ABSTRACT: Are Cyclic Sediments Periodic? Application of the Gamma Method to Cyclic Sediments of Pleistocene and Cretaceous Age

KOMINZ, MICHELLE A., University of Texas, Austin, TX, GERARD C. BOND, Lamont Doherty Geological Observatory, Palisades, NY, and R. K. GOLDHAMMER, Exxon Production Research Company, Houston, TX

The gamma method tests the assumptions that time is facies dependent in a cyclic stratigraphic section and that cycles are periodic (constant or quasi-constant duration). As such, this mathematical technique provides clues as to the relative accumulation rates of different facies in a cyclic sequence. It also allows a cyclic record to be "tuned," enhancing the likelihood that periodicities recorded in the sedimentary record can be determined by more conventional, spectral methods.

We have applied the method to deep-sea sediments of the North Atlantic in ODP core 609. Results of gamma analysis suggest that during glacial intervals the sediments accumulate less rapidly than during interglacials. Also, the gamma values tuned the time series, enhancing the 41 Ka obliquity and both the 19 and 23 Ka precessional peaks. This suggests that tuning via gamma analysis is as effective in enhancing the orbital signal as is direct tuning to the orbital variations as is commonly done with Pleistocene records.

Peritidal cycles of the Cupido Formation, a Lower Cretaceous carbonate section exposed in northeast Mexico, have also been analyzed via the gamma method. Gamma results are very stable and suggest that the deeper water more massive grey and white limestones accumulated at the highest rates while the peritidal cryptalgal laminites accumulated at the lowest rates. Oolitic facies, which represent cycle tops on many of the shallowing upward cycles are intermediate in accumulation rates. Spectral analysis of the tuned record, assuming that the fifth-order cycles record a response to precession shows strong precessional and eccentricity peaks, as well as a suggestion of a peak at the period of obliquity.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)