WILSON, ERIC, and PAUL KARL LINK,* Idaho State University, Pocatello, ID

ABSTRACT: New Evidence for Pull-Apart Origin of the Lower Mississippian Antler Flysch Trough in South-Central Idaho

Preliminary basin analysis of the coarse-grained submarine-fan strata of the Lower Mississippian Copper Basin Formation in the Pioneer Mountains north of the Snake River Plain suggests an origin within a transtensional pull-apart basin rather than a thrust-loaded foreland basin. Recent geologic investigations in the type area demonstrate over 5500 m of continuous section, as originally suggested by R. A. Paull and students. A major thrust fault (the recently deceased Glide Mountain thrust) does not duplicate the section, as has been previously mapped. Abrupt on-strike and upsection facies changes, however, are present.

In the Dry Canyon and Wildhorse Window areas, a sedimentary breccia (containing clasts of the lower two members of the Copper Basin Formation) was deposited on top of Devonian carbonates. This suggests Early Mississippian syndepositional movement on a high-angle fault that was draped by 2000 m of Lower Mississippian deep-water strata.

The eastern Copper Basin correlative (McGowan Creek Formation in east-central Idaho) thins and fines eastward, suggesting an asymmetrical (foreland?) basin. However, the lower Copper Basin Formation is entirely late Kinderhookian, suggesting a subsidence rate of 750 m/m.y. comparable with observed rates of 500 to 4000 m/m.y. for pull-apart basins, and analogous to Antler strike-slip tectonics in northeastern British Columbia. Our conclusions are compatible with the documented absence of either Antler flysch or thrust fault in Idaho south of the Snake River Plain. A discontinuous, transcurrent orogenic belt, containing both transpressional, thrust highland-foreland basin systems and transtensional pull-apart basins may be a more feasible model.

AAPG Search and Discovery Article #90993©1993 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah, September 12-15, 1993.