Lacustrine Hummocky Cross-Stratification in Green River Formation, Southern Uinta Basin, Utah: ABSTRACT

Abstract

Hummocky cross-stratification (HCS) is a common sedimentary structure in marine strata but has only been reported in a few lake deposits. However, recent field investigations in the southern Uinta basin show that HCS occurs within several different depositional environments and lithologies in the lacustrine Green River Formation (Eocene). HCS is well developed within the open-lacustrine informal S2 marker unit of the Parachute Creek Member, where it exhibits many of the characteristics common in marine HCS: numerous low-angle, second-order truncations; antiforms (hummocks); association with planar laminations and ripples; gently curved parallel to nearly parallel laminations; fanning of laminations into swales; and scattered dip directions of truncation surfaces and laminae. Hummock spacing ranges from 20 cm to approximately 2.75 m. Hummocks produced by laminae that thicken toward the hummock crest indicate that at least some of the HCS was produced by bed forms molded by the flow, and not by a random scour-and-drape mechanism. Common laminae downlap at set boundaries, and asymmetric hummocks indicate that at least some of the hummocks were subject to minor migration during growth.