T145 Soils and Paleosols across Space and Time I: A Celebration of Steven G. Driese

Tuesday, 11 October 2022
8:00 AM - 12:00 PM
207

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

Paper number: T145-4
Program: Technical Sessions
Time: 8:35 AM – 8:55 AM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-376979

© Copyright 2022 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.