High Precision U/Pb geochronology on the Cenomanian Dakota Formation, Utah: implications for paleobotany and the transgression of the Western Interior Seaway preceding Oceanic Anoxic Event 2 Laura Meyer Submitted to the Department of Earth, Atmospheric and Planetary Sciences in Partial Fulfillment of the Requirements for the Degree of Bachelor of Science in Earth, Atmospheric and Planetary Sciences at the Massachusetts Institute of Technology May 7, 2010 ?Jeg \ Copyright 2010 Laura Meyer. All rights reserved. The author hereby grants to MIT permission to reproduce and to distribute publicly paper and electronic copies of this thesis document in whole or in part in any medium now known or hereafter created. Author Signature redacted Department of Ftdh, Atmospheric and Planetary Sciences - 1102May 7, 2010 Certified by Signature redacted ay 7,2010 Samuel Bowring Thesis Supervisor bSignature redacted Samuel Bowring Chair, Committee on Undergraduate Program MASSACHSEITS INSTITUTE OF TECHNOWGY SEP 28 Z017 LIBRARIES ARCHIVES 1 Acknowledgements: I first like to thank Sam Bowring, my advisor, for his patience, humor, and encouragement not only on this project but throughout my undergraduate career. His guidance has been indispensable in my education. Special thanks also go to Kirk Johnson and Ian Miller, who opened their collections as well as their ideas to me without hesitation. I would also like to thank Matt Rioux who gave so much of his time, patience, optimism, and I am sure at times his sanity, in training me in at the MIT geochronology lab. 2 Table of Contents Abstract:........................................................................... 4 Introduction to the Dakota Formation Regional paleogeography and depositional environment.............4 Previous studies of the Dakota Formation...........................6 Radiometric dating introduction.......................................8 Site descriptions and geology of Henrieville and Westwater, Utah.........11 Methodology of high precision U/Pb dating techniques...................14 Results Paleobotany.............................................................. 17 Uranium-Lead .......................................................... 21 Discussion/ Interpretation.......................................................25 C onclusions...................................................................... 30 R eferences........................................................................ 31 Appendix A: Site photographs.....................................................36 B: Paleobotany data................................................... 37 C: Geochronology raw data........................................... 39 D. Accepted geological time scale with fossil correlations..... 40 3 Abstract The Dakota Formation was deposited during the Cenomanian, a time when the Western Interior Seaway spanned much of North America. The Dakota Formation contains a rich record of plant and animal fossils. Key to understanding their importance, it is imperative to precisely place the Dakota Formation within the geological time scale. Volcanic ash samples interlayed with fossil bearing sandstones of the Dakota were collected from two localities in Westwater, and Henrieville, Utah. Samples were dated using isotope dilution thermal ionization mass spectrometry (ID-TIMS). The samples collected from the Westwater, Utah localities have a weighted mean 206Pb/2 "U date of 97.656+0.082 Ma, while the samples taken from Henrieville, Utah have weighted mean 2 38 206Pb/ U dates of 95.170 0.056 Ma and 94. 941 0.032 Ma. Based on an analysis of stratigraphic, paleontological, and geochronological data, the Dakota Formation was then correlated across much of the Western Interior. These dates provide a base of the initiation of the transgression across Utah, and set a context for the CT boundary and OAE2 that follow. Introduction to the Dakota Formation During the early Cretaceous (145.5 +/- 4.0 to 99.6+/- 0.9 Ma; Gradstein et. al., 2004), a broad sea flooded continental North America from the north and the. By the end of the early Cretaceous, the Western Interior of North America was covered by a large, shallow marine seaway, the Western Interior Seaway (WIS) (Obradovich and Cobban, 1975). At its peak, the WIS extended through parts of Canada and the United States (Figure Figure 1: Cenomanian Paleogepgraphy of North America showcasing the Western Interior 1). To the west of the seaway lay the Sevier orogen, and the to the Seaway (adapted from Cobban and McKinney, USGS) east the cratonic platform (Kaufmann, 1984). The Dakota Formation, a series of alternating shales and mudstones bounded stratigraphically above and below by massive sandstone deposits, were deposited along the seaway, from the 