Abstract

Diamictites and matrix-supported wackes of the upper Cloverly Formation (Lower Cretaceous) represent rivers laden with volcaniclastic sediment (hyperconcentrated flows). The Cloverly Formation of central Wyoming is dominantly fluvial, lacustrine, and playa deposits. The sediments accumulated east of the Cordilleran foredeep during the early stages of the Sevier orogeny. The diamictites and wackes are decimeters thick and stacked vertically, forming rock bodies up to 11 m thick. Pebbles and cobbles within the diamictites and wackes occur as isolated clasts or in poorly defined layers. Many such clasts exhibit some degree of polish. Clast lithologies can be correlated to rocks that were exposed in the mountain belt to the west. Provenance data are consistent with Early Cretaceous movement on the Meade–Laketown–Paris–Willard thrust system. The clay fraction of the diamictites and wackes is dominantly illite–smectite and smectite characteristic of altered volcanic ash. Diamictites and matrix-supported wackes lack primary sedimentary structures (e.g., cross-stratification) but do exhibit soft-sediment deformation features indicative of dewatering. Erosional bases and bedding geometries are indicative of channelized flow.

The diamictites and matrix-supported wackes resemble debris-flow deposits. However, maintaining debris flows that behave as Bingham plastics on low depositional slopes 200–400 km from the mountain front is improbable. Rather, the diamictites and wackes represent hyperconcentrated flows. Deposition, either en masse by a Newtonian (or nearly Newtonian) flow that was turbulent throughout, or by progressive sedimentation from a stratified flow with a basal, incipient, granular mass flow overlain by a turbulent suspension is more consistent with the field data. Paleohydraulic calculations indicate that grain sizes up to small pebbles could have been transported in suspension, with larger clasts transported as bedload, consistent with sedimentological evidence. Much of the finer-grained suspended sediment was remobilized volcanic ash likely derived from the Idaho batholith to the west-northwest. The polished extraformational pebbles and cobbles, long regarded as dinosaur gastroliths (or "stomach stones"), are simply clasts associated with the hyperconcentrated-flow deposits. The polish exhibited by many of the clasts is attributable to transport in ash-laden flows. The occurrence of polished, extraformational stones in Cloverly diamictites and wackes may have implications for presumed gastroliths in other fluvial rocks of the western interior (e.g., the Cedar Mountain and Morrison formations).