DYNAMIC ERG MARGIN ENVIRONMENTS OF THE JURASSIC NAVAJO SANDSTONE, SOUTHEAST UTAH

The Jurassic Navajo Sandstone desert system has a well-exposed erg margin in SE Utah. The formation is ~ 34 m thick at Dewey Bridge, and ~111 m near Sevenmile Canyon, NW of Moab, Utah. The base interfingers with fluvial deposits of the Kayenta Formation, and the top has paleotopographic relief beneath the J-1 unconformity.

The Navajo Sandstone exhibits four major categories of lithofacies and environments, where wind-blown sand was a host for surface and groundwaters while providing habitats for diverse ecosystems of plants and animals: 1. Eolian dune deposits include: a) large (>10 m) sets of crossbedded fine sandstone, with common disturbed and contorted to massive beds; and b) small (m-scale) sets of crossbedded fine sandstone (small eolian dunes), and some minor horizontal laminated sandstone. 2. Dry to wet interdune records are highly variable, from first-order bounding surfaces to decimeter-scale deposits resulting from varying degrees of time represented at these boundaries, as well as fluctuations in the water table. These interdune expressions include small mud- to sand-cracked playas as well as rooted, burrowed, and pedogenically modified sandstones. 3. Surface-water lake and spring deposits are respectively distinguished by thin-bedded, platy limestone with chert, and irregular limestone buildups (tufa mounds) showing microbial structures, including lamination. Freshwater limestones are most prominent in the upper portions of the formation. 4. Interfingering extradunal deposits of rivers to small ephemeral streams show lenticular, subhorizontal, cm to m-scale sandstone with basal scours and rip-up clasts and some green clay partings in the upper portions. Tracing individual supersurfaces is problematic because stratigraphic correlation is so difficult.

The Navajo erg margin was much more than the once-imagined, wind-blown edge of a vast, dry desert. Instead, the sedimentary record preserves dynamic environments that were sensitive to rainfall, runoff from the ancestral Uncompahgre Uplift, springs, and episodic strong ground motion. At times, when sand supply and dune activity were limited, diverse biotas flourished at the desert’s edge.