Biostriatigraphic problem of the Ordovician/Silurian boundary

Abstract

The correct stratigraphic position of the Ordovician/Silurian boundary sediments as well as the exact boundary between these two systems have not as yet been definitely determined. This is connected with the presence of various fossil groups in the pelagic and neritic facies, which greatly hinders a correlation of profiles.

The orthostratigraphic division of the Upper Ordovician and the Lower Silurian is based on graptolites which place the currently accepted Ordovician/Silurian boundary at the bottom of the Glyptograptus persculptus zone. In profiles lacking graptolite remains the biostratigraphy of the border sediments is based on brachio-pods and trilobites which indicate the differentiation of the Hirnantian stage. In non-graptolithic facies the upper boundary of the Ordovician system is placed in the top of the Hirnantian stage.

During her studies on the Upper Ordovician of the Łeba elevation the present ,writer has also studied the Ordovician/Silurian boundary sediments in this area and compared them with those from other regions of Poland and from some classical areas of their world occurrence.

The sediments of the uppermost Ashgillian in the Łeba elevation are developed as grey mudstonesand-marls of the Mucronaspis mucronata zone with M. mucronata Brongniart, Phillipsinella sp., Hirnantia sagittifera (Davidson), Eostropheodonta hirnantensis (M'Coy), Orbiculoidea sp. In their top there occur black mudstones with graptolites of the Llandoverian Akidograptus ascensus zone. The fauna of the uppermost Ashgillian in the Łeba elevation has many features in common with the Hirnantia fauna of Wales.

Movements connected with the Taconian phase of the Caledonian orogeny are responsible for distinct facial changes throughout Poland. The muddy-marly Ashgillian deposits, bearing a benthonic fauna of trilobites and brachiopods, have practically everywhere been replaced by mudstones and black muddy shales with a rich Silurian graptolite fauna. Here and there on the Ordovician/Silurian boundary there are. sedimentary lacunae, however, only of a local significance. As a rule the sediments of the M. mucronata zone are in contact with rocks of the Llandoverian Akidograptus ascensus or A. acuminatus zones.

During the recent years the occurrence has been noted in various regions of the world (S Kazakhstan, the Perce district in Canada, Meifod-WelshpooI in Wales) of the Llandoverian G. persculptus zone together with the Hirnantian trilobites and brachiopods. This might reasonably suggest that the two stratigraphic units may, . at least partly, be regarded of the same age. On this assumption the top of the Hirnantian would correspond to the bottom of the A. ascensus zone.

Two conceptions are now currently accepted for the Ordovician/Silurian boundary:

a. In the bottom of sediments with a fauna in common, hence in the· bottom of the Hirnantian and of the G. persculptus zone.

b. In the top of the two above units, hence in the bottom of the A. ascensus zone. This latter conception seems more correct in view of the appearance in the bottom of the A., ascensus zone of new characteristic graptolite genera such as Akidograptus, Dimorphograptus and Monograptus.

The upper boundary of the Ordovician system as established in the bottom of the A. ascensus zone agrees with data from the Polish territory where this zone is ,readily distinguishable, while the G. persculptus zone has not -one case excepted -been reported from Poland.