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1.
Santanu Banerjee S.K. Bhattacharya Subir Sarkar 《Journal of Asian Earth Sciences》2007,29(5-6):823-831
Variations of carbon and oxygen isotopic ratios in response to cyclical sea level fluctuations have been documented from a Paleoproterozoic peritidal stromatolite succession. The upper division of the Kajrahat Limestone, Vindhyan Supergroup of central India consists of several shallowing upward stromatolite cycles identified by regular and systematic changes in stromatolite size. Normally, larger stromatolites are followed upward in the succession by smaller stromatolites and microbial laminites that occupy the top of the cycle. Desiccation cracks are found in all the facies indicating subaerial exposure. We investigated the stable isotope compositional variations across nine complete stromatolite cycles showing frequent subaerial emergence. Carbon and oxygen isotopic values of the limestones, in general, are comparable to contemporary marine values available from earlier studies but show regular depletion in response to shallowing of the water level. The δ13C and δ18O values of the limestones vary within an individual stromatolite cycle; depleted values characterize the topmost part of the cycles. The isotope pattern is explained by micritic carbonate deposition in different sub environments of the shallow marine domain having different salinity and variable duration of exposure. These variations also probably caused the observed scatter in δ13C and δ18O values of supratidal microbial laminites. 相似文献
2.
S. W. Mao J. M. Li W. Liu Z. C. Wang Z. L. Huang 《Australian Journal of Earth Sciences》2018,65(4):535-555
The Middle Ordovician Kelimoli Formation carbonate reservoirs in the northern Tianhuan area, Ordos Basin, China are main exploration targets. Subsurface core samples, logging, drilling and production data from the area were used to characterise the carbonate reservoir and to construct a genetic model for exploration. The sedimentary facies identified include trough–continental rise, upper–lower slope, platform margin reef-shoal, open platform and evaporation-restricted platform. The slope and platform margin facies are potential zones for high-quality reservoirs. Porosity in the study area comprises intergrain, intercrystal, intragrain and intracrystal pores, fractures and vughs. The Sr/Ba (0.40–4.87) and V/(V + Ni) (0.64–0.97) ratios indicate deposition in a brackish water-dominated environment under reducing conditions, associated with sea-level fluctuations during the deposition of the Kelimoli Formation. The Sr/Ba, V/(V + Ni), 87Sr/86Sr ratios, δ18O values and crystal texture of dolomite samples suggest that meteoric water was involved in the diagenetic fluid in the near-surface depositional environment. Isotopes (δ13C and δ18O) and trace elements (Fe and Mn) allowed the identification of seepage-reflux and mixed-water dolomitisation. The crystal textures of the samples consist of micritic, bioclastic and reefal limestones, and dolomite with gypsum, which were easily dissolved during the early diagenetic stage. The epidiagenetic stage was the key period for the development of high-quality reservoirs because of large-scale bedding-parallel karstification from meteoric water. The reservoir zones, dominated by partially filled and unfilled vughs, and fractures, are favourable exploration targets in the northern Tianhuan area. 相似文献
3.
ROBERT H. GOLDSTEIN 《Sedimentology》1991,38(1):67-77
The 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 subaerial exposure was unlikely. 相似文献
4.
The Thakkhola-Mustang Graben represents the extensional tectonic phase of the Tibetan Plateau uplift and whole Himalayan orogeny. It is situated at the northern side of the Dhaulagiri and Annapurna Ranges and south of the Yarlang Tsangpo Suture Zone. Stratigraphically, the oldest sedimentary units are the Tetang and Thakkhola Formations (Miocene), while the Sammargaon, Marpha and Kaligandaki Formations lying disconformably above these formations represent Plio-Pleistocene units. In this study, different lacustrine carbonates and calcretes were investigated within different lithological units and depositional environments to interpret the palaeoenvironmental and palaeoclimatological evolution of the area.Geological mapping, construction of columnar sections and carbonate sampling were carried out in the field, and stable oxygen and carbon isotope analyses and thin section analyses were done in the laboratory. Lacustrine facies contained abundant pelletal, charophytic algae, oncolitic algal micritic palustrine limestones with ostracods, and micritic mudstones with root traces. Stable carbon and oxygen isotope analysis from the carbonates show a range of δ13C values from −0.6‰ to 11.1‰ (V-PDB) and δ18O values from −13.5‰ to −25‰ (V-PDB).Discontinuous growth of oncolites and spherical pellets (25–40 μm in diameter) in micritic limestone, algal mats and charophyte algae indicate the presence of both shallow and deep water carbonates. Ostracods in dark micritic carbonates indicate quiet and calm water conditions. Microfabrics of the carbonates suggest that they were deposited in a flat and shallow lacustrine environment. The δ18O values of the investigated limestones of the Thakkhola-Mustang Graben suggest that it attained the current elevation level prior to the east-west extension of the Himalaya. 相似文献
5.
