| 1. |
- Novak, B, et al.
(författare)
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Late Pleistocene-early Holocene fluvial facies and depositional processes in the Fehmarn Belt, between Germany and Denmark, revealed by high-resolution seismic and lithofacies analysis
- 2002
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Ingår i: Sedimentology. - : Wiley. - 0037-0746 .- 1365-3091. ; 49:3, s. 451-465
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Tidskriftsartikel (refereegranskat)abstract
- In the narrow offshore border zone between Germany and Denmark, 550 km of high-density 3.5-kHz subbottom seismic reflection profiles were recorded within a 70-km(2) area in order to reconstruct the seismic stratigraphy of late Pleistocene to early Holocene lacustrine and fluvial environments. Using detailed line drawings, seismic facies analyses and a hierarchy of bounding surfaces, a depositional unit was recognized and subdivided into subunits 4(a) (oblique-parallel), 4(b) (mound, oblique-tangential), 4(c) (sigmoid, oblique), 4(d) and 4(e) (shingled and parallel). The base of this seismic facies association defines a wide U-shaped valley with well-defined scours and, in the valley sides, steps are located above deep steep-dipping reflections. Stratigraphic control was available from 32 coring sites (5- to 12-m-deep vibrocores). Subunit 4(b) represents coarsening-up silt and sand, and samples from subunit 4(d) show fining-up fine sand, silt and clay. The seismic facies association is proposed to have formed by a fluvial event of short duration some time in the period between 10.3 C-14 ka BP and 9.0 C-14 ka BP. Subunits 4(a) to 4(e) represent gradually decreasing flow power. A peak flow initiated the fluvial event, after which water discharge and level fell rapidly. Subsequently, the normal background discharge from the Baltic Sea area dominated the flow style. Reflections beneath the step-like valley side with high dip angles are interpreted as faults. This tectonic activity resulted in subsidence in the analysed area and could possibly have influenced the fluvio-dynamic development. The seismic stratigraphic succession reveals a high-resolution record of sediments in this area. In particular, the stepwise uncovering of the morphology of the subunits, preserved in high-resolution seismic facies associations, is proposed as a useful tool in modelling the dynamic development of the near sea-floor environment.
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| 2. |
- Sandström, Olof, et al.
(författare)
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Ludlow (Silurian) stromatoporoid biostromes from Gotland, Sweden: facies, depositional models and modern analogues
- 2002
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Ingår i: Sedimentology. - : Wiley. - 0037-0746 .- 1365-3091. ; 49:3, s. 379-395
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
- Gebhardt, Andrea Catalina, et al.
(författare)
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Seismic evidence of up to 200 m lake-level change in Southern Patagonia since Marine Isotope Stage 4
- 2012
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Ingår i: Sedimentology. - : Wiley. - 0037-0746 .- 1365-3091. ; 59:3, s. 1087-1100
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Tidskriftsartikel (refereegranskat)abstract
- Maar lake Laguna Potrok Aike is located north of the Strait of Magellan (south-eastern Patagonia). Seismic reflection profiles revealed a highly dynamic palaeoclimate history. Dunes were identified in the eastern part of the lake at approximately 30 to 80 m below the lake floor, overlying older lacustrine strata, and suggest that the region experienced dry conditions probably combined with strong westerly winds. It is quite likely that this can be linked to a major dust event recorded in the Antarctic ice cores during Marine Isotope Stage 4. The dunes are overlain by a series of palaeo-shorelines indicating a stepwise water-level evolution of a new lake established after this dry period, and thus a change towards wetter conditions. After the initial, rapid and stepwise lake-level rise, the basin became deeper and wider, and sediments deposited on the lake shoulder at approximately 33 m below present-day lake level point towards a long period of lake-level highstand between roughly 53.5 ka cal. bp and 30 ka cal. bp with a maximum lake level some 200 m higher than the desiccation horizon. This highstand was then followed by a regressional phase of uncertain age, although it must have happened some time between approximately 30 ka cal. bp and 6750 yrs cal. bp. Dryer conditions during the Mid-Holocene are evidenced by a dropping lake level, resulting in a basin-wide erosional unconformity on the lake shoulder. A second stepwise transgression between ca 5.8 to 5.4 ka cal. bp and ca 4.7 to 4 ka cal. bp with palaeo-shorelines deposited on the lake shoulder unconformity again indicates a change towards wetter conditions.
