Presented by Meyers, Stephen R..
Authors:
Sageman, Bradley B.,
Hinnov, Linda A..
Key words: Bridge Creek, ESA, OAE II
In Session 97 Climates and Oceans before the Quaternary Wednesday, 28-Oct-98 AM in Room: 703 at 9:45 AM for 15 minutes.
Abstract: Application of spectral techniques to detect sedimentary orbital cycles is commonly challenged by difficulties with time scales. In a recent study of the Cenomanian-Turonian Bridge Creek Limestone spectral analyses were limited to the upper half of the unit based on evidence of changes in sedimentation rates in the lower half. Although spectral analysis of the upper Bridge Creek Limestone yielded evidence of precession, obliquity, and eccentricity, restriction of the study to a shorter time series was not optimal. In this study, Evolutionary Spectral Analysis (ESA) was performed on Bridge Creek grayscale data from a core in order to evaluate proposed changes in sedimentation rates. Because orbital cycles have been documented within the unit, analysis of changes in the frequencies of those cycles provides an independent check on changes in sedimentation. A recently proposed interpolated time scale based on Ar-Ar dates and ammonite biozones was used to generate first order approximations of sedimentation rates. These sedimentation rates were applied to the spatial grayscale data set. ESA of the resulting time series resolve principal Milankovitch periodicities and linearized the spectrum. Shifts in the dominant orbital frequencies confirm that changes in sedimentation rates characterize the lower part of the section (Late Cenomanian), and
highlight a zone of increased sedimentation rates near the peak highstand of the Greenhorn Cyclothem (C-T boundary), just prior to termination of OAE II. Because detrital starvation is normally associated with maximum highstand, a short-term increase in the pelagic flux more likely accounts for this result. Nannofossil data from the literature appear to support the pelagic interpretation. Together with trends in organic carbon and macrofossil biofacies, these data illustrate the unique expression of OAE II in the Western Interior sea.
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© Copyright 1998 The Geological Society of America (GSA), all rights reserved.