorph data indicate a Late Carnian age for the base of J10 (Palain, 1976), but older deposits should occur within basinal areas (Table 1). Similarly, the saltprone Dagorda formation has been dated as Hettangian (Ribeiro et al., 1979) but it is considered to encompass the Late Triassic – Early Sinemurian (GPEP, 1986; Rasmussen et al., 1998; Witt, 1977). Evidence of partial synchronism between the Silves and Dagorda formations has also been suggested (Palain, 1976; Rasmussen et al., 1998; Witt, 1977). 4.2. J2 megasequence The J2 megasequence is bounded at its top by a high amplitude reflector characterised by baselap of the overlying reflections. Its lower boundary is coincident with a high amplitude reflector also marked by baselap (Fig. 4). The unit comprises gently folded, high amplitude internal reflections forming a single seismic sequence (J20, Fig. 2 and Table 1). It is equivalent to the upper MS1 of Wilson et al. (1989).

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4.2.1. Sequence J20, Sinemurian to Late Callovian Sequence J20 reaches more than 800 ms TWTT in basinal areas and is composed of three distinct seismic facies (Figs. 4 and 5), the lowermost of which (seismic facies J20.a) comprises 3 – 4 high to very high amplitude, sub-parallel continuous reflections. Seismic facies J20.b includes high amplitude, sub-parallel continuous reflections of moderate frequency. The uppermost seismic facies J20.c contains moderate amplitude, sub-parallel, continuous reflections with low frequency. Shallow to deep marine limestones, dolomites and sandy carbonates alternating with shales comprise the bulk of J20 (Coimbra and Brenha formations, Fig. 5). The high to moderate amplitude, continuous reflections that characterise the sequence are generated by contrasts in the acoustic impedance of shale- and carbonate-rich levels. Seismic facies J20.a has been attributed to the Early Sinemurian in the Northern Lusitanian Basin (Azereˆdo, 1998; Mouterde, Rocha, Ruget, & Tintant, 1979; Ribeiro et al., 1979; Watkinson, 1989). The seismic facies materialises the early syn-rift deposits of Rift 2 (Fig. 2), and it is associated on well data with a phase of increased subsidence in Central and Northern Lusitanian Basin after the Late Triassic – Hettangian post-rift (Stapel et al., 1996). Seismic facies J20.b and J20.c were deposited on a carbonate ramp during the late syn- and post-rift phases (Late Sinemurian –Late Callovian; Azereˆdo, 1998; Rasmussen et al., 1998; Watkinson, 1989). 4.3. J3 megasequence The top of megasequence J3 is coincident with an irregular reflector of moderate amplitude marking a basinwide erosional surface (Fig. 4). A high amplitude reflector showing baselap signs its base. The megasequence comprises thick (up to 1100 ms in TWTT) units developed in fault- and diapir-bounded depocentres (Fig. 4). Three seismic sequences (J30 – J50, Table 1) are identified within J3, itself equivalent to MS2 of Wilson et al. (1989). 4.3.1. Sequence J30, Late Early to Late Oxfordian The upper boundary of sequence J30 is coincident with a reflector of moderate amplitude marked by baselap of the overlying reflections. Its lower boundary is coincident with a high-amplitude, irregular reflector showing baselap (Fig. 4). The sequence is only visible north of the Marinha-Grande Fault and east of the 16A Fault (Fig. 1b),

Fig. 3. (a) Seismic section and (b) interpretation of line S84-08, located 4 km northwest of Figueira da Foz (see Fig. 1b for location). Fault-controlled subsidence in the Northern Lusitanian Basin resulted in growth of the Late Jurassic J30 and J40 towards the Moreia Fault. (c) Seismic section and (d) interpretation of line AR4-80, Arruda half-graben, Central Lusitanian Basin. For location see Fig. 1c. As in (a), fault-controlled subsidence in the Central Lusitanian Basin is signed by growth of the Oxfordian–Early Kimmeridgian pre- and syn-rift sequences J30 and J40 onto the Praganc¸a Fault.

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where it is composed of high amplitude, sub-parallel, continuous internal reflections (Fig. 4). It reaches 300 ms TWTT in thickness west of the Figueira da Foz Fault and more than 800 ms southeast of the same structure (Figs. 1b and 4). Well data show that J30 is composed of marine wackestones, packstones and grainstones, intercalated with lacustrine marls and anhydrites (Cabo Mondego formation, Fig. 5). The high amplitude and continuous reflections within the sequence are related to changes in acoustic impedance between the predominant carbonate beds and the evaporitic/marly intervals. Dinoflagellate data indicate the sequence is a lateral equivalent of the carbonate-rich Cabac¸os and Montejunto formations (Late Early to Late Oxfordian) (Reis et al., 2000). These latter units represent the Late Jurassic rift initiation phase in the Central Lusitanian Basin (Leinfelder & Wilson, 1998).

