UTRECHT MICROPALEONTOLOGICAL BUllETINS

BIOSTRATIGRAPHIC AND CHRONOSTRATIGRAPHIC
CORRELATIONS
The type-seq~ences of the Upper Cambrian and Ordovician in Great
Britain are for the greater part composed of dark shales and other terrigenous
clastical rocks. Their subdivision and correlation are mainly based
on graptolites and trilobites.
The scarcity of graptolites in the shelly limestone facies of the Baltoscandian
Ordovician and the lack of well documented chronostratigraphic
units in Britain always hindered detailed correlations with the type-Ordovician
of Great Britain.
As a consequence, a regional chronostratigraphic scale of the Baltic region,
based on type-sections from the shelly sequences of Estonia, was established
by Roomusoks (1960), and introduced in Sweden by Jaanusson (1960a, b).
In the author's opinion, the use of regional chronostratigraphic classifications
must be limited as much as possible and be restricted to those regions,
which have hardly any fossil in common with the internationally accepted
chronostratigraphic units. The latter condition is certainly not true for the
shelly limestone facies of the Lower Ordovician of Sweden, as may be seen
from the successful investigations of Tjernvik (1956). This author succeeded
in correlating important parts of the Swedish Lower Ordovician with the
British type-sequences, especially where these shelly sediments contain
graptolitic shale intercalations. Fig. 19 compares the chronostratigraphic
and some of the biostratigraphic units of the Cambro-Ordovician of Great
Britain with some of the biostratigraphic units distinguished in Sweden.
In his original definition of the OrdGvician System, Lapworth (1879)
excluded the Tremadoc Series from the Ordovician, according to Whittington
and Williams (1964). Most British geologists still consider the Tremadoc
as part of the Upper Cambrian (Whittington & Williams, 1964), but according
to Whittard (1956) and most Swedish geologists the Tremadoc Series belongs
to the Ordovician, following Moberg (1900).
In this chapter, first the zonations of Tjernvik (1956) and our conodontzonation
will be correlated on the basis of the bland data; they will be