Palaeozoic lithofacies correlatives and sequence stratigraphy of the Saharan Platform, Algeria

This paper attempts to unravel the interacting effects of eustacy, subsidence and sediment supply in controlling facies distributions in the Palaeozoic of the Algerian craton. These strata, particularly the Cambro-Ordovician section, contain no diagnostic index fossils, and the identification of sequence boundaries is therefore of paramount importance in defining a stratigraphic framework for the region. The wealth of outcrop and subsurface data in the Algerian Palaeozoic enables us to recognize six major sequences, the boundaries of which correspond to major events such as Taconic and Hercynian tectonism, the Ordovician glaciation and global-scale sea-level falls. Formations within these sequences exhibit thickness variations and lateral facies changes that are traceable and predictable in the subsurface. Unconformities of regional extent generally tie to significant facies changes and to inflection points on the global sea-level curve. These trends stem from the high sensitivity of this area, which was structurally stable over most of its Palaeozoic history, to eustatic sea-level fluctuations. The Cambro-Ordovician strata feature a high proportion of coarse clastic rocks, with subtle lateral and vertical facies variations, which were deposited within continental fluvial and shallow marine environments. The Silurian, Devonian and Early Carboniferous sequences show a typical pattern of marginal shallow marine sand belts, which migrate with changing sea-level and pass rapidly into deep marine shales, many of which are organic rich. The Upper Carboniferous strata consist of sediments deposited in continental and restricted basin settings, created as a result of tectonic movements heralding the Hercynian event. With respect to the occurrence of petroleum, the most important reservoirs are found in lowstand tracts in the Cambro-Ordovician and Siluro-Devonian sequences. Thick radioactive marine Silurian and Devonian black shales deposited during major transgressions provide both source and cap-rocks.