KINEMATIC ANALYSIS AND PROVISIONAL MIDDLE SILURIAN AGE CONSTRAINTS ON DECATURVILLE STRUCTURE, CENTRAL MISSOURI

The Decaturville structure is as a meteorite impact site located within Laclede and Camden counties, approximately 20 km north of Lebanon, Missouri. The structure is 5.5-km in diameter with a well-developed central uplift and surrounding moat regions. During the 1960s, the structure was interpreted as having a “cryptovolcanic” origin, and later, in the 1970s, structural and mineralogical analyses, breccia distributions, shatter cones, and shocked quartz supported an impact interpretation. Previous age dating of the impact has been controversial. Recent paleomagnetic results purport to constrain the origin to the Pennsylvanian or Permian.

New conodont biostratigraphic age constraints indicate a significantly older age of Middle Silurian, which is consistent with the youngest faunal ages previously reported. Middle and possibly Early Silurian conodonts were recovered from matrix and a large, isolated sandy limestone clast in a polymict breccia along a new road cut on Highway 5 northeast of the central uplift. These new ages are consistent with an impact event that pre-dated the widespread sub-Mississippian regional unconformity.

This study also examined structures along a newly widened 300-m-long road cut southeast of the central uplift, where the upper part of the lower Ordovician Jefferson City Dolomite crops out in a structurally deformed and depressed area. Extensive outcrop mapping as well as strike and dip measurements and stereonet plots show that there are four important structural components consistent with an impact origin: (1) a tight anticlinal fold, oriented radially to the central uplift; (2) thick accumulations of breccia on the crest of the anticline and along thrust faults; (3) at least six inward-directed thrust faults; and (4) normal faults. The radial fold is interpreted to have developed during the early modification phase, when the overriding forces were directed inward toward the central uplift. Both excavation and modification stages most likely led to extensive brecciation. Thrust faults were formed later during the modification stage because a thrust fault truncated the crest of the anticline. Finally, normal faults occurred during the late modification stage as a relaxation response after thrust faulting.