DISCOVERY OF THE BIG BALDY LANDSLIDE NEAR URBAN AREAS ALONG THE CENTRAL WASATCH RANGE, UTAH

Structural, bio- and litho-stratigrahic mapping and geomorphic studies reveal that a giant block (6.125 km³) of Mt. Timpanogos has slid toward what are now densely populated urban areas along the Wasatch Front of Utah. The block forms a prominent peak known as Big Baldy, which consists of steeply dipping and locally brecciated limestone and quartzite units that have slid over nearly horizontal shale. Preferential erosion of this shale out from under overlying limestone and quartzite cliffs is most likely the cause of the slide and others like it identified to the south. These giant slides are currently mapped as down-dropped normal fault blocks, but no traces are found of the large faults required to cause the 100’s of meters of offset observed.

The Big Baldy slide has produced geomorphological features, such as faceted spurs, landslide scarps, sackung ridges and hummocky terrain. Limestone and quartzite beds in the block are back-rotated by up to 90° and are locally disarticulated and brecciated. No evidence of hydro-fracturing is found in the breccia or multiple brecciation episodes, which indicates low vertical stresses (shallow emplacement) and perhaps a single event of slip. Structural reconstructions of the slide block, and interpolation of maximum down-cutting rates on nearby streams, indicate the slide initiated between 600-900 ka.

These results attest to the importance of recognizing the influence of surficial processes in the development of mountain front structure, and demonstrate the ongoing danger of mass wasting to communities along the seismically active Wasatch Front.