The stratigraphic record of many cratonic carbonate sequences includes thick successions of stacked peritidal deposits. Representing accumulation at or near sea‐level, these deposits have provided insights into past palaeoenvironments, sea‐level and climate change. To expand understanding of carbonate peritidal systems, this study describes the geomorphology, sedimentology and stratigraphy of the tidal flats on the Crooked‐Acklins Platform, south‐east Bahamas. The Crooked Island tidal flats extend continuously for ca 18 km on the platformward flank of Crooked Island, reaching up to 2 km across. Tidal flats include four environmental zones with specific faunal and floral associations and depositional characteristics: (i) supratidal (continuous supratidal crust and pavement); (ii) upper intertidal, with the mangrove Avicennia germinans and the cyanobacteria Scytonema; (iii) lower intertidal (with the mangrove Rhizophora mangal) and (iv) non‐vegetated, heavily burrowed subtidal (submarine). These zones have gradational boundaries but follow shore‐parallel belts. Coring reveals that the thickness of this mud‐dominated sediment package generally is <2 m, with depth to Pleistocene bedrock gradually shallowing landward. The facies succession under much of the tidal flat includes a basal compacted, organic‐rich skeletal‐lithoclast lag above the bedrock contact (suggesting initial flooding). This unit grades upward into rhizoturbated skeletal sandy mud (subtidal) overlain by coarsening‐upward peloid‐foraminifera‐gastropod muddy sand (reflecting shallowing to intertidal elevations). Cores from landward positions include stacked thin indurated layers with autoclastic breccia, root tubules and fenestrae (interpreted as supratidal conditions). Collectively, the data reveal an offlapping pattern on this prograding low‐energy shoreline, and these Holocene tidal flats may represent an actualistic analogue for ancient humid progradational tidal flats. Nonetheless, their vertical facies succession is akin to that present beneath channelled belt examples, suggesting that facies successions alone may not provide unambiguous criteria for prediction of the palaeogeomorphology, lateral facies changes and heterogeneity in stratigraphic analogues. 相似文献
6.
The carbonatic sequence of the Calabozo Formation (Lower Callovian) developed in southwestern Gondwana, within the northern area of the Neuquén basin, and is widespread in thin isolated outcrops in southwestern Mendoza province, Argentina. This paper describes the facies, microfacies and geochemical-isotopic analysis carried out in five studied localities, which allowed to define the paleoenvironmental conditions of a homoclinal shallow ramp model, highly influenced by sea level fluctuations, where outer, mid and inner ramp subenvironments were identified. The outer ramp subenvironment was only recognized in the south of the depocenter and is characterized by proximal outer ramp facies with shale levels and interbedded mudstone and packstone layers. The mid ramp subenvironment is formed by low energy facies (wackestone) affected by storms (packstones, grainstones and floatstones). The inner ramp subenvironment is the most predominant and is characterized by tidal flat facies (wackestones, packstones and grainstones) over which a complex of shoals (grainstones and packstones) dissected by tidal channels (packstone, grainstones and floatstones) developed. In the north area, protected environment facies were recorded (bioturbated wackestones and packstones). The vertical distribution of facies indicates that the paleoenvironmental evolution of the Calabozo Formation results from a highstand stage in the depocenter, culminating in a supratidal environment, with stromatolitic levels interbedded with anhydrite originated under restricted water circulation conditions due to a progressive isolation of the basin. δ13C and δ18O values of the carbonates of the Calabozo Formation suggest an isotopic signature influenced by local palaeoenvironmental parameters and diagenetic overprints. The δ13C and δ18O oscillations between the carbonates of the different studied sections are related with lateral facies variations within the carbonate ramp accompanied with dissimilar reactivities in relation to diagenetic fluids. The δ18O values of all sections exhibit a rather broad scatter which may be attributed to diagenesis and recrystallisation while the carbon isotopic composition has been less affected by those processes. Carbon isotope system has best retained the primary isotopic signal and δ13C values (0–3.9‰) are within the Callovian isotope range. The 87Sr/86Sr ratios of the bulk carbonates of El Plomo creek, La Vaina creek and Potimalal River sections are in agreement with the Callovian seawater Sr-isotope curve. 相似文献
7.
