Organic Matter Preservation in Paleocene Rocks at MoSU ridge in McKinney Roughs Nature Park: New Insights into Ancient Wetland Ecosystems Russel C. Rogers1, Jen O’Keefe1 (mentor), Thomas Demchuk2 (mentor), Chris Denison3 (mentor), Nicholas Cowey4 (mentor) 1Department of Earth and Space Sciences, College of Science, 2RPS Group, Houston, TX, 3Astra Stratigraphics, Bastrop, TX, 4McKinney Roughs Nature Park, Cedar Creek, TX

Introduction Results and Discussion Approximately 58.9 million years ago rivers transported siliciclastic sediment To date, fourteen of the eighteen samples have been analyzed for their mineral and from the distant southern Rocky Mountains (Mackey et al. 2012) to a near shore maceral content (Figure 5). Many of the samples did not have significant amounts of system in what is now known as the Wilcox Group, Bastrop County, eastern Tex- pollen or fungi, but were overall liptinite rich; much of this is likely telalginite, pro- as (Figure 1). The Wilcox Group is a geologic grouping of three formations, the duced by freshwater algae. Notable was the presence of exuding oil (exudatinite) in lowest formation being the informally named Hooper (Figure 2). The Hooper the coalified layers. formation, with its marine trace fossil assemblage, was likely deposited as a tidal Overall, the sampled layers are clay rich, containing over 50 % clay (Figure 6). This to near shore fluvially-influenced environment (Denison et al., 2016). Though means that the deposit is technically a carbonaceous shale, not a coal, although given the large amount of exudatinite present, geochemical analyses may be necessary to not known for thick coal deposits, the Hooper Formation contains thin and fully classify the deposit. The deposit ranges from inertinite-rich to liptinite-rich, the discontinuous coal deposits coincident with exuding oil in the area of McKin- latter primarily consisting of liptinitic plant pollen spores of terrestrial origin as well ney Roughs State Park. The lower Wilcox Group is a major reservoir of oil in the as freshwater algae. The presence of the liptinite maceral suberinite in the lighter col- Gulf of Mexico, and was deposited around the time of the hyperthermal event ored layers suggests the presence of bald cypress trees, often found in mire settings, known as the “Mid-Paleocene Biotic Event” (MPBE; Warwick, 2017; Hyland et and the morphology of the inertinite macerals found are generally more consistent al., 2015; Bralower et al. 2002; ). The focus of this study is the organic petrog- with herbaceous and emergent vegetation, rather than wood. Emergent vegetation raphy of the Hooper Formation, exposed at MoSU Ridge in McKinney Roughs is often found in mire margins and in shallow rheotrophic topogenous mires, like Nature Park, Bastrop County Texas. We know that provenance of coal macerals those found in Brazos Bend (Figure 7). Additionally, the increased inertinite content prevails through coal rank series (O’Keefe et al., 2013). Our overarching goal is, suggests increased fires and therefore, increased seasonal aridity, which is interest- therefore, to characterize the depositional environment using mineral matter ing considering the elevated levels of atmospheric carbon dioxide during this time and coal maceral contents and to supplement the climatic and decompositional (Hyland et al., 2015). information derived from concurrent palynology (Newman et al., this meet- Furthermore, the samples are hydrocarbon rich, which is interesting considering the ing). surrounding non-hydrocarbon rich coal deposits. While this deposit appears to be a coal in outcrop and hand sample, because in no case does it contain more than 50 Figure 3: Composite stratigraphic column for Figure 4: The MoSU Ridge coal ex- percent organic matter, it is clearly not a coal and should be classified as a carbona- the Hooper formation exposures at McKinney posed in its trench (top). Descrip- ceous shale. Roughs Nature Park. The coal at MoSU Ridge is tion of the exposure by students in WI From palynological study of the Hooper coals, we know that they are enriched in thinner than shown in the idealized section. This 2018: ESS 399. You can see in the top freshwater algae; these were difficult to distinguish from spores and pollen using re- section shows the relative position of samples ob- photo that the coal contains at least flected light petrography. They are the likely source of the exudatinite present in the tained for this study (1120-1137 relative to those two lightening-upward cycles, indica- coal and likely one of several sources of hydrocarbons in lower Wilcox Coals in the obtained for the O’Keefe et al., 2017 study. tive of drying upward (Korasidis et al., 2017). deepwater Gulf of Mexico.

