Abstract: Sequence Stratigraphy Applied to the Ordovician in Bourarhet Area (Illizi Bassin - Southern Algeria): Environments and Paleogeography

MAACHE, N.

The Ordovician period was a significant one in the evolution of the central Sahara. An important tectonic event, the Taconic phase, was accompanied by the development of a continental glacial cap. The sedimentological characteristics of the glacial deposits of the Upper Ordovician are clearly different from those of the Lower Ordovician detrital deposits. The Ordovician formations are commonly devoid of good index fossils in this part of the Sahara. However, correlation between outcrop and the subsurface allows the definition of three major sequences, which are described in the outcrop sequences of the Tassili N'Ajjer and Bourarhet areas. The first sequence, of probable Tremadocian to lower Arenigian age, is represented by fluvial to shoreface sandstones (LST) overlaid by transgressive sandstones (lower TST). The second sequence, as represented by the Hamra Quartzites of middle Arenigian age, represents lateral accretion fluvial bar deposits (LST). Marine sands and shales constitute the basin-wide transgressive system tract (TST) The sequence top is represented by the truncation surface of an incised valley fill network. These valleys are essentially filled by diamictite to coarse fluvioglacial sands (LST of the third sequence) evolving northward into a periglacial environment. The northeastern Illizi basin was essentially an area of glaciomarine sedimentation setting during the Ashgillian (TST), represented by deposition of sandy and shaly diamictite. The third sequence is locally eroded. Large hydrocarbon volumes should be expected to have been trapped in the Upper Ordovician reservoirs. The sedimentary complexity of the system however makes exploration in this section difficult.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria