Figure 5. A) Changes in the Earth's orbit such as eccentricity (change in shape), obliquity (change in axial tilt), and precession (chnage in axial orientation), result in modulation of solar input, as represented by the insolation curve for 40 degrees N latitude, calculated for 93-94 Ma (based on solution of Berger, 1978). These insolation changes may influence depositional pathways such that bedding cycles of limestone and mudstone develop in hemipelagic settings (see photo of Bridge Creek Limestone from Rock Canyon Anticline, CO; blue line represents a wgt. % CaCO3 curve). The preservation of orbital cycles in bedding patterns may be modified by non-linearities in the climate-sedimentation linkage, changes in local rates of sedimentation, and hiatuses (gaps in the record due to non-deposition or erosion). B) Spectral analysis of bedding cycles offers a powerful tool for identification of orbital signals. The Multi-taper method (MTM) of Thomson (1982) is among the best methods for short, noisy time series (i.e., geological data). However, changes in sedimentation rate during a study interval will result in multiple signals and a spectrum that is difficult to interpret (MTM power -background curve- for the entire hypothetical study interval above yields plateaus around significant frequencies reflecting spectral "leakage"; F-test results for individual harmonics -foreground red spikes- indicate the most statistically significant frequencies; the multiple F-test peaks reflect shifting frequencies due to changing sedimentation rates). Evolutive Harmonic Analysis (EHA), a moving window version of the MTM, is a new method for reconstructing sedimentation rate changes and hiatuses (Meyers et al. 2001). The color EHA plot above represents a modeled data series with obliquity/precession forcing (O1, P1-3), sedimentation rate changes, and hiatus. Tracking of statistically significant signals (note yellow dashed line) allows a detailed time-depth map to be produced, and thus a high-resolution orbital time scale.