STRATIGRAPHY AND GEOCHRONOLOGY THE TODD VALLEY, AN ABANDONED SEGMENT OF THE PLATTE RIVER VALLEY, EASTERN NEBRASKA

The Todd Valley (TV) is as an abandoned segment of the lower Platte River Valley between uplands eroded from loess-mantled pre-Illinoian tills. The valley is approximately 46 km long and 12 km wide and trends northwest to southeast. The TV was incised into sandstones and mudrocks of the Cretaceous Dakota Formation and was only partially filled with sediments after its abandonment. The TV fill ranges from 33 to 60 m in thickness, consisting primarily of well-sorted fine sand that overlies alluvial gravelly sand. The fine sand is largely eolian that was deposited as sand sheets and low-relief dunes which were produced through the deflation of alluvial sediments by strong northwesterly winds. It is absent at the very northern edge of the TV, but it thickens southward. There is also a thin cover of Peoria Loess atop the valley fill. The loess thins to the south, ranging in thickness from 9 to 11 m at the northern end, to 3 to 7 m in the south.

We collected OSL dating samples from cores, including ten from the eolian fine sand and three from the underlying alluvial sediments. Preliminary age estimates indicate deposition of the eolian sand sheet and dunes between 17 to 21 ka and deposition of the uppermost portion of the underlying alluvial sediments around 21 to 25 ka. The stratigraphy and geochronology indicate that the valley was abandoned ca 21 ka, and that eolian sand was moving on the former alluvial surface until 17 ka when the sand was stabilized and loess deposition began. Thicker loess on the northern edge of the TV probably reflects longer-term loess aggradation on the gravelly alluvium that was found in this portion of the valley. Toward the south, however, eolian sand continued to move, delaying landscape stabilization and resulting in a thinner loess cover.