NEW STRATIGRAPHIC INTERPRETATIONS OF THE JURASSIC “JUNCTION CREEK SANDSTONE,” UPPER GUNNISON BASIN, COLORADO

A Jurassic sandstone that commonly overlies Proterozoic basement crops out in discrete locations around Gunnison, Colorado, forming massive red to tan cliffs and hoodoos. The sandstone locally underlies the Salt Wash Member of the Morrison Formation. Ongoing controversy over the nomenclature of several Jurassic units on the Colorado Plateau, coupled with the absence of lower Jurassic strata on the uplifted Uncompahgre highlands has resulted in ambiguity of stratigraphic relations for several units in the region. This particular Jurassic sandstone has been referred to as the Junction Creek Sandstone, the Junction Creek Sandstone Member of the Wanakah Formation, or an eolian unit within the Tidwell Member of the Morrison Formation. The Junction Creek Sandstone was named as a member of the Morrison Formation for exposures between Junction Creek and the Animas River in La Plata County, Colorado (Goldman and Spencer,1941). Two separate units within the member were initially noted, a lower horizontally banded part, and an upper, large-scale cross-stratified part. Thick cross-bedded sandstone sets in the Junction Creek are indicative of eolian dune deposition, whereas, the thin flat-laminated beds suggest interdunal or local sabkha deposition. Hansen (1968) mapped the eolian sandstone that overlies and underlies unnamed parts of the Wanakah, as the Junction Creek Member of the Wanakah Formation in the Smith Fork area of the Black Ridge quadrangle, Gunnison County, Colorado. O'Sullivan (1992a) points out that the name Junction Creek Sandstone has not been applied to the same stratigraphic interval everywhere and suggests that extension of the name into the Gunnison County area is tenuous. O'Sullivan (1992b) redesignated units previously mapped as the Junction Creek Sandstone, in the Black Canyon area as an eolian facies contained within the upper beds of the Tidwell Member of the Morrison Formation. Detailed study of units mapped as Junction Creek Sandstone in the upper Gunnison basin, Colorado, suggests that, while in the same stratigraphic position as the type Junction Creek, it may be appropriately designated as an eolian unit within the Tidwell Member of the Morrison Fomation.