Foraminiferal morphogroups as a tool to approach the Toarcian Anoxic Event in the Western Saharan Atlas (Algeria)

摘要

The present contribution constitutes the first morphogroup analysis of foraminiferal assemblages applied to the Toarcian Oceanic Anoxic Event (T-OAE) in the Maghreb region, which represented the northern palaeomargin of Gondwana. The ecostratigraphic and palaeoecologic analysis of the foraminiferal assemblages has allowed interpretation of the evolution of the biotic crisis associated with the T-OAE in the Saharan Atlas deposits and the recovery of foraminiferal assemblages in the sea-bottom after the event. The foraminiferal assemblages of the Uppermost Pliensbachian (Emaciatum Zone) represent optimum ecological conditions (good oxygen and nutrient availability in the infaunal microhabitats) as evidenced by the high diversity of K-strategists and the low diversity of r-strategists (representing a minor proportion in the assemblages). The beginning of the Toarcian (Polymorphum Zone) shows an initial phase of unstabilization of the benthic environment, affecting some particular species while opportunistic forms proliferate (Epistomina and Lenticulina). Variations in the oxygenation degree affecting the infaunal microhabitat favoured stress-tolerant species with extensive niche such as Lenticulina and Eoguttulina. Later, a benthic azoic interval includes the lower part of the Levisoni Zone. In this interval, a slight increase of total organic carbon (TOC) values recorded can be related to dysoxic or anoxic conditions in low-oxygen bottom-water, beyond the ecological tolerance of opportunist forms. After the benthic mass extinction interval, a slow colonisation is registered (upper part of the Levisoni Zone) with a dominance of r-type strategists (Lenticulina, Reinholdella and Eoguttulina).