A Conodont-based Temporal and Spatial Framework for the Triassic of North America

By

M.J. Orchard (Geological Survey of Canada)

The last 20 years has seen significant advances in Triassic biozonation of North America, based largely on conodonts intercalibrated with ‘standard’ molluscan faunas. Excellent resolution is based largely, though not exclusively, on Arctic Lower Triassic, Nevadan Middle Triassic, and northeastern British Columbian Upper Triassic successions. The character of some faunas differ as a function of their paleolatitudinal/ longitudinal origins (e.g. Tethyan versus Boreal realms) and are thus also of value in determining Cordilleran terrane paleogeography. The conodont framework is consequently foundational for both basin analysis and paleogeographic reconstructions. In the far north, thick Lower Triassic clastic accumulations include rare carbonates with conodonts diagnostic for sedimentary cycles of the late Griesbachian, Dienerian, early and late Smithian, and late Spathian. Most of these conodonts are cosmopolitan in nature but some early Smithian taxa are unique to the high latitudes. In parts of the Yukon, Lower Triassic strata are preserved as extremely condensed successions which carry a similar fauna. Middle Triassic strata in the Arctic are also mostly argillaceous and, apart from characteristic Daonella bivalves, contain uncommon conodont indicators of both the Anisian and Ladinian stages, based on Nevadan successions. Some Cordilleran accreted terranes may have quite differentTethyan faunas of this age. Upper Triassic, Carnian conodonts from the Arctic attest to extensive carbonate deposition, which is reflected also throughout British Columbia. Norian conodonts are widespread outside the Arctic whereas Rhaetian conodonts are known only from the terranes; lower and middle Norian and Rhaetian endemics characterize certain regions.

AAPG Search and Discovery Article #90008©2002 AAPG Pacific Section/SPE Western Region Joint Conference of Geoscientists and Petroleum Engineers, Anchorage, Alaska, May 18–23, 2002.