Soft-Sediment Deformation Structures: Upper Cretaceous Lance Formation, Jonah Field, Wyoming

Alquhtani, Faisal A.¹, Frank G. Ethridge² (1) Colorado State Universtiy, Fort Collins, CO (2) Dept. of Geosciences Colorado State University, Fort Collins, CO

Jonah gas field is located in the northwestern Green River Basin, Sublette County, Wyoming. The field produces primarily from fluvial deposits of the upper Cretaceous, Lance Formation. This formation is composed of medium- to very fine-grained sandstones, silty-sandstones, siltstones, and mudstones that are divided into five lithofacies. Most primary sedimentary structures in the Lance sandstones have been altered to some degree by soft-sediment deformation processes.

Soft-sediment deformation structures in the Lance are classified into four main types following Lowe (1975), and include fluidization structures, liquefaction structures, soft-sediment intrusion, and soft-sediment folds. Common soft-sediment deformation structures include Type A and B pillars (dewatering pipes), dish structures, consolidation laminations, massive sandstones, load structures, convolute laminations and over-steepened cross beds. Type A pillars, dish structures and consolidation laminations are usually found together and are genetically related. Pillars are easily mistaken for burrows. These structures are related to water escape mechanisms and are widespread in turbidites. Over-steepened or overturned cross beds are common in fluvial deposits. Other soft-sediment deformation structures, while not common, have been recognized in fluvial sands and sandstones ranging in age from Precambrian to Holocene. One feature common to deposits with these types of soft-sediment deformation structures and to the Lance Formation is a strongly layered sequence consisting of interbedded sandstones and finer-grained deposits.

Similar lithofacies and soft-sediment deformation structures are present in outcrops of the upper Cretaceous, Laramie Formation, Cheyenne Basin, north-central Colorado. The Laramie Formation provides a useful outcrop analog for the subsurface Lance Formation at Jonah field.