Stratigraphy, Taphonomy and Paleoecology of the Upper Cretaceous Bearpaw Formation in Southern Alberta

Abstract

<p>The Upper Cretaceous Bearpaw Formation of southern Alberta was deposited in a western embayment of the Western Interior Seaway of North America. The formation records nearshore deposition in wave-dominated to fluvial-influenced settings and offshore deposition in storm-affected, shallow to deep basinal settings. Temporal variations in lithofacies are linked to third-order sea-level fluctuations, superimposed on the second-order, transgressive-regressive Bearpaw cycle.</p> <p>Faunal composition of benthic faunal assemblages was largely dictated by benthic oxygen levels, and substrate characteristics. Deep-infaunal, suspension-feeding benthos were restricted to well-oxygenated, sand-dominated shoreface and shallow-offshore environments. Dysaerobic, mud-dominated offshore environments hosted a scattered background fauna of deposit-feeding bivalves, large epifaunal suspension-feeding bivalves, and decapod crustaceans. Short-lived episodes of improved oxygenation permitted the periodic establishment of opportunistic benthic communities. These "event communities" were generally typified by dense. monospecific aggregates of epifaunal and shallow infaunal suspension-feeding taxa, although more diverse assemblages containing deposit-feeders and carnivores/scavengers were occasionally established.</p> <p>Morphofunctional analyses, and vertical and spatial distribution of ammonoid shells indicate that the Bearpaw ammonoids had diverse habitats. Most significantly, maximum habitat depths, inferred from septal strength estimates reveal a distinct depth zonation of ammonoid taxa. Adult Jeletzkytes, Hoploscaphites and Baculites ovatus were demersal, chiefly confined to the lower water column. Adults of the Baculites compressus group, and to a lesser extent, Rhaboceras lived at mid-water depths. Placenticeras is interpreted to have been nektic, living in the uppermost levels of the water column. Juvenile scaphitids and baculitids were strictly demersal and were particularly sensitive to oxygen fluctuations near the sea floor. Anomalously low δ¹⁸O signatures in Placenticeras nacre provides evidence of brackish conditions in the upper water column. Oxygen depletion in the lower to middle water column produced variations in the faunal composition of ammonite assemblages and differences in the spatial and temporal distribution of individual taxa.</p> <p>The origin of shell-rich concretions in offshore facies is linked to preferred calcium carbonate precipitation around shell-filled, storm-related scours. Local scours were formed due to augmented sea floor erosion around topographic irregularities, such as benthic community clusters. Variations in the faunal composition and biostratinomic characteristics of the shell-rich scour fills reflect differences in storm energy and modes of scour initiation. Similarly, exceptional preservation of the lobster Palaeonephrops is linked to preferential phosphate precipitation in the vicinity of phosphate-bearing decapod cuticle. Rapid burial of living lobsters was facilitated by the collapse of their unlined burrows, whereas the rapid burial of moults was restricted to periods of volcanic ashfall.</p>