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Middle Miocene Grounding Events on the Ross Sea Outer Continental Shelf, Antarctica

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Middle Miocene Grounding Events on the Ross Sea Outer Continental Shelf, Antarctica Louisiana State University LSU Digital Commons LSU Master's Theses Graduate School 2003 Middle Miocene grounding events on the Ross Sea outer continental shelf, Antarctica Juan Manuel Chow Louisiana State University and Agricultural and Mechanical College, [email protected] Follow this and additional works at: https://digitalcommons.lsu.edu/gradschool_theses Part of the Earth Sciences Commons Recommended Citation Chow, Juan Manuel, "Middle Miocene grounding events on the Ross Sea outer continental shelf, Antarctica" (2003). LSU Master's Theses. 2523. https://digitalcommons.lsu.edu/gradschool_theses/2523 This Thesis is brought to you for free and open access by the Graduate School at LSU Digital Commons. It has been accepted for inclusion in LSU Master's Theses by an authorized graduate school editor of LSU Digital Commons. For more information, please contact [email protected]. MIDDLE MIOCENE GROUNDING EVENTS ON THE ROSS SEA OUTER CONTINENTAL SHELF, ANTARCTICA A Thesis Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of the requirements for the degree of Master of Science in The Department of Geology and Geophysics by Juan Manuel Chow B.Arch., Louisiana State University, 1991 December 2003 DEDICATION I dedicate this work to the Earth. Without her, I would have nowhere to stand and contemplate her myriad manifestations. ii ACKNOWLEDGEMENTS I am beholden to my advisor, Dr. Philip J. Bart, and my committee members, Dr. Barun Sen Gupta and Dr. Lui-Heung Chan. Their help, patience, and support have made this study possible. I am grateful to the Department of Geology and Geophysics, its Faculty, and its Staff. My fellow students have helped keep in check the hectic and frantic life of graduate school education. I especially thank my family and friends, who endured my long-winded explanations of geological topics while using the arcane language of geology. This project would have been realized without the financial support of the National Science Foundation (NSF); the grant awarded to Dr. Bart enabled us to participate in data-gathering cruises on Antarctic waters. I am grateful for the technical and logistical support provided by Raytheon Polar Services Company (RPSC); Captain Joe Borkowski and the crew of the Research Vessel/Ice Breaker Nathaniel B. Palmer; and Edison-Chouest Offshore (ECO). iii TABLE OF CONTENTS DEDICATION . ii ACKNOWLEDGEMENTS . iii LIST OF TABLES . v LIST OF FIGURES . vi ABSTRACT . viii CHAPTER 1. INTRODUCTION . 1 CHAPTER 2. MATERIALS AND METHODS . 8 2.1 Types of Grounding Events . 9 2.2 Thickness Distribution of Middle Miocene Strata . 12 2.3 Dip Directions of Grounding-Zone Clinoforms . 12 2.4 Number of Grounding Events . 13 CHAPTER 3. RESULTS . 14 3.1 Middle Miocene Sediment Volume Estimates . 15 3.2 Thickness Distribution of Middle Miocene Strata . 16 3.3 Middle Miocene Grounding-Zone Progradation: Distribution and Orientations . 17 3.4 Middle Miocene Seismic Unconformities . 19 CHAPTER 4. DISCUSSION . 32 4.1 Were Ross Sea Grounding Events in the Middle Miocene Localized or Shelf-Wide? . 32 4.2 Thickness Trends on the Outer-Shelf/Upper Slope Depocenters in Northern and Eastern Basins . 35 4.3 Distribution and Orientation of Grounding-Zone Clinoforms . 38 4.4 How Many Grounding Events Occurred on the Ross Sea Shelf During the Middle Miocene? . 40 4.5 Do Shelf-Wide Grounding Events During the Middle Miocene Require a Full-Bodied Marine-Based WAIS? . 42 CHAPTER 5. CONCLUSIONS . 45 REFERENCES . 46 APPENDIX A. SEDIMENT VOLUME CALCULATIONS . 51 APPENDIX B. SEISMIC FACIES AND INTERPRETATIONS . 62 VITA . 76 iv LIST OF TABLES 1. Elevations and age range of middle Miocene strata sampled at DSDP sites 272 and 273 . 16 2. Volume and areal distribution of middle Miocene strata . 17 3. Middle Miocene units and bounding unconformities identified on the Ross Sea continental shelf . 19 v LIST OF FIGURES 1. Tectonic setting at the beginning of the middle Miocene, ca. 14 Ma . 2 2. Miocene eustatic sea level curve and δ18O curve from ODP site 747 . 3 3. Suggested middle Miocene ice sheet configuration . 4 4. Modern Antarctic ice cap and ice flow directions . 5 5. Bathymetric map of the modern Ross Sea continental shelf . 6 6. Structural contour map of RSU4 and distribution of seismic facies . 7 7. Map of the Ross Sea with seismic grid . 8 8. Age model and paleoenvironment at DSDP sites 272 and 273 . 11 9. Isopach map of middle Miocene unit RSS-5 . 13 10. Seismic profile PD 90-37 . 14 11. Seismic profile PD 90-30 . 15 12. Isopach map of the middle Miocene section on the Ross Sea . 18 13. Time-structure contour map of RSU 4 . 20 14. Time-thickness contour map of strata associated with RSU 3.5 . 22 15. Time-structure contour map of unconformity RSU 3.5 . 23 16. Time-thickness contour map of strata associated with RSU 3.4 . 