CARBONATE AND NONCARBONATE SPRINGS AND TREES IN THE EOLIAN NAVAJO SANDSTONE, NEAR TENMILE CANYON, SE UTAH

The Early Jurassic Navajo Sandstone consists primarily of eolian sandstone, including large-scale eolian crossbeds. Carbonate beds are a minor but significant lithology in the Navajo Sandstone, particularly in the area of Moab, Utah. The carbonate beds are primarily lacustrine with textures and mineralogy consistent with a range of salinities from relatively freshwater to highly saline. Penecontemporaneous weathering indicates periodic exposure of the carbonate beds. In some locations, the carbonate beds are contiguous with carbonate mounds up to 2 m high. The mounds show vuggy and brecciated textures, particularly in their cores, consistent with carbonate spring mounds. This is in contrast to many carbonate mounds in Capitol Reef NP, which are laminated throughout and lack brecciated cores. Lateral relationships indicate that the mounds built above the local surface and were subsequently onlapped and buried by sand. Many of the lacustrine carbonate beds contain or are stratigraphically associated with fossil trees as large as a meter in diameter. The fossil wood is poorly preserved, but is pycnoxylic, almost certainly from the Coniferopsida, and most likely from one of the following Mesozoic conifer families: Cheirolepidiaceae, Cuppressaceae, or Taxodiaceae. The wood lacks growth rings, indicating continuous growth through at least through the preserved radii. Disrupted sandstone occurs in two forms. One form consists of moderately disrupted sandstone that is slightly cemented with carbonate and that occurs as low mounds about 2 meters across embedded in otherwise undisturbed sandstone. We interpret these as noncarbonate spring mounds. The second form are larger features, up to several meters across and irregular in shape, consisting of highly disrupted sandstone and carbonate and surrounded by an area of disruption that decreases to undisturbed beds within a few tens of meters. We interpret these to be catastrophic dewatering structures. Similar structures have also been observed in Capitol Reef NP (Eisenberg, 2001, GSA Prog. Abstr. 33(5):2). In the Moab area, however, the catastrophic deposits are much more localized. Our interpretation is that in this area, the dewatering is the result of relatively localized loading. All these features have implications for the dynamics of the sedimentary environment and the ecosystem of the Navajo Sandstone.