Origin of Concretion-Hosted Shell Clusters in the Late Cretaceous Bearpaw Formation, Southern Alberta, Canada

Abstract

The widespread phenomenon of faunal clustering in concretions is examined in examples from the Late Cretaceous Bearpaw Formation of southern Alberta. Concretion-hosted shell clusters have traditionally been interpreted as biologically formed features. In contrast, sedimentologic, taphonomic and faunal evidence in the Bearpaw Formation suggests that shell clustering is inherently related to a common physical control. The clustered nature of the Bearpaw assemblages reflects the accumulation of shells in isolated storm scours. Differences in faunal composition resulted from variations in (1) the mode of scour initiation, (2) the faunal content of sea-floor sediments, and (3) the degree of shell transport experienced during storms. The formation of the isolated scours is attributed to locally augmented erosion in the vicinity of bed defects and objects exposed on the sea floor. The common restriction of fossils to concretions reflects preferred concretion growth around the shell-rich scour fills. Previously described examples of concretion-hosted shell clusters are re-interpreted in light of the scour-fill hypothesis.