227-8 Early Pleistocene Cosmogenic Burial Ages for the Upland Complex on Crowleys Ridge, Northeastern Arkansas

Session: Recent Advances in Glacial Geology, Geomorphology, and Chronology (Posters)

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Abstract:

The Mississippi River Valley (MRV) Upland Complex (UC) is a high-level fluvial gravel overlain by Pleistocene loess, extending from southern Illinois to Louisiana, including on the ~320 km-long upland in the MRV known as Crowleys Ridge. Widely regarded as a braided Pliocene gravel deposit from the ancestral Mississippi River, the UC may have formed a ~150 m-thick floodplain spanning over 100 km across the MRV. Today, it is typically less than 40 m thick, suggesting erosion occurred and much of the deposit may be missing. Despite decades of research, the UC is still profoundly important because it is the only significant sedimentary record of the Mississippi River between the late Eocene and Pleistocene.

Previous studies show that radiometric ages from eleven UC sites, taken from deposits along the valley walls and from Crowleys Ridge, range from ~3.6 Ma to ~1.4 Ma. This research applied ²⁶Al/¹⁰Be cosmogenic burial dating at six new UC sites along Crowleys Ridge, yielding the following ages, from north to south: 2.16 ± 0.45 Ma, 2.33 ± 0.30 Ma, 1.56 ± 0.45, 2.22 ± 0.25 Ma, 2.44 ± 0.33 Ma, and 1.36 ± 0.38 Ma. These ages suggest the upper 30-40 m of the UC on Crowleys Ridge is early Pleistocene or younger. Boulders exceeding 1 m in diameter, though uncommon, occur in most Crowleys Ridge gravel quarries and are best explained by ice rafting, supporting a Pleistocene age. Furthermore, a thin, gray, silty-clay deposit, interpreted as a weathered Yellowstone tephra (Huckleberry Ridge or Lava Creek), is found in the upper sections of many UC exposures, also supporting an early Pleistocene age.

From a continental-scale fluvial system perspective, ages from this research suggest the UC was likely deposited under Pliocene hydroclimatic conditions, and then, at least on Crowleys Ridge, early Pleistocene glacial meltwater eroded and replaced the Pliocene UC. This pattern mirrors late Pleistocene MRV dynamics, where the current alluvial fill composed of last-glacial outwash replaced earlier glacial deposits. Pliocene UC ages reported on the edges of the MRV may have escaped erosion by Pleistocene glacial meltwater. These new ages provide needed insights into the UC, which holds keys to understanding North American basin reorganizations, landscape evolution, the Pliocene-Pleistocene climate transition, early Pleistocene glacial cycles, and perhaps mechanisms driving intraplate seismicity.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10913

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