Iberian Ordovician and its international correlation

摘要

The regional chronostratigraphy of the British Ordovician, established mainly for shelly facies, is hard to correlate in the Iberian Peninsula, especially after the separation and drift of Avalonia from Gondwana by the early Middle Ordovician. The same applies to the Ordovician global scale, whose stratotypes involve deeper-water facies and faunas not recorded in the high-paleolatitudinal settings of southern peri-Gondwana. In order to solve the problem, an alternative regional scheme for the "Mediterranean" Ordovician was proposed in the 1970s. This comprises five regional stages plus the global Tremadocian and Hirnantian, which are largely based on the distribution of endemic shelly fossils combined with some graptolites and a good palynological record. This Ordovician scale presents precise correlation potential for southwestern and central Europe (Ibero-Armorica, Sardinia, Bohemia, Bulgaria) and may be extended around the Mediterranean region. Sporadic occurrences of graptolites and shelly faunas of Baltic or Avalonian affinities allow for indirect correlation with the global stages through their own regional scales.