The El Mizab formation (Talerhza Basin, north-west Morocco): Albian–early Cenomanian faunal diversity and paleoenvironment

Abstract

The Talerhza Basin is a small basin in north-west Morocco, in which the Albian–lower Cenomanian succession consists of three formations; in ascending order, the El Heifouf, Bab Lkarma and El Mizab formations. The first two units, of early–middle Albian age, comprise sandy marlstones and sandy limestones, respectively, and yield rare gastropods. The El Mizab Formation consists of an alternation of marlstones and argillaceous limestones and is richest in macrofauna, being divided into five informal members. The first member (silty marly limestone) is rich in small-sized debris of oyster shells and also yields gastropods and irregular echinoids; the second (lower shelly marly limestone) and the fourth (upper shelly marly limestone) members are rich in bivalves and contain, mainly the second member, gastropods, irregular echinoids and ammonites of late Albian age, representing the Mortoniceras pricei, Mortoniceras inflatum and Mortoniceras fallax zones; the third member (platy chalky limestone) is barren of macrofauna; the fifth one (marly limestone with Rhynchostreon), of early Cenomanian age, is rich in a monospecific bivalve, Rhynchostreon suborbiculatum. Twenty-eight species of bivalves, three irregular echinoids, ten gastropods and nine ammonites have been identified. The spatial distribution of bivalve faunas through the Cretaceous basin shows that some of these taxa were extremely abundant; mainly Lopha syphax in the upper Albian and Rhynchostreon suborbiculatum in the lower Cenomanian deposits. Most of the studied groups of macrofauna are common to several Cretaceous epicontinental basins of northern Gondwana, western and eastern margins of the Atlantic Ocean and the Cretaceous trans-Saharan marine corridor.