1,721,100 research outputs found
Ordovician stratigraphy of the Stumsnas 1 drill core from the southern part of the Siljan Ring, central Sweden
No full textThe Stumsnas 1 core, drilled in 2011 in the southern part of the Siljan Ring, provides new insights into the stratigraphy of the Ordovician succession in central Sweden. The core section shows evidence of the structural complications caused by the late Devonian meteorite impact in the area. In the core, about 90m of Tremadocian to Darriwilian strata are sandwiched between Proterozoic igneous basement rocks. At the lower contact, the sedimentary succession starts with glauconitic, fine-grained siliciclastics and carbonates directly resting on the weathered basement. The basal siliciclastic unit is overlain by a ca. 19-m-thick limestone succession, well-known from this part of the Baltoscandian Basin, includes strata corresponding to the Latorp Limestone through the upper Holen Formation. This succession is covered by a limestone-marl alternating sequence, representing some yet unknown deeper water facies, presumably coeval to the topmost Holen Formation through upper Dalby Limestone. The upper third of the sedimentary succession in the core includes numerous slices of partly overturned strata, including relatively thick successions of the Slandrom Formation, Fjacka Shale and Jonstorp Formation, as well as carbonate and siliciclastic units of yet unknown stratigraphic origin
Morphology, ontogeny and distribution of the Cambrian ellipsocephalid trilobite Strenuaeva spinosa from Scandinavia
No full textdelta C-13 chemostratigraphy in the upper Tremadocian through lower Katian (Ordovician) carbonate succession of the Siljan district, central Sweden
No full textBased on delta C-13 data from two drillcores recovered from the Siljan district, we present a first continuous carbon isotope record of the upper Tremadocian-lower Katian limestone succession of central Sweden. New names for some isotopic carbon excursions from the Cambrian-Ordovician boundary through the basal Darriwilian are introduced. The Mora 001 core from the western part of the Siljan impact structure ranges through the Lower-Middle Ordovician, whereas the Solberga 1 core from its eastern part ranges through the Middle-lower Upper Ordovician. Upper Tremadocian and Floian units are extremely condensed and include extensive stratigraphic gaps. Multiple hardgrounds, sometimes with minor karstic overprint, imply recurrent periods of erosion and/or non-deposition. Like in other parts of Sweden, the Dapingian and Darriwilian succession is characterized by a relatively complete sedimentary record and low sedimentation rates
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Stratigraphy and facies of Middle Silurian epicontinental carbonate platform deposits of Gotland, Sweden
No full textThe Silurian foreland of Scandinavia included a vast epicontinental carbonate platform system of which the island of Gotland (Sweden) is an erosional remnant. This thesis treats the Late Wenlock succession (Cyrtograptus lundgreni, Pristiograptus dubius parvus, Gothograptus nassa, Colonograptus(?) praedeubeli, and the Co(?) deubeli zones), and emphasises the sedimentology and biostratigraphy of these deposits. A new conodont zonation is introduced, encompassing the Kockelella ortus brevis, Ozarkodina bohemica longa and Ctenognathodus murchisoni zones. This interval is characterised by a diverse assemblage of lithofacies, reflecting deposition in a variety of carbonate platform sub-environments; the platform slope to basin transition, the subtidal shoal area, the reef area, and the rocky shore environment. Additionally, a more siliciclastic shelf-like setting evolved when coarser siliciclastics were transported to the basin during regression. These strata are preserved as a detached shoreline along the distal margin of the platform. The bulk of the sediments was deposited when the platform aggraded and prograded during two identified highstand systems tracts (HST). These periods of high sea-level favoured reef growth and skeletal carbonate production. A minor part of the succession accumulated during a forced regressive systems tract and a transgressive systems tract, both identified low in the O. b. longa Zone. Facies of these two stages reflect profound changes to the platform architecture during a 5th order sea-level chage, inferred to be of glacio-eustatic origin. A substantial drop in carbonate production probably preceded the onset of regression and siliciclastic influx. During this regression, storms were the most important process for offshore directed sediment redistribution. As sea-level fell, the shoreline was dislocated many kilometres seaward, resulting in one of the major unconformities in the basin. At the time of maximum lowstand an epikarstic intertidal flat fringed the west-southwest - east-northeast trending shoreline. Reefs did not manage to re-establish during the ensuing transgression ('give-up' scenario). Instead, non-skeletal carbonate shoal facies accumulated along the rocky shores sculpted in strata of the preceding HST. The first reefs re-appeared after a comparably long lag-phase, indicating deterioration of the environment during early transgression. The succeeding aggrading and prograding successions, both on the platform and along its distal slope, indicate that carbonate production phased or exceeded ('keep-up' scenario) the creation of accommodation space through the K. o. brevis and C. murchisoni zones. By utilising sequence stratigraphical concepts, the Late Wenlock sea-level curve is re-evaluated, excluding the long-term regressive trend often quoted in the literature. This reinforces the importance of studying genetic relations of sedimentary bodies, not individual sections, if sea-level curves should have any accuracy. Genetically, the studied interval encompasses two superimposed platforms separated by the epikarst and palaeotopography. The transitional strata are complex due to the depositional bias of the carbonate and siliciclastic depositional systems. Based on the idea that carbonate platforms essentially are major ecosystems, and on the close relationship of the succession and the oceanic Mulde Event, the two platforms are herein further subdivided into five evolutionary stages. Thereby, the relationship between sequence stratigraphy and event stratigraphy has been opened for study. It is shown that at least two of three extinctions among e.g., graptolites and conodonts, were not related to sea-level change. The new data accumulated indicate that the studied succession resulted primarily from regional to global processes. A numerical time-scale, based on graphic correlation, shows in detail the chronology of biotic extinctions and platform disruption, and their subsequent recovery
Silurian carbonate platforms and extinction events - ecosystem changes exemplified from Gotland, Sweden
No full textRecent and ancient carbonate platforms are major marine ecosystems, built by various carbonate-secreting organisms with different sensitivity for environmental change. For this reason, carbonate platforms are excellent sensors for changes in contemporaneous marine environments. A variety of ecosystem changes in carbonate platforms have previously been recognised in the aftermath of mass extinction events. This paper addresses how two Silurian extinction events among graptolites, conodonts, and pentamerid brachiopods can be related to changes in the style of carbonate production and general evolution of low latitude carbonate platforms in a similar way as previously reported from the major five mass extinctions of the Phanerozoic. Strata formed on Gotland during the Mulde and Lau events share remarkably many similarities but are strikingly different in composition compared to other strata on the island. The event-related strata is characterised by the sudden appearance of widespread oolites, deviating reef composition, flat-pebble conglomerates, abundant micro-and macro-oncoids, stromatolites, and other microbial facies suggesting decreased bioturbation levels in contemporaneous shelf seas. Importantly, these changes can be tied to high-resolution biostratigraphic frameworks and global stable isotope excursions. The anomalous intervals may therefore be searched for elsewhere in order to test their regional or global significance
A lowstand epikarstic intertidal flat from the middle Silurian of Gotland, Sweden
No full textThis paper describes the sedimentology, morphology and diagenesis of recently discovered Middle Silurian low-relief micro-epikarst from the Klintehamn area on western Gotland (Sweden). The epikarst represents the most distal portion of an unconformity that truncates stratigraphic units across a major part of the Baltic palaeo-basin. It formed in the intertidal area during the late stage of platform development and following a short period of regression and siliciclastic influx to the basin at the closing of the Cyrtograptus lundgreni Chron, An overlying oncolite shows signs of repeated exposure to meteoric and marine waters, supporting the interpretation of a karstic flat setting. Four microfacies with different stable isotopic signatures are recognised across the unconformity: dolomitic siltstone and altered siltstone in the topmost Frojel Formation, below the contact, and ferruginous crinoid algal packstone and ferruginous oncolite grainstone above the contact, in the basal lag of the transgressive Halla Formation. Endolithic structures of inferred fungal/algal origin occur frequently in the two transgressive microfacies. The set of observations pointing towards subaerial exposure and karstic processes include: (a) an understanding of the regional geology; (b) erosional relief with undercutting and an associated conglomerate; (c) vertical and subvertical conduits (karren), indicating gravitationally controlled waters; (d) widening of fissures and subsequent polyphase fillings (splitkarren); (e) eluviated, internal sediment (e.g,, crystal silt); (f) blackening; (g) repeated etching of carbonate allochems; (h) a fitted grain-fabric; (i) circum-granular cracking; (j) pendant nonluminescent cements; (k) abundant intercrystalline and intergranular (secondary) porosity, and (1) anomalous stable isotopes. A synsedimentary iron-crust at the contact was not related to karstification but formed a few centimetres below the sediment surface during the ensuing transgression. (C) 2002 Elsevier Science B.V. All rights reserved
Stratigraphy, facies development, and depositional dynamics of the Late Wenlock Fröjel Formation, Gotland, Sweden.
No full textA lithostratigraphic subdivision of the topmost Slite Beds on western Gotland is proposed, viz. the Fröjel Formation (9-11 m) including the Svarvare Mudstone Member (2-3 m) and the Gannarve Siltstone Member (7-8 m). The Fröjel Formation resulted from increased siliciclastic deposition within an intracratonic carbonate platform setting during the mid-Homerian (Wenlock, Silurian). The temporal and spatial development of sedimentary facies and the characters of formation boundaries can be explained with a sequence stratigraphic approach to depositional dynamics. The accelerating influx of siliciclastic material was connected to a sea-level fall which therefore initiated the formation of a gradationally based parasequence (the Gannarve Member). The depositional trend is supported by both the facies (upward decrease of accommodation space) and faunal development. The Svarvare Mudstone Member reflects a late highstand period, when hemipelagic deposition and weak, small-scale density currents contributed to sedimentation. The subsequent short period of shallowing resulted in deposition of the overlying Gannarve Siltstone Member, consisting of siltstone tempestites and associated fair-weather mudstones deposited in successively decreasing palaeodepth. The shallowing culminated with the formation of an unconformity, which marks the top of the Cyrtograptus lundgreni Biozone, and which is overlain by oolites. This unconformity correlates to the transgressive surface in offshore settings. The siliciclastic deposition is regarded as regional for this part of the Baltic Basin, influencing an area from offshore east Öland to Saaremaa west of Estonia. The depositional history of the Fröjel Formation is discussed in terms of relative and eustatic sea-level changes and with respect to regional correlation of the mid-Homerian eustatic regression
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