LOESS THICKNESS AND SOIL MORPHOLOGICAL VARIATION ACROSS A FORMER BRAIDED STREAM CHANNEL ON THE ROCK PRAIRIE, WI

The Rock Prairie is a distinctive geographic region in southern Wisconsin. The region formed as a high-level outwash plain during the Johnstown Phase of Wisconsin glaciation. Following abandonment, the outwash surface was covered by >1 m of loess; at the time of settlement vegetation was treeless tallgrass prairie that has since been converted to intensive agriculture. One of the most distinctive features of the Rock Prairie is the well preserved braided channel pattern apparent on aerial photos, and as subtle topographic lows in the field. This research investigates the thickness of loess across one channel segment, and how soil properties contribute to the appearance of the former channel pattern. Our study site is located in an agricultural field in eastern Rock County, WI, approximately 3.5 km south of the Johnstown Moraine and 7.5 km west of the Darien Moraine. We collected four soil cores with a Giddings probe along a transect that crossed a former channel segment between preserved bars, along with an additional core on another bar 500 m north. Modern surface relief across the channel is 0.7 m, and sub-loess relief is 1.1 m. Loess thickness in the three cores on the bars averages 1.28 m and in the channel averages 1.81 m. Loess was clearly redeposited into the channels, based on thickness and the occurrence of stratified sand and pebbly silt in the channel center. It is not clear if the loess was eroded from the adjacent bars, was first trapped in the moist channel, or was washed from further up in the fluvial system as flow waned. Outwash beneath the bars is coarse, with cobbles up to 12 cm in diameter. Outwash beneath the loess in the channel is sandy for several decimeters. Soil formation is largely confined to the loess, but dissolution of carbonate rocks and accumulation of illuvial clay occurs at the loess-outwash contact on the bars; in the channel center, structure development and illuvial clay features extend into the stratified sand and silt. Redoximorphic features indicating seasonal wetness occur in the lower solum of the soils in the channel. Soil organic carbon content and thickness of topsoil are greatest in the channel center. We conclude that the prominent appearance of the braided channel pattern on aerial photos is likely due to greater soil moisture storage and SOC content that enhances tonal differences.