Abstract

The late Miocene to early Pliocene Camp Davis Formation of northwestern Wyoming consists of conglomeratic lower and upper members separated by lacustrine limestone, siltstone, and tuff of the middle member. Detailed lithofacies analyses show the lower member (75-100 m [245-330 ft]) to be composed primarily of massive to horizontally stratified pebble to cobble gravel (Gm) and trough-cross-stratified granule to pebble gravel (Gt). Deposition occurred in a Scott-type braided stream system characterized by development of longitudinal bars (Gm) and filling of shallow channel scours (Gt). Paleocurrent data and presence of reworked Precambrian quartzite clasts and distinctive, yet previously unrecognized Tertiary plutonic clasts suggest deposition by a south- to southeastward-flowing basin-axis-parallel fluvial system (ancestral Snake River).

The upper member (1500 m [4900 ft]) is dominated by muddy, clast-supported, massive to horizontally stratified pebble to boulder conglomerate (Gm/Dcm) deposited by hyperconcentrated flood flows. Matrix-supported conglomerate (Gms/Dmm) is typically massive, may be inversely graded, and represents deposition by high-strength plastic debris flows. Low-angle trough crossbedded gravel (Gt) accumulated during shallow scour infilling within shallow bed-load streams and periods of unconfined sheet-flood. Collectively, these lithofacies suggest deposition on the proximal portion of a hyperconcentrated flood-flow dominated alluvial fan. Clast imbrication measurements indicate a southwestward paleoflow direction; clasts include Paleozoic and Mesozoic sedimentary and Precambrian crystalline rocks derived from the Hoback and Gros Ventre ranges to the northeast.

The lower and middle members are interpreted to have been deposited in an incipient Camp Davis basin prior to major movement on the Hoback listric normal fault. Upper member deposition was in response to major motion along the Hoback fault which may have occurred later and over a shorter period of time than previously thought.