Geochemical and palaeoecological record of the Cenomanian–Turonian Anoxic Event in the carbonate platform of the Preafrican Trough, Morocco

Abstract

Geochemical analyses (major and trace elements) and palaeontological studies were carried out based on rock samples and thin sections collected from three outcrop sections (Goulmima, Ziz and Tazzouguert) in the upper Cenomanian–lower Turonian in the carbonate platform of the Preafrican Trough (South Morocco), in order to determine if oxygen-depleted conditions affected the platform, and to highlight the possible impact of oxygen variation on carbonate production, particularly during the Oceanic Anoxic Event 2 (OAE2), which took place in the late Cenomanian.During this interval, the carbonate platform was composed of two contrasting environments. A shallow-water environment (peritidal to mid-ramp) was developed in the East (Ziz and Tazzouguert sections), characterized by a high diversity and abundance of benthic organisms, and in some cases, by a high diversity of planktonic organisms, associated with low concentrations of authigenic markers (U, Mo, V and Cr: redox proxies), indicating oxic conditions. In the West (Goulmima section), the environment was deeper (mid ramp/outer ramp) and redox proxies record a high enrichment, an indicator of reducing conditions, associated with a low diversity and abundance of organisms and the presence of Buliminidae. Bottom waters were dysoxic and the palaeoproductivity was low at the sea surface. Thus, during the late Cenomanian, oxygenation conditions were relatively good over the area of the Preafrican Trough, even during the OAE2, except in the deeper environments toward the West, where bottom waters were depleted in oxygen. During the Cenomanian transgression, and more precisely during the OAE2, this dysoxia was linked to the influx of poorly-oxygenated waters from the deeper anoxic basins of the Atlantic and/or Tethys oceans.Geochemical, palaeontological and sedimentological disturbances are recorded in the lower Turonian everywhere in the outer-ramp environments of the platform. Palaeontological proxies show a huge decrease in group diversity, with the disappearance of benthic foraminifera – except for the Buliminidae which proliferated – and only shallow-water planktonic foraminifera survived. Geochemical proxies of redox conditions and palaeoproductivity show a marked enrichment in all three outcrop sections, with higher values in the West. This dysoxia occurred in response to a climatic change, from warm with contrasting seasons to hot and wet, which induced an increase of chemical weathering in the Anti-Atlas to the southwest of the platform. This substantial input of nutrients increased the palaeoproductivity, which finally enhanced the consumption of O2 owing to the decay of organic matter. Moreover, the late Cenomanian transgression led to the supply of additional nutrients to the platform, due to the leaching of soils.