GEOLOGIC HAZARDS FROM LATEST PLEISTOCENE AND HOLOCENE DOWNCUTTING AND MASS MOVEMENT IN PART OF THE URBANIZED CENTRAL WASATCH FRONT, UTAH

New geologic mapping of the Kaysville 7-1/2 minute quadrangle reveals the extent of geologic hazards in this rapidly urbanizing area of the Wasatch Front. The area includes five cities and has a population of about 125,000, which increased almost 35% in the past decade. Although the active Wasatch fault zone bounds the area to the east, hazards other than earthquakes have caused more damage historically. These hazards result from downcutting and landsliding of latest Pleistocene Lake Bonneville deposits and the active Holocene erosion and deposition of debris flows.

As Lake Bonneville fell from its highest level beginning about 16,800 years ago, streams draining the Wasatch Range deeply incised the lake deposits. Snowmelt and precipitation infiltrated the deposits and perched on clay, triggering landslides along incised stream channels and steep delta bluffs. Parts of prehistoric landslide complexes along stream channels recently reactivated, aided by the addition of agricultural and landscape irrigation. Two of the recent landslides, the Heather Drive and Sunset Drive landslides, resulted in damage to 8 houses and economic losses of about $1.5 million since 1998. Potentially unstable delta bluffs covered by the southern extension of the South Weber landslide complex also threaten development in the quadrangle.

Canyons in the Wasatch Range have repeatedly generated debris flows, including severe episodes in the 1920s when rain from cloudburst storms fell on watersheds depleted of vegetation by overgrazing and wildfires, and in 1983-84 when debris was mobilized by rapid melting of a snowpack 160% of normal. Farmland, infrastructure, and houses were damaged regionally in the 1920s and 1980s, and locally in 2004 from the fire-related Compton Bench debris flow.

Two large, liquefaction-induced landslides were previously identified in the quadrangle. One, the Farmington Siding landslide complex, exhibits conclusive evidence of this origin. However, the West Kaysville feature apparently resulted from erosion of fine-grained Lake Bonneville deposits by streams on middle Holocene to late Pleistocene alluvial fans, and spring sapping of lake deposits along older lake shorelines, leaving features superficially resembling landslide hummocks and depressions.