Reports: ND851156-ND8: Sequence Stratigraphy, Detrital Zircon Geochronology, and Shale Geochemistry of Middle to Late Ordovician Quartzites in North America to Understand Provenance, Sediment Dispersal and Paleoclimate

Michael C. Pope, Ph.D., Texas A&M University

During the last year our lab had two students, 1 continuing Ph.D. and 1 new M.S., work on their theses on Detrital zircon geochronology of Middle-Late Ordovician quartz arenites in the western U.S. and U.S. Midcontinent. In the fall semester of 2012 Diar Ibrahim, a Ph.D. student from the University of Iowa came to Texas A&M to learn how to analyze zircons from the St. Peter Sandstone for provenance studies. I am co-chair of Diar's Ph.D. committee with Bill McClelland and all three of us are working closely with Brian Witzke who knows much about the St. Peter Sandstone from his 30 years of work and research at the Iowa Geological Survey and adjunct faculty position at the University of Iowa. Diar also attended courses in Petroleum Geology at TAMU that are not offered at the University of Iowa. Also in the fall of 2012 Sandra Marek was supported to help Diar with his laboratory prep of samples that Diar brought to TAMU and additional samples from the other students (Hutto, Wulf, Workman and Pickell) who had recently left our laboratory after doing detrital zircon geochronology on this topic. In the spring semester of 2013, Mario Lira a new M.S. graduate student in Geology (previously supported as an undergraduate field and laboratory worker), was supported on a research assistantship devising a thesis project to test whether the Tooele Arch in Nevada and Utah affected Middle Ordovician sedimentation. Mario is testing this idea by measuring a series of stratigraphic sections surrounding the Tooele Arch and determining if their detrital zircons are different than detrital zircons on either side (N or S) of the Tooele Arch. In May of 2013 Mario and I travelled to Utah and Nevada and I helped him measure a few sections. Subsequently, Mario and Taylor Law, an undergraduate student at Texas A&M measured the remainder of his sections. In the summer of 2013 Mario crushed his quartz arenite samples, then physically and chemically separate the detrital zircons within the samples. Mario has also imaged all of his zircons and is ready to run his samples on the LA-ICP-MS. Mario will continue to work on his zircons this semester, hopefully having them completely analyzed by December 2013. During the spring and summer of 2013 Diar Ibrahim described and correlated numerous cores of the St. Peter and Starved Rock sandstones in Iowa and made a number isopach and isolith maps. Diarsampled the cores and has separated zircons from all of his samples. Once he comes back to TAMU we will mount his zircons in pucks, polish and image the pucks and then analyze them on our LA-ICP-MS, hopefully by January of 2014.

Mario Lira submitted an abstract to be presented at the National AAPG Meeting in Houston, April 6-9, 2014 on his M.S. results.

The main results of the research so far are:

1) Detrital zircons for Middle-Late Ordovician quartz arenites of the western U.S. are remarkably similar indicating a uniform provenance from the Transcontinental Arch, mostly from the Trans-Hudson Orogen (1.8-1.9 Ga) and Archean terranes (2.5-2.9 Ga) such as the Wyoming Province. Almost all samples also include populations of 2.1 Ga zircons, and 0.8 – 1.2 Ga grains. The source for the 2.1 Ga grains is currently unknown, and the young zircons are derived from recycling Neoproterozoic-Cambrian quartz arenites.

2) Detrital zircons for the Middle-Late Ordovician quartz arenites of the U.S. Midcontinent are quite dissimilar from the samples on the other side of the Transcontinental Arch containing major populations of 0.9-1.2 Ga, 1.4-1.5 Ga, 1.6-1.7 Ga, 1.8-1.9 Ga, and 2.4-2.8 Ga detrital zircon grains. The amount of Grenville age zircons, and zircons from 1.6-1.7 Ga (Yavapai-Matzatzal) source terranesare much larger than in the western US samples.

3) Changes in detrital zircon populations between the base and top of a unit (see TOC) indicates long-term sea level fluctuations controlled the source(s) of sediment for these units by burying or exposing the basement rocks along the Transcontinental Arch available for erosion and subsequent deposition.

4) There appears to be little recycling of pre-existing siliciclastic units during deposition of the Middle-Late Ordovician quartz arenites.

Both Mario and Diar have completed much of the research for their graduate degrees, and during the past year we had hoped to finish most, if not all, of our detrital zircon analyses from the western U.S. and the U.S. Midcontinent. However, the LA-ICP-MS at Texas A&M has had a series of technical difficulties that until recently were difficult to diagnose and fix (see attached letter). However, it looks as if the repairs are now completed and we should have both Diar's and Mario's data completed by January, 2014. If there are further delays with using the Texas A&M facility we plan to make arrangements to use the LA-ICP-MS at Washington State University. Additionally, we are planning to analyze the few shale samples that occur within the quartz arenites to use their CIA index to determine paleoclimateduring deposition.

This grant has allowed me to follow an entirely new line of research, detrital zircon geochronology, and it has lead to numerous collaborations at Texas A&M between Dr. Brent Miller, Dr. Tom Yancey, and me. This grant has provided Research Assistantships for Mike Pickell and Mario Lira and will provide the bulk of the funding for the analyses for Mike Pickell, Mario Lira, and DiarIbrahim's detrital zircon samples, teaching them quantitative techniques and how to integrate these into fundamental stratigraphic correlation to solve problems. This skill will likely provide all three of them with a good background for successful careers in the petroleum industry.