依据实测的燕山地区(1.6～1.0Ga)高于庄组—景儿峪组114个碳、氧同位素数据,研究、讨论了中、新元古界碳酸盐岩碳、氧同位素组成、演化及其地质意义。研究表明,燕山中、新元古界由下至上的地层序列上,碳、氧同位素表现明显的旋回性演化特征,二者多显正相关关系;δ13C在-3‰～3‰区间低幅、高频振荡;δ18O则表现为-2‰～-8‰的高幅、高频波动;δ13C值的增大与沉积环境由潮间向潮下演变、海平面上升、海水变淡、生物量增多相关;降低的δ13C多代表潮间—潮上环境。高于庄组瘤状灰岩及洪水庄组页岩δ13C为低负值,代表最大海泛期沉积。氧同位素组成和变化指示研究区总体为咸化环境,杨庄组上部和雾迷山组下部古海水盐度最高,之后盐度逐渐降低,至雾迷山组上部又有所升高。研究区与天津蓟县和北京十三陵地区的中、新元古代碳酸盐岩碳、氧同位素组成与演化表现出明显的相似性,反映了它们共同受燕山裂陷槽发育的控制;同时,与北美Belt超群和俄罗斯乌拉尔里菲期碳酸盐岩碳、氧同位素组成、演变的高度协同性,又说明了中、新元古代碳酸盐岩中碳、氧同位素组成、演化响应于全球古海洋背景和地球化学条件。 相似文献
8.
Shallowing-upward sequences in Purbeckian peritidal carbonates (lowermost Cretaceous, Swiss and French Jura Mountains) 总被引：3，自引：0，他引：3
ANDRÉ STRASSER 《Sedimentology》1988,35(3):369-383
Purbeckian carbonates in the Swiss and French Jura (Goldberg Formation, lower Berriasian) comprise shallow-subtidal, intertidal, supratidal, low-energy, high-energy, marine, brackish, freshwater, and hypersaline facies. These facies are arranged in small (0–2–1.5 m thick) sequences which display a dominant shallowing-upward component, and which form the fundamental units of the highly structured Purbeckian sedimentary record. Six types of small-scale sequences can be recognized. A: intertidal to supratidal overprinting of shallow lagoonal facies; B: algal-marsh sequence with frequent dolomitization; C: sabkha sequence, often associated with collapse breccia; D: tidal-flat sequence with desiccation features; E: lacustrine sequence; F: terrestrial overprinting of subtidal or intertidal facies. Episodic event deposits such as tempestites are superimposed] Thin transgressive beds which rework elements of the underlying facies are frequently found at the base of the sequences. Green marls and black pebbles are common at the top and indicate long subaerial exposure. The sequences are often incomplete, as subtidal facies may be absent, or their upper part can be eroded. Lateral facies changes are common, which is due to the very shallow and partly emergent Purbeckian platform where various depositional environments were juxtaposed. However, many sequence boundaries are well developed and can be correlated over large parts of the study area. The Purbeckian shallowing-upward sequences were generated by climatically controlled sea-level changes. Autocyclic processes occurred locally, but were overprinted by drops of sea-level affecting the entire platform. The small-scale sequences are most probably related to the 20 000-year cycle of the precession of the equinoxes. Larger sequences with usually well-developed emersion surfaces are attributed to the 100 000 and 400 000-year eccentricity cycles of the Earth's orbit. Identification and correlation of sequence boundaries makes it possible to set up a framework of isochronous surfaces (which often cut across facies boundaries), and thus to interpret in detail the palaeogeographic, sedimentological and diagenetic evolution of the Purbeckian peritidal carbonate environments. 相似文献
9.
陆表海台地沉积充填模式及内克拉通碳酸盐岩勘探新启示 总被引：1，自引：0，他引：1
内克拉通陆表海盆地是古老碳酸盐建造形成的重要场所。除其顶部风化壳岩溶储层外,广泛分布的内幕非岩溶改造的白云岩勘探长期未受到足够的重视。近年来,随着深层、超深层勘探技术的进步,内幕陆表海白云岩陆续获得油气勘探新发现,但其预测难度远比镶边台地中的礁滩"移动靶"更大。为更好地揭示这类未经岩溶改造的白云岩储层的宏观发育分布规律,本文研究了鄂尔多斯盆地奥陶系马家沟期和四川盆地雷口坡期陆表海台地的高精度岩相古地理和沉积充填模式。结果表明:陆表海台地一般形成于温室期低纬度干燥气候下的稳定内克拉通盆地,与盐湖伴生的台地潮坪广泛分布是其标志性的沉积特征,微环境可进一步划分为潮上带泥云坪、上潮间带云坪、上潮间带微生物席、下潮间带席状潮缘滩、上潮下带灰云质澙湖、下潮下带膏云质澙湖;虽然内克拉通陆表海台地是一个相对静态的沉积环境,其沉积建造并非平板一块,海平面的升降变化往往会在相对平坦的陆表海内引起大幅度的相带迁移,造成潮上带、潮间带、潮下带频繁交互;相带频繁往复迁移过程中,潮间带的晶粒/颗粒/微生物白云岩受到盐湖、古隆起或潮坪岛的制约,形成大面积分布的席状富孔白云岩透镜体;陆表海沉积层序结构为典型的向上变浅序列,主要发育潮坪进积楔和潮坪岛拼嵌两种沉积充填模式,这两种模式形成的席状白云岩透镜体在层序结构和厚度都能很好地对比,通常延伸都在100km以上,甚至可跨越整个陆表海台地,是一个能够媲美台缘礁滩相的油气勘探新领域。 相似文献
10.
11.
S. Zeeh 《International Journal of Earth Sciences》1994,83(1):130-142
The carbonate platform of the upper Wetterstein Formation (Cordevol) of the Drau Range (Austria) shows a distinct facies differentiation which depends on the palaeotopography. Although most areas of the platform were permanently flooded, a periodically emergent palaeotopographic high (Bleiberg facies) occurred near to the centre of the platform. The resulting cyclic sequences are composed of subtidal and intertidal to supratidal carbonates and emersion horizons. Typical shallowing upward cycles occur, as well as deepening upward, symmetrical and incomplete cycles. These types of cycles, with long-term emersion phases, can be explained by eustatic sea-level oscillations. It can be shown, due to an excellent lithostratigraphy based on distinct emersion horizons, that the cycle-type varies along a marker bed while the vertical distance between the emersion horizons is retained. Therefore the cycles cannot be explained by eustatic sea-level oscillations alone; local events which produce a local relief are necessary.A compiled profile of the Bleiberg facies is simulated with the help of a one-dimensional, parametric computer model. The quantification of some parameters which result from this simulation is used to verify and quantify the effects of relief on sedimentation. 相似文献
12.
The paper presents the results of study of the Sr, C, and O isotope compositions in Upper Jurassic carbonate rocks of the Baidar Valley and Demerdzhi Plateau in the Crimean Mountains represented by different facies of the carbonate platform at the northern active margin of the Tethys. The 87Sr/86Sr value in them varies from 0.70699 to 0.70728. Based on the Sr chemostratigraphic correlation, the age of massive and layered limestones in the western part of the Ai-Petri and Baidar yailas (pastures) is estimated as late Kimmeridgian–early Tithonian, whereas the age of flyschoids of the Baidar Valley are estimated as late Tithonian–early Berriasian. The nearly synchronous formation of carbonate breccias of the Baidar Valley and Demerdzhi Plateau in late Tithonian–early Berriasian is substantiated. A summary section of Upper Jurassic rocks is compiled based on the Sr chemostratigraphic data. It has been established that δ18O values in the studied carbonate sediments vary from–2.9 to 1.3‰ (V-PDB). At the same time, shallow-water sediments in the internal part and the edge of the Crimean carbonate platform are depleted in 18O (–2.9 to +0.1‰) relative to sediments on the slope and foothill (–0.5 to +1.3‰). It is demonstrated that δ13C values do not depend on the facies properties and decrease in younger carbonate sediments from 3–3.5‰ to 1–1.5‰ in line with the Late Jurassic general trend. The δ13C values obtained for the Crimean carbonate platform turned out to be 0.5–1‰ higher than the values typical of the deep-water marine setting at the western margin of the Tethys. These discrepancies are likely related to peculiarities of water circulation and high bioproductivity in marine waters of the northern Peri-Tethys. 相似文献
13.
