Spatial characterization of the late Sinemurian (Early Jurassic) palaeoenvironments in the Lusitanian Basin
Résumé
The upper Sinemurian of the Lusitanian Basin (Portugal) is characterized by the deposition of carbonates (dolomitic limestones, limestones), sometimes enriched in organic matter (total organic carbon up to 22%). The main goal of this study is to understand the distribution of carbonates and organic matter-rich sediments in space and time, and to characterize the context of their deposition at the basin scale. Three sections located along a proximal-distal transect in the Lusitanian Basin and dated from the oxynotum to raricostatum ammonite zones (upper Sinemurian) have been studied for their microfacies and sedimentary structures, and correlated by means of ammonite and calcareous nannofossil biostratigraphy, and sequence stratigraphy. The proximal part of the basin is dominated by carbonates whereas organic-rich sediments and marl-limestone alternations are common in the distal part. Microfacies analysis shows a trend from high- to low-energy environments, which reflects a deepening trend in most of the basin within the raricostatum ammonite Zone. A forced regression took place in the earliest Pliensbachian, which is likely related to tectono-eustasy. This particular surface can be correlated at the regional level, to several basins of the western Tethys. The presence of black shales in the distal part of the basin and of framboidal pyrite in the proximal part is interpreted as resulting from an episode of pervasive dysoxia/anoxia at the basin scale during this time interval. The basin recorded conditions varying from dysoxic/anoxic phases in times of water column stratification, to full oxygenation in times of intense mixing by storms, attested by the presence of storm-related sedimentary structures in some carbonate-rich deposits. The occurrence of upper Sinemurian sediments rich in organic matter in other basins, in England and Spain, in the same time interval (upper Sinemurian, raricostatum ammonite Zone) suggests that palaeoclimatic and palaeogeographic conditions were prone to the development of a supraregional dysoxia/anoxia. (C) 2014 Elsevier B.V. All rights reserved.