Compositional and taphonomic gradients show fluctuating depositional conditions on Middle Jurassic brachiopod-rich accumulations from Northern Calcareous Alps (Austria–Germany)

Abstract

Brachiopod-rich accumulations of the well-known ‘VilserKalk’ calcareous units from the Middle Jurassic of Zehrer (Allgäuer Alps) have been recognised and analysed from a taphonomic standpoint. These brachiopod concentrations are frequent in the Jurassic deposits of various Alpine-Mediterranean localities, usually related to resedimentation processes generated by high-hydrodynamic episodes. In the four brachiopod-bearing accumulations recorded, high concentrations of mainly articulate shells are preserved in pocket-like deposits made up of brachiopod communities with quite different structure, size-frequency distribution, diversity and taxonomic composition. The analysed taphocharacters indicate different hydrodynamic regimes of diverse intensity for each accumulation, resulting in a gradation of the resedimentation processes. The inferred transport of the shells from the source area is minimal, the bioclastic levels constituting para-autochthonous assemblages without significant spatial averaging. The four defined taphofacies include different assemblages that chronostratigraphycally span the latest Toarcian-early Bajocian interval. These taphofacies are interpreted in terms of depth-related variations concurring with environmental disturbance events in several phases of the evolutionary history of the basin, progressively ranging from shallower proximal accumulations to shell-beds originated in more distal depositional environments.