SUBSIDENCE OF THE ATLANTIC MOROCCAN MARGIN DURING THE MESOZOIC

This paper presents a combined study based on seismic interpretation, sequence stratigraphy, and the evaluation of subsidence that aims to c haracterize the structure and development of the Essaouira Basin in Mo rocco. Located in the coastal Meseta adjoining the continental margin, this basin records an initial Carnian-Hettangian deformation phase du ring rifting in the central part of the North Atlantic region. The geo metry of the basin as a function of time shows a succession of half-gr abens and horsts that developed westwards from reactivated Hercynian s tructures. The postrift stage is characterized by an aggrading sedimen tary sequence, as shown by concordant seismic sequences stacking over the onshore part of the basin. The Upper Cretaceous coincides with a s equence showing a transition towards a prograding regime that leads to the topography of the present-day margin. Using the high-resolution a nalysis provided by sequence stratigraphy, it is possible to recognize fine-scale stratigraphic variations in the sedimentary succession. Th e well-to-well correlation of sedimentary cycles forms a dataset for e valuating subsidence. Residual subsidence curves reveal a differential behaviour between the present onshore and offshore areas. Although th e computed subsidence rates are low across the onshore zone, curves fo r the western offshore part of the basin follow theoretical lithospher ic cooling curves that are compatible with a stretch factor (beta) of nearly 1.4. Steep temporary gradients on the computed curves may be co rrelated with tectonic phases documented across the North Atlantic reg ion that exerted a tight control on the development of the Essaouira B asin from Triassic rifting until the uplift of the Atlas Mountains dur ing the Cenozoic.