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The AAPG/Datapages Combined Publications Database

Journal of Sedimentary Research (SEPM)


Journal of Sedimentary Research, Section A: Sedimentary Petrology and Processes
Vol. 64A (1994)No. 1. (January), Pages 42-54

The Influence of Lateral Basinal Slopes on Turbidite Sedimentation in the Annot Sandstones of SE France

H. D. Sinclair


The deep-water deposits of the lower Oligocene Annot Sandstones of SE France were deposited in topographically dissected basins. Axially derived turbidites show marked lateral onlap onto the underlying marls. The original lateral slopes were at least 250 m high, and up to 12° prior to compaction-induced steepening; they commonly show a stepped or undulatory profile. Lateral tracing of individual beds away from their position of onlap termination down the paleoslope reveals a characteristic succession of microfacies: thin-bedded, nongraded fine sandstones with upper-flow-regime planar lamination pass downslope into medium-bedded, partially graded medium sandstones with ripple and planar lamination. At the base of the depositional slope, the turbidites become thick to very thick-bed ed, fully graded (commonly stepwise) coarse sandstones to mudstones. The most likely interpretation of the lateral transition from lower-flow-regime to upper-flow-regime conditions from the axial trough to the lateral margin of the basin involves pulsatory flow conditions. Episodes of increased flow thickness and velocity deposited upper-flow-regime bed forms on the higher parts of lateral ramps, while periods of waning current deposited lower-flow-regime bed forms in the axial trough.

During accumulation of the sands, the underlying marls were differentially compacted, with maximum compaction under the axis of the basin, decreasing to zero compaction at the edges of the basin. This resulted in secondary slopes on the upper surface of the sandstones overlying the marl slopes, which may have been important in controlling the accumulation of successive sand packages.

Successive onlap and overstep of sand layers onto the marls resulted in a vertical profile that shows the same succession of facies as traced downslope in individual beds. The resulting vertical sections coarsen and thicken upwards over 5-10 m. The characteristics of the thin-bedded fine sandstones at the base of cycles may be used to distinguish between prograding and laterally onlapping turbidite packages. Sandstone packages in the basin axis are thick (30-60 m), sharp based, and sandstone rich. The laterally equivalent part of a package overlying a confining slope is thinner, asymmetric with a coarsening upward signature, and more mudstone rich.

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