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.

KEYWORDS:

• Brachiopods
• Western Tethys
• taphofacies
• Jurassic
• Northern Calcareous Alps
• depositional events

Acknowledgments

We thank Herr Dieter Gschwend (Vorderhindelang), who showed FGJ the Zehrer site. The field campaign was carried out in the framework of an Alexander von Humboldt Foundation Scholarship at the University of Munich during 1987-88 for FGJ, which is gratefully acknowledged. FGJ also thanks Professor Sixto Fernández-López for his continuous teachings and suggestions on Taphonomy. Helpful comments by A. Dulai (MNHN Budapest), J. Esteve (UCM Madrid) and chief-Editor G. Dyke are also acknowledged. This research is a contribution to the Research Groups PBM-910431 (Complutense University of Madrid) and VIGROB-167 (University of Alicante) and to the Projects IGCP-710 Western Tethys meets Eastern Tethys, and to the PID2019-104625RB-I00 project.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Statements and Declarations

We declare that this manuscript is original, has not been published before and is not currently being considered for publication elsewhere. We know of no conflicts of interest associated with this publication, and there has been no significant financial support for this work that could have influenced its outcome.