ERG TERMINATION – SEQUENCES AND PROCESSES IN NAVAJO AND PAGE SANDSTONES (JURASSIC) COLORADO PLATEAU (Invited Presentation)

The Jurassic Navajo and Page sandstones display excellent exposures that allow analysis of erg termination and the processes that produced erg demise. Three distinctive facies mark the upper Navajo across much of the Colorado Plateau: 1) large-scale simple to complex cross stratified sandstone – active erg deposits, 2) massive to contorted sandstone bodies, some completely structureless – disrupted, distorted, to fluidized originally cross-stratified sandstone, and 3) partly silicified limestone-capped surfaces, some with tens of meters of local relief below overlying units. In all locations the Navajo is sharply terminated and succeeded by younger eolian, sabkha, and shallow marine deposits. Well-studied polygonal cracks mark the uppermost beds of the Navajo commonly penetrating several meters into it. All aspects of the contact document abrupt termination and significant erosion of Navajo deposits by the transgressing lower Carmel seaway and adjacent coastal dunes of the Page Sandstone.

The Page Sandstone has a more complicated and prolonged termination. At some locations, large-scale eolian cross bed are sharply terminated by overlying sabkha redbeds but at many locations this initial truncation is succeeded by preserved eolian build-ups comprised of complex eolian strata with veniers of aqueous ripple-bedded strata; in places, several generations of build-ups are apparent. In other places, terminating sabkha beds are overlain by interbedded eolian and sabkha sandstone and the entire package has been tilted by presumed dissolution of evaporates. All aspects of this upper sequence document repeated eolian-sabka interactions as the upper Carmel marine sequence transgressed the Page erg.

Both erg terminations were caused by changes in climate, sand supply, and marine transgression as the Western Interior Seaway entered the region.