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high (Fig. 1b), but overlies sequence J30 in the southeastern part of the basin. Internally, J50 comprises low amplitude, discontinuous, irregular reflections (Fig. 4) and varies in thickness between less than 100 and 800 ms TWTT. Shallow marine to fluvial marls, sandstones and sandy limestones similar to those of J40 comprise J50 (Fig. 5). The low amplitude reflections of J50 are due to poor acoustic contrast between the sandstones and the marls. GPEP (1986) and Witt (1977) correlated the deposits of J50 with the Lourinha˜ formation (or gre´s superiores), a fluvial/continental unit attributed to the post-rift phase in the Lusitanian Basin (Early Kimmeridgian –Berriasian).

5. Structural evolution during Jurassic rifting 5.1. Sub-salt seismic sequence

4.3.2. Sequence J40, Late Oxfordian to Late Kimmeridgian The upper boundary of sequence J40 is marked by erosional truncation north of the Marinha-Grande Fault (Fig. 4). The base of the seismic sequence is coincident with a moderate amplitude surface showing baselap. The sequence comprises medium to low amplitude, irregular to divergent reflections of low frequency (Figs. 4 and 5). Southwest-tilting clinoforms are observed within the main Late Jurassic depocentres, where there are marked thickness variations in J40 associated with salt diapirs and basinmargin faults (Fig. 4). The sequence ranges from 1000 ms TWTT north of the Marinha-Grande Fault to less than 100 ms TWTT over the main halokinetic structures and fault-bounded structural highs (Figs. 1b and 4). Sequence J40 is composed of shallow marine to fluvial marls, claystones, minor sandstones and sandy limestone levels (Alcobac¸a formation, Kimmeridgian), grading laterally into alluvial/fluvial sandstones and marls forming the lower part of the Lourinha˜ formation (Kimmeridgian – Berriasian) (Fig. 5). On seismic data, the medium to low amplitude character of the reflections is due to low acoustic impedance contrast between the sandstones and marls. 4.3.3. Sequence J50, Early Kimmeridgian to Berriasian The upper and lower boundaries of J50 are coincident with low to moderate amplitude, irregular reflectors (Fig. 4). The upper boundary is marked by erosional truncation. Sequence J50 overlies J40 between the Marinha-Grande Fault and the Do-1/Mo-1 structural

5.1.1. Sequence J10, Late Triassic –Hettangian Fig. 6 depicts the sub-salt structure of the Northern Lusitanian Basin. The Palaeozoic –Triassic sub-salt units are dissected by two sets of normal faults, the dominant set striking NNE – SSW, roughly parallel to the coastline. These faults are 10 –25 km long and comprise the Western Fault Lineament, the Dourada Fault, the Moreia Fault, the Figueira da Foz Fault, the 16A Fault and the Sa˜o Pedro de Muel Fault (Fig. 6). The 35 km long, NE-dipping MarinhaGrande Fault is the most prominent structure of the secondary NW – SE striking set of faults (Fig. 6). Its strike follows that formed by the 13E-1, 13C-1 and 14A-2 exploration wells, which constrain the depth of the footwall block southwest of sub-basins 1, 3, 4 and 5 (Figs. 4 and 6). The Do-1/Mo-1 structural high and a number of other normal faults commonly less than 10 km long also show a NW – SE trend (Fig. 6). The structure shown in Fig. 6 highlights the presence of a NW – SE trough, 10 km long £ 3– 4 km wide, developed 12 km west of Figueira da Foz. This area comprises the main locus of Late Jurassic extension in the Northern Lusitanian Basin, being bounded to the southwest by a platform where Middle to Late Jurassic deposits are thin or absent. In sub-basins 1 –5, a series of distinctly tilted grabens and half-grabens lie beneath the latest Triassic – Hettangian salt. Sub-basins 1, 3, 4 and 5 comprise southwesterly tilted half-grabens mainly developed in the Late Jurassic due to movement of the Marinha-Grande Fault (Figs. 4 and 6). North of the Do-1/Mo-1 high, sub-basin 2 tilts

Fig. 4. (a) Seismic section and (b) interpretation of line S84-23, acquired 8 km southwest of Figueira da Foz (see Fig. 1b for location). Note the existence of lensoid sequences in the Cenozoic (sequence C20) and Late Jurassic (sequences J40 and J50), synchronously with the main phases of halokinesis in the Northern Lusitanian Basin. Surface propagation of basement faults is hindered by the latest Triassic– Hettangian salt. (c) Seismic section and (d) interpretation of line S84-29. For location see Fig. 1b. As in line S84-23, a major salt pillow formed in sub-basin 5 over the footwall of the MGF. Note the onlap and thinning of the pre-rift sequence J30 onto the SPM diapir, supporting the existence of Oxfordian halokinesis in the study area.

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Table 1 Summary of the principal features of the stratigraphic units in the Northern Lusitanian Basin Sequence

Age of base

TWTT thickness (ms)

Internal character, geometry and terminations

Lithology

C1

C20

Late Oligocene (Chattian?)

