Lower Silurian Black Shales of North Africa: The Role of Glaciation on the Distribution of Source-Rock Quality Facies

A key exploration risk in the Lower Palaeozoic of North Africa is the local absence of source rock facies in the lower Silurian shale. Following the Late Ordovician glaciation, decaying ice sheets left a residual topography that included a series of “underfilled” glacial incisions, carved by ice and/or meltwater. Some models explain the patchy distribution of high TOC shales as deposition in anoxic pools within this topography during Silurian post-glacial transgression. In some parts of North Africa, however, a high TOC shale “blanket” occurs, whereas in other parts a clear connection with fault activity is observed. We present data from Al Kufrah Basin in southern Libya, which includes a continuous section straddling the Mamuniyat (Late Ordovician) and Tanezzuft (Late Ordovician-Silurian) formations respectively. The topmost Mamuniyat Formation comprises glaciogenic sandstones, which pass upward into mixed facies of the Tanezzuft Formation. At its base, the Tanezzuft Formation includes a basal carbonate facies association, comprising bioturbated peloidal micrites and wackestones, bearing fragmented crinoids, bryozoa and orthocones (interpreted as reworked coolwater carbonates deposited under oxygenating conditions). Above, the remainder of the Tanezzuft Formation comprises hummocky cross-bedded and graded sandstone intervals intercalated with shale and siltstone (interpreted as storm influx onto a muddy shelf). These latter deposits, however, are interrupted by several lonestone-bearing intervals (interpreted as ice-rafted debris), a striated pavement (interpreted as glacial in origin), manganese crusts and concretions. The putative glacial deposits occur at the same stratigraphic level as high TOC shales elsewhere in North Africa. Deposition of the manganese concretions is interpreted as the result of a drop in sea level, as a result of glaciation, which was accompanied by a fresh water influx, flushing out the potential for anoxia to develop and hence high TOC shales to accumulate.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California