NEW GEOLOGIC MAPPING IN THE TENALQUOT PRAIRIE AND VAIL QUADRANGLES, WASHINGTON, INDICATES THAT CASCADE ARC VOLCANISM STARTED ABOUT 46 MA, ASSOCIATED VOLCANIC DEPOSITS LIKELY FILLED AN EOCENE FAULT-BOUNDED TACOMA BASIN NORTHEAST OF THE OLYMPIA STRUCTURE, PRE-VASHON DEPOSITS HERE EXTEND SEVERAL KILOMETERS FARTHER SOUTH THAN VASHON DEPOSITS, AND HIGH-VOLUME MELTWATER DISCHARGE MARKED THE GLACIAL ICE MARGIN

Geologic mapping of the Tenalquot Prairie quadrangle and the northern two thirds of the Vail quadrangle in Washington’s southern Puget Lowland provides insights into the onset of Cascade volcanism, the nature of the northwest-trending Olympia structure, and the extent of Vashon and pre-Vashon glaciations. Field mapping, 59 new whole rock analyses, eight new radiometric ages, and two northeast-trending gravity and magnetic profiles indicate that all igneous rocks in the map area are middle Eocene Northcraft Formation, including mafic to intermediate shield lavas, vent breccias, mafic dikes and stocks, and intermediate to silicic lava and pyroclastic flows. The oldest ⁴⁰Ar/³⁹Ar sample, at 45.8 ±0.07 Ma, indicates that Cascade arc magmatism overlaps Siletzia volcanism. Preliminary analysis of aeromagnetic and gravity potential fields suggests that the Olympia geophysical lineament is a normal fault and the Tacoma basin northeast of it contains several thousand ft-thick shallow igneous rocks that likely are Northcraft Formation. If both are true, then the Olympia fault has experienced no major northeast-down movement since the end of Northcraft Formation volcanism.

Vashon glacial outwash provides productive but easily polluted aquifers and hosts widespread, enigmatic Mima mounds. Hummocky drift east, north, and west of Tenalquot Prairie and along the hill front south of the Deschutes River forms terminal moraines from the eastern (Yelm) and western (Olympia) ice lobes. The distribution of moraines indicates that Tenalquot Prairie was ice-free between the lobes. Lacustrine sand above outwash gravel and terminal moraines between Tenalquot Prairie and Lake Saint Clair indicates that large outwash channels were later submerged by an ice-dammed lake with a 400 ft shoreline elevation. Penetratively weathered drift on both sides of Johnson Creek valley south of the Vashon ice limit indicates that at least one pre-Vashon Cordilleran ice advance extended several kilometers south of the Vashon ice limit. Bouldery outwash trains and denuded bedrock surfaces in the valleys of Johnson Creek and the Skookumchuck River document massive proglacial meltwater discharge south of all known ice limits.