Syntectonic Conglomerates in Southwestern Montana: Their Nature, Origin, and Tectonic Significance

Abstract

During the Late Cretaceous (late Albian-Cenomanian) through Paleocene and probably early Eocene, southwestern Montana and adjacent Idaho comprised the depocenter for thick accumulations of syntectonic quartzite conglomerate, limestone conglomerate, and minor amounts of sandstone collectively known as the Beaverhead Formation. In this study, the depositional and deformational history of the Beaverhead is documented in detail in an attempt to understand the tectonic and topographic development of southwestern Montana and east-central Idaho. Clast imbrication and composition measurements suggest that two fundamentally different source areas existed for the Beaverhead. The limestone conglomerate clasts were derived primarily from Mississippian and Triassic carbonate rocks exposed by uplift of the Black-tail-Snowcrest basement arch and the “Ancestral Beaverhead Range” along the present-day Montana-Idaho border. These deposits, which derived their detritus from an area less than 15 mi away, probably represent coalesced alluvial fans. Although the quartzite conglomerate beds were deposited simultaneously with the limestone clasts, the quartzite was derived from a more distant source. The vast quantity of Belt quartzite clasts which initially reached what is now southwest Montana in late Albian time apparently originated from the large area of Belt exposure on the northeast side of the Idaho Batholith approximately 50 mi to the west and northwest of the study area. Gradual expansion of this source area by active uplift continuously provided steep slopes needed for the transport of gravel by braided stream systems to the adjacent subsiding alluvial plain. Recycling of the Belt clasts owing to Late Cretaceous uplift in the present eastern Snake River Plain carried them as far east as western Wyoming, where they became incorporated in the Harebell and Pinyon Conglomerates of western Wyoming. The Beaverhead Formation exhibits two distinct structural patterns: (1) an earlier cratonic pattern composed of northeasterly trending, gently plunging, open folds and associated high-angle faults, and (2) a northwest-trending “geosynclinal” pattern characterized by major upthrusts such as the Tendoy, Cabin, and Fritz Creek faults, and large, gently plunging, open folds. The older structural pattern seems to be related to the tectonism of the Late Cretaceous to early Paleocene that created the Blacktail-Snow-crest basement arch. The younger, more pervasive northwest trend may have a causal connection with the large uplift in the region of the Idaho Batholith. It is plausible that gravitative energy created by this uplift was available not only for eastward fluvial transport of Belt clasts, but also for northeastward down-slope tectonic transport. According to this hypothesis, large sheets of bedded rock slid eastward along detachment surfaces within the Belt, and as these surfaces were folded and locked, new ones developed to the east. This eastward migrating tectonic front reached the Lima, Montana, region soon after major Beaverhead sedimentation had ceased during middle to late Paleocene or early Eocene time. At this time, the Beaverhead rocks, which in part had inherited the earlier northeast trends,