DETRITAL ZIRCON EVIDENCE FOR LATE ORDOVICIAN UPLIFT, PALEOKARSTING, AND DRAINAGE REORGANIZATION IN THE OZARK DOME AND VICINITY, SOUTH-CENTRAL MISSOURI

Isolated sandstone and high-temperature clay deposits exposed along the northern flank of the Ozark Dome in central Missouri have been interpreted for over a century as Pennsylvanian filled-sink deposits, primarily because of their roughly circular shape, chaotic bedding, and stratigraphic position as the first sandstones above Ordovician carbonate formations. However, stratigraphic and U-Pb detrital zircon geochronologic relations preclude a Pennsylvanian sink fill origin for these structures. For instance, “filled-sinks” are locally interbedded with Ordovician carbonate rocks and are overlain by Devonian and Mississippian strata. Furthermore, “filled sink” deposits contain exclusively Neoarchean and ca. 1.1 Ga detrital zircon age populations, most likely originating from the Superior Craton and Midcontinent Rift, respectively. In contrast, Pennsylvanian Warrensburg and Moberly channel-fill sandstones lack Neoarchean grains and contain abundant Grenville age (ca. 1.0-1.2 Ga) plus Paleozoic populations presumably derived from the uplifting and eroding Appalachian Mountains to the east. Existing data from Ordovician clastic strata (e.g. the Roubidoux and St. Peter sandstones) in central Missouri show zircon age distributions that are strikingly similar to those of “filled-sink” deposits. These relations lead us to suggest that the studied deposits are Upper Ordovician in age and are the products of backfilling of narrow, steep-sided solution valleys following base-level fall. The base-level fall invoked to explain the formation of solution valleys is at odds with evidence for eustatic sea-level rise in Late Ordovician time. To reconcile this discrepancy, we speculate that the Taconic orogeny led to modest uplift in central Missouri, resulting in karstification and formation of a significant disconformity. This far-field tectonism may also have been responsible for the observed Late Ordovician reorganization of clastic sediment dispersal pathways from the Superior Craton in the north to the growing mountains in the east.