DEPOSITIONAL TIMING AND PALEOCLIMATIC IMPLICATIONS OF SAND DUNES IN THE MEREDOSIA BOTTOMS, NORTHWESTERN ILLINOIS, USA

Recent attention has been given to eolian deposits and the information they provide about paleoclimates and geomorphological events. This study was undertaken to determine the ages and origin of eolian deposits in sand dunes of the Meredosia Bottoms of Rock Island and Whiteside counties in northwestern Illinois. This low-lying area of the ancient Mississippi Valley is the westward extension of the Green River Lowlands, characterized by very low relief and a deep bedrock valley filled with a thick succession of fluvial/glaciofluvial sands and gravels. Scattered eolian dunes on the surface are mapped as Parkland Sand and were previously interpreted to be sourced from deflation of the Henry Formation deposited more than 20 kyBP (Anderson, 1967; Willman and Frye, 1970). Recent work by Miao et al. (2009) found that optical dates gathered from dunes the nearby Green River Lowlands suggest the sources of the sands are linked to catastrophic drainage of Glacial Lake Scuppernong in southern Wisconsin around 17,500 ± 570 YBP. This event eroded and subsequently deposited sediments from the Rock River valley which without vegetation were subject to eolian transport. Holocene optical dates from Miao et al. (2009) produced scattered dates ranging between 400 and 9600 YBP which suggest localized aeolian remobilization.

This study investigates the stratigraphy of a large sand dune at (N 41° 40.887', W 090° 16.045'). We used NIU's Geoprobe 6600 direct push rig to take two continuous core samples approximately 20 m apart using the Dual Tube 22 method. The stratigraphic column consisted of a thin poorly developed soil layer overlying a 6 m layer of well-sorted sand progressing from fine to course-grained down the column. Below the sand layer was a poorly sorted layer with grain sizes ranging from clay to medium gravel suggesting a fluvial depositional environment. Below that was well sorted sand and gravel typical of the Henry Formation. Samples taken in 10cm intervals of the first meter of recovery were dominated by sand, with between 1% and 3% total silt and clay. Lack of silt and clay and weakly defined soil horizons point to late-Holocene reactivation, the cause of which could be frequent grass fires. This is supported by anecdotal evidence from the land owner. Localized Holocene reactivation further supports findings of Miao et al. (2009).