Early Paleozoic Depositional Cycles of Eastern Algeria; Sequence Stratigraphy and Paleogeography, and Insights into their Regional Correlation in Northern Gondwana

By

Sebastian Galeazzi¹, Olivier Point²

(1) Total Fina Elf, Paris - La Defence Cedex, France (2) TotalFinaElf,

 Cambrian-Silurian siliciclastic rocks are an important objective of petroleum exploration in eastern Algeria. They contain a major early Silurian source rock, and include sandstone reservoirs that hold over 20 BBOE. The Lower Paleozoic of SE Algeria is a 450 to 2000 m thick sand-prone depositional unit that fills the Berkine, and Illizi intracratonic depressions.

The Cambro-Ordovician interval consists of thick and laterally-extensive fluvial and shallow marine sandstone intervals separated by conspicuous offshore shale tongues, and major unconformities. The uppermost Ordovician are tillites, glaciogenic sandstones, and glaciomarine shales deposited during the short-lived Ashgill glaciation. The succession is followed by marine shales of early Silurian age deposited during the post-glacial transgression, covered by a mid-late Silurian fluvio-deltaic progradational sequence-set..

The series shows an overall parallel stratal-stacking pattern in its lower Cambro-Ordovician portion that changes to a low angle sigmoidal progradational architecture within the Silurian tracts. It lies over the late Proterozoic Pan-African igneous-metamorphic basement and its top is marked by the tectonically enhanced “Caledonian” Unconformity of terminal Silurian age. Low subsidence rates and a long-term early Paleozoic eustatic high created limited but widespread accommodation throughout the paleo-Tethys Gondwana margin. This, combined with a high sediment supply from the Pan African orogenic relief to yield widespread largely tabular sand-rich depositional packages. Major regional unconformities separate the series into five megasequences: Cam 1 (Early Cambrian), Cam 2 (Mid-Late Cambrian), Ord 1 (Late Cambrian- base Arenigian; peak transgression mid-Tremadocian), Ord 2 (Early Arenigian-Ashgillian, peak transgression Llanvirnian), and Sil 1 (Ashgill-Pridolian, peak transgression Llandoverian).

We have observed similar depositional cycles in Morocco and the Middle East and we postulate that they are Gondwana-wide cycles linked to eustasy and major tectonic events.