摘要:

ABSTRACTMore than 20 examples of fossil carbonate platform systems were compared for slope angle and sediment fabric. Plots of slope angle versus sediment fabric show that grainy, non‐cohesive, mud‐free sediments build steeper slopes than muddy, cohesive, sediments. Examples near the end‐members of grainy and muddy carbonate platform flanks are found in the Triassic of the Dolomites in northern Italy and in the Bahamas, respectively. They document the flank geometry and the processes readjusting the slope profile once the limiting slope angle is exceeded. The grainy flank sediments in the Dolomites, modified by shearing and avalanching, produce straight slope profiles with declivities up to 35°, whereas the muddy Bahamian flank sediments, modified by large‐scale creep and rotational to translational sliding and slumping, produce a concave‐upwards slope profile, inclined at less than 4°. The comparison between slope angle and sediment fabric indicates that the physical behaviour of sediments in the gravity field, angle of shearing and mode of readjustment processes, is linked to the composition of the slope sediment. Among the variables such as sea‐level, subsidence, climate, plate motion and oceanographic setting (windward‐leeward), sediment fabric is suggested to be a major, if notthemajor control on slope angle and slope curvature of carbonate platform flanks. Besides the recently documented tendency of carbonate sediments to build steeper slopes than siliciclastics, this proposed relation sheds new light on the analysis and quantification of the variables influencing the geometry and depositional evolution of carbonate systems. Furthermore, it provides an opportunity to deduce sediment composition from seismic lines and predict lithology pr

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01825.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTUppermost Triassic (Rhaetic) facies, as developed in the Southern Alpine region of Northern Italy, were deposited in a rapidly subsiding, fault‐dissected trough (the Lombardy Basin) bounded by carbonate platforms. The main part of the Rhaetic succession consists of 10‐m‐scale asymmetric cycles, each divided into three parts: a lower shale portion; a central rhythmic portion consisting of repeated marl‐limestone couplets, the limestone parts of which thicken upward; and an upper, wholly carbonate unit. A study of the diagenetic history of the series demonstrates that both the major asymmetric cyclicity and the limestone‐marl couplets of the central rhythmic member (together constituting a ‘compound’ cyclic form) are fundamentally depositional in nature. It is suggested that this compound cyclicity resulted from the superposition of a low‐frequency (approximately 100 000‐year periodicity) asymmetric carbonate mud signal with a higher‐frequency terrigenous mud signal.Field, petrographic, and geochemical investigations suggest that the basinal carbonate is predominantly allochthonous in origin, having been derived as relatively pure aragonitic mud from adjacent carbonate platforms. It is postulated that the asymmetric carbonate signal was linked to the ecological effects of eustatic fluctuation on platform carbonate systems. Repeated subaerial exposure of subtidal muds in shallow areas indicates that such sea‐level variations occurred. A model is presented in which the basinward export of carbonate was negligible in the deepening phase, increased to a maximum during shallowing and was finally halted by the emergence of large platform flats. In contrast, the higher frequency terrigenous mud signal of the basin is thought to have been climatically modulated; fluctuations of a shorter period than those predicted by the Milankovitch theory affected hinterland precipitation and runoff. Particularly rapid subsidence and high depositional rates may have allowed the preser

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01826.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTCarbonate cements (calcite, siderite, dolomite, and ankerite) formed throughout the diagenetic history of the Sag River and Shublik Formations. The trace element and isotopic geochemistry of these cements varies as a function of the timing of precipitation. Earliest calcites, formed prior to significant compaction of the sediment, are relatively enriched in Mg (up to 4·4 mol%), and have87Sr/86Sr values (mean = 0·707898) compatible with the original marine pore waters. Later calcites are relatively Fe‐rich (up to 5·0 mol%) and are characterized by increasing87Sr/86Sr values (up to 0·712823) and Sr content with decreasing age. The Fe content of zoned siderite and dolomite/ankerite rhombs increases towards the outside of the rhombs (i.e. increasing Fe content with decreasing age).These geochemical variations appear principally to result from changes in pore‐water chemistry during diagenesis. The increase in87Sr/86Sr and Sr content of the cements is most likely due to interaction between pore waters and87Sr‐rich clay and possibly feldspar in Ellesmerian mudrocks (whole rock87Sr/86Sr signatures for the mudrocks are>0·716). Pore‐water Fe2+concentration was probably controlled by diagenetic alterations involving Fe‐bearing minerals (e.g. pyrite precipitation). A reconnaissance examination of carbonate cements in the overlying Kingak Shale indicates that similar alterations occurred in the Kingak.The low δ18O value of some calcite cements (‐11·96% PDB) suggests that an influx of meteoric water may have occurred in the mid‐Neocomian, though the low value could also result from an abnormally high geothermal gradient associated with m

