Epibionts, their hard-rock substrates, and phosphogenesis during the Cenomanian–Turonian boundary interval (Bohemian Cretaceous Basin, Czech Republic)

Abstract

Hard inorganic substrates (rock clasts and rocky bottom) belonging to the Kaňk Member of the Korycany Formation (upper Cenomanian–?lower Turonian) and to the Bı́lá Hora Formation (lower Turonian) have offered a good opportunity for the study of encrusting faunas of the Cenomanian–Turonian boundary interval. Distributional features of cemented epibionts and the recorded period of phosphogenesis enabled the differentiation of two-phases in the Kaňk Member conglomerate formation. During the first phase, the rock substrates were occupied by a so-called A-association of encrusters (29 species, with dominance of oysters and bryozoans). This fauna partly changed during the subsequent phosphogenic period and not only survived the following period of reworking and the second phase of conglomerate formation, but also persisted until the onset of calmer sedimentation in the early Turonian. This changed community is named the Atreta-Bdelloidinacommunity (23 species). The opportunistic agglutinated foraminifers Bdelloidina cribrosaand/or Acruliammina longalocally dominated the respective communities during several physico-chemically stressed episodes. On sloping substrates, the bivalves Atretaand Spondylusattached themselves in a preferred orientation (so-called slope orientation). This feature indicates the original position in which some mobile substrates were colonized. Other questions, such as the taphonomy of encrusters and the character of their substrates are also briefly discussed.