Dissertation
Integrated high-resolution stratigraphy of the Devonian-Carboniferous Boundary and lower Mississippian Interval of the U.S. Midcontinent
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Summer 2022
DOI: 10.25820/etd.006677
Abstract
The tri-state area of Iowa, Illinois, and Missouri contains some of the best exposed Mississippian strata in the world, including the type area for the Mississippian subsystem. Strata here was deposited across a broad carbonate shelf and are thinnest along the central middle shelf, thickening both up-ramp and down-ramp, forming a complex dumbbell-like stratigraphic pattern rather than a simple clinoform geometry of thinning into the basin. Significant hiatuses mark the Devonian-Carboniferous Boundary (DCB) and Kinderhookian-Osagean Boundary (KOB) and as a result the precise temporal relationship between strata deposited across the region remain uncertain. Our new data demonstrate a complex relationship of complementary stratigraphic thicknesses where the DCB interval is thin or absent in the up-ramp inner-shelf setting and preserved in a significantly expanded interval in the central to distal middle shelf deposits of southeast Iowa and northeast Missouri. However, the overlying KOB interval is not preserved in this down-ramp setting but is preserved in significantly expanded strata in the up-ramp inner-shelf setting of central Iowa.
New high-resolution, major- and trace-element geochemistry from the Devonian-Carboniferous Boundary interval of southeast Iowa was produced from an expanded record of the Hangenberg Crisis. Similar to the classical European successions, elevated abundances of metals such as Cu, Co, Ni, and Pb occur within this stratigraphic interval in Iowa. Immobile/incompatible lithophile elements (Zr, Ti, Hf, W, U, and Th) show strong co-variation and increased abundance during this interval, suggesting they are likely associated with heavy minerals and a progressive increase of detrital input through the Hangenberg Crisis interval. Other redox-sensitive indicators such as Pb, Mo, P, and TOC demonstrate that patterns of redox change and TOC preservation are likely driven by enhanced primary productivity. Overall, our data suggest that enhanced delivery of continental materials increased primary productivity near the onset of the Hangenberg Excursion in Iowa, and that a complex feedback between sea level, volcanism, weathering, productivity, and anoxia may have played a significant role in the initiation of the Hangenberg Crisis.
The Black Hills, South Dakota, USA, expose thick packages of Mississippian carbonates and represent a shallow-water succession on the NW margin of the Transcontinental Arch. These carbonates are the up-ramp facies of the well-studied Devonian-Carboniferous Boundary succession in the Williston Basin, and are critical to the development of a detailed sequence stratigraphic history within the Williston Basin itself, and the early Mississippian of the U.S. Midcontinent in general. Conodont yields were generally poor, but the Mickelson Member of the Englewood Formation and lowermost two beds of Pahasapa Formation contain conodonts of the S. sandbergi Zone and the remaining lower portion of the Pahasapa contained S. crenulata Zone fauna. The Kinderhookian-Osagean boundary could not be placed precisely at the outcrop, however, there is no evidence that the Pahasapa Formation in the studied succession extends higher than within the global Tournaisian Stage.
Details
- Title: Subtitle
- Integrated high-resolution stratigraphy of the Devonian-Carboniferous Boundary and lower Mississippian Interval of the U.S. Midcontinent
- Creators
- Matthew Gregory Braun
- Contributors
- Bradley Cramer (Advisor)David Peate (Committee Member)Emily Finzel (Committee Member)Jeffrey Dorale (Committee Member)Brian Witzke (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Geoscience
- Date degree season
- Summer 2022
- Publisher
- University of Iowa
- DOI
- 10.25820/etd.006677
- Number of pages
- xi, 144 pages
- Copyright
- Copyright 2022 Matthew Gregory Braun
- Grant note
- Funding from the National Science Foundation (CAREER and EPSCoR Grants to Dr. Cramer) and the United States Geological Survey STATEMAP Program (funding through the Iowa Geological Survey).
- Language
- English
- Description illustrations
- illustrations, maps
- Description bibliographic
- Includes bibliographical references.
- Public Abstract (ETD)
- This study investigates marine rocks deposited during the Late Devonian and Early Mississippian interval, approximately 362 to 346 million years ago. The climate and ocean underwent significant changes during this time, as this interval contains the start of an ice age, a mass extinction, and two large carbon isotope excursions. Carbon has two stable isotopes, 12C and 13C, where the ratio between them is measured as 13C/12C (δ 13C), and a carbon isotope excursion is a sudden, sustained change in δ 13C and commonly reflects a large-scale change in the Earth’s climate. Conodonts are an extinct eel like animal who’s fossilized teeth can be used for correlation and to determine the relative age of the rocks that contain them. By combining conodont biostratigraphy and δ 13C chemostratigraphy we can precisely correlate similar rocks and determine the timing of climate events and mass extinctions in Earth history. Some of the best-preserved Devonian-Mississippian rocks in the world are located in the Midwestern and Western U.S., notably the Iowa, Missouri, and Illinois tri-state area. We collected conodont and δ 13C data from across these states to refine the correlation between rock units and create a chronostratigraphic framework (the age of rock formation) for the region. Within this framework, we collected geochemical data to investigate the cause of the Devonian Hangenberg mass extinction. Our results indicate the mass extinction may have been caused by enhanced riverine runoff promoting massive algae blooms, which in turn caused large dead zones to form in the ocean. We also examined age equivalent rocks in the Black Hills of South Dakota and collected conodont and δ 13C data. Our results here indicate a similar pattern of flooding and rock formation that we see in central Iowa, confirming these patterns are driven by large-scale global processes.
- Academic Unit
- Earth and Environmental Sciences
- Record Identifier
- 9984285247302771
Metrics
9 File views/ downloads
61 Record Views