--> Abstract: What Can Really Be Learned from the Gamma Method? Theory and Application to Albian Pelagic Piobbico Densitometer Cycles, by M. A. Kominz; #90987 (1993).

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KOMINZ, MICHELLE A., The University of Texas at Austin, Department of Geological Science, Austin, TX

ABSTRACT: What Can Really Be Learned from the Gamma Method? Theory and Application to Albian Pelagic Piobbico Densitometer Cycles

The gamma method makes and tests the assumptions that (1) time is facies dependent and (2) cycles are periodic (constant or quasi-constant duration). Because neither are true in a geologic data set forward models have been run to test the significance of gamma results, for a known input. Forward models have been constructed using cycle durations constrained by a Gaussian random noise generator or by constraining cycle durations and facies deposition by the Late Pleistocene precession index. In both cases, the gamma method is shown to provide values which improve the interpretation of the data by improving the time scale. However, gamma values are not exact, only an improvement over constant accumulation rates. Validation of the final result requires a predictable spectrum.

The gamma method has been applied to pelagic sediments of N. Italy. Previous work using spectral methods to tune the record to the 100 and 400 ky cyclicity of eccentricity, by Park and Herbert, suggested the presence of both eccentricity and obliquity. Gamma method tuning was done using the highest frequency cycles which were assumed to be a response to the precessional index. After tuning the spectra are suggestive of 3 of the predicted eccentricity peaks, the main peak of obliquity and one of the two precession index peaks. The obliquity peak and precession index peaks appear to be shifted to the lower period predicted by Berger and colleagues as a result of the increase in the earth-moon distance.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.