AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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200Myr of source-to-sink relations from a rift-passive continental margin: examining controls on sediment delivery

Abstract

A unique data set, comprising hundreds of low-temperature thermochronology measurements closely integrated with detailed analysis of exposed and subsurface sedimentary successions allows reconstruction of the source-to-sink evolution of the Moroccan Atlantic margin between the Variscan and Alpine orogenies (between 300 and 100Myr). This straddles the pre-, syn- and most of the post-rift evolution of the margin. We observe that the overall history of the margin bears little similarity with generally accepted tectono-stratigraphic models of rifted margins. At the very large spatial and temporal scales, the entire margin was characterized by regional, tectonically driven exhumation and erosion along a 100s km wide, NNE-SSW trending area, flanked to the W by a continuously subsiding basinal domain. During the first 150Myr (from the Permian until Early Jurassic), the highest rates of exhumation and erosion are recorded in the Reguibate shield and Anti-Atlas (Figure 1). Eroded section probably consisted of Palaeozoic carbonates and silts, with subordinate sands, which accumulated in the extensional basins forming along the margin (e.g. Tarfaya basin). During the Middle Jurassic to Cretaceous, high rates of exhumation in the western Reguibate and the (future) High Atlas and Meseta, which had previously been stable to subsiding in the Jurassic, gave high rates of sediment delivery (Figure 1). A regional westward tilting monocline developed, the exhuming area passing to the W into a domain of increased subsidence. The Anti-Atlas exhumation subsequently resumed in the Late Cretaceous, whilst the Meseta exhumation slowed down, marking a significant shift in source area. In the Agadir-Essaouira basin, at a smaller scale of 10s of kms and several Myrs, sediments from the exhuming High Atlas and Meseta were transported by a mid-sized river network towards the (paleo) shelf. Palaeo-drainge was influenced by a set of roughly E-W trending anticlines and N-S basin bounding faults, which at times had topographic expression. A newly developed high-resolution stratigraphic framework, that has been obtained through extensive outcrop logging and new biostratigraphy, documents the importance of even smaller wavelength variations, of kilometres Myrs, and provides control for the exhumation studies. Activation and deactivation of tectonic and salt-driven folds on the basin margin, which often had an expression on the seafloor, were a main factor controlling the sequence development (forced transgressions and regressions) and their identification allows improved modelling of the main fluvial systems delivering sand into the basin.