Abstract

The Goose Creek Basin is a north-trending, elongate intermontane basin situated along the northern margin of the Basin and Range province. Pre-Miocene igneous, metamorphic, and marine sedimentary rocks constitute the highlands that border the basin on the east, south, and west. Miocene rocks within the basin have an aggregate thickness of about 1,250 m (4,100 ft). The basal Miocene unit is rhyolite, overlain by ash-fall tuff interbedded with fluvial and lacustrine sedimentary rocks. Sheets of ash-flow tuff form much of the upper part of the Miocene rock sequence.

Development of the Goose Creek Basin resulted from regional Cenozoic extensional tectonism. Miocene sedimentation took place in a coalescing fluviolacustrine depositional system. Coarse clastic material from the pre-Miocene source rocks in the highlands accumulated in fan deltas along the basin margins; fine-grained clastic sediments and organic debris accumulated in shallow lakes within the basin. The entire region was repeatedly covered by large volumes of rhyolitic volcanic ash. The source of the pyroclastic material was located to the northwest, in the vicinity of the present-day Snake River Plain.

Reconstructions of Miocene paleogeography of the Goose Creek Basin and surrounding areas show that highlands bordered the basin on the east and west, and drainage was to the south. Fresh-water lakes containing fish and diatoms existed in the basin interior. Green algae, mollusks, ostracodes, and burrowing organisms occupied the distal portions of the fan deltas and stream inlets at the lake margins. A diverse cool-temperate-zone flora populated the marshes and lowlands within the basin, and forests of deciduous and coniferous trees grew in the surrounding highlands.