SYNSEDIMENTARY DEFORMATION AND SYNTECTONIC SEDIMENTATION IN MISSISSIPPIAN CARBONATES OF MISSOURI

Both syntectonic sedimentation and synsedimentary deformaton are common in post-rift to convergent tectonic settings and during events like meteorite impacts. Features such as paleoseismites, injectites, olistoliths, slides, and slumps sometimes are regarded as prima facie evidence, but most investigations require additional observations to make valid scientific arguments.

The Weaubleau impact structure of west-central Missouri provides examples of quasi-synsedimentary deformation. Bluffs on the Osage arm of Truman Reservoir expose a series of broken folds, thrust faults, and mélange zones within the Burlington-Keokuk Limestone (undivided). This area, near the confluence of the Osage and Sac rivers, is 3.5 km northwest of the rim of the main impact area and 7.8 km from the center. The preserved thickness of Burlington is ~35.0 m thick; it is overlain by a thin interval of impact breccia and younger Pennsylvanian Warner Sandstone. Axial sections of a few of the broken folds are thickened, indicating rapid strain rates. These are interpreted as flow folds in semi-consolidated sediment, cemented enough for beds to break, yet susceptible to plastic deformation. Apparent shortening ranges from 4.5 – 8.0 m in two of the broken folds. The excavation and modification phases of impact cratering produce. While a few thrust faults and high-angle reverse faults can be linked to major strike-slip faults across Missouri, folds and other features on the Osage arm have few feasible alternative explanations, especially in light of the proximity to a meteorite impact.

In contrast, interpretation of carbonate mounds in the Compton Limestone in McDonald and Stone counties of southwestern Missouri have been controversial. Three leading models include (1) reef mounds with some undetermined baffling or binding agent, (2) displaced reef mounds, or (3) slide and slump blocks, where the latter two explanations are associated with downslope movement. Among key observations are a few beds that appear to have been rafted or folded (rolled up) with emplacement. Also, mounds occur exclusively on the downthrown blocks adjacent to faults. Stable isotope signatures of mounds compared with non-mounded regularly stratified successions provide an argument in support of downslope movement of homogenized core facies within mounds.