--> Abstract: Stratigraphic Architecture from Hirnantian Glacial Tunnel Valley Infilling, Eastern Anti-Atlas, Alnif, Morocco, by Jean-Francois Buoncristiani, Eric Portier, Laurent Escare, Sebastien Laratte, and Christophe Rigollet; #90082 (2008)

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Stratigraphic Architecture from Hirnantian Glacial Tunnel Valley Infilling, Eastern Anti-Atlas, Alnif, Morocco

Jean-Francois Buoncristiani1, Eric Portier2, Laurent Escare2, Sebastien Laratte2, and Christophe Rigollet2
1CNRS Biogeosciences, University Burgundy, Dijon, France
2Direction Exploration Production -NAD, Gaz de France, PARIS, France

During the Late Ordovician (Hirnantian) an ice sheet covered a great part of the Gondwana. In Africa, several studies present the stratigraphy and the complexity of these glacial records.The different glacial landsystems correspond to several glacial cycles, related to rapid ice front oscillations and are grouped into two major ice-sheet advances, separated by a major ice sheet recession.
In this work, we present new data on glacial Hirnantian deposits in the Eastern part of the Anti-Atlas (Alnif area). We propose a new glacio-marine sedimentological model for a glacial tunnel valley infill. The glacial erosion in this area is well defined by an unconformity between pre-glacial Caradoc to Ashgil aged sandstones and Hirnantian glacial deposits. The unconformity is characterised by a SE-NW oriented palaeo-valley 5 km wide, 10 km long and up to 250 m deep. Its edges have an imbricated morphology which probably results from several incision phases.

The palaeo-valley has been infilled by two glacial depositional units, each containing several sedimentary cycles.Deposits are dominated by pure glacial and glacio-marine sediments, categorized into six facies associations. Glacial infillings present thrust and fold belt deformations, glacial pavements on soft sediments and clastic dykes. The Alnif tunnel valley infill is characterised by a complex mix between grounding line deposit systems (ice contact fan) and glacial deposit materials (lodgement till and flow till).

The glaciogenic Ordovician deposits constitute a proven oil and gas bearing reservoir on the North Gondwana margin, also known for their sharp and rapid facies changes. Also, such a study provides an excellent opportunity to understand and appraise the complex architecture and geometries of the reservoir sands bodies of this promising play.

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