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Caractéristiques de lobes sableux : exemples du Bassin du Karoo – Afrique du Sud

le 13-07-2012 à 10:45

Salle Stendhal, DGO, Bâtiment B18

Amandine PRELAT, Post-Doctoral Research Associate, Basins Research Group (BRG), Department of Earth Science and Engineering, Imperial College, Prince Consort Road, Londres

The LOBE project is an industry-funded research programme that aims to increase our understanding of the stratigraphic evolution and internal architecture of deep-marine lobe systems. Based on exceptionally large exposures of the Tanqua and Laingsburg depocentres in the South African Karoo Basin, and a recently cored ~550 m behind-outcrop borehole; bed stacking patterns, environments of deposition, and lobe dimensions and volumes have been studied. A hierarchy of depositional elements has been described and ranges in thickness from a few centimetres to more than 350 meters. Depositional elements are named, from small- to large-scale, bed, lobe element, lobe, lobe complex, lowstand systems track, sequence, composite sequence, and composite sequence set. This robust depositional hierarchy allows the comparison of dimensions and volumes of similar elements deposited within very different systems. Results show that two categories of lobe geometry exist between six studied systems: large and thin lobes deposited dominantly in unconfined setting and smaller but thicker lobes deposited in more confined settings. The two populations of lobe types, however, share similar volumes, in the order of 1 or 2 km3.
Mapping of individual lobes within the Tanqua and Laingsburg depocentres demonstrate that they are not a simple stack of continuous sand sheet but are however organised. A variety of bed stacking pattern is observed and contrary to several published models, thickening-upward is only one of several stacking patterns in lobes. Internal stacking patterns can change laterally from thickening-upward, thinning-upward, static thicknesses and thickening then thinning-upwards.
The main questions that will be discussed are: What is the architectural hierarchy of lobe systems ? What are the dimensions and map view geometries of those elements ? What is the facies distribution and the main environments of deposition at lobe scale ? How does the Tanqua system compare to other systems ? And finally, what are the controls (allo vs. autogenic) influencing lobe deposition ?
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