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Biostratigraphy and paleoclimatology of the Eocene-Oligocene boundary section at Toadstool Park, northwestern Nebraska, USA

Alessandro Zanazzi, Matthew J. Kohn and Dennis O. Terry
Biostratigraphy and paleoclimatology of the Eocene-Oligocene boundary section at Toadstool Park, northwestern Nebraska, USA (in The late Eocene Earth; hothouse, icehouse, and impacts, Christian Koeberl (editor) and Alessandro Montanari (editor))
Special Paper - Geological Society of America (2009) 452: 197-214


The stratigraphic section located at Toadstool Park, northwestern Nebraska, preserves a detailed sedimentological and fossil record of the Eocene-Oligocene boundary, yet chronologic control is limited to sparse ash dates and magnetostratigraphy. To improve the chronologic control of the Toadstool Park sediments, we examined the fossil collection at the University of Nebraska State Museum and recorded the stratigraphic level of some important biochronologic events. All stratigraphic levels were determined relative to the Upper Purplish White layer (UPW), a distinctive ash that is chemically and mineralogically correlated to an ash dated at 34.6+ or -0.1 Ma. The small artiodactyls Leptomeryx speciosus, Leptomeryx mammifer, and Leptomeryx yoderi co-occur at -34+ or -2 m, implying that the early Chadronian-middle Chadronian boundary (35.7 Ma) is located at this level. The first occurrence of L. mammifer (i.e., the middle Chadronian-late Chadronian boundary, 34.8 Ma) is located at -18+ or -2 m, and the first occurrence of the camel Poebrotherium wilsoni (34.2 Ma) is located at 0+ or -2 m. The first occurrences of two other artiodactyls (Hypertragulus calcaratus and Leptomeryx evansi), which define the Chadronian-Orellan boundary (33.9 Ma), are located approximately at the UPW, approximately 5 m lower than in the nearby type sections of Wyoming. This apparent diachroneity may reflect sampling bias in Nebraska, i.e., loose material washed down from higher outcrops. The first occurrences of the dwarfed oreodont Miniochoerus affinis and of the rodent Eumys elegans (i.e., late early Orellan, 33.6 Ma) are located at +8+ or -2 m and +11 m, respectively. Finally, the first occurrence of M. gracilis (i.e., early late Orellan, 33.2 Ma) is located at +13+ or -2 m. These time points, along with magnetostratigraphy and (super 40) Ar/ (super 39) Ar dates obtained for this section, were used to refine the age model for a previously published stable isotope record of fossil bone CO (sub 3) . The record is augmented here with 59 bone samples from federal lands near Toadstool Park spanning the critical time period of 33.8-33.4 Ma. The delta (super 18) O values in the complete data set show a step to higher values (from 23.0 ppm+ or -0.2 ppm to 24.5 ppm+ or -0.2 ppm) at ca. 34 Ma, synchronous with the marine record. In combination with unchanged enamel compositions, this increase in bone CO (sub 3) delta (super 18) O translates into a mean annual temperature drop of 7.1+ or -3.1 degrees C across the boundary. The time of onset of the climate change coincides with an increase in sedimentation rate from 19 to 37 m/m.y., confirming the strong role of climate on continental erosion and sediment yield.

ISSN: 0072-1077
EISSN: 2331-219X
Serial Title: Special Paper - Geological Society of America
Serial Volume: 452
Title: Biostratigraphy and paleoclimatology of the Eocene-Oligocene boundary section at Toadstool Park, northwestern Nebraska, USA
Title: The late Eocene Earth; hothouse, icehouse, and impacts
Author(s): Zanazzi, AlessandroKohn, Matthew J.Terry, Dennis O., Jr.
Author(s): Koeberl, Christianeditor
Author(s): Montanari, Alessandroeditor
Affiliation: University of South Carolina, Department of Geological Sciences, Columbia, SC, United States
Affiliation: University of Vienna, Department of Lithospheric Research, Vienna, Austria
Pages: 197-214
Published: 2009
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
ISBN: 978-0-8137-2452-2
Meeting name: Geological Sociaty of America (GSA) Penrose conference
Meeting location: Ancona, ITA, Italy
Meeting date: 20071003Oct. 3-6, 2007
References: 70
Accession Number: 2009-074339
Categories: StratigraphyIsotope geochemistry
Document Type: Serial Conference document
Bibliographic Level: Analytic
Annotation: NSF Grant ATM-0400532; includes appendix
Illustration Description: illus. incl. 2 tables, geol. sketch map
N42°52'00" - N42°52'00", W103°34'60" - W103°34'60"
Secondary Affiliation: Osservatorio Geologico di Coldigioco, ITA, ItalyBoise State University, USA, United StatesTemple University, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200940
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