Updating uniformitarianism: stratigraphy as just a set of frozen accidents
It has long been understood that the stratigraphic record is fragmentary. Barrell (1917) in a paper that was many years ahead of its time, was the first to clearly understood 1) the importance of accommodation, and 2) the very episodic way in which accommodation is created and removed by geological processes. He demonstrated that under typical conditions of base-level rise and fall only a fraction of geologic time is represented by accumulated sediment. This point was repeated in several influential books by Ager (1973, 1993). One of the most important, yet neglected, discoveries about the nature of the sedimentary record is the correlation between the duration of a sedimentary unit and its sedimentation rate (Sadler, 1981). Sedimentation rates range over more than eleven orders of magnitude. It is now recognised that the durations of stratigraphic gaps, the distribution of layer thicknesses, and sedimentation rates in stratigraphic successions are fractal. The fractal model provides an elegant basis for integrating our knowledge of the processes of accommodation generation with the data on varying sedimentation rates and the scales of hiatuses and the processes that operate over these time scales. This paper proposes the definition of a suite of Sedimentation Rate Scales to encompass the range of time scales and processes that can now be recognized from modern studies of the stratigraphic record. Assignment of stratigraphic units to the appropriate scale should help to initiate a potentially rich new form of debate in which tectonic and geomorphic setting, sedimentary processes and preservation mechanisms can be evaluated against each other, leading to more complete quantitative understanding of the geological preservation machine, and a more grounded approach than earlier treatments of “stratigraphic completeness”.
In Smith, D. G., Bailey, R., J., Burgess, P., and Fraser, A., eds., Strata and time: Geological Society, London, Special Publication 404 (in press).