Geoscientist 20.05 June 2011
What is stratigraphic cyclicity? Systems that combine to produce an output that we call the stratigraphic record are continuously and irreversibly changing in their effects, and never truly cyclic. So, does the term just mean: “In this section I keep encountering the same rock types”? Or are there really stratigraphic sections in which some lithological character varies in a predictable, strictly cyclic fashion?
MILANKOVITCH CYCLES
Stratigraphic cycles of both kinds have been attributed to periodic variations in the Earth’s orbit known as Milankovitch [M-] cycles. Each ‘cycle’ thus denotes some orbitally-related interval, opening up the possibility that a cyclic section could be chronologically calibrated, with a resolution up to 10 times finer than radiometric dating. But how realistic is this?
Where ‘cycle’ refers to repeated facies, the intervals between similar beds and the thicknesses of those beds usually vary unpredictably. Clearly, then, the only way that repetitions can achieve ‘cycle’ status, is by showing that they are all the outcome of M-forcing. Counting and ‘tuning’ aim to show that the number of cycles in a section of known time-span is consistent with this theory. For this to work the section must be free of significant hiatuses - something that can only be verified by a detailed time calibration. But in practice, the theory chases its own tail, concluding that there are no gaps, and M-forcing therefore proved, if the expected number of irregular cycles is counted!
Neogene deep-sea cores are presented as evidence that irregular, climate-related variations in sediment properties can be matched with M-cycles, leading to high resolution time-calibrations. But here the matching process relies on a robust, independent chronostratigraphic framework, and emphasises that orbital regularities need not translate into regularly cyclical deep-sea deposits. Such being the case, why search for regular cycles in older strata?
NOISY DATA
Sander’s Rule proposes that strict stratigraphic cyclicity most likely involves periodicity in sedimentary processes, suggesting M-forcing. Spectral analysis of sample series should detect such cycles; but encounters the problem of random data noise, which can give spectral peaks rather similar to those generated by ‘real’ cycles. When the chances that a spectral peak represents noise are thought to be lower than 10%, it is commonly assumed that it records a “real” cycle. But this requires an accurate statistical model of the noise. If that model is incorrect, it may lead to the recording of false cyclicities.
Hence, more apparent cycles may be detected than can be explained by orbital forcing; so only those that are consistent with the expected orbital model are taken to be “real”. Calibration then proceeds on the assumption that the “real” cyclic signal, extracted from the noisy background, characterises the entire interval, with no hiatuses. In practice, the possibility of undetected hiatuses means that it is unsafe to assume that the number of spatial cycles that will fit into an interval of known time-span is a measure of their time value.
So - if cycles are mostly suspect, and otherwise provide unreliable time calibrations, what exactly is their use?
Further reading
- Bailey, R J 2009: Cyclostratigraphic reasoning and time calibration. Terra Nova 21, 340-351.