--> Abstract: Incision, Filling and Abandonment of Coarse-Grained Turbidite Channels, Grés Du Champsaur, Alpine Foreland Basin, South-Eastern France: Facies Architecture and Process Analysis., by W. D. McCaffrey and S. Gupta; #90923 (1999)

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McCAFFREY, WILLIAM D., University of Leeds, and SANJEEV GUPTA, Huxley School, Imperial College.

Abstract: Incision, Filling and Abandonment of Coarse-Grained Turbidite Channels, Grés Du Champsaur, Alpine Foreland Basin, South-Eastern France: Facies Architecture and Process Analysis.

The relationship of channel elements to laterally bounding overbank successions is crucial to understanding the processes involved in channel incision and filling, but is generally poorly constrained from ancient coarse-grained channel complexes.We present a process-oriented study of the facies architecture and external geometry of an exceptionally well-exposed 2-km-wide turbidite channel complex that comprises both incisional and depositional elements.

The Priabonian Grès du Champsaur are a ~600-m-thick succession of turbidite sandstones deposited in the Lower Tertiary Alpine foreland basin in the Les Hautes Alpes region of southeastern France. It comprises three separate turbidite sub-systems, two allochthonous thrust-bound systems, and one autochthonous system. All the work described here relates to the autochthonous system, which was deposited in a confined basin, whose local long axis was oriented approximately WSW-ENE. Only the northern lateral basin margin is preserved. Although on the large scale this surface strikes WSW-ENE, in detail it consists of a series of promontories and embayments. The positions of the promontories can be related to the positions of culminations in the subcrop to the foreland basin sequence - which correspond to late Eocene paleotopographic highs, built of Mesozoic and basement rocks (Gupta and Allen, in press). Paleoflow is mostly from WSW to ENE, implying a tortuous dispersal pathway, given an Alpine sediment provenance.

The vertical succession can be broken down into three units:
1. a lower, apparently tabular, massive-sand - dominated unit,
2. a channelized unit,
3. an uppermost sheet unit with lower net-to-gross. The upper boundary of this higher sequence is unseen, as it is terminated by a later Alpine thrust.

The detailed sedimentological mapping and logging was concentrated on unit 2, where excellent exposure permits analysis of the channel complex along several different transects.

The study area lies above the village of Chaillol. Because the local topography is both deeply incised and complex, an excellent 3-D picture of the channel complex can be pieced together. The internal geometry of the channel complex was mapped and relationships with extra-channel levee elements defined. Our field studies permit reconstruction of the paleomorphology of the channel complex on a scale of several kilometers, providing clear evidence for multiphase episodes of incision and deposition.

Above the village is a large south-facing corrie with one encircling ridge on the west (Pt. Lingustier) and one on the east (Le Crest). The channelised unit crops out mid-height (c. 2000 m) within and around the corrie. To the west, the channel unit cannot be easily traced beyond Pt. Lingustier because of normal faulting. To the east however, the sequence can be traced on both sides of Le Crest, then through the intervening ridge and into the next valley. The principal chanellised unit comprises two channels (A and B). Each has an incisional channel axis sequence with a complex, dominantly sandy fill. In each case, the upper channel sand is transitional into - and continuous with - a laterally persistent sheetform overbank sand. Tabular, finer-grained intervals separate the channel sand bodies (except where they are amalgamated) and.also separate the channelised sequence from the overlying sheet sands. The right hand channel margins of both channels can be seen on the southern flank of Le Crest. The left-hand margin of this channel is scoured out by channel B, forming an amalgamated channel axis section on the northern flank of Le Crest. Moving further upstream and leftwards in the frame of reference of the channels, channel B can be seen incising what is interpreted to be the left-hand tabular overbank sequence of channel B on Pt. Lingustier.Thus both the left-hand and right-hand margins of the upper channel (B) are preserved. This channel can be traced along axis for at least 2km, and possibly for 3 more, as we believe it can be traced eastwards into the Tourand Valley, where it crops out in the valley wall. Channel A is apparently not exposed in this valley so it is difficult to constrain the plan form geometry of the channel. However, the geometry of channel B can be much more accurately constrained.

We analyse the sedimentary evolution of the channel complex and consider the paleotopographic controls on channelisation. Our study has important implications for identification and characterisation of the geometry of incisional turbidite channel complex deposits in the subsurface.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England