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AAPG Bulletin

Abstract


Volume: 52 (1968)

Issue: 1. (January)

First Page: 129

Last Page: 161

Title: Stratigraphy and Paleoenvironments of Cretaceous Rocks, Mitchell Quadrangle, Oregon

Author(s): W. D. Wilkinson (2), Keith F. Oles (2)

Abstract:

Cretaceous marine and continental sedimentary rocks, of Albian and possibly Cenomanian ages, crop out in the south half of the Mitchell quadrangle in central Oregon. More than 70 sq mi of Cretaceous rocks is exposed along a northeast-trending, doubly plunging anticline. The sequence, 9,023 ft thick as described along three principal reference sections, lies with angular unconformity on Permian metasedimentary rocks, and is overlain unconformably by Tertiary lava flows and volcanic sedimentary rocks. In part, the rocks have been faulted complexly and there are numerous fault-controlled and randomly oriented intrusions of Tertiary age.

These rocks are divided into two intertonguing formations. One, a widespread and thick sequence of marine mudstone with subordinate siltstone and sandstone, is defined herein as the "Hudspeth Formation." Intertonguing intricately with the marine rocks are conglomerate and sandstone; these, predominantly of fluvial and deltaic origin, are defined herein as the "Gable Creek Formation."

The total sequence consists of the Basal member of the Hudspeth Formation, a thin sandstone and conglomerate unit lying unconformably on the Permian basement rock; a thick mudstone and siltstone unit referred to as the "Main Mudstone member" of the Hudspeth Formation; and 11 numbered conglomerate and sandstone members of the Gable Creek Formation intertonguing with a like number of mudstone and siltstone members of the Hudspeth Formation. Seven tongues of the Gable Creek Formation wedge out southward into the marine facies of the Hudspeth Formation; the four other tongues thin southward. Three Hudspeth Formation marine tongues wedge out northward into the Gable Creek Formation; the six other tongues thin northward.

Shapes of tongues, textural and thickness variations, primary sedimentary structures, and current-flow directions indicate that during middle Cretaceous time there was a rising landmass on the north. Very large volumes of coarse sediment were delivered by a major river to a shallow-marine embayment, and extensive alluvial piedmont and delta plains projected into the sea. Swinging distributaries, episodic uplift of the source area, and intermittent subsidence of the basin caused the shoreline to fluctuate and produce a complex intertonguing of fluvial-deltaic sediments with marine sediments.

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