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.
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