摘要:

ABSTRACTA major palaeokarst erosion surface is developed within the middle Proterozoic Elu Basin, northwestern Canada. This palaeokarst is named the sub‐Kanuyak unconformity and truncates the Parry Bay Formation, a sequence of shallow‐marine dolostones that were deposited within a north‐facing carbonate platform under a semi‐arid climate.The sub‐Kanuyak unconformity exhibits up to 90 m of local relief, and also formed under semi‐arid conditions when Parry Bay dolostones were subaerially exposed during a relative sea‐level drop of about 180 m. Caves and various karren developed within the meteoric vadose and phreatic zones. Their geometry, size and orientation were largely controlled by northwest‐ and northeast‐trending antecedent joints, bedding, and lithology. Near‐surface caves later collapsed forming valleys, and intervening towers or walls, and plains. Minor terra rossa formed on top of highs. Karstification was most pronounced in southern parts of Bathurst Inlet but decreased northward, probably reflecting varying lengths of exposure time along a north‐dipping slope.The Kanuyak Formation is up to 65 m thick, and partially covers the underlying palaeokarst. It consists of six lithofacies: (i) breccia formed during collapse of caves, as reworked collapse breccia and regolith; (ii) conglomerate representing gravel‐dominated braided‐fluvial deposits; (iii) sandstone deposited as braided‐fluvial and storm‐dominated lacustrine deposits; (iv) interbedded sandstone, siltstone and mudstone of sheet flood origin; (v) dolostones formed from dolocretes and quiet‐water lacustrine deposits; and (vi) red‐beds representing intertidal‐marine mudflat deposits. Rivers flowed toward the northwest and northeast within karst valleys and caves; lakes were also situated within valleys; marine mudflat sediments completely cover the palaeokarst to the north.A regional correlation of the sub‐Kanuyak unconformity with the intra‐Greenhorn Lakes disconformity within the Coppermine homocline suggests that similar styles of karstification occurred over an extensive region. The Elu Basin palaeokarst, however, was developed more landward, and was exposed for a longer period of time th

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1991.tb01872.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

ABSTRACTStromatolitic crusts on stick‐like and platyPoritescorals forming Messinian reefs in Almería played an important role in supporting and binding the brittle corals. The crusts were previously regarded as probable marine cements. However, their clotted, peloidal, and micritic fabrics are directly comparable with those of stromatolites. They accreted allochthonous grains even on vertical faces, and include bushy fabrics closely comparable with those produced by cyanobacterial calcification. They contain numerous fenestrae, exhibit rapid fabric variation, and locally form micro‐columns and laminated domes. Their similarities to peloid micrite crusts in Recent reefs suggest that at least some of these Recent crusts are microbial in origin, even though they have widely been interpreted as marine cements.The sedimentary effects of crust development substantially affected both the morphology and relief of the reefs and the generation of reefal clasts. Binding of the reef‐frame in the Pinnacle and Thicket zones in the lower and middle parts of the reef created a rigid margin which shed large (commonly up to 5 m) cuboidal blocks of coral‐stromatolite frame. The blocks broke along planes of weakness provided by the verticalPoritessticks and were deposited on the Fore‐Reef Slope. In the uppermost parts of the reefs crusts dominate the structure, constituting 80% or more of the rock. Veneers up to 15 cm thick encrust thin irregularPoritesplates to create a solid Reef Crest Zone which has not been recognized before.The coral‐stromatolite framework is associated with echinoids, crustose corallines and halimedacean algae which, together with the scleractinians, indicate normal marine salinity throughout

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1991.tb01873.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

ABSTRACTThe Lower Permian Aldebaran Sandstone is the principal hydrocarbon reservoir in the Denison Trough (Bowen Basin), east‐central Queensland, Australia. It accumulated in a wide range of fluvio‐deltaic and nearshore marine environments. Detailed petrological study of the unit by thin section, X‐ray diffraction, scanning electron microscopy, electron microprobe and isotopic analysis reveals a complex diagenetic history which can be directly related to depositional environment, initial composition and burial‐temperature history. Early diagenetic effects included the precipitation of pyrite, siderite and illite‐smectite rims (δ18O (SMOW) =+8.9 to + 11.3‰). Deep burial effects included physico‐chemical compaction and the formation of quartz overgrowths, siderite (δ13C(PDB) =−34.0 to + 11.5‰, δ18O =−0.7 to +22.7‰), illite/illite‐smectite and ankerite (δ13C=−9.3 to −4.9‰) δ18O=+ 7.6 to + 14.4‰). Involved fluids were in part ‘connate meteoric’ water derived from compaction of the underlying freshwater Reids Dome beds. Important post‐maximum burial effects, controlled by deep meteoric influx from the surface, were ankerite and labile grain dissolution and formation of kaolinite (δ18O=+7.8 to +8.9‰, δD=−115 to −99‰), calcite (δ13C=−9.5 to +0.9‰, δ18O=+9.0 to +20.0‰) and dawsonite (δ13C=−4.0 to +2.3‰, δ18O=+9.8 to +19.8‰), the formation of dawsonite reflecting eventual stagnation of the aquifer. Entrapment of contained hydrocarbons was a relatively recent event which may be continuing today. Reservoir quality varies from marginal to good in the west to poor in the east, with predictable tre

