A standard time division close to the Middle Ordovician, the Darriwilian, has been recognized on the basis of graptolites.. Because of the high degree of regional differences in Ordovici
Trang 1however, and to date a type section has yet to be
chosen A standard time division close to the Middle
Ordovician, the Darriwilian, has been recognized on
the basis of graptolites The base of the Upper
Ordo-vician has been agreed to be the base of the widespread
Nemagraptus gracilis biozone This horizon coincides
with the base of the Caradoc ‘Series’ as now defined
A type section in the Fa˚gelsang section, Sweden, has
been selected for its definition Because this threefold
series subdivision has not yet achieved wide currency,
it is expedient to continue to use the fivefold
subdiv-ision in this chapter Strictly speaking, these should
no longer be referred to formally as ‘Series’, as they are
effectively regional stages The suffix ‘-ian’ is
there-fore employed below Because of the high degree of
regional differences in Ordovician strata, several
widely separated areas have become important in
de-fining Ordovician temporal subdivisions on a regional
to global scale Biozonal standards have been based on
graptolites, conodonts, and trilobites, but these are
found together in only a minority of rock sections
and they, too, show provinciality Much effort has
accordingly been expended in tying together
grapto-litic and conodont-based zonal schemes
Tremadocian
The small town of Tremadoc lies in North Wales at the
northern end of Cardigan Bay A series of dark shales
and mudstones of the Dol cyn Afon Formation – often
cleaved and weakly metamorphosed – overlie rocks of
the Upper Cambrian There is often a stratigraphical
break between the two, but in at least one section a
complete and conformable transition between the two
systems has been demonstrated The Tremadocian is
recognized by the incoming of net-like rhabdosomes of
the widespread graptolite Rhabdinopora flabelliformis
subspecies, the oldest planktonic graptoloid species
These rocks were deposited in comparatively open
shelf environmental conditions, and the graptolites
are accompanied by a variety of trilobites, of which
members of the Family Olenidae are a prominent
com-ponent, some species of which are also very
wide-spread The genus Jujuyaspis, for example, is known
from North and South America, Scandinavia, and
China in earliest Ordovician strata In North America,
the stage name Ibexian (approximately the same as
‘Canadian’ used by earlier authors) is employed for
Early Ordovician strata, based on well-exposed
sections in Utah in the Great Basin The Ibexian
em-braces all of the Tremadocian and part of the
Areni-gian Both the Tremadocian and Ibexian have been
further subdivided into chronostratigraphical
subdiv-isions of regional application The graptolite and
con-odont succession is particularly well-known in the
Baltoscandian area and indicates the presence there
of an upper part of the Tremadocian (Hunnebergian) that is not well-developed in Britain
Arenigian
Arenigian rocks overlie the Tremadocian rocks in North Wales, and were named after a mountain, Arennig Fawr, where they are well exposed However,
in North Wales there is an unconformity at the base of the Arenigian, which is marked by a transgressive sandstone, and so it is inadequate as a type area (Figure 1) The Arenigian succession is better de-veloped in South Wales and Shropshire, where thick sequences of mudstones, shales, sandstones, and turbidites have yielded diverse trilobite faunas of shallow-to-deep shelf aspect with, more locally, grap-tolites and brachiopods Because of the disparate bio-geography of the Early Ordovician, most of the species are of relatively local occurrence, and correl-ation of these strata interncorrel-ationally can be difficult For some years a de facto base of the Arenigian has been recognized at the base of the widespread grapto-lite Tetragraptus (Etagraptus) approximatus biozone, which can be identified in North and South America,
Figure 1 Ordovician (Arenigian) strata lying unconformably over Cambrian rocks on the Llyn Peninsula, North Wales.
PALAEOZOIC/Ordovician 177