Marine deposits, faunal assemblages and palaeoenvironments of the Late Cenomanian - Early Turonian interval in the lullemmeden Basin and the Northern Chad Basin (Niger)

摘要

The lullemmeden Basin and the Northern Chad Basin form a single West-African sedimentary area. During the Late Cenomanian-Early Turonian interval, they were invaded by a Tethyan transgression and a homoclinal ramp developed. The deposits consist of interlayered mudshales and limestones which recorded the effects of an equatorial climate and of sea-level fluctuations. Mudshales largely predominate and their characteristics indicate that bottom conditions were quiet and dysaerobic. Each limestone bed represents development of current-swept, oxygenated substrate for a short time interval which, in turn, corresponds to the maximum depth reached by each of the successive pulses of the transgression. The faunal content of the limestones is characterized by: (1) the absence of forms (rudistids, corals) commonly found on the Tethyan platforms, and of Pycnodonts; (2) the abundance of Gryphaeidae and Ostreidae. This faunal composition is mainly ascribed to: (1) a general decrease in the diversity of rudistids and corals during the Late Cenomanian-Early Turonian interval, and the lack of suitable shoals on the ramp for reefs to develop; (2) a severe competition from pioneer and opportunistic bivalves that quickly colonized the ramp and prevented the development of other forms.