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Zircon U-Pb Geochronology of the Great Valley Group: Recalibrating the Lower Cretaceous Time Scale

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Zircon U-Pb Geochronology of the Great Valley Group: Recalibrating the Lower Cretaceous Time Scale Zircon U-Pb Geochronology of the Great Valley Group: Recalibrating the Lower Cretaceous Time Scale Ayumi Shimokawa A thesis submitted to the faculty of the University of North Carolina at Chapel Hill in partial fulfillment of the requirements for the degree of Master of Science in the Department of Geological Sciences Chapel Hill 2010 Approved by Drew S. Coleman Tim J. Bralower Joseph G. Carter Abstract Ayumi Shimokawa Zircon U-Pb Geochronology of the Great Valley Group: Recalibrating the Lower Cretaceous Time Scale (Under the direction of Drew S. Coleman) Four new zircon U-Pb ages for volcanic horizons from the Great Valley Group require adjustments to parts of the Lower Cretaceous Time Scale: a Lower Aptian concordia age of 124.07 ± 0.13 Ma is in good agreement with the Global Time Scale 2008 Barremian- Aptian boundary age of 125.0 Ma, but Upper Berriasian (138.46 ± 0.15 Ma) and two Lower Valanginian ages (137.63 ± 0.14 Ma, 133.66 ± 0.14 Ma) shift the Berriasian-Valanginian and Valanginian-Hauterivian boundaries to 138 Ma and 133 Ma, respectively. These new boundary ages, relative to previous estimations, indicate longer Berriasian and shorter Valanginian stage durations, which 1) adjust the timing of the Valanginian δ13C positive excursion so that it is synchronous with the main pulse of the Paraná-Etendeka flood basalt volcanism, 2) require a faster seafloor spreading rate during the Valanginian than previously assumed, and 3) suggest a sharper calcareous nannofossil species diversity peak during the Hauterivian. ii ACKNOWLEDGEMENTS This project was enabled by funding from the UNC Chapel Hill Geological Sciences Martin fund, specifically for field and analytical expenses. I am grateful to the entire UNC geochronology group including Jesse Davis, Michael Tappa, Kevin Schrecengost, Ryan Mills, and Kyle Samperton, as well as Michael Ackerson for assistance inside and outside the lab. Committee members, Drs. Joseph Carter and Tim Bralower are acknowledged for helpful comments and advice. Lastly, my advisor Dr. Drew Coleman is deeply thanked for making this thesis possible, and for his strong support, both as a scientist and as an educator. iii TABLE OF CONTENTS LIST OF TABLES……………………………………………………………...............…….vi LIST OF FIGURES………………………………………………………………...………..vii CHAPTER I. Zircon U-Pb Geochronology of the Great Valley Group: Recalibrating the Lower Cretaceous Time Scale 1. Introduction…………………………………………………………………………....1 2. Geologic background. …………………………………………………………….......4 3. Methodology. …………………………………………………………….......…….....7 3.1 Sampling. ……………………………………………………………...........7 3.2 Sample processing…………………………...……………………….........11 4. Results. ……………………………………………………………..............……......12 5. Discussion……………………………………………………………..............……..18 5.1 Reverse discordance………………………………………………………..19 5.2 Lower Cretaceous time scale. ……………………………………………..22 5.2.1 Berriasian-Valanginianl………………………………………….22 5.2.2 Valanginian-Hauterivian…………………………………………24 5.2.3 Barremian-Aptian………………………………………………..24 5.3 Seafloor spreading rates…………………………………………................29 iv 5.4 Valanginian δ13C positive excursion……………………………………….31 5.4 Nannofossil evolution……………………………………………………...32 6. Conclusions……………………………………………………………………..........34 REFERENCES………………………………………………………………………………36 v LIST OF TABLES 3.1 Vestal Road sample data………………………………………………………….......9 4.1 U-Pb geochronology of the Great Valley Group….……………………………...…15 vi LIST OF FIGURES 1.1 Current Lower Cretaceous time scale (2008) compared with other recent studies.......3 2.1 Geologic map of Great Valley Group ...……………....………………………………5 3.1 Photographs of sample locations …………………..……………………………........8 3.2 Map of sample locations …………………………………………….……………....10 4.1 Concordia diagrams and weighted 206Pb/238U plots for all samples ..……………….14 4.2 MC873 concordia and 206Pb*/238U plots of all fractions …………………................13 5.1 Idealized discordance from Pb-loss and U-loss plotted on concordia.........................20 5.2 Common Pb correction concordia plots.......................................................................21 5.3 Integrated geochronobiomagnetic time scale...............................................................26 5.4 Calcareous nannofossil stratigraphy column for the Grindstone Creek section..........27 5.5 Calcareous nannofossil stratigraphy column for the McCarty Creek section……..…28 5.6 Comparison of seafloor spreading rates used in GTS2008 and recalculated…….......30 5.7 Timing of the late Valanginian δ13C positive excursion……..................................…31 5.8 Nannofossil species diversification plot adjusted with new time scale ………......