Age and provenance of upland gravels in Missouri, USA, and their relationship to Early Pleistocene glaciation


The age and origin of the Grover Gravel, which is preserved on stable uplands in St. Louis County, Missouri, USA, and similar gravels along the Mississippi River bluffs farther south, have been controversial for over a century. These gravels have some features consistent with a glacial origin but lack obvious erratics such as igneous rock fragments. New observations, however, show that the earliest tills within the Mississippi basin nearly lack such constituents as well, and that the Grover Gravel is a Pleistocene deposit derived from such a till. A Mississippi River terrace deposit in St. Louis County also includes typical clasts of this till and formed between 2.0–1.8 Ma, based on various dating methods. This deposit may correspond to a pulse of glacial meltwater recorded within sediment near the mouth of the Mississippi River and represent outwash from a previously unrecognized glaciation within the US midcontinent. The Mounds Gravel (or Upland Complex) is present farther south around the perimeter of the Mississippi Embayment and contains the same distinct clasts as the gravels in St. Louis County. Given that the age of the Mounds is <~3 Ma based on cosmogenic-nuclide ratios, the southern gravel could be the same age as the gravel terrace in St. Louis County. If so, these gravels could mark a geomorphic transition to glacial-dominated sedimentation farther south in and around the Gulf of Mexico. Regardless, all the upland gravels have insignificant amounts of zircon from provinces west of Hudson Bay. Therefore, detrital zircon provenance does not support a recent hypothesis that the Mounds (Upland Complex) was deposited by a giant Mississippi River with a drainage basin extending across broad areas west of Hudson Bay.


Geography, Geology, and Planning

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