STRATIGRAPHIC, LITHOLOGIC, AND GEOMORPHIC EVIDENCE FOR A TWO-PHASE ADVANCE AND RETREAT OF THE LATE WISCONSIN DES MOINES LOBE

Reconstruction of the Des Moines Lobe’s (DML) recessional history was completed by correlating landforms and sediments deposited during Greenland Stadial 2.1a (GS2.1a) (17.7 to 14.7 ka cal BP). Four well-dated ice margins provide chronologic control within the study area in central Iowa, southwestern Minnesota, and eastern South Dakota. Multiple lines of evidence suggest that the DML had two advance/retreat phases during GS-2.1a followed by widespread stagnation and rapid retreat upon the onset of Bølling-Allerød (B-A) interstadial (GI-1) (~14.6 ka cal BP).

The DML advanced to the Bemis margin ~17.0 ka cal BP. The ice then systematically retreated, producing a continuum of geomorphic features including the Altamont moraine (~16.2 ka cal BP) and associated landforms. This single-phased event in Iowa is further substantiated by the deposition of one homogenous basal till unit capped by discontinuous, interbedded sorted and unsorted sediment, formally known as the Dows formation, a stratigraphic relationship observed throughout the DML till plain of Iowa. Stratigraphic analysis using combined allostratigraphic and lithologic methodologies of 30 rotosonic drill cores in Minnesota indicates the DML retreated out of southern Minnesota during this phase.

At ~14.8 ka cal BP, the DML readvanced to the Algona moraine in northern Iowa. Geomorphic, sedimentologic, and lithologic relationships associated with the Algona phase establish that (1) this is the second and the last phase of the DML to occur in southern Minnesota, (2) sustained ice flow perpendicular to the inferred regional flow direction occurred along the lateral margins at the base of topographic highs, (3) change in ice flow direction indicates the formation of lateral shear margins (4) bed topography influenced ice flow dynamics, and (5) changes in till lithology observed in continuous landforms along the lateral margins indicate compositional variability within a single DML till sheet and not separate advances.

The onset of the B-A occurred soon after ice advance to the Algona, resulting in widespread stagnation and rapid retreat to the Big Stone margin (~14.1 ka cal BP) at a rate of 250 meters/year. The scale of stagnation and resulting surficial landforms is a unique response to unprecedented global warming associated with the B-A interstade.