Stromatoporoid palaeobiology and taphonomy in a Silurian biostrome on Gotland, Sweden
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Taphonomy
Paleobiology
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Kershaw, Stephen 1980 10 15: Cavities and cryptic faunas beneath non-reef stromatoporoids. Lethaia, Vol. 13, pp. 327–338. Oslo. ISSN 0024–1161. Stromatoporoids from level-bottom shales and argillaceous limestones in the Silurian of Gotland, Sweden, form substrates for a variety of encrusting and boring organisms. Overturned stromatoporoids have encrusters and borers on both upper and lower surfaces, while coenostea preserved in situ have encrusters on both surfaces but borings on upper surfaces only. This suggests that cavities, now infilled, existed below coenostea. The stromatoporoids are isolated and not part of a reef framework where growth alone could have created overhangs and cavities. The scouring activity of currents removing sediment from around and beneath the edges of coenostea, and small current-controlled movements leaving stromatoporoids imperfectly settled on the uneven substrate or partly overlying skeletal debris, are invoked to explain the presence of cavities and hence encrusters on stromatoporoid lower surfaces. Both processes probably operated on many specimens. The lower surfaces of these stromatoporoids also show basal concavities which range from shallow to deep and reflect the topography of the substrate and the success of stromatoporoids growing on positive features from which they could shed sediment easily. Overturned stromatoporoids and coenostea with deep, encrusted, basal concavities, point to violent environmental events, such as storms, more powerful than currents producing scour and small movements of coenostea.
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Biota
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Silurian stromatoporoid-dominated reefs of Gotland, Sweden, are bioherms and biostromes formed during phases of reduced clastic supply. Well-exposed examples are in the early Wenlock Högklint Formation and middle Ludlow Hemse Group. Reef development culminated in shallow water biostromes, laterally expanded over a stabilized substrate and associated with frequent erosion surfaces. Högklint reefs began as bioherms, to be replaced by biostromal phases as water shallowed, with vertically zoned communities. Biostromal phases are considered equivalent to the whole of individual stromato-poroid biostromes (without biohermal phases) of the Hemse Group, which developed in shallow, low to moderate energy water, on stabilized substrate. Low clastic supply associated with these reefs reduced not only the clay input, which is otherwise common in most non-reef environments on Gotland, but also is interpreted to have lowered available nutrient levels. Modern reefs develop best in conditions of low nutrients and also show (a) suppressed bioerosion, and (b) increased symbiosis between corals and algae which conserves nutrients in the reef biota. Högklint reefs show reduced bioerosion while Hemse reefs show almost complete lack of bioerosion. Symbiosis between stromatoporoids and corals, interpreted as a response to the need for nutrient conservation, occurs in the reefs, but is much more common in Hemse reefs. Both Högklint and Hemse reefs are regarded as having grown in nutrient-deficient conditions in line with modern reefs although the effect on Hemse reefs is believed to have been greater because they appear to have grown in quieter conditions on a broad shelf and may have had reduced access to nutrients in a regressive regime throughout the Ludlow. Reduced clastic supply while the Gotland reefs grew could have been due to regional tectonism controlling local sea-level change. However, the Högklint Formation and Hemse Group reef phases coincide with times of proposed eustatic sea-level falls. Reef growth on Gotland could therefore relate to eustatic tectonism by subduction-related or plate flexure processes, or climatic control. An oceanographic model for the Silurian proposed alternating wet and dry episodes, interpreted as CO 2 -driven, and could have controlled clastic input, permitting the development of fossiliferous reefs and the carbonate platforms on which they formed. These interpretations provide a revised framework for examining the reef growth.