Huminite Macerals: organic matter derived from wood. Mineral Matter: minerals in the coal Figure 1: MoSU Ridge is located near Coyote Creek Trail, northeast of Hooper sites 1 and 2 and west of Hooper site 3, in the McKinney Roughs Nature Park, east of Austin, TX.

a b c j k Figure 2: The Hooper formation Liptinite Macerals: organic matter derived from lipids, such as pollen, cuti- m of the Wilcox Group was depos- cle, cork, resin, and algae. These fluoresce under blue light illumination.. ited across the Selandian-Thane- tian boundary, approximately e 58.9 ma. l Figure 7: A peat-producing wetland at Brazos Bend State d f Materials/Methods Park showing an open-water rheotropic mire surround- Inertinite Macerals: organic matter that has been oxidized. MoSU ridge was located during a reconaissance hike in December 2018. A me- ed by emergent vegetation, ringed by bald cypress trees. g This type of wetland is highly variable, with tiny changes ter-deep vertical trench was dug to expose a fresh coal face. This trench extended in land surface height supporting radically different plant through the seat-earth of the coal and through the roof-rock into the overlying groups, and producing very different peat types across nFigure 5: Macerals and minerals in the coal. oA) soil horizon (Figure 4). The total vertical exposure was approximately 2 meters. small areas. Attrinite in clay with a piece of volcanic glass; Samples were then collected every five cm, beginning at a depth of 1.75m verti- h i B) Attrinite in clay with rainbowing caused by cally until modern topsoil was reached. Samples were then dried, crushed and exudatinite; C) Ulminite in clay; D) Resinite and Sporinite in attrinite; E) Suberinite; F) seved using a 1 mm sieve. Two part epoxy was then mixed with the samples, Resinite and Alginite in clay; G) Fusinite; H) Fusinite; I) Secretinite; J) Oxidized Pyrite; K) which were then poured into 1.25-inch diameter phenolic ring moulds to pro- Pyrite; L) Zircon; M) Sanadine; N) Rutilated Quartz; O) Clay. duce petrogrpahic pellets. Once cured, the pellets were polished to a 0.05-mi- crometer finish. Maceral counts and sample microphotography were completed using a Leitz Ortholux II-pol reflected light microscope with plane-polarized Figure 6: The samples collected from the coal exposed at MoSU Ridge are extremely rich in white and blue light illumination at a total magnification of 500x and equipped mineral matter (left), most of which is clay. The lowermost sample contained no macerals with a Leica MC170 HD camera and LAS software. Following laboratory stan- and was omitted from the coal maceral percentages chart (right). The coal maceral percent- dard and a 0.3 mm spacing, approximately 500 points were counted on each ages chart shows the general lightening upward sequences: high inertinite content produces pellet. darker coal than high liptinite content (Korasidis et al., 2017). • Korasidis, V., et al., 2017. The Significance of Peatland Aggradation in Modern and Ancient Environments. Palaios 23658-671. References • Mackey, G, et al., 2012. Provenance of the Paleogene-Eocene Wilcox Group, western Gulf of Mexico basin: Evidence for integrated drainage of the southern Laramide Rocky Mountains and Cordilleran arc. 124:1007-1024. • O’Keefe, J., et al., 2013. On the fundamental difference between coal rank and coal type. 118: 58-87 Acknowledgements • Bralower, T., 2002. Proceedings of the Ocean Drilling Program, Initial Reports Leg 198 (http://www-odp.tamu.edu/publications/198_IR/198ir.htm). • O’Keefe, J. et al., 2017. Organic Petrography of Hooper Formation Coals, Central Texas, USA. 2017 Annual Meeting of the Geological Society of America. https://gsa.confex.com/gsa/2017AM/webprogram/Paper305547.html. This project was completed during the WI 2018: ESS 399 and Spring 2019: ESS 476. We would like to • Denison, C., et al., 2016. Paleocene-Eocene Stratigraphy and Paleontology of East-Central Texas: Wilcox Group-Claiborne Group-Jackson Group, Post-Meeting Field Trip Guidebook. 49th Annual Meeting of AASP-The Palynological Society. • Warwick, P.D., 2017. Geologic assessment of undiscovered conventional oil and gas resources in the lower Paleogene Midway and Wilcox Groups, and the Carrizo Sand of the Claiborne Group, of the northern Gulf Coast region: U.S. Geological Survey Open-File • Hooke, R., et al., 2011.Paleocene coal deposits of the Wilcox Group, central Texas, in P.D. Warwick, A. K., M. Merrill, and B. J. Valentine, eds., Geologic assessment of coal in the Gulf of Mexico coastal plain, U.S.A: AAPG Discovery Series No. 14/AAPG Studies Report 2017–1111, 67 p., https://doi.org/10.3133/ofr20171111. thank McKinney Roughs Nature Park for access to the site and MADU Hives Field Station (Chris & Kathy in Geology No. 62, p. 179-191. Denison) for housing us during our stay. • Hyland, E., et al., 2015. Terrestrial evidence for a two-stage mid-Paleocene biotic event. 417: 371-378.