Facies distribution and deposit preservation in relation to accommodation potential within a high-resolution framework. The example of condensed sections from the late Sinemurian Burgundy Spur (France)

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-frame of 14 horizons in order to identify and explain variati ons in sediment accommodation potential. The condensed sections are valuabl e inasmuch as they fit into a simplified model in which accommodation space is not modified by the invariably negligible sedimentary input and palaeob athymetric variations directly reflect relative sea level variations. Four facies associations correspond to lower offshore, and upper offshore enviro nments. Facies stacking in each section is analysed and set against biostra tigraphic data from three viewpoints. 1 - Facies stacking between two discontinuity surfaces, corresponding eithe r 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 18 9 levels in the 20 sections surveyed. In this narrowly constrained biostrat igraphic context where decimetric beds can be correlated over large distanc es, each maximum biohorizon extension is considered to be a maximum transgr ession. A Wheeler diagram and a frequency variation curve of horizon occurr ence bring out the transgressive-regressive variations. The transgressive m axima are dated to H2, H5, H10 and H13. They coincide with the transgressiv e maxima at the scale of northern Europe. 2 - Facies stacking between two deepness maxima encompassing a maximum of f acies regression or minimum depth allows us to identify regressive-transgre ssive cycles or generic units. By setting the genetic units against the hor izon boundaries, it is possible to date the deepening maxima or to determin e whether a biohorizon contains a deepening maximum locally. The percentage of sections where a given horizon contains a deepening maximum allows us t o 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 O xynotum zones) is characterized by synchronism between deepening maxima and transgression maxima and therefore the absence of any local tectonic facto r. The second (Raricostatum zone) is characterized by diachronism between t he two maxima and attests to the occurrence of local tectonic movements (co rresponding to the "Lotharingian crisis" of the literature) and the rejuven ation 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 intensit y of condensation and the amplitude of the sedimentary gaps. These correspo nd to three types of sedimentary preservation area related to reactivated b lock faulting of thr platform with differential subsidence.