Stratigraphy, palaeoenvironment and mollusks (gastropods and bivalves) of the Toarcian succession of the Central Unit of the South Riffian Ridges (northern Morocco)

Abstract

In the Central Unit of the South Riffian Ridges, the Toarcian succession is represented by two thick terrigenous detrital formations, Sned and Lmachkha formations, each one is divided into two members. The lower member of the Sned Formation, often rich in belemnite rostra and ammonites of the Tenuicostatum Zone, consists of grey marlstones, with rare marly limestone intercalations. The upper member consists of silty marlstones, with frequent thin limestone beds and contains rare ammonites of the Serpentinus Zone. Its upper part is locally rich in terebratulids. The lower member of the Lmachkha Formation consists of a silty marlstone and silty limestone alternation, and is the richest in benthic macrofauna, composed of rhynchonellids, gastropods, common bivalves, and rare ammonites of the Bifrons Zone. Seven taxa of gastropods and six taxa of bivalves were recorded from this member and described for the first time from the study area. The upper member of the Lmachkha Formation consists of silty marlstone and silty limestone alternation, and contains rare bivalves and rare ammonites of late Toarcian age. The lithofacies and pelagic macrofauna of the lower member of the Sned Formation characterise a relative deep marine environment unfavourable to benthic macrofauna. The second member indicates a gradual decrease in sea-level allowing from its upper part the recurrence of benthic macrofauna, especially terebratulids. The lithofacies and macrofauna of the lower member of the Lmachkha Formation indicate a shallow terrigenous shelf environment. The second member, containing coral reefs in the western and southern peripheries of the Central Unit, characterises a still shallow marine environment. This significant but gradual decrease in sea-level of the Central Unit from the latest early Toarcian to late Toarcian is likely related to the acceleration of sea-floor spreading in the Central Atlantic Ocean.