CYCLOSTRATIGRAPHY OF THE PARACHUTE CREEK MEMBER: ANALYSIS OF VERTICAL AND LATERAL FACIES AND INORGANIC GEOCHEMICAL VARIABILITY IN THE GREEN RIVER FORMATION OF THE UINTA BASIN, UTAH

Detailed lithologic description of the Parachute Creek Member of the Green River Formation deposited in Lake Uinta reveals a hierarchy of cyclicity. Decameter-scale cycles of organic-rich and organic-lean lacustrine mudstone and sandstone contain meter-scale and decimeter-scale lake expansion and contraction cycles. These higher frequency cycles are more apparent within organic-rich zones. This is either due to a decrease in lake level fluctuation during deposition of lean zones or because cycles are amalgamated within lake margin deposits, common in these zones. Many cycles can be correlated between two cores separated by 16 km in the eastern Uinta Basin, confirming that these are basin-scale events rather than local facies alternations. Several volcanic tuff layers are interbedded with the lacustrine deposits, which can also be correlated between the two wells, providing chronostratigraphic control. Future geochronology studies of these tuffs can be used to estimate the periodicity of lake expansion-contraction cycles in Lake Uinta. Together, cycle correlations and tuff beds allow us to test if the contact between organic-rich and organic lean zones are time lines as commonly stated, or if they represent time transgressive surfaces.

The stratigraphic correlation produced in this study provides a template to place pXRF-based elemental concentration data in depositional and lithologic context. Elemental abundances were measured every 5 cm on the two cores studied in this project. They provide proxies for detrital input, clay minerology, carbonate type and abundance, redox conditions, and organic matter content. With each data point constrained by lithology, vertical and lateral trends of these proxies within high frequency lake expansion-contraction cycles can be examined, and longer-term trends within individual depositional environments can be studied.

The results of this study help to better understand the evolution of Lake Uinta and the potential driving mechanisms on various scales of lacustrine cyclicity. These contributions to a relatively well-studied lacustrine basin provide useful analogues for more poorly constrained basins.