Sedimentology of the chalk of coastal Haute Normandie, France

Abstract

This project presents an integrated study of the lithostratigraphy and sedimentology of the Haute Normandie chalks (Upper Cenomanian to Santonian), northern France. To enable a detailed lateral correlation across the region a local lithostratigraphic framework is proposed in which the succession is divided up into five basic Formations. Members and marker horizons are also identified. Comprehensive field studies and sedimentary analyses shows the existence of twelve facies. Facies are defined on sediment textures, biogenic composition, resedimented textures, terrigenous content and degree of early cementation. Analysis of spectacular discordant bedding structures, previously interpreted as accretionary "banks", indicates that they are formed from erosion and infill of channels and scours on the sea-floor. Channel and scour structures usually have concave-up basal truncation surfaces with a sedimentary infill of complex fining-upward facies sequences.These structures are compared with erosional furrows, scours and channels of modern continental shelves. The development of channels and scours in the Haute Normandie chalks is attributed to the convergence of oceanic and tidal currents flowing from the proto Atlantic into the epicontinental sea of northwest Europe. A re-examination of shallow seismic refection profiles indicates that the distribution pattern of channels and scours is related to the local influence of tectonic highs within the shallowing chalk sea. Evidence from gravity-flow deposits and turbidites infilling some channel structures may reflect contemporaneous tectonic activity.Dolomitized and dedolomitized chalks occur in the basal horizons of channel and scour structures. Detailed petrographic analysis supplemented with isotope data suggests that dolomitization took place early in the burial history, resulting from a hydrological regime of mixed meteoric and marine ground waters. Late diagenetic dedolomitization is also evident, resulting from the recrystallization of original dolomite crystals within a near surface weathering environment.