Saint-Antonin Conglomerate in the Maritime Alps: A Model for Coarse Sedimentation on a Submarine Slope

Abstract

The Upper Eocene to Lower Oligocene Saint-Antonin Conglomerate, a formation more than 1000 m thick well exposed in the French Maritime Alps, about 30 km north of the Mediterranean coast, comprises coarsening-upward successions, or megasequences, of silty shale-siltstone, sandstone and conglomerate sections. The megasequences include coarse channelized deposits associated with coarse lenticular and fine-grained sheet facies that are identified as migrating channels and lobe and channel overflow deposits. Microfossils in the finer-grained units indicate dispersal in an open marine, outer shelf to upper bathyal environment where minimal depths ranged from 100 to 200 m. The spatial and temporal distribution patterns of facies successions, assemblage of stratification types and sedimentary structures, and petrology of the various textural grades indicate submarine progradation on a slope, or in a slope basin, seaward of a fan delta system. The Saint-Antonin Conglomerate is more similar to alluvial fans than to some of the gravel-rich submarine fan deposits that accumulate on a gentle gradient at the base of a slope. The coarsening-upward megasequences record a strong tectonic overprint, including a northward shift of the basin margin on which these strata were deposited, concurrent andesitic flows and structurally-induced fan delta switching on the adjacent land. This latter phenomenon was largely responsible for the irregular back-and-forth migration of the sandstone and gravel-rich tongues on the upper slope. Emplacement of poorly sorted (disorganized) conglomerates and pebbly sandstones, and of strata displaying crudely stratified inverse grading or preferred clast fabric, was largely by debris flow and associated high-concentration dispersions. Slumping, turbulent flows with some bed-load traction and turbidity currents also were effective mechanisms for the transport of sediment to proximal depositional sites on the slope. Modern counterparts of the Saint-Antonin Conglomerate are probably to be found on the leading edge of plates, rift margins and other tectonically-active coastal chain-bounded margins where coarse terrigenous sediments bypass narrow shelves and are transported directly on steep mobile slopes.