Abstract

The Colter Formation, composed largely of pyroclastic flow, lahar, surge, and coarse air-fall deposits, provides evidence that volcanoes were active in Jackson Hole, Wyoming, from the early to middle Miocene (late-early Arikareean to late Barstovian land-mammal age, correlating with about 24 to 13 my ago). The formation is the only record of relatively continuous early and middle Miocene volcanism known from the northern Rocky Mountains. The distribution of the Colter, its varying thickness, and facies changes within it suggest that most vents were located in the vicinity of the presently active Teton Fault. An early episode of compressional-type volcanism, lasting from at least 24 to 18 my ago, is inferred from the lower 500 m of andesitic, trachytic, and latitic tuff and ignimbrite, which are newly defined as the Crater Tuff-breccia Member. Extensional-type volcanism apparently followed by 13 my ago, as interpreted from the overlying 1000 m of rhyolitic tuff, ignimbrite, and conglomerate, the Pilgrim Conglomerate Member. The change from regional compression to extension between 18 and 13 my ago coincides with the development of the Mid-Tertiary unconformity in the northern Rockies, but the unconformity is less prominent in Jackson Hole than in southwestern Montana. The more complete depositional sequence in Jackson Hole probably is due to proximity of volcanoes that produced abundant detritus. Downfaulting, possibly as the magma chamber emptied, preserved the section. Additional movements along the Teton Fault since about 9.4 my ago tilted the Colter Formation westward and stepfaulted it into at least three major blocks that dropped eastward up to 1000 m per block.