Abstract

The Upper Sinemurian condensed sections of the edge of the Morvan (France) are known to have been part of an outer carbonate platform that was little affected by subsidence. In this paper, the facies distribution is arranged within a time of 14 horizons in order to identify and explain variations in sediment accommodation potential. The condensed sections are valuable inasmuch as they fit into a simplified model in which accommodation space is not modified by the invariably negligible sedimentary input and palaeobathymetric variations directly reflect relative sea level variations. Four facies associations correspond to lower offshore, and upper offshore environments. Facies stacking in each section is analysed and set against biostratigraphic data from three viewpoints. 1 - Facies stacking between two discontinuity surfaces, corresponding either to erosion or to non deposition, allows us to identify preserved sediment volumes, which have been dated (i.e. horizon identified) for 178 of the 189 levels in the 20 sections surveyed. In this narrowly constrained biostratigraphic context where decimetric beds can be correlated over large distances, each maximum biohorizon extension is considered to be a maximum transgression. A Wheeler diagram and a frequency variation curve of horizon occurrence bring out the transgressive-regressive variations. The transgressive maxima are dated to H2, H5, H10 and H13. They coincide with the transgressive maxima at the scale of northern Europe. 2 - Facies stacking between two deepness maxima encompassing a maximum of facies regression or minimum depth allows us to identify regressive-transgressive cycles or genetic units. By setting the genetic units against the horizon boundaries, it is possible to date the deepening maxima or to determine whether a biohorizon contains a deepening maximum locally. The percentage of sections where a given horizon contains a deepening maximum allows us to construct a frequency curve of maximum deepening against time. Comparison of the frequency curves of horizon occurrence and maximum deepening leads us to divide the late Sinemurian into two periods. The first (Obtusum and Oxynotum zones) is characterized by synchronism between deepening maxima and transgression maxima and therefore the absence of any local tectonic factor. The second (Raricostatum zone) is characterized by diachronism between the two maxima and attests to the occurrence of local tectonic movements (corresponding to the "Lotharingian crisis" of the literature) and the rejuvenation of differentially subsiding fault-bounded blocks. 3 - Finally, facies stacking of the overall condensed section of each site allows us to distinguish three types of sections, depending on the intensity of condensation and the amplitude of the sedimentary gaps. These correspond to three types of sedimentary preservation area related to reactivated block faulting of the platform with differential subsidence.

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