摘要:

ABSTRACTWell exposed, often laminated gypsum sequences occur in many Quarternary salt lakes in southern South Australia. The gypsum in the salt lakes is classified by increasing grain size into gypsite, gypsarenite and selenite. The salt lakes are classified by age and hydrological setting into coastal salinas which are Holocene sea‐water fed groundwater lakes, and continental playas which are late Pleistocene endorheic basins. A study of the relationships between coastal salina hydrology and the associated gypsum deposition has shown the different types of gypsum form under distinct hydrological regimes. As the hydrology above a coastal salina depositional surface changes through time so does the type of gypsum deposited. Application of a gypsum depositional model derived from a study of the coastal salina gypsum to those portions of a continental playa gypsum unit where deposition is no longer occurring confirms the applicability of the model to non‐salina gypsum depos

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1982.tb00071.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY

摘要:

ABSTRACTPyrite occurs both in normal clays and shales with a benthic fauna (Oxford Clay, England, and Lias ε, Germany) and in highly bituminous shales (Lias ε, Germany). In normal shales it is present in small quantities as early framboids, but more conspicuously as internal moulds of fossils, especially ammonites. The pyrite in these is petrographically varied; several types of internal sediments and chamber linings are described and illustrated by reflected‐light and scanning electron microscopy. Most striking are pyrite stalactites, suspended from the roofs of ammonite chambers, which were later filled by calcite or baryte. Pyrite formed in reducing micro‐environments, while the sediment generally was not wholly anoxic. Most pyrite pre‐dates compaction of sediment, breakage of fossils and solution of shell aragonite. Variable rates and conditions of reduction of sea water sulphate are reflected in δ34S values ranging from −55 to +44. Stalactites probably started to form when the ammonite chambers were partially gas‐filled.In the bituminous Lias ε shales pyrite occurs abundantly as early framboids and micro‐nodules. Larger nodules show a variety of forms, some of which post‐date compaction of the sediment. Pyrite is not associated with the abundant flattened ammonites. δ34S values in shales are grouped about a mode near −20. Pyrite formed over a long time‐span, and throughout the sediment, not just in protected cavities.Contrasts in pyrite types can be related to differing depositional environments and organic contents of the shales. Pyrite is an important mineral in diagenetic mineral parageneses which can be deduced by studying fossil void‐fillings and concretions, and which help define the diage

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1982.tb00072.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY

摘要:

ABSTRACTThe Lower Jurassic sequence exposed at Makhtesh Ramon, in the south‐western Negev of Israel, comprises cyclic fluviatile sediments derived almost exclusively from the reworking of an older laterite terrain. A succession of palaeosols, having the characteristics of modern, red‐brown earths, has developed during periods of sub‐aerial exposure upon the fine grained overbank sediments. A marked seasonal palaeoclimate with accompanying poor sub‐surface drainage conditions, and some stagnation of ground water, is reflected by the distribution and mottling patterns of the iron minerals (with varying degrees of hydration), precipitation of illuviated carbonate and sulphate minerals and the development of soil microrelief.A complex history of pedogenesis and ground water alteration is shown by the rapid lateral facies change within this sequence, into one dominated by high‐alumina flint clay and bauxite. Relict pedogenetic and sedimentological textural features of the combined palaeosol/fluviatile sequence, have been identified and mapped within the flint clay/bauxite lithologies. These features include irregular voids, produced by solution of pedogenic sulphate nodules, slickensided curviplanar surfaces of soil microrelief, and depositional bedding. The field relationships of these relicts indicate the overprinting of a younger, chemical weathering event giving rise to the high‐alumina flint clay and bauxites.The geometric shape of the upper bounding surface of the ‘bauxite’ alteration zone, is irregular and has an ‘intrusive’ character with respect to the younger, overlying, unaltered sediments. This, together with abrupt changes in mottling patterns near the contact zone and subtle changes in mineralogy from a bauxitic assemblage at depth to one containing mixed‐layer clay minerals and carbonates nearer the contact, leads to the conclusion that the later stages of chemical weathering occurred within a confined system, where ground water movements were induced by intake o

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1982.tb00073.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY

摘要:

ABSTRACTThe carbonate mineral suite of the modern offshore bottom sediment of the South Basin of Lake Manitoba consists mainly of high magnesian calcite and dolomite with minor amounts of low‐Mg calcite and aragonite. The high‐Mg calcite is derived from inorganic precipitation within the water column in response to supersaturation brought about by high levels of organic productivity in the basin. Both dolomite and pure calcite are detrital in origin, derived from erosion of the surrounding carbonate‐rich glacial deposits. Aragonite, present only in trace amounts in the offshore sediments, is bioclastic in origin. The upward increase in the amount of magnesian calcite in the post‐glacial sediment record is attributed to increasing photosynthetic utilization of CO2in the lake. Stratigraphic variation in the amount of magnesium incorporated into the calcite lattice is interpreted as reflecting a variable magnesium input to the lake from ground water and surface runoff, and possibly variable calcium removal in the precipitating lake water. The effects of long‐term chemical weathering at the source and size segregation explain the changes in dolomite content throughout th

