EARLY TOARCIAN SOURCE-ROCK POTENTIAL IN THE CENTRAL HIGH ATLAS BASIN (CENTRAL MOROCCO): REGIONAL DISTRIBUTION AND DEPOSITIONAL MODEL

The early Toarcian is characterized by the occurrence of widespread black shale deposits, recorded extensively across Europe and interpreted to be related to an Oceanic Anoxic Event. In order to investigate the source-rock potential of lower Toarcian deposits in Morocco, detailed sedimentary logging accompanied by outcrop spectral gamma-ray measurements were carried out on four sections representative of different palaeo-depositional settings in the Central High Atlas. Authigenic uranium enrichment is used as a proxy for oxygen-depleted depositional conditions, and may thus indicate potential organic-rich lithologies. Of the four measured sections, only one (the Amellago section, which represents the deepest marine setting) shows relative authigenic uranium enrichment. Complementary TOC analyses (with maximum values of 3.24%) over this interval confirm an associated enrichment trend in organic matter. The organic matter recovered from the outcrop samples is however highly oxidised, and thus the absolute value is not regarded as being a true record of the rocks' organic content. It is concluded from this study that early Toarcian potential source rocks were deposited within restricted sub-basins in the Central High Atlas. It is therefore possible that organic-rich deposits of Toarcian age are present in similar palaeogeographic settings in other areas of Morocco. This increases the exploration potential of Moroccan basins in which Jurassic deposits are potential source rocks.