Chemostratigraphy and Mineral Evolution of the Green River Formation

Abstract

The Eocene Green River Formation (GRF) in Colorado, Utah and Wyoming contains the largest oil shale deposits in the world and is also noted for its unusual economic mineral resources. As an example of lacustrine depositional systems, the GRF provides an opportunity to define the chemical evolution of this rich source rock. Understanding the chemistry and mineralogy of these rocks is critical to evaluating the relationship between inorganic mineral association and organic matter deposition. A systematic profile of major & trace elements was generated by Handheld X-ray Fluorescence (HHXRF) measurements made on excavated outcrop surfaces across the stratigraphic section at Douglas Pass in western Colorado, on the margin of the Piceance Basin portion of ancient Lake Uinta. Major elements define lithologic trends and indicate mixed deposition of siliciclastic and dolomitic siltstone/mudstone, shale, sandstone, and limestone. Si, Ca+Mg, Al+K are good proxies for quartz, calcite-dolomite and feldspar/clay, respectively. Si/Al ratio reflects sand/clay content in mixed beds. Trace elements, especially trace metal elements (Mo, U, V, Cr, Ni, Cu, etc.), characterize the redox conditions of the basin. Zr/Nb is good indicator for grain size. Mineral distribution diagrams based on X-ray diffraction (XRD) analyses, show that clay minerals, feldspar and analcime are more abundant in the basin margin than in the center. The key factor controlling these mineral distributions is the lake/sediment water chemistry. Geochemists WorkbenchTM was used to model mineral stability relations, extending work by Surdam and Parker (1972) in Wyoming. The results indicate a systematic pattern of evolution, with illite representing brackish to mesosaline conditions in the early lake, succeeded by dawsonite + albite + k-feldspar replacing clay minerals as conditions became hypersaline and elevated in bicarbonate/ carbonate, then nahcolite and later halite precipitation reflecting still further salinity increase, and eventually dawsonite disappearance/analcime appearance indicating an increase in silica activity, and possibly a decrease in bicarbonate concentration.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016