29 research outputs found

    Lateral discrepancies of cycle patterns in the LatemĂ r, Triassic, Italian Dolomites

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    The well-known cyclic carbonate succession of the Middle Triassic Latemàr Massif in the Italian Dolomites reveals significant lateral variability in cycle numbers in platform-interior strata. Within an interval of 60 m, a 25% increase in the number of marine flooding surfaces was detected when approaching the several-hundred-meters-wide tepee belt in the backreef area, which represents the maximum elevation of the isolated Latemàr buildup. The impact of high-frequency– low-amplitude sea-level fluctuations on this elevated zone resulted in the development of spatially restricted intermittent emergence and marine flooding surfaces bounding small-scale upward-shallowing cycles. It is postulated that these alternations of submergence and subaerial exposure have favored tepee formation. Sediment collecting in the saucer-shaped tepee megapolygons further expedited upward shallowing of small-scale cycles. Conversely, deeper parts of the lagoon remained largely unaffected by high-frequency, lowamplitude sea-level oscillations: marine flooding surfaces disappear and cycles amalgamate. It is concluded that tepee structures are generally confined to topographically elevated areas where low-amplitude sea-level fluctuations were recorded. Lateral variations in cycle stacking pattern should be commonplace in shallow carbonate buildups throughout the geological record, where paleorelief existed in the platform interior

    Sea level changes versus hydrothermal diagenesis: Origin of Triassic carbonate platform cycles in the Dolomites, Italy: Discussion

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    In his recent paper, Blendinger (2004) presents a new and unconventional hypothesis for the formation of the Triassic Latemàr carbonate cycles. He assigned the cycles to intermittent hydrothermal influence alternating with normal marine deposition. Fluids with a composition similar to normal seawater were forced by elevated heat flow from an underlying hydrothermal field to circulate to the seafloor. These fluids are said to produce stratiform diagenetic features including tepees, and they favoured early dolomitization and the growth of cyanobacterial mats. This interpretation deviates strongly from the conventional models where these cycles are considered to be the result of relative sea-level fluctuations in a shallow-marine to subaerially exposed environment (Hardie et al., 1986, Goldhammer et al., 1987, Goldhammer et al., 1990 R.K. Goldhammer, P.A. Dunn and L.A. Hardie, Depositional cycles, composite sea-level changes, cycle stacking patterns, and the hierarchy of stratigraphic forcing: examples from Alpine Triassic platform carbonates, Geol. Soc. Amer. Bull. 102 (1990), pp. 535–562.Goldhammer et al., 1990, Goldhammer et al., 1993, Brack et al., 1996, Mundil et al., 1996, Egenhoff et al., 1999, Preto et al., 2001, Mundil et al., 2003 and Zühlke et al., 2003). We welcome this entirely new approach, as it further stimulates a discussion on the validity of criteria used to detect subaerial exposure from carbonate facies. Furthermore, it highlights the importance of microbial growth and cementation processes for the development and the geometrical maintenance of post-extinction carbonate platforms such as the Latemàr

    Lateral discrepancies of cycle patterns in the LatemĂ r, Triassic, Italian Dolomites

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    The well-known cyclic carbonate succession of the Middle Triassic Latemàr Massif in the Italian Dolomites reveals\ud significant lateral variability in cycle numbers in platform-interior strata. Within an interval of 60 m, a 25% increase in the number\ud of marine flooding surfaces was detected when approaching the several-hundred-meters-wide tepee belt in the backreef area,\ud which represents the maximum elevation of the isolated Latemàr buildup. The impact of high-frequency– low-amplitude sea-level\ud fluctuations on this elevated zone resulted in the development of spatially restricted intermittent emergence and marine flooding\ud surfaces bounding small-scale upward-shallowing cycles. It is postulated that these alternations of submergence and subaerial\ud exposure have favored tepee formation. Sediment collecting in the saucer-shaped tepee megapolygons further expedited upward\ud shallowing of small-scale cycles. Conversely, deeper parts of the lagoon remained largely unaffected by high-frequency, lowamplitude\ud sea-level oscillations: marine flooding surfaces disappear and cycles amalgamate. It is concluded that tepee structures\ud are generally confined to topographically elevated areas where low-amplitude sea-level fluctuations were recorded. Lateral\ud variations in cycle stacking pattern should be commonplace in shallow carbonate buildups throughout the geological record,\ud where paleorelief existed in the platform interior

    Introduction

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    The complexity of mudstone diagenesis–some insight from the Tøyen Shale, Lower to Middle Ordovician, southern Sweden

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    The Lower to Middle Ordovician Tøyen Shale in southern Sweden, a biostratigraphically well-dated siliciclastic mudstone unit, shows 18 distinct authigenic cements that include sulfides, carbonates, silicates, clays, and phosphates. Marcasite, sphalerite, galena, and six texturally distinct types of pyrite characterize the sulfides whereas only one type of dolomite and three different generations of calcite are observed in this unit. Quartz, phosphate, and organic matter occur as only one generation each. Authigenic clay minerals are represented by chlorite and kaolinite. The paragenetic sequence of cements is subdivided into the two pre-burial carbonates, succeeded by ten relatively early burial cements, and six late burial cements, the kaolinite being the latest of them all and potentially being of Cretaceous age. Based on textural relationships, the paragenetic sequence of alterations started with dolomite precipitation followed by calcite, and then five different generations of pyrite. All eleven other phases post-date these initial seven cements in the Tøyen Shale

