Browsing by Subject "STRATIGRAPHY"

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  • Kröger, Björn; Aubrechtová, Martina (2018)
    The cephalopods of the reef limestones of the Vasalemma Formation, northern Estonia, are highly diverse and comprise 22 species belonging to 10 families and seven orders in a sample of >300 specimens. Most of the specimens were collected from shell concentrations in synsedimentary cavities and are interpreted as parautochthonous, washed in from nearby habitats. Nearly all of the shells are fragmented and nearly 15% are partially encrusted by epibionts. The assemblage is dominated by small (mostly less than 30mm wide), straight-shelled actinocerids and orthocerids; in addition, coiled tarphycerids are common. The high-level taxonomic composition of the Vasalemma cephalopod assemblage, with a dominance of actinocerids and an absence of endocerids, is in agreement with deposition in a warm-water (tropical or subtropical), shallow, subtidal regime. At the species level the assemblage is highly endemic, but the generic composition allows for a statistical comparison with other faunas. A cluster analysis of contemporary assemblages reveals a high degree of similarity with late Sandbian cephalopod faunas of epicontinental Laurentia. The palaeogeographical distribution pattern is similar to that of brachiopods, which supports earlier interpretations of these clusters as mainly controlled by water temperature and depositional depth. Several of the Vasalemma genera became conspicuous elements of epicontinental Laurentia during the Katian, which emphasizes that immigration towards Laurentia was an important factor in Late Ordovician diversity dynamics. Of the described taxa, the following are new: Beloitoceras cautis sp. nov., Curtoceras abditus sp. nov., Hemibeloitoceras arduum sp. nov., H. molis sp. nov., Hoeloceras muroni sp. nov., Isorthoceras cavi sp. nov., I. maris sp. nov., I. padisense sp. nov., I. vexilli sp. nov., Ordogeisonoceras tartuensis sp. nov., Orthonybyoceras isakari sp. nov., O. moisense sp. nov., Pleurorthoceras organi sp. nov., Rummoceras rummuensis gen. et sp. nov. and Trocholites gennadii sp. nov.
  • Kröger, Björn; Penny, Amelia (2020)
    During the late Cambrian–Early Ordovician interval the predominant non-microbial reef builders were sponges or sponge-like metazoans. The lithological and faunal composition of Cambro-Ordovician sponge-dominated reefs have previously been analyzed and reviewed. Here we take the relationship between reef aggregation pattern at reef to seascape scale into account, and look for changes during the Early–Middle Ordovician interval, in which metazoans became dominant reef builders. In a comparison of sponge-rich reefs from eight sites of the Laurentia paleocontinent three different seascape level reef growth patterns can be distinguished: (1) mosaic mode of reef growth, where reefs form a complex spatial mosaic dependent on hard substrate; (2) episodic mode, where patch reefs grew exclusively in distinct unconformity bounded horizons within non-reefal lithological units that have a much larger thickness; and (3) belt-and-bank mode, where reefs and reef complexes grew vertically and laterally as dispersed patches largely independent from truncation surfaces. The distinct modes of growth likely represent specific reef forming paleocommunities, because they differ in content and abundance of skeletal metazoan framebuilders, bioturbation intensity of non-skeletal reef sediment matrix, and in association of reef growth with underlying hard substrate. We suggest, based on a review of Laurentian reef occurrences, that the mosaic mode dominated in Early Ordovician strata and that the dominance shifted toward the belt and bank mode from Middle Ordovician strata onward.
  • Veikkolainen, Toni Henri Kristian; Kukkonen, Ilmo Tapio; Näslund, Jens-Ove (2019)
    In northern Europe, radiogenic heat production of surface rocks has been extensively studied in Finland and Norway alike. This paper presents a heat production analysis of Sweden, based on a rock outcrop data compilation obtained from the Geological Survey of Sweden (SGU). The study area comprises Precambrian Shield, Caledonian and platform cover areas. Altogether 39933 samples with uranium, thorium and potassium concentration (C-U, C-Th and C-K) and density () data were available. Heat production (HP) was calculated using raw point data, binning on a regular grid, and averaging by bedrock units in the geological map. Methods based on raw point data and grid-based binning resulted in HP values of 2.5 +/- 4.1 and 2.5 +/- 5.6 Wm(-3), respectively, while averaging by lithology produced a lower value of 2.4 +/- 1.7 Wm(-3). Limiting the lithology-based averaging to Precambrian bedrockareas resulted in heat production of 2.4 +/- 1.6 Wm(-3). Due to the small geographic extent of area covered by sediments, this is similar to the Precambrian-only value. Regardless of the calculation method, heat production in Sweden is considerably higher than the corresponding value for Finland. The Swedish platform cover had apparently the lowestheat production (1.0 +/- 1.8 Wm(-3)) of all units but the presence of Precambrian rocks below the sediments means that this value strongly misleads if used to represent the entire upper crust. Svecokarelian (Svecofennian) and Sveconorwegian rocks, which comprised 94.0 per cent of all individual observations, had heat production values of 2.6 +/- 1.8 and 1.7 +/- 1.4 mu Wm(-3), respectively. Although the Swedish data still have large spatial gaps when compared to Finnish data, most bedrock units in Sweden are covered. It is obvious that the higher heat flow of Sweden compared to that of Finland is caused by near-surface (i.e. upper crustal) heat production, and crustal differentiation in Sweden is also larger.
  • Kröger, Björn; Aubrechtová, Martina (2019)
    The cephalopods collected from the mud mounds of the Kullsberg Limestone Formation, late Sandbian-earliest Katian(?), south central Sweden, are highly diverse and comprise 26 identifiable species of 12 families and six orders in a sample of c. 180 specimens. The assemblage is strongly dominated by orthocerids in abundance and diversity. In contrast, the time-equivalent assemblage of the reef limestone of the Vasalemma Formation of Estonia is dominated by actinocerids and less diverse. Only one-third of the species co-occur in these two palaeogeographically relatively close assemblages. The taxonomic composition of the Kullsberg assemblage is, on the order level, more similar to that of the late Katian-early Hirnantian Boda Limestone Formation of south central Sweden, which represents a similar relatively deep depositional environment. The high local differentiation of cephalopod reef faunas exemplifies the importance of the emergent Baltic reef habitats in diversification processes during the early Late Ordovician. Of the described taxa, the following are new: Beloitoceras thorslundi sp. nov., Cameroceras motsognir sp. nov., Clothoceras thornquisti gen. et sp. nov., Danoceras skalbergensis sp. nov., Discoceras amtjaernense sp. nov., D. nilssoni sp. nov., Endoceras naekki sp. nov., Furudaloceras tomtei gen. et sp. nov., Isbergoceras consobrinum gen. et sp. nov., I. niger gen. et sp. nov., Isorthoceras nikwis sp. nov., I. sylphide sp. nov., I. urdr sp. nov., I. verdandi sp. nov., Kullsbergoceras nissei gen. et sp. nov., Ordogeisonoceras uppsalaensis sp. nov. and Valkyrioceras dalecarlia gen et sp. nov.
  • Kroger, Bjorn; Finnegan, Seth; Franeck, Franziska; Hopkins, Melanie J. (2017)