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1982.tb00074.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY

摘要:

ABSTRACTMapping of heavy mineral associations and diagnostic accessories in Oligocene sediments of the eastern Ebro Basin has revealed two major provinces and several sub‐provinces. ThePyrenean Provinceis characterized by an apatite‐tourmaline‐garnet association with diagnostic staurolite in the west (western Pyrenean Sub‐Province) and micaceous composite grains plus chloritoid in the east (eastern Pyrenean Sub‐Province). TheCatalanides Provinceis characterized by an ultrastable angular brown tourmaline‐staurolite association with kyanite in the south‐west (Montsant Sub‐Province), faceted garnets and rounded tourmalines in an intermediate position (St Miquel del Montclar Sub‐Province) and in the north‐east rounded and green tourmalines admixed with clinopyroxene, apatite and micas derived from the Pyrenees (Montserrat Sub‐Province).Palaeodischarges have been estimated from 30 channels distributed throughout both major provinces. A gradual decrease in river discharge through Oligocene times is indicated for the rivers of Catalanides parentage, although the discharges of high‐sinuosity streams alone remain uniform. This decrease through time of Catalanides drainage competitiveness is reflected in the greater areal extent of the Pyrenean petrological province together with a progressive displacement south or south‐eastwards of basinal lacustrine facies. The increasing dominance of Pyrenean drainage during the Oligocene may be due to more prolonged tectonic activity in the Pyrenees compared to the m

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1982.tb00075.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY

摘要:

ABSTRACTSedimentary rocks of late Mesozoic age exposed at Camp Hill, northern Antarctic Peninsula, are associated with calc‐alkaline volcanic rocks. They represent deposition on a fault‐controlled floodplain, with marginal alluvial fans, on a volcanic arc. Finely laminated mudstone and occasional graded laminae were deposited from suspension and by density underflow currents, respectively, in small shallow lakes. Thickening‐ and coarsening‐upward sandstone bodies overlying the lake deposits are interpreted as lacustrine deltas of which two types are preserved: (1) Gilbert‐type with steep foresets and (2) mouth‐bar type which lack steep foresets. Sections through the latter type reveal the presence of sub‐environments characteristic of fluvial‐dominated marine deltas, i.e. prodelta, distributary mouth‐bar and distributary channel. Abandoned mouth‐bars resulting from avulsion are recognized. It is suggested that the processes which operated during formation of the mouth‐bar deltas resulted from hyperpycnal flow. By contrast, the Gilbert‐type delta is thought to be the consequence of a reduced inflow of suspended sediment causing homopycnal flow, and thorough mixing of th

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1982.tb00076.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY

摘要:

ABSTRACTSurface textures of quartz grains have been examined from five samples from the Laurentian Fan and Sohm Abyssal Plain, representing varied transport distances and power of the depositing turbidity current. The grains retain their primary irregular shape derived from glacial erosion, and glacial surface textures are preserved in dish‐shaped depressions. These features have been superimposed by a slight rounding of edges and an abundance of collision‐induced markings, particularly mechanical V‐forms. The most intense current modification of this sort occurs in mid‐Wisconsinan or earlier sands that have been transported over 1000 km to the distal Sohm Abyssal Plain by turbidity currents. Collision textures probably develop during grain flow on the steep continental slope: delicate resedimented shelf foraminifera are preserved in the same turbidites and most have been transported exclusively in sus

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1982.tb00077.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY

摘要:

ABSTRACTA pilot study with Holocene fluvial sands was undertaken in order to evaluate the effects of source rock composition and climate on natural abundances of rare elements (REE) in the first leg of the sedimentary cycle. We have analysed the medium grained sand fraction of samples collected from first order streams exclusively draining granitic plutons in Montana (semi‐arid), Georgia (humid), and in South Carolina (humid). Despite compositional differences between parent plutons the REE distribution patterns (but not the total absolute abundances) of the daughter sands are very similar. Averages of the three areas have a La/Lu ratio of ∼ 103 showing a depletion of heavy REE with respect to an ‘average granite’ (La/Lu = 79) or the composite of North American Shales (NAS; La/Lu = 55). Also, the Eu/Sm ratio in sands from these areas is ∼ 0·22 which is very close to the NAS ratio of 0·21. However, the overall REE distribution of these sands is not similar to that of NAS in any way. We conclude that the major rock type, but neither its minor subdivisions nor the climate (in the source area), controls the REE distribution patterns in first cycle daughter sands. But, the total and the parent rock‐normalized abundances of REE in sands from the humid areas are much lower than those in sands from t

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1982.tb00078.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1982.tb00079.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1982.tb00080.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY