SEDIMENTOLOGIC, CLIMATIC, AND DUNE SAND SOURCE IMPLICATIONS OF A LATE WISCONSIN TERRACE OF THE NIOBRARA RIVER, NEBRASKA

A prominent high terrace and associated pebbly sand deposit, 1-9 km wide and 2-25 m thick, occurs between 30 and 50 m above the Niobrara River in northern Nebraska. The deposit, informally called the “Connely Flats beds” (CFb), ranges from cross-bedded, medium-grained sand with clayey and silty lenses to pebbly gravels. Pebbles of acidic and mafic igneous rocks comprise 80-95% of the non-locally derived clasts and were likely reworked from the Miocene Runningwater Formation of northwestern Nebraska. Chert pebbles comprise 5-20% of the clasts and were probably derived from the Paleozoic carbonates in the Hartville Uplift. The CFb contains a late Rancholabrean (late Wisconsin) fauna dominated by burrowing mammals and indicative of a full-glacial climate. Locally, the CFb overlies lacustrine and fluvial deposits yielding radiocarbon ages older than 32,000 yBP. A soil developed on top of the CFb yielded a radiocarbon age of 9,300 yBP, and three radiocarbon ages on gastropods from within silty sands of the CFb range from 14,300 to and 27,700 yBP. The CFb is interpreted to have been deposited in a bedload-dominated stream system that experienced significant periods of no-flow and in a drainage basin similar to that of the modern Niobrara River.

Geochemical analyses of the sand-fraction of the CFb and adjacent dune sand of the Nebraska Sand Hills suggest that some of the previously reported mineralogical maturity of dune sands in the northern Sand Hills can be explained by southward eolian transport of sand from the CFb, rather than appealing to ballistic impact as a means to remove feldspars.