THE ESCALANTE BAY OF LATE PLEISTOCENE LAKE BONNEVILLE, UTAH: SHORELINES IDENTIFIED FARTHER SOUTH THAN PREVIOUSLY RECOGNIZED

New geologic mapping in the Escalante Desert of southwestern Utah suggests that the shorelines of late Pleistocene Lake Bonneville extend approximately 12 km south of the town of Lund, where previous workers have recognized them. The newly mapped features are along the northwestern side of Table Butte at an elevation of 1,560 m. This is the same elevation as well exposed shorelines at and south of Milford, Utah. Although obscure, the shoreline features include beach deposits, spits, and offshore bars. The beach deposits are composed of pebbles, sand, and silt and are as much as 200 m long and 50 m wide. The bar features, located along the shore, are composed of sinuous sand and gravel ridges as much as 0.5 km long and 100 m wide. Both the beach and bar deposits contain subrounded to rounded pebbles less then 3 cm long and consist of chert, vein quartz, early Tertiary (Claron Formation) and Paleozoic limestone, and distinctive but minor (about 5 percent) late Tertiary basaltic scoria. Although most of the rounding of the pebbles could have taken place during Claron deposition, the rounding of the scoria and some other lithologies is inferred to result from wave action and longshore drift along the shoreline.

Several enigmatic ridges occur on the northeast side of Table Butte. They are similar in size to the Bonneville bars, but are oriented perpendicular to the shoreline, splay to the west, and are at an elevation higher than the shoreline. They may represent stream channels formed by floods originating from a contemporaneous lake in Cedar Valley that spilled over a drainage divide at about 1,675 m and then cascaded westward through Iron Springs Gap and then into the Escalante Bay. The study of the Bonneville shorelines, and related deposits and features is continuing.