Stig M. Bergstroem

First record of the Hirnantian (Upper Ordovician) delta C-13 excursion in the North American Midcontinent and its regional implications

The most prominent of the two major global delta C-13 excursions in the Ordovician, the Hirnantian delta C-13 excursion (HICE), which is previously recorded from the uppermost Ordovician in a few sections in Nevada, Quebec, Arctic Canada, Baltoscandia, Scotland and China, is documented for the first time from the North American Midcontinent. Samples through the Girardeau Limestone and Leemon Formation in Missouri and Illinois show elevated delta C-13 values of + 4 parts per thousand to + 5 parts per thousand. Although not determined precisely, the beginning of the HICE is likely to be in the upper part of the Orchard Creek Shale, and it ends in the upper Leemon Formation. Being extraordinarily useful chronostratigraphically, the presence of the HICE makes it possible to provide a firm dating of the study interval, whose age has long been controversial. Comparison between the study sections and coeval HICE sequences in North America and Europe show striking similarities, especially in sea-level history, indicating that major local lowstands reflect eustatic sea-level changes. A comparison with Hirnantian diamictite successions in North and South Africa and Argentina suggests that these lowstands correspond to two major Gondwanan glacial episodes. (Less)

First documentation of the Ordovician Guttenberg δ13C excursion (GICE) in Asia: chemostratigraphy of the Pagoda and Yanwashan formations in southeastern China

The only published VC data from the Ordovician of China are from the Lower and Upper Ordovician, and only the latter records include a significant excursion, namely the Hirnantian excursion (HICE). Our recent chemostratigraphic work on the Upper Ordovician (Sandbian-Katian) Pagoda and Yanwashan formations at several localities oil the Yangtze Platform and Chiangnan (Jiangnan) slope belt has resulted in the recognition of a positive delta C-13 excursion that has values of similar to+1.5 parts per thousand above baseline values. This excursion starts a few metres above a stratigraphic interval with B. alobatus Subzone conodonts as well as graptolites of the N. gracilis Zone. The distinctive conodonts Amorphognathus aff. Am. ventilatus and Hamarodus europaeus first occur at, or very near, the excursion interval. Because these conodonts appear in the stratigraphic interval of the Guttenberg VC excursion (GICE) in Estonia, we identify the Chinese excursion as the GICE. This is the first record of the GICE in the entire Asian continent. It confirms that GICE is a global excursion and provides an illustration of how VC chemostratigraphy, combined with new biostratigraphic data, solves the problem of the previously controversial age of the Pagoda Formation and how this classical stratigraphic unit correlates with the Baltoscandian and North American successions.

A new upper Middle Ordovician–Lower Silurian drillcore standard succession from Borenshult in Östergötland, southern Sweden: 1. Stratigraphical review with regional comparisons

A recent drilling at Borenshult near Motala resulted in discovery of the stratigraphically most complete succession through the upper Darriwilian–Rhuddanian interval known in Östergötland. The approximately 70 m long drillcore succession is subdivided into eight formations, the oldest being the late Darriwilian Furudal Limestone and the youngest being the Rhuddanian Motala Formation. Conodonts are used for a detailed biostratigraphic classification of the Borenshult drillcore into three subzones of the Pygodus serra Zone, two subzones of the Pygodus anserinus Zone, and three subzones of the Amorphognathus tvaerensis Zone. The base of the Amorphognathus superbus Zone is taken to be ∼10 m above the Kinnekulle K-bentonite, that of the Amorphognathus ordovicicus just below the Fjäcka Shale, and that of the Ozarkodina hassi Zone at the base of the Middle Member of the Loka Formation. Because of its unique lithology and paleontology and its wide geographic occurrence, this member is formally named herein the Skultorp Member. The previously uncertain stratigraphical position of the internationally known “Borenshult fauna” is shown to correlate with the Skultorp Member. A regional comparison of the Borenshult drillcore succession shows it to be most similar to coeval successions in Västergötland and Dalarna but there are some significant regional differences. The average rate of net rock accumulation during late Darriwilian and Sandbian time is calculated to be ∼3–4 mm/ka.