200–500

C10

Paleocene–Early Eocene

200–250

Transparent to low-amplitude reflections. Sigmoidal on the shelf-edge. Baselap on its base. Fills rim synclines Moderate- to high-amplitude, sub-parallel, continuous reflections. Baselap on its base

Coarse sand, gravel and clays. Intercalations of mudstone and lignite. Continental to shelf deposits Shallow marine/lacustrine sandy dolomites. Sand and gravel intercalated with red clays of an alluvial/fluvial realm

K3

K40

Late Campanian

120–150

Moderate- to high-amplitude irregular reflections

Shallow marine to lacustrine dolomitic sandsome, dolomite and sandy limestone

K2

K30

Turonian

200–330

K20

Late Aptian

200–450

Moderate- to high-amplitude, low-frequency reflections. Three seismic facies. Downlap on its base Low-amplitude, low-continuity irregular reflection. Toplap of underlying reflections on its base. Truncates J40

Fine to coarse sandstones, limestones, shale and sand of marine (limestones) and fluvial/lacustrine origin Coarse sandstones, conglomerates, red claystones and marls of a fluvial/alluvial realm

K1

K10

Not identified in the NLB

J3

J50

Early Kimmeridgian Late Oxfordian

Low-amplitude, discontinuous, irregular reflection. Baselap on its base Moderate- to low-amplitude, irregular, sub-parallel to divergent reflections. Baselap on its base High-amplitude, sub-parallel reflections with occasional baselap on its base

Red marls, sandstones and sandy limestones of a fluvial/lacustrine realm Red marls, claystones and sandy limestones. Shallow marine/lacustrine to fluvial environments Limestones (wacke-, pack- and grainstones) with micaceous marls/anhydrite. Shallow marine to lacustrine realm

J40

0–800 0–1000

J30

Late Early Oxfordian

0–300

J2

J20

Sinemurian

100–800

Moderate- to very high-amplitude reflections. Onlap occasionally seen on its base. Otherwise concordant

Marine limestones, dolomites and sandy carbonates alternate with hemipelagic and turbiditic shales

J1

J10

Late Triassic (Carnian?)

200–1300

Moderate- to high-amplitude, low-continuity, reflections. Baselap on its base

Red shales, evaporites and limestones of a continental to restricted marine realm

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Megasequence

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Fig. 5. Correlation panel between the main lithostratigraphic units on well data and the interpreted seismic packages. See Figs. 4 and 6 for detailed location of wells and seismic sections.

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Fig. 6. Structural map of Near Top Late Triassic (facies J10.b) for the Northern Lusitanian Basin. Bold lines represent the main normal faults active during the Jurassic extensional phases.

eastwards in association with movement on the Moreia and Figueira da Foz faults (Fig. 3a and b). Despite poor seismic resolution of the sub-salt units it is possible to identify a main evaporite depocentre east of the Dourada Fault within which J10 is thick (. 500 ms TWTT) (Figs. 7a and 8a). This contrasts with the area west of the latter fault, where the evaporites (seismic facies J10.c/10.d)

are less than 100 ms TWTT. This area (the Nazare´ plateau, Fig. 8a) forms the northward extension of the Berlengas Horst (Fig. 1a), a marginal basement high that marked the western boundary of the Central Lusitanian Basin during the Jurassic rift phases (Leinfelder & Wilson, 1998). Fault-controlled subsidence during the Late Triassic –

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Hettangian Rift 1 accounts for the localised development of seismic facies J10.c/10.d east of the Dourada Fault, as shown by the lateral seismic facies and thickness variations in sequence J10. For instance, Fig. 9 shows that relatively thin evaporites (100 – 150 ms TWTT) were deposited over the Mo-1 structural high, in clear contrast with sub-basin 2 where the thickness of salt-prone seismic facies J10.c/10.d are at least 400 – 500 ms TWTT thick. This suggests that the Do-1/Mo-1 high represented a fault-bounded basinal high in the latest Triassic – earliest Jurassic. Similar half-graben/ graben fills are identified in sub-basins 1, 3 and 5 (Figs. 4 and 13). 5.2. Post-salt seismic sequences 5.2.1. Sequence J20, Sinemurian – Late Callovian The Sinemurian – Late Callovian evolution of the Northern Lusitanian Basin is marked by moderate halokinesis, particularly in the areas of relatively thick evaporites. These controlled the geometry of the postsalt sequences in association with basement extension (Fig. 10). In contrast, away from salt-prone areas, faultrelated subsidence was the main factor controlling the location of the Sinemurian – Late Callovian depocentres (Fig. 3a and b). Lines S84-08 (Fig. 3a and b) and S84-16 (Fig. 11), together with the isochron data in Fig. 7b, show the Western Fault Lineament and the Moreia Fault to constitute major boundaries to a NE – SW trending Sinemurian – Late Callovian depocentre. These data agree with onshore evidence for fault movement (indicated by the deposition of fine-grained carbonaterich turbidites), and with the suggested location of a Ntrending horst bounding the Northern Lusitanian Basin to the west in the Early – Middle Jurassic (Azereˆdo, 1998; Watkinson, 1989). We consider the Western Fault Lineament to bound a by-pass margin along the western side of the Sinemurian – Late Callovian depocentre, as indicated by Azereˆdo (1998) and Watkinson (1989). In addition, the eastern margin of the Sinemurian – Late Callovian depocentre is bounded by the Moreia Fault, across which there is pronounced thinning of J20 onto the Aveiro Horst to the east (Fig. 8b). In the areas of thin latest Triassic – Hettangian salt (i.e. northeast of the Do-1/Mo-1 high and southwest of the Marinha-Grande Fault), sequence J20 was deposited in fault-bounded depocentres geometrically similar to the sub-salt grabens/half-grabens observed on seismic data (Fig. 3a and b). Elsewhere in the basin, faultcontrolled extension was replaced by combined salt-/ fault-controlled (thin- and thick-skinned) subsidence, particularly south and west of Figueira da Foz and east of the Western Fault Lineament (Figs. 7b and 11). Nevertheless, halokinesis was relatively moderate in this latter region prior to the Late Jurassic, as shown by the

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minor growth observed within J20 on the flanks of salt structures (Fig. 10). 5.2.2. Sequence J30, Late Jurassic pre-rift phase The Northern Lusitanian Basin contains up to 2700 m (1100 ms TWTT, Fig. 12a) of Upper Jurassic sediments, which infill the inherited rift- and growth-related topography developed during Rift 3 (Fig. 7c and d). Isochron data from sequences J30 to J50 indicate that the filling of the Late Jurassic sub-basins 1 – 5 was not synchronous and varied in thickness within the study area (Fig. 12). In addition, the seismic data show that as early as the Oxfordian, salt-controlled subsidence influenced the thickness and lateral distribution of the Late Jurassic sequences. Late Early – Middle Oxfordian (pre-rift) subsidence was centred on sub-basin 5, where J30 is up to 700 ms TWTT (, 1750 m) thick (Fig. 12b). Onlap of the internal reflections in sequence J30 onto the 14A and SPM salt pillows demonstrates that these were growing during the Oxfordian (Figs. 4c,d and 10). A marked change in halokinetic activity between the Sinemurian – Late Callovian and the Late Jurassic pre-rift is also inferred in sub-basin 5 (Fig. 10). Thickening of J20 towards the northeast suggests that transverse salt withdrawal towards the southwest occurred in subbasin 5 prior to the Late Jurassic, probably accompanying along-strike flow of salt. In contrast, the Late Jurassic J30 thickens southwestwards across the 14A structure, denoting salt withdrawal from sub-basin 5 towards the northeast (Fig. 10). Salt-related subsidence was accompanied by moderate growth of salt pillows, since erosion and folding of the post-Jurassic sequences over the 14 structure reveals that most of its present day geometry is due to Cenozoic compression rather than a result of Late Jurassic halokinesis (Fig. 10). Halokinesis was apparently rare elsewhere in the study area during the deposition of J30, with the Figueira da Foz Fault separating the subsiding sub-basin 5 from sub-basins 1 – 4 (Fig. 12b). In these latter subbasins, sequence J30 is thinner than 300 ms TWTT (, 750 m). 5.2.3. Sequence J40, Late Jurassic syn-rift phase Isochron data for sequence J40 are depicted in Fig. 12c. This unit shows maximum development within sub-basins 3 and 4, 10 km west of Figueira da Foz, where it forms a NW – SE trending 8 km long £ 5 km wide depocentre filled with more than 2000 m (800 ms TWTT) of sediments (Figs. 4a,b and 12c). The activity of basement faults during the Late Jurassic syn-rift phase is marked in sub-basin 2 by thickening of J40 into the basin-bounding faults (Fig. 3a and b). In salt-rich areas of the Northern Lusitanian Basin, basement fault-controlled subsidence resulted in growth of salt pillows over the basin-margin structural

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Fig. 8. Tectono-stratigraphic evolution of the Northern Lusitanian Basin during the (a) Late Triassic–Hettangian and, (b) in the Sinemurian–Late Callovian.

highs. This is demonstrated by the pronounced thinning of sequence J40 over the main diapir highs and the contemporaneous thickening of strata towards the centre of bowl-shaped salt withdrawal depocentres (Figs. 4 and 12c). In parallel with salt pillow growth, the hard basement – cover linkage of fault systems was suppressed in salt-prone areas during the Late Jurassic syn-rift, leading to a different basin form during the deposition of J40. Salt-prone areas present therefore a central axis of subsidence distant from the sub-salt basin-margin faults (Fig. 13). Fig. 14 shows one example of Late Jurassic halokinesis in the Northern Lusitanian Basin. No major overburden faults can be identified within sub-basins 3 and 4. In the

latter sub-basins, three distinct seismic units (A, B and C) can be distinguished within J40, based on their internal character and reflector terminations. Unit A, the oldest, is only visible southeast of the S20 salt pillow, indicating that subsidence was restricted to sub-basin 4 at the start of the syn-rift phase. Onlap of internal reflections of unit A onto the S20 salt pillow suggests growth of this structure accompanied by subsidence of sub-basin 4, probably due to transverse flow of salt towards northwest. Unit B is widespread in sub-basins 3 and 4, most likely due to movement of the Marinha-Grande Fault (Figs. 6 and 14). The relative thinning of unit B over the S20 structure denotes salt pillow growth. Similarly, thinning of unit B

Fig. 7. Two-way travel-time (TWTT) structural and isochron maps for the Northern Lusitanian Basin. (a) Location of salt structures in the study area. (b) Isochron map for sequence J20. Note that the main depocentre of J20 is north–northeast-striking, roughly extending from the 13E-1 to the Ca-1 wells. Other depocentres are identified 2 km north of the 14A-1 well and 3 km west of S.P. Muel. (c) TWTT structural map of the Top Middle Jurassic (sequence J20). The figure highlights the location of the main Late Jurassic depocentres in the Northern Lusitanian Basin. They are roughly located within a 25 km long £ 7 km wide, northwest-striking depocentre. (d) TWTT structural map of the Base Cretaceous (sequence K20). The infilling of the Late Jurassic depocentres is apparent on isochron data, with the Northern Lusitanian Basin forming an extensive low-relief basin with relatively little topographic expression.

742 T.M. Alves et al. / Marine and Petroleum Geology 19 (2002) 727–754 Fig. 9. (a) Seismic section and (b) interpretation of line S84-102, located 4 km west of Figueira da Foz next to the Do-1/Mo-1 structural high. For location see Figs. 1b and 6. Halokinetic structures are absent in sub-basin 2, with fault-controlled subsidence prevailing over salt pillow growth. Next to the Mo-1 well, growth and lateral changes of seismic facies (LCSF) in sequence J10, facies J20.a and facies J20.b are due to fault-controlled movement of the Do-1/Mo-1 structure.

T.M. Alves et al. / Marine and Petroleum Geology 19 (2002) 727–754 Fig. 10. (a) Seismic section and (b) interpretation of line S84-27 (see Figs. 1b and 6 for location). Note the relative thickening of sequence J30 southwest of the 14A-1 well contrasting with the apparent growth of sequence J20 northeast of this same well. Halokinetic structures are particularly developed between Figueira da Foz and the Nazare´ fault. These have asymmetric profiles due to (Miocene –Present) compression.

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744 T.M. Alves et al. / Marine and Petroleum Geology 19 (2002) 727–754 Fig. 11. (a) Seismic section and (b) interpretation of line S84-16, intersecting sub-basins 1 and 3 (see Figs. 1b and 6 for location). The Western Fault Lineament and the Do-1/Mo-1 high are visible in this seismic line. Gentle folding of the post-salt overburden, a character inherited from Miocene–Present compression, is also highlighted. As during the Jurassic, the salt-prone sequence J10 constituted a detachment horizon during the Cenozoic compression, separating the gently deformed cover sequences from the underlying fault-bounded basement.

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onto the northwest indicates growth of the 13 salt structure due to transverse flow of salt. Salt withdrawal subsidence was reduced by the time unit C deposited, as it shows only minor thickness variations and no onlap terminations of the internal reflections onto the 13 and S20 salt pillows (Fig. 14). Unit C is thicker within sub-basin 3 (200 ms TWTT against , 150 ms TWTT in sub-basin 4) suggesting that a minor volume of salt was flowing from this area at the end of the syn-rift phase, probably both along-strike and towards the salt pillow 13. At the same time, salt withdrawal ceased southeast of the S20 structure. 5.2.4. Sequence J50, Late Jurassic post-rift phase Sequence J50 fills salt withdrawal basins (Fig. 10), but in comparison with J40 relatively minor growth of strata is observed (Fig. 11). In addition, the isochron data in Fig. 12d demonstrate the development of multiple depocentres east of the Western Fault Lineament during the post-rift phase. Sequence J50 thins away from salt withdrawal depocentres and shows gentle folding associated with salt pillow development (Figs. 10 and 13). However, since erosional truncation of the reflections in J50 by the overlying sequence K20 is common, but growth strata are poorly developed, halokinesis was most likely minor during the deposition of J50 (Fig. 14). In contrast with the Late Jurassic syn-rift phase, the majority of the post-rift sub-basins (Fig. 12d) did not form discrete areas of active, local subsidence, but are probably part of a wider sag area gently folded during the latest Jurassic– Early Cretaceous (Figs. 11 and 13). This latter event was followed in the Early Cretaceous by widespread erosion and truncation of diapirrelated highs in association with regional, transient inversion (e.g. Terrinha et al., 2002).

6. Discussion 6.1. Relation between basement extension and halokinesis The seismic – stratigraphic and structural interpretations presented here provide additional information on the evolution of salt-prone sedimentary basins on Atlantictype margins. In particular, four main factors responsible for the present-day structure of the Northern Lusitanian Basin can be identified. † Similarly to other salt-prone regions where the initiation of salt tectonic episodes is linked to major basement/ regional tectonic events (Nielsen, Vendeville, & Johansen, 1995; Stewart & Clark, 1999; Vendeville & Jackson, 1991), and to onshore areas of the Central Lusitanian Basin (Leinfelder & Wilson, 1998; Reis et al., 2000; Wilson et al., 1989), Jurassic extension was the main trigger of halokinesis in the study area.

