by Hugh A. Hurlow

ABSTRACT

The Kamas-Coalville region is in the Middle Rocky Mountains physiographic province, about 30 miles (48 km) east of the Wasatch Front urban area. Rapid population growth and increased water use are the impetus for a collaborative study of water resources in the Kamas-Coalville region, which includes geologic (this study) and hydrologic components.

This study describes the geologic framework of the Kamas-Coalville region, emphasizing geologic features that most strongly influence ground-water occurrence, flow, and development. The main topics include: (1) the stratigraphy and structural geology of bedrock, (2) the nature and geometry of unconsolidated deposits in Kamas Valley, (3) the hydrostratigraphy of the study area, and (4) the structure of bedrock units below Kamas Valley.

Kamas Valley is a depositional basin bounded on its east and west margins by normal faults. New gravity data, while not definitive, suggest that this basin is asymmetric, thickening to the east, and that the combined thickness of Tertiary Keetley Volcanics plus younger sediments locally exceeds 3,500 feet (1,067 m) adjacent to the East Kamas Valley fault zone, which forms the eastern structural boundary of the basin.

Quaternary deposits in the northern half of Kamas Valley are chiefly alluvium, glacial outwash, and alluvial-fan deposits. Alluvial fans emanate from the western Uinta Mountains, grading into alluvium in the central and western parts of the basin. Alluvial fans are absent in the southern half of the valley, which is dominated by Pleistocene outwash.

The hydraulic conductivity of unconsolidated deposits in Kamas Valley likely varies with age and depositional environment. Deposits formed during and immediately after glacial episodes have relatively high clay content, possibly resulting in lower hydraulic conductivities compared to similar deposits formed during interglacial periods. Alluvial-fan deposits are more homogeneous and likely have lower average hydraulic conductivities, due to poorer sorting, than alluvium. Based on these relations, the average hydraulic conductivity in unconsolidated deposits in Kamas Valley likely decreases with depth due to increased clay content and compaction, and increases from east to west as the relative proportion of alluvium to alluvial-fan deposits increases.

The hydrogeologic properties of bedrock units in the study area depend strongly on lithology and fracture characteristics, attributes that vary significantly among the different stratigraphic units. The stratigraphic column can be divided into stratigraphic ground-water compartments (SGWCs) and low-permeability units.

SGWCs are composed of formations, groups of formations, or individual members within formations. Heterogeneous SGWCs consist of complexly layered rock sequences whose individual beds have variable thickness, lateral extent, and hydraulic conductivity. SGWCs and clustered sandstone beds in heterogeneous SGWCs should be the primary targets for future water wells.

Regionally continuous fault zones may partition the Kamas-Coalville region into large-scale structural groundwater compartments. Compartment boundaries are internally complex and likely restrict transverse flow of ground water due to severing of SGWCs and the presence of finegrained, impermeable fault rock along the fault plane.