202 D31. Recent Advances in Stratigraphy (Posters)

Tuesday, 24 September 2019
9:00 AM - 6:30 PM
Hall AB, North Building

PATTERNS IN FLOODPLAIN DRAINAGE DURING THE EARLY EOCENE IN THE SOUTHERN BIGHORN BASIN (WYOMING, U.S.A.)

Paper number: 202-7
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

The Eocene Willwood Formation in the Bighorn Basin of Wyoming is an alluvial stratigraphic formation characterized by fluvial channel sandbodies, heterolithic avulsion deposits (i.e. crevasse splay-rich lithofacies), and fine-grained overbank deposits. The overbank facies consists mainly of mudrocks with moderate to strong pedogenic development, which is documented by complex mottling patterns, ped structures, clay cutans, rhizoliths, and slickensides amongst other sedimentary structures characteristics of soil horizonation. The degree of paleosol development has been attributed to variations in sedimentation rate, floodplain drainage, and overarching climate within the basin by previous researchers. Specifically, previous researchers documented better-drained conditions during Paleocene-Eocene Thermal Maximum (PETM) at ~56 Ma, which was a severe global warming lasting ~200,000 years, using a semi-quantitative soil morphology index. This study focuses on a ~80 meter stratigraphic section that post-dates the PETM in the early Eocene, and establishes a longer history of floodplain drainage in the basin.

The index includes measurements of matrix chroma, pedogenic carbonate, and ferruginous nodules. Matrix chroma, or color intensity, decreases with increasing duration of soil saturation. The presence of reduced iron gives periodically or seasonally waterlogged soils a grey color (low chroma), while brightly colored soils, often red, indicate oxidation under well-drained conditions (high chroma). Paleosol chroma was described in the field using Munsell color charts on dry samples in natural light. Pedogenic carbonate forms in higher abundance in drier conditions, and absence of soil carbonate indicates soil moisture above a certain threshold. In the field, numerical values were assigned to paleosol profiles according to the amounts of carbonate. Ferruginous, yellow-brown nodules form during changing redox conditions and increase in abundance with soil wetness, indicating frequent waterlogging. The scores for chroma, carbonate, and ferruginous nodules were added together for each paleosol profile to obtain scores ranging from waterlogged soils to very well drained soils.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-336988

© Copyright 2019 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.