Abstract

The genesis of quartz-arenite sand bodies preserved above unconformities has recently been brought into question. Until now their interpretation has been regarded as simply transgressive, without a full appreciation of the variability of their fades associations and their environments of deposition. The lower Trias of the Western Alps, France, provides an excellent opportunity to document the origins of this type of sand body. The succession unconformably overlies Carboniferous/Permian cover rocks and Hercynian basement massifs. The succession comprises a laterally extensive quartz-arenite member of conglomerate and sandstone, trasitionally overlain by a red bed member dominated by siltstone and mudstone. The lower part of the quartz-arenite member developed in response to a marine transgression involving the landward migration of beach, ephemeral inlet, and shoreface environments with the eventual stabilization of a tidal shelf in areas between the basement massifs. The upper part of the quartz-arenite member and overlying red beds reflects deposition of prograding mesotidal barrier islands backed by extensive tidal flats.

Additional complexity is introduced by topography on the unconformity in the vicinity of basement massifs. Where the quartz-arenite oversteps the basement, thicknesses are drastically reduced and certain facies associations are no longer preserved. The vertical and lateral facies composition of this example confirms the complex origin of quartz-arenite sand bodies developed above unconformities.