CARBONATE MOUND STRUCTURES IN THE JURASSIC NAVAJO SANDSTONE OF SOUTHEASTERN UTAH

The Jurassic Navajo Sandstone is the largest erg deposit preserved in the North American geologic record. The exposed portions of the Navajo Sandstone in southeastern Utah include carbonate beds that were interdune lakes in the erg. The carbonate beds are interbedded with the sandstone at various locations throughout the region. Although these carbonate beds have previously received attention, one important aspect has, until recently, received minimal study: carbonate mounds within or adjacent to some of the carbonate beds. Nine carbonate spring mounds have been identified at seven different locations in southern and southeastern portions of Utah, specifically around Canyonlands National Park and the town of Moab. The carbonate mound structures were found in the Spring Canyon, Dewey Bridge, The Knoll, Mineral Canyon, Dubinky Wash, and Ten Mile Point 7.5 minute quadrangles. Samples were collected from each mound and the surrounding bedding at each location. Between 5 and 22 samples were collected per location. Based on the sampling, further detailed analysis was conducted on two mounds. Sixty total samples were analyzed: 20 from one small mound, and 40 from a larger mound nearby. Thin section examination of the 20 samples associated with the small carbonate mound has revealed high quantities of ostracods (5-25% per sample), peloids (up to ~40%) and brown algae (30-40%). The algal and peloidal microfacies occur in random to laminated structures. Lamination occurs predominately in the carbonate bedding, whereas random growth of algae and dispersal of peloids occurs in the mound structure. Porosity of the samples varies: thin sections contain vug structures and shrinkage typical of subaerial exposure. Calcite and calcisilt fill pore spaces >1.5-2 mm in size. Secondary dolomitization and calcification are common across the sampling range. In addition, we performed stable isotopic analyses on the mounds and associated carbonates. The analyses revealed no structure in the data, in contrast to previous work.