Outcrop Analogues for Deep Water Channel and Levee Genetic Units from the Grès d'Annot Turbidite System, Southeast France

Clark, J.D. and A.R. Gardiner, Department of Petroleum Engineering, Heriot-Watt University, Edinburgh, UK

Abstract

Large-scale exposures of the Grès d'Annot in south-east 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, in general, have quite distinctive bed thickness characteristics and sandstone-to-shale proportions. Field studies in the Grand Coyer, Trois Evêchés, and Col de la Cayolle outcrop areas have identified, in addition to the sheet-like packets, a common type of turbidite channel succession. These channels are characterised by a relatively high aspect ratio; channel dimensions range from 900-4000 m wide and 14 to 110 m deep. The channel fill is 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 Grés d'Annot basin has previously been underestimated.

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, a nd planar-laminated, and beds are commonly discontinuous over relatively short distances (showing pinch-out in both directions). Cross bedding is commonly found in the thicker beds, and trace fossils such as Ophiomorpha and Thalassinoides a re abundant. Sandstones containing lignite and charred wood fragments are commonly found in these intervals. Megascours, and small-scale channel-fill sandstone bodies can also be found within these intervals. Together, these facies, and their association with the channels, suggest that the thin-bedded packets represent the levees to sandier channelised deposits.

These levees formed from the aggradation of sand and shale, 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 Grés d'Annot, where channels are not exposed. It is not, however, possible to interpret all thin-bedded packets in the Grés 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.

The Grés d'Annot channel and levee deposits are characteristic of this turbidite system. These channels appear more common in the proximal part of the system (in the Grand Coyer outcrop area), and may have developed basinwards fr om highly confined flows that were channelled though the Annot Sub-basin. The channels are found interbedded with laterally continuous sheet sandstones, have a sheet-like fill, and relatively high aspect ratios, suggesting that they broadened out downstr eam to form connected depositional lobes. This type of channel-levee system may provide a new model for analogous channels in other turbidite systems.