Lowland Creek Volcanics, an Upper Oligocene Formation near Butte, Montana

Abstract

A thick sequence of quartz latitic volcanic rocks of late Oligocene age crops out in the region north and west of Butte, Montana. The formation, here named Lowland Creek volcanics, comprises six major units whose aggregate thickness is nearly 6,000 feet, and whose areal extent is more than 600 square miles. During the earliest phase of volcanism a mixture of tuff and detritus from older rocks accumulated on the lower parts of an erosion surface having a relief generally of 1,000 to 1,500 feet, but locally of 3,000 or 4,000 feet. Rocks of this basal unit were slightly tilted and eroded before the extrusion of a thick sequence of ash flows, most of which are now sheets of welded tuff. These eruptions were followed by block faulting and tilting, and subsequent erosion produced a surface of moderate relief upon which a breccia unit accumulated. The basal part of the breccia unit commonly consists of moderately well-sorted sandstone and granule conglomerate, which grades upward into breccias that contain boulders and blocks of quartz monzonite and other pre-Tertiary rocks, and rocks of the welded tuff unit. Higher up, instead of these, the breccia contains blocks of porphyritic quartz latite as much as 20 feet in diameter. Some of the breccia is probably vent agglomerate. Block faulting recurred during deposition of these coarse deposits. The breccia unit and older rocks are overlain by two unconformable successions of quartz latite lava, locally separated by a perlitic crystal-rich quartz latite vitrophyre. Many small intrusive bodies, and a few large ones, were emplaced after and perhaps during the extrusion of the lower lava unit. These intrusives, and the lavas which they cut, were eroded before the eruption of the vitrophyre and the lavas of the upper unit. In some places, the lavas of the upper unit are unconformably overlain by rhyolite of a still younger unnamed volcanic formation.