REINTERPRETING THE JUNIATA PALEO-VERTISOL (LATE ORDOVICIAN) AT BEANS GAP IN EASTERN TENNESSEE

A 1 m thick red, slickensided claystone in the upper Juniata Fm. (Upper Ordovician) exposed in an outcrop at Beans Gap, eastern TN, was previously interpreted (1991, 1992) as a paleo-Vertisol formed by pedogenic shrink-swell processes acting upon a smectitic clay substrate (low-energy coastal mudflat elevated and subaerially exposed during low stand). Distinct reduced mottles were interpreted as localized reduction around Glossifungites ichnofacies traces formed by marine invertebrates, as the paleosol was submerged by rising sea-level. No evidence for soil flora or fauna were identified in the field or in thin sections. However, reexamination of the thin-sections using UV fluorescence, to supplement standard microscopy, revealed evidence for filamentous and thread-like remains of non-vascular plants, as well as bioturbation and mixing by soil animals. Re-examination of XRF data using more sophisticated methods and proxies indicate that only the upper part of the claystone experienced weathering, so claystone at a depth of 50 cm was chosen as the protolith. Juniata colloidal estimates based on modern Vertisol elemental oxide relations indicate high total clay content (68-77) with about half as fine clay (33), typical of Vertisols, together with high COLE (0.16) and a CEC/clay ratio (0.70) consistent with a montmorillonite mineralogy. The soil is non-sodic and non-saline with a moderate amount of pedogenic iron oxide. % Base saturation (71-74) and pH (5.7) are low, consistent with the absence of soil carbonates, and are clearly within the range of the modern League soils of East Texas, except that the Juniata has a substantially shallower solum. After estimating MAP (1000-1500 mm/yr) using PPM 1.0 & 2.0 and comparing these properties to our modern Vertisols data set we assign original taxonomic classification as: fine, montmorillonitic, thermic Chromic Hapludert. The paleosol experienced K-diagenesis, and relative to a pre-weathering, reconstructed concentration of K₂O of 2.24% in the claystone, the 90-cm thick profile records an addition of 6.76 g/cm² K₂O, a total increase in K₂O of 137%. MAT estimates are skewed to abnormally low temperatures (6-12 ^oC) because of the K₂O additions and were discounted. Correlation to the end-Ordovician eustatic low stand would be aided by absolute dating of paleosol zircons.