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01827.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTComplex aluminium phosphate‐sulphate minerals of the hinsdalite group have been identified as early diagenetic precipitates in Late Permian sandstones of the Northern Calcareous Alps, western Austria. According to their chemical composition they can be regarded as solid solutions between woodhouseite, svanbergite, crandallite and goyazite. The model proposed for their origin involves the dissolution of detrital apatite in a low‐pH environment and subsequent precipitation of aluminium phosphate‐sulphate minerals, which clearly pre‐date syntaxial quartz cementation of the sediment. Their occurrence probably has been overlooked in other sandstones showing diagenetic apatite dissolution. The recognition of such minerals could provide a significant insight into early phosphate dia

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01828.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTA unique instance of Holocene estuarine phosphogenesis has been investigated in Raglan Harbour, on the west coast of North Island, New Zealand. Nodular calcareous concretions, which formed within surficial estuarine muds between 6500 and 3600 years BP, were superficially impregnated by carbonate fluorapatite between 3600 and 1600 years BP. The phosphatized nodules lie beneath a late Quaternary shore platform, which is reinforced by a mass of intertwined bioturbation structures and tubular rootlet casts impregnated by limonite and goethite. Large polygonal cracks, indicative of a desiccatory regime, are locally conspicuous on the platform surface and in the immediately subjacent muds.The phosphogenesis in Raglan Harbour is here related to an episode of lowered sea‐level, during which a paludal environment succeeded the estuarine, to be followed, in turn, by a positively and interlude. The subsequent rise of sea‐level to its present position over the last 1600 years or so has reinstated the shore platform in its present intertidal situat

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01829.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTThe lower Visean Belle Roche breccia (east Belgium) displays a number of features that indicate brecciation by evaporite dissolution collapse: the sharp lower contact of the breccia, the gradual transition into the overlying strata, the presence of semi‐continuous beds within the breccia giving it a crude ‘stratification’, and the existence of several types of (calcite, dolomite and silica) evaporite pseudomorphs. Furthermore, the majority of the breccia fragments indicates hypersaline to lagoonal sedimentation conditions. Most of these fragments display an interlocking fabric. The interpretation is also supported by the existence of continuous evaporite beds replaced by carbonates (calcite and dolomite) both under and overlying the breccia.The brecciation history is characterized by gradual subsidence. Multiple brecciation episodes are recognized, and are best seen in the lower breccia which underwent at least two major brecciation episodes. Here, brecciated and veined breccia fragments occur within a microsparite (neomorphosed mud) matrix. Brecciation of these strata was due to the dissolution of interlayered evaporites. The second brecciation event relates to infiltration of meteoric water and to the dissolution of the remaining evaporites. This infiltration was probably triggered by the orogenic event at the end of the Visean (Sudetic orogenic phase). The whole breccia was finally cemented by a blocky calcite.The different lithologies and cements were characterized by their trace element (Mg, Sr, Na, Fe, Mn, K), insoluble residue and organic matter content. Carbon/oxygen isotope data of the cements and replaced evaporite layers helped to place the multiple collapse episodes within a general diagenetic model. Solution‐reprecipitation processes within the original aragonite‐dominated mud fragments, as well as in the early diagenetic dolomite fragments, have been recognized. The geochemical data show that these transformation processes occurred in equilibrium with the same fluid. These processes may have occurred within a freshwater lens very early in the diagenetic evolution or under shallow burial conditions. Cementation of blocky calcite occurred in a meteoric realm under burial c

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01830.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTThe Permanente terrane, part of the Franciscan accretionary complex of California, contains an unusual fault‐bounded packet bearing lithic‐volcanic sandstones and conglomerates. Based on petrographical, sedimentological, stratigraphical and geochemical studies, it is concluded that these rocks were deposited on the flank of an oceanic rise or plateau by sediment gravity flows probably below the carbonate compensation depth. The bulk geochemistry of the sandstones is similar to the chemistry of greenstones in the Permanente terrane, suggesting that the greenstones or rocks similar to them represent the source of the lithic‐volcanic sandstones, and that the sandstones geochemically ‘remember’ the chemistry of their source rocks. This ‘sedimental memory’ is impressive: geochemical variation in the sandstones is co‐linear with presumable magmatic variation trends in the greenstones. Sandstone geochemistry, plotted on Zr/Y vs. Y and V vs. Ti diagrams, shows that compositions occupy similar fields as greenstones in the Permanente terrane and oceanic crustal compositions in general.Because oceanic crustal sequences can become accreted to continental margins, oceanic crust should be considered a potential source of sediment, especially in continental margin settings. However, differentiating sandstones derived from oceanic arcs and from oceanic crust is extremely difficult unless discriminating geochemical cr

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01831.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTTurbidity currents, initiated from spring runoffs of an influent river, were observed in the upper region of a reservoir in Hokkaido, Japan, by measuring water temperature, velocity and suspended‐sediment concentration. Their profiles offer some physical parameters for the sedimentary conditions, assuming the turbidity currents to be quasi‐uniform. The bottom sediment deposited by the turbidity currents was then collected by a portable core sampler. The bottom sediment consists of more than 90% silt and clay, and thus offers a hydraulically smooth bed for shear flow; a plane bed as a bed configuration was formed on the reservoir bed, probably because of the low shear velocity and small grain size of sediment. Using a graphic method with log‐normal probability paper, the bottom sediment is divided into several overlapping log‐normal subpopulations. Grain‐size analysis indicates that the bottom sediment may be regarded as cohesionless; criteria for ‘complete deposition’ of transported grains can then be incorporated into the ‘extended Shields diagram’ giving the minimum shear stress to erode bottom sediment. Applying the new diagram to the grain size distribution of the bottom sediment, it is suggested that each of the log‐normal subpopulations was deposited in each of four different ‘modes of deposition’, i.e. ‘traction’, ‘saltation (or intermittent suspension)’, ‘suspension’ and ‘suspension under equilibrium’. The last mode may be observed under a sedimentary condition where upward flux of suspended sediment by eddy diffusion is almost equal to its depositional flux due to gravity. The mean and critical grain sizes for bottom sediment and each of the corresponding subpopulations decrease consistently with an increase of Ψ=Fd2log10Re(Fdis the densimetric Froude number andReis the flow Reynolds number). Ψ correlates inversely with shear velocity, which bears a linear relationship to mean velocity. These results lead to the conclusion that relatively fine suspended sediment is deposited as a result of decreasing bottom friction wit

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01832.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTBoth a modern and a raised, late Holocene gravelly beach have been studied along the cliffed coast near the village of Chora Sfakion in southwestern Crete, Greece. The Holocene beach shows the record of regional, well‐documented relative sea‐level changes, in the form of regressive and transgressive sequences. The texture, fabric and depositional architecture of these sequences are described and compared with those of the modern, microtidal beach there. The depositional history of the raised beach is discussed in terms of the sediment transport processes and relative sea‐level changes.The regressive sequences of both the modern and the Holocene beach resemble those of the mesotidal beaches in the Sker district of southwest Wales, described by Bluck (1967). The transgressive sequence of the Holocene beach resembles the mesotidal beach sequence near Newton (southwest Wales), recognized by Bluck as another type of progradational (regressive) beach. Therefore, transgressive beach sequences in the stratigraphic record might easily be overlooked or misinterpreted as regressive. The stratigraphic depositional patterns of gravelly beach systems are, apparently, still poorly under

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01833.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTThe Eocene flysch of Middle Dalmatia comprises several beds that are interpreted to have been deposited from reflected sediment gravity flows. Their compositions are similar and two bed types are differentiated: complex beds that are debrite‐plus‐turbidite couplets, and turbidites.The sequence alternations in the turbidite part of the bed, opposing ripples within the same bed, and opposite flow directions indicated by flutes and ripples are indicative of flow reflections. The influence of seiches is suggested by the occurrence of symmetrical (oscillation) ripples.The palaeotransport directions of reflected flows show wide dispersal. A geometry of small, fault‐controlled sub‐basins with centripetal palaeotransport patterns is p

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01834.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY