Stig M. Bergström

Isotopic evidence for geochemical decoupling between ancient epeiric seas and bordering oceans: Implications for secular curves

Isotopic analysis of conodonts and their host limestones sampled between two regionally extensive, altered volcanic ash layers in eastern Laurentia shows that a 454 Ma epeiric sea maintained large lateral differences in Nd and C isotope compositions. This is consistent with inferred temperature-salinity‐defined epicontinental water masses and restricted circulation between epicontinental and oceanic environments. Because the majority of old marine fossils and sedimentary rocks are known from epeiric seas, some isotope excursions in ancient marine strata may originate from expansion and contraction of geochemically distinct epicontinental water masses, rather than global-scale changes in the state of the earth-ocean system.

Conodonts as paleotemperature tools in Ordovician rocks of the Caledonides and adjacent areas in Scandinavia and the British Isles

Abstract Based on collections from 110 localities, the temperature-induced color change associated with low-grade metamorphism in conodonts is used for the first regional assessment of the degree of heating of Ordovician rocks in Scandinavia and the British Isles. Baltoscandian Platform rocks without intrusives in southern Sweden have apparently not been heated above 90°C. The autochthonous rocks in Jamtland appear to have been heated to a locally specific temperature between 110 and 400°C (perhaps even higher in some areas). The samples from the Oslo region, as well as those from the allochthon in the Trondheim region and the eastern nappes in Jamtland, suggest heating temperatures in excess of 300°C. Most collections studied from the British Isles indicate heating to more than 300°C, but those from the Girvan area and the Welsh Borderland show little, if any, thermal effects. In most cases, the conodont temperature indications fit well into, and clarify, the known geologic history of the study areas but...

Nomenclature, stratigraphy, chemical fingerprinting, and areal distribution of some Middle Ordovician K‐bentonites in Baltoscandia

Abstract Based on biostratigraphic position, chemical fingerprinting, and lithic characteristics such as relative thickness, several of the numerous K‐bentonite beds, or complexes of beds, in the Middle Ordovician of Baltoscandia are shown to be traceable over large areas. Because the type of volcanic eruptions that produced such widespread ash beds lasts only a short time (a couple of weeks, or less), the individual ash layers represent as close equivalents of time planes as one is likely to find in the Lower Paleozoic stratigraphic record. Although these clay beds were recorded in section descriptions as far back as in the 1880s, their volcanic nature was recognized in Baltoscandia only in the mid‐1940s. Subsequent research has added important data on the occurrence and chemical composition of the Middle Ordovician K‐bentonites, but the present study is the first detailed regional investigation of these beds across their entire distribution area in Baltoscandia. Four beds, or complexes of beds (the Gr...

Large-magnitude Middle Ordovician volcanic ash falls in North America and Europe: Dimensions, emplacement and post-emplacement characteristics

Abstract Middle Ordovician K-bentonites represent some of the largest known fallout ash deposits in the Phanerozoic Era. They cover minimally 2.2 × 106 km2 in eastern North America and 6.9 × 105 km2 in northwestern Europe, and represents the coeval accumulation of plinian and co-ignimbrite ash on both Laurentia and Baltica during the closure of the Iapetus Ocean. The three most widespread beds are the Deicke and Millbrig K-bentonites in North America and the Kinnekulle K-bentonite in northwestern Europe. The vents were located near the Laurentian margin of Iapetus on an arc or microplate undergoing collision with Laurentia. The volume of ash preserved in the stratigraphie record converted to dense rock equivalent (DRE) of silicic magma is minimally estimated to be 943 km3 for the Deicke, 1509 km3 for the Millbrig and 972 km3 for the Kinnekulle. The Millbrig and Kinnekulle beds are coeval and possibly equivalent, yielding a combined DRE volume of nearly 2500 km3. Some unknown but probably large amount of additional ash fell into oceanic regions of the Iapetus, but these areas became subducted and the ash is not preserved in the geologic record. The symmetry of the thickness contours is suggestive that one or more ash clouds interacting with equatorial stratospheric and tropospheric wind patterns dispersed pyroclastic material to both the northwest and southeast in terms of Ordovician paleogeography. Based on grain size measurements and thickness/area 1 2 plots we conclude the three beds were each formed from co-ignimbrite or possibly phreatoplinian eruption columns. Analyses of melt inclusions in primary quartz crystals indicate the parental magma contained approximately 4% dissolved water at the time of the eruption. This water provided the explosive energy during the initial gas thrust phase. The implied fragmentation pressure on the magma would have reduced much of the ejecta to small particles, forming a deposit composed largely of single crystals and glassy dust. Conversion of the ash to K-bentonite resulted in a mass loss of approximately 35%, mostly in the form of Si with lesser amounts of Na and K.

The Lower Silurian Osmundsberg K-bentonite; Part II, Mineralogy, geochemistry, chemostratigraphy and tectonomagmatic significance

The Lower Silurian Osmundsberg K-bentonite is a widespread ash bed that occurs throughout Baltoscandia and parts of northern Europe. This paper describes its characteristics at its type locality in the Province of Dalarna, Sweden. It contains mineralogical and chemical characteristics that permit its regional correlation in sections elsewhere in Sweden as well as Norway, Estonia, Denmark and Great Britain. The < 2 µm clay fraction of the Osmundsberg bed contains abundant kaolinite in addition to randomly ordered (RO) illite/smectite (I/S). Modelling of the X-ray diffraction tracings showed the I/S consists of 18 % illite and 82 % smectite. The high smectite and kaolinite content is indicative of a history with minimal burial temperatures. Analytical data from both pristine melt inclusions in primary quartz grains as well as whole rock samples can be used to constrain both the parental magma composition and the probable tectonic set- ting of the source volcanoes. The parental ash was dacitic to rhyolitic in composition and originated in a tec- tonically active collision margin setting. Whole rock chemical fingerprinting of coeval beds elsewhere in Baltoscandia produced a pronounced clustering of these samples in the Osmundsberg field of the discriminant analysis diagram. This, together with well-constrained biostratigraphic and lithostratigraphic data, provides the basis for regional correlation and supports the conclusion that the Osmundsberg K-bentonite is one of the most extensive fallout ash beds in the early Phanerozoic. The source volcano probably lay to the west of Baltica as part of the subduction complex associated with the closure of Iapetus.

Did intense volcanism trigger the first Late Ordovician icehouse

Oxygen isotopes measured on Late Ordovician conodonts from Minnesota and Kentucky (United States) were studied to reconstruct the paleotemperature history during late Sandbian to Katian (Mohawkian–Cincinnatian) time. This time interval was characterized by intense volcanism, as shown by the prominent Deicke, Millbrig, and other K-bentonite beds. A prominent carbon isotope excursion (Guttenberg δ 13 C excursion, GICE) postdates the Millbrig volcanic eruptions, and has been interpreted to reflect a drawdown of atmospheric carbon dioxide and climatic cooling. The oxygen isotope record in conodont apatite contradicts this earlier interpretation. An increase in δ 18 O of 1.5‰ (Vienna standard mean ocean water) just above the Deicke K-bentonite suggests an abrupt and short-lived cooling that possibly initiated a first short-term glacial episode well before the major Hirnantian glaciation. The decrease in δ 18 O immediately after the mega-eruptions indicates warming before the GICE, and no cooling is shown in the GICE interval. The coincidence of the Deicke mega-eruption with a cooling event suggests that this major volcanic event had a profound effect on Late Ordovician (late Mohawkian) climate.

The Ordovician Sebree Trough: An oceanic passage to the Midcontinent United States

The Sebree Trough is a relatively narrow, shale-filled sedimentary feature extending for several hundred kilometers across the Middle and Late Ordovician carbonate platform of the Midcontinent United States. The dark graptolitic shales within the trough stand in contrast to the coeval bryozoan-brachiopod-echinoderm– rich limestones on the flanking platforms. We infer from regional stratal patterns, thickness and facies trends, and temporal relations established by biostratigraphy and K-bentonite stratigraphy that the Sebree Trough initially began to develop during late Turinian to early Chatfieldian time (Mohawkian Series) as a linear bathymetric depression situated over the failed late Precambrian–Early Cambrian Reelfoot Rift. Rising sea level and positioning of a subtropical convergence zone along the southern margin of Laurentia caused the rift depression to descend into cool, oxygen-poor, phosphate-rich oceanic waters that entered the southern reaches of the rift from the Iapetus Ocean. The trough apparently formed in a system of epicontinental estuarine circulation marked by a density- stratified water column. Trough formation was accompanied by cessation of carbonate sedimentation, deposition of graptolitic shales, development of hardground omission surfaces, substrate erosion, and local phosphogenesis. The carbonate platforms on either side of the trough are dominated by bryozoan-brachiopod- echinoderm grainstones and packstones that were deposited in zones of mixing where cool, nutrient-rich waters encountered warmer shelf waters. Concurrently, lime mudstone and wackestone were deposited shoreward (northern Illinois, Wisconsin, Iowa, Minnesota, Michigan) in warmer, more tropical shallow seas. Coeval upward growth of the flanking carbonate platforms sustained and enhanced development of the trough shale facies. Five widespread diachronous late Mohawkian and Cincinnatian omission surfaces are present in the carbonate facies of the Midcontinent. These surfaces include sub-Deicke K-bentonite, DS1; top of Black River Limestone, DS2; base and top of the Guttenberg Limestone Member of the Decorah Formation, DS3 and DS4; and top of the Trenton Limestone, DS5. Some of the surfaces correspond to previously described depositional sequence boundaries. All five surfaces, which embody deepening phases on top of highstand-systems tracts, converge in the Sebree Trough, indicating that the trough was a long-lived feature and was the source of eutrophic waters that episodically spread across the adjacent platforms, terminating carbonate production. Late Turinian and early Chatfieldian incipient drowning episodes were followed by a final drowning event that began in the Sebree Trough during the late Chatfieldian ( Climacograptus spiniferus Zone) and reached southernmost Minnesota and other regions far within the platform interior by Richmondian time ( Amorphognathus ordovicicus Zone).

Chemostratigraphy in the Swedish Upper Ordovician: Regional significance of the Hirnantian delta C-13 excursion (HICE) in the Boda Limestone of the Siljan region

Abstract Samples from the Boda Limestone and immediately overlying strata at Osmundsberget in the Siljan region have produced an excellent Hirnantian isotope excursion (HICE) curve, in which the excursion interval is about 22 m thick and the HICE reaches maximum δ13C values of between +5‰ and +6‰. Both the HA and HB stratigraphic unconformities are recognized in the study succession as well as in the coeval stratigraphic interval in the Loka Formation in Västergötland and at Meifod in central Wales. A comparison between these and some other shallow-water successions in North America and northern Europe shows striking similarities in the δ13C curve, the lithology, and the stratigraphic sequence development. This is taken as support for the idea that glacio-eustacy exercised a major control on the depositional pattern in shallow-water successions during Hirnantian time. This applies also to the Osmundsberget succession where the formation of carbonate mounds was stopped by the HA regression that was caused by a major glaciation in Gondwana.

Ordovician K-bentonites in the Argentine Precordillera: relations to Gondwana margin evolution

Abstract Ordovician K-bentonites have now been recorded from >20 localities in the vicinity of the Argentine Precordillera. Most occur in the eastern thrust belts, in the San Juan Limestone and the overlying the Gualcamayo Formation, but a few ash beds are known also from the central thrust belts. The oldest occur in the middle Arenig I. victoriae lunatus graptolite (Oe. evae conodont) Zone, and the youngest in the middle Llanvirn P. elegans (P. suecicus) Zone. Mineralogical characteristics, typical of other Ordovician K-bentonites, include a matrix of illite/smectite mixed-layer clay and a typical felsic volcanic phenocryst assemblage: biotite, beta-form quartz, alkali and plagioclase feldspar, apatite, and zircon, with lesser amounts of hornblende, clinopyroxene, titanite and Fe-Ti oxides. The proportions of the mineral phases and variations in their crystal chemistry are commonly unique to individual (or small groups of) K-bentonite beds. Glass melt inclusions preserved in quartz are rhyolitic in composition. The sequence is unique in its abundance of K-bentonite beds, but a close association between the Precordillera and other Ordovician sedimentary basins cannot be established. The ash distribution is most consistent with palaeogeographical reconstructions in which early Ordovician drifting of the Precordillera occurred in proximity to one or more volcanic arcs, and with eventual collision along the Andean margin of Gondwana during the mid-Ordovician Ocloyic event of the Famatinian orogeny. The Puna-Famatina terrane northeast of the Precordillera might have served as the source of the K-bentonite ashes, possibly in concert with active arc magmatism on the Gondwana plate itself.

The middle Darriwilian (Ordovician) δ13C excursion (MDICE) discovered in the Yangtze Platform succession in China: implications of its first recorded occurrences outside Baltoscandia

Abstract: The middle Darriwilian δ13C excursion (MDICE), one of the least known of the Ordovician δ13C excursions, has previously been recorded only from the Middle Ordovician of Baltoscandia. Analysis of many recently collected limestone samples from the Guniutan Formation at two Yangtze Platform localities show elevated δ13C values in the same biostratigraphic interval (Microzarkodina ozarkodella Conodont Subzone) as the MDICE in Baltoscandia, which justifies identification of the Chinese δ13C excursion as the MDICE. These occurrences, which are in strata that show striking lithological and conodont faunal similarity to the Swedish Holen Limestone and some coeval units in Estonia, indicate that the MDICE, the stratigraphically oldest of the named Ordovician δ13C excursions, is likely to have a world-wide distribution and to have great potential for local and long-range chemostratigraphic correlations.