40Ar/39Ar ages and geochemical characterization of Cretaceous bentonites in the Nanushuk,Seabee, Tuluvak,and Schrader Bluff formations,North Slope,Alaska |
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| Institution: | 1. Geology Department, Whitman College, 345 Boyer Avenue, Walla Walla, WA, 99362, USA;2. Geophysical Institute, 903 Koyukuk Drive, University of Alaska Fairbanks, Fairbanks, AK, 99775, USA;3. Department of Geosciences, 900 Yukon Drive, University of Alaska Fairbanks, AK, 99775, USA;4. Alaska Division of Geological and Geophysical Surveys, 3354 College Road, Fairbanks, AK, 99709, USA;1. Museum of Natural History, University of Wroc?aw, Sienkiewicza 21, 50-335, Wroc?aw, Poland;2. Schmalhausen Institute of Zoology, National Academy of Sciences of Ukraine, Bogdana Khmel''nitskogo 15, Kiev, 01601, Ukraine;1. Institut de Systématique, Évolution, Biodiversité, ISYEB, UMR 7205, CNRS, MNHN, UPMC, EPHE, Muséum national d''Histoire naturelle, Sorbonne Universités, 57 Rue Cuvier, CP 50, Entomologie, F-75005, Paris, France;2. State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, 210008, PR China;1. School of the Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China;2. Dinosaur Tracks Museum, University of Colorado Denver, PO Box 173364, Denver CO 80217, USA;3. Regional Geological Survey Team, Sichuan Bureau of Geological Exploration and Development of Mineral Resources, Chengdu, China;4. Peace Region Palaeontology Research Centre, Box 1540, Tumbler Ridge, British Columbia V0C 2W0, Canada;5. Saurierwelt Paläontologisches Museum, Alte Richt 7, D-92318 Neumarkt, Germany;6. Department of Biological Sciences, University of Alberta 11455 Saskatchewan Drive, Edmonton, Alberta T6G 2E9, Canada;7. No.208 Hydrogeological and Engineering Geological Team, Chongqing Bureau of Geological and Mineral Resource Exploration and Development, Chongqing 400700, China;1. State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China;2. University of the Chinese Academy of Sciences, Beijing 100049, China;3. Key Laboratory of Economic Stratigraphy and Palaeogeography, Nanjing Institute of Geology and Palaeontology Chinese Academy of Sciences, Nanjing 210008, China;4. Steinmann Institute, University of Bonn, Bonn 53115, Germany;5. Department of Earth Sciences, The Natural History Museum, London SW7 5BD, UK |
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| Abstract: | Diagenetically altered volcanic ash deposits (bentonites) found in Cretaceous terrestrial and marine foreland basin sediments have the potential to be used for chronostratigraphy and subsurface correlation across Alaska's North Slope. Detailed age and geochemical studies of these volcanogenic deposits may also shed light on the tectonic evolution of the Arctic. Though these bentonites have been previously studied, there are few published results for regional bentonite ages and geochemistry due to challenges of dating weathered volcanic ash. We analyzed mineral separates from cored bentonites recovered from wells in the National Petroleum Reserve Alaska. The analyses confirm that an intense period of volcanic ash deposition on Alaska's North Slope began by the late Albian and persisted throughout the Cenomanian, an interval of rapid progradation and aggradation in the Colville basin. These results also add to a sparse record of radioisotopic ages from the Nanushuk Formation. A bentonite preserved in delta plain sediments in the upper Nanushuk Formation dates to 102.6 ± 1.5 Ma (late Albian), while a bentonite near the base of the overlying Seabee Formation was deposited at 98.2 ± 0.8 Ma, in the early Cenomanian. The two ages bracket a major flooding surface at the base of the Seabee Formation near Umiat, Alaska, placing it near the Albian-Cenomanian boundary (100.5 Ma). Several hundred feet up-section, the non-marine Tuluvak Formation contains bentonites with 40Ar/39Ar ages of 96.7 ± 0.7 to 94.2 ± 0.9 Ma (Cenomanian), several million years older than previously published K–Ar ages and biostratigraphic constraints suggest.Major and trace element geochemistry of a sub-sample of six bentonites from petroleum exploration wells at Umiat show a range in composition from andesite to rhyolite, with a continental arc source. The bentonites become more felsic from the late Albian (～102 Ma) to late Cenomanian (～94 Ma). A likely source for the bentonites is the Okhotsk-Chukotka Volcanic Belt (OCVB) of eastern Siberia, a continental arc which became active in the Albian and experienced episodes of effusivity throughout the Late Cretaceous. Chronostratigraphically anomalous 40Ar/39Ar ages coincide with peaks of magmatic activity in the OCVB, suggesting that these anomalously old ages may be due to magmatic contribution of xenocrysts or recycling of detrital minerals from older volcanic events. An alternative explanation for the chronostratigraphically anomalous ages is mixing of bentonites with detrital sediment derived from unroofing and erosion of metamorphic rocks in the Brooks Range, Herald Arch, and Chukotka throughout the mid to Late Cretaceous. |
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| Keywords: | Alaska Colville foreland basin Bentonite Argon–argon dating Geochemistry |
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