John Isbell

Professor

328 Lapham Hall
(414) 229-2877
jisbell@uwm.edu

PhD: The Ohio State University
MS: Northern Illinois University
BS: Augustana College

Specializations: Sedimentary geology, historical geology, Antarctica

Research:

Late Paleozoic-Mesozoic Fauna, Environment, Climate, and Basinal History: Beardmore Glacier Area, Transantarctic Mountains (2002 to 2005)

The 4 km thick sequence of sedimentary rocks in the Beardmore Glacier area records 90 million years of Permian through Jurassic history of this high-paleolatitude sector of Gondwana. It accumulated in a foreland basin with rates of subsidence approximately equal to rates of deposition. The deposits have yielded diverse vertebrate fossils, in situ fossil forests, and exceptionally well preserved plant fossils. They provide a unique glimpse of glacial, lake, and stream/river environments and ecosystems, and preserve an unparalleled record of the depositional, paleoclimatic, and tectonic history of the area. Studies done to date provide a firm base of information for investigating more specific questions.

This is a collaborative study of the Beardmore stratigraphic section that integrates sedimentologic, paleontologic, and ichnologic observations to answer focused questions, including: (1) What are the stratigraphic architecture and alluvial facies of Upper Permian to Jurassic rocks in the Beardmore area?; (2) In what tectonostratigraphic setting were these rocks deposited?; (3) Did vertebrates inhabit the cold, near-polar Permian floodplains, as indicated by vertebrate burrows, and can these burrows be used to identify, for the first time, presence of small early mammals in Mesozoic deposits?; and (4) How did bottom-dwelling animals in lakes and streams use substrate ecospace, how did ecospace use at these high paleolatitudes differ from ecospace use in equivalent environments at low paleolatitudes, and what does burrow distribution reveal about seasonality of river flow and thus about paleoclimate?

Answers to these questions are clarifying the paeloclimatic, basinal, and tectonic history of this part of Gondwana, elucidating the colonization of near-polar ecosystems by vertebrates, is providing new information on the environmental and paleolatitudinal distributions of early mammals, and is allowing for semi-quantitative assessment of the activity and abundance of bottom-dwelling animals in different freshwater environments at high and low latitudes.

Collaborative Research: Permian-Triassic Basin History of Southern Victoria Land and the Darwin Mountains (2000-2004)

This is a collaborative sedimentological, palynological, and paleomagnetic study of Permian and Lower Triassic strata in southern Victoria Land (SVL) and the Darwin Mountains (DM), Antarctica. Results of the study are helping to constrain the paleoenvironmental, tectonic, biotic, and paleogeographic histories of southern Pangea and provide a unique polar view of the world during an icehouse to greenhouse transition.

The assembly and drift of Pangea resulted in heightened orogenic activity and associated development of numerous depositional basins. One of the largest basins was the 10,000+ km long “Gondwanide foredeep” that extended across southern South America, South Africa, the Falkland Islands, Antarctica, and Australia. Antarctica’s centralized location between South Africa and Australia, make SVL and DM key areas for testing paleogeographic and paleoclimatic models.

Upper Paleozoic and Lower Mesozoic rocks in SVL and DM were deposited during Gondwanaland’s drift across the south pole. Based on present plate reconstructions, SVL and DM were located higher than 75 degrees S from 320 to 210 Ma. Despite the putative high latitude position, SVL and DM sedimentary successions record a change from Lower Permian glacial deposits, to Permian fluvial coal measures, to Lower Triassic non-carbonaceous fluvial deposits, and finally to Middle and Upper Triassic fluvial coal measures, with well-developed vegetation during much of this time. Present climatic simulations suggest seasonal climatic extremes within Pangea’s polar interior. Discrepancies between the geological evidence and the climate simulations need to be resolved, and may be magnified by incomplete understanding of the influence of paleotopography, large lakes, and river systems at the time of deposition, as well as by incomplete documentation of paleoenvironmental conditions. Furthermore, Late Permian and Triassic mean pole positions for Gondwanaland are not tightly constrained. Paleomagnetic signatures will be recovered from Permian and Triassic petrified wood, silicified peat, and coal, which were cemented during early diagenesis (preliminary results indicate stable remanent magnetizations). Palynological analyses will provide time control for the succession.

Other Projects:

Analysis of fluvial and tidal signatures within late Paleozoic rocks in the Appalachian and Illinois Basin.

Analysis of Upper Carboniferous and Permian Glacial, Post-Glacial and Fluvial Coal-Measure Deposits in Antarctica.

Fluvial sedimentology of the Wisconsin River.

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