Milankovitch Climate Cycles in ODP Wireline Logs

R. D. Jarrard, X. Golovchenko

Orbitally induced climatic rhythms can cause cyclical variations in the physical properties or mineralogy of deep-sea sediments. Regional variations in the climatic factors controlling sedimentation (e.g., rainfall, temperature, or current patterns) are likely to cause regional variations in the mineralogic signature of Milankovitch orbital cycles. Downhole geophysical logs, routinely recorded during ODP, continuously sample changes in sediment properties and mineralogy with depth. Thus, logs form an important methodology for detecting these changes and have the potential of indicating the local sedimentary response to the Milankovitch forcing functions. Cores lack a complete stratigraphic record of the well because of incomplete core recovery. The log, however, returns a continuous set of measurements every 15 cm with standard logs and every 1.5 cm with neutron activation logs, allowing one to extend the study of Milankovitch cycles to older sediments than are accessible by core analyses.

Examples of Milankovitch cycles in logs are available from the Labrador Sea (ODP Site 646), Baffin Bay (ODP Site 645), Weddell Sea (ODP Site 693), and Oman Margin (Leg 117). Porosity variations for sediments at ODP Site 646 (9-2 Ma) and ODP Site 693 (8-4.5 Ma) indicate major periods of 90,000-110,000 years and 40,000-45,000 years, concordant with the periods of eccentricity and obliquity. In contrast, at ODP Site 645 (3.0-1.8Ma) the dominant periods are eccentricity and 19,000-23,000 years (precession). The introduction of logging tools with higher vertical resolution in the near future will permit this type of study to be extended to sites with lower sedimentation rates.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.