Surficial geologic map of the Nephi segment of the Wasatch fault zone, eastern Juab County, Utah (M-170)

By: K. M. Harty, W. E. Mulvey, and M. N. Machette

The Nephi segment of the Wasatch fault extends 33 kilometers from north of Payson to south of Nephi. Quaternary deposits along the segment are middle Pleistocene to late Holocene in age, and consist primarily of coalesced alluvial fans and Lake Bonneville lake-bottom sediments. The Bonneville lake cycle spanned approximately 16,000 years, but the lake occupied Juab Valley for only about 1,000 years during the lake's highstand. Other Quaternary deposits in the mapped area include stream alluvium, landslides, glacial till, eolian deposits, and colluvium.

Three surface-faulting earthquakes have occurred on the Nephi segment during the middle to late Holocene. Morphology of the scarp formed by the most recent event and its lack of vegetation indicate that it may be the youngest surface-faulting event on the Wasatch fault. Radiocarbon dates from trenches indicate that it is younger than about 1,200 years, but the surficial expression leads researchers to suggest the most recent event may be as young as 300 to 500 years old. Based on trench data, the two previous events occurred less than about 4,000 years ago and between about 4,000 and 5,300 years ago. Little is known of the paleoseismic history during latest Pleistocene time, except that slip rates appear to have been much lower than in the Holocene.

This map extends USGS 1 :50,000-scale surficial geologic mapping south to include all of the most active central segments of the Wasatch Fault Zone. The goal of this study, as for previous USGS mapping studies along the Wasatch Fault Zone, is to provide basic geologic data needed for accurate assessments of paleoseismic history and earthquake hazards associated with the Wasatch fault Zone.

Other Information:
Published: 1997
Pages: 14 p.
Plates: 1 pl.
Scale: 1:24,000
Location: Juab County
Media Type: Paper Map