POSSIBLE ISOSTATIC RESPONSE TO QUATERNARY EROSION IN THE CENTRAL MISSISSIPPI RIVER VALLEY

Discontinuous high-level terrace remnants of the ~3.2 Ma ancestral Mississippi River system locally called the Upland Complex (UC), Mounds gravel, pre-loess gravel, etc. have been mapped from Louisiana north into Illinois. The Pliocene Mississippi River floodplain (UC) was left as a terrace during an early Pleistocene sea level low stand and was subsequently buried by Pleistocene loess. The Pliocene Mississippi River drained much of southern Canada, had a discharge that was 6 to 8 times greater than the current average discharge at Vicksburg, Mississippi, and within the northern Mississippi embayment has a base-of-terrace elevation 70 m higher than the base of the Holocene Mississippi River floodplain. The UC average thickness is currently 10 m; however, we interpret the UC to be the basal sand and gravel erosional remnant of an upward fining, originally much thicker, Pliocene floodplain.

Pliocene sea level was +25 m requiring that the UC has risen 45 m (70 m - 25 m) within ~3.2 Ma. We believe this 45 m uplift is isostatic. However, 15 m of overlying Pleistocene loess deposition would result in 13 m of post-loess subsidence resulting in an original isostatic uplift of 58 m (45 m + 13 m). An 85% isostatic uplift response to erosion of the UC upper sand/silt/clay facies indicates that 68 m (58 m/0.85) of the UC was eroded. Since the current average thickness of the UC is 10 m then its original thickness was 78 m (68 m + 10 m).

Geomorphology of the central Mississippi River valley supports regional Wisconsinan through Holocene uplift. Wisconsinan Mississippi and Ohio/Mississippi terrace distribution reveals that these rivers shifted westward and eastward respectively relative to the down-valley axis (Crowley’s Ridge) during the Wisconsinan. Basin asymmetry analysis also indicates Wisconsinan and Holocene tributary valley migrations away from the axis of the Mississippi River valley. This apparent isostatic response to Pleistocene erosion of the Mississippi River Valley may be a contributing factor in the Quaternary reactivation of the underlying Reelfoot Rift faults and its New Madrid seismic zone.