NON-SURGE ADVANCE OF THE DES MOINES LOBE DURING ITS ALTAMONT PHASE

Three successive advances of the late-Wisconsinan Des Moines Lobe formed sequentially in Iowa, from south to north, the Bemis, Altamont and Algona end moraines. These advances are believed to have been three distinct surge episodes of the Des Moines Lobe. In addition to the deposition of end moraines, the lobe’s advances deposited washboard (minor) moraines. Previous studies indicate that these low-relief moraines formed as a result of basal till squeezed into transverse crevasses, which opened in response to high rates of longitudinal extension associated with surging. Upon stagnation debris contained within crevasses melted out, depositing sediment in a “washboard” pattern.

Using 1 m LiDAR data over the Des Moines Lobe and a newly constructed map of 49,333 minor moraines, we analyze depositional features of the Des Moines Lobe. The mean amplitude of the minor moraines that lie between the Altamont and Algona end moraines is reduced by ~30% in comparison with minor moraines to the north and south. The spacing of minor moraines within 10 km of either side of the Altamont moraine have a Z-score of 1.4, indicating they are statistically the same populations. Many formerly unrecognized longer wavelength moraines have been mapped and interpreted as recessional moraines, and 80% of these lie between the Altamont and Algona end moraines.

Based on minor and recessional moraine records we postulate that unlike the Bemis and Algona advances the Altamont advance was not a surge advance with subsequent stagnation, and hence did not deposit its own set of washboard moraines. Rather, the earlier Bemis surge advance likely deposited the minor moraines between the Altamont and Algona moraines. This conclusion is supported by three lines of evidence: 1) the smaller amplitudes of minor moraines between the Altamont and Algona end moraines likely indicate overriding and erosion of these minor moraines during the Altamont advance 2) the similarity of minor moraine spacing north and south of the Altamont moraine provides no evidence for the moraines being the result of two distinct advances, 3) the concentration of recessional moraines between the Altamont and Algona moraines supports that the lobe’s retreat from the Altamont moraine was by progressive recession of the margin, rather than by widespread stagnation and downwasting after a surge.