THE AGE OF THE EAST TRAVERSE MOUNTAIN LANDSLIDE, UTAH: DISRUPTION OF MIOCENE LAKE SEDIMENTS AND U-PB AGES OF FRACTURE-HOSTED OPAL

The East Traverse Mountain (ETM) , located in Utah, is believed to have been emplaced by a mega-landslide. We have used two independent methods to determine its age. One method uses the age of a fallout tuff in the sediments of the Jordan River Narrows Member (T_jrn) of the Salt Lake Formation. The T_jrn is a white marlstone that was deposited in a lacustrine environment during the Miocene. The ETM landslide is projected to have slid into the lake where the T_jrn was forming and disrupted its deposition to form the Camp Williams Member. The fallout tuff layer is several centimeters thick and occurs 30m below the Camp Williams Member and has an apatite fission track age of 6.5 +/- 0.5 Ma (Naeser, 1983). The tuff provides an approximate age for the ETM landslide since it appears to have been deposited a few hundred thousand years before the landslide. Further research on the tuff is being done for a more precise age. Moreover, the Camp Williams Member is a pebble conglomerate to sandstone near its base but grades upward quickly to siltstone and mudstone. Our survey of the composition of over a hundred clasts of the basal conglomerate accords with that of Slentz (1955) and shows that the conglomerate includes clasts of the same stratigraphic units and rock types as those included as clasts and blocks in the ETM. This suggests that the ETM was the source of the pebble conglomerate.

The second method for determining the age of the ETM landslide is U-Pb ages of fracture-hosted opal in the ETM. Previous studies on U-bearing opals show they typically form at low temperatures from hydrothermal alteration of silicic tuff or rhyolitic lava. The ETM landslide is projected to have been emplaced in standing water. The U-bearing opal is believed to have been produced as this water percolated through the landslide block’s fractures. We dated 11 samples using sensitive high-resolution ion microprobe techniques. Individual samples yielded ages between 5.3 and 39.9 Ma. A weighted average of the youngest 5 opal samples came out at 6.1 ± 0.9 Ma (MSWD = 2.1) with the others yielding imprecise ages ranging from ~20 and ~40 Ma. The two methods suggest that the age of the ETM is about 6.1 to 6.5 Ma.