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The Sunset Prairie Formation: Designation of a New Middle Triassic Formation between the Lower Triassic Montney Formation and Middle Triassic Doig Formation in the Western Canada Sedimentary Basin, Northeast British Columbia
Revised lithostratigraphic correlations reveal that the current stratigraphic nomenclature for the Middle Triassic within the subsurface of the Western Canada Sedimentary Basin needs modification. This paper introduces a new formation, the Sunset Prairie Formation, for an interval of interbedded light grey, pervasively bioturbated sandstone and dark grey, minimally bioturbated to non-bioturbated siltstone, which sits stratigraphically between the Lower Triassic Montney Formation and the Middle Triassic Doig Formation within northeastern British Columbia, Canada. The Sunset Prairie is sedimentologically, ichnologically and paleontologically distinct from the overlying and underlying formations and warrants a new formational name to be established.
The Sunset Prairie consists of seven lithofacies which are interpreted to have been deposited within offshore, offshore transition and lower shoreface settings. There is limited preservation of physical sedimentary structures due to pervasive bioturbation but, where preserved, include planar-, pinstriped-, planar-wavy and wavy laminae, low and high angle planar cross laminae, asymmetric ripples, hummocky cross stratification and penecontemporaneous deformation structures. Trace fossils present within the interval include Phycosiphon, Rosselia, Cylindrichnus, Teichichnus, Asterosoma, Scolicia, Helminthopsis, Palaeophycus, Chondrites, Planolites, Diplocraterion, Rhizocorallium, Thalassinoides, Skolithos and Zoophycos. Glossifungites-demarcated discontinuity surface (firm ground assemblage) are also common throughout the unit. Macrofossils present within the unit include bivalves, gastropods, lingulid brachiopods, spiriferid brachiopods, terebratulid brachiopods, echinoid skeletal elements (particularly spines) and crinoid ossicles. Based on conodont biostratigraphy and the macrofossil assemblage, the unit is Middle Triassic (Anisian) in age.
The transgressive, onlapping wedge is thickest to the west (basinward) and has been erosionally removed or not deposited within the east. Erosional surfaces occur at the top and base contacts, which suggest the unit is unconformity bound. These unconformities are interpreted to represent flooding surface sequence boundaries. Structural features active during Triassic deposition greatly influence the distribution and preservations of the Sunset Prairie Formation, and the thickest (80 m) preserved interval is located within the Fort St John Graben system and Hudson Hope Low. However, the interval is areally extensive and is mappable throughout the Western Canada Sedimentary Basin.
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