Controls on the architecture of turbidite sandstone in MTD dominated settings; The Vocontian paleomargin, SE France
Leeuw, Jan de
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The Aptian succession of the Vocontian paleomargin (SE France) consists of marl-prone mass transport deposits, turbidite sandstone and pelagic marl. A detailed stratigraphic framework, which was established by previous workers, allows correlation of individual event beds for up to 80 kilometres in the down-dip direction. One thick-bedded turbidite sandstone that is interbedded between two mass transport deposits is studied in several outcrops. The thickness of this sandstone bed is very variable. This paper aims to reconstruct the origin of these thickness variations. Detailed outcrop descriptions, with a focus on the nature of the contact of the turbidite with the underlying and overlying mass transport deposits, are presented. Multiple origins for sandstone thickness variation are recognised. Loading and injection are restricted to a relatively small scale and have only resulted in minor sandstone thickness variations. Infill of mass transport related topography acted on a much wider range of scales and resulted in thickness variations with an amplitude of up to ten meters. The elevated topography at the surface of the mass transport deposits is associated with compressional deformation zones within these deposits. The inference that the geometry a turbiditic sandstone is, to a large extend, controlled by the surface morphology of mass transport deposits is contrasting an earlier interpretation, in which the heterogeneity was attributed to the presence of multiple parallel turbidite channels. This reinterpretation yields that the planform geometry of the sandstone may be different than previously thought. Several types of field data that indicate directly that surface topography, created by mass transport deposits, indeed affected the flow dynamics of turbidity currents. A scour hole upstream of an obstruction indicates that erosion by turbidity currents was locally enhanced. Deviating flute directions on the flanks of obstructions indicates that flows were deflected.