摘要:

ABSTRACTThe Paraná Basin (1 600 000 km2) is the largest intracratonic basin in southern South America and contains a thick (1300 m) Permo‐Carboniferous glacial succession (the Itararé Group). This paper describes over 1700 m of drill core recovered during recent exploration for oil and gas. Itararé Group sediments consist of massive and stratified diamictites interbedded with massive and graded sandstones, and massive and laminated mudstones. Facies are interpreted as the product of sediment gravity flows in a glacially influenced marine basin.Three stratigraphic formations can be defined across the basin, each consisting of a lowermost sandstone‐rich member overlain by a diamictite‐rich member. Examination of Itararé Group rocks both in core and outcrop shows that depositional processes were influenced by active faulting and downslope resedimentation on relatively steep and unstable substrate slopes. Primary glacial deposits such as tillites and associated striated pavements occur along the present eastern outcrop belt which probably coincided with the eastern basin margin during deposition of the Itararé Group. Ice masses fringing the eastern (southern African) and western (Bolivian) basin margins supplied sediment to the basin in the form of fluvio‐glacial deltas, fans and floating ice tongues. This sediment was then resedimented downslope as debris flows and turbidites.Both stratigraphic relationships and the regional distribution of facies types identify a clear pattern of basin subsidence and step‐wise expansion by outward faulting within Late Proterozoic mobile belts. The position of successive basin margins can be related to specific lineament structures in the underlying basement. Asymmetric expansion of the Paraná Basin occurred along the northern and southern basin margins during deposition of the Itararé Group; this expansion probably reflects shallow crustal adjustments activated by collisional movements along the Andean margin of South America during the H

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1993.tb01087.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTAt Tylicz, near Krynica Spa (Polish Carpathians), spelean deposits fill fissures and caverns in Eocene flysch rocks. They occur as: (1) clastic cave sediments transformed into hard crusts due to cementation by finely crystalline low‐Mg calcite, (2) drusy calcite that covers crust surfaces and fills voids in the crust and (3) colloform calcite.Two varieties of drusy calcite are distinguished: acicular and columnar. The acicular calcite is built up of crystallites forming spherulitic fans or cones. In places it is syntaxially covered with colloform calcite. The drusy calcite is low‐Mg ferroan calcite with non‐ferroan subzones, whereas the colloform calcite is a low‐Mg non‐ferroan variety.The columnar calcite crystals form fan‐like bundles. Cross‐sections cut perpendicular to the c‐axes of columnar crystals are equilateral triangular in shape, although some have slightly curved edges. The columnar crystals have steep rhombic terminations and most have curved triangular faces, i.e. gothic‐arch calcite. Saddle crystals have also been observed.The columnar crystals are composed of radially orientated crystallites whose long dimension is parallel to thec‐axis. The curved crystal faces of such polycrystals are interpreted as a result of differential growth rates of the crystallites.The spelean calcites precipitated from CO2‐saturated water. The high rate of CaCO3precipitation is thought to be responsible for the formation of radial structures.Finely crystalline calcite formed within pore spaces of clastic sediments close to the water‐air interface, drusy calcite crystallized beneath the water‐air interface, and colloform calcite precipitated

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1993.tb01088.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTA characteristic of fluvial conglomerates is a gradual decrease in pebble size in the direction of the palaeocurrents. However, for the ancient braided stream investigated in this study, a deviation from this norm was found.The Middelvlei gold placer is a quartz pebble conglomerate that is mined in a South African goldfield, known as the West Wits Line, situated west of Johannesburg. Stratigraphically the placer occurs near the base of the Central Rand Group of the Witwatersrand Supergroup.In previous studies, the placer and the underlying sequence have been shown to be fluvial in origin, constituting a coarsening upward genetic increment of strata. The complete conformable succession, however, is only preserved in the distal parts of the depositional environment. In most parts of the goldfield the placer is separated from the underlying sequence by a progressive unconformity, which disappears deeper into the basin. This unconformity is thought to have formed during a progradation.Palaeocurrents of the Middelvlei placer are towards the south‐southeast in the western parts of the study area and towards the south‐east in the east. Maximum pebble size, however, decreases towards the west‐southwest, that is in a direction roughly perpendicular to the palaeocurrents.A reconstruction of the palaeotopography of the base of the placer in the western parts of the study area revealed considerable variations in palaeoheight with a gradient of about 0·002 along depositional strike.The anomalous lateral pebble size variation of the placer is explained by postulating an eastward migration of the depositing braided stream, simultaneous with its progradation. During the eastward migration, erosion of the underlying sequence became progressively more severe. This created an eastward‐thinning wedge of underlying strata.The results of the study do not invalidate lateral pebble size variation as a regional palaeoslope indicator. However, the findings suggest that caution should be exercised in the application of this i

