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ABSTRACT: Lowstand Wedge Deposits: Comparison of Examples from the Quaternary Rhone Shelf and the Cretaceous Viking Formation (Alberta)

M. Tesson, B. Gensous, G. P. Allen, H. W. Posamentier, C. Ravenne

Shallow seismic profiling on the Rhone continental shelf (southeastern France) has shown the existence of stacked prograding deltaic wedges on the outer shelf forming a complex up to several hundred meters thick. The stacked wedges onlap a mid-shelf unconformity and are interpreted as individual lowstand wedges deposited during successive Quaternary glacio-eustatic lowstands. At the shelf/slope break, the wedges are sheared off by slumping, furnishing large volumes of sediment to the slope and basin during lowstands.

Internally, the wedges are characterized by prograding clinoforms and are bounded at the top by a toplap surface that steps systematically downward in a seaward direction. This stepwise lowering of the upper surface of each wedge suggests successive lowering of relative sea level rather than a gradual continuous fall associated with each wedge. Each step or lowering of relative sea level results in erosion and truncation of the upper surface of the immediately preceding step (i.e., an unconformity). Consequently, this successive lowering of relative sea level yields a series of abrupt basinward shifts of the shoreline and is associated with sedimentary bypass of the shelf. This seaward shift of the shoreline occurring in response to relative sea level fall is referred to as a forced r gression.

The Lower Cretaceous (Albian) Viking Formation is characterized by similar geometries. Interpreted relative sea level fall has resulted in a succession of forced regressions. Similar to the Rhone shelf lowstand wedges, these lowstand shoreface deposits are basinward of and isolated from the immediately preceding shoreface deposits. Interpretation of these deposits as lowstand shorelines rather than offshore bars impacts hydrocarbon exploration strategies with regard to anticipated internal facies architecture.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990