EVIDENCE FOR MILANKOVITCH PERIODICITIES IN CENOMANIAN-TURONIAN LITHOLOGIC AND GEOCHEMICAL CYCLES, WESTERN INTERIOR USA

The limestone/marlstone bedding couplets of the Bridge Creek Limestone Member, Cenomanian-Turonian Greenhorn Formation, were analyzed by app lying spectral techniques to high-resolution lithologic and geochemica l 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 eccentricit y were also observed. The development of the bedding couplets is inter preted to have resulted from a combination of factors, including insol ation-controlled changes in higher-latitude precipitation leading to d ilution/redox cycles, and in lower-latitude evaporation, leading to ch anges in surface water conditions and productivity cycles in the calca reous plankton. The data interpreted to reflect redox cycles appear to be more strongly influenced by obliquity, and show a weak precessiona l signal. In contrast, trends in the carbonate record show the opposit e response. The complex bedding pattern observed in the Bridge Creek L imestone is interpreted to result from the competing influences of dif ferent orbital cycles expressed through different pathways of the depo sitional system, and was also affected by changes in sedimentation rat es related to relative sea level fluctuations, aperiodic dilution by v olcanic ash, and changes in organic matter production and redox condit ions related to a global ''oceanic anoxic event''. These factors compl icate cycle analysis in the lower part of the member but leave a relat ively undisturbed record in the upper Bridge Creek Limestone.