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Geology topics

Robert B. O’Sullivan

Publications and source records attributed to Robert B. O’Sullivan.

5 recordsLinked to original sources
USGS1997

The Jurassic section along McElmo Canyon in southwestern Colorado

In McElmo Canyon, Jurassic rocks are 1500-1600 ft thick. Lower Jurassic rocks of the Glen Canyon Group include (in ascending order) Wingate Sandstone, Kayenta Formation and Navajo Sandstone. Middle Jurassic rocks are represented by the San Rafael Group, which includes the Entrada Sandstone and overlying Wanakah Formation. Upper Jurassic rocks comprise the Junction Creek Sandstone overlain by the Morrison Formation. The Burro Canyon Formation, generally considered to be Lower Cretaceous, may be Late Jurassic in the McElmo Canyon area and is discussed with the Jurassic. The Upper Triassic Chinle Formation in the subsurface underlies, and the Upper Cretaceous Dakota Sandstone overlies, the Jurassic section. An unconformity is present at the base of the Glen Canyon Group (J-0), at the base of the San Rafael Group (J-2), and at the base of the Junction Creek Sandstone (J-5). Another unconformity of Cretaceous age is at the base of the Dakota Sandstone. Most of the Jurassic rocks consist of fluviatile, lacustrine and eolian deposits. The basal part of the Entrada Sandstone and the Wanakah Formation may be of marginal marine origin.

Colorado↗
USGS1965

Geology of the Cedar Mesa-Boundary Butte area, San Juan County, Utah

The Cedar Mesa-Boundary Butte area lies within the Colorado Plateau and includes about 650 square miles in southern San Juan County, Utah. Altitudes ranges from 3,890 feet on the westward-flowing San Juan River, the major and only perennial stream, to more than 6,400 feet on Cedar Mesa in the northwest. Bare rocks, high mesas, sheer cliffs, and deep canyons characterize the area. Comb Ridge, a prominent hogback of eastward-dipping rocks, trends north through the middle part of the area and is the most conspicuous topographic feature. The only permanent settlements are Bluff in the east and Mexican Hat in the west, both on the San Juan River.

Utah↗