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Abstract: Facies Model and Sequence Stratigraphy of an Incised Valley Fill, Turonian-Coniacian Boundary, South-East France

Fabrice Malartre, Serge Ferry

In the middle Rhone valley (so-called "rhodanian Provence"), the Turonian-Coniacian boundary is marked by a great downwardshift of facies. Indeed, field observations and biostratigraphic correlations with basinal sections lead to the conclusion that uppermost Turonian Sands rest on weathered sandstones and fill incised valley. This structure is composite; there are two erosional surfaces between which a thin transgressive cycle is spreading out. The first infilling lies on weathered shoreface Sandstones (Boncavail sandstones) and is composed of beach gravel deposits, Sandstones with beach lamination and marly limestones to silty marls capped by a weathered level The second infilling rests on the weathered marls and comprises an assemblage of deposits ranging from non-mari e through estuarine to open marine: braided-stream sands, coastal plain sandstones (with significant tidal evidences), mudstones and finally glauconitic sandstones which are the more open-marine environment. So it represents a classical and well-known case of composite incised valley. In terms of sea level variations, incised valleys form and fill in two phases. The first phase consists of erosion, sediment bypass through the eroded valleys. The second phase consists of deposition within the valleys in response to a relative rise in sea-level, generally during the late lowstand or transgressive systems tracts. Correlations with seaward deposits located close to the shelf break or in the basin, in the Vocontian Trough, show that both erosional surfaces (with evidences of emergences) and t e two infilling stages may have occurred during the third order transgressive system tract at the Turonian-Coniacian boundary. In fact, it represents high frequency sea level oscillations enhanced during the third order rise in relative sea level and not third order sequence boundaries as it looks like. Here, the base level falls responsible of sequence boundaries, Exxon-sense, at the end of third-order progradational phases do not occur as single events. They often occur as sets of successive short time events (a few paracycles, not to use the term of parasequence). More disturbing, these events are coincident with the deposition of third-order transgressive systems tracts, as defined in slope to basin carbonates.These observations also suggest that the simplistic assumption of decrease amplitude of sea level variations with increase of the cycle frequency is probably not valid throughout geologic time. We believe that high-frequency cycles could have episodically high amplitude similar to Quaternary glacial fluctuations or that tectonic hinge point moved very rapidly across the shelf with a very rapid uplift rate of isostatic origin. Consequently, high order true depositional sequences can be developed within whatever third-order system tract and can modified the stacking pattern of third-order cycles. This suggestion result from different field studies of carbonate and siliciclastic depositional systems.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France