Kansas Geological Survey, Subsurface Geology 12, p. 35
M. A. Kominz1 and G. C. Bond2
1The University of Texas at Austin
2Lamont-Doherty Geological Observatory
A new technique, gamma analysis (where gamma is defined as time/unit thickness), tests for constant period in cyclic sediments even in the absence of age control. The method utilizes the individual cycles themselves and their facies distribution to determine if the cycles represent constant time and/or that accumulation rates are constant within facies. Since sedimentation rates are conventionally assumed to be constant in order to assign time to strata, it is important to test whether this is true even within a specified facies. Because sedimentary cycles are often assumed to be constant period in order to assess what that period is, it is also crucial to test if cycles have a constant period. Both sets of information are extremely difficult to obtain using conventional methods but can be derived from gamma analysis.
Results from the Middle Cambrian Trippe Formation from the Wah Wah Range of central Utah indicate that shoaling-upward, shallow-marine strata there have essentially constant gamma values and were probably deposited in response to a periodic process. The results of gamma analysis are consistent with field observations of lack of evidence of hiatuses within this section. Abundant hiatuses would be likely to result in calculation of highly variable facies-accumulation rates and gamma values.
When Fourier analysis of the Middle Cambrian cyclic interval in the Wah Wah Range was performed using the conventional assumption of constant accumulation rates in all facies, the spectral pattern, although composed of distinct peaks, was not diagnostic of any known climatically induced mechanisms and showed no periodicity at the scale of the measured cycles. However, Fourier analysis of the sections, in conjunction with time corrections derived from the gamma analysis, produced a spectral pattern that is strikingly similar to that predicted by the orbital or Milankovitch model of climate forcing. Specifically, the gamma-corrected time series strongly indicates that the primary cycles have precessional periods (19/23 Ka) and that significant periodic components are also present at both 100 Ka and 400 Ka, the periods of eccentricity. The 41-Ka period, or tilt component of orbital forcing, is weak or absent, however. This pattern of Milankovitch signals, precessional cycles modulated by eccentricity, is thought to be characteristic of low-latitude, monsoon-dominated climate. Although Middle Cambrian time was not a time of glacial climate, the sequences of interest were deposited in low latitudes which are most likely to be affected by monsoonal climate. Thus, studies of the periodicity of strata incorporating gamma analyses have the potential to increase our understanding of the controls of climate in these ancient environments.
Relative accumulation rates derived from the results of gamma analysis indicate that, on average, cryptalgalaminites and parted limestones had the highest accumulation rates, while calcareous grainstones had the lowest accumulation rates. Assigning relative time based on the assumption that the cycles are precessional results in average accumulation rates of 20 cm/Ka for the cryptalgalaminites and parted limestones and 8 cm/Ka for the calcareous grainstones.
Kansas Geological Survey
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Web version May 11, 2010. Original publication date 1989.