Jurassic ammonite biochronology of Greenland and the Arctic

􀀬e 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 fauna! 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 dis­tinguished. 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. The geographical limits of time-correlation by means of fossils are often set by bioprovincial endemisms of the organisms of which the fossils are the remains. The biochronology, and any standard chronostratigraphical scale based upon it, has therefore to be worked out in each Province separately, and such provincial scales correlated in regions of provincial overlap, if known. An excellent example is found in the Middle and Upper Jurassic of East Greenland. Its ammonite biochronology is today represented by some 100 fauna! horizons. But the ammonites are largely confined to a sharply segregated Arctic, Boreal Province, for which they now provide a standard zonation. Detailed correlations with the primary standards of Europe continue to range from the problematical to the impossible.