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1.
Paleoweathering in the Sergi Formation has been classified and analyzed to ascertain its origin and relationship with stratigraphic evolution. The Sergi Formation belongs to the pre-rift sequence of the Recôncavo Basin (northeastern Brazil) and comprises a complex association of eolian and fluvial sandstones and lacustrine mudstones. This formation can be subdivided into three depositional sequences bounded by regional unconformities. Four paleoweathering types, each one related to a distinct origin, have been described in the Sergi Formation: (1) textural mottling, which is distinguished by alternating rock colors as a result of the iron oxide mobilization within mineral phases that evolved under alternating oxidation (yellowish, brownish and reddish shades) and reduction (grayish or greenish hues) conditions; (2) non-textural mottling, which displays a discoloration pattern that is independent of the original rock texture; (3) carbonate concentrations, usually related to carbonate nodule formation, which display a massive internal structure that reveals their origin through continuous growth or crystallization; and (4) banded carbonates (silicified), associated with the beginning of regular surface formation due to the chemical precipitation of carbonates within lacustrine environments. Both mottling color motifs and carbonate accumulation usually represent groundwater oscillation rather than pedogenesis. Only carbonate intraclasts and banded carbonate (silicified) have their origin ascribed to pedogenesis sensu stricto, although the carbonate intraclasts do not represent soil deposits in situ, but calcretes eroded from areas close to channels, and the banded carbonates (silicified) have strong diagenetic modifications. Therefore, it is reasonable to assume that fluvial and meteoric water have controlled paleoweathering evolution as well as deposition, yet both aspects are ruled by the same mechanisms (relief, sedimentation rate and, above all, climate).  相似文献   

2.
The non-marine Campins Basin developed in the Oligocene, during a period of early rifting of the Catalan Coastal Ranges. Lacustrine deposits, interbedded between two alluvial units, comprise shallow and deep lacustrine facies. The lower, shallow lacustrine facies are made up of microbialite buildups and thin limestone beds. In the studied area, these facies are overlain by deep lacustrine facies which consist of alternations of several, metre-thick carbonate- and mudstone-dominated intervals. In addition to calcite, which is characteristic of the shallow lacustrine facies, aragonite and abundant dolomite are present in the deep lacustrine facies. This mineralogical change in the sequence reflects an overall increase in the Mg/Ca ratio of the lake waters. The deep lacustrine sequences are interpreted as having formed in a hydrologically closed basin that was subject to changes in the Mg/Ca ratio of the water, probably related to variations in the evaporation/precipitation rate. The sedimentological, mineralogical and isotopic characteristics of the Campins Basin dolomites suggest that, in general, they are primary in origin. The stable isotope data show an approximate covariance between δ13C and δ18O in the lower shallow lacustrine carbonates (calcite) which suggests that they formed during the onset of closure of the lake. The δ13C and δ18O values of the deep lacustrine carbonates display three different clusters that are roughly related to the carbonate mineralogy. Normalisation with respect to calcite of the isotopic compositions of dolomite and aragonite from the deep lacustrine carbonates allows the integration of all these isotope values into one covariant trend. The sequential appearance of different carbonate minerals and the isotopic covariant trend may indicate an overall evaporative concentration of the lake waters. The change in slope of the covariant trend for the isotope values between the shallow and the deep lacustrine carbonates might reflect the change in the waterbody morphology recorded in the basin fill sequence.  相似文献   

