Stratigraphy, Biogeography and Climatic Relationships of the Alpine Palaeozoic

Abstract

The combined evidence from palaeomagnetism, climate-sensitive sediments and biogeography suggests that the plates on Earth have been in constant movement relative to the poles and relative to each other during the Phanerozoic and probably during older periods as well. Since the palaeogeographic position of the Alps during the Palaeozoic is not well-constrained by palaeomagnetic data, the best evidence to infer the palaeolatitudinal framework and to reconstruct the complex geological history for this area is provided by lithic palaeoclimatic indicators, e.g. the distribution of carbonate rocks and carbonate buildups, oolites, evaporites, coal beds, red beds, and certain fossil data.

During the Ordovician, the former supposed relationship with northern Africa decreased. Instead, the affinity to the warm water north European realm evidently became closer, suggesting that the Alps were positioned in a moderately warm climate and within the carbonate belt of not more than 45° S as opposed to the conclusions reached by Scotese and McKerrow (1990) and others.

In the Silurian, the relationship of faunas from the Eastern Alps to coeval faunas from southern, central and southwestern European regions continued. As for the Ordovician, the affinity with northern Africa was remarkably loose during the Silurian. Corals, ooids and other distinct faunal and lithic occurrences in the Eastern and Southern Alps indicate (1) a setting of about 35°S for Silurian rocks in the Alps and (2) the option of two separate terranes for this interval of time.

Similarily, during the Devonian, faunas and floras showed strong affinities wilt Hercynian-Bohemian and Uralo-Tienshan occurrences and were less closely related to northern Africa. Moreover, similarities did also exist with the Eifel Mountains, Belgium, France and Great Britain. Available faunal, floral and lithic evidences are best explained by supposing (1) a palaeolatitudinal setting within the tropical belt of some 30° or less, (2) the operation of equatorial gyres which aided the dispersal of many groups of organisms, and (3) the possible existence of at least two separate terranes during the Devonian.

In the sedimentary sequences, orogenic paroxysm was reached in the Middle Carboniferous, i. e. between the late Namurian Gastrioceras Zone and the Upper Miatchkovo. While Lower Carboniferous faunas of the Southern Alps are only of limited biogeographic significance, the famous “Carboniferous of Nötsch” located north of the Periadriatic line appears to be of great interest (Fig. 1). Its Early Namurian fossil assemblage is closely related to the Western European Kohlenkalk facies and comparable with coeval occurrences in Spain and North America. Furthermore, exotic limestone clasts indicate the existence of a vast shallow platform environment in this or adjacent areas of the Alps of which, however, only small remnants have been preserved. In addition to others, the Veitsch Nappe might have belonged to this shelf facies which characterized the Central Alps during the Carboniferous in contrast to the Southern Alps. Consequently, both domains may have represented two different microplates during the Carboniferous Period, and thus seem to confirm the suggested fragmentation of the predecessors of the Alps during the Lower Palaeozoic. After the Carboniferous amalgamation, the biogeographic pattern of the Southern Alps closely matched those from the former settings in the Central Alps and migration paths developed between the Southern Alps and Western Europe. The major part of the post-Variscan fauna, however, exhibits a cosmopolitan aspect with similarities to coeval occurrences along the margin of the Tethys Sea. As for the Devonian, dispersal of faunas and floras was much aided by the oceanic circulation system.

From palaeoclimatic sensitive data, it is inferred that the Central Alps were mainly confined to a humid equatorial belt during the Carboniferous, with the Southern Alps located further to the south. Progressive northward movement during the Late Carboniferous and the Permian resulted in semi-arid and arid conditions that began north of the Periadriatic line apparently in the Lower Permian and in the Southern Alps in the Middle Permian. The Alps may thus have crossed the equator at different times between the Carboniferous and the Lower Permian depending on the plate position on either side of the Periadriatic line.