USING EXPOSURE SURFACES AS CHRONOSTRATIGRAPHIC SURFACES IN THE LACUSTRINE CASTLE PEAK MEMBER OF THE EOCENE GREEN RIVER FORMATION, UINTA BASIN, UTAH

The rapid development of the Uinta Basin horizontal oil play has resulted in many new cores becoming available for study in the last several years. This includes cores of the Castle Peak Member, which has become one of the most commonly targeted members of the Green River Formation, with 97 (update) horizontal wells drilled within it in the last several years. We analyzed the available Castle Peak core to identify depositional facies and group them into stratigraphic sequences stretching across the ancient Lake Uinta system and up the connected alluvial slope. A dominant feature of the lacustrine beds in core are surfaces that mark apparent long-term exposure and drying of the sediments. Features of these exposure surfaces include pedogenic altering of the sediment, including clay alterations and the breakdown of lithic grains. Perhaps the easiest features to identify, and most diagnostic, are the development of vascular roots through the sediments below the exposure surfaces, extending up to five feet below. These roots are identifiable by a carbon residue around the cast and their branching nature. Exposure surfaces on carbonates results in dissolution and karsting, with some root and clay alterations also present. We identified each exposure surface and then correlated them across the play fairway and to major paleosols at the southern outcrop belt. Minor exposure surfaces were visible in proximal cores, but not present in distal cores where lake conditions may never have dried completely. The largest and best developed of the exposure surfaces lies just below the Long Point lacustrine transgression, perhaps marking a period when ancient Lake Uinta entirely dried up. These exposure surfaces are valuable as timelines in correlating Castle Peak sequences and in better understanding the variable nature of lake conditions during this period of Green River time.