Sedimentation
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Stacked stromatoporoid‐dominated biostromes of the Ludlow‐age Hemse Group (Silurian) in eastern Gotland, Sweden, are 0·5–5 m thick and a few tens of metres to >1 km in lateral extent. They form one of the world's richest Palaeozoic stromatoporoid deposits. This study compiles published and new data to provide an overall facies model for these biostromes, which is assessed in relation to possible modern analogues. Some biostromes have predominantly in‐place fossils and are regarded as reefs, but lack rigid frameworks because of abundant low‐profile non‐framebuilding stromatoporoids; other biostromes consist of stromatoporoid‐rich rudstones interpreted here as storm deposits. Variation between these two `end‐members' occurs both between interlayered biostromes and also vertically and laterally within individual biostromes. Such variation produces problems of applying established reef classification terms and demonstrates the need for the development of terminology that recognizes taphonomic destruction of reef fabrics. An approach to such terminology is found in all four categories of a recent biostrome classification scheme that are easily recognized in the Hemse biostrome facies: autobiostromes (>60% in place); autoparabiostromes (a mixture of in‐place and overturned reef‐building organisms, 20–60% in place); parabiostromes (builders are overturned and damaged, <20% in place); and allobiostromes (transported and detrital reef material, nothing in place). These categories provide a broad taphofacies scheme for the Hemse biostromes, which are mostly autoparabiostrome to allobiostrome. The biostromes developed on crinoidal grainstone sheets and expanded laterally across relatively flat substrates in a marine setting of low siliciclastic input. Planar erosion surfaces commonly terminate biostrome tops. Three broadly similar modern analogues are identified, each of which has elements in common with the Hemse biostromes, but none of which is an exact equivalent: (a) laterally expanded and coalesced back‐barrier patch reefs behind the Belize barrier, an area influenced by limited accommodation space; (b) a hurricane‐influenced shelf, interpreted for Grand Cayman, where reef cores consist of rubble and lack substantial framework; the wide distribution of rounded pebbles and cobbles of stromatoporoids in the Hemse biostromes most probably resulted from hurricanes; (c) coral carpets in 5–15 m water depth of the northern Red Sea, where lateral expansion of low‐diversity frames dominated by Porites coral has produced low‐profile biostromes up to 8 m thick and several km long. Such carpets accumulated large amounts of carbonate, with little export, as in the Hemse biostromes, although the latter did not build frameworks because of the nature of growth of the stromatoporoids. The notable lack of algae in the Hemse biostrome facies is also a feature of Red Sea coral carpets; nevertheless, coral carpets are ecologically different. Hemse biostromes lack evidence of a barrier reef system, although this may not be exposed; the facies assemblage is consistent with either a storm/hurricane‐influenced mid‐ to upper ramp or back‐barrier system.
Grainstone
Devonian
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Paleoecologists studying Paleozoic reef-builders have interpreted their growth forms as responses to conditions of depth and turbulence in reef complexes. Comparison of the shapes of Paleozoic stromatoporoids and corals with the growth forms of modern scleractinians has been used to reconstruct Paleozoic conditions. A review of shape zonation on modern reefs indicates that no general pattern is applicable to all reefs and variations in shape are the result of the interaction of many environmental factors with the genetically dictated growth pattern of the coral. In most zones of a reef a wide range of shapes co-exist. The growth forms of corals on modern reefs are not a simple vegetative response to the many environmental parameters that have been shown to influence form, but are constrained by phylogenetic and developmental influences as well as functional ones. Interpretations of the environments of western Canadian and other mid-Paleozoic reefs have been based on the growth forms of stromatoporoids. The environmental significance of the shapes has been deduced from comparison with the shapes of modern scleractinians, functional morphology, nature of the enclosing sediment, position of growth, position within the reef, and diversity gradients. The validity of these criteria is open to question and considerable doubt remains concerning the significance of the growth forms. The shapes of reef animals are not specific guides to environments of modern reefs and should not be expected to be guides for ancient ones.
