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Zebra dolomitization as a result of focused fluid flow in the Rocky Mountains Fold and Thrust Belt, Canada 总被引：4，自引：0，他引：4
VEERLE VANDEGINSTE RUDY SWENNEN SARAH A. GLEESON† ROB M. ELLAM‡ KIRK OSADETZ§ FRANÇOIS ROURE¶ 《Sedimentology》2005,52(5):1067-1095
Discordant zebra dolomite bodies occur locally in the Middle Cambrian Cathedral and Eldon Formations of the Main Ranges of the Canadian Rocky Mountains Fold and Thrust Belt. They are characterized by alternating dark grey (a) and white (b) bands, forming an ‘abba’ diagenetic cyclicity. These bands developed parallel to both bedding and cleavage. Dark grey (a) bands consist of fine (< 300 μm) non-planar crystalline impure dolomite. The white (b) bands are composed of coarse (up to several millimetres) milky-white pure saddle dolomites (b1) which are often covered by pore-lining zoned dolomite (b2). The b phases often possess a saddle-shaped morphology. In contrast to the replacement origin of the a dolomite, the zoned b2 dolomite rims are interpreted as a cement formed in open cavities. The b1 dolomite is interpreted as the result of recrystallization with diagenetic leaching of non-carbonate components. All the zebra dolomites studied are (nearly) stoichiometric and are characterized by enriched Na and depleted Sr concentrations. Fe and Mn concentrations in these dolomites differ depending on the sample locality. Fluid inclusion data indicate that the dolomites formed from relatively hot (TH = 130–200 °C), saline (20–23 wt% CaCl2 eq.) fluids. A diagenetic high temperature origin is also supported by depleted δ18O values (−20 to −14‰ VPDB). A contribution of 87Sr-enriched fluids is reflected in the 87Sr/86Sr values (0·7091–0·7123). Zebra dolomite development is explained by focused fluid flow, which exploited areas of structural weaknesses (e.g. basin-platform, rim areas, faults, etc.). Expulsion of hot basinal brines in a tectonically active regime generated overpressures, which explains the development of secondary porosity during zebra dolomitization as well as the intra-zebra fracturing at decimetre to micrometre scale. 相似文献
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The S1 pedocomplex, correlated to Oxygen Isotope Stage 5 of the ocean cores, has been traced west of the Loess Plateau on to the extreme northeastern margin of the Qinghai-Xizang (Tibetan) Plateau in China. Detailed micromorphological analysis of closely spaced thinsections from the pedocomplex at this site provides the basis for reconstruction of the sequence of pedosedimentary processes and associated palaeoenvironmental changes occurring during this time interval. The six pedosedimentary stages identified are interpreted in terms of temporal variations in depositional rates, size of particles transported and availability of moisture for pedogenic alteration, as determined by changing balances in dominance of winter and summer monsoonal forces. Two main ‘soil-forming intervals’ are identified: the more extensive of the two in terms of resultant pedological features reflects pedogenic alteration at a relatively ‘stable’ land surface under a semi-arid climate during the later part of pedosedimentary stage 2. The other main period corresponds to pedosedimentary stages 4 and 5, when leaching and bioturbation processes were active at aggrading surfaces, leading to development of an accretionary unit without clear differentiation of horizons. Pedosedimentary stages 1, 3 and 6 were characterised mainly by rapid rates of coarse loess accumulation, with synsedimentary modification restricted to surface slaking and crust formation, and minor localised redistribution of calcite. 相似文献
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DAVID R. MARCHANT GEORGE H. DENTON JAMES G. BOCKHEIM SCOTT C. WILSON REW R. KERR 《Boreas: An International Journal of Quaternary Research》1994,23(1):29-43
