Outcrop Analogues for Deep-Water Channel and Levee Genetic Units from the Grès d'Annot Turbidite System, SE France
J. D. Clark, A. R. Gardiner, 2000. "Outcrop Analogues for Deep-Water Channel and Levee Genetic Units from the Grès d'Annot Turbidite System, SE France", Deep-Water Reservoirs of the World, Paul Weimer
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Large-scale exposures of the Grès d’Annot in southeast France provide excellent opportunities to study detailed architecture of sand-rich turbidite systems deposited in a relatively confined basin. The deposits are dominated by laterally extensive sheet-like sandstone packets which have, in general, quite distinctive bed thickness characteristics and sandstone-to-shale proportions. In addition to these sheet-like packets, field studies in the Grand Coyer, Trois Evêchés, and Col de la Cayolle outcrop areas have identified a common type of turbidite channel succession. The channels occur at a variety of scales, with channel dimensions ranging from 900 m to 4000 m wide and 14 m to 110 m deep, and are characterized by a relatively high aspect ratio. The channel fills are sand-rich, moderately to highly amalgamated, but the sandstones show largely planar bedding architecture. The relatively low relief of the channels and their sheet-like fill makes them difficult to distinguish in areas without good lateral continuity of exposure, and it is possible that their importance within the fill of the Grès d’Annot basin has previously been underestimated. Likewise, it is possible that the occurrence of analogous channel-levee complexes may be underestimated in subsurface reservoirs that are otherwise dominated by sheet-like turbidite sands.
Several channel exposures allow detailed examination of channel margin architecture, and clearly demonstrate several periods of reactivated channel activity. Typically, the margins show the most complex sedimentary architecture.
Laterally away from the channel margin, and/or stratigraphically above or below the channel-fill, thin-bedded sandstones and shales form distinctive packets. Within these packets, the thin-bedded sandstones are relatively coarse-grained, cross-laminated, and planar-laminated, and beds are commonly discontinuous over relatively short distances. Cross bedding is common in the thicker beds, and trace fossils, including Ophiomorpha and Thalassinoides, are abundant. Sandstones containing wood fragments and lignite are commonly found. Megascours and small-scale channel-fill sandstone bodies are also present in these intervals. This combination of features, and their association with the channels, suggest that the thin-bedded packets represent the levees to sandier channelized deposits. The levees formed by the aggradation of sand and mud which were deposited from flows, or parts of flows, that spilled from the channels. This facies association has been used to interpret other levee deposits elsewhere in the Grès d’Annot, where channels are not exposed. It is not, however, possible to interpret all thinbedded packets in the Grès d’Annot succession as levee facies; for this interpretation to be valid, the packets must include some or all of the distinctive features described above.