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The Collisional Orogeny in the Scandinavian Caledonides (COSC) project focuses on processes related to the closure of the Iapetus Ocean, causing the Ordovician-Silurian continent-continent collision between Baltica and Laurentia. The rock succession in the second drill core (COSC-2) from the Jämtland County, central Sweden, provides the base for detailed sedi - mentological, stratigraphic, geophysical, geochemical, geothermal and structural studies. The basement, comprising 1.66-1.65 Ga Transscandinavian Igneous Belt porphyries intruded by 1.47 Ga and 1.27-1.26 Ga mafic dykes and sills, is heavily weathered towards the top. Here it grades into typical saprock and saprolite (including immature soil reflecting the sub-Cambrian peneplain). The overlying sedimentary sequence starts with basal conglomerates and heterogeneous sediments with shell fragments, indicating an early Cambrian rather than a Neoproterozoic age for the marine transgression in the area. The developing early Cambrian basin was rapidly filled, initially by mostly coarse-grained sediment gravity flows. These strata are covered by sandstone turbidites that show an upward transition into the Alum Shale Formation, representing a tectonically quieter period (mid-Cambrian/Maolingian to Early Ordovician/Tremadocian). The upper part of the Alum Shale Formation is overlain by a late Early Ordovician turbidite succession. Local sources of sediments below the Alum Shale Formation and the extended deposition period may indicate continuous sedimentation in a pull-apart basin pre - served in a window beneath the Caledonian thrust sheets. [ABSTRACT FROM AUTHOR]

Based on a new deep drilling on southern Gotland (Sweden), this study is the first to document the carbon isotope chemostratigraphy of the Upper Ordovician through lowermost Silurian sedimentary record from the central parts of the Baltic Sea subsurface. The lithological record of the Stora Sutarve core has similarities with adjacent successions in both the Viki core in the East Baltic area and the Borenshult core in the Swedish mainland. The core includes the Kahula (including 13 bentonites in the "Kinnekulle K-bentonite complex"), Hirmuse(?), Rägavere, Paekna, Jonstorp, Loka, Motala and Kallholn formations. Several of the internationally recognized upper Ordovician δ13C excursions have been identified in the core, including the Guttenberg Isotope Carbon Excursion (GICE), Moe excursion, Hirnantian Isotope Carbon Excursion (HICE), and an undefined early Silurian carbon isotope excursion, presumably the Early Aeronian Carbon Isotope Excursion (EACIE). We particularly address the microstratigraphy of the Hirnantian Stage and the finer details of the HICE, which is not fully complete and entirely confined to the 2.09-m-thick Loka Formation. This formation yields abundant, but low-richness brachiopod faunas characteristic of the Hirnantian Stage and is, based on erosional-depositional surfaces and facies, interpreted as reflecting interglacial warming and eustatic transgression related to ice-sheet contraction in Gondwana. Based on very dense carbon isotope sampling we identify four clusters of δ13C values that aid to define significant changes in the preserved record of the HICE and which may facilitate global correlation of the succession and the herein interpreted sea-level record. [ABSTRACT FROM AUTHOR]

Erosional outliers are an important component of the Ludlow (Late Silurian) stratigraphy of Gotland, Sweden. However, due to the poor degree of exposure, outcrop studies have not revealed a detailed section from any of these outliers. The first complete stratigraphy of such an erosional remnant, the Burgen outlier, is presented here, based on the recently recovered Burgen- 1 drillcore. Stratigraphical units encountered are, from oldest to youngest, the uppermost Hemse Group (> 35 m), Eke Formation (0.4 m), Burgsvik Formation (3.4 m) and the basal Hamra Formation (12 m preserved). Based on carbon stable isotopes and lithological correlation to nearby outcrops that are zoned by conodonts, it can be concluded that the most profound late Ludlow sedimentary changes took place shortly after the onset of the globally recognized Lau Event ([delta][sup.13]C values reach 8.7 [per thousand] in the lower Hamra Formation). Sedimentary changes include development of discontinuity surfaces, widespread occurrence of an intraclastic conglomerate, increased microbial activity (Rothpletzella and Wetheredella), and influx of clastics to the basin. These changes are linked to substantial base-level changes during the event. A general comparison with the Prague basin suggests contemporaneous sedimentary changes on Baltica and on cratonic elements from peri-Gondwana during the event. Keywords: Lau Event, stable isotopes, microbial, Silurian, Gotland, Baltica.

