STRATIGRAPHIC AND DEPOSITIONAL FRAMEWORK OF THE LOWER TRIASSIC VEGA AND PHROSO MEMBERS OF THE SULPHUR MOUNTAIN FORMATION IN THE WAPITI-BELCOURT AREA, NORTHEASTERN BRITISH COLUMBIA

The Lower Triassic Sulphur Mountain Formation in the Wapiti - Belcourt region of Alberta & British Columbia was deposited in a low-angle siliciclastic ramp setting on the western margin of the North American craton. In the study area the Sulphur Mountain Formation sits unconformably upon Upper Paleozoic strata of the Mowitch Formation. Where the contact between the two formations was observed it was characterised by a chert pebble / phosphate granule lag or by a high density / low diversity Glossifungites trace fossil assemblage consisting of fine-to medium sandstone & chert granule-filled Skolithos and Thalassinoides. The dominant lithofacies in the Sulphur Mountain Formation in the study area consist primarily of laminated silty shale with siltstone & very fine-grained sandstone interbeds. An interval dominated by thick (0.25 to 1.3 m), well-sorted, normally graded, very fine-grained sandstone beds (total thickness between 5 to 25 metres) occurs in the medial part of the study interval, approximately 45-50 metres above the top Paleozoic surface.

The thick sandstone package that occurs in the medial part of the study interval is interpreted to be an offshore turbidite fan complex deposited in channel, levee/overbank, & lobe/sheet sand facies associations. Sandstone channels have a lenticular geometry, 40:1 width/thickness ratios, & grade laterally to levee/overbank deposits of interlaminated very fine-grained sandstone, siltstone & shale. Current ripple laminae & convolute bedding are common. Lobe/sheet sand bodies consist of laterally extensive, amalgamated very fine-grained sandstone beds. Individual sand sheets are primarily massive and homogenous. Sand/shale ratios vary from 9:1 to 1:1 in proximal through distal facies.

Detailed biostratigraphy, using conodonts & ammonoids, provides clear evidence that the study interval was deposited during the Dienerian (upper Induan) & Smithian (lower Olenekian) stages. The lowermost Triassic strata preserved in the study area are earliest Dienerian in age. Griesbachian strata are apparently absent in the study area. Absence of earliest Triassic strata and anomalous thickness changes in Triassic strata supports the interpretation that Triassic deposition was strongly influenced by extensional tectonics in western Canada.