CARBONATE DEPOSITS IN THE LOWER JURASSIC NAVAJO SANDSTONE, SOUTHERN UTAH AND NORTHERN ARIZONA, USA

摘要

Carbonate strata constitute a minor proportion of the widespread eolian Lower Jurassic Navajo Sandstone but are vital for understanding the hydrological and paleoecological dynamics of the erg system. The carbonate lithologies are in the form of tufa (carbonate spring) mounds and lacustrine beds, predominantly limestone and/or dolomite with minor constituents of chert and clastics. The tufa mounds and lacustrine carbonate beds occur separately or together; where together, the springs are interpreted to have sourced water to the lakes. Where they are not associated with tufa mounds, the lacustrine carbonate beds may have formed where the water table intersected the topographic surface through seepage. Many of the deposits exhibit three common characteristics: (1) clastic facies in the carbonate beds that indicate proximity to the lake margin; (2) deformation of the lower beds of the carbonate units by catastrophic dewatering events; and (3) a variety of weathering features and evaporite crystal molds that indicate periodic subaerial exposure. These deposits were also the habitats of a rich terrestrial biota, including vertebrates, aquatic and terrestrial invertebrates, and plants, including large trees. In addition to the tufa mounds, several other mound-like structures occur in the carbonate beds as stromatolite mounds, carbonate beds draped over primary topography, tepee structures, and accumulations around tree trunks.