AN OCCURRENCE OF LATE-PLEISTOCENE SPRUCE MACROFOSSILS IN A PLATTE RIVER TERRACE FILL, SOUTH-CENTRAL NEBRASKA, USA

The Platte River in central Nebraska occupies a broad valley that is composed of multiple low alluvial surfaces and fills that range in height from less than 2 m up to 9 m above the modern channel. For this study, which was initiated as a geological mapping project of the Kearney 7.5-minute USGS quadrangle, we investigated several of these abandoned surfaces through coring. One of the alluvial surfaces found to the north of the river stands approximately 4.6 m above the modern channel. Our core collected from this surface revealed approximately 6.3 m of Peoria loess that overlies 2.6 m of fine-grained alluvium. The core terminated in alluvial sand and gravel at a depth of 9.1 m below the ground surface. Between the depths of 8.2 to 8.7 m the fine-grained alluvial sediments are dark brownish gray and contain well-preserved plant material. Our initial screenings demonstrate that the plant materials are almost entirely spruce needles, with lesser amounts of fine wood fragments and one sedge seed. The spruce needles are mostly broken bases and tips, and both their quantity and condition suggest they were deposited on a spruce forest floor. Two samples of spruce needles from the upper and lower portions of this deposit yielded AMS ¹⁴C ages of 22,900-22,440 cal yr B.P., and 23,680-23,100 cal yr B.P. (2σ) respectively. This interval is near the onset of Peoria loess deposition in central Nebraska (~25,000 years ago) and indicates the presence of spruce forest on the Platte valley bottom just prior to or around the last glacial maximum. Previous research has reported spruce pollen and charcoal from late Pleistocene deposits on the central Great Plains, including Nebraska and Kansas. However, well-preserved spruce macrofossils from this region are rare. Our continuing investigation will include additional drilling to further document the extent of the bed, additional age control, and further identification of plant materials from this deposit.