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1991.tb01874.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

ABSTRACTSedimentation from radially spreading gravity currents generated at the top of ascending sediment‐laden plumes is described by a model which assumes that sediment is dispersed homogeneously by turbulence in the gravity current, resulting in an exponential decrease in the concentration of sediment with time as particles settle out of the lower boundary of the current. For radial spreading this model predicts a Gaussian distribution of sediment accumulation away from the source with an exponential constant,B, which depends on flow rate,Q, and particle settling velocity,v(B=nv/Q). In the experiments described, sedimentation occurs from gravity currents generated by ascent of buoyant, particle‐laden plumes of fresh water in a tank of salty water. The sediment accumulation shows close agreement with the theoretical model, and the Gaussian decay constant,B, can be determined from a maximum in the accumulated mass of sediment per unit distance and from the slope of the line In(S/S0) = ‐Br2, whereris the radial distance,Sis the sediment mass flux per unit area andS0is the value ofSatr=0. Data from the dispersal of volcanic ejecta from a large (c. 24 km high) plinian eruption column in the Azores also show good agreement with the theory, confirming that it is general and independent of scale and the nature of the fluid. The experimental data also show a change in sedimentation behaviour at distances from the source corresponding to the corner of the plume where it diverts into a lateral gravity current and there is an abrupt decrease in vertical velocity. Sedimentation of coarse grain sizes, between the source and the corner, occurs from the inclined plume margins and does not behave as predicted by the theoretical

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1991.tb01875.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

ABSTRACTNodular chert from the middle and upper Arbuckle Group (Early Ordovician) in the Slick Hills, SW Oklahoma, was formed by selective replacement of grainstones, burrow fillings, algal structures, and evaporite nodules. Chert nodules are dominantly microquartz with minor fibrous quartz (both quartzine and chalcedony), megaquartz, and microflamboyant quartz. Lepisphere textures of an opal‐CT precursor are preserved in many (especially in finely‐crystalline) chert nodules. The δ18O values of microquartz chert range from +23.4 to + 28.80/00(SMOW), significantly lower than those of Cenozoic and Mesozoic microquartz chert formed both in the deep sea and from near‐surface sea water. The δ18O values of chert decrease with increasing quartz crystal size.Silicification in the Arbuckle Group occurred during early diagenesis, with the timing constrained by the relative temporal relationships among silicification, burial compaction, and early dolomite stabilization. Silica for initial chert nucleation may have been derived from both dissolution of sponge spicules and silica‐enriched sea water. Chert nucleation appears to have been controlled by the porosity, permeability, and organic matter content of precursor sediments. This conclusion is based on the fact that chert selectively replaced both porous grainstones and burrows and algal structures enriched in organic matter. Growth of chert probably occurred by a maturation process from opal‐A(?), to opal‐CT, to quartz, as indicated by the presence of opal‐CT precursor textures in many chert nodules. Although field and petrographic evidence argues for an early marine origin for chert in the Arbuckle Group, the light δ18O values are inconsistent with this origin. Meteoric resetting of the δ18O values of the chert during exposure of the carbonate platform best explains the light δ18O values because: (i) the δ18O values of chert nodules decrease with decreasing δ18O values of host limestones, and (ii) chert nodules from early dolomite, which underwent more extensive meteoric modification than associated limestones, have lighter δ18O values than chert nodules from limestones. Increasing recrystallization of chert nodules by meteoric water resulted in progressive18O depletion and (quartz

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1991.tb01876.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

ABSTRACTDuring Serravallian through Messinian time, marine carbonates flanked topographic highs that rimmed Neogene basins in the Western Mediterranean. Middle to upper Miocene carbonate strata in the Las Negras and Nijar areas (southeastern Spain) are 50‐150 m thick and display 50‐200 m of shelf‐to‐basin relief over 1‐2 km. Detailed studies in those areas document the effects of relative sea‐level change on sedimentation, biotic composition, and reef development. We identify three previously unrecognized, regionally correlatable depositional sequences (DS1, DS2, DS3) that occur between the underlying basement and the overlying Terminal Carbonate Complex. The lower depositional sequences (DS1, DS2) are mostly normal marine shelf (ramp) carbonates deposited on the flanks of basement highs. The basal part of DS2 locally contains some megabreccia reef blocks composed ofTarbellastraeaandPorites. These blocks are the first evidence of reef growth in the area and represent a previously unrecognized period of reef development prior to the fringing reef development. The reef blocks probably formed as upslope patch reefs that were eroded and transported to distal slope locations. The upper sequence (DS3) is characterized by clinoform strata of aPorites‐dominated fringing reef complex that prograded basinward in a downstepping style with successively younger reefs forming in a topographically lower and more basinward position as a result of a net sea‐level drop.Regional correlation of Miocene shallow‐marine strata between basins in Spain and elsewhere in the western Mediterranean is complicated because basins were semi‐isolated from adjacent basins making physical correlation impossible. In addition, age‐definitive biostratigraphic markers are poorly preserved in most of the Miocene shallow‐water strata; basinal sediments that are more easily dated by microfossils do not typically interfinger with the shallow‐marine strata in outcrop. Even where datable microfossils are found, resolution of dating is poor.Our studies in the Las Negras and Nijar areas illustrate the usefulness of integrating sedimentological, geometric and biotic data with locally derived relative sea‐level (accommodation space) curves for correlation. The relative sea‐level curves for each area show remarkable similarities in shape and magnitude of sea‐level changes. These curves indicate several relative sea‐level fluctuations during Miocene carbonate deposition prior to the major sea‐level drop at the end of DS3 deposition that culminated in the exposure of the basin margin deposits and the deposition of evaporites in basinal areas during the Messinian.The depositional sequences in the Las Negras and Nijar areas may correlate with depositional sequences of similar age throughout the southern Cabo de Gata area, in Mallorca some 600 km to the northeast, and possibly in other Mediterranean locations. The widespread occurrence and possible correlation of the depositional sequences suggest regional processes such as eustacy or tectonism for their formation.The integration of sedimentological, palaeontological and sequence stratigraphic studies, and the construction of relative sea‐level (accommodation space) curves may help in the interpretation of depositional histories of shallow‐marine carbonate complexes and correlation of these strata between isolated areas. Other dating methods, in addition to microfossil dating, may allow for better age determination of the sequences and aid in identifying the importance of eustac

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1991.tb01877.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

ABSTRACTThe sandbodies of the Bearreraig Sandstone Formation (Inner Hebrides, UK) are cemented by two generations of calcite. The first generation, an inhomogeneous ferroan calcite (0.05−3.28 mol% FeCo3) formed during sulphate reduction (δ13C =−24 to −32%o PDB) in marine porewaters (δ18O of cement from −1 to −4%o PDB) at very shallow burial depths (a few centimetres). These cements are rare but form millimetre‐scale clusters of crystals which acted as nuclei to the later, concretionary cements.The second generation of cements are more homogeneous ferroan calcites (mean 1−58% mol% FeCo3) which evolve to progressively higher Fe/Mg ratios. They are sourced by shell dissolution (δ13C of cement from +1 to −3%o PDB) into meteoric (δ18O of cement from −6 to −10%o PDB) or mixed marine meteoric waters (δ18O of cement from −4 to −6%o SMOW). These were introduced into the formation either during Bathonian times as a freshwater lens, or, subsequent to partial inversion, by confined aquifer flow. Corroded feldspars within the concretions suggest that an interval of at least 8 Ma separated the deposition of the sediments from the onset of concretion growth.Abundant concretions are preferentially developed at certain horizons within the sandbodies, where the early generation of ferroan calcite cements provided nuclei. The latter formed close to the sediment‐water interface, the concentration of cement within the sediment being related to sedimentation rate. The relatively high concentrations of the first generation of cement, upon which the concretionary horizons are nucleated, formed during pe

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1991.tb01878.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

ABSTRACTDetailed study of marine shales (the Ostracod zone) within a Cretaceous, third‐order transgressive‐regressive sequence in the Alberta Foreland Basin reveals a systematic association between shell beds and parasequence‐scale flooding surfaces, including surfaces of maximum flooding. The Ostracod zone (a subsurface lithostratigraphic unit known as the Calcareous Member in outcrop) consists of 10‐20 m of black shale and bioturbated sandstones with many thin, fossiliferous limestones. Parasequences (shallowing‐up cycles 2–3 m thick) were delineated within this transgressive unit based on lithology, sedimentary structures, degree of bioturbation, dinoflagellate diversity, total organic carbon and carbon/sulphur ratios; many flooding surfaces are firmgrounds or hardgrounds.Shell‐rich limestones occur in three different positions relative to these flooding surfaces, and each has a distinctive bioclastic fabric and origin. (i) Base‐of‐parasequence shell beds (BOPs) lie on or just above flooding surfaces in the deepest water part of a parasequence; they are thin (up to a few centimetres), graded or amalgamated skeletal packstones/wackestones composed of well‐sorted granular shell, and are interpreted as hydraulic event concentrations of exotic shell debris. (ii) Top‐of‐parasequence shell beds (TOPs) are capped by flooding surfaces at the top, shallowest water part of a parasequence; they typically are several decimetres thick, are physically amalgamated packstones/grainstones or bioturbated wackestones, and contain abundant whole as well as comminuted shells; these are composite, multiple‐event concentrations of local shells. (iii) Mid‐sequence shell beds rest on as well as are capped by firmgrounds or hardgrounds, and are intercalatedbetweenparasequences in the deepest water part of the larger sequence; they are laterally extensive lime mudstones a few decimetres thick, with sparse shells in various states of dissolution, recrystallization and replacement; these beds are terrigenous‐starved hiatal concentrations and record maximum flooding within the Ostracod zone. Offshore sections of the Ostracod zone typically contain several starved mid‐sequence shell beds, underscoring the difficulty of identifying a single‘maximum flooding surfa

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1991.tb01879.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

ABSTRACTA remarkable suite of shallow‐water, gravity‐flow deposits are found within very thinly‐bedded siltstones and storm‐generated sandstones of member 2 of the Chapel Island Formation in southeast Newfoundland. Medium to thick siltstone beds, termed unifites, range from non‐graded and structureless (Type 1) to slightly graded with poorly developed lamination (Type 2) to well graded with lamination similar to that described for fine‐grained turbidites (Type 3). Unifite beds record deposition from a continuum of flow types from liquefied flows (Type 1) to turbidity currents (Type 3). Calculations of time for pore‐fluid pressure dissipation support the feasibility of such transitions. Raft‐bearing beds consist of siltstone with large blocks or‘rafts’ of thinly bedded strata derived from the underlying and adjacent substrate. Characteristics suggest deposition from debris flows of variable strength. Estimates of debris strength and depositional slope are calculated for a pebbly mudstone bed using measurable and assumed parameters. An assumed density of 2.0 g cm‐1and a compaction estimate of 50% gives a strength estimate of 79.7 dyn cm‐2and a depositional slope estimate of 0.77°.The lithologies and sedimentary structures in member 2 indicate an overall grain‐size distribution susceptible to liquefaction. Inferred high sediment accumulation rates created underconsolidated sediments (metastable packing). Types of sediment failure includedin situliquefaction (‘disturbed bedding’), sliding and slumping. Raft‐bearing debrites resulted from sliding and incorporation of water. Locally, hummocky cross‐stratified sandstone directly overlies slide deposits and raft‐bearing beds, linking sediment failure to the cyclical wave loading associated with large storms.The gravity flows of the Chapel Island Formation closely resemble those described from the surfaces of modern, mud‐rich, marine deltas. Details of deltaic gravity‐flow deposition from this and other outcrop studies further our understanding of modern deposits by adding a third dimension to studies pr

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1991.tb01880.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Book Reviewed in this article:Karst Geomorphology and Hydrology, by Derek Ford and Paul WilliamsBasin Analysis: Principles and Applications, by P. A. Allen and J. R. AllenPetroleum Basin Studies, by P. M. Shannon and D. NaylorSoils and Micromorphology in Archaeology, by A. M Courty, P. Goldberg and R. MacphailStratigraphy: Principles and Methods, by Robert M. Schoch, Van Nostrand ReinholdClassic Petroleum Provinces, ed. by J. BrooksNorthwest European Micropalaeontology and Palynology, ed. by D. J. Batten and M. C. KeenPaleobiology of the Dinosaursed. by J. O. FarlowGeological Map of Bangladesh, scale 1: 1000 000, Mohamed Khurshid Alam, A. K. M. Shahidul Hasan, Mujibur Rhaman Khan and J. W. Whitney

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1991.tb01881.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY