| 1. |
- Ahlberg, Anders, et al.
(författare)
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Triassic-Jurassic weathering and clay mineral dispersal in basement areas and sedimentary basins of southern Sweden
- 2003
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Ingår i: Sedimentary Geology. - 0037-0738. ; 161:1-2, s. 15-29
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Tidskriftsartikel (refereegranskat)abstract
- The basement rocks of southern Sweden were deeply weathered in Late Triassic through Cretaceous times, and possibly even earlier. Sub-Mesozoic saprolites formed, presumably due to long-term warmth and humidity, in conjunction with decomposition of abundant organic matter and subsequent infiltration of acidic water. The upper parts of the weathering profiles were subjected to intense leaching and kaolinitisation whereas the less evolved deep weathering front is enriched in smectite. Thus. any erosion prior to full regolith development, or extraordinary deep erosion, released less developed, smectite-rich weathering material to the Late Triassic-Jurassic receiving basins. Mesozoic sedimentary successions in Southern Sweden show high detrital smectite and/or kaolinite contents, moderate illite contents and low chlorite contents, compatible with expected mid-latitude warm equable greenhouse conditions. Norian and older Triassic haematite-cemented continental arkoses and smectite-dominated clays (smectitemuch greater thankaolinite) were clearly formed in and settings. Front the Rhaetian and throughout the Jurassic, humid onshore conditions sustained abundant plant and peat accumulation, early diagenetic meteoric flushing, and intense chemical weathering. Kaolinite is therefore a dominating detrital clay mineral in these sedimentary successions (kaolinite = illitemuch greater thanchlorite and smectite (I/S)). Minor quantitative variations in clay mineralogy within the "humid climate" Rhaetian-Jurassic sedimentary successions are difficult to interpret. In deltaic coal-bearing successions, the detrital clay mineral composition is conspicuously constant (kaolinite = illite-chlorite), possibly due to post-depositional clay mineral transformation. Conversely, in shallow marine deposits, some quantitative variation is notable (kaolinite approximate to illite>variable smectitemuch greater thanchlorite). This may reflect that the original detrital composition was better preserved in shallow marine settings. Smectitic clay minerals associated with gypsum and caliche nodules ambiguously indicate increased aridity towards the end of the Jurassic. (C) 2002 Elsevier Science B.V. All rights reserved.
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| 2. |
- Calner, Mikael
(författare)
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A lowstand epikarstic intertidal flat from the middle Silurian of Gotland, Sweden
- 2002
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Ingår i: Sedimentary Geology. - 0037-0738. ; 148:3-4, s. 389-403
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Tidskriftsartikel (refereegranskat)abstract
- This 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.
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| 3. |
- Fredin, Ola, et al.
(författare)
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Relict lateral moraines in northern Sweden : evidence for an early mountain centered ice sheet
- 2002
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Ingår i: Sedimentary Geology. - 0037-0738 .- 1879-0968. ; 149:1-3, s. 145-156
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Tidskriftsartikel (refereegranskat)abstract
- Glacial geomorphology along the eastern rim of the Scandinavian mountain range includes glacial landforms from the last deglaciation as well as from earlier glacial stages. One of the most prominent landform groups from earlier glacial stages, and the most diagnostic for ice sheet reconstruction, is a set of lateral moraines. In this paper, we describe these lateral moraines within a key area around Kvikkjokk, northern Sweden. Position of these lateral moraines in relation to the last deglaciation patterns indicates that they were formed before the last glacial maximum (LGM). The location and morphology of moraines show that they were deposited by a mountain centred ice sheet with outlet glaciers along major valleys, emanating from the highlands west of the Kvikkjokk area. This ice sheet was likely less than 170-km wide and no more than 600-m thick. Climatologically and glaciologically, we expect the relict lateral moraines to have been deposited before 75 ka BP (marine oxygen isotope stage 4). Their preservation is a consequence of subsequent overriding of nonerosive cold-based ice. Ice-marginal landforms and deposits from mountain centred ice sheet configurations in Fennoscandia are scarce. Therefore, the relict lateral moraines are important tools for reconstructing these elusive early glacial stages, possibly correlated to the ice sheet inception.
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| 4. |
- Ahn, Soo Yeun, et al.
(författare)
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Microorganism-mediated preservation of Planolites, a common trace fossil from the Harkless Formation, Cambrian of Nevada, USA
- 2012
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Ingår i: Sedimentary Geology. - : Elsevier BV. - 0037-0738. ; 263, s. 30-35
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Tidskriftsartikel (refereegranskat)abstract
- Specimens of Planolites, a tubular ichnofossil, from the Harkless Formation (Cambrian of Nevada) are commonly rimmed by thin layers of limonite, a probable pseudomorph after pyrite, suggesting that bacterially induced biomineralization played a key role in the preservation of the burrows. Organic material secreted by a vermiform, infaunal tracemaker is inferred to have formed the substrate for a biofilm that facilitated iron sulfide biomineralization by bacteria under localized anaerobic or dysaerobic conditions. Rapid, microbially mediated biomineralization of mucus-lined burrow walls is inferred to have promoted three-dimensional preservation of burrows by strengthening them against sediment compaction, and possibly providing sufficient support for burrows to allow time for sediment infill. Decomposition of organic materials in Planolites also left evidence in sediment layers immediately above the burrows: undulose sediment surfaces and small, round, pimpleform gas escape structures. (C) 2011 Elsevier B.V. All rights reserved.
