SILICIFIED OOIDS FROM THE MID-MISSISSIPPIAN SHORT CREEK OÖLITE (?) PRESERVED IN WEAUBLEAU IMPACT STRUCTURE, WEST-CENTRAL MISSOURI

Recent re-examination of drill core from Weaubleau structure revealed lithostratigraphic evidence for the age and timing of impact that is consistent with previous chronostratigraphic ages. Weaubleau structure is located the west-central Missouri. Two eccentric circular stream drainages outline features interpreted as the main impact area (8 km dia.) and tectonic rim (19 km dia.). Widespread breccias are found across both. Structural deformation is common in the inferred down-trajectory direction area. Seven varieties of breccia are recognized in the structure. Abundant, multi-directional planar deformational features (PDFs) in quartz-grains from the uppermost breccia indicate exposure to shock pressures and are considered diagnostic of meteorite impacts. Conodont and echinoderm faunas recovered from the breccias indicate a mid-Mississippian age, either latest Osagean or earliest Meramecian epoch. The trajectory of this low-angle marine impact is inferred to have been from southwest-to-northeast (paleo-west-to-east).

Approximately 800 m of drill core were recovered, split, scanned digitally, and are housed at Missouri State University. Light-to-medium brown silicified ooid grainstone clasts commonly are found as clasts in injection breccias and in target rock successions in the upper Jefferson City Dolomite. The SMSU-MoDOT Iconium 1 core was drilled down-trajectory from the main impact area. Short segments of core composed of white chert, were recovered between 7.5 – 10.0 m. These are not clasts but are composed of silicified ooid grainstone above yellowish crinoid grainstone that contain clasts of yellow siltstone. This facies overlies coarsely clastic breccias that characterize most of the impact. The Short Creek Oölite, the uppermost member of Osagean Burlington-Keokuk Limestones (undivided), is the oldest ooid-bearing stratigraphic unit in Mississippian strata. Alternatively, ooid grainstone also is found in the St. Louis Limestone (middle Meramecian Series) which crops out on the western flank of the Ozarks near the Montevallo and El Dorado. Given the proximity to outcrops of Burlington-Keokuk and their abundance in both breccias and target rocks, we regard the former as the most likely scenario for the origin of these ooids.