    Pyritization history in the late Cambrian Alum Shale, Scania, Sweden: Evidence for ongoing diagenetic processes

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    Detailed diagenetic studies of the late Cambrian Alum Shale in southern Sweden were undertaken across an interval that includes the peak Steptoean Positive Carbon Isotope Excursion (SPICE) event to evaluate the pyrite mineralization history in the formation. Samples were collected from the Andrarum-3 core (Scania, Sweden); here the Alum was deposited in the distal, siliciclastic mudstone-rich end of a shelf system. Abundant cryptobioturbation is observed in the Alum, which points to oxic–dysoxic conditions prevailing during deposition. Petrographic examination of polished thin sections (n = 65) reveals the presence of numerous texturally distinct types of pyrite, including matrix framboids, two different types of framboid concretions (those with rims of iron-dolomite and those lacking rims), disseminated euhedral pyrite crystals, concretions of euhedral pyrite crystals, overgrowths of pyrite on these different pyrite generations, anhedral pyrite intergrown with bedding parallel mineralized fractures (i.e., “beef”), and massive vertical/subvertical accumulations of pyrite.Paragenetic relationships outline the relative timing of formation of the texturally distinct pyrite. Framboids and framboid concretions formed prior to precipitation of any euhedral pyrite crystals, and these pyrite generations precipitated prior to the pyrite overgrowths on them. As Alum Shale sediments are all distorted by these texturally different pyrite generations, they are likely to have formed early in the postdepositional history of the formation. In contrast, pyrite associated with “beef” is likely temporally related to the onset of hydrocarbon generation, which in this part of Sweden is thought to have been many tens of millions of years after deposition. Because vertical/subvertical massive pyrite features distort “beef,” they clearly postdate it. Of all these pyrite textures, only framboid concretions appear to be restricted to the SPICE interval.The texturally distinct nature of the pyrite generations, along with evidence of their formation at different times in the postdepositional history of the Alum Shale, is the key outcome of this petrographic study. Because the petrographic data presented herein point to a postdeposition origin for all generations of pyrite, diagenetic processes—not those processes associated with deposition—were responsible for the complex pyritization history observed in the Alum, in the Andrarum-3 core

    Sedimentology of SPICE (Steptoean positive carbon isotope excursion): A high-resolution trace fossil and microfabric analysis of the middle to late Cambrian Alum Shale Formation, southern Sweden

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    The Cambrian Alum Shale Formation in the Andrarum-3 core from Scania,southern Sweden, consists of black siliciclastic mudstone with minor carbonate intercalations. Four facies comprise three siliciclastic mudstones and one fine-grained carbonate. The facies reflect deposition along a transect from deep ramp to basin on a Cambrian shelf. The three mudstone facies contain abundant clay clasts and laterally variable siltstone laminae. Bed-load transport processes seem to have dominated deposition on this deep shelf. These sedimentary rocks record mainly event deposition, and only relatively few, thin laminae probably resulted from suspension settling. The Alum Shale Formation deep shelf did not show a bioturbation gradient, but fecal strings are common and Planolites burrows are rare in all mudstone facies. Evidence for biotic colonization indicates that this mudstone environment was not persistently anoxic, but rather was most likely intermittently dysoxic. The Alum Shale Formation in the Andrarum-3 core shows an overall decrease of grain size, preserved energy indicators, and carbonate content upsection interpreted to reflect a deepeningupward. The succession can also be divided into four small-scale fining-upwardcycles that represent deepening, and four overlying coarsening-upward cycles that represent upward shallowing

    New Ordovician-Silurian drill cores from the Siljan impact structure in central Sweden: An integral part of the Swedish Deep Drilling Program

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    New drill cores from the largest known impact structure in Europe, the relict of the Siljan meteorite crater, provide new possibilities to reconstruct Early Palaeozoic marine environments and ecosystems, and to document changes in sedimentary facies, sea level and palaeoclimate in Baltoscandia. The impact crater is an important target of the project "Concentric Impact Structures in the Palaeozoic" within the framework of the "Swedish Deep Drilling Program". Two core sections, Mora 001 and Solberga 1, have been analysed. The sedimentary successions of these core sections include strata of late Tremadocian through late Wenlock ages. Our preliminary studies show not only that several of the classical Palaeozoic units of Sweden are represented in the area, but also that other significantly different facies are preserved in the Siljan district. An erosional unconformity representing a substantial hiatus occurs between Middle Ordovician limestone and a Llandovery-Wenlock (Silurian) shale succession in the western part of the Siljan structure and suggests an extended period of uplift and erosion. This may be related to forebulge migration due to flexural loading by the Caledonian thrust sheet to the west. Thus, this part of Sweden, previously regarded as a stable cratonic area, presumably was affected by the Caledonian collision between Baltica and Laurentia. © 2012 Copyright Geologiska Föreningen
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