Southern Tethys shelf evolution around the Cenomanian–Turonian OAE2: The Saharan Atlas (Algeria) depositional model

Abstract

The present work, based on lithostratigraphy and biostratigraphy, is focused on ammonites, microfacies, and chemostratigraphic study (δ¹³C and δ¹⁸O) of the late Cenomanian–early Turonian sequence in the El Bayadh area (Saharan Atlas, Algeria). Five informal stratigraphic units (units 1 to 5) have been defined within the Rhoundjaïa Formation, which is subdivided into four ammonite biozones: the Neolobites vibrayeanus, the Metoicoceras geslinianum, the Vascoceras cauvini, and the Choffaticeras sinaiticum zones. The Cenomanian–Turonian boundary (CTB) is located at the limit between units 2 and 3. Field data and microscopic analysis have resulted in the recognition of fourteen microfacies types (MT 1 to MT 14) that are described in terms of depositional environments, and grouped into six main limestone and limestone–marl facies associations (FA 1–6), representing two different palaeosettings: (i) open shelf/ramp-type carbonate platform and (ii) isolated carbonate platform. The sequence stratigraphic analysis revealed two third-order depositional sequences (DS1 and DS2) covering a period of about 2.8 million years. The depositional model for the uppermost Cenomanian shows the presence of a rudist- and microbial-rich isolated carbonate platform, away from terrigenous influence, and surrounded by deep-water open shelf facies. The comparison of the isotopic fluctuation profile with those of various marine sites in the Tethyan Ocean shows synchronous global events.