A Genetic Stratigraphic Framework of the Green River Formation, Uinta Basin, Utah: The Impact of Climatic Controls on Lake Evolution
Birgenheier, Lauren P.; Plink-Bjorklund, Piret; Vanden Berg, Michael D.; Rosenberg, Morgan; Toms, Leah; Golab, James A.
The Eocene Green River Formation, host to abundant conventional and unconventional hydrocarbon resources, is one of the most heavily cited ancient fluvial-lacustrine systems. Despite this, a stratigraphic model that separates tectonic from climatic controls on deposition and resource potential is lacking from the Uinta Basin, Utah. A detailed core and outcrop based study of Green River Formation, spanning 150 km across the Uinta Basin, was performed. This was coupled with construction of an organic carbon isotope paleoclimate proxy record from one studied outcrop region. Seven facies associations were defined: littoral, sublittoral, and profundal carbonates; littoral and sublittoral siliciclastics; fluvial; saline; and volcanic deposits. The evolution of Lake Uinta is separated into Phases A - G based on combinations of sediment supply, organic matter preservation, salinity, and overall lake depth. The long term evolution of the lake phases was tectonically driven (sensu Carroll and Bohacs, 2001). However, the impact of climate on lake evolution has been underappreciated. Organic lean (L) zones below the Mahogany Zone (the richest organic interval and record of the highest lake level) contain siliciclastic dominated facies with sedimentologic indicators of episodic high sedimentation rates. These intervals record negative carbon isotope excursions associated with early Eocene abrupt, transient global warming events (hyperthermals). Hyperthermals caused a change to a highly seasonal climate and flashy fluvial discharge regime that resulted in increased siliciclastic sediment supply and delivery to the lake. Organic rich (R) zones below the Mahogany Zone contain carbonate dominated facies and record stable climate periods between hyperthermals with lower fluvial siliciclastic sediment supply. The stratigraphic progression upwards to the Mahogany and R8 zones represents the global transition out of the pulsed hyperthermal climate regime, at which time, the lake became sediment starved and permanently stratified, which encouraged the preservation of organic matter and later precipitation of saline minerals. Through the use of coupled sedimentary and geochemical analysis, this study has been able to separate tectonic and climatic controls on the distribution of facies in the Green River Formation, Uinta Basin and provide a predictive stratigraphic framework for further resource development in the basin and analogous lacustrine systems.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013