Marine Triassic Stratigraphy in Eastern Great Basin

Abstract

The Lower Triassic rocks of the eastern Great Basin provide a record of the last marine deposition in the Cordilleran miogeosyncline. Sections are present in eastern Nevada, particularly in Elko County, and in the northwestern Utah. Most of the eastern Great Basin sections can be divided into (1) a lower shaly limestone, (2) the Meekoceras limestone zone, (3) an upper shaly limestone, and (4) an upper thick-bedded limestone. Thicknesses range from 700 to 3,000 feet. Redbeds are present only in the lower part of the western Utah sections and above the uppermost Triassic limestones near Currie. The Meekoceras zone contains many ammonoids, nautiloids, and conodonts. Pelecypods, gastropods, brachiopods, echinoderms, crinoids, foraminifers, and fish remains occur in beds above the Meekoceras interval. The Triassic rests unconformably on Permian rocks throughout the area of study, but no angular discordance was noted. The uppermost Triassic beds are overlain by Tertiary or Recent material. Age determination for the Triassic sections is based on the fossils of the Meekoceras zone which occurs near the base and represents middle Scythic of the European standard. Correlation with adjacent sections in Idaho, Utah, and western Nevada indicates that equivalents of additional Lower Triassic zones are probably present in the eastern Great Basin. Clastics above marine limestone in the Currie area may be Middle Triassic. Names for stratigraphic units have been established in adjacent areas. The Candelaria, Tobin, China Mountain, and Dixie Valley formations of western Nevada and the Moenkopi, Dinwoody, and Thaynes formations of Utah and Idaho can be correlated with the eastern Great Basin Triassic. Early Triassic seas invaded the northeastern Great Basin from Idaho and transgressed Nevada from east to west. Marine connections with the eugeosynclinal part were probably through California into southern Nevada. As the rate of subsidence varied in the miogeosyncline, marine tongues extended over the more stable craton toward the east. Seas were not uniformly present over western Utah and eastern Nevada and in northeastern Nevada a high area shed coarse detritus into the Lower Triassic sea. Redbeds accumulated in western Utah during early Triassic time; the source for this material was probably the Nye-Lincoln geanticline, which extended from southern California to the vicinity of Ely, Nevada. During the Middle Triassic, this feature was widened and fused to the craton, occupying t e position of the former geosyncline.