EVALUATING THE STRATIGRAPHIC COMPLETENESS OF DEEP-TIME RECORDS: A NEW QUANTITATIVE APPROACH

Thick stratigraphic records reflect both high accommodation, typically created by greater subsidence, and high sediment supply, such that a relatively temporally complete record of deposition is preserved. However, thin records can form through multiple possible interactions between accommodation and sediment supply. Thin records can be (1) miniaturized, i.e. thinner, but equally complete; (2) comparable in thickness and quality where the sedimentary record is preserved, but certain portions of the record are notably absent; or (3) so invariably preserved that the stratigraphic packages are qualitatively different compared to a coeval thick record.

Here, I present a method for discriminating among these distinct modes of stratigraphic thinning, which each have different consequences for the quality and temporal resolution of the preserved sedimentary record. Starting with stratigraphic building blocks of lithofacies and meter-scale cycles, this quantitative approach can be used to compare two (or more) ancient sedimentary records, which differ in terms of net rock accumulation rates (typically measured over 10⁶-year time scales), but not in terms of completeness given available chronostratigraphic resolution. The methodological approach is as follows: First, assess the relative contribution of (1) different average cycle thickness between the two records versus (2) differences in the numbers of cycles preserved over the entire stratigraphic interval. Next, evaluate the degree to which differences in cycle thickness can be accounted for by (3) differences in facies thickness versus (4) numbers of facies per cycle. Finally, (5) consider how the composition of facies and meter-scale cycles contributes to the observed differences in thickness.

When applied to Devonian carbonate sedimentary records, this quantitative analysis revealed the important role of suppressed subtidal sedimentation rates in creating a thin cratonic interior record relative to a coeval record on the continental margin. This approach can be applied to other geologic time periods and settings to better constrain the resolution of stratigraphic, paleoenvironmental, and paleobiological information available and the comparability of studies conducted across distinct basins.