BIOEROSION OF OYSTERS IN THE TYPE CAMPANIAN (UPPER CRETACEOUS) OF SOUTHWESTERN FRANCE

Henri Coquand established the Campanian Stage of the Upper Cretaceous in 1857. His type area was a series of fossiliferous marly carbonates in the Charente and Charente Maritime departments of southwestern France. Shell beds of the cosmopolitan gryphaeid oyster Pycnodonte vesiculare (Lamarck, 1806) are very common in these rocks. These oysters lived primarily on soft sediments, creating islands of hard carbonate surfaces for the settling larvae of sclerobionts, both encrusters and borers. We collected several hundred specimens of P. vesiculare from outcrops in southwestern France of the Biron, Barbezieux, and Aubeterre Formations (in ascending order) and then identified the encrusters and borings on and in these shells. The encrusters (bryozoans, foraminiferans, polychaete worms, bivalves and calcareous sponges) are covered in a separate study. Here we describe the bioerosion ichnofauna: sponge borings (Entobia), worm borings (Maeandropolydora and Caulostrepsis). barnacle borings (Rogerella), phoronid borings (Talpina), predatory borings (Oichnus and Belichnus), and grazing traces (Gnathichnus and Radulichnus). The Biron, Barbezieux, and Aubeterre Formations are a shallowing upwards shelf sequence of chalky marls, all with common P. vesiculare. The bioerosion trace fossils show a distinct relationship to these depositional facies. The deeper-water Biron Formation oysters have a low-diversity ichnofauna consisting mostly of Entobia with relatively large chambers. The shallower-water Barbezieux Formation oysters have more diverse and abundant borings, including an Entobia with small chambers, Maeandropolydora, Gnathichnus, and Rogerella. The shallowest-water Aubeterre Formation has all the above ichnogenera plus Caulostrepsis, Talpina, Belichnus, and Oichnus. The bioerosion ichnodiversity increases stratigraphically upwards with the shallowing paleoenvironments. These trace fossils show a transition from the Entobia ichnofacies in the Biron Formation upwards to the Gnathichnus ichnofacies in the Barbezieux and Aubeterre Formations. This study shows that bioerosion trace fossils are useful for determining relative paleobathymetry of deposits, and that the marine bioerosion ichnofacies have predictive value. This is also the first bioerosion study of the Type Campanian.