HYPERCONCENTRATED FLOWS IN THE EARLY CRETACEOUS CEDAR MOUNTAIN FORMATION, EAST-CENTRAL UTAH

The Early Cretaceous Cedar Mountain Formation of east-central Utah hosts spectacular vertebrate fossils with previously proposed entombment mechanisms including debris flows, foundering, and quicksand. Our sedimentological examination of the Cedar Mountain Formation documents the presence of hyperconcentrated flows as a previously unrecognized component of the stratigraphy. Recent studies have documented hyperconcentrated flows to play an important role in the preservation of vertebrate fossils and have been used to explain the preponderance of “gastroliths” in Mesozoic strata.

In east-central Utah, the Cedar Mountain consists of three members, oldest to youngest, Yellow Cat, Poison Strip, and Ruby Ranch. In each member, hyperconcentrated flows were recognized and were recorded by UAV images. All three occurrences are in close stratigraphic association with lacustrine deposits. The beds are within the upper Yellow Cat, uppermost Poison Strip, and near the middle of the Ruby Ranch Member. The core of all beds is composed predominantly consist of structureless siltstone to fine-grained sandstone with floating granules to 2-3 cm pebbles and are highly variable in thickness, 0 to 2.5 m. The basal portion consists of intraformational conglomerate lenses. Margins and tops of this flow are capped by current-ripple deposits. Load and fluid escape structures are present but rare.

The vertical and lateral sequences of structures indicate deposition from hyperconcentrated flow conditions as demonstrated by the structureless pebbly sandstones and siltstones. Rapid sedimentation of the hyperconcentrated flow deposits diluted the flow to normal stream flow conditions as indicated by the capping tractional flow deposits. Although no fossils were found in this study, the recognition of hyperconcentrated flow deposits in the Cedar Mountain Formation presents a potential new sedimentological prospect for vertebrate remains.