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| 5. |
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| 6. |
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| 7. |
- Bábek, Ondřej, et al.
(författare)
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Redox geochemistry of the red ‘orthoceratite limestone’ of Baltoscandia : Possible linkage to mid-Ordovician palaeoceanographic changes
- 2021
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Ingår i: Sedimentary Geology. - : Elsevier BV. - 0037-0738. ; 420
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Tidskriftsartikel (refereegranskat)abstract
- The orthoceratite limestone of the Ordovician epicontinental sea of Baltoscandia is one of the oldest Phanerozoic examples of pelagic marine red beds (MRBs). Being enriched in authigenic haematite, MRBs are considered to be sensitive palaeoceanographic redox indicators. In this paper, the origin of the reddening of the Ordovician MRBs and its timing and redox conditions were addressed at two sections, and in a drill core in the Kinnekulle area, Sweden, through the application of diffuse reflectance spectroscopy, sedimentary petrology (microfacies and electron microprobe), bulk-rock and in-situ element geochemistry (laser-ablation ICP-MS) and molybdenum stable isotope systematics. Enrichment in haematite (up to ~0.05 wt%) occurred during very early diagenesis under low sedimentation rates (~5 mm/kyr), which is comparable to several examples of Phanerozoic MRBs. The reddening was associated with the mm-scale, in-situ mobility of Fe, Mn, As, Mo and U, due to Fe–Mn redox cycling between primary and secondary minerals and pore water under oxic and suboxic to anoxic conditions; it was not related to significant changes in seawater chemistry. Stratigraphic red-to-grey transitions likely coincided with two mid-Darriwilian global regressions (the so-called Täljsten and at the base of the Gullhögen Formation), likely due to the effects of changing sedimentation rates and sub-bottom redox potentials. The Ordovician MRBs coincided with a positive δ13C excursion during the middle Darriwilian, which is different to MRB examples cited from the Devonian to the Cretaceous age, which frequently coincide with shifts to lower values of δ13C. Although MRBs are believed to show a time-specific occurrence in greenhouse or transitional greenhouse-to-icehouse climatic modes by several authors, our study suggests that causal links between MRBs and global carbon cycle remain unclear.
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| 10. |
- Egenhoff, Sven, et al.
(författare)
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Sedimentology and sequence stratigraphy of a pronounced Early Ordovician sea-level fall on Baltica - The Bjorkasholmen Formation in Norway and Sweden
- 2010
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Ingår i: Sedimentary Geology. - : Elsevier BV. - 0037-0738 .- 1879-0968. ; 224:1-4, s. 1-14
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Tidskriftsartikel (refereegranskat)abstract
- The Bjorkasholmen Formation consists of interbedded carbonates, shales, and glauconitic beds and is characterized by heavy bioturbation and few preserved sedimentary structures. The unit shows five facies shale, glauconitic packstone, and three predominantly mud-dominated carbonate facies. Carbonates and shales are arranged in small-scale deepening-upward cycles. A minimum of fourteen of these small-scale cycles are recognized in the Bjorkasholmen Formation. They are arranged in stacks of 3 to 5, forming a total of four medium-scale cycles separated by decimeter-thick shale units. Based on the predominance of mud-rich facies the succession is interpreted to have been deposited in an overall tranquil setting during one mayor sea-level fall and subsequent initial rise of third order. Time-estimates suggest that the 14 small-scale cycles fall into the Milankovitch band of precessional forcing, and the overriding medium-scale cycles likely represent short eccentricity. The sequence stratigraphic interpretation shows that the Bjorkasholmen Formation is characterized by failing stage, lowstand and initial transgressive systems tracts. Consequently, the contact between the Bjorkasholmen and the underlying Alum Shale Formation represents the basal surface of forced regression. The maximum regressive surface is defined by a hiatus in the Oland sections and by shallow-marine packstones within mud-rich distal ramp carbonates in Norway. The top of the Bjorkasholmen Formation represents a flooding surface at the base of the transgressive systems tract. A comparison of time-equivalent successions worldwide suggests that the Bjorkasholmen Formation represents a tectonically-enhanced lowstand with two overriding short-term Milankovitch eustatic signals. Although deposition of the Bjorkasholmen Formation coincides with the initiation of a foreland basin in the Caledonides of Norway it remains unclear how these tectonic movements may have lead to the widespread Bjorkasholmen lowstand during the Early Ordovician. It is Suggested in this Study that a combination of compressional forces from Avalonia and the Caledonian margin may have acted in concert to produce an uplift of larger parts of the Baltica plate for a time-span of approximately 0.5 Myr.
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