Glacial drifts perched alongside outlet glaciers that drain through the Transantarctic Mountains constrain inland polar plateau ice elevations. The Taylor Glacier, which heads in the Taylor Dome (a peripheral dome of the East Antarctic Ice Sheet), drains East Antarctic ice into the Dry Valleys sector of Transantarctic Mountains and terminates in central Taylor Valley, about 24 km west of the Ross Sea. Five gravel-rich drifts (including 39 distinct moraine ridges) fringe a lateral lobe of the Taylor Glacier in the lower Arena Valley, Quartermain Mountains, southern Victoria Land. 3 He and 10 Be exposure age dating (from Brook et al . 1992), together with Arena Valley stratigraphy and soil morphologic data, provide chronologic control for these drifts and constrain maximum Quaternary thickening of the inland Taylor ice dome to less than 160 m. These minor Quaternary expansions of Taylor Glacier were out-of-phase with outlet glaciers that pass through the Transantarctic Mountains and terminate in the Ross Sea north and south of the Dry Valleys region. Textural analyses suggest that drift deposition occurred from cold-based ice, even though Taylor Glacier advances most likely occurred during global interglaciations. The thermal regime of former Taylor Glacier ice lobes, the character of geomorphic features superimposed on individual drifts, the chemical composition of soils developed on Taylor drifts, and the stability of in situ moraine ridges on steep valley walls suggest that the present cold-desert climate in Arena Valley has persisted for at least the last 2.2 Ma. 相似文献
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Diagenetic signatures of stratal surfaces in the Upper Jurassic Fulmar Formation, Central North Sea, UKCS 总被引：2，自引：0，他引：2
FIONA E. BURNS STUART D. BURLEY ROB L. GAWTHORPE JOHN E. POLLARD 《Sedimentology》2005,52(6):1155-1185
Sedimentation rate and changes in relative sea level affect early diagenetic cementation along key stratal surfaces within the deeply buried Upper Jurassic Fulmar Formation, South Central Graben, UKCS. As a result of the bioturbated nature of the shallow marine strata, variations in ichnofabrics and cross-cutting relationships between trace fossils are important in identifying and correlating stratal surfaces. Two transgressive and two regressive surfaces/sequence boundaries were studied, using petrographical, mineralogical and geochemical techniques. Cements, including early diagenetic pyrite framboids, grain-rimming apatite, microcrystalline dolomite and ankerite, occur at, above and immediately below both transgressive and forced regressive surfaces/sequence boundaries. Breaks or subdued rates of sedimentation associated with both types of surfaces meant that the sediment package resided within early diagenetic zones for prolonged periods of time, enhancing diagenetic reactions at and below the surfaces. The distribution of ankerite, despite being a deep-burial cement, was primarily controlled by concentrations of bioclasts that are particularly abundant at transgressive surfaces. The diagenetic character of the forced regressive surfaces/sequence boundaries is more complex than that of the transgressive surfaces, both in terms of mineralogy and paragenesis. This is attributed to the superimposition of diagenetic assemblages as a result of erosion, re-sedimentation and sediment by-passing. Although the diagenetic signature of the surfaces has been modified by dissolution/replacement reactions during deep burial, early diagenetic signatures can still be distinguished. 相似文献
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Silurian pinnacle reefs in the subsurface of the south‐western Ontario portion of the Michigan Basin display a variety of laminated carbonates (laminites) within predominantly muddy reef‐capping facies in the upper part of the Guelph Formation and the overlying A‐1 Carbonate of the Salina Group. Laminites, which are limestone, dolomite or partially dolomitized limestones, have a range of morphologies, from simple planar to a variety of wavy and serrated forms. Individual laminae are composed mainly of micrite, microspar or replacive dolomite, and vary internally from isopachous and continuous over the diameter of the core to non‐isopachous and often discontinuous. Clotted and peloidal micrite, sometimes defining small knobs and chambers, is interpreted as being microbial in origin and occurs within all types of laminites. Fibrous cement locally comprises laminite clasts in breccias or coats clasts in breccias, and also occurs as spherulites in the interparticle spaces in breccias. Although similar laminites have been described from elsewhere in the Michigan Basin and interpreted as caliche, travertine and abiotic subtidal stromatolites, the laminites in south‐western Ontario are most realistically regarded as microbial. The causes for the variations in morphology and characteristics of the constituent laminae are uncertain, although fluctuations in local microenvironmental conditions would have been important, set against a backdrop of an increasingly restricted overall setting. Caliche or travertine origins for these laminites are unlikely in general, except perhaps locally at the subaerial exposure surface at the tops of pinnacle reefs. 相似文献
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ROB WESTAWAY 《Boreas: An International Journal of Quaternary Research》2009,38(2):261-284
Prior to its disruption during the Anglian glaciation (MIS 12), the Ingham or Bytham River used to flow eastwards across central England and East Anglia into the southern North Sea. It thus had a much larger catchment than any extant river system in Britain; its headwaters may well have been as far away as North Wales and/or NW England. Terrace deposits of this former river system crop out across East Anglia and, as for any other river, can be used to investigate uplift, landscape evolution and the physical properties of the underlying continental crust. However, such an investigation has hitherto been hampered by inconsistencies between different authors' terrace schemes; furthermore, and controversially, one such scheme has formed the basis for the inference that the region was affected by a pre‐Anglian (MIS 16) glaciation. By re‐examining the raw data, the Ingham River deposits are shown to be disposed in three terraces, inferred to date from MIS 16, 14 and 12. The evidence previously attributed to pre‐Anglian glaciation is associated with the youngest of these terraces, and thus marks the MIS 12 (i.e. Anglian) glaciation; the argument for glaciation of the region in MIS 16 is thus an artefact of previous miscorrelation of the terrace deposits. It is inferred that development of the very large Ingham River was synchronous with decapitation of the former ‘Greater Thames’, or ‘High‐level Kesgrave Thames’ river, some time between MIS 18 and MIS 16. Uplift histories at representative localities across East Anglia have been modelled using composite data sets, combining the terrace deposits of the Ingham River and of the post‐Anglian rivers Lark and Waveney. The sites modelled are typefied by much faster uplift in the early Middle Pleistocene than in the late Middle Pleistocene; this effect is shown to be a consequence of the relative thinness (no more than ～7–8 km thick) of the mobile lower‐crustal layer, itself a consequence of the low surface heat flow in the London Platform crustal province. The post‐Early Pleistocene uplift tapers eastward, consistent with the observed downstream convergence of the Ingham and Waveney terraces, and is close to zero near the modern coastline around Lowestoft and Great Yarmouth. Stratigraphic relationships between the Ingham terrace deposits and temperate‐stage marine and terrestrial deposits in this coastal area allow sites to be dated; thus, Pakefield and Corton date from MIS 15, whereas Norton Subcourse dates from MIS 17. The oldest known Lower Palaeolithic sites in the region, characterized by flake artefacts, are Pakefield (MIS 15) and Hengrave (?MIS 14); younger pre‐Anglian sites that have yielded handaxes and/or fossil material of the water vole Arvicola cantiana date from MIS 13. The minimal vertical crustal motion in this coastal area, where temperate‐stage deposits from different climate cycles crop out close to present‐day sea level, does not imply high crustal stability; instead, it indicates a ‘hinge zone’ between the uplifting hinterland and the subsiding depocentre in the southern North Sea. 相似文献
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VEERLE VANDEGINSTE RUDY SWENNEN MARK H. REED ROB M. ELLAM KIRK OSADETZ FRANÇOIS ROURE 《Sedimentology》2009,56(7):2044-2060
The Upper Devonian carbonate reefs in West‐central Alberta are important petroleum reservoirs that are well‐known for their extensive secondary porosity. An outcrop analogue study indicates that an early matrix‐selective dolomitization event occurred which is characterized by a major Late Devonian sea water component with increased salinity because of evaporation. It is interpreted that the matrix (replacive) dolomite formed during the Famennian as the result of a combination of both seepage and latent reflux dolomitization, although an additional type or overprinting of later intermediate burial dolomitization cannot be excluded. Formation of the moulds is attributed mainly to the dissolution of undolomitized fossil cores, most typically stromatoporoids. Geochemical modelling indicates that carboxylic acid fluids have the highest potential for dissolving residual calcite in this case. Geochemical models consistent with this analysis and interpretations can reproduce the secondary porosity and suggest a viable dolomitization process for the localities studied. 相似文献