24 17. Time-structure contour map of unconformity RSU 3.4 . 25 18. Time-thickness contour map of strata associated with RSU 3.3 . 26 19. Time-structure contour map of unconformity RSU 3.3 . 27 20. Time-thickness contour map of strata associated with RSU 3.2 . 28 21. Time-structure contour map of unconformity RSU 3.2 . 29 22. Time-thickness contour map of strata associated with RSU 3.1 . 30 vi 23. Time-structure contour map of unconformity RSU 3.1 . 31 24. Interpreted subglacial delta . 63 25. Interpreted till tongues . 65 26. Interpreted till tongues . 66 27. Interpreted inter-ice stream ridge . 67 28. Interpreted seismic facies (Bartek et al., 1997) . 68 29. Acoustic signatures (Alonso et al., 1992) . 70 30. Interpreted proglacial fan . 70 31. Examples of facies A, B, and C (De Santis et al., 1995) . 72 32. Interpreted off-bank progradation wedge . 74 vii ABSTRACT The middle Miocene δ18O enrichments from deep-sea data and eustatic sea level falls are traditionally attributed to expansion of the East Antarctic Ice Sheet. Interpretations of such data have led many to conclude that the West Antarctic Ice Sheet (WAIS) was not well-developed until the late Miocene. In such a scenario, middle Miocene glaciation on the Ross Sea shelf would have had to be minimal, perhaps in the form of ice caps, to be consistent with proxy data. New seismic-stratigraphic analysis of the Ross Sea outer continental shelf suggests that at least five grounding events (ice sheet advances into the marine environment, in contact with the sea floor) occurred in the middle Miocene. Because West Antarctica constitutes a major part of the drainage basin for the Ross Sea, these results do not support the long-standing assumption that West Antarctica was substantially ice-free, although the number of WAIS grounding events generally matches the number of extreme δ18O enrichments and eustatic lowstands. Rather, the seismic-stratigraphic evidence from the Ross Sea shelf documents waxing and waning of a well-developed WAIS in the marine environment at least on the Pacific sector of the West Antarctic continental shelf, and suggests a WAIS that was more robust in the middle Miocene than has previously been deduced from proxy data. viii CHAPTER 1. INTRODUCTION During the early Miocene (~24 Ma to ~16 Ma), thermohaline circulation was much different than today because low-latitude inter-oceanic passages (i.e., through the Isthmus of Panama and Tethys Seaway) permitted well-developed equatorial circulation. This equatorial surface flow produced warm saline water masses (i.e., Tethyian Indian Saline Water; TISW) that sank to intermediate depths and flowed southward (Woodruff and Savin, 1989). The upwelling and subsequent refrigeration of TISW in the Southern Ocean probably was a major agent in the meridional heat transport that helped maintain relatively warm climates in Antarctica during the early Miocene (Woodruff and Savin, 1989; Wright et al., 1992; Flower and Kennett, 1995). At the start of the middle Miocene (i.e., ~16 Ma), closure of the Tethys at the eastern portal of the Mediterranean Sea severely interrupted equatorial circulation (Hsu and Bernoulli, 1978; Steininger et al., 1985) and resulted in reduced production of TISW (Woodruff and Savin, 1989) (Figure 1). According to conventional interpretations of deep-sea proxy data, the reduced meridional heat transport led to climatic cooling in the Southern Ocean, which thereafter fostered rapid growth of the Antarctic Ice Sheet. Indeed, the large magnitudes of δ18O enrichments (Shackleton and Kennett, 1975; Woodruff et al. 1981; Woodruff and Savin, 1989,1991; Wright et al., 1992; Zachos et al., 2002) and eustatic falls (Haq et al. 1987) in the middle Miocene (~16 Ma to ~10 Ma) suggest significant long-term cooling and several stepped expansions of ice volume on Antarctica (Figure 2). Primarily on the basis of these deep-sea proxy and eustatic records, it is generally assumed that: 1) middle Miocene ice volume increases were associated with the large land-based East 1 Antarctic Ice Sheet (EAIS) attaining physical dimensions similar to that existing on East Antarctica today (Savin et al., 1975; Woodruff et al. 1981; Wright et al., 1992); and 2) West Antarctica, a much smaller block of low-lying continental crust and volcanic highlands, remained substantially ice-free until the latest Miocene (Kennett and Barker, 1990, Prentice and Matthews, 1991, Mercer, 1977) (Figure 3). Although the view of an ice-free West Antarctica during the middle Miocene is widely accepted, eustatic and deep-sea proxy records do not uniquely define the specific locations on Antarctica where the ice-volume fluctuations occurred. Moreover, direct geologic evidence suggests that ice cover on West Antarctica may have been more extensive than has traditionally been deduced from proxy data. For Eastern Tethys Drake Passage Figure 1. Tectonic setting at the beginning of the middle Miocene, ca. 14 Ma (modified from www.scotese.com/earth.htm).
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