Floodplain processes in the shaping of fossil bone assemblages: an example from the Late Miocene, Bahe Formation, Lantian, China

Abstract

Vertebrate remains are frequently preserved and recovered from floodplain deposits. The composition of such fossil assemblages is expected to vary considerably with the processes active during and after deposition. In this paper, vertebrate fossil assemblages from three separate localities (Late Miocene, Bahe Formation, Lantian County, Shaanxi, China) were studied to assess how fluvial processes may have influenced the shaping of these. Although, the three localities represent floodplain deposits, all were formed in different fluvial subfacies. The localities were logged in detail and the degree of surface exposure of the collected specimens was determined through analysis of their stage of weathering. Combining these data, the following interpretation was made: Locality 31 represents a crevasse splay, an avulsion that instantaneously buried a group of animals. Locality 6 was formed on an overbank area during repeated, unchannelised flooding. This assemblage represents an attritional accumulation, ranging from fresh to reworked remains, accumulated during several events representing a long time period. Locality 42 represents a bone accumulation laid down in a topographic depression. The bulk of this assemblage is likely to represent remains accumulated over a very short time period, along with some reworked remains. The assemblages analysed were all formed in non-exceptional floodplain subfacies, representing a wide range of time periods ranging from instantaneous catastrophic events to long periods of time. This work provides a context for the these remains and contributes to the understanding of the events that gave rise to vertebrate fossil assemblages in the Bahe Formation and fossil accumulations in floodplain environments in general.