Periodic changes in depositional environments due to Milankovitch astronomical climate cycles can cause cyclic patterns in sedimentation properties as recorded by logging data. Ocean Drilling Program Site 801 recovered Callovian (upper Middle Jurassic) through Valanginian (Lower Cretaceous) clayey radiolarites, originally deposited in a near-equatorial setting. Cycles of variable concentration of radiolarians and clay, and associated changes in degree of silicification were apparent in the geophysical logs, especially in the gamma-ray signal and the Formation MicroScanner. Three-dimensional spectral analysis was performed on the gamma-ray log signal using a 40-m sliding window. The dominant spectral peaks maintain the same relative ratios in frequency as the 413-k.y., 123-k.y., and 95-k.y. Milankovitch periods of eccentricity. The wavelengths of these eccentricity-modulated cycles were used to determine rates and discontinuities in sedimentation with depth.Two sharp discontinuities in sedimentation rate were inferred: (1) Callovian alternations of red radiolarite and claystone, with a sedimentation rate of approximately 14.5 m.y., is terminated by a Callovian/Oxfordian boundary hiatus, and the overlying upper Oxfordian through lowest Tithonian clay-rich radiolarites and the lower Tithonian banded chert have an average sedimentation rate of 7 m/m.y.; (2) a discontinuity of probable late Tithonian-early Berriasian age terminates the Tithonian banded chert; the overlying Berriasian through Valanginian radiolarite has a mean sedimentation rate of 11.5 m/m.y. These computed sedimentation rates and interpreted discontinuities are consistent with the stratigraphy of the recovered sediments and the uncertainties in the geological time scale.Eccentricity cycles identified in the gamma-ray signal were matched to corresponding features on the Formation MicroScanner high-resolution imagery of clay content and degree of silicification. Eccentricity cycles are manifested as groupings of beds of higher radiolarian content and silicification. Milankovitch climate cycles may affect the intensity of equatorial Pacific upwelling, hence the surface productivity of radiolarians, and the amount of eolian dust and clay contributed to the sediments.

Ocean Drilling Program Leg 129 recovered Jurassic and Lower Cretaceous clayey radiolarite in Hole 80IB in the Pigafetta Basin of the western equatorial Pacific. Two runs of the Formation MicroScanner logging tool provided high-resolution downhole imagery of relative resistivity at this site. The Formation MicroScanner imagery was analyzed by using Schlumberger-processed gray-scale printouts and by using various enhancement methods on a Macintosh II color workstation. The processed images enabled continuous observation at approximately 1-cm vertical resolution of variations in clay and radiolarian content, of discontinuities in sedimentation, of the degree of silicification, of nodularity of porcellanite and chert, of bedding thickness and spacing, of cyclic sedimentation exhibiting Milankovitch climatic periodicities, and of the dip of structures. The detailed Formation MicroScanner imagery was interpreted with the aid of other geophysical and dip-meter logs, and compared to the limited core recovery in order to compile a comprehensive stratigraphic column.The lower 20 m of lower Callovian strata consist of a clay-rich series of slump structures and debris flows, with a possible breccia at the basal contact (467 mbsf) to the basaltic basement. Undisturbed but tilted upper Callovian beds of alternating radiolarite and claystone extend for less than 6 m, then are terminated by an unconformity to Oxfordian-lower Tithonian interbedded chert and clayey radiolarite of lithologic Subunit IVb. Clay content and degree of silicification varies periodically within Subunit IVb. A mid-Tithonian bedded radiolarian chert, which may be a Pacific analog to "ribbon radiolarite," spans an 18-m interval (396-378 mbsf) at the base of lithologic Subunit IVa, and is overlain by a 5-m bed of claystone. A discontinuity in sedimentation is suspected at this Jurassic-Cretaceous boundary. The Berriasian-Valanginian consists of clayey radiolarite with periodic bands of chertification. Ahardground, possibly silicified, is at 314 mbsf, overlain by 13 m of distal clay-rich volcaniclastic turbidites. Radiolarite sedimentation resumes for another 20 m, then a second and major pulse of volcaniclastic turbidites begins at 281 mbsf.The Formation MicroScanner stratigraphy was augmented with a synthetic plot of chert/clay ratio, as computed from cross-correlation of density and geochemical logs.

Geochemical well logs were obtained in sediment at Sites 800, 801, and 802 of Ocean Drilling Program Leg 129. Corrections have been applied to the logs to account for variations in hole size, drilling-fluid composition, and drill-pipe attenuation. Oxide and calcium carbonate weight percentages have been calculated from the processed logs and are compared with the available measurements of core. Log-derived CaCO 3 measurements correlate well with shipboard CaCO 3 core measurements from Holes 800A and 802A. Log measurements are compared with X-ray fluorescence measurements from Holes 800A and 801B. Because of selective sampling from silica-rich beds, discrepancies between log and core data occur. The silica logs show values higher overall than those of the core measurements; the aluminum logs show values lower overall.