CRETACEOUS SEDIMENTARY AND TECTONIC EVOLUTION OF THE NORTHERN SECTOR OF THE LUSITANIAN BASIN (PORTUGAL)

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 the long 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 Nazare-Lousa 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 Nazare-Lousa 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.