4 Era Period Epoch Stage Age (Ma) -65-50' Maastrichtian 70.60 Late Albian to Turonian ages (Figure 2). While the Campanian Dakota Formation was deposited during a time of Late 85.53 Santonian 8585 Coniacian 8927 major marine transgression, Haq et.al (1987) found Turonian 0 93.55 - Cenamanian 99.6 these to be superimposed by regressive phases N Albian (marked by shoreline deposits such as sandstones) 112.0 that were periodically deposited within the more Aptian Early 125.0 phase of the seaway (marked Barremnlan expansive transgressive 130.0 Hauterivian in deep water areas by marine shales). The 136.4 Valanginian 1401 Berriasian 14r r. I deposition of the Dakota Formation, as considered in Figure 2: Geological time scale for the Cretaceous (adapted from Gradstein et. this paper, began in the early Late Cretaceous near the al, 2004) base of the Cenomanian. At this time, deposition was occurring in coastal environments as the base level of rivers rose related to the transgression of the inland Western Interior Seaway. The Dakota Formation, often overlain by the Tropic Shale Formation, marks a rise in sea level and the beginning of a minor mass extinction event around the Cenomanian-Turonian boundary. This extinction is coincided with a negative carbon isotopic excursion and sea level rise (Sageman, 2006; Barclay 2010). This interval of time and associated perturbation of the carbon cycle is also known as an "Oceanic Anoxic Event" (OAE2), and is characterized by an accumulation of organic rich sediments (Schlanger and Jenkyns, 1976). Cretaceous OAEs, and specifically OAE2 occurred on a warm planet with high concentrations of atmospheric CO 2. OAE2 (-93.5 Myr ago (Gradstein et al, 2004)) peaks in the uppermost Cenomanian, and is the most widespread of the three recognized oceanic anoxic events in the mid-Cretaceous. The 5 increased deposition and burial of organic material can be explained through enhanced primary productivity and/or different preservation possibilities, but the causal trigger of the ocean anoxic event has not been clearly identified. Despite the controversy, it is undeniable that OAE2 marks a major change in climate including a 40-80% reduction in CO2 levels (Kuypers et. al., 1999) although an exact sequence of events has never been established. This disturbance stressed the ecosystem, forcing a major biotic crisis (Hallam and Wignall, 1997). The onset in the Western Interior was abrupt, and in central Colorado resulted in 6"Ccarb and 8"Corg records which display positive excursions of up to 2%o-4%o (Sageman 2006; see Figure 14). The two main hypotheses for the cause of the carbon isotope excursion are a period of enhanced seafloor spreading and orbitally forced climate change (Schlanger and Jenkyns, 1976; Kerr, 1998; Kuroda, 2007). This paper focuses on the age and paleontology of the Dakota Formation, in order to provide an environmental context for the boundary and OAE2, as well as place an age on the initiation of the transgression of the Western Interior Seaway. The Dakota Formation has historically been subject to controversy and misunderstandings surrounding its age and flora. Many of these arguments stem from the incorrect usage of the term "Dakota flora", which makes assigning an exact age difficult at best. The type section near the Nebraska-Iowa border was first studied by Meek and Hayden (1858). This section has since been referred to as the "Dakota Formation", the "Dakota Sandstone", the "Dakota Group", the "Dakota Conglomerate", and simply as the "Dakota". When referencing the Dakota fauna, this diversity of names can lead to confusion and misinterpretations. Often, the Dakota is used to mean any Cretaceous flora with dicot leaves older than the Benton Formation (Berry, 1920). But Witzke and 6 Ludvigson (1994) suggested a simplification and unification of terms, stating that "The Dakota Formation should define as a body of rock of eastern provenance, primarily nonmarine fluvial to marginal marine and deltaic deposits, that were deposited during transgressive phases of the Greenhorn marine cyclothem." Taking this into account, Figure 3 spans the western United States and correlates the Dakota Formation as it is defined in this paper to other formations and localities. S"t j K " Utah % *"Il-w (Ameueai Kkoh& CI&Ka", (AIL111ma 1 '4 Pa-t 364w-6Mak. Mfl nGram-f Granm Graner -- _____ I~~aL..t D Sh"ah. -~ SuJ. gnred tratgrpi cunS:a t.i ale CooaPla (Young, Nrhae S%) C Shoa(le%) Da~w Figure 3: Correlation of geologic rock formations across the western interior of the US. Adapted from Lane (1963) with generalized stratigraphic columns: Utah (this paper), Colorado Plateau (Young, 1960), Northwest Colorado (Lane 1963), North Front Range Foothills (Waage, 1955), South-Central Colorado (Waage, 1953),