Abstract

The late Quaternary lacustrine sedimentary record in the Bonneville lake basin in the eastern Great Basin provides an excellent example of sequence stratigraphy. Two sequences, referred to as the Little Valley and Bonneville Alloformations, are exposed in the bluffs of the Sevier River where it has entrenched its Pleistocene delta between Leamington and Delta, Utah. Both alloformations contain offshore marl units and fine-grained deltaic or underflow-fan deposits. They can be identified and mapped by tracing the unconformity separating them and employing a number of geochronometric tools, including amino acid epimerization in fossil gastropods, radiocarbon and thorium-230 ages, and basaltic tephrochronology. Thin transgressive sand of the Little Valley Alloformation is overlain by deeper-water marl and down-lapping regressive-phase deltaic silt. The Bonneville Alloformation lies unconformably above the Little Valley deposits. Fine-grained deltaic sediments deposited during the transgressive phase of Lake Bonneville fill the entrenched Sevier River valley that was eroded subsequent to the Little Valley lake cycle. Marl deposited during the deep-water phase is overlain by down-lapping deposits of the regressive phase below the Provo shoreline but is the uppermost unit in the altitudinal range where the lake was lowered catastrophically during the Bonneville Flood.

The sequence stratigraphic interpretation leads to the conclusion that the Sevier River delta as a whole is probably made up of a number of sediment sequences, each composed of several facies. Recognition of this complexity could be important in potential applications of the stratigraphic model.

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