GEOCHRONOLOGIC CONSTRAINTS ON THE AGE AND DEPOSITIONAL MECHANISMS OF THE GROVER GRAVEL IN WESTERN ST. LOUIS COUNTY, MISSOURI

The Grover Gravel in St. Louis County, Missouri often has been correlated with the “Lafayette-type” gravels, which mantle upland surfaces along the margins of the Mississippi Embayment several hundred kilometers to the south. Here we report new data for these deposits from tephrostratigraphy and cosmogenic-nuclide burial dating.

Cosmogenic-nuclide burial dates for the southern “Lafayette” gravels indicate a late Pliocene age, in agreement with evidence that they predate continental glaciation. Type-Grover deposits in western St. Louis County, however, postdate initial formation of the Laurentide Ice Sheet. First, the Grover contains large erratic boulders, which must have been transported at some point by glacial ice. Secondly, the Grover locally overlies a pediment surface that truncates a highly weathered remnant of the (2.4 Ma) Atlanta till. Finally, the base of the Grover fills ice-wedge casts within substrate materials, demonstrating that it was deposited under permafrost conditions during a continental glaciation. The stratigraphic constraints are consistent with a single limiting burial date at the type section, which provides a maximum age of 0.85 +/- 0.4 Ma for the Grover. This age, combined with a normal remanent magnetization within a conspicuous white silt layer near the base, indicates that the type-Grover is probably early Middle Pleistocene (< 0.78 Ma) in age.

The white silt is a weathered volcanic ash. Pumice shards within the ash far outnumber platy or bubble-wall shards, and biotite is abundant within the silt’s heavy-mineral fraction. This composition is inconsistent with ash from Yellowstone eruptions, but closely matches that of the Bishop ash (0.77 Ma). This age also coincides with burial ages for the oldest normal-polarity till in Missouri.

Other features indicate that the Grover is not a typical fluvial deposit. The Grover is present widely atop numerous upland remnants, but there is no evidence that deposition was restricted to discrete valleys or channels. Moreover, clasts are usually matrix supported, indicating deposition from mass flows, not traction currents. Thus, we posit that the Grover Gravel in the type area was deposited during a time of widespread landscape instability, which was triggered by a unique combination of melting permafrost mantled by volcanic ash.