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Dissertation
Analysis of fluvial-tidal transition zones using sedimentology, geochemistry, palynology, and modern analogs
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Autumn 2023
DOI: 10.25820/etd.007130
Abstract
Fluvial-tidal transition zones (FTTZ) represent one of the most complex depositional systems within ancient strata due to their dynamic nature. The Lower Cherokee Group in southeastern Iowa records a previously undocumented Lower-Middle Pennsylvanian aged FTTZ. We use a combination of detailed core descriptions and geochemical analysis obtained with a handheld X-ray Fluorescence spectrometer (pXRF) to describe 10 distinct geochemical zones that can be identified across the study area. The geochemical data provide a framework within which palynological and RockEval pyroloysis data are integrated to develop a biogeochemical framework for the Lower Cherokee Group. The results suggest that the study area was dominated by lycopod forest swamps throughout the mid-late Atokan. Sedimentary structures, trace fossils, and palynology suggest intermittent marine conditions within the study area during deposition of the Kilbourn Formation, but that relative sea level fall persisted throughout much of the deposition of the Kalo and Floris formations. The Floris Formation coincides with the arrival of continental-scale fluvial systems to the region originating from the Alleghenian Orogeny to the east as the basin bounding arches separating the Forest City Basin, Illinois Basin, Michigan Basin, and Appalachian Basin were overtopped by sedimentation. This was followed by the first full marine transgression coinciding with deposition of the Oakley Shale. Abundant charcoal throughout the Lower Cherokee Group suggests frequent wildfires in the region during Early-Middle Pennsylvanian time.
Within ancient FTTZs it is challenging to differentiate sedimentary strata deposited under marine, marginal marine brackish-water, or freshwater conditions. Presented here are sediment geochemistry transects from along the length of two modern fluvial-tidal transition zones: the Ogeechee River and estuary in Georgia, USA and the Petitcodiac River and estuary in New Brunswick, Canada. These data are used to test the effectiveness of two previously proposed paleosalinity proxies and explore for other relationships between sediment geochemistry and salinity. Our results indicate that Sr/Ba is moderately effective but strongly dependent on grainsize, whereas S/TOC is ineffective due to the various controls exerted on S and TOC concentrations in the depositional environment. Multivariate discriminate analysis (MDA) of the full geochemical dataset demonstrates a strong relationship between grainsize and geochemistry. as well as the specific river and the sediment geochemistry, but no relationship between sediment geochemistry and water salinity. Principal component analysis (PCA) does not identify any elements that strongly covary with salinity. We suggest that paleosalinity proxies may not work across all depositional systems due to geochemical controls unique to each region, such as grainsize, mineralogy, and sediment provenance. We recommend a very measured and reserved approach to utilizing paleosalinity proxies that should include multiple proxies when possible. This also necessitates understanding the unique controls exerted on each system before developing a localized paleosalinity indicator.
Details
- Title: Subtitle
- Analysis of fluvial-tidal transition zones using sedimentology, geochemistry, palynology, and modern analogs
- Creators
- Dustin Shawn Northrup
- Contributors
- Emily Finzel (Advisor)David Peate (Committee Member)Brad Cramer (Committee Member)Jeff Dorale (Committee Member)Kevin Bohacs (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Geoscience
- Date degree season
- Autumn 2023
- Publisher
- University of Iowa
- DOI
- 10.25820/etd.007130
- Number of pages
- xvi, 198 pages
- Copyright
- Copyright 2023 Dustin Shawn Northrup
- Grant note
- This work was generously supported and made possible by grants from the American Chemical Society (ACS) and their Petroleum Research Fund, the Geologic Society of American (GSA) and their student research grants, the Society for Sedimentary Geology (SEPM) and their student research grants, and from funding from the University of Iowa Department of Earth and Environmental Sciences.
- Language
- English
- Date submitted
- 11/27/2023
- Description illustrations
- illustrations (chiefly color), tables, graphs, maps
- Description bibliographic
- Includes bibliographical references.
- Public Abstract (ETD)
- Identification of sedimentary rocks deposited under either fully freshwater or fully marine conditions is relatively straightforward. It is significantly more challenging, however, to identify sedimentary strata deposited under brackish-water conditions, such as where a river enters an ocean basin. This is because the fluvial-tidal transition zone (FTTZ) of coastal rivers is one of the most complex environments on Earth due to the complicated interplay among terrestrial and marine sedimentologic, biologic, and environmental processes occurring there. The first part of this thesis examines ancient sedimentary rocks in central southeastern Iowa deposited in an ancient FTTZ around 300 Ma. Utilizing detailed descriptions of the sedimentary rocks, analysis of fossilized pollen spores, whole rock geochemistry, and statistical analysis, we suggest that the region was dominated by swampy conditions with gradually falling sea level. The second part of this thesis examines sediments in two modern FTTZs in New Brunswick Canada and Georgia, USA. Various elemental ratios have been proposed by previous researchers as potential paleosalinity proxies that may be used to understand salinity conditions at the time of deposition of ancient marine sediments. Geochemical analysis of sediment from the two modern FTTZs was used to test these paleosalinity proxies. Our results suggest that the proposed paleosalinity proxies are not reliable in every depositional system.
- Academic Unit
- Earth and Environmental Sciences
- Record Identifier
- 9984546542002771
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