摘要:

ABSTRACTShelf limestones are widely distributed in New Zealand Cenozoic sequences and are especially well developed in the Oligocene. Detailed field and laboratory work on several Oligocene occurrences, and reconnaissance field‐work at most other sections have elucidated the major characteristics of the environment, texture, composition and diagenesis of these sediments. Several generalizations emerge which contrast with the commonly accepted characteristics of shallow marine carbonate sedimentation established from studies of tropical and subtropical deposits.The limestones are either calcarenites or, less commonly, calcilutites and, in general, these two lithologies are mutually exclusive, both in time and space. The allochems and interparticle carbonate mud (where developed) in calcarenitic limestones consist almost exclusively of fragmented skeletal material derived primarily from bryozoan, echinodermal, benthic foraminiferal, barnacle, brachiopod, bivalve and coralline red algal tests. The calcilutitic limestones consist mainly of whole and disintegrated tests of pelagic foraminifers and coccolithophorids. Non‐skeletal carbonate components such as ooids, pellets and aggregates are conspicuously absent from both lithologies. Reefal structures are also absent or rare and are mainly oyster reefs.The limestones commonly contain a significant content of terrigenous material and/or glauconite and at the stratigraphic level the limestones are intimately associated with terrigenous formations. The distribution of the carbonate sediments has been governed mainly by rate of supply of river‐derived terrigenous material, by subsequent dispersal patterns of this material over the shelf, and by current sorting. As a consequence of selective grain transport, bedding in the limestones is often defined by the cyclic alternation on a wide range of scales of carbonate units that are relatively enriched and relatively impoverished in terrigenous material.The primary (carbonate) mineralogy of the carbonate sediments was completely dominated by magnesium calcite and/or calcite with only small amounts of aragonite and no dolomite or associated evaporite minerals. The metastable magnesium calcite and aragonite grains were probably altered on, or close below, the shallow sea‐floor. Among other factors, transformation was encouraged by the absorption of magnesium in pore waters by montmorillonitic clays and by the complete oxidation of all organic matter in the bottom sediments. Magnesium calcite grains were stabilized by texturally non‐destructive incongruent dissolution, but aragonite was often dissolved without trace from the sediment, especially in grainstones. Thus submarine diagenesis has been characterized by selective dissolution phenomena.Cementation by granular and syntaxial rim orthosparite of calcite and/or ferroan calcite composition occurred mainly during shallow subsurface burial and was associated with the intergranular solution of calcitic skeletal fragments, especially at those levels in the sediment relatively enriched in terrigenous material. This lithification process has worked to accentuate and modify original litho‐logic differences and sedimentary structures in the primary sediments and has produced a kind of rhythmic vertical alternation of less well cemented, microstylolitized, impure limestone beds (‘cement‐donor’ beds) and well cemented, more open textured, purer limestone beds (‘cement‐receptor’ beds).The New Zealand limestones formed between latitudes 60° S and 35° S under generally cool temperate to warm temperate climate conditions. Oxygen isotopes suggest that surface waters were mainly significantly cooler than 20°C, so that shelf waters may have experienced extended periods of undersaturation with respect to calcium carbonate. Generally open circulation patterns maintained near normal salinity values over the entire shelf platform. Calculated sedimentation rates for the New Zealand carbonate sediments are generally very low (<5 cm/1000 years). Periods of more active deposition commonly alternated with longer periods of non‐deposition and by‐passing or erosion. It is concluded that many characteristics of the New Zealand shelf limestone occurrences are explained best by a temperate latitude model of shallow

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1978.tb00328.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

ABSTRACTDebris transported by glacier is derived either supraglacially from nunataks and valley sides or from erosion of the subglacial bed. Debris produced above the glacier by fracturing of rock walls has a dominant coarse fraction with angular boulders. Subsequent englacial or supraglacial transport is relatively passive and little comminution occurs. Debris eroded from subglacial bedrock is initially transported in a basal zone of traction, where particles frequently come into contact with the glacier bed and are retarded by it so that large forces may be generated between particles and the bed and at interparticle contacts. The material introduced into this tractional zone may be subglacial bedrock which has undergone a crushing‐plucking event and which has a dominant coarse fraction, or supraglacially derived material which finds its way to the glacier bed. These parent debris assemblages are further comminuted by failure in response to locally concentrated compressive stresses, and attrition at shearing interfaces. Boulders transported through the tractional zone will tend to be rounded and bear several directions of striation. Large boulders embedded in lodgement till will tend to be streamlined with striae parallel to glacier flow and with an abruptly truncated distal extremity, rather like aroche moutonnée.Textural and boulder shape characteristics can be used to help distinguish different types of ti

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1978.tb00329.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

