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Abstract
Journal of Sedimentary Research, Section
B: Stratigraphy and Global Studies
Vol. 67 (1997)No.
2. (March), Pages 286-302
Evidence for Milankovitch Periodicities in Cenomanian-Turonian Lithologic
and Geochemical Cycles, Western Interior U.S.A.
B. B. Sageman (1), J. Rich (1), M. A. Arthur (2), G. E.
Birchfield (1), W. E. Dean (3)
ABSTRACT
The limestone/marlstone bedding couplets of the Bridge Creek Limestone
Member, Cenomanian-Turonian Greenhorn Formation, were analyzed by applying
spectral techniques to high-resolution lithologic and geochemical data
from a core. The results suggest that the Bridge Creek contains a complex
record of orbital cyclicity. The dominant signal appears to be obliquity,
but signals corresponding to precession and eccentricity were also observed.
The development of the bedding couplets is interpreted to have resulted
from a combination of factors, including insolation-controlled changes
in higher-latitude precipitation leading to dilution/redox cycles, and
in lower-latitude evaporation, leading to changes in surface water conditions
and productivity cycles in the calcareous plankton. The data nterpreted
to reflect redox cycles appear to be more strongly influenced by obliquity,
and show a weak precessional signal. In contrast, trends in the carbonate
record show the opposite response. The complex bedding pattern observed
in the Bridge Creek Limestone is interpreted to result from the competing
influences of different orbital cycles expressed through different pathways
of the depositional system, and was also affected by changes in sedimentation
rates related to relative sea level fluctuations, aperiodic dilution by
volcanic ash, and changes in organic-matter production and redox conditions
related to a global "oceanic anoxic event". These factors complicate cycle
analysis in the lower part of the member but leave a relatively undisturbed
record in the upper Bridge Creek Limes one.
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