An engineering-geological investigation for a proposed reservoir in Pleasant Grove (eastern Utah County), Utah identified a composite landslide interpreted to be part of a previously unrecognized, 1.5 sq. km landslide complex. The site, located in the foothills of the Wasatch Mountains, is at an elevation of about 1,532 m.

Two main geologic issues were of concern: the possible presence of the landslide-prone, Manning Canyon Shale at shallow depth, and; the potential for seismically induced ground rupture, based on previous mapping of an on-site, “active” fault.

Our investigation included geologic mapping, interpretation of aerial-photographs, and exploratory trenching. Evidence of faulting was not observed. However, we did identify an approximately 0.2 sq. km composite landslide. Trenching exposed multiple, gravity-driven slip surfaces associated with both pre- and post-Bonneville landslide events. Along two slip surfaces, intensely sheared lower Pennsylvanian to upper Mississippian Manning Canyon shale was thrust over late Pleistocene Bonneville lake cycle sediments. We also documented several high-angle reverse faults near the landslide toe. Landslide deposits and associated shear surfaces were not observed in the initial engineering borings drilled at the property.

The topographic location of these features suggests a driving mechanism and a deep-seated detachment surface associated with a larger landslide complex emanating from the mountain front to the east. Physiographic evidence of such a landslide complex is visible on aerial photographs.

Similar geomorphic features and geologic relationships occur in other areas of east Utah County. These features are particularly evident in areas where Manning Canyon shale is indicated on geologic maps, suggesting the presence of other relatively large, previously unrecognized landslide complexes. As urban growth encroaches into foothill areas of eastern Utah County, recognition of these relatively large landslide features and their implications for development become ever more critical. We therefore recommend that engineering geologists recognize the landslide potential in such terrain, particularly in light of on-going hillside development.