ABSTRACT

Cedar Valley is a north-northeast-trending topographic depression on the southeastern margin of the Basin and Range Province in Iron County, southwestern Utah. The towns of Cedar City and Enoch, and adjacent parts of the valley, experienced a 105 percent population increase and a 110 percent increase in public-supply water use between 1980 and 2000, creating potential water-supply and water-quality problems.

This report addresses the geology of the Cedar Valley drainage basin and its influence on the storage and transport of ground water; it represents part of a cooperative, multidisciplinary study of the hydrogeology of Cedar Valley by the Utah Geological Survey and the U.S. Geological Survey, designed to help local officials address future watersupply issues.

The principal aquifer in the Cedar Valley drainage basin consists of Tertiary sedimentary basin-fill deposits, chiefly interbedded sand, gravel, silt, and clay. Most recharge is derived from infiltration of Coal Creek into alluvial-fan deposits near Cedar City. Coal Creek drains much of the Markagunt Plateau east of Cedar Valley; this highland receives the majority of the precipitation that falls in the drainage basin. The drainage basin is closed to surface outflow except during extreme precipitation events, but minor underflow of ground water occurs in places along its northwestern and southern margins.

Miocene- to Holocene-age normal faults bound the eastern and western margins of the Cedar Valley depositional basin. The eastern basin-bounding fault system (EBBFS) is physically more continuous and accommodated significantly greater displacement than faults along the western basin margin. Subsidence of the EBBFS hanging wall created the Cedar Valley depositional basin, which accumulated up to 3,800 feet (1,160 m) of basin-fill sediment. This sediment was derived chiefly from the uplifting footwall, and was deposited in alluvial-fan environments along the basin margins and in fluvial and lacustrine environments in the basin interior.

Interpretation of seismic-reflection data collected by Mobil Exploration and Production Services U.S., Inc. reveals that the Tertiary basin fill contains three unconformity- bounded units and has a complicated subsurface structure that is not entirely reflected by present-day topography. These relations indicate a complex evolution of coupled faulting and basin subsidence during Tertiary time.

The transmissivity of the basin-fill aquifer, estimated from aquifer-test and specific-capacity test data, is greatest in coarse-grained alluvial-fan deposits along the eastern and southwestern basin margins, and gradually decreases toward the basin center as sedimentary deposits become progressively finer grained.

Bedrock units are presently of secondary importance for water supply, but they are hydrologically connected to the basin-fill aquifer and include several hightransmissivity units that are important aquifers in other parts of southwestern Utah.

INTRODUCTION

This report describes aspects of the geology of Cedar Valley and adjacent areas, located in Iron County, southwestern Utah, that most directly influence the occurrence and flow of ground water. The report focuses primarily on unconsolidated to semi-consolidated basin-fill sediments of Quaternary-Tertiary age beneath Cedar Valley because they are presently the most important aquifer for the valley, and secondarily on bedrock units because they are hydrologically connected to the basin fill and are the target of increasing ground-water development.

A digitally compiled geologic map (plate 1) and accompanying cross sections (plate 2) illustrate the geology of the region. The cross sections and related isopach maps show the large-scale geometry and stratigraphy of the basin fill, and are based primarily on interpretations of 11 seismic-reflection lines obtained from Mobil Exploration and Production Services U.S., Inc. (now part of ExxonMobil).

The work summarized herein is part of a cooperative, multidisciplinary project by the Utah Geological Survey and the Water Resources Division of the U.S. Geological Survey to characterize the budget, flow, and chemistry of ground water in the Cedar Valley drainage basin.

The goal of the project is to provide tools to help local and state officials manage ground-water development to sustain reserves and maintain high chemical quality. Such tools are necessary because the population of the study area increased by about 105 percent and water use from public suppliers increased by about 110 percent between 1980 and 2000, and groundwater levels have gradually declined in most of the valley since 1945 (Utah Division of Water Rights, 1982, 2000; Burden, 2000; Utah Governor’s Office of Planning and Budget, 2001a).

The principal conclusions of this project are as follows. Unconsolidated Quaternary-Tertiary-age sediment in the Cedar Valley depositional basin forms two distinct subbasins in the northeastern and southwestern parts of the valley. Both sub-basins comprise asymmetric, east-thickening wedges that terminate against the eastern basin-bounding fault system (EBBFS), a Quaternary-Tertiary-age normalfault zone responsible for basin formation.

The basin-fill sediment grades from coarse alluvial-fan deposits near the basin margins to finer grained alluvial and playa deposits in the basin center. This facies variation largely controls the distribution of transmissivity within the basin-fill aquifer, with values ranging from over 20,000 square feet per day (>1,860 m2/d) along the basin margins to less than 5,000 square feet per day (<465 m2/d) in the basin center.

The best prospective bedrock aquifers in the area include fractured volcanic rocks below and adjacent to the southwestern basin margin, and fractured sedimentary rocks southeast of the basin.

To assist non-geologists in reading this report, many technical or specialized geologic terms are defined in a glossary located after the references. Geologic ages are reported with the abbreviations ka for thousands of years before present and Ma for millions of years before present. For example, the phrase “the Pleistocene epoch lasted from 1.6 Ma to 10 ka” means that the Pleistocene epoch began 1.6 million years before present and ended 10,000 years before present.