3D reconstitution of the Lower Jurassic Lusitanian Basin (western Portugal) and associated organic rich deposits

Abstract

The Lower Jurassic series of the Lusitanian Basin provide an excellent case study of lateral and vertical variations through time. That basin was a branch of the Tethys Ocean located on the western part of the Iberian Massif. As the depositional conditions of this carbonate ramp system evolved, some marly organic rich intervals get deposited and they are particularly well recorded and outcrop now along the Portuguese coast. Many published studies aim to characterize the whole series and provide paleogeographical settings, environmental frame and biostratigraphical constraints. However, only a few studies have proposed paleo-reconstitutions and explained the processes linked to the organic rich deposits at the basin scale. We develop a numerical stratigraphic forward modeling approach based on the integration of newly acquired data (age, mineralogy, TOC, HI) with all the data currently available in the literature. The principal aim of this project was to determine and to quantify the parameters describing the whole sedimentary basin from the middle Sinemurian (193 Ma) to the late Toarcian (174 Ma) on a 60 km² area with a 200 ka time step. We especially look at the horizons with high Organic Matter (OM) content in order to test the impact and the viability of several hypotheses and to rank the different associated processes. Here we present the obtained 3D numerical model and explain from this reconstitution how the primary production, sources, sedimentation rate, mineralogy, paleogeography, basin morphology and environment deposit conditions such as water depth or anoxia, can trigger organic rich deposits and their heterogeneities like in the Davoei/Margaritatus zone interval. This study addresses questions on the repartition and variability of OM content at global scale. The subsidence and sedimentation rates appear to be the key factors. The same approach for the whole Lower Jurassic interval of the European domain is our current challenge.

AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015