Cretaceous bivalves of the "Marnes et calcaires lumachelles a huitres" Formation (Upper Albian-Lower Cenomanian) of the South Rifian Ridges (Moulay Idriss Zerhoun area, northern Morocco)

The "Marnes et calcaires lumachelles A huitres" Formation (Oyster shelly marls and limestones Formation) of the South Rifian Ridges domain, Upper Albian-Lower Cenomanian age, is very rich in bivalves. Twenty-seven species belonging to eighteen genera were identified and are described and illustrated herein. Seven of them are particularly abundant: Rhynchostreon suborbiculatum (Lamarck, 1801), Lopha syphax (Coquand, 1854), Amphidonte conica (Sowerby, 1813), Cucullaea (Idonearca) they estensis (Coquand, 1862), Cucullaea (I.) trigona (Seguenza, 1882) and in a lesser degree, Protocardia hillana (Sorwerby, 1813) and Aphrodina (Aphrodina) dutrugei (Coquand, 1862). Most of the listed species are common to several epicontinental middle Cretaceous basins throughout the world. From a paleoenvironmental point of view, the first member of the studied formation, the Silty manly limestones, Diploceras cristatum zone?, is poor in bivalves; it contains mainly gastropods and echinoids. The second member, the Lower shelly manly limestones, Mortoniceras (M.) pricei-base of Mortoniceras (M.) fallax zones and the fourth one, the Upper shelly marly limestones, Mortoniceras (M.) fallax-Mortoniceras (M.) rostratum? zones, are rich in various species of bivalves. The third member, the Platy limestones, base of Mortoniceras (M.) fallax zone, is azoic. It marks a short period of total destruction of the marine ecosystem which was favorable to the proliferation of the bivalve faunas, probably related to an eustatic elevation of the sea level. The fifth member, the Manly limestones with Rhynchostreon, Lower Cenomanian, extremely rich in Rhynchostreon suborbiculatum (Lamarck, 1801), evokes peculiar palaeoecological conditions. The overlying formation, the Yellow marl and marly limestones, Middle-Upper Cenomanian, is azoic. It marks a rapid transition to a deeper and confined marine environment unfavorable to the benthic macrofauna. (C) 2016 Elsevier Masson SAS. All rights reserved.