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1993.tb01089.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTFabrics of phosphatized calcium carbonate particles in various phosphorites have been studied using scanning electron microscopy coupled with X‐ray dispersive microanalysis. Replacement of calcium carbonate by apatite has been observed in bivalve shell fragments and in foraminiferal tests; replacement proceeds at constant volume with excellent preservation of the original microtextures. In some deposits, replacement of carbonate by apatite is the main phosphogenic process. However, in general, the process seems to be far less important than might be believed purely on the basis of thin section observations. In many phosphorites, internal or external apatite moulds of bioclasts are common, including very small particles such as coccoliths in phosphatized chalks. Apatite precipitation was typically followed by carbonate dissolution. Later apatite precipitation within the dissolution voids may produce partial or total phosphate pseudomorphs of the original carbonate grain. In these examples direct replacement of carbonate by phosphate cannot be demonstrate

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1993.tb01090.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTBalancing the sediment budget of the Atlantic shoreline of Long Island has proved to be problematic because more sediment has been found to be moving westward in littoral drift than was estimated to be supplied by wave erosion of the Ronkonkoma terminal moraine at Montauk Point. There is a need for the existence of an additional, possibly offshore source. As part of a larger Atlantic shore study, 11 beach samples from Fire Island were compared with samples representing the known glacial source at Montauk Point, and also with four offshore samples to the south. Comparisons were based upon quartz sand grain surface texture patterns derived by scanning electron microscope checklist analysis. Initial evidence of an offshore‐onshore link was provided by qualitative results examining surface texture variability between samples, illustrated by photographic evidence and written notes. This evidence was confirmed consistently during quantitative analysis. In particular, canonical variate analysis grouped two offshore samples representing deposits of buried glacial to fluvioglacial lobes with beach samples from Fire Island in adjacent onshore locations or slightly downdrift. In addition, canonical variate analysis and factor analysis using the spssx package, together with cluster analysis using the ARTHUR package, depicted the main factors and texture variables largely responsible for sample discrimination. The significance of both the offshore‐onshore link and the associated factors influencing quartz grain surface texture was revealed by the fact that they are strongly related to each other and to the regional geomorphol

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1993.tb01091.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTThe excellently preserved metre‐scale, linear bedforms in an aeolian horizon of the Proterozoic Dhandraul Quartzite, India, show oppositely dipping strata arranged in a zigzag pattern. The strata are dominantly of translatent type, deposited by along‐crest migrating ripples preserved on the flanks of dunes. The bedforms thus may be interpreted in a morphodynamic sense as longitudinal (seif) dunes.In order to determine the regional palaeoflow pattern, the migration directions of ripples preserved at the top of sheet sandstones that are associated with the dune cross‐strata and internally show subhorizontal translatent strata were measured. A directionally varying flow with a mean direction nearly parallel to the mean axial trend of the dunes is indicated. The kinematics of the dunes were thus largely the result of alternate operation of two oblique flow components, each of which was deflected at a dune crest into an along‐crest flow on the downwind flank of the dune. The deflected flow formed along‐crest migrating ripples, which in turn deposited climbing ripple strata. Alternate deposition on the two opposite flanks resulted in near vertical accretion of the dunes, as is indicated by the zigzag pattern of stratal ar

