XENOCLAST BIOSTRATIGRAPHY: EVIDENCE FOR MID-MISSISSIPPIAN AND LATE ORDOVICIAN IMPACTS AT WEAUBLEAU AND DECATURVILLE STRUCTURES, MISSOURI

Biostratigraphy is important for determining ages of impacts. The youngest fauna in deformed or brecciated strata indicate maximum ages; the oldest fauna in overlying undeformed strata provide minimum ages. It is hard to constrain ages of terrestrial impacts because of subsequent erosion or non-deposition, but marine impacts more likely have conformable successions. Even so, unconformities, palimpsest strata, and xenoclasts provide important clues for understanding age relationships.

Weaubleau was a marine impact, and conodont and echinoderm faunas in impact breccias constrain a mid-Mississippian age (latest Osagean or earliest Meramecian), and overlying strata are early Pennsylvanian (Atokan). Meramecian brachiopods, bryozoa, and trilobites are in chert blocks in red clay residuum that overlies the impact breccia and underlies Pennsylvanian strata, indicating that Salem or St. Louis Limestone was deposited after the impact. A xenoclast of Late Devonian Chattanooga Shale in the impact breccia pre-dated development of the regional sub-Mississippian unconformity. An estimated 75-150 m of strata were removed below this pre-impact unconformity. Post-impact uplift and pre-Pennsylvanian erosion removed Meramecian and possibly younger strata, leaving the Weaubleau impact breccias mostly intact.

In the Decaturville structure, xenoclasts from impact breccias provide biostratigraphic age constraint. Early geological investigations in the Decaturville area reported blocks of Burlington Limestone (Osagean), but later investigations did not confirm this. New road cuts along MO Hwy. 5 expose polymict breccias and isolated boulders and cobbles. New collections include crinoid grainstone blocks that look like Burlington Limestone but yield abundant Late Ordovician (Cincinnatian) conodonts comparable to faunas of the Cape Limestone. Resurge breccias from several areas yield similar faunas. Abundant Late Ordovician fauna and a complete lack of younger fauna helps to support the idea that Decaturville impact is considerably older than Weaubleau. The sub-Mississippian or an older unconformity likely removed Cincinnatian strata from surrounding areas but left xenoclasts in Decaturville breccias. The different biostratigraphic ages of these two impacts do not support a serial impact hypothesis.