Hirnantian (latest Ordovician) delta C-13 chemostratigraphy in southern Sweden and globally: a refined integration with the graptolite and conodont zone successions

The δ13Corg chemostratigraphy of the Hirnantian and lower Rhuddanian in the biostratigraphically well-controlled Röstånga-1 drillcore from west-central Scania is used for an improved integration of the Hirnantian Isotope Carbon Excursion (HICE) with the standard graptolite zonation. In this drillcore succession, the end of the HICE corresponds to the top of the range of Metabolograptus persculptus. Baseline δ13Corg values occur in the uppermost Hirnantian Avitograptus avitus Faunal Interval as well as in the Rhuddanian Akidograptus ascensus Zone, and the isotope curve is also tied to the Swedish uppermost Katian and Hirnantian trilobite zonation. Chemostratigraphic data from sections in Västergötland confirm that the beginning of the HICE is at, or very close to, the base of the Skultorp Member of the Loka Formation. The biostratigraphically less precisely controlled end of the HICE is at least locally in the Upper Member of the same formation. The graptolite biostratigraphy in the Mt. Kinnekulle succession indicates that the lowermost Kallholn Formation, which has long been known as the Leonaspis (formerly Acidaspis) Shale, is of Hirnantian rather than earliest Silurian age which is consistent with the age of the lowermost Kallholn Formation in the Röstånga-1 drillcore. Comparisons with Hirnantian sections in the United Kingdom, North America and China make it possible to improve the calibration of the HICE with conodont and graptolite biostratigraphy and confirm the usefulness of δ13Corg chemostratigraphy for detailed correlations. The upper Katian carbon chemostratigraphy in key sections in North America and eastern Baltoscandia indicates that the Elkhorn and Paroveja excursions are the same. Available data are used for a new Hirnantian eustasy-climate-faunal evolution model.

Upper Ordovician (Sandbian–Katian) graptolite and conodont zonation in the Yangtze region, China

The graptolite and conodont biostratigraphy of the latest Darriwilian, Sandbian and early and middle Katian succession on the Yangtze Platform is reassessed based on numerous new fossil collections and previously published data. At least at some localities, the lowermost Miaopo Formation is of pre-Nemagraptus gracilis Zone age (uppermost Pygodus serra and lowermost P. anserinus conodont zones). The rest of this formation (except for the uppermost part, which lacks diagnostic graptolites) has a diverse graptolite fauna of the N. gracilis Zone. The uppermost part of the Miaopo Formation contains few biostratigraphically diagnostic graptolites but the occurrence of conodonts of the Baltoniodus alobatus Subzone of the Amorphognathus tvaerensis Zone suggests equivalence with part of the Climacograptus bicornis graptolite Zone. The conodont succession of the Datianba Formation is virtually identical with that of the Miaopo Formation, confirming that these units are coeval. The lowermost part of the overlying non-graptolitic Pagoda Formation represents the upper A. tvaerensis Zone, and its upper part represents the A. superbus Zone. The occurrence of Dicellograptus elegans in the overlying Chientsaokou Formation (equivalent to the Linhsiang Formation in Yangtze Gorges region) suggests equivalence with the Pleurograptus linearis Zone in Scotland, which is consistent with the relatively non-diagnostic conodont fauna in these Chinese units. The biostratigraphic data are in good agreement with the delta C-13 chemostratigraphy and permit the establishment of precise correlations with the Baltoscandic and North American successions.

New Ordovician-Silurian drill cores from the Siljan impact structure in central Sweden: an integral part of the Swedish Deep Drilling Program

New drill cores from the largest known impact structure in Europe, the relict of the Siljan meteorite crater, provide new possibilities to reconstruct Early Palaeozoic marine environments and ecosystems, and to document changes in sedimentary facies, sea level and palaeoclimate in Baltoscandia. The impact crater is an important target of the project “Concentric Impact Structures in the Palaeozoic” within the framework of the “Swedish Deep Drilling Program”. Two core sections, Mora 001 and Solberga 1, have been analysed. The sedimentary successions of these core sections include strata of late Tremadocian through late Wenlock ages. Our preliminary studies show not only that several of the classical Palaeozoic units of Sweden are represented in the area, but also that other significantly different facies are preserved in the Siljan district. An erosional unconformity representing a substantial hiatus occurs between Middle Ordovician limestone and a Llandovery-Wenlock (Silurian) shale succession in the western part of the Siljan structure and suggests an extended period of uplift and erosion. This may be related to forebulge migration due to flexural loading by the Caledonian thrust sheet to the west. Thus, this part of Sweden, previously regarded as a stable cratonic area, presumably was affected by the Caledonian collision between Baltica and Laurentia.