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† Growing salt pillows developed over major basin-margin structural highs during the deposition of the J2 and J3 megasequences. † Basement-related faulting was accommodated by the latest Triassic – Hettangian salt in the areas where this was thick enough to cause decoupling between the overburden rocks and the sub-salt basement. † In salt-poor areas of the Northern Lusitanian Basin, half-graben/graben structures formed over the latest Triassic– Hettangian units during active rifting. These suffered minor reactivation during the Cenozoic in response to the absence of a salt-rich detachment layer in depth. This study indicates that the last two of these factors were crucial to the Jurassic evolution of the Northern Lusitanian Basin, controlling the structural style, geometry and the subsidence history of the basin. The two end-member structural styles observed in the Northern Lusitanian Basin show similarities with the physical models of Withjack and Callaway (2000) (Fig. 15). Their experimental models show that decoupling between the deep and shallow deformation is enhanced by increasing the thickness of the viscous layer and the ductility of the overburden, a phenomenon identical to that identified in the Northern Lusitanian Basin. Models 3, 4 and 12 of Withjack and Callaway (2000) replicate the three main structural styles developed in the Northern Lusitanian Basin during the Jurassic (Fig. 15). Models with a brittle overburden and without a viscous putty layer underneath the cover sequences deform in a similar way to that observed in the saltpoor sub-basin 2 (Model 3 and seismic line S84-08, Fig. 15a). In the experimental models, no monocline developed in the cover sequence and a graben formed above the master normal fault (Withjack & Callaway, 2000). This propagated directly into the overburden, forming one of the major bounding faults of the developed graben, a phenomenon also observed on seismic data (Fig. 15a). Decoupling between the cover sequences and the basement is not observed in sub-basin 2 and direct propagation of the basement master normal faults (Moreia and Figueira da Foz faults) into the Jurassic cover sequences occurred (Fig. 15a). By introducing a 2-cm thick layer of viscous silicone putty over a master normal fault suffering moderate displacement (1.41 cm), Model 4 simulated the existence of a thick salt layer below a brittle cover sequence with the same physical properties to that of Model 3 (Fig. 15b) (Withjack & Callaway, 2000). In Model 4, a broad monocline formed in the cover sequence, but no faulting was observed above the master normal fault. Instead, a detached graben at the edge of the model formed near the hanging-wall edge of the putty layer due to the model boundary conditions, i.e. close to the edge of the viscous putty horizon (Withjack & Callaway, 2000). The structure

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observed in this model is similar to that recorded in areas of the Northern Lusitanian Basin where the displacement of basement-related normal faults is apparently moderate, or where the overburden rigidity was enough to prevent the development of broad extensional folds (Fig. 15b). That is the case of the Western Fault Lineament, of the Do-1/Mo-1 high, and of the area east of the 16AF (Fig. 6). The structural style of sub-basins 3, 4 and 5, particularly during the Late Jurassic syn-rift phase, is similar to that developed in Model 12 of Withjack and Callaway (2000) (Fig. 15c). In this model, the 1-cm thick silicone putty was chosen to represent a salt layer with moderate thickness, underlying a ductile, cohesive overburden. The displacement of the master normal fault below the silicone putty was increased from 1.41 cm in Model 3 and 4 – 5.66 cm in Model 12. During the experiment, the geometry of the overburden showed that the relative displacement of the sub-salt normal fault, together with the ductility and cohesive strength of the overburden, controlled the style and distribution of the cover sequence (Withjack & Callaway, 2000). Moreover, the few, minor normal faults that cut the cover sequence near the hinges of the monocline in Model 12 accompanied basement – cover decoupling (Fig. 15c). This is in agreement with the seismic data crossing the Marinha-Grande Fault, illustrating a broad monocline over the Sinemurian – Late Callovian sequence J20 (geometrically similar to that in Model 12) in sub-basins 3, 4 and 5 (Figs. 4 and 15c). In addition, the models of Withjack and Callaway (2000) allow us to consider that the post-salt overburden (sequence J20) was relatively ductile and cohesive during the main phases of Jurassic halokinesis, possibly reflecting presence of clay-rich deposits in sequence J20. The abundant lime mudstones and shales drilled in the latter sequence (Fig. 6) apparently confirm this interpretation. The relatively limited faulting of the cover sequences observed on the seismic sections, as well as corroborating the models of Withjack and Callaway (2000), also replicates the structural setting of the North Sea salt basins. Here, as in the Northern Lusitanian Basin, hard basement – cover normal fault linkage was controlled by the relative thickness and viscosity of the underlying Zechstein salt layers, and by the location of growing salt structures in relation to the underlying master fault (Davison et al., 2000; Stewart & Clark, 1999; Stewart, Harvey, Otto, & Weston, 1996).

