Depositional processes in Beaverhead Formation, southwestern Montana and northeastern Idaho, and their tectonic significance
The Upper Cretaceous to Palocene(.) Beaverhead Formation is a thick sequence of interbedded and intertonguing synorogenic conglomerates, sandstones, and limestones. In the vicinity of Lima, Montana, the conglomerates carry two distinct clast assemblages, one dominated by well-rounded quartzite clasts derived from Precambrian and early Paleozoic rocks to the west, and the second by locally derived, angular to rounded limestone clasts of Mississippian to Jurassic age. Recent observations on the details of depositional facies, clast composition, fining away sequences, and the structure of deformed strata beneath the Beaverhead unconformity, however, suggest that the limestone conglomerates represent a complex of deposits with sources both in the thrust belt and the foreland terrane. In the Antone Peak area the limestone conglomerate of the Beaverhead lies uncomformably on rocks deformed solely by foreland deformation. The sequence is characterized by cycles up to 10 m (30 ft) thick beginning with laterally extensive lenses of sandy, clast supported, well stratified but poorly sorted cobble and boulder conglomerate grading upward into cross-bedded pebble conglomerate, pebbly sand, and flat laminated and rippled sandstone. In contrast, the limestone-rich conglomerates near Dell display a dominance of matrix-supported conglomerate and pebbly mudstones incised by steep walled channels filled with well-stratified, better sorted, clast-supported conglomerate lenses interbedded with thin discontinuous lenses of flat laminated coarse sandstone. Recognition of the link between depositional style and source terrane in the Lima area may provide a powerful tool for distinguishing different deposits of compositionally similar conglomerate.
- Research Organization:
- Johns Hopkins Univ., Baltimore, MD
- OSTI ID:
- 6433056
- Report Number(s):
- CONF-8309274-
- Journal Information:
- Am. Assoc. Pet. Geol., Bull.; (United States), Journal Name: Am. Assoc. Pet. Geol., Bull.; (United States) Vol. 67:8; ISSN AAPGB
- Country of Publication:
- United States
- Language:
- English
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