BASINWIDE CLAY-CARBONATE CYCLES AND DEPOSITIONAL MODEL OF THE LACUSTRINE UTELAND BUTTE MEMBER, GREEN RIVER FORMATION, UINTA BASIN, UTAH

The Eocene Green River Formation represents lacustrine deposition in intermontane basins in Utah, Colorado, and Wyoming. In the Uinta Basin of Utah, the 15- to 65-m-thick informal Uteland Butte member is the basal unit of the Green River Formation and represents dominantly freshwater deposition during the first widespread transgression of Lake Uinta. This study assesses the basinwide expression of the Uteland Butte member using detailed measured sections, cores, organic and inorganic geochemical data, outcrop gamma ray logs, and petrophysical well logs. Twenty lithofacies comprise seven facies associations interpreted to represent lacustrine, palustrine, and deltaic environments. Five 4- to 12-m-thick shallowing upward cycles are identified and correlated across the basin. Each cycle is defined by a >1.5-m-thick basal clay-rich interval and is capped by a thicker carbonate-dominated interval. Clay-rich intervals consist of finely laminated organic-rich (3%–16% TOC) mudstone (profundal to sublittoral lacustrine) and/or silty mudstone (deltaic and littoral). Carbonate intervals are composed of complexly interbedded bivalve wackestone, ostracodal grainstone, laminated to massive dolomite, and coal (littoral to palustrine). Each of the five clay-carbonate shallowing upward cycles is present across proximal and distal basin locations, which signifies an allogenic control that resulted in distinct cycling of clay- or carbonate-rich lake phases. We interpret climate to be the dominant control on depositional cyclicity. During relatively humid periods, increased fluvial input of siliciclastic sediment and fresh water resulted in higher relative lake levels and clay distribution across the basin. In contrast, more arid periods resulted in decreased fluvial input and evaporative conditions that lowered lake levels and promoted widespread carbonate accumulation and subsequent dolomitization. Climatically driven depositional cycles within the Uteland Butte member likely reflect, at a smaller scale, the cycles of fluvial and lacustrine sedimentation observed at broader member and formation levels of Paleocene–Eocene stratigraphy in the Uinta Basin. Importantly, this sub-member-level study showcases how variation of fluvial input can impact lacustrine sedimentation at basinwide scales.