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Abstract A stromatoporoid reef in the Ludlovian Sundre Beds at the sea stack field of Holmhällar I on southernmost Gotland was subjected to subaerial exposure during its period of deposition. The sea stack field shows a subcircular belt 500 m in diameter and 50 m wide which exposes part of a reef complex. One distinct erosional surface can be traced through all the localities. The surface cuts the stromatoporoid framework of a lower unit and undulates with a general dip 0–15° away from the centre of the subcircle. On the low‐lying parts of the undulating surface, limestone boulders and sediments containing a number of subordinate erosional surfaces occur. The geometry of the platform results in a complicated facies mosaic of the upper unit composed of stromatoporoid boundstone and its lateral facies, crinoidal debris. The boundstone generally rests on the topographic rises of the inner part of the platform. The crinoidal debris largely occurs on the outer part. Stromatoporoid skeletons occupy 50–65 per cent of volume of the boundstone. This value far exceeds the proportion of framebuilders in Quaternary reefs. The stromatoporoid assemblages are recognized. The assemblage of the boundstone is strongly dominated by Plectostroma scaniense which is laminar‐domical and mostly larger than 50 cm in basal diameter. In the other facies, the commonest species is Parallelostroma typicum which often exhibits a certain growth form as a result of growth on soft bottom and periodical sedimentation.
Paleoecology
Subaerial
Deposition
Devonian
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Silurian reefs of northern Europe occur in cratonic sedimentary sequences which have been relatively well documented stratigraphically and paleontologically although the reefs have generally been less closely examined than the level bottom communities Reef development adjacent to the Caledonian Belt was restricted by siliciclastic sedimentation and this is also reflected in the low proportions of carbonate rocks in the Welsh Borderland and Oslo successions Marine sequences in the Baltic areas of Gotland and Estonia are relatively thin and contain much higher proportions of both carbonates and reefs Four main types of reef are recognizable on Gotland Axelsro previously termed Upper Visby and Hoburgen Reefs are essentially tabulate coral and stromatoporoid dominated bioherms of moderate to high diversity Their dense structure and argillaceous matrix made them locally unstable and prone to marginal collapse and internal displacement Important accessory reef builders include rugose corals calcareous algae Prohlematica and bryo zoans Similar bioherms particularly of the smaller tabulate rich Axelsro type are well developed in the Wenlock Limestone of the Welsh Borderland of England where good examples occur at Wenlock Edge They are also present in the Oslo Region of Norway together with tabulate dominated biostromes and Rothpletzella Wetheredella bioherms and occur at several horizons sometimes very extensively in the Llandovery and Wenlock In Gotland Hoburgen reefs are especially widespread at numerous horizons but a unique feature is the occur rence of Kuppen and Holmhallar type stromatoporoid biostromes which have rigid dense to frame structures and relatively low diversity They are interpreted as shallow water high energy linear reefs which developed prefer entially in the cratonic interior The Estonian Silurian sequence shows close similarities to that in Gotland Reefs are developed at a number of horizons but are generally little documented in detail Reef geometry and organic composition were controlled by environmental factors The size and morphology of the organisms in turn determined the internal structure of the reefs The bioherms show internal displacement and differential compaction of adjacent sediments in response to their own weight and to subsequent overburden The biostromes behaved more rigidly and compaction was taken up mainly by stylolitization of adjacent large skeletons
Siliciclastic
Devonian
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Kershaw, Stephen & Riding, Robert 1978 07 15: Parameterization of stromatoporoid shape. Lethaia, Vol. 11. pp. 233–242. Oslo. ISSN 0024–1164. Stromatoporoid cross-sectional shape can be considered as a product of the interaction of three morphological variables: the relative proportions of basal, vertical and diagonal dimensions. The gross arrangement of internal lamination can be superimposed upon the resulting outlines. These shapes are taken to represent cross sections in any vertical plane through the centre of the coenosteum. This simple parameterization scheme is presented in triangular arrays which include the stromatoporoid morphotypes laminar, domical and bulbous, and varieties of them together with forms not utilized by stromatoporoids. Dendroid and irregular forms are too complex to be readily included in the scheme. Smooth and ragged varieties of laminar and domical forms are distinguished and related to sedimentation on the flanks of the coenosteum. Two types of mutual arrangement of latilaminae within the coenosteum are recognized: enveloping, where they completely overlap previous latilaminae, and non-enveloping, where they do not. Laminar-domical-bulbous forms represent a series generated mainly by reduction of the basal dimension. They commonly exhibit enveloping latilaminae except in ragged varieties and some extended domes. Measurement of defined dimensions in the field allows stromatoporoid morphotypes to be plotted onto the triangular arrays and provides a rapid method of displaying the range of forms present.
Lamination
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