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Lusitanian Basin Evolution and Petroleum Systems Analysis Margins Exploration Group

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Lusitanian Basin Evolution and Petroleum Systems Analysis Margins Exploration Group

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Article
The Mesozoic and Cenozoic sedimentary record of the western Portuguese passive margin is about 5 km thick. In the Mesozoic deposits, several unconformity-bounded stratigraphic units (UBS) are defined: UBS1 Triassic-Callovian; UBS2 middle Oxfordian-Berriasian; UBS3 Valanginian-lower Aptian; UBS4 upper Aptian-lower Campanian and UBS5 upper Campanian-Maastrichtian. Onshore, the Cretaceous northern sector of the Lusitanian Basin is up to 500 m thick, and developed mainly in terrestrial siliciclastic facies. Three stages are recognised in its tectonic and sedimentary evolution. (1) During the late Berriasian to early Aptian, marine and terrestrial sedimentation was restricted to central and southern sectors of the basin. (2) The late Aptian-early Campanian stage began with tectonic activity corresponding to the uplift of the western and eastern borders of the basin, along with an important enlargement the area of the sedimentation. Siliciclastic facies derived from the intense erosion of the Hesperic Massif are interpreted as coalescent wet alluvial fans passing southwestwards into marginal marine sediments and a carbonate platform. Local drainage to the east is related to the uplift of the Berlenga Horst during the late Aptian. An important fall in sea-level followed to the term Albian-Cenomanian transgression resulting in progradation and, later, strong incision of the depositional systems. The top of the siliciclastic record of this stage (UBS4) is marked by an important silcrete, indicating weathering during a long period of non-deposition (Santonian ?—early Campanian). (3) The late Campanian-Maastrichtian period is marked by the main Mesozoic reactivation of the Nazaré-Lousã Fault, significant volcanic activity (emplacement of subvolcanic complexes, basaltic extrusives and associated dykes) and diapirism. Onshore, UBS5 consists of yellowish quartzarenites and red mudstones, interpreted as a meandering fluvial system draining to the northwest, changing distally to transitional and marine environments. The fluvial sediments are interpreted as second cycle, with the main provenance being previous Cretaceous sediments (UBS4) whose erosion can be related to uplift of the southern block of the Nazaré-Lousã fault. Correlative diapiric reactivations built up coalescent peridiapiric alluvial fans, transverse to N-S diapiric ruptures, that consist of calcareous conglomerates and red mudstones. Offshore, very shallow marine fine clastics and dolostones are the main facies.
Article
The study provides a regional seismic interpretation and mapping of the Mesozoic and Cenozoic succession of the Lusitanian Basin and the shelf and slope area off Portugal. The seismic study is compared with previous studies of the Lusitanian Basin. From the Late Triassic to the Cretaceous the study area experienced four rift phases and intermittent periods of tectonic quiescence. The Triassic rifting was concentrated in the central part of the Lusitanian Basin and in the southernmost part of the study area, both as symmetrical grabens and half-grabens. The evolution of half-grabens was particularly prominent in the south. The Triassic fault-controlled subsidence ceased during the latest Late Triassic and was succeeded by regional subsidence during the early Early Jurassic (Hettangian) when deposition of evaporites took place. A second rift phase was initiated in the Early Jurassic, most likely during the Sinemurian–Pliensbachian. This resulted in minor salt movements along the most prominent faults. The second phase was concentrated to the area south of the Nazare Fault Zone and resulted here in the accumulation of a thick Sinemurian–Callovian succession. Following a major hiatus, probably as a result of the opening of the Central Atlantic, resumed deposition occurred during the Late Jurassic. Evidence for Late Jurassic fault-controlled subsidence is widespread over the whole basin. The pattern of Late Jurassic subsidence appears to change across the Nazare Fault Zone. North of the Nazare Fault, fault-controlled subsidence occurred mainly along NNW–SSE-trending faults and to the south of this fault zone a NNE–SSW fault pattern seems to dominate. The Oxfordian rift phase is testified in onlapping of the Oxfordian succession on salt pillows which formed in association with fault activity. The fourth and final rift phase was in the latest Late Jurassic or earliest Early Cretaceous. The Jurassic extensional tectonism resulted in triggering of salt movement and the development of salt structures along fault zones. However, only salt pillow development can be demonstrated. The extensional tectonics ceased during the Early Cretaceous. During most of the Cretaceous, regional subsidence occurred, resulting in the deposition of a uniform Lower and Upper Cretaceous succession. Marked inversion of former normal faults, particularly along NE–SW-trending faults, and development of salt diapirs occurred during the Middle Miocene, probably followed by tectonic pulses during the Late Miocene to present. The inversion was most prominent in the central and southern parts of the study area. In between these two areas affected by structural inversion, fault-controlled subsidence resulted in the formation of the Cenozoic Lower Tagus Basin. Northwest of the Nazare Fault Zone the effect of the compressional tectonic regime quickly dies out and extensional tectonic environment seems to have prevailed. The Miocene compressional stress was mainly oriented NW–SE shifting to more N–S in the southern part.
Article
In the Lusitanian Basin, which is located along the western Iberian margin, rapid subsidence in the Oxfordian-Tithonian resulted from extension related to the opening of the North Atlantic. In the Santa Cruz area, located in the western part of the Lusitanian Basin, the Kimmeridgian syn-rift, marine Abadia Formation and the overlying alluvial Kimmeridgian-Tithonian Praia da Amoreira member of the Lourinhã Formation, were deposited in the middle part of the hangingwall dip slope of a larger half-graben that constitutes the Turcifal subbasin. The marine part of the succession consists of base-of-slope and lower slope mudstones and sandy turbidites, middle slope mudstones, upper slope to shelfal/?shoreline mudstones and sandstones, and fan-deltaic deposits. Two intermediate-scale fining-to-coarsening (FUCU) units are present in the marine strata. The lower FUCU unit represents a normal, base-of-slope/lower slope to upper slope, shoaling-upward succession. The unit reflects the progradation of a hangingwall shelfal ramp and infilling of an inherited rift bathymetry during a period of relative tectonic quiescence. The upper FUCU unit represents an intermediate-scale deepening-shoaling-upward or backstepping-forestepping sequence which is related to a single rift event; the backstepping segment was deposited during a period of increasing basinal subsidence and basin floor tilt rates, the early syn-rotational stage. It consists of submarine channel-fill incised into slumped upper slope sediments, and adjacent overbank deposits. The backstepping segment terminates with a clay-prone marine interval with abundant soft-sediment deformation features, which represent a period of maximum sea-level stand during the rotational climax. The overlying forestepping segment consists of a shoaling-upward, slope to fan-deltaic succession which terminates with alluvial deposits. It represents an interval with decreasing basinal subsidence and basin floor tilt rates, and renewed overbalance of sediment supply, which led to a regression during the late syn-rotational stage. In medial reaches of the hangingwall dip slope, periods of increasing-to-decreasing rift activity are recorded by a backstepping-forestepping package, or a threefold sandstone-mudstone-sandstone lithosome motif. The formation of smaller-scale units superimposed on the intermediate-scale FUCU syn-rotational motif were controlled predominantly by minor fault-controlled subsidence events or higher-frequency faulting events.