Early Triassic geologic history of northeastern Nevada

Conodont biostratigraphic and lithostratigraphic studies of Lower Triassic rocks in northeastern Elko County, Nevada, and adjacent parts of Idaho and Utah provide new information about regional geologic history. A sequential summary of Early Triassic events in this area follows: (1) rapid transgression of the Griesbachian sea to limiting barriers on the south (Oquirrh-Uinta axis) and west (Humboldt highland.). (2) Although the initial Triassic transgression may have persisted farther south and west than present-day evidence indicates, a period of progradation during the Dienerian limited marine sedimentation to northeastern-most Nevada and adjacent states. (3) In Smithian time, a widespread transgression spilled south and west over the earliest Triassic basin margin. (4) The southward flood is characterized by locally spectacular basal conglomerates followed by shallow marine deposits of the Thaynes Formation. (5) The transgression to the west was facilitated by tectonic removal of the restrictive barrier during the Smithian. This resulted in a slope-basin environment that accumulated a thick sequence of shale and calcareous siltstone with interbeds of turbidite conglomerates, olistostromes, and exotic blocks derived from Permian formations in northern Nevada or adjacent Idaho. (6) During a regional progradation in early Spathian time, marine conditions persisted in northeastern Nevada. (7) A final depositionalmore » episode is documented by the progressive westward spread of carbonate rocks of the Thaynes Formation. (8) Withdrawal of Triassic seas from northeast Nevada occurred post-latest Early Triassic, since a carbonate sequence of more than 300 m overlies the youngest dated interval.« less