SILCRETE DEVELOPMENT WITHIN FLUVIAL CHANNELS OF THE CEDAR MOUNTAIN FORMATION (EARLY CRETACEOUS), GREEN RIVER, UTAH

The Cretaceous Cedar Mountain Formation (Aptian) near Green River, Utah, is renowned because of the extensive preservation of channel belt sandstones that allow for plan views of the fluvial systems. Segments of the channel belts occur as inverted topography and are up to 9 km long. Portions of these channel belts were imaged using UAV technology. The original goal of this work was to document the depositional systems using the higher-resolution imagery acquired by the UAVs. Pixel resolutions of 1 to 10 cm allowed documentation of lateral and downstream accretion features. During analysis of the UAV imagery and field verification, patches of darker colored areas of sandstone were observed that were more indurated. These zones are silica-cemented orthoquartzite and this study focused on their distribution and origin.

Orthoquartzite was observed on UAV imagery from all 13 flights covering approximately 180 acres. The channel belts imaged were at different stratigraphic levels within the Cedar Mountain Formation suggesting the orthoquartzite was not limited to a single channel belt. The patchy orthoquartzite is primarily in zones (10’s to 100’s of square meters) along the margins of the channel belts. It is found as thinner lenses capping sandstone and as layers up to 30 cm thick. Crossbedding is preserved within the orthoquartzite and extends into the underlying sandstone. The silica-cemented nature of the orthoquartzite makes it more resistant to weathering and erosion. Where it occurs, it forms topographically higher areas that are 50 cm to 1 m of relief. The orthoquartzite is matrix supported with chalcedonic cement.

These orthoquartzites have characteristics similar to documented silcretes. We interpret the orthoquartzite within the Cedar Mountain Formation as silcretes formed from groundwater silicification processes. The variable distribution of the orthoquartzite both laterally and vertically and its occurrence on multiple channel belts suggests that silicification was not a single event. In addition, the presence of these orthoquartzites along the margins of the channel belts is analogous to other documented silcretes of groundwater origin. The resistant nature of the orthoquartzite possibly contributes to the inverted topography and resulting preservation of these systems.