Chemostratigraphic Analysis of Green River Formation in Douglas Pass, Piceance Basin: A Tool for Unraveling the Behavior of Lacustrine Source Rocks

Abstract

This study will elucidate geochemical variations in the development of Lake Uinta (Eocene Green River Formation – GRF), in both marginal and basin center environments in Colorado through elemental geochemistry at Douglas Pass, and later on cores at the U. S. Geological Survey Core Research Center. Major and trace element abundances can be compared to existing geochemical and mineralogic data to define the salinity, alkalinity, silica activity, redox potential and trace element character of sediments of the GRF. X-ray fluorescence analyses were conducted on excavated outcrop surfaces and samples removed from outcrop with a Niton (XL3 Analyzer) handheld spectrometer over part of a previously described stratigraphic section in Douglas Pass. Outcrop surfaces present challenges to effective measurement, particularly due to the potential for visually fresh surfaces to be contaminated by recent precipitation (e. g. gypsum) from ground water, due to the relatively small depth of penetration. Results of our first season analysis show the tool to be an effective means of rapid dense sampling and give an enhanced view of the chemical variability of marginal sedimentation. Repeatability was confirmed by regular measurements of a single sample, especially before and after battery changeout. Even where potential contamination was identified after analysis was complete, element ratios provide a means to understand chemical variation. Major element chemistry identifies much of the section as mixed clastic and carbonate siltstone/mudstone, shale, with interbedded sandstone and carbonate beds that are recognized markers of stratigraphic boundaries. Si/Al ratio faithfully reflects sand/clay content in mixed beds, and is especially distinct in thick sandstone near the top of the measured interval. Ca/Mg ratio is relatively constant, with high Ca/Mg spikes, suggesting calcite rich intervals in a background of constant calcite/dolomite. Fe concentration declines at the top of the interval, primarily due its low abundance in the sandstones that mark base of the R4 zone. Fe and Al are weakly correlated, suggesting iron arrives with clastic components. Zr/Nb effectively identifies sandier intervals in a section notable for the difficulty of hand specimen grain size estimation. V/Cr ratio ranges from 1.5–3, in good agreement with previous results, indicating oxic to dysoxic conditions.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015