SEQUENCE STRATIGRAPHY OF LACUSTRINE DEPOSITS - A QUATERNARY EXAMPLE FROM THE BONNEVILLE BASIN, UTAH

The late Quaternary lacustrine sedimentary record in the Bonneville la ke basin in the eastern Great Basin provides an excellent example of s equence 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 Leamingto n and Delta, Utah. Both alloformations contain offshore marl units and fine-grained deltaic or underflow-fan deposits. They can be identifie d 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 Allofor mation is overlain by deeper-water marl and down-lapping regressive-ph ase deltaic silt. The Bonneville Alloformation lies unconformably abov e 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 l ake cycle. Marl deposited during the deep-water phase is overlain by d own-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 st ratigraphic interpretation leads to the conclusion that the Sevier Riv er delta as a whole is probably made up of a number of sediment sequen ces, each composed of several facies. Recognition of this complexity c ould be important in potential applications of the stratigraphic model .