MICHAEL M. JOACHIMSKI 《Sedimentology》1994,41(4):805-824
Purbeckian (lowermost Cretaceous) peritidal carbonates are characterized by open marine, lagoonal, intertidal and lacustrine facies arranged in Milankovitch-type shallowing upward sequences. Shallowing upward sequences typically consist of 2–6 individual beds. The sequences may be (i) complete, (ii) incomplete or (ii) pedogenetically overprinted, reflecting the duration of subaerial exposure and/or the extent of erosion and pedogenetic modification at the cycle tops. The stable isotopic composition of the peritidal micrites reveals homogenous δ18O values attributed to diagenetic stabilization in a meteoric, water-buffered system. Carbon isotopes show three distinctly different carbon isotope patterns dependent on the completeness of the shallowing upward sequences. Complete shallowing upward sequences consist of 4–6 individual carbonate beds. The carbon isotope values show a facies-dependent pattern: open marine carbonate muds record enriched δ13C values of +0·28‰ while lagoonal (−0·82‰), intertidal (−2·46‰) and lacustrine micrites (−2·96‰) are increasingly depleted. This distinct pattern is explained by carbonate mud deposition in environments of differing salinity and marine influence. Incomplete sequences (2–5 carbonate beds) are characterized by depleted δ13C values below subaerial exposure surfaces that become progressively enriched in 13C with increasing depth. Pedogenetically overprinted sequences (1–3 carbonate beds) show strong 13C depletion throughout the sequence with little variation in the carbon isotopic composition. The depleted values (−4·5‰) of the pedogenetically altered micrites suggest that modification during subaerial exposure was associated with equilibration with meteoric solutions enriched in isotopically light soil gas CO2. The duration of subaerial exposure is the most crucial factor determining the extent of pedogenetic alterations, the completeness of the shallowing upward sequences and the carbon isotope pattern. The recorded patterns clearly illustrate that micrites have a good potential for the preservation of their primary carbon isotopic composition if the duration of subaerial exposure is rather brief. Otherwise, the recorded carbon isotope patterns may support sequence stratigraphic analysis by providing a refinement of the time-stratigraphic interpretation. 相似文献
14.
《Sedimentary Geology》2001,139(3-4):171-203
Carbonates in the upper member of the Mesoproterozoic Victor Bay Formation are dominated by lime mud and packaged in cycles of 20–50 m. These thicknesses exceed those of classic shallowing-upward cycles by almost a factor of 10. Stratigraphic and sedimentological evidence suggests high-amplitude, high-frequency glacio-eustatic cyclicity, and thus a cool global climate ca. 1.2 Ga.The Victor Bay ramp is one of several late Proterozoic carbonate platforms where the proportions of lime mud, carbonate grains, and microbialites are more typical of younger Phanerozoic successions which followed the global waning of stromatolites. Facies distribution in the study area is compatible with deposition on a low-energy, microtidal, distally steepened ramp. Outer-ramp facies are hemipelagic lime mudstone, shale, carbonaceous rhythmite, and debrites. Mid-ramp facies are molar-tooth limestone tempestite with microspar-intraclast lags. In a marine environment where stromatolitic and oolitic facies were otherwise rare, large stromatolitic reefs developed at the mid-ramp, coeval with inner-ramp facies of microspar grainstone, intertidal dolomitic microbial laminite, and supratidal evaporitic red shale.Deep-subtidal, outer-ramp cycles occur in the southwestern part of the study area. Black dolomitic shale at the base is overlain by ribbon, nodular, and carbonaceous carbonate facies, all of which exhibit signs of synsedimentary disruption. Cycles in the northeast are shallow-subtidal and peritidal in character. Shallow-subtidal cycles consist of basal deep-water facies, and an upper layer of subtidal molar-tooth limestone tempestite interbedded with microspar calcarenite facies. Peritidal cycles are identical to shallow-subtidal cycles except that they contain a cap of dolomitic tidal-flat microbial laminite, and rarely of red shale sabkha facies or of sandy polymictic conglomerate. A transect along the wall of a valley extending 8.5 km perpendicular to depositional strike reveals progradation of inner-ramp tidal flats over outer- and mid-ramp facies during shoaling. The maximum basinward progradation of peritidal facies coincides with a zone of slope failure that may have promoted the development