Characteristics of Sandy Hyperpycnite Deposits on the Shallow, Southern Margin of Eocene Lake Uinta, the Green River Formation of Northeastern Utah

Abstract

Gravity flows play an important role in distributing clastic sediments into lacustrine basins at the front of fluvial-deltaic systems due to the tendency of sediment-laden rivers to plunge along the bottoms of lower-density lake waters. Fluvial derived underflows can be relatively long lived, perhaps on the scale of weeks, and distribute substantial volumes of sand far into the profundal zone of lacustrine systems that are otherwise characterized by chemical sediments and clay and silt sized clastics. This paper examines the properties of these sandy hyperpycnites in the Eocene Lake Uinta (Green River Formation) of the Uinta Basin in Northeastern Utah using outcrop, core, and wellbore data. These deposits are of particular interest due to the developing tight oil play which exploits them using horizontal wellbores. Facies typical of gravity flows display recurring sedimentary structures that represent different flow conditions. A typical bed consists of well sorted, fine and very fine grained sandstone with a flat base that may include clay chips (rip-ups) within massive or low angle cross beds, with planar beds often overlying them. These are overlain by thick beds of climbing ripples with individual ripple trains reaching over two feet in thickness. Climbing ripples are a consistent characteristic of these deposits, occurring even when other sedimentary structures typical of these beds are absent. In proximal areas supercritical climbing ripples can comprise the bulk of the deposit, with subcritical climbing ripples being more common in distal portions of the flow. Thin planar sands and interbedded silts and claystone top out the deposit, with the entire gravity flow ranging from less than a foot to over 15 ft. in thickness. Soft-sediment deformation is common, particularly in the basal portions of the deposit, with typically including ball and pillow, flame structures and convolute bedding. Hyperpicnal sandstone lobes of up to a mile in width and several miles in length have been mapped using well data. Individual lobes branch off larger feeder channels, forming larger fans and fan complexes. These extensive, well sorted sandstone complexes are unlikely to originate in surge-like gravity flows, instead probably represent seasonal sediment-laden fluvial underflows.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018