PLIOCENE GEOLOGY IN MID-CONTINENT NORTH AMERICA: THE ARC RIVER STORY CONTINUED

The Upland Complex (aka Lafayette Gravel) is the only sedimentary body deposited between the end of the Eocene and the onset of glaciation in the lower Mississippi River valley. The story of its origin provides insight into geologic processes that occurred in the heartland within this 30 million year interval. The Upland Complex is a terrace gravel deposited by Arc River (the Pliocene ancestor of the present Mississippi River) in the Mississippi Embayment. Modern exposures suggest that the Upland Complex initially extended in a belt up to 200 km wide with a thickness of 100m that extended at least from the confluence of the modern Mississippi and Ohio Rivers to the Gulf of Mexico. Abundant fist-sized (10 cm) cobbles and meander scars up to 20 km in diameter (two to three times those of the modern Mississippi River meanders) suggest a powerful torrent of water flowed south ultimately forming a thick gulf coast delta. The source of all this gravel and sand is simultaneously obvious and baffling. Examination of 20 exposures in Arkansas, Kentucky, Missouri, Mississippi, and Tennessee suggest a pattern of changing fluvial conditions; alluvial fan, braided and meandering as one progresses south. Upland Complex chert clasts have textures and fossils similar to those of midcontinent Silurian to Mississippian bedded and nodular chert. Chert clast texture is similar both east and west of the present Mississippi River. Sand-sized grains have five textures, monocrystalline quartz, polycrystalline quartz, chert, goethite, and feldspar. East of the Mississippi River monocrystalline quartz is markedly more abundant than polycrystalline quartz and feldspar is absent. In contrast, west of the river the sand-sized grains have a greater proportion of polycrystalline quartz and 5 to 7% feldspar (both plagioclase and K-spar). We conclude that the sources of the sand components of the Upland Complex were different. Much of the quartz sand on Crowley’s Ridge came from the St. Francois Mountains of Missouri whereas sand east of the Mississippi came from uncertain sources to the north or east. Chert is the puzzle. Alluvial fan characteristics require a proximal high relief source to the east that is no longer present.