Resurgence of microbialites and anachronistic facies in the aftermath of Phanerozoic extinction events has hitherto only been documented among the 'major five.' Based on the classical Silurian carbonate platform strata of Gotland, Sweden, a 'minor' biotic event is herein for the first time related to a substantial increase in anachronistic facies. Strata deposited during and shortly after the Lau Event in the Baltic paleobasin (Eke and Burgsvik Formations) include evidence for reduced grazing and infaunal activity in contemporaneous shallow low-latitude seas, e.g., flat-pebble conglomerates, normal-marine stromatolites, mass occurrence of oncoids, and subtidal wrinkle structures (e.g., Kinneyia and microripples). The latter structures have recently been suggested to be restricted to Proterozoic-Cambrian and Early Triassic time and therefore are regarded as anomalous for the Phanerozoic. The co-occurrence of these facies and global extinctions among pentamerid brachiopods and conodonts suggests that minor extinction events also resulted in anachronistic periods, less devastating but fully comparable to those in the aftermath of the end-Ordovician and end-Permian mass extinctions. A global positive stable carbon isotope excursion and the last appearance datum of the conodont Polygnathoides siluricus may facilitate identification of the Late Silurian anachronistic period elsewhere. Keywords: anachronistic facies, wrinkle structures, Silurian, extinction, Gotland.

Evidence from sedimentology and conodont biostratigraphy is used to reinterpret the mid-Homerian (Late Wenlock) succession on Gotland, Sweden. A new conodont zonation includes from below: the Ozarkodina bohemica longa Zone (including five subzones), the Kockelella ortus absidata Zone and the Ctenognathodus murchisoni Zone (two taxa are named, Ozarkodina bohemica longa and Pseudooneotodus linguicornis). These new zones are integrated with facies in order to correlate strata and infer the major depositional environments and the controls on deposition during the mid-Homerian Mulde Event. Reef-associated and skeletal carbonate deposition predominated before and after the event, i.e. during the uppermost O. s. sagitta Zone and, again, in the C. murchisoni Zone. These periods are characterized by the expansion of reefs and shoal facies across marls in the topmost Slite Group on eastern Gotland and in the lower parts of the Klinteberg Formation on western Gotland, respectively. The intervening O. b. longa and K. o. absidata zones are initially characterized by rapid facies changes, including siliciclastic deposition, and later stabilisation of a carbonate depositional system. The composition of sediments and depositional rates are closely related to the creation and destruction of accommodation space and reflects a classical case of depositional bias of the carbonate and siliciclastic depositional systems. Based on coastline migration, stratal boundaries, and the stratigraphic position of major reef belts, several facies associations can be fitted into a sequence stratigraphic model for platform evolution. A highstand systems tract (HST) situation prevailed prior to, and during the early part of the event; the upper Slite Group including the lower Frojel Formation. This HST was characterized by prolific skeletal production and regional reef development except for during the latest stage when carbonate production declined at the onset of the Mulde Event. Platform growth was inhibited during a following regressive systems tract (RST) when regional siliciclastic deposition predominated; the Gannarve Member. The subsequent lowstand resulted in regional emersion and karstification, i.e. a complete termination of the platform. The post-extinction transgressive systems tract (TST) is exclusively composed of non-skeletal carbonates; the Bara Member of the Halla Formation. Re-occurrence of reefs and a prolific skeletal production marks platform recovery during a second HST; the remaining Halla and the lower Klinteberg formations. Integration of high-resolution biostratigraphy and sequence stratigraphy reveals that the major physical control on platform evolution was a 5th order eustatic sea-level change during an early part of the Mulde Event, and that the bulk of the strata accumulated when the platform aggraded and prograded during the highstand systems tracts. Thus, Silurian oceanic events and associated sea-level changes had profound impact on the neritic carbonate system. The Gotland-based middle and late Homerian sea-level curve shows two rapid regressions, both leading to truncation of highstand systems tracts. The first lowstand occurred at the very end of the C. lundgreni Chron, and the second at the end of the Co.? ludensis Chron. The intervening interval was characterized by stillstand or possibly slow transgression. Keywords: carbonate platforms, Conodonta, mass extinctions, sequence stratigraphy, Silurian.

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