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The Cenomanian Turonian boundary on the Saharan Platform (Tunisia and Algeria)

Grosheny Danièle, Ferry Serge, Jati Mohamed , Ouaja Mohamed , Bensalah Mustapha , Atrops François , Chikhi-Aouimeur Fettouma , Benkerouf-Kechid Fatiha , NEGRA Mohamed Hédi, Aït Salem Hamid, 2013

Cretaceous Research, 42, 42, 66-84, Juin 2013

Résumé

Several transects made of correlated stratigraphic sections and well logs have been constructed spanning southern Tunisia and the Algerian Sahara (Tinrhert) for comparison with earlier results obtained in the Saharan Atlas. The study is based on facies analysis, sedimentology, biostratigraphy focused on ammonites and foraminifers) as well as whole rock geochemistry (d13C). These suggest that the entire northern Sahara Platform underwent marine flooding that commenced just prior to the onset of the global positive d13C shift documented for the CenomanianeTuronian boundary. This flooding occurred in two phases. The first phase is expressed by the deposition of deeper-water, light-coloured bioturbated mudstones overlying the shallow-water deposits comprising the local Cenomanian successions. But in some places in the Central Sahara (Hassi Messaoud area, Tihemboka Arch) as well as in the Saharan Atlas, shallowwater carbonates kept up locally with the relative sea-level rise to build up isolated carbonate platforms. The topographic lows or saddles between these areas could have been formed through differential accumulation rates. During the second phase, flooding resumed and black shales were deposited over the mudstones in the saddles. The occurrence of black shales in these saddles is limited to the northern edge of the platform (Saharan Atlas of Algeria, Gafsa Trough in southern Tunisia). On the platform, this phase is represented by the same kind of mudstones deposited during the first phase of the flooding (southern Tunisia), or by ammonite-rich chalks in the intra-cratonic basin of the Tinrhert (southern Algeria). Blackshale deposition ceased in the early Turonian. Based on the d13C curve, the latest Cenomanian flooding of the Sahara Platform is roughly coeval with that documented for the US Western Interior. During the first phase of the transgression, that is before the occurrence of the large Whiteinella of the W. archeocretacea Zone in the black shale unit, planktic foraminifers are dominated by small globulose

forms of the Hedbergella delrioensis type, associated with Heterohelicidae. Keeled forms (rotaliporids, dicarinellids) are scarce and always very small when present. Perhaps these dwarfed forms were adapted to the restricted environments of the extensive intracratonic seaways crossing the Saharan Platform to the Benoué Trough in Nigeria.