of the stromatolitic reefs.The sea-level history of the Victor Bay Formation is represented by three hectometre-scale sequences. An initial flooding event resulted in deposition of the lower Victor Bay shale member. Upper-member carbonate cycles were then deposited during highstand. Mid-ramp slumping was followed by late-highstand reef development. The second sequence began with development of an inner-ramp lowstand unconformity and a thick mid-ramp lowstand wedge. A second transgression promoted a more modest phase of reef development at the mid-ramp and shallow-water deposition continued inboard. A third and final transgressive episode eventually led to flooding of the backstepping ramp.Overall consistent cycle thickness and absence of truncated cycles, as well as the high rate and amount of creation of accommodation space, suggest that the periodicity and amplitude of sea-level fluctuation were relatively uniform, and point to a eustatic rather than tectonic mechanism of relative sea-level change. High-amplitude, high-frequency eustatic sea-level change is characteristic of icehouse worlds in which short-term, large-scale sea-level fluctuations accompany rapidly changing ice volumes affected by Milankovitch orbital forcing. Packaging of cyclic Upper Victor Bay carbonates therefore supports the hypothesis of a late Mesoproterozoic glacial period, as proposed by previous workers. 相似文献
15.
MARIA JENSEN EILIV LARSEN IGOR N. DEMIDOV SVEND FUNDER KURT H. KJæR 《Boreas: An International Journal of Quaternary Research》2006,35(3):521-538
Deposits from a Middle Weichselian transgression, the Mezen Transgression, are found in coastal sections in the Mezen and Chyorskaya Bays, northwestern Russia. The marine event is bracketed between two ice advances from the Barents and Kara Sea shelves and dated by Optically Stimulated Luminescence (OSL) to around 60 kyr BP. The deposits represent a shallowing upward succession from offshore marine to intertidal coastal environments. Relative sea-level maximum was at least 40 m above the present owing to significant isostatic subsidence. The sedimentary record is dominated by shallow-marine, subtidal deposits bounded below by an erosion surface representing a downward shift in facies and above by subaerial exposure. The succession reflects deposition during forced regression due to isostatic uplift. A rapidly aggrading succession of subtidal deposits at one site suggests a relative sea-level rise or stillstand superimposed on the isostatically controlled sea-level fall. The rhythmic tidal deposits allow identification of semi-monthly to yearly cycles, providing an estimate of the sedimentation rate of 39 cm/year. This implies a high sediment yield and a rapid relative sea-level rise. We correlate this signal with the rapid eustatic sea-level rise at the end of OIS 4 known from deep-sea records. 相似文献
16.
Quartz geodes and nodular chert have been found within middle–upper Campanian carbonate sediments from the Laño and Tubilla del Agua sections of the Basque‐Cantabrian Basin, northern Spain. The morphology of geodes together with the presence of anhydrite laths included in megaquartz crystals and spherulitic fibrous quartz (quartzine‐lutecite), suggest an origin from previous anhydrite nodules. The anhydrite nodules at Laño were produced by the percolation of marine brines, during a period corresponding to a sedimentary gap, with δ34S and δ18O mean values of 18.8‰ and 13.6‰ respectively, consistent with Upper Cretaceous seawater sulphate values. Higher δ34S and δ18O mean values of 21.2‰ and 21.8‰ recorded in the Tubilla del Agua section are interpreted as being due to a partial bacterial sulphate reduction process in a more restricted marine environment. The idea that sulphates may have originated from the leaching of previously deposited Keuper sulphate evaporites with subsequent precipitation as anhydrite, is rejected because the δ34S, δ18O and 87Sr/86Sr values of anhydrite laths observed at both the Tubilla del Agua and Laño sections suggest an origin from younger marine brines. Later calcite replacement and precipitation of geode‐filling calcite is recorded in both sections, with δ13C and δ18O values indicating the participation of meteoric waters. Synsedimentary activity of the Peñacerrada diapir, which lies close to the Laño section, played a significant role in the local shallowing of the basin and the formation of quartz geodes. In contrast, eustatic shallowing of the inner marine series of the Tubilla del Agua section led to the generation of morphologically similar quartz geodes. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
17.
