MORPHOLOGICAL CHARACTERIZATION OF CARBONATE MOUND-LIKE STRUCTURES IN THE JURASSIC NAVAJO SANDSTONE, SOUTHEASTERN UTAH

Carbonate beds make up a small portion of the Jurassic Navajo Sandstone; they are concentrated in the Moab region in Utah, where the Navajo erg margin is well preserved. The carbonate occurs as interdune lake deposits and a variety of mound structures. The mounds fall into four categories: tufa mounds, stromatolite mounds, draped small-scale topography, and accumulations around standing trees. The tufa mounds were formed by springs of carbonated water that precipitated calcite upon emergence. Fully preserved, they comprise (1) mound-like shape significantly thicker than the surrounding carbonate, if any; (2) vuggy or massive internal texture, usually with (3) laminated carbonate on the lower flanks; (4) smooth upper surfaces on the flanks; and (5) brecciated cores that were the vents for spring water. They range from ~1-10 m in diameter, or larger, if they are clustered and overlapping. They may contain chert and/or sparry calcite with large (> 1 cm) crystals. The other three types of mounds occur in lacustrine carbonates, which may or may not be associated with tufa mounds. Stromatolite mounds are generally < 1 m in diameter, and consist of laminated limestone slightly thicker than, and contiguous with, the adjacent lake carbonates; they are less than two meters in diameter. Where observed, the contiguous lake carbonates are always also stromatolitic. Draped mounds occur on uneven surfaces; lateral dimensions are meter scale. The importance of these mounds is that they show that not all interdune lakes evolved from long-standing, flat interdune playa environments, but represent times when the interdune was flooded quickly enough that irregularities in the sand surface were not leveled before carbonate deposition began. These mounds are otherwise indistinguishable from flat-lying lacustrine carbonates. Accumulations around standing trees have stromatolitic flanks that slope away from the fossilized trees, which originate in the underlying clastics. The mounds are < 1 m in diameter. In some cases, the wood structure is preserved but, in others, the core is either amorphous, fractured chert replacing wood or the core has been hollowed out by erosion. These tree-related buildups can be mistaken for tufa mounds, but the small sizes are diagnostic.