Jurassic ammonite biochronology of Greenland and the Arctic

• Main Author: John H. Callomon
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 1994
• Subjects: Arctic; Greenland
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.559.9053; http://2dgf.dk/xpdf/bull41-02-128-137.pdf

The standard biochronological chronostratigraphy of the Phanerozoic and of its conjugate time-scale has been refined over a century and a half by a process of top-down subdivision in a hierarchy of successively smaller units. The finest units currently accepted, at the seventh level of the hierarchy, are the Subzones widely used in the Jurassic, thanks to that System's exceptional guide-fossils, its ammonites. But the time-resolution even at this level is not yet at the limits attainable through biostratigraphy. The ultimate observable is a characteristic faunal horizon, defined as a fossiliferous stratum or succession of strata within whose specified fossil assemblages no further evolutionary- as opposed to compositional- changes can be dis-tinguished. Such a horizon represents effectively a biochronological instant. The fossil record is resolved into a succession of such instants, recognizable perhaps in as little as a single section and separated by time-gaps of unknown duration. The time-intervals between the ages t of successive horizons represent the limits of temporal resolution, bt, discernible by means of fossils. They depend strongly on the fossils employed and may be expressed in terms of their secular resolving-power, R = tlbt. Some estimates selected from the Mesozoic and Palaeozoic are compared in a Table.