ORIGIN OF HOLOCENE LOESS IN THE CENTRAL GREAT PLAINS: A RECONSIDERATION

Thick Bignell Loess of the central Great Plains provides a high-resolution stratigraphic record of episodic Holocene dust deposition that has few if any rivals among loess sequences worldwide. Thick Bignell Loess occurs mainly where the underlying Late Pleistocene Peoria Loess is also thickest, at an abrupt boundary between loess and dunefields or wind-scoured surfaces farther upwind (northwest). My colleagues and I have argued that those dunefields and erosional surfaces were the major immediate source of Bignell Loess, when aeolian sand activity released dust temporarily stored there. The stored dust was ultimately derived from a variety of dryland dust sources in semiarid landscapes farther upwind (Mason et al., 2003, QR 60:330-339).

Here I consider an alternative (or complementary?) conceptual model. The thickest Bignell Loess is typically found at the brink of high west- to north-facing escarpments, with upper slopes cut in Peoria Loess (often 30-50 m thick). Bignell Loess could have been deflated directly from exposed Peoria Loess on the escarpment slopes and from reworked loess on ephemeral stream beds or fans at the foot of the escarpments. Sweeney and Mason (2013, JGR-ES, 118:1-12) demonstrated the susceptibility of coarse Peoria Loess to direct wind entrainment, as required for this model. Thick Bignell Loess is coarser than underlying Peoria Loess and displays clear evidence of enhanced “clifftop” deposition where flow separation occurs at the escarpment brink, both observations consistent with short-distance, low-level transport from the escarpment itself. Finally, new data indicate very similar Nd-Sr isotopic compositions for Bignell and Peoria loess (Yang et al., in review). On the other hand, earlier work found that volcanic glass and Zr content and clay mineralogy differ between Bignell and Peoria Loess (Mason and Kuzila, 2000, QI 67:119-131; Jacobs and Mason, 2007, GSAB 119:462-475). Dust from the escarpments alone may be inadequate to account for the extensive thin mantle of Bignell Loess across the Central Plains. Thick Bignell Loess in some localities is hard to explain with either model, probably requiring additional kinds of sources, such as river floodplains. Both models discussed here are consistent with interpretation of Bignell Loess as a record of regional climatic change.