ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1991.tb01851.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

ABSTRACTThe infill of a small Variscan basin shows a complex succession of lacustrine and alluvial sediments whose pattern of distribution through time appears to have changed in response to contemporaneous volcanism. Following initial deposition of pyroclastic breccias and extrusion of lavas, small lakes were established on the floor of the basin. Their infills were probably locally controlled by relief on top of the lavas but the pattern was disrupted by phases of mass movement from the basin margin which emplaced debris flows and a sedimentary mélange containing large blocks of limestone and led to extensive soft‐sediment deformation. This complex sequence of events culminated in the emplacement of a widespread ash‐flow bed.An alluvial plain, established during a period of rather uniform subsidence, was rapidly drowned to give an extensive lake whose progradational infill led to re‐establishment of an alluvial plain. The deposition of a widespread airfall tuff coincided with the onset of differential subsidence during which major channel sandstones were stacked in association with poorly drained palaeosols in the east of the area, whilst a mudstone‐dominated sequence characterized by well‐drained palaeosols developed in the west. A return to more uniform subsidence appears to have coincided with the cessation of volcani

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1991.tb01852.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

ABSTRACTThe late Quaternary development of part of the lower continental rise off Western Sahara has been determined from an investigation of short (<2 m) gravity cores collected from a deep‐sea channel, the interchannel areas and an abyssal hill, between 30 and 33°N. Stratigraphic analysis is based on systematic variations in abundances of particular coccolith species and pelagic sediment types, referenced to the oxygen isotope time‐scale. During the last 73 000 years deposition in the channel has included volcaniclastic sand/silt turbidites and minor marl turbidites as well as pelagic sediments. The interchannel area has fewer turbidites, and the sands present were probably deposited from turbidity currents which spilt over the channel sides. The last‘event’ to give rise to sands in the channel and interchannel area occurred about 45 000 years ago. Although the channel has been inactive as an area of turbidity current deposition for the last 20 000 years, sands were deposited elsewhere on the lower rise, indicating that turbidity current transport routes have varied in time.Turbidity current deposition on the abyssal plain and low‐lying continental rise appears to be related to distinct sliding events involving transport of material from various sources. Thin marl turbidites are interbedded with pelagic sediments in the area of sediment drape. There is a strong correlation between these and the thick marl turbidites on the abyssal plain, suggesting that the same turbidity current‘events’, occurring about once every 25 000 years, gave rise to both sets of deposits. The thinner units probably represent deposition from the outer parts or tails of the large turbidity flows. The turbidites occur at glacial/interglacial transitions, suggesting that the slides that created them were triggered by mechanisms related to climatic change. Several volcaniclastic sand/silt units within the channel and in interchannel areas occupy mid‐stage stratigraphic positions, perhaps indicating a different triggering mechanism for slides around volcanic islands. A debris flow deposit (debrite), between 30°N, 21°W and 31°N, 24°W, is related to the Saharan Sediment Slide, a major mass movement feature on the continental slope over 1000 km to the southeast. Stratigraphic correlations indicate that this slide produced a large turbidity current as w

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1991.tb01853.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

ABSTRACTThe Swan Hills Formation (Middle‐Upper Devonian) of the Western Canada Basin is host to several NW‐SE‐trending gas fields developed in massive replacement dolostone. One of these, the Rosevear Field, contains two major dolostone trends along opposing margins of a marine channel that penetrates into a platform‐reef complex. Dolostones consist predominantly of branching and bulbous strdmatoporoid floatstones and rudstones with well‐developed moldic and vuggy porosity. Replacement dolomite is coarsely crystalline (100‐600 μm), inclusion‐rich, composed of euhedral through anhedral crystals and has a blotchy to homogeneous red cathodoluminescence. Geochemically, replacement dolomite is characterized by (i) nearly stoichiometric composition (50.1‐51.1 mol% CaCO3), (ii) negative δ18O values (mean=‐7.5‰, PDB) and (iii) variable87Sr/86Sr ratios ranging from values similar to Late Devonian‐Early Mississippian seawater (∼0.7082) to radiogenic compositions comparable to saddle dolomite cements (>0.7100).Dolomitization began after widespread precipitation of early, equant calcite spar and after the onset of pressure solution, implying that replacement dolomite formed in a burial environment. Oxygen isotope data suggest that dolomite formed at 35‐75°C, temperatures reached during burial in Late Devonian through Jurassic time, at minimum depths of 450 m. The linear NW‐SE orientation of most dolomite fields in the Swan Hills Formation is suggestive of fault control on fluid circulation. Two models are proposed for fault‐controlled circulation of dolomitizing fluids at the Rosevear Field. In the first, compaction‐driven, updip fluid migration occurred in response to basin tilting commencing in the Late Palaeozoic. Deep basinal fluids migrating updip were focused into channel‐margin sediments along fault conduits. The second model calls upon fault‐controlled convective circulation of (i) warm Devonian‐Mississippian seawater or (ii) Middle Devonian residual evaporitic brines.The overlap in87Sr/86Sr and δ18O compositions, and similar cathodoluminescence properties between replacement and saddle dolomites provide evidence for neomorphism of some replacement dolomite. Quantitative modelling of Sr and O isotopes and Sr abundances suggests partial equilibration of some replacement dolomite with hot radiogenic brines derived during deep burial of the Swan Hills Formation in the Late Cretaceous‐Palaeocene. Interaction of replacement dolomite with deep brines led to enrichment in87Sr while lea

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1991.tb01854.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

ABSTRACTThe Holder Formation (Pennsylvanian, Virgilian) of southern New Mexico, USA, consists of limestones interbedded with siliciclastics. It was deposited during times of glacio‐eustatic sea‐level change and was exposed subaerially during multiple sea‐level lowstands. Microcomponents and whole‐rock samples of limestones were analysed for δ13C and δ18O values to examine the method of whole‐rock isotopic analysis for detecting subaerial exposure events and to determine the diagenetic processes acting during subaerial exposure. Whole‐rock isotopic shifts are not consistently present across petrographically identified subaerial exposure surfaces. Apparently, whole‐rock isotopic shifts do not result from wholesale replacement of the host sediment during soil formation. However, the isotopic shifts are present in calcareous, soil‐precipitated microcomponents, such as rhizoliths, laminated crusts, and soil‐precipitated cements. The components are heterogeneous in isotopic composition, but converge on a meteoric calcite line at about δ18O=−5.5‰. These microcomponents are heterogeneous in distribution and may either dominate or be a minor constituent of the whole rock at a single stratigraphic horizon. Without petrographic selection of palaeosol components, the detection of whole‐rock isotopic shifts may depend on the selection or chance sampling of a rock containing abundant microcomponents precipitated in a soil environment. Only minor whole‐rock isotopic shifts come from those rocks bearing no evidence of exposure and bearing lithological characteristics suggesting sub

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1991.tb01855.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

ABSTRACTConcretions from the Kimmeridge Clay Formation are of three types: calcareous concretions, septarian calcareous concretions and pyrite/calcite concretions and nodules, which occur within different mudstone facies. Isotopic and chemical analysis of the concretionary carbonates indicate growth in the Fe‐reduction, sulphate‐reduction and decarboxylation zones.The septarian concretions show a long and complex history, with early initiation of growth and development spanning several phases of burial, each often resulting in the formation of septaria. Growth apparently ceased in the transitional zone between the sulphate‐reduction and the methanogenesis zones. Very early growth in the Fe‐reduction zones is also seen in one sample. The non‐septarian concretions began growth later within the sulphate‐reduction zone and have had a simpler burial history while the pyrite/calcite concretions show carbonate cementation in the sulphate‐reduction‐methanogenesis transition zone. A ferroan dolomite/calcite septarian nodule with decarboxylation zone characteristics also occurs.Development of concretions appears to be indirectly controlled by the sedimentation rate and depositional environment, the latter determining the organic matter input to the sediments. Calcareous concretions predominate in swell areas and during periods of low sedimentation rate in the basins with poor organic matter preservation and deposition of calcareous mudstones. Pyrite/calcite concretions occur in organic‐rich mudstones deposited under higher sedimentation rates in the basins, while the ferroan dolomite nodule grew under very high se

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1991.tb01856.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

ABSTRACTAnalysis of water and associated carbonate precipitates from a small, warm‐spring travertine system in SW Colorado, USA, provide an example of the: (i) great variability of the geochemical parameters within these dynamic systems, and (ii) significance of the microenvironment in controlling mineralogy and morphology of carbonate precipitates. Waters emerged from the springs highly charged in CO2, with an initialpCO2of 1.2 × 105Pa.Degassing of the CO2from the waters decreased the pH from 6.1 to 8.0, resulting in an increase of 8%‰ in δ13C values downflow in the total CO2in solution and an increase in theISATfrom 2.1 to as high as 63 times supersaturation with respect to calcite. Due to changes in the stable isotopic composition of the waters downflow as well as changes in the degree of supersaturation, stable isotopic analyses range greatly from locale to locale within this small system.Near the spring vents, at relatively lowISATlevels, well‐developed rhombohedra of calcite formed as biotically induced precipitates around diatom stalks and other algae as well as abiotic crusts. In contrast, near the distal end of the system, very highISATlevels were reached and resulted in the precipitation of skeletal‐dendritic crystals of calcite on copper substrates, floating rafts of laterally linked hemispheres of aragonite crystals, and bimineralic carbonate‐encrusted bubbles. Microenvironmental parameters control the mineralogy and habit of these p

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1991.tb01857.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

ABSTRACTFor several years, palaeoecological research has been conducted on micritic limestones of late Kimmeridgian age in the southern Jura Mountains. The sedimentary environment is that of a lagoon with an irregular bottom, which was repeatedly exposed. Between two stages of lime deposition, a microbial mat grew over the muddy surface, giving cohesion to the sediment, restraining erosion and preserving fossil remains and reptile tracks. Various structures at the microbial mat surface can be observed: crescentic wavy, radial wavy, torn, petee and mixed structures. They imply the presence of desiccation periods and a slight bottom slope leading to a downward sliding of the microbial mat. Such features may also be generalized to explain superficial microbial structures of other shallow carbonate‐mud environment

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1991.tb01858.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

ABSTRACTManganese carbonate deposits in Japanese Jurassic sedimentary rocks were studied petrogeochemically. The deposits are characteristically composed of spheroidal micronodules, up to 1 mm in diameter, and always contain well‐preserved radiolarian shells. Chemical elemental composition and mineralogical characteristics indicate that the micronodules contain rhodochrosite in a mixed carbonate phase composition (Mn86.7−92.2Ca2.2−2.9Mg2.6−6.7Fe2.6−5.6)CO3Carbon and oxygen isotope values, which range from −7.99 to −4.78‰ and −4.05 to 0.28‰ relative to PDB, respectively, suggest that the manganese carbonate was precipitated in a suboxic zone. The micronodules closely resemble agglutinated benthic foraminifera in shape. We suggest that agglutinated foraminiferal tests composed of radiolarian shells accumulated selectively on the sediment surface during redeposition of bottom sediments and were replaced by manganese carbonate in suboxic diagenetic conditions of

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1991.tb01859.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

ABSTRACTArcuate mud‐drapes have been observed in an interdune area in southern Tunisia. They are believed to have been formed by deposition from ponded water around aeolian dunes during flood events. They are preferentially preserved on the stoss‐side of the dunes as a result of subsequent dune migration, and exhibit extensive evidence of animal tracks. They are useful in illustrating dune‐interdune dynamics and recording the type of animals pr

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1991.tb01860.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY