PROVENANCE OF THE UPLAND COMPLEX: WHERE DID ALL THAT PLIOCENE GRAVEL AND SAND COME FROM?

The Upland Complex, a gravel and sand terrace exposed along the Mississippi Embayment axis, was deposited by Arc River, a Pliocene ancestor to the present Mississippi River. Where did all its gravel and sand come from? To answer this question we sampled 10 locations in Arkansas, Tennessee and Mississippi and analyzed the petrology of gravel and sand components. Most gravel clasts are fine chert impregnated with goethite (FeO(OH)) to a depth of one to two mm giving the gravel its medium brown color. Many chert clasts are replaced carbonate with recognizable mid-Paleozoic fossils and oolite. Spiculiferous chert with goethite and chert replaced dolomite rhombs is common. Partial to complete replacement of chert by goethite forms a fourth texture. Sand fraction composition varies with location. East of the Mississippi River (Chickasaw Bluffs) monocrystalline quartz is markedly more abundant than polycrystalline quartz. In contrast, west of the river (Crowley’s Ridge) the sand has a greater proportion of polycrystalline quartz grains and 5 to 7% feldspar (both plagioclase and K-spar). Other details combine to suggest that at least some Crowley’s Ridge sand came from the St. Francois Mountains. Previous studies suggested that the Arc River a drainage basin was about twice the area of the modern Mississippi and could have extended to the Arctic Circle. We conclude that much (perhaps most) of the chert gravel of the UC came from formations in central Canada that were subsequently removed by glacial erosion whereas the accompanying quartz sand came from closer, American, sources.