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Regions of the West Central Graben where normal fault systems were reactivated below thick Zechstein evaporites also show high amplitude folding of the Meso-Cenozoic overburden with very limited basement – surface fault propagation (Errat, 1993; Stewart & Clark, 1999; Stewart & Coward, 1995; Stewart et al., 1996; Williams, 1993). Differences between the North Sea and the Northern Lusitanian Basin occur, however, because Jurassic halokinesis in the latter was moderate compared with the former. As in the Central Lusitanian Basin, Jurassic halokinesis above the basin-margin footwalls of the Northern Lusitanian Basin was restricted to salt pillow growth, generating areas of positive topographic relief. In contrast, the fault-bounded sub-basins southwest of the Marinha-Grande Fault and northeast of the Do-1/Mo-1 high record hard basement –cover linkage of fault systems (Fig. 3a and b), reproducing the structural style of halfgraben basins of the Northern North Sea (i.e. North Viking Graben, Inner Moray Firth, Murchinson – Statfjord Fault Zone; Stewart & Clark, 1999; Young, Gawthorpe, & Hardy, 2001). 6.2. Controls on sedimentation during the Late Jurassic The Late Jurassic seismic – stratigraphic record of the Northern Lusitanian Basin illustrates the role of salt tectonics on the depositional evolution of a rift basin. As in the Central Lusitanian Basin (Ellis, Wilson, & Leinfelder, 1990; Wilson et al., 1989), Late Jurassic extension of the basement triggered the development of salt-related structural highs in the Northern Lusitanian Basin, forming barriers to the progradation of sediment from marginal areas. At the same time, accommodation space was enhanced within bowl-shaped diapir-bounded basins in association with fault-controlled subsidence of the basement and with salt withdrawal due to sediment loading. The generation of accommodation space within diapirbounded depocentres was most likely a complex process dependent both on the amount of extension of the pre-salt basement and on the load of the Late Jurassic sediment sequences. The Late Jurassic syn-rift and post-rift sequences (Rift 3) in the Northern Lusitanian Basin (Fig. 16a) comprise SW-prograding fluvio-deltaic units (Bernardes, 1992; Bernardes et al., 1998), imaged on seismic data by SW-dipping clinoforms (sequence J40, Fig. 4a and b). Well, isopach and seismic data indicate that hanging-wall

Fig. 12. Isochron maps for the Late Jurassic seismic units in the Northern Lusitanian Basin. (a) Isochron map of the Late Jurassic J3 megasequence. Up to 1100 ms TWTT of Late Jurassic sediments are deposited in sub-basin 3, the main locus of subsidence in the Northern Lusitanian Basin. (b) Isochron map of the pre-rift sequence J30. Sub-basin 5 constituted a main diapir-bounded depocentre during the pre-rift phase. The 800 ms TWTT of sediments identified here contrast with the less than 400 ms TWTT observed elsewhere in the study area. (c) Isochron map of the syn-rift sequence J40. More than 800 ms TWTT accumulated in sub-basin 3 during the Late Jurassic syn-rift phase. A main northwest-striking depocentre formed between the 13E-1 and 14A-1 wells in relation to the growth of the 13 salt pillow and to fault-controlled subsidence of the Marinha-Grande Fault. (d) Isochron map of the post-rift sequence J50. Subsidence was more widespread during post-rift when compared with the previous sequences. Main depocentres are seen in sub-basins 3 and 4.

748 T.M. Alves et al. / Marine and Petroleum Geology 19 (2002) 727–754 Fig. 13. (a) Seismic section and (b) interpretation of line S84-23, intersecting sub-basins 2 and 3. For location see Figs. 1b and 6. Note the relatively thin Late Jurassic sequences in the two sub-basins. In contrast, the Early Middle Jurassic facies J20.a to J20.c are well developed. Also note the marked truncation of the Jurassic units by the Cretaceous sequence K20.

T.M. Alves et al. / Marine and Petroleum Geology 19 (2002) 727–754 Fig. 14. (a) Seismic section and (b) interpretation of line S84-20. For location see Figs. 1b and 6. The seismic line illustrates the importance of salt withdrawal in controlling Late Jurassic deposition. Cenozoic compression resulted in reactivation of the 13 salt pillow as a thrust front and in the formation of a gentle anticline over the S20 salt structure. Over this, the existence of thick overburden units (more than 1800 ms TWTT) limited the formation of a similar thrust to that adjacent to well 13E-1.

