EXCEPTIONAL TRACE FOSSIL PRESERVATION AND ENIGMATIC SUBSTRATE CONDITIONS IN THE CAMBRIAN OF THE GREAT BASIN, WESTERN USA

The majority of the Phanerozoic normal marine stratigraphic record is characterized by biogenically reworked sediments and complex ichnofabrics (e.g., Bromley and Ekdale, 1986). The Cambrian trace fossil record, in contrast, is renowned for its extraordinary quality of preservation and the anomalous capture of shallow-tier structures and bioglyphs by the stratigraphic record. We describe the ichnology of the Lower-Middle Cambrian Pioche Formation of the Great Basin, western USA, in order to characterize taphonomic and facies controls upon ichnofabric. Further, we use this succession to track the regional pace of mixed layer development.

The Pioche Formation and correlative strata are extensively exposed, stratigraphically well-constrained (Hintze and Robison, 1975; Palmer, 1971) and characterized by thinly (mm- to dm-scale) bedded mudstones, siltstones and very fine- to medium-grained sandstones. Mixing is not prevalent in the Pioche Formation; beds are largely populated by discrete trace fossils. Certain beds are characterized by dense assemblages of centimetric-scale burrows, cast in hyporelief. However, other beds are characterized by dense assemblages of millimetric-scale trace fossils, bioglyphs and physical sedimentary structures, preserved in both epi- and hyporelief. Both types of trace assemblage are crisply preserved and associated with simple, non-disruptive ichnofabrics, indicating that, in spite of significant increases in infaunal motility, sediment mixing and thus ichnofabric were still poorly developed. Using this approach, it is clear that the onset of significant sediment mixing was a protracted process and that the taphonomic window for delicate preservation of shallow-tier biogenic and physical sedimentary structures was still open as late as the Middle Cambrian. The anactualistic lack of complex ichnofabrics and rarity of even simple ichnofabrics in the Pioche Formation and similarly-aged strata may hold important implications for contemporaneous taphonomic and geochemical phenomena accompanying the radiation of early metazoan ecosystems. Detailed study of the trace fossil record, by providing a semi-quantitative log of substrate conditions, may give us our most direct means of testing and resolving competing models for Paleozoic substrate evolution.