SEDIMENTOLOGY OF TERTIARY SEDIMENTARY ROCKS NEAR SALMON, IDAHO (STRATIGRAPHY, PALEOGEOGRAPHY, LEMHI COUNTY)

This paper presents the results of a detailed sedimentologic field investigation of the Tertiary basin-filling sequence located in the Salmon and Lemhi River Valleys of east-central Idaho. The primary objectives of the study were to develop a revised lithostratigraphy for the Paleogene part of the sequence and to determine the nature of the depositional environments for these deposits.;The sequence includes resistant conglomerates and breccias, vitric quartz sandstones and siltstones and poorly consolidated lignitic and bentonitic mudstones. Volcanic ash is a major component. Eight depositional facies are recognized, comprising proximal alluvial fan and mid-fan environments, proximal gravelly streams, distal sandy and shallow braided streams, shallow lakes, and proximal and distal mixed-load streams and flood basins. Atypically high suspended-load to mixed-load ratios, probably related to the abundance of volcanic ash, are noted in the preserved lithofacies in some proximal deposits.;Distribution of the facies, together with their relationship to faulting and volcanic deposits, demonstrates that the basin was established almost identically to its present form in early Oligocene and probably late Eocene time.;The facies recognized include favorable sites for uranium deposition as well as small-scale coal and gas accumulation. Fossil placer gold deposits may be present in coarse lithofacies.;Although Anderson (1956, 1957, 1959, 1961) divided the Tertiary rocks into four formations, these units were inappropriately named and have not been widely used. I propose new lithostratigraphic units that I believe are lithologically coherent and should replace Anderson's units. These new units are the Kriley Gulch Formation, the Salmon City Formation, locally divided into the Carmen Creek and Mulkey Creek members, and the Wimpey Creek Formation. On the basis of field relationships with the Challis volcanics, radiometric dates from associated volcanic rocks and paleobotanical data, these rocks are inferred to be of late Eocene-early Oligocene age.