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drainage systems derived from the north and northwest prograded into the basin during the Oxfordian – Kimmeridgian, flowing southwards into the subsiding sub-basins 1 – 5 (Fig. 16a). Similar drainage systems probably fed southward-flowing axial drainage systems during the postrift phase (Fig. 16c). This interpretation is in agreement with the Late Jurassic facies distribution in the Central Lusitanian Basin, where gradual filling occurred after the latest Oxfordian (rift climax) due to progradation of axial drainage systems from the north (Leinfelder & Wilson, 1998; Wilson et al., 1989). In contrast with the faultbounded Turcifal and Arruda sub-basins of the Central Lusitanian Basin (Fig. 1c), footwall-derived sediment (i.e. sourced from the Western Basin-Margin Horst) is apparently scarce in main depocentre areas of the Northern Lusitanian Basin as no easterly pro-grading clinoforms are identified on seismic data (Figs. 4 and 16). As in similar saltdominated extensional margins (Alexander & Flemings, 1995; Lawton, Vega, Giles, & Domı´nguez, 2002; Morton & Suter, 1996; Rowan, Hart, Nelson, Flemings, & Trudgill, 1998; Rowan & Weimer, 1998; Smith, Hodgson, & Fulton, 1993; Weimer et al., 1998), fluvial channels flowed along structurally low areas between salt-related highs, whilst the 13, 14A. Mo, S20, Leirosa and SPM salt structures (Fig. 7a) constituted topographic barriers to the southwest progradation of fluvial/deltaic systems in sub-basins 4– 7 (Fig. 16c). Although halokinetic structures generally occur over basinmargin structures in the Lusitanian Basin (Rasmussen et al., 1998; Wilson et al., 1989; Zbyszewski, 1959), evidence of similar control of growing salt pillows on the thickness and distribution of the Late Jurassic deposits is also recorded in the depocentre areas of sub-basins 3 – 5, probably in association with sub-salt structures (e.g. S20 salt pillow, Fig. 14). In contrast with the Central Lusitanian Basin (Ellis et al., 1990), there is no evidence on seismic sections or from outcrop data of developed post-Oxfordian salt pillowrelated carbonate build-ups in the study area.

7. Conclusions During the Jurassic, NW- and NNE-trending basement faults, together with their associated salt structures, divided the Northern Lusitanian Basin in distinct sub-basins separated by basin highs. Growth of salt pillows was prominent during the Late Jurassic and was contemporaneous with salt withdrawal from basinal areas that created

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four bowl-shaped depocentres between the Marinha-Grande Fault and Figueira da Foz. Salt movement accommodated most of the Jurassic basement extension imposing in the Northern Lusitanian Basin a mixed thick- and thin-skinned structural style. † In salt-prone areas, sub-salt basement faults did not propagate into the post-salt overburden during extension. † Basement faults propagated into the post-salt overburden in regions where salt thickness was insufficient to form a deep detachment layer to accommodate extension. The data from the Northern Lusitanian Basin suggest that the structural response of the cover to basement faulting is strongly controlled by the relative thickness of sub-cover evaporites (Stewart & Clark, 1999), and is similar to structural studies of extensional-forced folding (Withjack & Callaway, 2000). The seismic and well data indicate that the thickness and depositional facies variations in the Late Jurassic pre-rift units were already controlled by halokinesis. The salt withdrawal sub-basin 5 was filled with marine carbonates at this time, but lacustrine deposits predominate elsewhere. Siliciclastic units were deposited by southwest-prograding hanging-wall and southeast-prograding axial drainage systems during the syn- and post-rift phases. Over basinmargin footwall blocks, growing salt pillows constituted topographic barriers to the prograding deltaic/fluvial deposits. This work shows the effect of salt tectonics on the tectono-stratigraphic evolution of rift-related basins. The structural style and sedimentation processes are controlled in these basins by the spatial distribution and thickness of overlying evaporite units.

Acknowledgments The authors would like to acknowledge the permission conceded by the IGM (Instituto Geolo´gico e Mineiro) and NPEP (Nu´cleo para a Pesquisa e Prospecc¸a˜o de Petro´leo) for the use of the data included in this paper. We thank R.C.L. Wilson and an anonymous reviewer for the meticulous comments that much improved the early versions of this paper. We also thank the prolific discussions held with the colleagues G. Manuppella, C. Moita, and J. Pacheco. This work is part of the project Praxis XXI (BD 13903/97) of the

Fig. 15. Diagram illustrating the main salt-controlled structural styles in the Northern Lusitanian Basin, formed in association to Jurassic rifting. The physical models of extensional forced folds published in Withjack and Callaway (2000) are compared with key seismic lines from the study area. (a) Brittle overburden overlying thin or no salt suffers direct propagation of sub-salt faults during basement extension. (b) Faulting in the cover sequences is absent when a thick salt layer separates the latter from a moderately subsident fault-bounded basement. (c) When overlain by a ductile layer of salt, basement normal faults showing high displacement during extension form subsident monoclines above cohesive cover sequences. Basement–overburden decoupling is enhanced when both the thickness and ductility of the salt layer, and the cohesion/ductility of the overburden are increased.

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Fig. 16. Tectono-stratigraphic evolution of the Northern Lusitanian Basin during the Late Jurassic (a) pre-rift, (b) syn-rift and, (c) post-rift phases.

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Fundac¸a˜o para a Cieˆncia e Tecnologia (FCT/MCT), Programa Cieˆncia e Tecnologia.

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