ICHNOLOGY AND DEPOSITIONAL ENVIRONMENTS OF THE UPPER ORDOVICIAN STONY MOUNTAIN FORMATION IN THE WILLISTON BASIN, CANADA: REFINING ICHNOFACIES AND ICHNOFABRIC MODELS FOR EPEIRIC SEA CARBONATES

Epeiric sea carbonates in intracratonic basins of the Ordovician Laurentia are enigmatic due to their unique depositional settings and the absence of modern analogs. The origin of common carbonate-evaporite cycles in the Williston Basin is still under debate; some studies termed them “shallowing-upward” sequences, others named them “brining-upward” sequences. Since existing models of epeiric seas cannot adequately explain facies variation in the Upper Ordovician Stony Mountain Formation, reappraisal of the models is crucial, and multiple approaches are essential to better understand depositional settings. Six subenvironments, neritic marine, nearshore marine, open lagoon, restricted lagoon, peritidal bar and peritidal flat, are interpreted along the depositional profile. The Cruziana ichnofacies occurs in storm beds and fair-weather deposits formed in neritic (between fair-weather and storm wave bases) and nearshore (around fair-weather wave base) marine environments. The depauperate Cruzianaichnofacies in open and restricted lagoon environments indicates a shift from fully marine to stressed conditions. In the open lagoon, composite ichnofabrics related to omission surfaces illustrate low net-sedimentation rate interrupted by episodic deposition and early cementation. The decrease in burrow size in the restricted lagoon is attributed to a slightly reduced oxygenation under stagnation and probable salinity dilution. The peritidal complex includes high energy sand bars and low energy tidal flats. Sporadic bioturbation characterizes these environments. Whereas rapid sedimentation in tidal bar settings may have prevented bioturbation, the hostile condition of periodically exposed tidal flats may have also been detrimental for organisms. Monospecific colonization took place only in subtidal environments during transgressive events. Overall, ichnofacies analysis records changes from open to restricted conditions along the epeiric ramp, whereas ichnofabrics yield insights into depositional dynamics within subenvironments. The utility of ichnology to enhance a refined interpretation of depositional environments is illustrated for the Stony Mountain Formation. These results are expected to be applied to other ancient Paleozoic epeiric sea carbonates.