Abstract

The penultimate deep lake in the Bonneville basin occurred before 100,000 yr ago and was followed by a long interval of lake levels below 4,530 ft. The last, or Bonneville, deep-lake cycle had begun by 25,000 yr ago and included: (1) The Stansbury Shoreline-forming oscillation 23,000 to 20,000 yr ago, (2) Zenda Threshold control of the Bonneville Shoreline, interrupted by the Keg Mountain oscillation, from about 16,000 to before 14,000 yr ago, (3) the Bonneville Flood before 14,000 yr ago, (4) Red Rock Pass Threshold control of the Provo Shoreline from immediately after the Flood to before 13,000 yr ago, (5) climatic regression leading to near desiccation by 12,000 to 11,000 yr ago, and (6) a minor transgression to the Gilbert Shoreline 11,000 to 10,000 yr ago. In the Old River Bed type area of Gilbert, the yellow clay is a shallow-water transgressive facies and the white marl is a deep-water facies of the Bonneville lake cycle. Alpine and Draper formations are problematic in basin-wide retrospect. Following the 1983 North American Stratigraphic Code, sediments of the last deep-lake cycle comprise the Bonneville Alloformation and time represented by those sediments comprises the Bonneville Episode. Isostatic deformation originating in the Bonneville basin apparently deformed the adjacent Lake Waring basin. High shorelines in the Waring basin may be equivalent to the threshold-truncated Bonneville record (i.e., the Bonneville and Provo shorelines). Termination of the last deep-lake cycle was probably synchronous in the two basins.