Distribution of Heavy-Mineral Placer Deposits in Upper Cretaceous McCourt Tongue of Rock Springs Formation, Rock Springs Uplift Area, Wyoming: ABSTRACT

Abstract

The stratigraphic position, depositional environments, and paleogeographic distribution of heavy-mineral placer deposits in the McCourt Tongue were investigated at five localities on the southeastern flank of the Rock Springs uplift and on the northern flank of the Uinta Mountains. The placer deposits are discontinuous along 40 mi of a northeast-trending strand-plain shoreline of the Upper Cretaceous (Campanian) interior seaway. The placers consist of black, fine to medium-grained, ferruginous sandstones that form elongated lenses up to 6.5 ft thick and as much as 2000 ft long. The placers are normally intercalated with either gray or tan quartzose sandstones. They occur in six distinct depositional settings: (1) at the mouths of fluvial channels where they entered the seaway; (2) on beach berms; (3) on the upper parts of forebeaches (swash zones); (4) in bioturbated parts of lower forebeaches; (5) in surf zones; and (6) in the upper parts of middle shoreface deposits. Primary sedimentary structures in the sandstones indicate that the concentrating mechanisms were partly eolian and fluvial currents but mostly southwest-trending longshore currents.