THE PRECAMBRIAN MIDDLE RUN FORMATION IN KENTUCKY: EVIDENCE FOR FLUVIAL AND TIDAL FACIES
The detrital zircon age spectrum from the core contains Meso/Paleoproterozoic age modes, a single Archean age, and a major peak at ca. 1125 Ma, but only two grains younger than that: 1022 and 1031 Ma. The ca. 1000-1050 “Ottawan” age mode, so characteristic of Neoproterozoic through Phanerozoic sediments derived from Grenville basement in the Appalachian orogen, is mostly absent. The two young ages require that the Middle Run is not younger than ca. 1000 Ma, and is likely a Grenville synorogenic clastic unit.
The Middle Run is a brick-red, fine-grained, lithic arkose, with an illite clay matrix, and hematite cement. Petrographic analyses from nine thin sections shows sixty percent of grains are rhyolite lithic fragments derived from underlying Granite-rhyolite province igneous rocks and sparse grains of metamorphic rocks derived from Grenville basement. Quartz and feldspar grains are angular suggesting a local source, whereas lithic grains are rounded, suggesting a more distant source.
The brick red color of grains in the unit make differentiation of sedimentary features difficult in some parts of the core, but fluvial and tidal facies are interpreted from visible sedimentary structures in the core; including crossbeds, flaser bedding, reactivation surfaces, scours, mudstone rip-up clasts, and tidal rhythmites. Tidal rhythmites provide evidence that the Middle Run basin was open to the sea at least during highstands. Also, at ca. 1025, the tidal rhythmites are some of the oldest known from Midcontinent North America.