Palaeozoic low-oxygen, high-latitude carbonates: Silurian and Lower Devonian nautiloid and scyphocrinoid limestones of the Anti-Atlas (Morocco)

High latitude Silurian and lowermost Devonian basinal limestones of northern Gondwana are characterised by mass accumulations of orthocone nautiloids, bivalves and planktonic crinoids (scyphocrinoids). Extensive exposures in the Anti-Atlas of Morocco reveal this basinal facies passes up-gradient into an iron-rich bioclastic facies associated with distal shelf siliciclastics and into crinoid–brachiopod dominated bioclastic blankets and shoals. The limestones formed in a cool-water setting in which upwelling of nutrient-rich waters during sea-level lowstands is interpreted to have triggered the abundance of nautiloids and scyphocrinoids, preservation of organic matter in under- and overlying graptolite shales as well as iron-precipitation in the neighbouring, shallower-water sub-oxic zone.The nautiloid limestones proved to be good correlation markers on a regional to global scale. Contemporaneous siliciclastic supply into the neighbouring facies belt as well as new biostratigraphic data are taken as evidence that these limestones can be correlated to 2nd-order eustatic sea-level lowstands and the following transgression.The low oxygen basinal facies disappeared more or less abruptly during the Early Devonian (Pragian) and was replaced by a well oxygenated basinal lowstand facies, suggesting the end of upwelling. At the same time, the first reefal build-ups (coral-biostromes and mud-mounds) appear in North Africa, indicating a significant rise in water temperatures. The change from low oxygen lowstand mass accumulations to well oxygenated basinal facies with a concomitant rise in water temperature is in accordance with recent models of a cool Silurian to earliest Devonian (Lochkovian) ocean with oscillating oxic to anoxic (ventilated to poorly circulated) oceanic states and a warm ocean with anoxic to euxinic (poorly ventilated to stagnant) ocean waters from the beginning of the Pragian onwards.