Abstract

Most of the geothermal resource of Utah is in the eastern part of the Basin and Range physiographic province and the adjacent transition zone with the Middle Rocky Mountains and Colorado Plateau physiographic provinces. The high-temperature part of this resource is in an area of about 25,000 km² in the southwestern part of the state. At least seven hydrothermal systems with reservoir temperatures ranging from 97°C to more than 250°C have been identified in this area. Hot water and steam from two of these high-temperature systems, Roosevelt Hot Springs and Cove Fort - Sulphurdale, are being used to generate electricity, and another system, Newcastle, has been developed for space heating.

From Precambrian through Jurassic time a thick sequence of sedimentary rocks was deposited over the region, and these rocks form the cores of most of the ranges. Some of these rocks have been metamorphosed. Most of the exposed pre-Tertiary rocks are in overthrust sheets of the Cretaceous Sevier orogenic belt. The rocks underlying these overthrust sheets are only occasionally exposed.

Cenozoic volcanic and intrusive rocks are abundant in the southern part of the area and in a north-trending zone in the northern part, Calderas and other volcanic structures are associated with Cenozoic igneous rocks. The basin and range structures that produced the present topography developed in Neogene time. Cenozoic sedimentary rocks, locally over 1,000 m thick, underlie most of the valleys. Evidence of young volcanic activity includes rhyolites as young as 400,000 years, basalt flows less than 1,000 years old, and possibly the arching of 15,000 year old Lake Bonneville shorelines.

With the exception of Newcastle, all of the known high-temperature systems are reflected at the surface. Some of the geothermal systems appear to involve primarily hydrothermal convection along normal fault zones in areas of relatively high thermal gradients associated with high regional heatflow. Other systems appear to be related to local heat sources associated with recent igneous activity. Reservoir rocks include all of the major rock types of the region. The geothermal systems have been extensively explored by a variety of surface and subsurface techniques. Much of the information on these geothermal systems was obtained through programs supported by federal and state government agencies and is thus available to the public. Important information on some of the systems obtained by industry is also available to the public.