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

Abstract

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.

The Cambrian-Ordovician succession in the Ougarta Range (western Algeria, North Africa) and interference of the Late Ordovician glaciation on the development of the Lower Palaeozoic transgression on northern Gondwana

“…Pioneering works of Menchikoff (1933), Poueyto (1952), Arbey (1962) and Gomez Silva et al (1963) proposed the lithostratigraphic nomenclature that is still currently used (Legrand 1985, Fekirine & Abdallah 1998Figs 3, 4). The definition of lithostratigraphic units was based on both lithologies and observed bounding discontinuities.…”

“…The occurrence of a ferruginous, conglomeratic and oolitic horizon that is reported in the literature within the Kheneg el Aatène Fm. (Gomez Silva et al 1963, Fekirine & Abdallah 1998) is used to tentatively differentiate the correlative surface of a subaerial unconformity (Fig. 8).…”

“…If present, they correspond to a thin prograding succession (unit of the Vire du Mouflon of Eschard et al 2005). In subsurface of Algeria, the regressive system tract comprises the El Gassi shales and El Atchane sandstones (Fekirine & Abdallah 1998).…”

“…Based on its prominent erosional character, this bounding surface is generally considered to be a major unconformity that includes a tectono-stratigraphic component and separating two major, long-term depositional sequences (Sharland et al 2001, Carr 2002, Eschard et al 2005. The overlying glacially related strata are sometimes regarded as an individual major depositional sequence (Fekirine & Abdallah 1998, Eschard et al 2005. However, using a system of sequence hierarchy based on boundary frequency, we consider that this surface has no greater significance than that of other sequence boundaries documented in the underlying Cambrian-Ordovician succession.…”

“…These strata were dominated over North Gondwana by fluvial to inner-shelf deposits. Correlative biostratigraphic data are relatively sparse with two exceptions: (1) the Anti-Atlas (southern Morocco, e.g., Destombes et al 1985, ElaouadDebbaj 1988, Gutiérrez-Marco et al 2003, Bourahrouh et al 2004) and the Ougarta Range (western Algeria, e.g., Fekirine & Abdallah 1998, Fabre & Kazi-Tani 2005, two areas that displayed abundant macrofaunas; (2) palynomorphs (chitinozoans, acritarchs) mainly from drill cores in sedimentary basins (Taugourdeau & de Jekhowsky 1960, Vecoli 1999, Vecoli & Le Hérissé 2004 and references therein). Correlation of these fossil-bearing strata with barren fluvial (or tidal) sandstones in more proximal, southern parts of the platform is still debated even though it is of prime importance in the understanding of the development of Palaeozoic sedimentary basins on the North African cratonic platform.…”