Geologic framework and stratigraphy of Cretaceous and Tertiary rocks of the Southern Ute Indian Reservation, southwestern Colorado

Abstract

Cretaceous and Tertiary rocks on the Southern Ute Indian Reservation in southwestern Colorado represent a variety of depositional environments. The sediments of the lower Cretaceous Burro Canyon Formation were deposited on an alluvial plain. Upper Cretaceous rocks consist of marine, coastal, and alluvial deposits that accumulated in or adjacent to the Western Interior seaway. A drop in sea level at the end of the Early Cretaceous caused streams to cut valleys into the top of the Burro Canyon, forming a regional unconformity. Regression of the Early Cretaceous sea was followed by a transgression into southwestern Colorado that resulted in the deposition of the fluvial, deltaic, and marginal-marine sediments of the Upper Cretaceous Dakota Sandstone and overlying marine Mancos Shale. The Mesaverde Group forms a generally northeasterly prograding deltaic and strand-plain wedge that intertongues with the upper part of the underlying Mancos and the lower part of the overlying marine Lewis Shale. The marginal-marine sediments of the Pictured Cliffs Sandstone, which overlie and interfinger with the upper part of the Lewis Shale, were deposited during the final regression of the Western Interior sea from southwestern Colorado near the end of Late Cretaceous time. The Pictured Cliffs is overlain by the alluvial, paludal, and lacustrine deposits of the uppermost Cretaceous Fruitland Formation and Kirtland Shale. Tertiary rocks on the Reservation include parts of the Animas, Nacimiento, and San Jose Formations. These rocks, which are composed of sediments that are mostly fluvial in origin and have northerly sources, are the result of episodic uplift north of the San Juan basin during the early part of the Laramide orogeny. Tertiary dikes, sills, Manuscript approved for publication, April 24, 1989. and stocks which are composed of basalt, diabase, and andesite, intrude the sedimentary rocks on the eastern side of the Reservation.

Geologic framework for the national assessment of carbon dioxide storage resources—Southern Rocky Mountain Basins: Chapter M in <i>Geologic framework for the national assessment of carbon dioxide storage resources</i>

“…The Upper Cretaceous Lewis Shale and Mesaverde Group SAU consists mostly of the Mesaverde Group, a diachronous, northeastwardly thinning, regressive to transgressive wedge of siliciclastic rocks deposited in marine, marginal-marine, and nonmarine environments (Aubrey, 1991;Molenaar and others, 2002). The basal Mesaverde is gradational with the underlying marine Mancos Shale, whereas the upper contact intertongues with the overlying marine Lewis Shale ( fig.…”

Mineral resource potential and geology of the San Juan National Forest, Colorado

“…Over the past century, Cretaceous rocks have been assigned various formation names by geologists as an aid in description, mapping, and stratigraphic studies of deposits. The sedimentary rock units are of both continental and marine origin, and they show that, during Late Cretaceous time, the western shoreline of an epicontinental sea, which trended north-south across North America, advanced toward the southwest and retreated toward the northeast several times across Colorado and adjacent areas in response to varying sediment supply and rate of subsidence of the ocean basin (Sears and others, 1941;Pike, 1947;Molenaar, 1983;Molenaar and Baird, 1991;Aubrey, 1991). Figure 52 shows a generalized geometric configuration of the stratigraphic succession and the names applied to the coastal, marine, and continental deposits that accumulated during these marine oscillations.…”

“…The basal Upper Cretaceous unit, the Dakota Sandstone, was laid down across fluvial rocks of the Lower Cretaceous Burro Canyon Formation during the initial advance of the sea into this area. The Dakota Sandstone, about 200ft thick in the Forest area, is fluvial at the base and deltaic to marine shoreface in upper parts and includes sandstone, conglomerate, and subordinate interbedded lenticular claystone, carbonaceous mudstone, and coal (Pike, 1947;Zapp, 1949;Wood and others, 1948;Barnes, 1953;Haynes and others, 1972;Aubrey, 1991 ).…”

“…The Upper Cretaceous Mancos Shale, which conformably overlies the Dakota Sandstone, is about 2,000 ft thick in the Forest area (Barnes, 1953) and consists of dark-gray, sparsely fossiliferous marine shale containing a few thin limestone beds (Cross and Purington, 1899;Zapp, 1949;Haynes and others, 1972). Aubrey (1991) discussed a regional unconformity within the unit and pointed out that limestone development was a function of distance from shore.…”

Heavy-mineral placer deposits of the Ute Mountain Ute Indian Reservation, southwestern Colorado and northwestern New Mexico

“…Deposits located in this study are plotted on l:24,000-scale topographic base maps. Condon (1992) and Aubrey (1991) describe the overall stratigraphy of the Reservation. Of most interest to this study are the Point Lookout Sandstone and the overlying Upper Cretaceous Menefee Formation.…”