Paleoecology of the burgess shale formation at The Monarch, Southern Rocky Mountains, Canada

Abstract

The Middle Cambrian Burgess Shale Formation 1S a unique deposit hosting a remarkable diversity of soft-bodied and skeletal organisms. This unit onlaps the shallow-water dolomites of the Cathedral Formation which have been cut by a near vertical truncation surface known as the Cathedral Escarpment, a feature continuous for nearly 100 km. An abundant and diverse fauna is known from the classic quarries on Fossil Ridge between Mount Field and Mount Wapta, but other fossiliferous localities are present along the strike of the Cathedral Escarpment. In order to understand the ecological and paleontological basis for lateral variations in faunal composition within the Burgess Shale, exposures 60 km to the southeast of the type area were examined at The Monarch, a mountain straddling the British Columbia/Alberta border. In four closely spaced sections, faunal elements are present only in the basal 30 m of the silty shales immediately adjacent to the Cathedral Escarpment; fossil occurrence decreases dramatically barely tens of meters away. The fauna is dominated by priapulid worms, hyolithids, and phosphatic and calcareous brachiopods, which together constitute more than 75% of specimens counted. Trilobites (mainly Olenoides serratus), sponges (mainly Vauxia spp.) and phosphatic tubes (Byronia annulata) are also significant components of the fauna. The fauna at The Monarch is much less diverse than that of the classic Burgess Shale on Fossil Ridge. A prominent difference is the absence of non-trilobite arthropods (e.g. Marrella splendens and Anomalocaris canadensis) and most soft-bodied elements. While all four sections at The Monarch are equally fossiliferous, differences between localities are evident in the distribution of the rarer biotic elements. Specifically, the arthropod Sidneyia inexpectans, the sponge Takakkawia sp. nov., and the probable lepadomorph barnacle Priscansermarinus barnetti occur, albeit rarely, only on the southern exposures. Fault-related fluid seeps along the base of the Cathedral Escarpment are a possible explanation for the localized biota. Sporadic and variably active seeps could have generated localized and intermittent increases in biologic productivity, especially bacteria, which in tum would have resulted in patchy concentrations of fauna close to the escarpment, a pattern consistent with the contrast in assemblages between The Monarch and the type area.