High-resolution Analysis of the Ichnological Diversity, Depositional Dynamics, and Stratigraphic Architecture of the Middle Triassic Sunset Prairie Formation

Abstract

The Sunset Prairie Formation is a newly established lithostratigraphic formation within the Middle Triassic (Anisian) of western Canada. The interval had commonly been overlooked or placed within the underlying Montney Formation or overlying Doig Formation, or was considered to be partially within the two formations. However, the Sunset Prairie Formation is sedimentologically, ichnologically and palaeontologically distinct from the overlying and underlying formations. The Sunset Prairie Formation is an unconformity-bound unit, characterized by interbedded pervasively bioturbated and diminutively bioturbated siltstone, fine-grained sandstone, phosphatic sandstone with locally abundant bivalves, gastropods, lingulid brachiopods, spiriferid brachiopods, terebratulid brachiopods, echinoid skeletal elements, and crinoid ossicles. Trace fossils present within the interval, listed from most abundant to least abundant, include Phycosiphon, Rosselia, Cylindrichnus, Teichichnus, Asterosoma, Scolicia, Helminthopsis, Palaeophycus, Chondrites, Planolites, Diplocraterion, Rhizocorallium, Thalassinoides, Skolithos and Zoophycos. Seven facies are identified in the formation and record deposition within the offshore, offshore transition and lower shoreface of a sheltered coastal setting. Facies stacking patterns exhibit up to three shoaling-upward parasequences. These parasequence exhibit a back-stepping or retrogradational stacking pattern, and are assigned to a transgressive systems tract. Structural features associated with the Western Canada Sedimentary Basin greatly influenced the deposition and preservation of the formation.The Sunset Prairie Formation is the first stratigraphic occurrence of a pervasively bioturbated interval that is regionally extensive following the end-Permian mass extinction in Western Canada. The unit provides a unique dataset to investigate physico-chemical stresses influencing the distribution of trace fossils, and thereby infaunal biota, within a marine realm during post-extinction faunal recovery. Through the comparison of process ichnology data (bioturbation intensity, burrow diameter, ichnodiversity, and Size Diversity Index) and geochemical proxies for redox-sensitive elements (Mo, V, and V/Cr) collected at a high-resolution (10 cm intervals), vacillating redox conditions are interpreted to have occurred during the deposition of the Sunset Prairie Formation. The recognition and investigation of the Sunset Prairie Formation provides new sedimentological, ichnological, palaeontological and geochemical evidence to unravel the palaeoenvironmental and palaeoecological transition from the Lower Triassic to the Middle Triassic within Western Canada.