QUATERNARY LANDSCAPE AND DRAINAGE EVOLUTION IN THE DRIFTLESS AREA

The timings and mechanisms for deep entrenchment of the upper Mississippi River and its tributaries remain a debated topic in the Quaternary science community. Most recent investigators agree that the upper Mississippi River course was established across the Niagara (Silurian) and Galena-Platteville (Ordovician) dolomite cuestas during early Quaternary drainage diversions along the eastern margin of pre-Illinoian glacial ice. However, exact timing remains in question. Magnetic polarity of sediments in deeply incised valleys of western and southwestern Wisconsin indicates that approximately 200 m incision, representing approximately three-quarters of post-Cretaceous incision of bedrock valleys, had occurred by about 790 ka. The post-Cretaceous long-term average incision rate of about 0.0033 mm/yr significantly under estimates Quaternary incision rates. Elevation comparisons between a modern bedrock valley bottom and the bedrock valley bottom of an adjacent cut-off valley meander whose basal fill correlates with 500 ka glacial deposits in NE Iowa indicate valley bedrock incision averaged about 0.048 mm/yr over the last 500 ka. A similar comparison for another bedrock valley meander that was cutoff following Illinoian age (OIS 6-8) valley backfilling indicates an average incision rate of 0.046 mm/yr since about 130 ka. Driftless Area valleys experienced massive aggradation with sandy gravel during late Wisconsin (OIS 2) tundra conditions, and this aggradation caused valley bottom bedrock incision to cease since then in drainages larger than a few square kilometers. A few meters of strongly weathered alluvial gravel underlies late Wisconsin (OIS 2) alluvium. Possible contributors to the relatively active bedrock incision throughout much of the Quaternary include: lowered regional base level associated with Mississippi River drainage displacement, long-term regional uplift, and landscape forebulge in front of nearby glacial advances.