The Pony Express basaltic ash is locally a useful stratigraphic marker in Lake Bonneville sediments in west-central Utah. The ash was erupted from a vent in the Sevier Desert basin soon after Lake Bonneville had transgressed high enough to flood into the basin about 24,000 years ago. The ash is found at or near the base of the Bonneville marl below altitudes of 1400m (4600 ft) in part of the Sevier Desert basin and the southernmost part of the Great Salt Lake basin. The chemical composition of the ash is similar to that of other basalts in the Sevier Desert. Possible source vents are in the Pahvant Butte area or a maar near Smelter Knolls.
This 20-page report summarizes observations of sediments and shorelines of the Gilbert episode in the Bonneville basin of northwestern Utah. Lake Bonneville dropped to altitudes similar to those of modern Great Salt Lake by 13,000 years ago, remained low for about 1400 years, then rapidly rose about 50 ft (15 m) during the Gilbert episode (about 11,600 years ago). The Gilbert lake was probably less extensive than shown by previous mapping of the Gilbert shoreline. The lake reached altitudes of 4250-4255 ft (1295-1297 m), and its shoreline, which is not well defined anywhere in the basin, was probably not deformed by residual isostatic rebound associated with removal of the Lake Bonneville water load. Holocene Great Salt Lake has not risen as high as the Gilbert-episode lake.
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