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| 4. |
- Klaminder, Jonatan, et al.
(författare)
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Post-deposition diffusion of 137Cs in lake sediment : implications for radiocaesium dating
- 2012
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Ingår i: Sedimentology. - : Wiley-Blackwell. - 0037-0746 .- 1365-3091. ; 59:7, s. 2259-2267
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Tidskriftsartikel (refereegranskat)abstract
- Peak activities of radiocaesium (137Cs) in lake sediments have frequently been used to infer the ages of sediments deposited in the 1960s (137Cs derived from nuclear bomb testing) or in 1986 (Chernobyl derived 137Cs). Records of the vertical distribution of 137Cs in sediments can thus be used to provide accurate dates for a critical period in which palaeoecological reconstructions often overlap contemporary monitoring data. However, knowledge regarding how the distribution of 137Cs in sediments is affected by post-depositional processes is limited to interpretations based on the 137Cs distribution in sediments sampled at a single given date. This study assesses the extent to which the 137Cs record in annually laminated (varved) lake sediments is affected by post-depositional diffusion, using 11 archived sediment cores sampled between 1986 and 2007. The sediment record reveals how Chernobyl 137Cs incorporated into the 1986 varve diffused downwards in the core at a decreasing rate over time, whereas the surface sediments continued to receive inputs of 137Cs mobilized from the catchment soils or lake margin. In spite of these processes, all cores post-dating the Chernobyl accident had a clear and well-resolved peak in the 1986 varve, justifying the use of this feature as a fixed chronostratigraphic feature. Because of the very high levels of Chernobyl fallout at this site, downwards migration of Chernobyl 137Cs has, however, completely masked the nuclear weapons 137Cs fallout peak that had been clearly preserved in the 1964 varve of a pre-Chernobyl core sampled just three weeks before the Chernobyl accident. In consequence, the weapons fallout marker is likely to be of little use for determining 137Cs dates in areas strongly affected by high levels of Chernobyl fallout.
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| 5. |
- Plink Björklund, Piret, 1969
(författare)
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Stacked fluvial and tide-dominated estuarine deposits in high-frequency (fourth-order) sequences of the Eocene Central Basin, Spitsbergen
- 2005
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Ingår i: Sedimentology. - : Wiley. - 0037-0746 .- 1365-3091. ; 52:2, s. 391-428
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Tidskriftsartikel (refereegranskat)abstract
- Eighteen coastal-plain depositional sequences that can be correlated to shallow- to deep-water clinoforms in the Eocene Central Basin of Spitsbergen were studied in 1 x 15 km scale mountainside exposures. The overall mud-prone (> 300 m thick) coastal-plain succession is divided by prominent fluvial erosion surfaces into vertically stacked depositional sequences, 7-44 m thick. The erosion surfaces are overlain by fluvial conglomerates and coarse-grained sandstones. The fluvial deposits show tidal influence at their seaward ends. The fluvial deposits pass upwards into macrotidal tide-dominated estuarine deposits, with coarse-grained river-dominated facies followed further seawards by high- and low-sinuosity tidal channels, upper-flow-regime tidal flats, and tidal sand bar facies associations. Laterally, marginal sandy to muddy tidal flat and marsh deposits occur. The fluvial/estuarine sequences are interpreted as having accumulated as a series of incised valley fills because: (i) the basal fluvial erosion surfaces, with at least 16 m of local erosional relief, are regional incisions; (ii) the basal fluvial deposits exhibit a significant basinward facies shift; (iii) the regional erosion surfaces can be correlated with rooted horizons in the interfluve areas; and (iv) the estuarine deposits onlap the valley walls in a landward direction. The coastal-plain deposits represent the topset to clinoforms that formed during progradational infilling of the Eocene Central Basin. Despite large-scale progradation, the sequences are volumetrically dominated by lowstand fluvial deposits and especially by transgressive estuarine deposits. The transgressive deposits are overlain by highstand units in only about 30% of the sequences. The depositional system remained an estuary even during highstand conditions, as evidenced by the continued bedload convergence in the inner-estuarine tidal channels.
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| 6. |
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| 7. |
- Tafesse, Solomon, et al.