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1993.tb01092.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTFaunally restricted argillaceous wackestones from the Middle Jurassic of eastern England contain evidence of early diagenetic skeletal aragonite dissolution and stabilization of the carbonate matrix, closely followed by precipitation of zoned calcite cements, and precipitation of pyrite. Distinctive cathodoluminescence and trace element trends through the authigenic calcites, their negative δ13C compositions and the location of pyrite in the paragenetic sequence indicate that calcite precipitation took place during sequential bacterial Mn, Fe and sulphate reduction. Calcite δ18O values are compatible with cementation from essentially marine pore fluids, although compositions vary owing to minor contamination with18O‐depleted ‘late’cements. Mg and Sr concentrations in the calcites are lower than those in recent marine calcite cements. This may be a result of kinetic factors associated with the shallow burial cementation microenvironments.Bicarbonate for sustained precipitation of the authigenic calcites was derived largely from aragonite remobilization, augmented by that produced through anaerobic organic matter oxidation in the metal and sulphate reduction environments. Aragonite dissolution is thought to have been induced by acidity generated during aerobic bacterial oxidation of organic matter. Distinction of post‐oxic metal reduction and anoxic sulphate reduction diagenetic environments in modern carbonate sediments is uncommon outside pelagic settings, and early bacterially mediated diagenesis in modern platform carbonates is associated with extensive carbonate dissolution. High detrital Fe contents of the Jurassic sediments, and their restricted depositional environment, were probably the critical factors promoting early cementation. These precipitates constitute a unique example of calcite authigenesis in shallow water limestones during bacterial Mn and Fe

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1993.tb01093.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTCombined petrographic and geochemical data document several kinds of detrital carbonate in subsurface Miocene‐Oligocene mudstones of the Texas Gulf Coast. In the extreme south of Texas, in muddy sediments deposited by ancient precursors to the modern Rio Grande, mudstone carbonate is dominated by extrabasinal detritus derived from Cretaceous limestones of the Edwards plateau. Further north, Oligocene mudstones contain carbonate that is mostly syndepositional skeletal material. Minor amounts of authigenic carbonate are found as replacements of silt sized feldspars in all the mudstones. Depth‐related shifts in δ18Ocalciteand δ13Ocalcitesuggest that small amounts of authigenic carbonate also form by replacement of detrital carbonate (‘recrystallization’), although this cannot be demonstrated petrographically. Pervasive carbonate cementation in primary pores in mudstones is not generally observed. Textural evidence of carbonate loss through pressure dissolution is widespread, providing a mechanism for the depth‐related decline in carbonate content observed in many Gulf Coast wells. Carbonate dissolution in mudstones, and export of the CaCO3, implies a massive acid source, probably resulting from silicate reactions within the mudstones during burial

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1993.tb01094.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTThe Cercadillo Sandstone and Siltstone (CSS) Formation forms the upper part of the Buntsandstein fluvial facies in the western margin of the Iberian Basin. Sedimentation was controlled by two major pre‐Triassic tectonic systems: the Iberian Ranges System (NW‐SE) and the Central System (NE‐SW). Fluvial sedimentation during this period took place in coexisting braided and meandering channels, together with associated ephemeral episodes. This unusual association was due to a combination of structural control by the main normal faults coupled with large discharge fluctuations.The CSS Formation consists of two major fluvial sequences with very similar vertical patterns comprising low sinuosity stream deposits that pass upwards into high sinuosity deposits associated with extensive floodplain sediments. A widespread calcrete horizon is developed at the top of the lower sequence and a surface with pedogenic features is present at the top of the upper sequence. These horizons are interpreted to be the result of subaerial exposure during a period of no subsidence and equilibrium in the erosion‐sedimentation processes in the area, probably due to quiescent periods on the master faults that delineate the basin.Minor changes within the sequences are probably due to discharge fluctuations of a climatic origin whereas the general vertical trend of the sequences is suggested to be tectonically controlled. Palaeocurrent data show a general flux of sediments to the south‐east, parallel to the axis of the Iberian Basin, but the final part of the upper sequence palaeocurrents to the north‐east are interpreted to indicate a local reactivation of secondary, transverse faults in the south‐east part of t

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1993.tb01095.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1993.tb01096.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY