The Sahara from the Middle Jurassic to the Middle Cretaceous: Data on environments and climates based on outcrops in the Algerian Sahara

Abstract

The period investigated, succeeded the Upper Triassic and Liassic-Lower Dogger arid sedimentation which was characterized by the development of large evaporitic deposits. The north-eastern part of the Algerian Sahara is specially interesting as it constitutes part of the few dated continental Jurassic outcrops in North Africa (except in the Maghreb). Continental Lower-Middle Cretaceous is also well developed in the same region. The grading of these continental series into marine series particularly in the north-eastern part of the Maghreb has been investigated from an adequate number of well spaced wells. The Middle-Upper Jurassic (Lower Taouratine) consists of an argillaceous-sandy-ferruginous series, often grey coloured and rich in filicean wood and vertebrate remains in some levels. The ferralitic alteration products denote a more humid climate than that which prevailed during the Liassic-Lower-Dogger times. These products alternate with sandy conglomeratic events confirming the influences of alternating important run-off and aeolian winnowing. The large continental Lower and Middle Cretaceous detrital nappes are discussed. They constitute the uppermost part of the series, south of the Tinrhert Hamada (Alba-Vraconian series with the more detrital lateral equivalent), west of Tademait (Gara Samani). Data on lithology and paleobiology (fishes, Dinosaurs, other vertebrates, pelecypods, gastropods, wood debris, palynoflora) permit the reconstruction of the palaeo-environment and the conditions of deposition. This is similar to the sub-desertic terrestrial sedimentation by violent and ephemeral hydrodynamism model based on the modern Darfour example. The desertic conditions which prevailed in these Saharan areas during the Cretaceous are reflected as well as their possible consequences on the sedimentation of nearby oceanic areas (Atlantic and Tethys). This detrital sedimentation is finally reviewed in the context of its tectonic origins and an attempt is made to define the different environmental conditions (such as spreading of meandering streams, aeolian modelling phases etc.) A critical view of the deltaic sedimentation concept was applied to these epicontinental platform deposits is presented.