Browsing by Subject "VASALEMMA FORMATION"

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  • Kröger, Björn; Hints, Linda; Lehnert, Oliver (2017)
    The widespread growth of reefs formed by a framework of biogenic constructors and frame-lacking carbonate mounds began on Baltica during Ordovician time. Previously, Ordovician reef and mound development on Baltica was considered to be sporadic and local. A review of all known bioherm localities across the Baltic Basin reveals a more consistent pattern. Ordovician bioherms grew in a wide E-W-aligned belt across the Baltic Basin and occur in several places in Norway. Substantial reef development began simultaneously across the region during the late Sandbian - early Katian interval and climaxed during the late Katian Pirgu age. The current spatiotemporal distribution of bioherms is a result of interdependent factors that involve original drivers of reef development such as relative sea level, climate during the time of deposition and effects of post-depositional erosion. Oceanographic conditions were likely more favourable during times of cooler global climates, low sea level and glacial episodes. At the same time, the likelihood that bioherms are preserved from long-term erosion is higher when deposited during low sea level in deeper parts of the basin. A main factor controlling the timing of the reef and mound evolution was Baltica's shift toward palaeotropical latitudes during Late Ordovician time. The time equivalence between initial reef growth and the Guttenberg isotope carbon excursion (GICE) suggests that global climatic conditions were important.
  • Kröger, Björn; André, Desrochers; Andrej, Ernst (2017)
    Bryozoans, stromatoporoid sponges, and tabulate corals, all colonial metazoans with lamellar, encrusting growth forms, developed and simultaneously diversified during the Great Ordovician Biodiversification Event (GOBE). After revisiting some classic Lower, Middle, and Upper Ordovician reef localities in Laurentia (Franklin Mountains, west Texas, Mingan Islands in eastern Canada, and Champlain Valley in northeastern United States) and Baltica (northern Estonia) and reviewing the literature, we demonstrate that during the Ordovician a newly emerging consortium of sheet-like bryozoans, stromatoporoid sponges, and tabulate corals locally bound together by microbes, automicrite, and cement and solidly rooted in sediment became the dominant reef-builders globally. The diversification of these sheet-like metazoans (SLM), however, clearly lagged behind the first appearance of their respective skeletal ancestors. Their habitat expansion can be exemplified as a case of simultaneous ecological fitting and ecosystem engineering when the independently evolved shared traits were simultaneously co-opted and became advantageous under globally different environmental conditions. This interaction led to the evolutionary diversification of colonial metazoans during the GOBE and to the expansion of novel reef habitats in previously soft-surface settings; a transformation that forever changed marine reefal ecosystems.