Early Devonian bioclastic contourites in the High Atlas: a plastered drift recording the convergence between Gondwana and Laurussia (Sub-Meseta Zone, Morocco)

Abstract

Bottom-current deposits, formed in association with ancient oceanic gateways, preserve records of past global oceanic circulation, plate-tectonic events and palaeogeographical change. This outcrop study showcases a Pragian–early Emsian carbonate contourite drift, plastered on the northern upper margin of Gondwana, which recorded the convergence of Laurussia and Gondwana and the (temporal) closure of the nearby American Gateway. The results underscore the value of bed-scale features, including microfacies and primary sedimentary structures, in interpreting bottom-current dynamics. The documented contourite-specific bi-gradational sequences, 40–80 cm thick, are largely complete and closely comparable with the standard facies sequence identified in modern oceans. The bioclastic contourites show subtle variations in grain size and mud content in the lower part, distinct sequences of inverse to normal grading at different scales in the middle and a large variety of traction structures in the upper part of the succession. This long-term coarsening-upward trend was caused by a successive increase in maximum bottom-current strength during drift growth. Traction structures such as current ripples and dunes document a westward-directed palaeocirculation of surface or intermediate water during the Pragian–Emsian. They provide evidence of enhanced bottom-current episodes towards the top of the drift, before the contourites were buried by late Emsian shales.