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Stratigraphy and Time

Zalasiewicz et al. (2004) have proposed fundamental changes in the definition and application of stratigraphic terminology involving time-rock and geologic-time units that fly in the face of widely accepted principles that have been tried and tested in establishing an international unified stratigraphic hierarchy and nomenclature. Their adoption would create major disruption to established procedures.

The principal problem with the paper is that the authors appear to be confused about how time is related to stratigraphy; this misunderstanding starts with the title, it permeates the whole paper, and confuses arguments. Major dictionaries (e.g., Oxford, Webster's, Chambers) define stratigraphy as the study of strata and their succession and relative position; there is no mention of time there. Time itself cannot have a stratigraphy as implied in their title, which is why we do not use it in our comments and we cannot agree with their use of simplifying in this context.

The authors' failure to grasp the essential difference between time and rock is demonstrated by their Table 1, where the features labeled Chronostratigraphy are in fact units of standard chronostratigraphy, that is classification of rocks by age measured on a standard time scale (i.e., the geological column comprising units of system, series, stage rank, etc.), commonly referred to as time-rock units. The hierarchy of units applied here is defined ultimately by the bases of the units set by marker points (golden spikes) in rock within type-sections in a global stratotype section and point (GSSP); the defined base of each unit of a higher rank corresponds automatically with the base of the lowest unit at the next lower rank of the hierarchy. Thus the marker point coinciding with the lowest occurrence of Monograptus uniformis in the type-section at Klonk, Czech Republic, marks the base of the Lochkovian Stage, and also the bases of the Lower Devonian Series, the Devonian System, and the Upper Paleozoic Suberathem. The important point about all these units is that they apply to rocks, can be examined in the field, and one can hammer a golden spike into them.

The items under the heading Geochronology in Table 1 are geologic-time units; these are arranged in a chronologic order and have a hierarchy of units (in the same way as year, day, hour, minute, etc.). Geochronology, on the other hand, is an arrangement of events by age that has nothing to do with time itself nor with a hierarchy of units. Geologic-time units include the familiar periods, epochs, etc., but they do not have an exactly corresponding stratigraphy. How stratigraphy at any point relates to time is a matter of interpretation. Each unit in every part of the geologic-time unit hierarchy is bounded by time instants (i.e., points on a linear time scale) and comprises a total record of time without gaps or overlaps. These units can neither be seen in the field nor can you hammer a golden spike into them. The only point at which there is known correspondence with time-rock boundaries is at the golden spikes. Thus these geologic-time units differ in principle from time-rock units and their distinction is necessary for the purposes of language and communication.

Having thus defined units, it is possible to appreciate why a dual terminology is needed. It is to maintain clarity of thought. The Zalasiewicz et al. (2004) proposal that a GSSP should define both the base and the top of a unit clearly reveals why it is important to maintain a difference between chronostratigraphy and time units. A GSSP is a point in rock coinciding with a point in time that defines the base of a unit at one locality and provides a standard with which other sections can be correlated and calibrated. A GSSP is thus defined at a point at which some event occurred, believed to be time diagnostic; this may be based on such factors as a biostratigraphic criterion (e.g., the appearance of a time-significant fossil), or on time-significant lithostratigraphic criteria (e.g., an iridium-enriched clay band signifying an impact event). In selecting a point for a GSSP, the event should be chosen preferably to allow the possibility of correlation as closely as possible with other sections across the same time interval.

Even if the rock successions at GSSPs are believed to be “in continually [continuously] deposited sections,” it would be remarkable if this were so; time is inevitably unrepresented by sediments at some part of any section, whether on land, in an Ocean Drilling Program core, or wherever. Bedding planes themselves may represent a break in sedimentation, so that time is likely to be unrecorded there. Such appreciation led to the decision to fix GSSPs by their bases only. In fact, long before the concept of a GSSP was introduced for the base of the Devonian System (McLaren, 1977), the same method had been used in Jurassic stratigraphy for a great number of years (Callomon, 1995). GSSPs cannot thus define tops as well as bases of time units, but they may correlate with the tops of chronostratigraphic units. It should also be pointed out that they need not be used to define the tops of standard chronostratigraphic units because these coincide automatically with the bases of the succeeding standard chronostratigraphic units.

The authors refer to Hedberg's (1976) stratigraphic guide, and the principles of distinction between rocks and time have also been enunciated clearly by other authors (e.g., Callomon, 1995). The rock-time duality has been seen to be essential for over 150 years; its evolution has led to a stratigraphic classification that has been both practical and broadly accepted internationally. To abandon that duality now would be a retrograde step that will simply confuse stratigraphic thought. Zalasiewicz et al. (2004) may well be correct in saying that the distinction between the duality of time and rock is “not clear to the greater part of the professional (or student) geological community,” but this is no reason to abandon it! What is needed is clearer understanding of the principles, and not an abandonment of methodologies and definitions that now do so much in promoting global correlation of geological events.