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The AAPG/Datapages Combined Publications Database

Wyoming Geological Association

Abstract


Cretaceous and Lower Tertiary Rocks of the Bighorn Basin, Wyoming and Montana; 49th Annual Field Conference Guidebook, 1998
Pages 137-165

Late Cretaceous, Paleocene, and Early Eocene Paleogeography of the Bighorn Basin and Northwestern Wyoming

David Seeland

Abstract

Fluvial drainage patterns of the Bighorn Basin in Late Cretaceous, Paleocene and early Eocene time have been reconstructed from studies of crossbedding in the Upper Cretaceous Lance and Paleocene Fort Union Formations and from studies of crossbedding and particle sizes and shapes in the lower Eocene Willwood Formation. Crossbedding in the Lance Formation in the Bighorn and adjacent basins in general shows an easterly paleoslope and east-flowing streams, but a divergence in sliding average paleocurrent directions in the eastern part of the Bighorn Basin suggests initial positive tendencies in the area of the north-central Bighorn Mountains. Fort Union Formation crossbedding indicates that the basin was defined by surrounding uplifts and that drainage was not southeasterly out of the southern part of the basin as has been previously suggested, but rather northward much like the streams of the overlying Willwood Formation but with a basin exit between the Pryor and Beartooth Mountains. In the Willwood these stream patterns were inferred from (1) a moving-average paleocurrent map based on crossbedding vector means for 104 localities, (2) size and shape isopleth maps based on sand grains from 120 other localities, and (3) maximum pebble length at each crossbedding locality. The sand-grain shape and size factors determined for each locality were: regularity (area/perimeter2), elongation (width/length), and mean length. The resulting isopleth maps reveal areas of maximum stream competence.

The paleogeographic reconstruction for the Willwood Formation in the Bighorn Basin suggests a system of three major basin-margin tributary streams and one trunk stream. Two of these tributaries entered the western side of the basin: one near Clarks Fork Canyon, the other near the Shoshone River west of Cody. A third tributary, which drained the highest part of the Bighorn Mountains, entered the eastern side of the basin near Paint Rock Creek. The coincidence of present major drainages and Eocene drainages suggests a constancy of major upland drainages since early Eocene time. The early Eocene trunk stream of the Bighorn Basin rose west of the Washakie uplift, perhaps as far west as central Idaho. The trunk stream crossed the ancestral Washakie uplift and entered the southwestern part of the basin. From there, it flowed northward west of the present Bighorn River but east of the basin's structural axis, and flowed northeastward out of the basin through the structural and topographic saddle between the Bighorn and Pryor Mountains near the present course of the Bighorn River, but at a higher level. The trunk stream carried minor amounts of sedimentary and igneous clasts and an abundance of very coarse quartzite roundstones about two-thirds of the way across the extent of the basin during Eocene time. The roundstones were either locally derived from older conglomerates in Jackson Hole or were "new" roundstones transported from eastern and central Idaho, The westernmost outcrops of lower Eocene rocks west of Cody do not show the westerly paleoslopes that should occur if these localities are on the east margin of the proposed Absaroka basin; either the east edge of the basin is west of these outcrops or the basin does not exist.


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