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Figure 2. Paleogeographic map of the western U.S. during the Late Jurassic when the Bluff/Junction Creek Sandstone was being deposited as dunes migrated northeast through Colorado. From Peterson, 1994. “UPPER UNIT” “LOWER UNIT” Figure 3. Typical Junction Creek outcrop in the Gunnison area. Yellow line marks approximate position of the contact between the “lower” flat-bedded unit and “upper” cross-stratified unit. Note that highway lies directly on the contact of Proterozoic basement rocks. Metamorphic basement Morrison Formation Salt Wash Member Junction Creek Sandstone Brushy Basin Member Burro Canyon Formation Cretaceous Jurassic Figure 4. Stratigraphic column showing members of the Jurassic Morrison Formation in the Upper Gunnison basin, Colorado. Overlying and underlying units as well as unconformities are also shown. Nonconformity Proterozoic Wanakah Formation Morrison Formation Tidwell Member Salt Wash Member Bluff/ Junction Creek Sandstone Recapture Member Westwater Canyon Member Brushy Basin Member Burro Canyon Formation J-5 Unconformity Cretaceous Jurassic Figure 1. Stratigraphic column showing members of the Jurassic Morrison Formation in the Four Corners region. Overlying and underlying units as well as unconformities are also shown. A. B. C. D. Figure 5. Photographs of sedimentary features within the Junction Creek Sandstone in the Gunnison, Colorado area. A.) Symmetrical ripple marks near bottom of lower unit. B.) Coarse sand grains and chert lags along bedding plane at bottom of upper unit. C.) Contact between “lower” flat bedded silty sandstone and “upper” large scale cross stratified sandstone. D.) Cross-stratified dune-form within “upper” unit. 1m ABSTRACT 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 these units lie in the same stratigraphic position and are of the same lithology as the type Junction Creek. Based on the findings of this study, extension of the name Junction Creek Sandstone into the Upper Gunnison basin is appropriate and provides simplicity and clarity to the nomenclature of Jurassic units in the area. STRATIGRAPHIC RELATIONS The Upper Jurassic Junction Creek Sandstone is exposed in southwestern Colorado and reaches 300 to 500 feet thick at the type area near Durango, Colorado. The Junction Creek was informally divided into three units in the Four Corners area near Ute Mountain, Colorado, by Ekren and Houser (1965). Their lower unit is 40 feet of reddish-orange, fine to medium grained sandstone, with alternating flat-bedded and cross-stratified cosets. Laterally extensive horizontally-stratified beds are characteristic of this unit. The middle unit consists of 250 feet of pink to orange, fine to medium grained sandstone. Coarse grains and chert pebbles are commonly concentrated on bedding planes. This unit is thick bedded with very large scale cross-stratification. The middle unit thins eastward in the subsurface. The upper unit is grayish-red, fine grained, argillaceous sandstone consisting of thin to medium horizontal beds. This unit is irregular in thickness, but 30 feet was measured by Ekren and Houser (1965). The Junction Creek Sandstone thins northward to about 25 feet near Dunton, Colorado north of the La Plata Mountains (O’Sullivan, 1992). The Junction Creek Sandstone is equivalent to the Bluff Sandstone Member of the Morrison Formation, which is recognized in Utah, Arizona, and New Mexico (Figure 1). Use of dual nomenclature is unfortunate, but arose before the extensive network of well logs made subsurface correlations possible. The Bluff/Junction Creek laterally interfingers with both the Tidwell and Salt Wash Members of the Morrison Formation (O’Sullivan, 1995). The lower contact of the Junction Creek with Jurassic Wanakah Formation is defined by the J-5 Unconformity. The Morrison Formation overlies the Bluff/Junction Creek Sandstone region-wide. DEPOSITIONAL SETTING During the earliest deposition of the Morrison Formation, the Sundance Seaway extended south through Wyoming and into southern parts of Utah and Colorado (Turner and Peterson, 1998). As the seaway retreated, streams that drained highlands to the west of the depositional basin carried gravel, sand, and mud eastward forming an extensive alluvial plain (Figure 2). These deposits are the Salt Wash Member of the Morrison Formation. In central Colorado, remnants of the ancestral Rocky Mountains consisted of scattered low hills drained by small and likely intermittent streams that were unrelated to the Salt Wash system. During times when streams in the Colorado Plateau region were dry, winds from the south and west removed sediment from the dry stream beds and deposited it downwind to the north forming extensive dune fields in the Four Corners region (Turner and Peterson, 1998). Deflation of previously deposited shallow marine sands was also likely contributor of sediment to the eolian systems. These eolian deposits are represented today by the Bluff and Junction Creek Sandstones. NEW STRATIGRAPHIC INTERPRETATIONS OF THE JURASSIC “JUNCTION CREEK SANDSTONE,” UPPER GUNNISON BASIN, COLORADO Casey G. Dick, Department of Natural and Environmental Sciences, Western State College of Colorado, Gunnison, CO 81230 JUNCTION CREEK IN THE GUNNISON AREA Outcrops of red to white, fine grained, flat-bedded to large scale cross- stratified sandstone line road cuts and creek beds in the area surrounding Gunnison, Colorado (Figure 3). These resistant beds form cliffs and steep slopes over 60 feet high in some locations. The sandstone bodies lie nonconformably atop uplifted Proterozoic basement rocks and underlie the Jurassic Morrison Formation (Figure 4) . The underlying nonconformity is thought to be related to coalescing of several major Jurassic unconformities due to the thinning of sedimentary units onlapping the ancestral Rocky Mountain uplift. Although the Junction Creek is only sporadically exposed in the Gunnison area, marker beds persistent in several measured sections suggest lateral continuity between the units during deposition. Two distinct units within the Junction Creek, an “upper” and “lower”, are recognized in the area. The lower unit consists of grey to red, siltstone to fine grained sandstone. Bedding is typically thin and horizontal with abundant wavy laminations and symmetrical ripples. Shaley siltstone beds are also common within the lower unit. A marker bed near the top of the lower unit consisting of a grey-red-green shale succession is present at several localities. The upper unit consists of light tan to red, fine grained sandstone. This unit is dominated by large scale (0.5 m – 2 m) planar-tabular cross-stratification. Paleocurrent data, from the Gunnison area, suggest a transport direction of eolian sediments to the north. Coarse sand and chert pebbles are concentrated along bedding planes near the bottom of the upper unit at some localities. Exposures of the upper unit reach 40 to 50 feet thick near Gunnison.
