摘要:

ABSTRACTThe quantitative model presented simulates the development of a two‐dimensional alluvial sedimentary succession beneath a floodplain traversed by a single major river. Several inter‐related effects which influence the distribution of channel‐belt sand and gravel bodies within overbank fines are accounted for. These are (a) laterally variable aggradation, (b) compaction of fine sediment, (c) tectonic movement at floodplain margins, and (d) channel avulsion. Selected experiments with the model show how the interconnectedness and areal density of channel‐belt deposits decrease with increasing floodplain width/channel‐belt size, mean avulsion period, and channel‐belt aggradation rate. Separation of stream patterns based on interconnectedness and channel deposit density is difficult. Tectonic movements do not have a significant influence upon the successions unless a preferred direction of tilting is maintained (half‐graben). Then channel‐belt deposits showing offlap tendencies tend to cluster adjacent to the active floodplain margin, leaving dominantly fine‐grained alluvium to accumulate on the inactive side. Individual channel‐belt deposits thicken during aggradation, although a self‐regulating limit to such thickening is likely to operate. ‘Multistorey’features resulting from aggradation may be difficult to tell apart from those arising through superposition of distinct channel‐belt

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1979.tb00935.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

摘要:

ABSTRACTThe marginal carbonate facies of the Miocene Ries meteorite crater lake in southern Germany contain bioherms up to 7 m high and 15 m across built by the green algaCladophorites.The algae were externally encrusted during life by micrite, probably precipitated in response to photosynthetic uptake of CO2, which produced tufts of fine (100 μm diameter), calcareous tubes. Coalescence of tufts, together with incorporation of peloidal and skeletal sand, created nodules and cones of algal tufa which in turn formed larger masses some of which are in the form of compound cones up to 2 m high. The bioherms are constructed by beds and groups of these cones and masses, and are surrounded by poorly cemented peloid, ostracod and gastropod sands. Five depositional and diagenetic stages of development can be distinguished: (1) growth and calcification (probably calcitic) ofCladophoritesin shallow fresh‐ or slightly brackish water; (2) emergence due to a temporary fall in lake level and veneering of the algal tufa and adjacent sediments by laminated sinter; (3) resubmergence and deposition of peloidal and skeletal sands; (4) burial and partial phreatic dolomitization, together with dissolution of aragonite and penecontemporaneous deposition of thin isopachous rims of dolomite rhombs; (5) local vadose cementation by rhombs, spar and spikes of low‐magnesian calcite. Stages 1–3 probably occurred several times. Most of the biohermal and surrounding sediments were produced by biological processes in the lake; subaerial sinter deposition and meteoric cementation have contributed relatively minor amounts of material and the majority of the sediments retain porosities of

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1979.tb00936.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

摘要:

ABSTRACTForty beds of authigenic carbonate were identified from the deep Bering Sea in cores taken on Leg 19 of the Deep Sea Drilling Project. Carbonate minerals were mainly high‐magnesium calcite and protodolomite, less commonly siderite, rhodo‐chrosite, low‐magnesium calcite, and manganosiderite. Authigenic carbonates cement and replace diatom ooze, ash and bentonite beds, and, less commonly, clastic beds. Replacement zones are as much as 60 cm thick. Eighty‐five per cent of carbonate beds occurred below 400 m sub‐bottom depth and 70% in sediment older than 4 m.y. δ13C values averaged ‐17.200/00PDB and δ18O ranged from 18.59 to 34–110/00SMOW. The carbon was derived from oxidation of organic matter under anaerobic conditions during bacterial reduction of sulphate, or from CO2produced in concert with CH4during degradation of organic matter. The cations (Ca, Mg, Fe, Mn) were derived from alteration of ash beds. In Bering Sea deposits, ash beds altered to smectite within about 3–5 m.y. Carbonate precipitated simultaneously at different stratigraphic levels within the 627–1057 m sections at temperatures of 7–85°C. No apparent calcite precursor of biogenic origin was found for these

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1979.tb00937.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

摘要:

ABSTRACTHeavy mineral studies on Pleistocene tills from North America, Upper Palaeozoic tillites of South Africa and Australia, and late Precambrian tillites of South Australia show that the heavy mineral suites of the Pleistocene tills are dominated by amphiboles, the Upper Palaeozoic tillites by garnet, and the late Precambrian tillites by zircon and tourmaline. About half of the garnets in the Upper Palaeozoic tillites show evidence of having been rounded, and retain delicate surface chattermark trails, which indicates that these garnets have not undergone chemical attack since deposition. Although the remainder of the garnets show, by way of etching, that intrastratal solutions were active in the sediments, it is suggested that amphiboles, pyroxenes and epidote, which must have been present in the original Upper Palaeozoic heavy mineral suites, were lost primarily by the action of sorting and mechanical abrasion in beach environments prior to, and during interglacial periods. The absence of garnet and the etching of tourmaline and zircon in the late Precambrian tillites is attributed to the action of alkaline intrastratal solutions over the long time interval during which the tillites were buried in the Adelaide Geosyncline.

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1979.tb00938.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

摘要:

ABSTRACTMultiple intraformational discordances, recognized in the Tortonian (Upper Miocene) type area, occur at the top of a shelf sequence, directly overlain by slope sediments. The discordances display smooth, concave upward lower surfaces and are covered by sediments identical to the surrounding strata. Inferred lateral extent of single discordances is 500–700 m. Thickness of sediments overlying the discordances ranges from 5 to 25 m. The intraformational discordances are interpreted as scars formed by downslope sliding of whole packets of strata. The slumps may have been triggered by tectonic oversteepening of shelf sediments due to tilting of the sea floor. The slump scars appear comparable in position and size to some small scale modern examples recorded on margins near the shelf‐br

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1979.tb00939.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

摘要:

ABSTRACTEuhedral, tetragonal, dipyramidal crystals of weddellite, an authigenic calcium oxalate, occur in samples from the topmost core of Deep Sea Drilling Project Site 210 in the Coral Sea Basin. Opinions in the literature differ as to whether weddellite is formedin situor during sample storage. Comparison of smear‐slides made of samples after a 4 year storage period with shipboard smear‐slides made immediately after core retrieval, indicates that weddellite was formedin situ.Possible further growth during storage is not obvious. The presence of many corroded crystals, and the absence of weddellite in deeper cores, suggest that weddellite is formed soon after deposition, but dissolves again in time and with increased burial, due to changing geochemical paramet

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1979.tb00940.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

摘要:

The Encyclopedia of Sedimentology.

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1979.tb00941.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1979.tb00942.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY