A POSSIBLE VOLCANIC ASH WITHIN THE GROVER GRAVEL, ST. LOUIS COUNTY, MISSOURI

The Grover Gravel in western St. Louis County, Missouri is generally inferred to be Pliocene in age, but it contains large crystalline erratics and fills ice-wedge casts in the subjacent substrate, indicating a Pleistocene age. Recent exposures at the type section reveal a white silt layer within the Grover, which resembles a volcanic ash. If this bed is an ash, it could provide a definite age for the Grover.

The white silt contains vitric grains, but at low concentrations. Therefore, the heavy mineral content of the silt and gravel were studied, first to verify that the two units have different provenances, and second to compare the characteristics of the silt with those of common volcanic ashes in the U.S. Relative to the underlying Grover Gravel, the silt is enriched in the unstable minerals biotite, allanite, and magnesium-rich chain silicates, and is depleted in the ultrastable minerals zircon, tourmaline, and rutile. Furthermore, the glass shards in the ash are primarily pumiceous. The difference in mineralogical makeup between the ash bed and the gravel indicates that the two units have different provenances, and the heavy-minerals within the silt are characteristic of a W-type rhyolitic ash (i.e. the Bishop ash from Long Valley, CA), not G-type ashes from Yellowstone eruptions. To date, no chemical or chronologic analyses have been performed on the silt, although a concurrent study found the silt to be magnetically normal and provided a cosmogenic-nuclide burial date of 0.85 +/- 0.4 Ma for the base of the gravel. Therefore, the mineral composition, type of glass shards and age constraints of the silt are consistent with the Bishop ash (0.77 Ma) from the Long Valley caldera in California.