…33 6.1 Comparison of time scales from this study, Geologic time scale 2008, and Fiet et al., (2006)...................................................................................................35 vii 1. INTRODUCTION The stratigraphic record preserves a history of Earth’s biologic, climatic, and geologic evolution. As our understanding of this history improves, thanks to improved analytical techniques and the simple accumulation of data, we are able to ask increasingly complex and detailed questions. Answering these questions often demands a more precise and higher- resolution Geologic Time Scale than has been necessary in the past. Such a high-precision time scale is essential for studying the timing, duration, rates, and causal relationships between biologic, climatic, and tectonic processes. In the most recent time scales – Geologic Time Scale (GTS) 2004 and 2008 – the Lower Cretaceous Time Scale global boundary stratotype sections and points (GSSPs) are still undecided. Although candidates based on ammonites and calpionellids exist in land sections of France, Spain, and Italy, the provincialism of these fossils make the establishment of a global type section for this time period difficult (Ogg et al., 2004a, 2008). Further complicating the construction of the Lower Cretaceous time scale is a dearth of accurate high-precision radiometric ages. This deficiency, save a single 40Ar/39Ar age at the base of the Aptian, requires the use of a Pacific seafloor spreading model to interpolate the entire Lower Cretaceous time scale from the one age and results in the greatest stage boundary age uncertainties of the Phanerozoic (Gradstein et al., 2004; Ogg et al., 2004a; Pringle and Duncan, 1995). The poor state of the Lower Cretaceous Time Scale is problematic, especially because the Lower Cretaceous was an interval during which many important global events – such as the formation of large igneous provinces, rapid climate change and associated faunal changes, and oceanic anoxia (Jenkyns, 2003) – were recorded in the sedimentary record. Defining the temporal succession of closely spaced events tied to different time scales – biostratigraphic, magnetostratigraphic, chronometric – is often critical for conclusions of cause and effect, but with the poorly defined Lower Cretaceous Time Scale, such conclusions are tentative at best. Since the publication of GTS2004 and the call for Lower Cretaceous high-precision radiometric ages (Ogg et al., 2004a), several studies have contributed new Lower Cretaceous ages (Fig. 1.1). Mahoney et al. (2005) gave a 40Ar/39Ar age for the Shatsky Rise, 144.6 ± 0.8 Ma, which corroborates GTS2008 base Berriasian age of 145.5 Ma. Fiet et al. (2006) proposed an entirely new Lower Cretaceous time scale with stage boundary ages that different from those of GTS2004 by up to 12m.y. (Fig. 1.1), on the basis of K-Ar glauconite ages supplemented with cyclostratigraphy in the Vocontian basin, France. Several studies following this work, however, raised doubts about the drastic time scale changes proposed. For example, Aguirre-Uretta et al. (2008) published a zircon U-Pb sensitive high resolution ion microprobe (SHRIMP) age of 132.5 ± 1.3 Ma for a tuff corresponding to the Subsaynella sayni ammonite zone (early upper Hauterivian) from the Neuquén Basin, Argentina in good agreement with GTS2008; He et al. (2008) produced an 40Ar/39Ar age of 121.2 ± 0.5 Ma for reversely magnetized lava flows from the Yixian Basin, China and concluded that this age approximates the base of magnetochron M0r. Selby et al. (2009) dated zircons from a tuff that puts the younger limit for the Aptian-Albian boundary at 113 ± 0.3 Ma. These recent studies contribute ages spanning Hauterivian-Aptian; however none have dated any rocks from the Valanginian-Berriasian interval. 2 Here I present new zircon U-Pb ages for bentonites from the Great Valley Group in northern California that span Berriasian to Aptian. The Great Valley Group, with its well documented calcareous nannofossil biostratigraphy and abundant tuff horizons, serves as an excellent location for the task of improving the Lower Cretaceous Time Scale. Figure 1.1: Comparison of GTS2008 Lower Cretaceous time scale with time scale suggested by Fiet et al., (2006). Other Lower Cretaceous radiometric ages from recent studies shown to the left of GTS2008; gray boxes represent age uncertainty. 3 2. GEOLOGIC BACKGROUND The Great Valley Group is a 15km thick sedimentary section that was deposited throughout the Cretaceous in a deep-water forearc basin (Ingersoll, 1978; Surpless et al., 2006). This north-south oriented basin is bordered on the west by the Franciscan ophiolite subduction complex and on the north and east by the Klamath-Sierran volcanic arc, the main source for Great Valley sediments (Fig. 2.1; Ingersoll and Dickinson, 1981; Ingersoll, 1982). Mudstone, shale, siltstone,
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