The cretaceous Urgonian Carbonate platform of the NW subalpine chains of SE France: a sequence stratigraphic approach

Abstract

This study interprets the stratigraphic evolution of the mid-Cretaceous Urgonian Carbonate Platform sensu lato exposed in the northwestern Subalpine Chains of southeast France within a framework of dynamic environmental and climatic changes including relative sea-level fluctuations. Macroscopic observations of stratal geometries, especially stratal packaging within the succession revealed by differential weathering patterns have been integrated with the petrographic examination of limestone samples to assist in the interpretation of the carbonate succession. Seven hundred and twenty-eight rock samples were examined under the microscope as either thin sections or acetate peels and twenty five component parameters were used to define sediment composition within the data set. New and emerging analytical techniques have been used to interpret these data rather than the more traditional static fixed facies belts. Sequence stratigraphic concepts have been used to interpret platform evolution within the context of relative sea-level changes. In order to define microfacies and identify underlying environmental gradients computer optimized Jaccard's similarity coefficients matrices have been calculated between samples and component parameters using two computer programs SedUtil and JaccMat run on an IBM compatible Pentium personal computer. Computer optimized Jaccard's similarity coefficients similarity matrices are able to model both gradational and abrupt changes in facies composition in response to palaeogeographic and stratigraphic controls. Computer optimized similarity matrices between limestone compositional parameters from a series of logs show differences in the spatial and temporal nature of variations in sediment composition in response to environmental changes across the platform. The contrasts between the configurations of optimized similarity matrices are interpreted as indicating a palaeogeography at the beginning of the Urgonian characterised by a more strongly subsident basin to the southeast of the field area connected to the topographically-higher, tectonically more stable parts of the platform by low-angle slopes; the basin was eliminated by the end of the Urgonian. In addition optimized similarity matrices calculated between samples have been used to assist in defining eight-eight conventional microfacies assemblages which are described and illustrated using photomicrographs and pie-diagrams of modal composition. Despite the apparently conformable nature of the succession as observed in individual exposures, changes in the stratigraphy of the Urgonian platform sensu lata as traced along two southwest-northeast transects constructed through the Aravis and Bargy Chains indicate the existence of a low-angle distally steepened ramp topography at the top of the Hauterivian basement that sloped into a more strongly subsident basin. The Urgonian succession is divided into two third-order depositional sequences termed Depositional Sequence 1 and 2. In Depositional Sequence 1 all three systems tract are developed whereas Depositional Sequence 2 only contains transgressive and highstand systems tracts. The lowstand systems tract of Depositional Sequence 1 preserved in the southwest of the Aravis and Bargy Subalpine Chains is volumetrically significant, being dominated by hemipelagic deposits that shallow laterally to intertidal deposits to the northeast and also contain calciturbidites and two spectacular megabrecciahorizons. The two megabreccia horizons are interpreted as thin surficial collapse horizons formed by net extension on a very-low angle, previously kinematically stable slope, during fourth-fifth order relative sea-level falls. The lower sequence boundary of Depositional Sequence 1 passes laterally from being conformable in the southwest (deep-water basin) to being a major unconformity in the northeast ('inner' platform). However, this sequence boundary appears planar and concordantly bedded in the main and it is only in the northeastern Bargy Chain that it is observed as a erosional angular unconformity. The top of Depositional Sequence 1 is marked by the end of rudist lagoonal sediments and the deposition of beach facies which contain evidence of grain dissolution, overlain by Orbitolinidae foraminiferal rich horizons. By the top of Depositional Sequence 1 the low angle slope to the southwest had been totally subdued by sediment infilling and/or a decrease in the rate of differential subsidence. Depositional Sequence 1 was terminated by a comparatively minor relative sea-level fall followed by an immediate return to a major third-order relative sea-level rise and the basal transgressive deposits of Depositional 2. Depositional Sequence 2 re-establishes rudist sediment lagoonal deposition on the platform. It is often difficult to differentiate the transgressive and highstand systems tracts from one another in Depositional Sequence 2 and the boundary may be transitional. The upper sequence boundary of Depositional Sequence 2 is marked by a subaerial karst overlain by phosphatic greensands interpreted as reflecting a major relative sea-level fall and subsequent reflooding. Factors unique to carbonate depositional systems and their implications for the use of 'traditional' sequence stratigraphic models for carbonates are also discussed.