USING CONODONTS AND OTHER FOSSILS TO DETERMINE THE AGE OF MISSOURI'S 38TH PARALLEL STRUCTURES AND SOME "LOST HORIZONS" OF THE OZARK DOME

The Weaubleau, Decaturville, Crooked Creek, and Avon structures are aligned west to east across Missouri's Ozark dome. The Rampino-Volk hypothesis suggests that they formed as a serial impact. Associated breccias contain fossils redeposited from pre-impact strata and from uncemented ocean-floor sediments. The Weaubleau structure in western Missouri, partly buried by Pennsylvanian clastics, involves deformed Precambrian through middle Mississippian (Osagean) rocks. The marine resurge facies of the "Weaubleau Breccia" contains well-preserved crinoids and blastoids that were redeposited from uncemented Burlington-Keokuk Limestone, plus corals and brachiopods; redeposited conodonts include rare Colaptoconus (Early Ordovician), Icriodus (Devonian), and Palmatolepis (Late Devonian). Redeposited Mississippian conodonts include Kinderhookian and Osagean Gnathodus, Polygnathus, Pseudopolygnathus, Siphonodella, and Taphrognathus. The youngest conodonts are common G. texanus and rare T. varians (late Osagean to Meramecian). Conodonts of these ages typically are mixed together in samples of the resurge breccia. Residual chert from the lower Meramecian Warsaw Formation overlies the breccia and gives an upper limit for the time of impact.

The Decaturville structure, farther east, is eroded down to the middle Lower Ordovician. Its fallback breccia has Middle to Upper Ordovician conodonts redeposited from Mohawkian (Amorphognathus, Curtognathus, Erismodus, Panderodus, Phragmodus) and Cincinnatian strata (Protopanderodus insculptus).

Little research on breccia fossils has been done at the Crooked Creek structure in central Missouri, but the Early Ordovician conodont Chosonodina was found at the same locality as float containing the middle Mississippian brachiopod Orthotetes keokuk. Breccia at the Avon structure in eastern Missouri contains the early Late Cambrian phosphatic brachiopod Angulotreta missouriensis and Early Devonian macrofossils.

Conodonts from these breccias have a Color Alteration Index of 1 to 1.5, indicating that they have not been affected by high temperature.

Apatite fossils in breccias identify Mohawkian, Cincinnatian, and Devonian strata that were eroded away across most of the Ozark dome after impact. These strata are some of Missouri's "lost horizons."