INFLUENCE OF NEOGENE EXTENSIONAL STRUCTURE AND STRATIGRAPHY ON THE HYDROGEOLOGY OF CEDAR VALLEY, SOUTHWESTERN UTAH

Cedar Valley is a Neogene extensional basin in the southeastern Basin and Range Province. Neogene-Quaternary synextensional basin-fill deposits form the valley's principal aquifer. Rapid increases in population, industrial development, and water use potentially jeopardize the quantity and quality of Cedar Valley's ground water, requiring improved understanding of the valley's hydrogeologic system. The stratigraphy and structural geometry of the Cedar Valley synextensional basin and adjacent consolidated rocks influence the hydrogeology of the drainage basin in the following ways.

Neogene synextensional basin fill forms an east-thickening asymmetric wedge bounded by the eastern basin-bounding fault system (EBBFS). The synextensional basin evolved as two longitudinal sub-basins adjacent to the main segments of the EBBFS, separated by a transverse intrabasin high. This dual-basin structure persists as two distinct playa basins and corresponding closed lows in the potentiometric surface. The basin fill contains three unconformity-bounded units; only the upper unit, 0 to 400 m thick, has significant permeability. Transmissivity is greatest near the eastern basin margin and decreases rapidly toward the basin center, following facies changes within the basin fill from alluvial-fan to interbedded alluvial and playa deposits. Confined conditions exist inward of the facies transition due to abundant, laterally discontinuous clay layers in the alluvial and playa deposits.

The EBBFS footwall, 1,200-1,500 m above the valley floor, receives the majority of precipitation in the drainage basin. Most recharge to the basin-fill aquifer is from infiltration of Coal Creek, which drains the footwall. Footwall rocks are Mesozoic sandstone and shale with heterogeneous permeability. The low-permeability shale units preclude a simple flow path from bedrock to basin fill, and produce springs feeding Coal Creek. Significant underflow across the EBBFS may occur only where highly permeable formations abut the basin fill.