Detailed information on semi‐arid, palustrine carbonate–calcrete lithofacies associations in a sheetwash‐dominated regolith setting is sparse. This is addressed by studying the Lower Limestone of the Lameta Beds, a well‐exposed Maastrichtian regolith in central India. The general vertical lithofacies assemblage for this unit comprises: (a) basal calcareous siltstones and marls with charophytes, ostracods and gastropods; (b) buff micritic limestones associated in their upper parts with calcretized fissure‐fill sandstones; (c) sheetwash as fissure‐fill diamictites and thin pebbly sheets, locally developed over a few metres; and (d) sandy, nodular, brecciated and pisolitic calcretes at the top. The sequence is ‘regressive’, with upsection filling of topographic lows by increased sheetwash. Lateral lithofacies change is marked, but there are no permanent open‐water lake deposits. In topographic lows close to the water table, marshy palustrine or groundwater calcretes formed, whereas on better drained highs, brecciation and calcretization occurred. Prolonged exposure is implied, suggesting that shrinkage was the main cause of brecciation. Evidence for rhizobrecciation and other biological calcrete fabrics is sparse, contrasting with the emphasis on root‐related brecciation in many studies of palustrine lithofacies. Stable isotope (δ18O and δ13C) values are consistent with the palustrine limestones being fed from meteoric‐derived groundwater with a strong input of soil‐zone carbon. There is overlap of both δ18O and δ13C values from the various palustrine and calcrete fabrics co‐occurring at outcrop. This suggests that, in groundwater‐supported wetlands, conversion from palustrine carbonate to calcrete need not show isotopic expression, as the groundwater source and input of soil‐zone carbon are essentially unchanged. Cretaceous–Tertiary δ18O and δ13C values from palustrine lithofacies and associated calcretes appear to be strongly influenced by the inherited values from lakes and wetlands. Hydrologically closed lakes and marine‐influenced water bodies tend to result in low negative palustrine δ18O and δ13C values. During brecciation and calcretization, the degree of isotopic inheritance depends on whether or not alteration occurs in waters that are different from those of the original water body or wetland. Marked biological activity (e.g. rhizobrecciation or root mat development) during calcretization may lower δ13C values where C3 plants are abundant but, in shrinkage‐dominated systems, δ13C values will be largely inherited from the palustrine limestones. 相似文献
18.
Bulk carbonate samples of hemipelagic limestone–marl alternations from the Middle and Upper Triassic of Italy are analysed for their isotopic compositions. Middle Triassic samples are representative of the Livinallongo Formation of the Dolomites, while Upper Triassic hemipelagites were sampled in the Pignola 2 section, within the Calcari con Selce Formation of the Southern Apennines in Southern Italy. Triassic hemipelagites occur either as nodular limestones with chert nodules or as plane‐bedded limestone–marl alternations which are locally silicified. In the Middle Triassic Livinallongo Formation, diagenetic alteration primarily affected the stable isotopic composition of sediment surrounding carbonate nodules, whereas the latter show almost pristine compositions. Diagenesis lowered the carbon and oxygen isotope values of bulk carbonate and introduced a strong correlation between δ13C and δ18O values. In the Middle Triassic successions of the Dolomites, bulk carbonate of nodular limestone facies is most commonly unaltered, whereas carbonate of the plane‐bedded facies is uniformly affected by diagenetic alteration. In contrast to carbonate nodules, plane‐bedded facies often show compaction features. Although both types of pelagic carbonate rocks show very similar petrographic characteristics, scanning electron microscopy studies reveal that nodular limestone consists of micrite (< 5 μm in diameter), whereas samples of the plane‐bedded facies are composed of calcite crystals ca 10 μm in size showing pitted, polished surfaces. These observations suggest that nodular and plane‐bedded facies underwent different diagenetic pathways determined by the prevailing mineralogy of the precursor sediment, i.e. probably high‐Mg calcite in the nodular facies and aragonite in the case of the plane‐bedded facies. Similar to Middle Triassic nodular facies, Upper Triassic nodular limestones of the Lagonegro Basin are also characterized by uncorrelated δ13C and δ18O values and exhibit small, less than 5 μm size, crystals. The alternation of calcitic and aragonitic precursors in the Middle Triassic of the Dolomites is thought to mirror rapid changes in the type of carbonate production of adjacent platforms. Bioturbation and dissolution of metastable carbonate grains played a key role during early lithification of nodular limestone beds, whereby early stabilization recorded the carbon isotopic composition of sea water. The bulk carbonate δ13C values of Middle and Upper Triassic hemipelagites from Italy agree with those of Tethyan low‐Mg calcite shells of articulate brachiopods, confirming that Triassic hemipelagites retained the primary carbon isotopic composition of the bottom sea water. A trend of increasing δ13C from the Late Anisian to the Early Carnian, partly seen in the data set presented here, is also recognized in successions from tropical palaeolatitudes elsewhere. The carbon isotopic composition of Middle and Upper Triassic nodular hemipelagic limestones can thus be used for chemostratigraphic correlation and palaeoenvironmental studies. 相似文献
19.
