ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1980.tb01154.x

出版商:Blackwell Publishing Ltd    

年代:1980

数据来源: WILEY

摘要:

ABSTRACTThis study examines the morphology, sedimentology and genesis of the point bars and floodplain of the Beatton River. The formation of point bars occurs in distinct stages. An initial point bar platform composed mainly of coarse sediment is formed adjacent to the convex bank of a migrating meander bend, and is the base on which develops a single scroll bar of fine traction and suspended load. With continued sedimentation, the scroll bar grows, eventually supporting vegetation and becoming a floodplain ridge. Scroll bars form with greatest size and frequency in rapidly migrating bends, and the shape of the meander bend appears to determine both the location of the initial bar deposit, and its direction of growth up or downstream. Approximately one‐half of the floodplain sediment is derived from suspended load, and the initiation of a scroll bar appears to be due to excessive deposition of suspended load in a zone of flow separation over a point bar platform. The critical flow condition for the initiation of a scroll bar does not occur with the same recurrence interval on different shaped meander bends, however, the average recurrence interval within the study reach is approximately every 30 years. Sedimentation rates on point bars and on the floodplain indicate two relatively distinct stages of floodplain alluviation. The most rapid is for surfaces less than 50 years old, although sediment accumulation still persists on surfaces up to 250 years in age. Although frequently flooded, surfaces older than this accumulate very little sediment. Despite 2–3 m of overbank deposition, the amplitude of floodplain ridges is maintained by secondary currents which sweep sediment from the swales towards the ridge cre

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1980.tb01155.x

出版商:Blackwell Publishing Ltd    

年代:1980

数据来源: WILEY

摘要:

ABSTRACTTurbidite muds in cores from the outer Scotian continental margin, off eastern Canada, contain abundant thin silt laminae. Graded laminated units are recognized in parts of this sequence. These represent single depositional events, and show a regular decrease in modal grain size and thickness of the silt laminae through the unit. A similar fining trend is shown by both silt and mud layers over hundreds of kilometres downslope. Textural analysis of individual laminae allows the construction of a dynamically consistent physical model for transport and sorting in muddy turbidity currents. Hydraulic sorting aggregates finer material to the top and tail regions of a large turbidity flow which then overspills its channel banks. Downslope lateral sorting occurs with preferential deposition of coarser silt grains and larger mud flocs. Depositional sorting by increased shear in the boundary layer separates clay flocs from silt grains and results in a regular mud/silt lamination. Estimates can be made of the physical parameters of the turbidity flows involved. They are a minimum of several hundreds of metres thick, have low concentrations (of the order of 10−3or 2500 mg 1−1), and move downslope at velocities of 10‐20 cm s−1. A 5 mm thick, coarse silt lamina takes about 10 h to deposit, and the subsequent mud layer ‘blankets’ very rapidly over this. A complete unit is deposited in 2‐6 days which is the time it takes for the turbidity flow to pass a particular point. These thick, dilute, low‐velocity flows are significantly different from the ‘classical’ turbidity current. However, there is mounting evidence in support of the new concept from laboratory observations and direct

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1980.tb01156.x

出版商:Blackwell Publishing Ltd    

年代:1980

数据来源: WILEY

摘要:

ABSTRACTPliocene and Holocene siltstones and silts in outcrops from the Boso Peninsula, Japan and in cores from the East China were studied to determine distinctive characteristics of the turbiditic (Bouma E‐Division) and hemipelagic siltstones and silts. Weathering characteristics, colour, grain size, and organic carbon‐organic nitrogen ratio, plus clay fabric proved valuable in characterizing each unit. Clay and non‐clay mineral content was uniform throughout. Clay fabric differences are pronounced. Random clay flake orientation prevails in the turbiditic interval while the hemipelagic unit has more preferred orientation. The fabric reflects different conditions of sedimentation. The turbiditic clay was deposited more rapidly in the flocculated state while the interturbidite hemipelagic clay may have formed from more slowly sedimented dominantly dispersed clay. Results suggest that clay fabric may be useful in combination with other sedimentary features in the study of mud‐turbidite sedime

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1980.tb01157.x

出版商:Blackwell Publishing Ltd    

年代:1980

数据来源: WILEY

摘要:

ABSTRACTThe Maastrichtian Kathikas mélange is shown to be of sedimentary origin, being a succession of undeformed, submarine, matrix‐supported debris‐flow deposits up to 270 m thick. Internal sedimentological features include beds emphasized by colour or clast size variation, pelagic chalk interbeds, planar clast fabrics and channels. A trend of upwards‐thinning beds in the mélange is interpreted as due to debris‐flow initiation on gradually increasing slopes. Debris was shed locally from the deformed and fragmented Mamonia Complex, a series of disrupted gravity‐slide sheets of Mesozoic sedimentary rocks and deformed igneous rocks. All Mamonia lithologies are represented in the mélange, and local facies variations permit identification of individual sources. The mélange probably pre‐dates emplacement of serpentinite into the Mamonia Complex. There was also local inter‐mixing of material from the adjacent and underlying Troodos sequences. The mélange rests unconformably on both Mamonia and Troodos sequences, and formed after the main deformation episode of the Mamonia Complex. The degree of resedimentation increases gradually away from the disrupted Mamonia source rocks. The thickness and volume of the Kathikas mélange are comparable with those of recent submarine debris flow deposits on unstable or seismically active

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1980.tb01158.x

出版商:Blackwell Publishing Ltd    

年代:1980

数据来源: WILEY

摘要:

ABSTRACTHistorical records demonstrate that the Lake of Tunis, Tunisia, was an open bay that became separated from the Mediterranean by an accreting barrier spit, forming a lagoon. Closure of the lagoon was caused by increased sedimentation as a consequence of Roman deforestation of the Medjerda River drainage basin to the north and subsequent accelerated longshore transport. The separation of the lagoon from the Mediterranean was completed in the early 1500s. At present, the 48 km2lagoon averages less than 1 m in depth and reaches eutrophic conditions in the late summer. Consistent with the historical records, the lagoon sedimentary column shows three distinct successive environments of deposition: (1) an arid continental environment; (2) an open marine bay; and (3) the present brackish to hypersaline lagoon. These depositional environments are represented by the lower grey layer, which is less than 0‐5 m thick, the middle olive‐grey layer, which varies from 1 to 5 m in thickness, and the upper black layer, which is 1 m thick. All of the strata are predominantly silt plus clay, but usually contain at least 10% sand. The lower grey layer consists of pitted quartz sand, with very few abraded, broken molluscan fragments and benthic foraminifera with thick tests. An arid, subaerial depositional environment of latest Pleistocene time best explains these sediment and fossil assemblages. In the middle olive‐grey layer, coral, coralline algae, open marine graeses and the dominance of foraminifera over ostracods (expressed as a low percentage of ostracods/ostracods plus foraminifera) attest to a depositional environment of an open marine bay. A sharp increase in the percentage of ostracods/ostracods plus foraminifera and organic carbon from the middle olive‐grey layer to the upper black layer signifies a major change in depositional environment. The predominance of ostracods over foraminifera, abundance of gastropods characteristic of eutrophic conditions, high organic carbon content and absence of macro‐fossils characteristic of open marine conditions clearly indicate that the upper black layer was deposited in a brackish to hypersaline, eutrophic lagoon which has persisted to the present. Seismic records indicate a karstic bedrock surface underlying the lagoon. The surface is marked by considerable relief, and shows a linear depression which may represent an early sixteenth century ship canal. Heavy metal analyses of total samples in five cores demonstrate that Mn and Fe vary randomly, and are apparently derived from natural sources only. Cd, Cr, Pb, Cu and Zn are typically highest in the uppermost unit, which reflects levels of human contamination since closure of t

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1980.tb01159.x

出版商:Blackwell Publishing Ltd    

年代:1980

数据来源: WILEY

摘要:

ABSTRACTSandyhaven Pill is a ‘drowned valley’ type of estuary. Thus the deposits differ from most other described estuarine deposits which are of ‘tidally influenced river’ type. The surface sediments may be divided broadly into wave‐dominated deposits (22% of area), tide‐dominated deposits (65%), deposits related to marginal cliff collapse (12%) and river‐dominated deposits (1%). Further subdivision shows that the subenvironments are nested in a progression up the estuary with trends to finer sediment size, reduced sorting and increased biogenic activity. The latter relates to a marine to estuarine faunal change and a strong relationship between the distributions of biota and depositional subenvironments. Over a 29 day period, reduction in wave height was reflected in wave‐dominated areas by shoreward movement of some subenvironment boundaries and by improved definition of symmetrical ripples. The tidal cycle had only a limited effect on the tide‐dominated sediments. The most reliable indicators of estuary trend are channels and asymmetrical ripples; but coring shows that ripples and other minor structures are rarely preserved. Heavy mineral analysis indicates that most of the estuarine sand came from offshore. Gradual sediment build‐up will result in a regressive sequence. If this were preserved under a later transgression, the resultant deposit would be an elongate sediment body bounded laterally by a coarse marginal facies. The sediment sequence would be inward and upward fining with diachronous facies boundaries sloping upwards towards the offshore end and towards t

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1980.tb01160.x

出版商:Blackwell Publishing Ltd    

年代:1980

数据来源: WILEY

摘要:

ABSTRACTThe calcite fossils of the Derbyhaven Beds, Isle of Man, have δ13C values (+ 1·8 PDB) similar to modern, shallow‐water marine skeletons, but the δ18O values (−6·1 PDB) are much lighter than modern skeletons. The light oxygen values indicate either re‐equilibration with isotopically light water before cementation started, or Carboniferous sea water with δ18O of −6‰. Aragonite dissolution was followed by precipitation of zoned calcite cement. In this cement, up to six intracrystalline zones, recognized in stained thin sections, show isotopic variation. Carbon varies from + 3‐8 to + 1‐2‰. and oxygen from − 2‐6 to − 12‐4‰. with decreasing age of the cement. This trend is attributed to increasing temperature and to isotopic evolution of the pore waters during burial. The zoned calcite is sequentially followed by dolomite and kaolinite cements which continue the trend towards light isotopic values. This trend is continued with younger, fault‐controlled dolomite, and is terminated by vein‐filling calcite and dolomite. The younger calcite, interpreted as a near‐surface precipitate from meteoric waters, is unrelated to the older sequence of carbonates and has distinctly differen

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1980.tb01161.x

出版商:Blackwell Publishing Ltd    

年代:1980

数据来源: WILEY

摘要:

Books review in this article:Framework, Facies and Oil‐Trapping Characteristics of the Upper Continental Margin,Ed. by A. H. Bouma, G. T. Moore and J. M. ColemanGeological and Geophysical Investigations of Con‐ tinental Margins (AAPG Memoir 29), Ed. Joel s. Watkins, Lucien Montadert, and Patricia W. Dickerson.Geochemistry, by A. H. Brown

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1980.tb01162.x

出版商:Blackwell Publishing Ltd    

年代:1980

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1980.tb01163.x

出版商:Blackwell Publishing Ltd    

年代:1980

数据来源: WILEY