(författare)
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Evaluation of image analysis methods used for quantification of particle angularity
- 2013
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Ingår i: Sedimentology. - : Wiley. - 0037-0746 .- 1365-3091. ; 60:4, s. 1100-1110
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Tidskriftsartikel (refereegranskat)abstract
- Angularity is an important parameter in the characterization of particle morphology that is used to interpret the transport history of particles in sedimentary deposits. In the past, visual classification using silhouette charts was widely used to determine particle angularity, but this approach is subjective and time-consuming. With advances in modern image analysis techniques and low-cost software packages, it is possible to rapidly quantify particle angularity more objectively than using visual classification methods. This study re-examines the performance of three existing image analysis methods and one new image analysis procedure, applied to six rock and sediment samples that were visually different in angularity. To facilitate comparison between the angularity results, measurements were reduced to rankings for each aggregate sample. These results show that the four image analysis methods rank the angularity of the samples differently, and that none rank the mean angularity index in the same order as the angularity ranking using visual classification. Therefore, further research is needed to develop an image analysis method that can quantify the angularity of sedimentary particles more precisely.
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| 8. |
- Bouton, Anthony, et al.
(författare)
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External controls on the distribution, fabrics and mineralization of modern microbial mats in a coastal hypersaline lagoon, Cayo Coco (Cuba)
- 2016
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Ingår i: Sedimentology. - : Wiley. - 0037-0746 .- 1365-3091. ; 63:4, s. 972-1016
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Tidskriftsartikel (refereegranskat)abstract
- Active, carbonate-mineralizing microbial mats flourish in a tropical, highly evaporative, marine-fed lagoonal network to the south of Cayo Coco Island (Cuba). Hypersaline conditions support the development of a complex sedimentary microbial ecosystem with diverse morphologies, a variable intensity of mineralization and a potential for preservation. In this study, the role of intrinsic (i.e. microbial) and extrinsic (i.e. physicochemical) controls on microbial mat development, mineralization and preservation was investigated. The network consists of lagoons, forming in the interdune depressions of a Pleistocene aeolian substratum; they developed due to a progressive increase in sea-level since the Holocene. The hydrological budget in the Cayo Coco lagoonal network changes from west to east, increasing the salinity. This change progressively excludes grazers and increases the saturation index of carbonate minerals, favouring the development and mineralization of microbial mats in the easternmost lagoons. Detailed mapping of the easternmost lagoon shows four zones with different flooding regimes. The microbial activity in the mats was recorded using light-dark shifts in conjunction with microelectrode O-2 and HS- profiles. High rates of O-2 production and consumption, in addition to substantial amounts of exopolymeric substances, are indicative of a potentially strong intrinsic control on mineralization. Seasonal, climate-driven water fluctuations are key for mat development, mineralization, morphology and distribution. Microbial mats show no mineralization in the permanently submersed zone, and moderate mineralization in zones with alternating immersion and exposure. It is suggested that mineralization is also driven by water-level fluctuations and evaporation. Mineralized mats are laminated and consist of alternating trapping and binding of grains and microbially induced magnesium calcite and dolomite precipitation. The macrofabrics of the mats evolve from early colonizing Flat mats to complex Cerebroid or Terrace structures. The macrofabrics are influenced by the hydrodynamic regime: wind-driven waves inducing relief terraces in windward areas and flat morphologies on the leeward side of the lagoon. Other external drivers include: (i) storm events that either promote (for example, by bioclasts covering) or prevent (for example, by causing erosion) microbial mat preservation; and (ii) subsurface degassing, through mangrove roots and desiccation cracks covered by Flat mats (i.e. forming Hemispheroids and Cerebroidal structures). These findings provide in-depth insights into understanding fossil microbialite morphologies that formed in lagoonal settings.
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| 9. |
- Fielding, Christopher, et al.
(författare)
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Sedimentology of the continental end-Permian extinction eventin the Sydney Basin, eastern Australia
- 2021
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Ingår i: Sedimentology. - : International Association of Sedimentologists. - 0037-0746 .- 1365-3091. ; 68, s. 30-62
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Tidskriftsartikel (refereegranskat)abstract
- Upper Permian to Lower Triassic coastal plain successions of the Sydney Basin in eastern Australia have been investigated in outcrop and continuous drillcores. The purpose of the investigation is to provide an assessment of palaeoenvironmental change at high southern palaeolatitudes in a continental margin context for the late Permian (Lopingian), across the end‐Permian Extinction interval, and into the Early Triassic. These basins were affected by explosive volcanic eruptions during the late Permian and, to a much lesser extent, during the Early Triassic, allowing high‐resolution age determination on the numerous tuff horizons. Palaeobotanical and radiogenic isotope data indicate that the end‐Permian Extinction occurs at the top of the uppermost coal bed, and the Permo‐Triassic boundary either within an immediately overlying mudrock succession or within a succeeding channel sandstone body, depending on locality due to lateral variation. Late Permian depositional environments were initially (during the Wuchiapingian) shallow marine and deltaic, but coastal plain fluvial environments with extensive coal‐forming mires became progressively established during the early late Permian, reflected in numerous preserved coal seams. The fluvial style of coastal plain channel deposits varies geographically. However, apart from the loss of peat‐forming mires, no significant long‐term change in depositional style (grain size, sediment‐body architecture, or sediment dispersal direction) was noted across the end‐Permian Extinction (pinpointed by turnover of the palaeoflora). There is no evidence for immediate aridification across the boundary despite a loss of coal from these successions. Rather, the end‐Permian Extinction marks the base of a long‐term, progressive trend towards better‐drained alluvial conditions into the Early Triassic. Indeed, the floral turnover was immediately followed by a flooding event in basinal depocentres, following which fluvial systems similar to those active prior to the end‐Permian Extinction were re‐established. The age of the floral extinction is constrained to 252.54 ± 0.08 to 252.10 ± 0.06 Ma by a suite of new Chemical Abrasion Isotope Dilution Thermal Ionization Mass Spectrometry U‐Pb ages on zircon grains. Another new age indicates that the return to fluvial sedimentation similar to that before the end‐Permian Extinction occurred in the basal Triassic (prior to 251.51 ± 0.14 Ma). The character of the surface separating coal‐bearing pre‐end‐Permian Extinction from coal‐barren post‐end‐Permian Extinction strata varies across the basins. In basin‐central locations, the contact varies from disconformable, where a fluvial channel body has cut down to the level of the top coal, to conformable where the top coal is overlain by mudrocks and interbedded sandstone–siltstone facies. In basin‐marginal locations, however, the contact is a pronounced erosional disconformity with coarse‐grained alluvial facies overlying older Permian rocks. There is no evidence that the contact is everywhere a disconformity or unconformity.
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| 10. |
- Fontana, Simone, et al.
(författare)
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Fluid-rock interactions associated with regional tectonics and basin evolution
- 2014
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Ingår i: Sedimentology. - : Wiley. - 0037-0746 .- 1365-3091. ; 61:3, s. 660-690
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Forskningsöversikt (refereegranskat)abstract
- An integrated approach consisting of fracture analysis, petrography, carbon, oxygen and strontium-isotope analyses, as well as fluid-inclusion micro-thermometry, led to a better understanding of the evolution of fluid-rock interactions and diagenesis of the Upper Permian to Upper Triassic carbonates of the United Arab Emirates. The deposited carbonates were first marked by extensive early dolomitization. During progressive burial, the carbonates were affected by dolomite recrystallization as well as precipitation of vug and fracture-filling dolomite, quartz and calcite cements. After considerable burial during the Middle Cretaceous, sub-vertical north-south oriented fractures (F1) were cemented by dolomite derived from mesosaline to hypersaline fluids. Upon the Late Cretaceous maximum burial and ophiolite obduction, sub-vertical east-west fractures (F2) were cemented by dolomite (Dc2) and saddle dolomite (Ds) derived from hot, highly saline fluids. Then, minor quartz cement has precipitated in fractures from hydrothermal brines. Fluid-inclusion analyses of the various diagenetic phases imply the involvement of increasingly hot (200 degrees C) saline brines (20 to 23% NaCl eq.). Through one-dimensional burial history numerical modelling, the maximum temperatures reached by the studied rocks are estimated to be in the range of 160 to 200 degrees C. Tectonically-driven flux of hot fluids and associated diagenetic products are interpreted to have initiated during the Late Cretaceous maximum burial and lasted until the Oligocene-Miocene compressional tectonics and related uplift. The circulation of such hydrothermal brines led to partial dissolution of dolomites (Dc2 and Ds) and to precipitation of hydrothermal calcite C1 in new (mainly oriented north-south; F3) and pre-existing, reactivated fractures. The integration of the obtained data confirms that the diagenetic evolution was controlled primarily by the interplay of the burial thermal evolution of the basin and the regional tectonic history. Hence, this contribution highlights the impacts of regional tectonics and basin history on diagenetic processes, which may subsequently affect reservoir properties.
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