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, PhD, Texas A&M University

During the last year our lab had four students complete M.S. theses on Middle-Late Ordovician quartz arenites in the western U.S. and U.S. Midcontinent. Some of detrital zircon geochronology samples for each of these students were supported by this grant. In addition, one of the students, Michael Pickell, was supported on a research assistantship in the spring semester of 2012. We presented a poster at the American Association of Petroleum Geologists Annual Convention in Long Beach, April 2012 about our research to date. It was well received and I am trying to get each of the students theses submitted for publication by the end of this year. Throughout the duration of the grant Mario Lira, an undergraduate Geology major at Texas A&M has worked in the field and in the laboratory learning techniques to sample and crush quartz arenite samples, then to physically and chemically separate out the detrital zircons within the samples. Mario worked on and off throughout the academic year and hopefully he will be attending Texas A&M this coming spring semester (2013) as a graduate student to continue working on this project.

The main results of the research so far are: 1) detrital zircons for the 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 Ð 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 terranes is much larger than in the western 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.

During the coming year of the grant we will be finishing most, if not all, of our detrital zircon analyses from the western U.S. and will be continuing to analyze detrital zircons from the U.S. Midcontinent. Additionally, we are planning to analyze the few shale samples that occur within the quartz arenites to use their CIA index to determine paleoclimate during deposition.