INTERDUNE LAKES OF THE LOWER JURASSIC NAVAJO SANDSTONE, UTAH, USA

Interdune lakes form in complex and variable sedimentary environments influenced by migrating dunes and flashy fluvial discharge. These active hydrologic processes are preserved in the Jurassic Navajo Sandstone of SE Utah. The interdune lakes formed by seepage and/or were fed by springs; in both cases, carbonate deposition was the rule, although a few lacustrine deposits are siliciclastic. All Navajo lacustrine carbonate deposits have at least some of the following: (1) generally small size, < 1 km²; (2) carbonate deposition preceded by colonization of the surface by plants, including trees; (3) proximal shoreline and distal deposits; (4) evidence of exposure during deposition; (5) event horizons consisting of thin laminae of sandstone attributed to sand storms; (6) termination by drying, sometimes accompanied by formation of evaporite minerals, or by dune migration; and (7) water supplied by springs. Lacustrine carbonate beds are commonly contiguous with tufa mounds interpreted as deposits formed by springs that fed the adjacent lakes. Although stratiform stromatolites are present, the Navajo deposits lack domal, cylindrical, or columnar stromatolitic mesostructures typical of other carbonate lakes. Carbonate facies are mostly laminated fenestral mudstone or peloidal, but other facies are also present. Lacustrine deposits are generally stratigraphically isolated, but are recurrent in a few places; this is likely a function of dune dynamics and/or local control of runoff or recharge.

The lakes in the Navajo Sandstone were important centers for biodiversity. In addition to tree growth that preceded many lakes, the lake deposits and paludal areas have tracks of both vertebrates and invertebrates, burrows, stems, root mats, and leaves. Invertebrate trackways of caterpillars; vertebrate tracks are therapods (Eubrontes isp.) and ornithopods (Iguanodontipus isp.). Burrows (Arenicolites, Skolithos) in the lake deposits are microscopic in scale and uncommon. Massive fenestral mudstone could be due to complete bioturbation or may be an original texture. Stems include possible cycadeoid stems, silicified stems of small, reed-like plants, and Neocalamites(?). Leaves are mostly the cycadeoid form genus Ptilophyllum(?) and conifer needles. Microfossils are ostracodes and charophyte oogonia.