Abstract

During the summer of 1969, an integrated geophysical study was completed in Cache Valley near the state line between Utah and Idaho. The survey covered about 30 sq mi and was designed to provide data on unconsolidated sediments and basin structure in a study of the ground-water resources of the area. Gravity, magnetic, resistivity, and seismic reflection methods were employed to obtain the desired information.

Detailed gravity data revealed that the west side of Cache Valley is underlain by a trough which is about 5 mi wide and which is filled with as much as 7,000 ft of Quaternary and Tertiary sedimentary units. The magnetic data revealed a probable mafic dike intruded into Paleozoic and Precambrian bedrock in the Lewiston area and possibly a buried volcanic flow unit in the Tertiary sediments. Interpretations of 38 vertical electrical soundings supported by three well logs showed that an area of about 8 sq mi in the center of the surveyed area is underlain by conductive clays (resistivity less than 10 ohm-meters) that are as much as 400 ft thick. Most of the area surveyed is underlain at depths of 100 to 500 ft by more resistive materials (20 to 40 ohmmeters) interpreted to contain a higher percentage of coarse-grained sediments. These sediments may constitute a potential aquifer.

Seismic reflection data further confirmed the existence of the massive clays in the central part of the surveyed area, revealed complex sedimentary geology at the valley margins, and defined subtle structures within the valley fill.

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