HIGH-RESOLUTION STRATIGRAPHY OF THE SUB-MISSISSIPPIAN UNCONFORMITY IN SOUTHWESTERN MISSOURI AND NORTHERN ARKANSAS

The sub-Mississippian unconformity in southwest Missouri and northern Arkansas records erosion and subsequent marine sedimentation. The truncation of units below the sub-Mississippian unconformity seemingly was associated with flexure and an early phase of tectonism during the Ouachita orogeny. Although the Ouachita orogeny and the uplift of the Ozark dome are considered to be Pennsylvanian in age, this model proposes development of a peripheral bulge north of the Ouachita proto-foreland basin in Arkansas that directly was related to the tilting and cutting out of units below the unconformity.

Roadcuts along U.S Highway 65 expose cyclic facies within the lower Ordovician (Ibexian Series) Cotter Dolomite. It consists of thin-bedded, laminated silty dolomite with alternating beds of structureless dolomite and sucroisic structureless dolomite, characteristic of tidal flat deposition. Above the unconformity, the Bachelor Formation consists of thin beds of quartz sandstone and shale that record the initial sea-level rise during the lowermost Mississippian (Kinderhookian Series). Sandstone from the Bachelor Formation fills paleokarst fissures and cavities developed below the base of the unconformity in the Cotter Dolomite.

Approximately 15 km north of Springfield, MO, the Bachelor Formation rests with angular discordance on the Cotter Dolomite; about 2.5 m of strata below the unconformity are cut out over a horizontal distance of 83.5 m (29 m/km). Approximately 80 km north of Springfield, other investigators have shown that the Cotter Dolomite has been cut out entirely. Furthermore, northerly tilting of the unconformity indicates deformation during later stages of tectonism in the Pennsylvanian Period.

Middle and upper Ordovician strata are present in northern Arkansas but have been partly cut out by erosion at the base of the Mississippian. The lower to middle Ordovician Powell and Everton formations and St. Peter Sandstone are preserved in the eastern reaches along the Buffalo River. The upper Devonian (Famennian Series) Chattanooga Shale is preserved only along thin outcrops near the Missouri-Arkansas state line and in paleokarst developed on the lower Ordovician in southwestern Missouri.