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Sedimentation Mechanisms Approach of Calcareous Gravity Deposits; Lithologic Prevision and Computer Analysis

Bernard Offroy

The analysis of the exceptional calcareous gravity deposits of the North Pyrenean Upper Cretaceous flysch (France) has led to recognition of a genetic longitudinal sequence composed of slump (and/or debris flow or mud flow) with only a poor differentiation at the top (arenite); slump (and/or debris-flow or mud-flow) with developed arenitic and lutitic turbidite; and arenitic and lutitic turbidite.

The variable organization of colored Paleozoic fragments gives evidence of the hydrodynamic conditions of transport (sorting, winnowing, concentrations). This organization infers that the upper deposed deposits (turbidite) are created by differentiation of the original (evolutive) mass flow by incorporation of ambient water from base and top.

The general term of "megaturbidite" currently used to define such deposits seems to be ambiguous because only the graded arenites and lutites are of turbiditic origin (hydrodynamic acception).

Computer treatments and their application to similar deposits of other basins show that: this mechanism occurs for deposits that are less than 1 m to more than 100 m thick; thickness of the lutitic deposit (relative to the total turbidite thickness) is a recognition criteria of gravity-flow maturity. At a section scale, it is a tool characterizing deposition environments and their evolutions; some exceptional events depict either maxima of cyclic megasequences or out-system events. Such key beds define very sharp isochrons and, in petroleum geology, allow good subsurface correlations.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.