The scale-dependence of strata-time relations:Implications for stratigraphic classification

The establishment of chronostratigraphic units such as geological Systems and Series depends upon an ability to equate succession in rock strata with the passage of time, and upon a pervasive Law of Superposition. These assumptions hold true at a gross scale. But, at fine scales of stratigraphic resolution, they commonly break down. Thus, bioturbation in Phanerozoic marine deposits typically homogenizes sedimentary packages spanning millennia, affecting biostratigraphic, isotopic and paleomagnetic signals, and post-burial mass transport phenomena such as large-scale sedimentary slumps and intra-stratal diapirs locally disrupt superpositional relationships on a larger scale. Furthermore: the multi-stage transport of microfossils prior to final burial complicates the relationship between depositional and biostratigraphic ages; paleomagnetic signals, imposed at shallow burial depths, may be distinct from depositional ages; and high precision zircon U-Pb dates from tuff layers determine time of crystallization in the magma, rather than depositional age. In such circumstances, depositional units cannot be unambiguously equated with time units: because they include multiple temporal components, they cannot be subdivided precisely into time-rock units. By contrast, the different phenomena which have contributed to constructing sedimentary deposits, pre-, syn- and post-depositional, may be effectively accommodated within a unitary geological time framework