A reconnaissance study of Upper Jurassic sediments of the Lusitanian Basin

Abstract

Key-words: Upper Jurassic - Carbonate facies - Terrigenous clastic fades - Structural control of sedimentation - Rifting phase of continental separation The Upper Jurassic evolution of the Lusitanian Basin is shown to be linked to the rifting phase which preceded the separation of Iberia and the Grand Banks. Structural controls on sedimentation include both NNE-SSW trending faults in the Hercynian basement. and contemporaneous movement of salt diapirs. At the beginning of Upper Oxfordian times, the entire basin had been levelled to within a few metres of sea level. so that the freshwater algal marsh and marginal marine facies of the Cabaços and Vale Verde Beds rest on Triassic to Callovian strata. In the latter part of the Upper Oxfordian. carbonate sedimentation continued. with fluctuating salinity lagoons in the north (Pholodomya protei Beds) separated from shallow open marine carbonates in the south (Montejunto Beds) by the Caldas da Rainha diapir-barrier island complex. The commencement of rifting is recorded in the Kimmeridgian by the sudden influx of terrigenous c1astics (developed in both f1uviatile and deltaic/submarine fan environments) and accelerated depositional rates in excess of 10 cm/1000 yrs in association with contemporaneous faulting along the SE margin of the. Arruda sub-basin. The Caldas-Santa Cruz chain of diapiric structures continued to influence the distribution of carbonate and clastic sediments. In the Portlandian. a simpler facies pattern occurs, with fluviatile clastics interfingering to the south with shallow low energy carbonates.