Upper Paleozoic Paleogeography of Idaho: ABSTRACT

In south-central Idaho, Mississippian rocks were deposited in flysch-trough and carbonate-bank environments. Lower Mississippian detritus shed from the Antler highland accumulated to a thickness of more than 3,000 m in an adjacent elongate flysch-trough and End_Page 785------------------------------ spread eastward to the inner craton margin. In southeastern Idaho, less detritus was available from the Antler highland in Early Mississippian time, and a starved basin developed between the foreland basin on the west and the inner craton margin on the east. Carbonate-bank deposition followed on both sides of the Snake River plain in Late Mississippian time, as the northern flysch basin ceased subsidence. By Middle Pennsylvanian time, the flysch deposits rose to form a highland from which coarse detritus was shed west, east, and south to interfinger with the fine-grained craton-derived, in part subarkosic, sands of the Wood River and Sublett basins, which gradually deepened through Late Pennsylvanian into Early Permian (pre-Phosphoria) time. Carbonate-bank deposition continued into Early Permian time east of the Copper Basin highland. The Lower and Upper(?) Permian Phosphoria Formation is recognized throughout southeastern Idaho and in south-central Idaho as far west as the Lemhi Range. West of the Lemhi Range, mollusk-rich fine-grained Phosphoria-equivalent rocks are known from one locality in the White Knob Mountains and from two localities in the Pioneer Mountains. Fine-grained, banded and graded siltites of Phosphoria age, which resemble continental-rise contourites, are present in the Boulder Mountains. Phosphoria-equivalent andesitic and dacitic volcanic rocks in western Idaho record a contemporaneous volcanic arc west of the continent. End_of_Article - Last_Page 786------------