THÔNG TIN TÀI LIỆU
178 PALAEOZOIC/Ordovician Scandinavia, 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 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 biostratigraphical standard This disparity of stratigraphical criteria together with regional differences have meant that a plethora of stratigraphical subdivisions of Arenigian strata has grown up over the years, and different 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 Arenigian, for example, the appearance of biserial graptolites 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 nineteenth 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 multitude of ‘tuning fork’ graptolites (e.g Didymograptus artus, D murchisoni, and related forms), appearing in the Aber Mawr Formation, which have been recognized widely in continental Europe and Scandinavia 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 inspected 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 Llandeilo ‘Series’) has recently been incorporated into the upper part of the Llanvirnian The flaggy limestones 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 Llanvirnian 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, England, 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 succession of mudstones, sandstones, and siltstones (from which several formations are mapped), a great variety of trilobites and brachiopods appear, which were originally 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 represent deeper water and the sections are without unconformities 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 Nemagraptus gracilis biozone, which can be traced into many areas including the Laurentian platform edge, Bohemia, continental Europe, and the standard graptolitic sequences of Australia and New Zealand Nemagraptus 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 correlation of Caradocian strata is less of a problem than is the case with Arenigian and Llanvirnian strata Nonetheless, 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
Ngày đăng: 26/10/2022, 10:55
Xem thêm: