Low-Mg calcite marine cement in Cretaceous turbidites: origin, spatial distribution and relationship to seawater chemistry   总被引：2，自引：0，他引：2  

JAMES P. HENDRY  NIGEL H. TREWIN†  ANTHONY E. FALLICK 《Sedimentology》1996,43(5):877-900

Lower Cretaceous (Hauterivian) bioclastic sandstone turbidites in the Scapa Member (North Sea Basin) were extensively cemented by low-Mg calcite spars, initially as rim cements and subsequently as concretions. Five petrographically distinct cement stages form a consistent paragenetic sequence across the Scapa Field. The dominant and pervasive second cement stage accounts for the majority of concretions, and is the focus of this study. Stable-isotope characterization of the cement is hampered by the presence of calcitic bioclasts and of later cements in sponge spicule moulds throughout the concretions. Nevertheless, trends from whole-rock data, augmented by cement separates from synlithification fractures, indicate an early calcite δ¹⁸O value of+0·5 to -1·5‰ PDB. As such, the calcite probably precipitated from marine pore fluids shortly after turbidite deposition. Carbon isotopes (δ¹³C=0 to -2‰ PDB) and petrographic data indicate that calcite formed as a consequence of bioclastic aragonite dissolution. Textural integrity of calcitic nannoplankton in the sandstones demonstrates that pore fluids remained at or above calcite saturation, as expected for a mineral-controlled transformation. Electron probe microanalyses demonstrate that early calcite cement contains <2 mol% MgCO₃, despite its marine parentage. Production of this cement is ascribed to a combination of an elevated aragonite saturation depth and a lowered marine Mg²⁺/Ca²⁺ ratio in early Cretaceous ‘calcite seas’, relative to modern oceans. Scapa cement compositions concur with published models in suggesting that Hauterivian ocean water had a Mg²⁺/Ca²⁺ ratio of ≤1. This is also supported by consideration of the spatial distribution of early calcite cement in terms of concretion growth kinetics. In contrast to the dominant early cement, late-stage ferroan, ¹⁸O-depleted calcites were sourced outwith the Scapa Member and precipitated after 1–2 km of burial. Our results emphasize that bioclast dissolution and low-Mg calcite cementation in sandstone reservoirs should not automatically be regarded as evidence for uplift and meteoric diagenesis.  相似文献   

Burial dolomitization of the Woo Dale Limestones Formation (Lower Carboniferous), Derbyshire, England     

KEVIN SCHOFIELD  ANTHONY E. ADAMS 《Sedimentology》1986,33(2):207-219

Dolomite occurs in open-shelf packstones and grainstones of the Vincent House Member of the Woo Dale Limestones Formation (Lower Carboniferous) in the Wye Valley area, east of Buxton, Derbyshire, Central England. Two stages of dolomitization can be recognized. Stage 1 consists of limpid, non-luminescent dolomite lacking detectable iron and manganese. Stage 2 comprises cloudy, inclusion-rich, orange-red luminescing dolomite with significant iron and manganese. The contact between the two stages is irregular and suggests a hiatus between dolomite growth stages. Dolostones lacking any unreplaced limestone may contain up to 35% calcite cement rilling intercrystal porosity. This cement shows a characteristic zonal sequence and is normally in optical continuity with the surrounding dolomite crystals. In some cases, precementation compaction of the dolomite mosaic caused fracturing of the crystals. Sherds of dolomite on the floors of pore-spaces then provided the nucleus for growth of calcite cement. The petrographic character and stratigraphic distribution of the dolomite suggest that dolomitization occurred at depth, during burial of the Woo Dale Limestones in the Late Carboniferous. Dolomitizing solutions, expelled from basinal shales, moved up-dip along permeable limestones or along the sub-Carboniferous unconformity. Fluids responsible for the generation of stage 2 dolomite may have been contaminated by ions such as iron and manganese released from the alteration of volcanic rocks which occur beneath the Carboniferous Limestone in the Wye Valley area.  相似文献   

RELATIONSHIPS BETWEEN CATCHMENT ATTRIBUTES AND HYDROLOGICAL RESPONSE CHARACTERISTICS IN SMALL AUSTRALIAN MOUNTAIN ASH CATCHMENTS     

DAVID A. POST  ANTHONY J. JAKEMAN 《水文研究》1996,10(6):877-892

Sixteen small catchments in the Maroondah region of Victoria, Australia were analysed using rainfall, temperature and streamflow time series with a rainfall–runoff model whose parameters efficiently characterize the hydrological response of a catchment. A set of catchment attributes for each of these catchments was then compared with the associated set of hydrological response characteristics of the catchments as estimated by the model. The time constant governing quickflow recession of streamflow (τ_q) was related to the drainage network and catchment area. The time constant governing slowflow recession of streamflow (τ_s) was related to the slope and shape of the catchment. The parameter governing evapotranspirative losses ( f ) was related to catchment gradient and vegetative water use. Forestry activities in the catchments changed evapotranspirative losses and thus total volume of streamflow, but did not affect the rate of streamflow recession.  相似文献   

Jurassic septarian concretions from NW Scotland record interdependent bacterial, physical and chemical processes of marine mudrock diagenesis   总被引：3，自引：0，他引：3  

JAMES P. HENDRY  MICHAEL J. PEARSON†  NIGEL H. TREWIN‡  ANTHONY E. FALLICK§ 《Sedimentology》2006,53(3):537-565

Septarian concretions in the Staffin Shales Formation (Kimmeridgian, Isle of Skye) allow controls on concretion rheology and septarian cracking to be investigated. Stratabound concretions consist of anhedral ferroan calcite microspar enclosing clay and minor pyrite. Intergranular volumes range from 77% to 88%, and calcite δ¹³C and δ¹⁸O values in most concretion bodies range from ?10·0‰ to ?17·3‰ and +0·3‰ to ?0·6‰ respectively, consistent with rapid and pervasive cementation in marine pore fluids. Septarian rupture occurred during incipient cementation, with a sediment volume reduction of up to 43%. Crack‐lining brown fibrous calcite records pore fluid re‐oxygenation during a depositional hiatus, followed by increasing Fe content and δ¹³C related to bacterial methanogenesis. Brown colouration results from an included gel‐like polar organic fraction that probably represents bacterially degraded biomass. A new hypothesis for concretion growth and septarian cracking argues that quasi‐rigid ‘proto‐concretions’ formed via binding of flocculated clays by bacterial extracellular polysaccharide substances (EPS). This provided rheological and chemical conditions for tensional failure, subcritical crack growth, volume contraction, calcite nucleation, and incorporation of degraded products into crack‐lining cements. Bacterial decay of EPS and syneresis of host muds provided internal stresses to initiate rupture at shallow burial. Development of septarian (shrinkage) cracks in muds is envisaged to require pervasive in situ bacterial colonization, and to depend on rates of carbonate precipitation versus EPS degradation and syneresis. Subsequent modification of septarian concretions included envelopment by siderite and calcite microspar, hydraulic fracturing associated with Cretaceous shallow burial or Palaeogene uplift; and cementation by strongly ferroan, yellow sparry calcite that records meteoric water invasion of the host mudrocks. An abundance of fatty acids in these spars indicates aqueous transport of organic breakdown products, and δ¹³C data suggest a predominantly methanogenic bicarbonate source. However, the wide δ¹⁸O range for petrographically identical cement (?1·3‰ to ?15·6‰) is difficult to explain.  相似文献   

The simulated response of dimethylsulfide production in the Arctic Ocean to global warming   总被引：2，自引：0，他引：2  

ALBERT J. GABRIC  BO QU  PATRICIA MATRAI  ANTHONY C. HIRST 《Tellus. Series B, Chemical and physical meteorology》2005,57(5):391-403

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Oxygen Isotope Geochemistry of Oceanic-Arc Lavas   总被引：19，自引：11，他引：8  

EILER  JOHN M.; CRAWFORD  ANTHONY; ELLIOTT  TIM; FARLEY  KENNETH A.; VALLEY  JOHN W.; STOLPER  EDWARD M. 《Journal of Petrology》2000,41(2):229-256

Variations of oxygen isotope ratios in arc-related lavas canconstrain the contributions of subducted crustal igneous rocks,sediments, and fluids to the sub-arc mantle. We have measuredoxygen isotope ratios in 72 arc and back-arc lavas from fiveocean–ocean subduction zone systems using laser-fluorinationanalyses of olivine and other phenocrysts and glass. Eightypercent of our samples have  相似文献   

Tectono-sedimentary analysis of alternate-polarity half-graben basin-fill successions: Late Devonian-Early Carboniferous Horton Group, Cape Breton Island, Nova Scotia     

ANTHONY P. Hamblin  BRIAN R. Rust † 《Basin Research》1989,2(4):239-255

Abstract The Devono-Carboniferous Horton Group of Cape Breton Island was mostly deposited in two fault-bounded asymmetric sub-basins which were part of a large intracontinental rift system. This system lay at a palaeolatitude of about 10–15^o S–a warm, semi-arid climate. The half-graben sub-basins had opposed polarity, were approximately 100 times 50 km in size and were separated by a narrow zone of elevated Acadian basement. These features are common to the adjacent structural segments of known rifts, and are unlike those of transtensive pull-apart systems. Sedimentation occurred in four successive depositional systems which reflect a tectonic evolution of increased and then decreased extensional subsidence through the 8–12 Myr interval represented. Post-Acadian sedimentation began with System 1 bimodal volcanics and grey distal braided fluvial sediments deposited in a slowly subsiding broad linear sag basin. System 2 consists of reddened braidplain sediments near fault-bounded margins and mudflat/playa sediments in sub-basin centres, deposited in two discrete asymmetric sub-basins with a general upward-fining trend. Gradual expansion of the mudflat setting and confinement of coarse marginal fades is interpreted as a response to increasingly rapid and deep fault-bounded subsidence. Depositional System 3, is a complex of grey lacustrine offshore, shoreline and fan delta facies deposited in two adjacent half-graben segments with opposed polarity of asymmetry. An increased rate of tectonic subsidence allowed a large standing body of water to accumulate lacustrine sediments along the axis of each sub-basin during this phase of maximum subsidence. System 4 consists of reddened proximal alluvial fan, medial fluvial and distal grey meandering fluvial/floodplain sediments which accumulated in sub-basins with fault-bounded margins and asymmetry identical to those of earlier systems, indicating a continuation of tectonic style. However, an overall coarsening-upward trend indicates waning of active fault-related subsidence and consequent progradation of marginal coarse wedges to fill the sub-basins. Rapid marine transgression and deposition of Windsor Group carbonates, evaporites and elastics continued within a more extensive rift basin during renewed fault-bounded subsidence.  相似文献   

Geochemistry of the J-M Reef of the Stillwater Complex, Minneapolis Adit Area II. Silicate Mineral Chemistry and Petrogenesis   总被引：1，自引：0，他引：1  

BARNES  STEPHEN J.; NALDRETT  ANTHONY J. 《Journal of Petrology》1986,27(4):791-825

The platinum group element (PGE) rich J-M Reef of the StillwaterComplex is associated with the reappearance of olivine and chromiteas cumulus phases within a sequence of gabbroic and noriticcumulates. The Olivine-Bearing Subzone (OBZ I) which hosts theReef is interpreted as the result of fresh influxes of magmainto the Stillwater chamber. Cumulus pyroxenes from the Norite Subzone (No II) overlyingOBZ I are enriched in Fe over Mg, depleted in Cr and have similarNi contents relative to pyroxenes from the underlying GNo I.The most primitive pyroxene compositions are found in PBc andBc layers within OBZ I, and the most evolved as oikocrysts inOBZ I anorthositic layers. Plagioclase An content correlatesclosely with pyroxene MgO/FeO, except within OBZ I where a verticalAn/En trend is observed. These observations may be synthesized into a model involvingreplenishment of the chamber by high pressure differentiatesof a liquid P, representing the parental magma to the UltramaficSeries of the Stillwater. Liquid P crystallizes olivine at thepressure of the Stillwater chamber, but owing to migration ofthe olivine-orthopyroxene phase boundary is saturated with bronziteat higher pressures. The spectrum of cumulus mineralogy andmineral compositions within and above OBZ I may be generatedby a series of magma influxes having compositions derived byvarious degrees of high pressure fractionation from liquid P. The characteristic textures of the heterogenous plagioclase-olivinecumulates of OBZ I may be explained by mixing of P-type replenishingliquid with a fractionated resident liquid containing suspendedplagioclase crystals. Anorthositic layers within OBZ I formedfrom influxes of liquid derived by approximately 30 per centfractionation of P. Model density calculations indicate that the liquids shouldhave entered the chamber as a serious of buoyant plumes. Lateralvariations in OBZ I stratigraphy arise from pulses of differentsize spreading to varying distances from the feeder axis. TheReef formed from a single layer transgressing earlier faciestransitions.  相似文献   

Pleistocene occurrences of the European pond tortoise (Emys orbicularis L.) in Britain     

ANTHONY JOHN STUART 《Boreas: An International Journal of Quaternary Research》1979,8(3):359-371

Emys orbicularis is the most northerly-breeding chelonian in the world, present in southern and central Europe, but absent from the British Isles at the present day. Mean July temperatures well in excess of 18C appear necessary for the living species to breed successfully. In England records of E. orbicularis are now available from the Cromerian Hoxnian, Ipswichian and Flandrian temperate stages, suggesting mean July temperatures at least 2C warmer than now. Most of the available material is of Ipswichian (= Eemian) age, spanning most of this temperate stage and indicating generally high summer temperatures even near the end. On the other hand there is only one English E. orbicularis of Flandrian age, dating from the climatic optimum, compared with numerous finds in southern Scandinavia, probably reflecting the prevalence of cooler and damper summers in the more oceanic northwest.  相似文献   

Tephrochronology and the late Holocene volcanic and flood history of Eyjafjallajökull,Iceland     

ANDREW J. DUGMORE  ANTHONY J. NEWTON  KATE T. SMITH  KERRY‐ANN MAIRS 《第四纪科学杂志》2013,28(3):237-247

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