CARBONATES IN THE JURASSIC NAVAJO SANDSTONE, MOAB AREA, UTAH

The Jurassic Navajo Ss is the remnant of the largest erg in Earth history and is known for its spectacular eolian structures representing large dunes. Although the Navajo system has been characterized as a hyperarid desert, our recent research shows that parts of the Navajo Ss near Moab, UT, exhibit an active paleohydrological system. Notable evidence is recorded in the surface groundwater formation of paludal and lacustrine carbonates and tufa mounds. These were centers for abundant life from small insects to large dinosaurs, as well as trees, shrubs, and ground cover.

As part of a larger project to describe the paleohydrology and paleoecosystems of the Navajo, we are mapping and studying in detail these carbonates, which occur in several stratigraphic intervals. The carbonates can be divided into three categories: (1) paludal and lacustrine carbonates; (2) tufa mounds, which can be isolated or complex; and (3) tufa mounds associated and contiguous with paludal and/or lacustrine carbonate.

The paludal and/or lacustrine carbonates exhibit a variety of features including karstification, dehydrated algal mats, stromatolites, gypsum crystal casts, animal tracks, root mats, and other traces of plants. Some lacustrine carbonates contain chert as nodules or beds. These lakes may represent simple surfacing of the water table, or may be erosional remnants of mound-lake systems. Tufa mounds associated with paludal and/or lacustrine carbonates represent complex environments in that surfaces within the tufa mounds can be traced into the adjacent carbonate beds, indicating that the source of the water for the paludal and/or lacustrine environments was, at least in part, the springs. In one case, a complex mound is underlain by stromatolitic carbonate beds, possibly suggesting surfacing of the water table prior to mound formation. However, evidence for spring-vent migration in another instance indicates that the three-dimensional evolution of springs does not preclude the possibility that mounds underlain by carbonate beds migrated over previously deposited, spring-fed lake carbonates. Finally, isolated tufa mounds may represent springs that did not flow enough to create the paludal and/or lacustrine environments, or that were overtaken by deposition and migration of sand dunes before the lakes could form.