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176 7.1. Map showing the distribution of the White River Group and correlative strata across the northern Great Plains. B = Badlands National Park, South Dakota; D = Douglas, Wyoming; F = Flagstaff Rim, southwest of Casper, Wyoming; L = Little Badlands of southwest North Dakota; P = Pawnee Buttes of northwest Colorado; R = Reva Gap, east of Buffalo, South Dakota; S = Scotts Bluff National Monument, Nebraska; T = Toadstool Geologic Park, north of Crawford, Nebraska. 177 7 The Big Badlands in Space and Time The Eocene–Oligocene Boundary The Eocene and Oligocene epochs are specific intervals of time recognized across the globe, but in order to understand their significance, it is important to understand how they are identified. The geologic history of our planet is stored within the archives of various types of rock bodies and strata, and the plant and animal remains preserved within them. Early scientists were aware that changes in plant and animal life had occurred, that the types of plants and animals preserved in a type of rock varied even if the rock type was the same, and that the existence of fossil remains in rock layers high on the tops of mountains suggested that the landscape had not always been as it appeared. Other than methods that allowed early geologists to establish the relative ages of various rock bodies, such as superposition, which states that in a sequence of horizontal strata the oldest layers of rock are on the bottom, these early scientists had no way to quantify the ages of strata and their associated fossils, when deposition started, how long it lasted, or when it stopped. Relative ages could be established across broad regions by tracing laterally continuous rock units or occurrences of particular fossils that allowed these early scientists to construct a relative geologic order of important paleontological and geological events, such as the appearance and extinction of the dinosaurs , but absolute ages for these events were unavailable. This changed with the development of radiometric dating of geologic materials and the discovery of magnetic signatures preserved in rock. The combination of radiometric dating plus paleomagnetism in sedimentary rock strata around the globe has allowed the establishment of a chronologic framework within which the patterns of evolution and geologic events portrayed in the geologic column can be placed. The Eocene–Oligocene boundary represents the most dramatic change in global paleoclimatic conditions since the extinction of the dinosaurs. This change, also referred to as the Hothouse to Icehouse Transition (Prothero, 1994; Prothero, Ivany, and Nesbitt, 2003; Koeberl and Montanari, 2009), is associated with dramatic changes in global temperature , large drops in sea level, the establishment of permanent glacial ice on Antarctica, and extinctions in the nonmarine and marine fossil record. The causes of this dramatic climate change are still a matter of debate. Hypotheses to explain this climate change have included plate tectonic reorganization and opening of marine passageways between South America, Australia, and Antarctica; uplift of the Himalayan Mountain Range; and multiple extraterrestrial impact events during the late Eocene. Any attempt to understand the causes of this transition must take into account both terrestrial and marine records across the globe during this period of time. Our particular interval of interest, the Eocene–Oligocene boundary, is one of several divisions of epochs within the Tertiary Period of the Cenozoic Era. For each of these divisions within the geologic column, one particular place on the globe is used as the global reference against which all other locations of the same age can be compared. These scientifically agreed-upon stratigraphic levels and locations are referred to as a Global Stratotype Section and Point (GSSP) and are formalized for many of the boundaries in the geologic column (Alvarez, Claeys, and Montanari, 2009). For the Eocene–Oligocene boundary, the global standard is a section of shallow marine limestones exposed along the Adriatic coast near Massignano, Italy, which contains several layers of impact ejecta, volcanic ashes, evidence of magnetic reversals, and abundant marine fossils (Premoli and Jenkins, 1993; Hyland et al., 2009). The Eocene–Oligocene boundary is currently dated at approximately 33.8 Ma and is associated with a particular set of normal and reversed paleomagnetic signatures referred to as 13N (normal) and 13R (reversed) polarity when compared to today (Hilgen and Kuiper, 2009). The numbers for polarity events increase with older rocks, counting backward from 1N/R for our modern record (Fig. 2.2). Within the various epochs of the Tertiary such as the Eocene and Oligocene, smaller units of time can be...

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Additional Information

ISBN
9780253016089
Related ISBN
9780253016065
MARC Record
OCLC
909368973
Pages
240
Launched on MUSE
2015-05-21
Language
English
Open Access
No
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