Refining the Depositional Environments of Epeiric Sea Carbonates Through Applying Ichnologic Toolbox: A Case Study of the Upper Ordovician Stony Mountain Formation in the Williston Basin, Canada

Abstract

Refining the Depositional Environments of Epeiric Sea Carbonates through Applying Ichnologic Toolbox: A Case Study of the Upper Ordovician Stony Mountain Formation in the Williston Basin, Canada

Charlie Y.C. Zheng^a,b, M. Gabriela Mángano^b, Luis A. Buatois^b

^a Department of Geological Sciences, Jackson School of Geosciences

^b Department of Geological Sciences, University of Saskatchewan

Epeiric sea carbonates in deep-geological time remain enigmatic due to their unique depositional settings and the absence of modern analogs. Whereas previous deposition environmental interpretations relied on the body-fossil record, the excellent trace fossil preservation in the Upper Ordovician Stony Mountain Formation in the Williston Basin makes this unit ideal for testing the utility of ichnology in the Ordovician epeiric sea carbonates. Integrated ichnologic and sedimentologic analysis allows recognition of six subenvironments in the formation along the depositional profile, including neritic marine, nearshore marine, open-lagoon, restricted-lagoon, peritidal sand-shoal and peritidal-flat subenvironments. The marine (neritic and nearshore) environments are dominated by the Cruziana ichnofacies, whereas the depauperate Cruziana ichnofacies characterizes the lagoonal environments, indicating a departure from fully marine conditions. The open lagoon is characterized by the Glossifungites and Trypanites ichnofacies in firmground and hardground, respectively, suggesting rapid early cementation. In the restricted lagoon, the decreased size of discrete burrows implies the stagnated water circulation that typifies the restricted condition. In the most proximal deposits, bioturbation is absent. Only monospecific colonization is recorded during subtidal conditions. Furthermore, ichnofabric characterization yields insight into depositional dynamics within subenvironments. Alternative preservation of the dominated deep-tier infauna and shallow-tier burrows is evidence of episodic sedimentation in the marine environments. The occurrence of shallow tier burrows allows delineating neritic (between fair-weather and storm wave bases) from nearshore (around fair-weather wave base) marine subenvironments based on the degree of erosion in the subenvironments. In the open lagoon, the composite ichnofabrics related to omission surfaces illustrate continuous background sedimentation and early cementation interrupted by storm events. In the restricted lagoon, intercalation of two types of ichnofabrics illustrates the alternation of contrasting environmental conditions, restricted, stressful fair-weather condition and refreshing, higher energy events. It is

expected that this study would encourage further ichnologic examination of other ancient Paleozoic epeiric sea carbonates.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019