Quaternary uplift in the lower mississippi river valley

Abstract

Discontinuous high-level terrace remnants of the ∼3.1 Ma ancestral Mississippi River floodplain, locally called the Upland Complex (UC), are mapped from Louisiana into Illinois. We interpret the UC to be the basal sand and gravel erosional remnant of a much thicker Pliocene floodplain. The Pliocene Mississippi River, at the latitude of Tennessee, has a base-of-terrace elevation 70 m higher than the base of the Holocene Mississippi River floodplain. This difference cannot be attributed solely to changes in sea level and suggests Quaternary uplift that may be ongoing. Borehole data reveal that the base of the UC and that of the Mississippi River alluvium have been tilted southeast. The geomorphology of the central Mississippi River Valley also supports regional Quaternary uplift. Mississippi and Ohio/Mississippi river terrace distribution reveals that these rivers shifted away from the down-valley axis during the Wisconsinan, and basin asymmetry analysis also indicates Quaternary tributary valley migrations away from this axis. Pliocene sea level is estimated to have been at +25 m, requiring that the UC has risen 45 (70-25) m within the past ∼2.4 My. We believe this 45-m uplift may be isostatic. However, 15 m of overlying Pleistocene loess deposition would result in 8 m of isostatic subsidence, suggesting an original isostatic uplift of 53 (45 + 8) m. A 53-m isostatic uplift response indicates that 141 m of the UC was eroded. Since the current average thickness of the UC is 10 m, its original thickness was 151 m. This proposed isostatic response to Pleistocene erosion of the Mississippi River Valley supports the denudation model for Quaternary reactivation of the underlying Reelfoot rift faults and its New Madrid seismic zone and suggests that uplift mechanisms should be considered in the denudation model.

Publication Title

Journal of Geology

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