STRATIGRAPHY, SEDIMENTOLOGY AND FAUNAL DISTRIBUTION OF TRIASSIC-JURASSIC BOUNDARY SECTIONS IN THE ROCKY MOUNTAIN FRONT RANGES OF NORTHEASTERN BRITISH COLUMBIA: IMPLICATIONS FOR BIOSTRATIGRAPHY AND PALEOECOLOGY

The nature and placement of the Triassic-Jurassic (T-J) boundary in the Rocky Mountain Front Ranges and Foothills of northeastern British Columbia has proven to be a contentious issue in recent years. Many investigators have inferred that the T-J boundary in this area is characterized by a disconformity coincident with the contact between the Pardonet and Fernie formations (i.e. top of the Monotis beds).

Uppermost Triassic / lowermost Jurassic strata in the study area contain a moderately diverse array of body fossils (bivalves, nautaloids, ammonoids, anthozoans, gastropods, crinoids, fish and ichthyosaurs). Despite this dataset the precise level of the T-J boundary at these localities has yet to be precisely determined. Several horizons have been picked, based alternatively on bivalves, ammonoids and conodonts.

The T-J boundary at some sites (Pine Pass and Crying Girl Prairie) is clearly characterized by an erosional unconformity. However biostratigraphic evidence and detailed stratigraphic correlation indicate that localities along Williston Lake (Black Bear Ridge, Ne-parle-pas Rapids and Pardonet Creek) contain variably thick post-Monotis Triassic successions (~2.5-21.0 m, thickening to the west). These strata are characterized by the conodonts Epigondolella mosheri and Norigondolella sp. and the ammonoid Rhacophyllities debilis indicating Rhaetian deposition. The ‘pecten beds' (characterized by Agerchlamys and Ochotochlamys) occur below and with psiloceratid ammonoids and occur above Epigondolella . These strata are overlain by a succession characterized by Hettangian ammonoids (i.e. Psiloceras, Schlotheimia, and phylloceratiids). The erosional unconformity previously interpreted to coincide with the T-J boundary is now recognized to coincide with the Norian-Rhaetian boundary.

Sedimentologic and ichnologic observations provide evidence that the Rhaetian-Hettangian boundary section at Williston Lake was deposited in an overall deepening upwards succession grading from proximal offshore / offshore transition to proximal / distal offshore. Rhaetian strata are characterized by a series of submarine discontinuity surfaces characterized by low diversity Glossifungites trace fossil assemblages and phosphate pebble lags. These surfaces occur primarily within the Triassic and are absent in overlying strata.