In North America, strata equivalent to the Arenigian embrace the upper part of the Ibexian and the lower part of the Whiterockian, and have been documented in largely calcareous successi
Trang 1Scandinavia, Australia, and New Zealand The most
varied succession of graptolite faunas is found in the
shales of the region of Bendigo, Victoria, Australia;
this has achieved the status of a global standard, and a
fine-scale division into biozones (and stages) has been
established, which has been widely applied elsewhere
In south-western China sequences span facies from
inshore to deep-water, as do sequences in Scandinavia
and the Russian Platform In North America, strata
equivalent to the Arenigian embrace the upper part of
the Ibexian and the lower part of the Whiterockian,
and have been documented in largely calcareous
successions in the Great Basin In many limestone
sequences, conodonts have become the
biostratigra-phical standard This disparity of stratigrabiostratigra-phical
cri-teria together with regional differences have meant
that a plethora of stratigraphical subdivisions of
Are-nigian strata has grown up over the years, and
differ-ent regional names are used in, among others, China,
Scandinavia, Britain, Australia/New Zealand, and
North America Correlation between these schemes
is difficult However, certain horizons have proved to
have international utility Near the top of the
Areni-gian, for example, the appearance of biserial
grapto-lites marking the austrodentatus biozone has been
recognized on most palaeocontinents, and this will
form one basis of future standardization
Llanvirnian
The Llanvirn ‘Series’ was recognized in the
nine-teenth century in black slates exposed on the coast
in south-western Wales, where it takes its name from
an insignificant farmhouse It is typified by a
multi-tude of ‘tuning fork’ graptolites (e.g Didymograptus
artus, D murchisoni, and related forms), appearing
in the Aber Mawr Formation, which have been
rec-ognized widely in continental Europe and
Scandi-navia The type area is complicated by volcanics and
difficult structure, as is the area around Fishguard,
and the succession of strata in Britain is better
in-spected in Shropshire and central South Wales In
Scandinavia, the Upper Didymograptus Shales and
their equivalents in limestone strata have enabled
comparisons to be made with a rich conodont fauna,
which provides an international basis for correlation
The Llandeilan (the lower part of the original
Llan-deilo ‘Series’) has recently been incorporated into
the upper part of the Llanvirnian The flaggy
lime-stones around the town of Llandilo in central South
Wales were already well known by Murchison’s time
and are among the most fossiliferous rocks of the
British Ordovician Similar problems to those of
the Arenigian apply to the international correlation
of subdivisions of the Llanvirnian The Whiterockian
of the North American standard includes the Llanvir-nian but extends downwards into the Arenigian Across Europe and Asia the base of the Llanvirnian
is marked by a transgressive event, indicated by a deepening in the biofacies and the appearance of graptolites
Caradocian The type area of the Caradocian is in Shropshire, Eng-land, and it takes its name from a prominent hill, Caer Caradoc, in the vicinity of Church Stretton In that area the Caradoc is transgressive and its base is defined
by an unconformity at the base of the Hoar Edge Grit
As the sequence deepens upwards into a varied succes-sion of mudstones, sandstones, and siltstones (from which several formations are mapped), a great variety
of trilobites and brachiopods appear, which were ori-ginally studied in detail by B B Bancroft He divided the Caradocian into fine subdivisions (‘stages’) based
on faunal turnover, which, while useful at a local level, are of limited service internationally In the Shelve Inlier, in South Wales, and near Builth the facies repre-sent deeper water and the sections are without uncon-formities However, the effect of the Caradocian transgression is to bring in many graptolite species of stratigraphical utility, including Dicellograptus and Nemagraptus, replacing the restricted fauna of the Llandeilan teretiusculus biozone beneath This event
is of worldwide significance, and the graptolite species concerned can be widely employed for correlation purposes
The base of the Caradocian (and of the Upper Ordovician) is defined as the base of the Nemagrap-tus gracilis biozone, which can be traced into many areas including the Laurentian platform edge, Bohe-mia, continental Europe, and the standard graptolitic sequences of Australia and New Zealand Nemagrap-tus gracilis itself is probably associated with relatively
‘oceanic’ conditions, and its appearance in basinal successions is probably not entirely synchronous, but the co-occurrence of some of its attendant species
of Dicellograptus usually places correlation on a sound footing This, together with the beginnings of
a breakdown of the faunal provinciality typifying the earlier Ordovician, means that international correl-ation of Caradocian strata is less of a problem than is the case with Arenigian and Llanvirnian strata None-theless, there are still distinctive fossil faunas and concomitant separate stratigraphical schemes in the platform limestone successions (Trentonian and Blackriverian) of North America and the Baltic areas In China, a distinctive diachronous red often nautiloid-rich deep-water formation, the Pagoda Limestone, appears in the Caradocian and continues
178 PALAEOZOIC/Ordovician