RECALIBRATING THE LOWER CRETACEOUS TIME SCALE WITH U-PB ZIRCON AGES FROM THE GREAT VALLEY GROUP

SHIMOKAWA, Ayumi, Department of Geological Sciences, University of North Carolina at Chapel Hill, CB #3315, Mitchell Hall, Chapel Hill, NC 27599-3315, COLEMAN, Drew S., Department of Geological Sciences, Univ of North Carolina at Chapel Hill, CB# 3315, Mitchell Hall, Chapel Hill, NC 27599-3315 and BRALOWER, Timothy J., Department of Geosciences, Pennsylvania State University, 503 Deike Building, University Park, PA 16802, ayumish@email.unc.edu

The Geologic Time Scale is the primary resource for age control for most geologic studies. Therefore, the reliability of many conclusions based on chronology depends on the accuracy and precision of the Geologic Time Scale, which in turn is dependent on the precision of the radiometric ages used in the construction of the time scale, and the accuracy of the correlation between these ages and the stratigraphic record.

The paucity of high-precision radiometric ages from the Lower Cretaceous was highlighted in the 2004 publication of the Geologic Time Scale, and high stage boundary age uncertainties reflected the challenges inherent in the construction of a time scale from so few tie points. However, U-Pb geochronology of volcanic horizons from the Great Valley Group, a group of rocks with well-documented macrofossil and calcareous nannofossil biostratigraphy, can supply much needed radiometric ages for the Lower Cretaceous Time Scale.

Four new zircon U-Pb ages from the Great Valley Group require adjustments to parts of the Lower Cretaceous Time Scale: a Lower Aptian concordia age of 124.07 ± 0.13 Ma is in good agreement with the Geologic Time Scale 2008 Barremian-Aptian boundary age of 125.0 Ma, but Upper Berriasian (138.46 ± 0.15 Ma) and two Lower Valanginian ages (137.63 ± 0.14 Ma, 133.66 ± 0.14 Ma) shift the Berriasian-Valanginian and Valanginian-Hauterivian boundaries to approximately 138 Ma and 133 Ma, respectively. These new boundary ages lengthen the duration of the Berriasian from 5.3 m.y. to 7.5 m.y. and shorten the duration of the Valanginian from 6.3 m.y. to 5 m.y. These results 1) adjust the timing of the Valanginian δ¹³C positive excursion so that it is synchronous with the main pulse of the Paraná-Etendeka flood basalt volcanism, 2) require a faster seafloor spreading rate during the Valanginian than previously assumed, and 3) suggest a sharper calcareous nannofossil species diversity peak during the Hauterivian.

Session No. 160

T66. Geochronology of the American West: In Honor of the Careers of Bill Cobban and John Obradovich and the Roots of EARTHTIME

Tuesday, 2 November 2010: 8:00 AM-12:00 PM

Room 205 (Colorado Convention Center)

Geological Society of America Abstracts with Programs. Vol. 42, No. 5, p.393

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