Time from fossils: S. S. Buckman and Jurassic high-resolution geochronology

Abstract

AbstractChronostratigraphical classification of rocks can be approached from two directions. The first is a ‘top-down’ process of subdivision of the geological column in a hierarchy of successively finer units. These units are thereforedefinedby their boundaries, which are time-planes, and form complete continuous series or scales. They are chosen to be widelyrecognizable, and hence correlatable, by means, in the Phanerozoic, of their contained guide-fossils,i.e.by their characteristic biozones. This was the approach of d’Orbigny and Oppel in the Jurassic, leading to astandard chronostratigraphydown to Subzonal level notably espoused by Arkell and widely adopted today. The second approach is one of ‘bottom-upwards’ integration: the assembly into time-ordered sequences of the most minutely distinguishable local faunal horizons—distinguishable in the sense of evolutionary change—which may or may not subsequently be found to have more widespread value for time-correlations and biochronology. This was the method introduced by Buckman a hundred years ago to describe the ammonite biostratigraphy of the Inferior Oolite of Dorset, in response to the need for the finest attainable time-resolution in phylogenetic palaeontology. The time-equivalents of such faunal horizons were termedhemerae.Polyhemeral chronostratigraphy went into abeyance with Buckman’s death in 1929, but its equivalent, in terms of the faunal horizons themselves, has been revived. A faunal horizon is defined as a stratigraphical entity within which no further biochronological subdivision can be made, so that the bed or beds embodying that horizon must, on the evidence of the fossils alone, be regarded as internally isochronous. A succession of faunal horizons becomes the record of well-spaced instants: the record is presumeda priorito be full of gaps of unknown duration waiting to be filled by new discoveries. The measure of chronostratigraphical finesse is the average time-interval between the moments represented by the faunal horizons, δ̅t̅, thesecular resolution. The relative ability of groups of guide-fossils to resolve time-intervalsδtin rocks of agetis theirsecular resolving-power, R = t/δt.The current state of Jurassic chronostratigraphy is reviewed. The guide-fossils of choice are the ammonites, whose secular resolving-power exceeds that of any other group and which can give time-resolutions of 150000 years in rocks of age 150 million years (it > 1000). These figures are compared with those attainable elsewhere in the Mesozoic and Palaeozoic. Resolution-analysis of the Jurassic shows that, at the level of resolution of ammonite faunal horizons, the geological record is highly incomplete, nowhere more so than in the Inferior Oolite. As Buckman concluded, the more complete the fossil record of a system becomes, the more incomplete turns out to be its lithochronology. This has important consequences in sedimentology, and in sequence stratigraphy.