Depositional Processes in Beaverhead Formation, Southwestern Montana and Northeastern Idaho, and their Tectonic Significance: ABSTRACT

The Upper Cretaceous to Paleocene(?) Beaverhead Formation is a thick sequence of interbedded and intertonguing synorogenic conglomerates, sandstones, and limestones located at the juncture of a northeast-southwest-trending foreland uplift and the northwest-southeast-trending thrust belt in Beaverhead County, Montana, and Clark County, Idaho. 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. Based on clast imbrication data by Ryder and Scholten in 1972, the latter assemblage has long been thought to represent a deposit sh d radially from the southwest end of the foreland uplift. 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. Where deposits from these two uplifts can be distinguished, they display markedly different sequences of sedimentary structures and fabrics attributed to significantly different modes of deposition. In the Antone Peak area the limestone conglomerate of the Beaverhead lies unconformably 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. Such sequences are characteristic of perennial braided stream deposits. Clast composition, distance from the thrust belt, coarse clast size, and the nature of the unconformity dictate a foreland source for these conglomerates. 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. These features are consistent with those observed on modern debris-flow-dominated alluvial fans in the Basin and Range province of the Western United States. Ubiquitous recycled sheared quartzite clasts strongly suggest a thrust belt origin for these conglomerates. 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. With careful mapping of these deposits and precise dating by pollen and fossils, the chronologic relationships of the various deposits may be established, enabling us to better understand the timing of the two uplifts. In addition this relationship suggests a general model that can and should be tested in other parts of the Cordillera where synorogenic deposits are found in both foreland uplift and thrust belt settings. End_of_Article - Last_Page 1340------------