Lower Katian (Upper Ordovician) delta(13)C chemostratigraphy, global correlation and sea-level changes in Baltoscandia

A long-standing problem in the Ordovician stratigraphy of south-eastern Norway has been to the relations between the Mjøsa Formation in the Lake Mjøsa region and coeval strata in the Oslo region. The recent discovery of the globally distributed Guttenberg δ13C excursion (Guttenberg Isotopic Carbon Excursion) in the Mjøsa region provided the impetus to search for this excellent chemostratigraphic marker in the classical Oslo region succession, where it was found in the Frognerkilen Formation. Another positive δ13C excursion, which we identify as the Kope excursion, was discovered in the Solvang Formation. The new data show that the lower Katian δ13C chemostratigraphy in the Oslo region is closely similar to that from south-eastern and southern Estonia. This permits detailed correlations across Baltoscandia, which are useful for recognising the Baltic stage boundaries in the Oslo region succession. Both the Lake Mjøsa and Oslo regions study successions can be chemostratigraphically correlated with those in North America and eastern Asia. The newly established stratigraphic relations in the Oslo region are also used for a re-assessment of lower Katian local and eustatic sea-level changes.

Global Upper Ordovician correlation by means of delta C-13 chemostratigraphy: implications of the discovery of the Guttenberg delta C-13 excursion (GICE) in Malaysia

Apart from a single study of the early Katian δ 13 C chemostratigraphy in two regions in China, no investigations of the Sandbian and Katian chemostratigraphy have been published from anywhere in Asia. A recent study of the conodont biostratigraphy of the classical Ordovician succession on Langkawi Islands, peninsular Malaysia, showed the presence there of strata coeval with those having the Guttenberg Carbon Excursion (GICE) on the Yangtze Platform. In an effort to establish for the first time the presence of this widespread δ 13 C excursion in southern Asia, a series of samples from the upper part of the Kaki Bukit Formation was isotopically analysed. This resulted in the discovery of a conspicuous δ 13 C excursion with peak values of ~ 2 ‰ above the baseline values. The excursion is located just above the Baltoniodus alobatus Subzone and near the level of the first appearance of Hamarodus europaeus , hence the same stratigraphic position as the GICE on the Yangtze Platform. Using the GICE, the Malaysian study interval is closely correlated with the GICE intervals at three localities representing an approximately 23 000 km long transect from Malaysia across Baltoscandia to central North America. This shows the usefulness of δ 13 C chemostratigraphy to clarify previously obscure stratigraphic relationships between geographically very widely separated localities.

Conodont biostratigraphy, and delta C-13 and delta S-34 isotope chemostratigraphy, of the uppermost Ordovician and Lower Silurian at Osmundsberget, Dalarna, Sweden

The previously established graptolite and chitinozoan Hirnanian-Telychian biostratigraphy in the unique Osmundsberget North outcrop in the Siljan region, south-central Sweden, is integrated with new conodont biostratigraphy and δ13Corg, δ13Ccarb and δ34Spyr chemostratigraphy. At this locality, the middle Hirnantian (latest Ordovician) topmost part of the Boda Limestone is overlain by the latest Hirnantian Glisstjärn Formation, and the late Aeronian–early Telychian (Llandovery) Kallholn Formation rests unconformably on the Glisstjärn Formation. Previous conodont work showed that the Glisstjärn Formation belongs to the lower Ozarkodina hassi Zone. New samples from calcareous interbeds in the dominantly shaly Kallholn Formation, some of which contain hundreds of condont elements, yielded Distomodus staurognathoides, Aspelundia fluegeli and other taxa indicating the D. staurognathoides Zone. In the East Baltic succession, the coeval interval, which is in the uppermost Raikküla–lowermost Adavere stages just below the geographically widespread Osmundsberg K-bentonite, has yielded a conodont fauna similar to that of the lower Kallholn Formation. A regional review of the D. staurognathoides Zone shows that there are possible equivalents to our study interval also in Norway and the Welsh Borderland, but equivalent strata are missing in large parts of North America, or have not produced diagnostic conodonts. The δ13Corg values from the study section are relatively uniform (mostly ranging between − 29‰ and − 30‰), and the late Aeronian and Valgu positive excursions have not been recognized.