G. E. Williams 《Australian Journal of Earth Sciences》2013,60(7-8):377-386
Thin (<10 m), extensive dolostones conformably capping late Precambrian glaciogenic rocks in the Kimberley region and the Adelaide Geosyncline commonly comprise micritic dolomite apparently of ‘primary’ or early diagenetic origin. Their aphanitic texture, the presence of algal and cryptalgal lamination, tepee structures and intraclastic rocks, and the lack of saline evaporites or their pseudomorphs, suggest deposition mainly in supratidal to lagoonal environments under a seasonally evaporitic/humid climate. Less common oolitic or pelletal rocks and a finely laminated shaly dolostone facies also suggest peritidal to marine influences. The ‘cap’ dolostones (omitting possible large erratics) have a δ18O mean of —6.4 ± 1.9% vs. PDB and a δ13C mean of —1.9 ± 2.1% vs. PDB. The oxygen mean is comparable to that determined for other late Precambrian ‘primary’ or early diagenetic dolostones not associated with glaciogenic rocks and presumably deposited under warm conditions. The carbon mean, in contrast, is lighter than that determined for other late Precambrian dolostones. The sedimentological and oxygen‐isotope data are consistent with relatively high formation‐temperatures for the cap dolostones. Abrupt climatic warming at the close of late Precambrian glacial epochs is implied. 相似文献
20.
Isotope signatures associated with early meteoric diagenesis 总被引：12，自引：0，他引：12
The environments in which carbonate diagenesis proceeds have been documented in previous studies of Holocene and late Pleistocene sediments and limestones on Barbados, West Indies. Variations in the carbon and oxygen isotopic composition of limestones, produced during early freshwater diagenesis, have been observed in this study to occur in specific patterns. Six potentially useful patterns emerge when one views stable isotope data within a stratigraphic framework: (1) the subaerial exposure surface is characterized by strongly 12C-enriched limestones. δ13C compositions of underlying limestones grow progressively heavier with increasing depth; (2) the subaerial exposure surface may also be marked by slight 18O-enrichment; (3) an abrupt shift in δ18O values may differentiate sediments above the exposure surface from those below; (4) sediments altered in the marine-meteoric mixing zone may be characterized by positive covariance between their δ18O and δ13C compositions; (5) the vadose-phreatic boundary may be marked by a sharp increase in δ13C values in the seaward portions of a fresh groundwater system; and (6) samples contemporaneously altered in a single fresh groundwater system within an areally restricted region should display a narrow range of δ18O and a wide range of δ13C compositions. Analysis of samples from five Palaeozoic and Mesozoic formations, which contained petrographic evidence of early freshwater diagenesis, showed that isotope patterns similar to those observed in Barbados limestones have been preserved in rocks as old as Mississippian. These isotope patterns could prove to be useful for identifying diagenetically induced porosity trends in carbonate rocks. They might be used to identify limestones diagenetically altered in meteoric environments, to identify mixing zone cements and dolomites, and to trace the regional and vertical distributions of early meteoric groundwater systems in ancient carbonate formations. 相似文献