Variability of Depositional Systerms Related to the Latest Ordovician Glacial Events in Northern Gondwana

Jean-Loup Rubino¹, Jean-François Ghienne², Max Deynoux², Julien Moreau², and Daniel Le Heron^3
1 TotalFinaElf, Pau, France
² EOST Strasbourg University, Strasbourge, France
³ CASP, Cambridge University, Cambridge, England

The late Ordovician Hirnantian succession of northern Gondwana shows very rapid lateral and vertical facies changes, reflecting four main phases of glacial advance and retreat. Large-scale glacial valleys, both isolated and vertically stacked into fairways, are the major hydrocarbon reservoir intervals. Valley fill is highly variable, but outcrop and seismic data allow the recognition of four major types. 1) Glacio-marine systems, including shelfal sediments dominated by distal diamictites, as well as proximal deltas located at the mouth of tunnel valleys. Depending of their location, these systems record maximum ice advance and the beginning of ice retreat coupled with a transgression. 2) Terrestrial glacial-dominated systems, including sandur complexes at the base of the valleys, and braid plains dominated by sheet flood units or ribbon channels developed within outwash plains. Sandur complexes are related to the onset of ice retreat, while the channels, comprising sheet-like units formed by amalgamated climbing dunes, occur either at the end of interglacial periods or at the onset of a new glacial advance. 3) Deltas, including Gilbert-type fans deltas, as well as classical wave, and fluvial dominated deltas. These systems show the classical features of deltas, but are confined to the valley width; they record the main phase of progradation during interglacial periods. 4) Conventional fluvial systems, including braided rivers, and meander belt complexes with crevasse channels. These systems occur during interglacial progradation and the beginning of ice advance. Outcrop analogues and good 3D seismic data are necessary in order to assess the internal complexity of these hydrocarbon reservoirs.