Documenting the Reliability and Utility of the Gamma Method as Applied to Cyclic Sections Using Forward Modeling

The problem of determining if cyclic sections which are older than 5 m.y. in age were deposited in response to orbital control is discussed. Gamma analysis, a method which uses the assumption that effective accumulation rates are facies dependent to minimize differences in cycle duration, is presented as a possible solution to this problem. While the assumptions of gamma analysis are generally not met by geologic data, the effective accumulation rates estimated may be instrumental in recovering a spectrum which reveals orbital control. We address the problem of assessing the reliability of gamma results by performing analyses on three cyclic time series generated by a forward model. Our results show that exact accumulation rates are not likely to be obtained by the method. However, the accumulation rates obtained markedly improve the time scale, resulting in a spectrum from which one could distinguish orbital forcing, if present. Only when spectra of cyclic data show predicted orbital peaks can gamma analysis be considered to yield effective accumulation rates which are more nearly like the true effective accumulation rates than the assumption of a single effective accumulation rate for all facies.