FOSSIL ASSEMBLAGES IN LOWER CAMBRIAN MICROBIAL MATGROUND SEDIMENTS AND IMPLICATIONS FOR EARLY BENTHIC AND INFAUNAL ECOLOGIES

Bacterial mats were likely found in a range of subtidal shelf paleoenvironments during the Proterozoic and Early Cambrian, as demonstrated by microbially-mediated sedimentary structures (e.g., wrinkle structures), but were subsequently constrained to intertidal and restricted facies during the remaining Phanerozoic. Metazoans preserved on Early Cambrian wrinkle structure bedding planes offer a glimpse into interactions between early animals and a rapidly disappearing Precambrian-style mat-covered seafloor. In this study, wrinkle structures from the Lower Cambrian Poleta and Harkless Formations were examined at the outcrop, hand sample, and thin section scale for evidence of metazoan activity.

Inarticulated brachiopods (preserved as molds) were preferentially associated with wrinkled surfaces. Agglutinated mm-scale conical fossils (possibly Volborthella) were also preferentially preserved on and beneath wrinkle covered bedding planes. Assemblages of trace fossils on wrinkle covered surfaces, and on the laminated sediments beneath, appear to be limited to bedding-parallel traces such as Planolites. However, vertically-oriented traces (e.g., Skolithos) occur in other parts of the Poleta and Harkless Formations, cross cutting massively bedded sediments rather than the thin laminae found beneath wrinkle structures. The absence of vertical burrowing in wrinkle-structure covered sediments and the distinctive fauna associated with these sediments may have resulted from anoxic conditions that are characteristic of sediments beneath microbial mats.

Anaerobic or possibly euxinic pore-waters may have permeated subsurface ecospace beneath the microbial mats with a variety of possible consequences for metazoan habitation. Some organisms may have been excluded from infaunal activity, while others may have been able to take advantage of redox conditions in the geochemically-stratified zones just beneath the sediment-water interface. For example, the construction of Volborthella from pyrite may indicate that the organism responsible for these agglutinated structures thrived in euxinic waters where sulfide was actively forming. These observations suggest that Cambrian matground substrates may have been important, yet temporally fleeting, habitats for protists and early metazoans.