Milankovitch Cycles from Fourier Analysis of Logs from Sites 865 and 866

Abstract

Ocean Drilling Program (ODP) Sites 865 and 866, at Allison and Resolution guyots, respectively, were logged during Leg 143. Shipboard analysis of recovered core materials indicated rhythmic repetition of shallowing-upwards facies, however, recovery at both sites averaged less than 16% (l%-2% in the shallow-water limestones). The original depth of the recovered core material was not known with certainty, which precluded visual quantification of Milankovitch cycles. The geophysical logs, which provided continuous records of lithologic changes, were analyzed in an attempt to detect Milankovitch periodicities. Age controls were not sufficient to provide time constraints for the logged interval from 102.5 to 867.0 mbsf of Site 865; however, the entire depth interval is of late Albian age. Logs exhibit strong cyclicity in the depth interval from 250 to 490 mbsf that is mainly controlled by variations in porosity and, to a lesser extent, clay content. Sedimentation rate increases slightly downhole. Because of the high, although variable, accumulation rate (about two times the Albian sedimentation rate at Resolution Guyot), the spectral analysis revealed high-amplitude frequencies corresponding to eccentricity and obliquity. The cycle frequency corresponding to a Milankovitch period of 413 k.y. dominates all spectra; the frequency corresponding to 123-95 k.y. has high-amplitude for the interval from 330 to 490 mbsf. Vertical resolution was not high enough to resolve frequencies corresponding to precession. At Site 866, the logged interval, from 78.0 to 1679.4 mbsf, corresponds to a time interval of about 33 m.y. Two depth intervals showed strong cyclicity in the geophysical logs: 430-670 and 935-1135 mbsf. Sedimentation rate is highly variable and increases downhole from about 30 m/m.y. in the middle-to-late Albian section to about 80 m/m.y. in the Barremian section. Vertical resolution was not sufficient to resolve frequencies corresponding to precession; the 41-k.y. obliquity peaks are resolvable only in the lower portion of Hole 866A. The 413-k.y. eccentricity cycle dominates all spectra. The 123-to 95-k.y. cycles, which are probably the alternations between wackestone and more coarse-grained packstones, likely represent the combined influence of obliquity and eccentricity orbital perturbations on sea level.

Aptian‐Albian sea level history from Guyots in the western Pacific

“…Our sequence stratigraphy interpretations must be based on a single 10-cm diameter "outcrop" at each guyat. Comparison of the recovered lithologies with Formation Microscanner imagery of the corresponding cored intervals suggests that core recovery was preferentially from relatively homogenous, partially cemented wackestone-packstone, especially near the top of each 10-m cored interval [e.g., Cooper, 1995;Ogg, 1995]. Intervals of very low resistivity facies, such as poorly cemented to friable wackestone that characterized transgressive "deepening," tended to have negligible recovery, and very high resistivity facies, such well-cemented horizons that capped "sequences," probably shattered into unrecoverable pieces during drilling.…”