ABSTRACTVarious types of pelagic sediments occur on a dolomitic basement located at some 1000 m depth on the eastern slope of the Tyrrhenian Sea, i.e. in the most internal, collapsed and presently submerged portion of southern Apennines. The deposits include laminated limonites, muds with manganosiderite nodules, radiolarian clays, opal chert, gypsiferous muds, lutites with calcareous plankton, and all are of Quaternary age. They are interpreted as products of the interactions between submarine hydrothermal activity and deep‐sea sediments, and represent the first documented case of submarine hydrothermal sediments not directly connected to active ridges or volcanic buildings and deposits. The iron‐rich sediments are very similar to many other reported examples of submarine thermal activity. The radiolarian clay displays very abundant and almost exclusively radiolarian tests, and no calcareous fossils, but shows numerous dissolution traces of carbonate skeletons. It has been interpreted as deriving from a primary bloom of siliceous plankton followed by an extensive leaching of the calcareous tests on the seafloor. The radiolarian chert shows a very early and yet unknown Opal‐A cement. The immediate source of the cement is biogenous silica, whereas the ultimate source is the bloom of siliceous plankton triggered by the hydrothermal and volcanic activity. The gypsum muds are the result of a hydrothermal chemical remobilization of the Messinian sulphate beds which overlie the dolomitic bas

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1978.tb00330.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

ABSTRACTBoth spring‐summer and fall‐winter sand transport have been observed on the Long Island, New York, inner shelf at water depths of 20‐22 m using a radio‐isotope sand tracer system. The extent of dispersal of the tagged, fine sand was measured at 3 week intervals in two 70 day experiments. In the late spring and early summer, movement was primarily diffusive in nature, extending 100 m around the line of tracer injection, while late fall‐winter patterns had strong advective features, including an ellipsoidal outline extending approximately 1500 m westward of the injection points after the passage of several storms with strong northeasterly winds. Near‐bottom current observations made with Savonius rotor sensors identify the event responsible for the bulk of the transport over the 135 day observation period as a storm flow of 2 days duration. Tracer and current observations together suggest that westward winter storm flow along the Long Island shelf is the major mechanism of sand transport at these depths on a yearly time scale. A least‐squares fit of several of the observed winter patterns with a plume model yields average sediment mass flux lower bounds of 3.2 × 10−3gm/cm/sec and 1.7 × 10−1gm/cm/sec for ‘typical’ and extrem

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1978.tb00331.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

ABSTRACTThe replacement by ferroan calcite with preservation of the original structures can be used as a new criterion for identifying skeletons originally composed of high‐magnesian calcite. This applies to bryozoa, rugose corals, echinoderms, many foraminifera, most ostracods, red algae, and serpulids. On the other hand, skeletons originally composed of low‐magnesian calcite were never replaced by ferroan calcite, as shown by belemnites, brachiopods, and most of the pelecypods.Using this criterion, an original low‐magnesian calcite composition is inferred forTentaculitesand some ostracods and foraminifera, whereas a previous high‐magnesian calcite composition is inferred for trilobites,oligosteginaand certain ooids. Chemical instability of high‐magnesian calcite is suggested to be the driving force of the replacement by ferroan calcite. In most of the thirty‐seven samples investigated, of Oligocene to Devonian age, the ferrous iron concentration of the interstitial fluid increased during diagenesis, as shown by well established sequences of cement A and B and fissure fill. This offers a relative time scale for diagenetic processes.Ferroan calcites contain up to 6 mol % FeCO3and up to 5 mol % MgCO3. In this range of concentration, the distribution coefficients for Fe and Mg between calcite and solution at about 25°C are about 1 to 0‐03, respectively, according to experiments. Possible sources of iron are iron oxides and hydroxides as well as clay minerals including glauconite.Though a submarine origin below the sediment surface is conceivable for ferroan calcite, there are serious limiting conditions such as low Eh and, at the same time, lack in sulphate‐reducing bacteria. On the other hand, ferroan ‘dedolomite’, compositional zonality in individual ferroan calcite overgrowths, low δ18C and δ18O values, and low Mg concentrations point more to a meteoric‐phreatic origin of many fer

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1978.tb00332.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

ABSTRACTA temporary section in the Cuisian Bagshot Beds, which has been mapped in detail, displayed estuarine sediments with interlayered sands and muds, fine sands, channel‐fill sands and intraformational (mainly mud clast) conglomerates. The facies show rapid lateral and vertical changes in grain size and bed form and a restricted suite of trace fossils includingOphiomorpha nodosaandArenicolitessp.The sequence is shown to have been deposited in a subtidal channel where tidal, wave and fluvial processes were dominant at different times. The following points are considered to be characteristic of sedimentation in this environment: (i) correlation is difficult and facies predictability is low; (ii) there are frequent lateral facies changes from the channel to the subtidal bank environment; (iii) pene‐contemporaneous erosion removes considerable amounts of sediment; (iv) load structures may be exposed, eroded, buried and reactivated; and (v) muddy layers and bioturbated horizons offer similar resistance to penecontemporaneous eros

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1978.tb00333.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

ABSTRACTSurge marks have previously been described as hieroglyphs and more recently as erosional rills caused by the turbulent bank‐swashing of storm waters upstream of obstructions. This paper presents evidence that surge marks have various forms directly related to the steepness of sediment slope. These features are due, at least in part, to deformation and not necessarily to erosion by surging water. The previous term dendritic surge mark may now only apply to one member of the surge mark formatio

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1978.tb00334.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY