INSOLUBLE MINERAL ANALYSIS OF THE ST. JOE-BOONE FORMATIONS (LOWER MISSISSIPPIAN) AND THE REGOLITH PROBLEM, NORTHWESTERN ARKANSAS

A thick regolith dominated by chert gravel with subordinate sand and clay fractions mantles the Springfield Plateau in the conterminous Arkansas-Oklahoma-Missouri region of the southern midcontinent of North America. The chert contribution to the regolith is obviously derived mostly from the chert-bearing carbonates of the Boone Formation (Lower Mississippian), and to a lesser extent the underlying St. Joe Limestone, both of which form the plateau. However, insoluble residue analysis of the St. Joe and Boone interval suggests that not all the sand and clay fractions in the regolith can be attributed to in situ accumulation of Pleistocene weathering products. Standard insoluble residue analysis using 10% by volume hydrochloric acid revealed that the average insoluble fraction of the Boone carbonates is only 16% by volume. Examining insoluble relations stratigraphically, the upper St. Joe averages 1.7% insoluble, while in the Boone Formation, the lower member averages 31%, while the upper member averages only .75%. These insoluble percentages reflect the transgressive-regressive history of the interval, and support our contention that most of the Boone insoluble content, which includes volcanic ash, fell out of the air. These data also demonstrate clearly that the sand and clay fractions of the regolith cannot be derived entirely from solutioning of the Boone during the Pleistocene, but must include a contribution of eluvial material as well.