Complex fluvio-lacustrine stratigraphic interactions in the Renegade Tongue, Green River Formation, Uinta Basin, Utah

Abstract

The early Eocene Renegade Tongue of the Green River Formation in the Uinta basin, northeast Utah, is a fluvial siliciclastic dominated interval, interpreted as a fluvial megafan depositional system, interbedded with carbonate and siliciclastic lacustrine facies. Stratigraphy of the Green River Formation is poorly understood due to complex and variable spatial and temporal relationships between fluvial and lacustrine deposits. The complexity of these fluvial-lacustrine deposits is assigned to frequent climate-driven lake level fluctuations, as combined with longer-frequency allogenically controlled progradation and aggradation of the fluvial fans system, and lower-frequency autogenic processes, such as fan lobe and channel avulsions. Through the analysis of detailed outcrop measured sections, photomosaics, and optical thin section microscopy twenty two sedimentary facies and nine facies associations have been identified. These associations are grouped into fluvial, deltaic and lacustrine associations based on lateral and vertical facies association relationships. The fluvial associations include: FA 1.1 Amalgamated channels, FA 1.2 Isolated channels, FA 1.3 Tabular splay sandstones, and FA 1.4 Floodplain mudstones and siltstones. The deltaic associations include: FA 2.1 Distributary channels and mouth bars, FA 2.2 Tabular delta front turbidites, and FA 2.3 Prodelta mudstones and siltstones. The lacustrine carbonate associations include: FA 3.1 High energy lacustrine carbonates, and FA 3.2 Low energy lacustrine carbonates and microbialites. This study combines a detailed dataset across a triangle of three measured sections, each approximately one kilometer apart, in Nine Mile Canyon, Utah, with sub-regional scale correlations across approximately 130 kilometers. The detailed facies and facies associations provide a better understanding of vertical trends, lateral variability, and correlation of the fluvial, deltaic, and lacustrine complexities within the Renegade Tongue. In understanding this variability, intervals of allogenic and autogenic changes have been identified in correlations and complexity of the interaction between fluvial, deltaic, and lacustrine deposition was illustrated in 3D paleogeography time slices. The descriptions and interpretations of the interval can be compared with other fluvial intervals within the Uinta basin or across the globe, which provide valuable insights into the variability observable in other fluvio-lacustrine interfaces.