Spatial-Textural Study of Siliciclastics in a Navajo Sandstone Carbonate Lens

Flat-lying carbonate deposits in the Lower Jurassic Navajo Sandstone have long been interpreted as interdune lacustrine deposits. This dissertation critically assesses such inference through the study of an exceptionally well-preserved carbonate deposit in the Trough Canyon area, northwest of Moab, Utah. Thickness measurements along multiple cross-sectional exposures were used to construct an isopach map, approximating the original size and lenticular geometry of the deposit. Dip directions were recorded from foreset strata in cross-bed sets stratigraphically overlying the deposit to infer the approximately coeval direction of siliciclastic transport across the carbonate basin. These data sets yielded a contextual framework for studying the spatial distribution of detrital siliciclastics within the carbonate deposit. X-ray diffraction analysis of stratigraphically correlated carbonate samples measured wt% SiO2 as a proxy for the amount of detrital quartz sediment, yielding unexpectedly low levels of dispersed quartz content (0.8-11.4 wt% SiO2). Textural analysis of siliciclastic grains within carbonate samples was conducted using an AI-driven image analysis application and used to investigate spatial trends of Navajo siliciclastics within the carbonate basin, showing a general absence of strong directional trends in relation to mean, median, and maximum grain size. In addition, there was an absence of coarser grain sizes in carbonate samples compared to cross-bedded Navajo Sandstone samples, the presumed source of detrital sediment. Nearly all carbonate samples, including those closest to the upwind margins, bore grain sizes that exceeded the maximum threshold for significant eolian transport as identified from a literature review. This result raises questions about the proximity of dunes to “interdune” carbonate lenses in the Navajo Sandstone. The spatial distribution of siliciclastics according to composition and texture was unexpected for an interdune lacustrine environment dominated by eolian transport. Additionally, the surfaces bounding the carbonate deposit extend laterally into Navajo Sandstone sets, maintaining a flat geometry and preserving a thin draping of mudstone. It is suggested that instead of representing a deposit formed in an interdune depression by gravitational settling of lime mud and eolian siliciclastic input from nearby dunes, the lower carbonate deposit accumulated under sustained subaqueous flows onto a flat, expansive terrain.