CRITICAL STRATIGRAPHIC DATA FROM REWORKED CONODONTS IN IMPACT BRECCIAS ACROSS MISSOURI'S OZARK DOME

The Weaubleau, Decaturville, and Crooked Creek structures are impact features aligned across Missouri near the 38th parallel. One hypothesis of their origin suggests they were a serial impact. Impact breccias at these structures have reworked conodonts that provide provenance data that are unobtainable using only in situ faunas.

Mississippian and Pennsylvanian strata crop out in and around the Weaubleau structure. Breccia at Weaubleau in the western Ozark dome contains common Early Ordovician conodonts, rare latest Devonian conodonts, and abundant Mississippian (Kinderhookian to Osagean) conodonts. Conodonts of all these ages occur in one sample, together with reworked late Osagean to earliest Meramecian crinoids and blastoids. The youngest conodonts are Gnathodus texanus and Taphrognathus varians. Latest Osagean to Meramecian megafossils occur in chert regolith derived from limestones that formerly covered the breccia. Collectively, these faunas indicate an impact near the Osagean–Meramecian boundary.

The nearby type area of the Osagean Series lacks uppermost Osagean strata, but echinoderms of this age are abundant in breccia at Weaubleau. Impact-generated resurge currents may have blended unconsolidated latest Osagean and early Meramecian crinoid-bearing sediment together with clasts of older strata that were ejected from the impact crater to form the breccia at Weaubleau. The type Osagean is incomplete, most likely because these currents removed part of the stratigraphic record, but it can be reconstructed using reworked fossils.

The Decaturville and Crooked Creek structures in the central Ozark dome are eroded down to the Lower Ordovician. Breccia samples at Decaturville yielded conodonts that occur in Upper Ordovician (Mohawkian and Cincinnatian) strata exposed only east and west of the Ozark dome. Reworked conodonts in the breccia at Decaturville are the only evidence that these strata were deposited in the central Ozark dome.

Crooked Creek is the easternmost impact structure. A single breccia sample contains Early Ordovician conodonts, including Chosonodina herfurthi, which is known from only one other locality in Missouri. A float block contains the latest Osagean brachiopod Orthotetes keokuk, suggesting that the time of this impact may be similar to that of the Weaubleau structure.