Ar/Ar geochronology of the Eocene Green River Formation, Wyoming
نویسندگان
چکیده
The deposits of Eocene Lake Gosiute that constitute the Green River Formation of Wyoming contain numerous tuff beds that represent isochronous, correlatable stratigraphic markers. Tuff beds selected for Ar/Ar analysis occur within laminated mudstone, are matrix supported, and lack evidence of reworking. These tuffs contain 2%–15% euhedral phenocrysts of quartz, plagioclase, sanidine, biotite, and minor amphibole, pyroxene, and zircon, encased in a matrix of altered glassy ash. Air abrasion and handpicking under refractiveindex oils were required to obtain clean, unaltered phenocrysts of sanidine. Ar/Ar age determinations from single-crystal and !1 mg multigrain aliquots of sanidine and biotite allowed the identification and exclusion of xenocrystic contamination. Laserfusion experiments on phenocrysts from the Rife, Firehole, C Bed, Grey, Main, Sixth, and Analcite tuff beds from the Tipton, Wilkins Peak, and Laney Members yielded weighted-mean ages (!2" analytical uncertainties) of 51.25 ! 0.31 Ma, 50.70 ! 0.14 Ma, 50.56 ! 0.26 Ma, 50.39 ! 0.13 Ma, 49.96 ! 0.08 Ma, 49.70 ! 0.10 Ma, and 48.94 ! 0.12 Ma, respectively. Ages for sanidine and biotite from the Main tuff are indistinguishable when presumably xenocrystic contaminants are excluded from the age calculation. Moreover, the Ar/Ar ages are consistent with the stratigraphic order of the tuff beds and with provenance in the Absaroka and Challis volcanic fields. Our Ar/Ar-based age model indicates that sediment accumulated three times more rapidly (327 ! 86 #m/yr) during the Corresponding author e-mail: bsinger@geology .wisc.edu. evaporative Wilkins Peak phase than the freshwater to saline Tipton (88 ! 34 #m/ yr) and Laney (104 ! 18 #m/yr) phases. The much lower accumulation rates for the Tipton and Laney Members are permissive of an annual origin for !1-mm-thick laminae and precessional forcing of 1–3-mthick depositional cycles in these units. However, previously described cycles in the Wilkins Peak Member have average durations that are significantly shorter than the 19–23 k.y. precessional modes. The Green River Formation encompasses an "5 m.y. period between ca. 53.5 and 48.5 Ma, spanning magnetic chrons 24n through 21r. The Green River Formation was therefore deposited during the warmest period of the Cenozoic corresponding to the early Eocene climatic optimum as defined by the global marine O isotope record. Deposition of bedded evaporites (trona) of the Wilkins Peak Member began at ca. 51 Ma, or "2 m.y. after the onset of the highest inferred temperatures of the climatic optimum. The Bridgerian–Wasatchian faunal turnover occurred subsequently during Wilkins Peak time, at ca. 50.6 Ma. Thus, our Ar/Ar ages strongly suggest that Wilkins Peak evaporite deposition and the turnover from Wasatchian to Bridgerian fauna were not directly caused by the initiation of maximum greenhouse conditions.
منابع مشابه
Discussion and reply: 40Ar/39Ar geochronology of the Eocene Green River Formation, Wyoming Discussion
Smith et al. (2003, hereafter referred to as Smith et al.) present seven new isotopic ages from the Green River Formation in the greater Green River Basin. These data provide welcome new information in the ongoing effort to develop a coherent and reliable geochronological framework for this large and stratigraphically complex basin. Several of their main conclusions are based on a revised age e...
متن کاملSpectral analysis of the lower Eocene Wilkins Peak Member, Green River Formation, Wyoming: Support for Milankovitch cyclicity
This study is the first to employ spectral analysis to examine meter-scale sedimentary cyclicity in the Wilkins Peak Member of the lower Eocene Green River Formation ofWyoming. Generally regarded as the classic example for orbital forcing of lacustrine sediments at eccentricity and precession time scales, this long-standing interpretation was recently contested, with a much shorter duration (≤1...
متن کاملAn Early Eocene Oilbird from the Green River Formation of Wyoming (caprimulgiformes : Steatornithidae)
The first diagnostic fossils of an oilbird (Steatornithidae : Caprimulgiformes), two nearly complete skeletons, are described as a new genus and species from the early Eocene Green River Formation of Wyoming. The fossil form is smaller than the living Oilbird (Steatomis caripensis) but had the characteristically shortened tarsometatarsus and distinctive mandibular morphology. The sternum and pe...
متن کاملSalamander Tracks (ambystomichnus?) from the Cathedral Bluffs Tongue of the Wasatch Formation (eocene), Northeastern Green River Basin, Wyoming
VERTEBRATE TRACKS are comparatively rare in Tertiary deposits of the western United States. Unlike the deposits of the Mesozoic in this region, in which each formation often has several dozen known tracksites, there are only a few known sites in Paleocene units of the region (Lockley and Hunt, 1995), and though the Eocene Green River Formation contains relatively numerous tracks, especially tho...
متن کاملNew specimens of the early Eocene frigatebird Limnofregata (Pelecaniformes: Fregatidae), with the description of a new species
Four additional specimens from the Green River Formation of Wyoming are referred to the Eocene frigatebird Limnofregata azygosternon Olson, originally described from a nearly complete skeleton and two partial paratypes. Two skulls with mandibles and a partial postcranial skeleton are described as a new species, Limnofregata hasegawai, characterized by much larger size and a proportionately long...
متن کاملذخیره در منابع من
با ذخیره ی این منبع در منابع من، دسترسی به آن را برای استفاده های بعدی آسان تر کنید
عنوان ژورنال:
دوره شماره
صفحات -
تاریخ انتشار 2008