Shallow marine sedimentary facies in the earliest Triassic (Griesbachian) Cordilleran miogeocline, U.S.A.

Abstract

Permian history in the Cordilleran miogeocline, U.S.A., ended with an erosional interval of 1 to 22 million years. Previous studies of this event focused on unrepresented time, faunal extinction, regional truncation, or physical evidence in local areas. Another way to view this unconformity is to examine the nature of shallow marine sediments deposited on the erosional surface during the earliest Triassic (Griesbachian) transgression. As part of this study, the rate of the initial Mesozoic flood was estimated by determining the extent of the oldest Triassic conodont biozone within the study area. The calculated rate of transgression over 194,000 km 2 occupied by this biozone, within the 270,000 km 2 depositional area of the earliest Triassic Dinwoody Formation, is orders of magnitude greater than those suggested for eustatic sea-level change related to glaciation or plate tectonic processes. The rapidity of transgression, paucity of reworked material at the base of the Triassic, a general lack of local relief, and nearly conformable relations of Permian and Triassic rocks throughout the region suggest that the Triassic sea advanced across a featureless plain. The earliest Triassic sediments described in this study are restricted to the basal 2 m of the Dinwoody Formation. The generalized geographic distribution of facies within the depositional basin follows: evaporites are restricted to the northeast, Lingula-bearing dolomite characterizes the north-central, sandstone is locally present in the northwest and central areas, the southern part is dominated by shaly-bedded siltstone with some thin limestone interbeds, and calcareous, silty shale is present in the west toward the basin center. The similarity of basalmost Triassic depositional environments to those of the earlier Permian documents tectonic rejuvenation of a previous paleogeographic regime that strongly controlled Early Triassic sedimentation.