3.
This paper describes and interprets the mineral and facies assemblages that occur in carbonate–evaporite shallow lacustrine deposits, considering the importance of the processes pathway (i.e. dolomitization, gypsum calcitization and silicification). The Palaeogene deposits of the Deza Formation (Almazán Basin, central‐northern Spain) are selected as a case study to determine the variety of physicochemical processes taking place in carbonate–evaporite shallow lakes and their resulting diagenetic features. Dolostones are the predominant lithology and are composed mainly of dolomite with variable amounts of secondary calcite (5 to 50%), which mainly mimic lenticular gypsum (pseudomorphs). Five morphological types of dolomite crystal were identified as follows: dolomite tubes, dolomite cylinders, rhombohedral dolomite, spheroidal and quasi‐rhombohedral dolomite, and cocoon‐shaped dolomite. The dolomite cylinders and tubes are interpreted as the dolomitized cells of a widespread microbial community. The sequence of diagenetic processes started with growth of microlenticular interstitial gypsum in a calcareous mud deposited on the playa margin mudflats, and that sometimes included microbial sediments. Immediately following growth of gypsum, dolomite replaced the original calcite (or possibly aragonite) muds, the microbial community and the gypsum. Partial or total replacement of gypsum by dolomite was related mainly to the biomineralization of endolithic microbial communities on gypsum crystals. Later calcitization took place under vadose, subaerial exposure conditions. The development of calcrete in distal alluvial settings favoured the release of silica and subsequent silicification on the playa margin mudflats. Stable isotope compositions of calcite range from ?9·02 to ?5·83‰ δ13CPDB and ?7·10 to 1·22‰ δ18OPDB; for the dolomite, these values vary from ?8·93 to ?3·96‰ δ13CPDB and ?5·53 to 2·4‰ δ18OPDB. Quartz from the cherts has δ18OSMOW values ranging from 27·1 to 31·1‰. Wide variation and relatively high δ18OSMOW values for dolomite indicate evaporitic and closed hydrological conditions; increased influx of meteoric waters reigned during the formation of secondary calcite spar.  相似文献   

4.
Stromatolites are abundant in Upper Palaeocene to Eocene fluvial deposits from the Eastern Ebro Basin, whereas they are scarce in the lacustrine facies. The fluvial stromatolites display a variety of growth forms: oncoids, domes, laminated crusts and locally irregular bioherms. These morphologies are related to the hydrological behaviour of the fluvial systems. Elongate domes, dome heads and ovoid to subspherical oncoids accreted in flowing water. Laminated crusts and small-sized cylindrical to lenticular oncoids formed during low-discharge and ponding phases. In the lacustrine environments only small-sized ovoid oncoids occur. The main characteristics of the Ebro Basin non-marine stromatolites are: (1) laminations and concentric banding exhibited by all morphologies; (2) the occurrence of well-preserved cyanobacterial filaments in several laminae, suggesting that these microbiotas are the major contributors in growth; (3) the asymmetrical growth of cortices in domes and oncoids with polar thickening on the upper side suggesting in situ growth under low energy conditions. δ13C values of carbonate stromatolites range from - 11.3%o (Late Palaeocene) to — 4.4%o (Middle-Late Eocene). During the Late Palaeocene, under tectonically quiescent conditions, the δ13C values mainly reflect the influence of soil-derived CO2. During the Eocene, the progressive uplift of the surrounding catchment areas led to an increase of exposures of Mesozoic rocks. Consequently, the δ13C values of the Eocene stromatolites reflect the influence of the CO2 derived from the dissolution of Mesozoic marine carbonates. There is a 2.5%o shift in δ18O values of stromatolites from the Late Palaeocene to Middle-Late Eocene which is consistent with the variation in δ18O of precipitation due to changes in altitude of the catchment area during this time. The isotopic values of all Middle-Upper Eocene laminated crust samples show distinct covariant trends, suggesting that these stromatolites were formed either in ponding zones of fluvial channels or in disconnected pools developed during low-discharge episodes.  相似文献   

5.
利用野外露头、岩心、测井录井和分析化验资料,对柴达木盆地西部(简称“柴西地区”)新生界干柴沟组湖相碳酸盐岩进行了研究,划分了其沉积微相类型,研究了其分布规律,分析其形成环境和控制因素,并建立了相应沉积模式。该区湖相碳酸盐岩在垂向上与碎屑岩频繁互层,湖相碳酸盐岩包括颗粒灰岩、藻灰岩、泥晶灰岩和混积岩4大类11种,划分出了灰泥坪、颗粒滩、藻丘(礁)、浅湖湾以及(半)深湖泥灰岩相等5种沉积微相。通过分析不同碳酸盐岩及其微相时空展布特征,认为其发育主要受控于湖盆构造运动、湖平面变化、陆源碎屑注入、古气候与古水介质条件、古地貌与古水深环境,并在此基础上建立了柴西湖相碳酸盐岩的沉积模式。研究认为柴达木盆地西部干柴沟组沉积时期,湖盆为典型咸化湖盆,构造活动相对稳定,湖平面上升达到峰值。碳酸盐岩主要发育在湖侵期,高频湖平面变化形成了碳酸盐岩与碎屑岩频繁互层。在枯水期,盆地坡折处发育碎屑岩滩坝或三角洲前缘沉积;在湖侵期,盆地坡折处发育了鲕粒滩及藻灰岩,盆地洼陷区发育泥灰岩或灰质泥岩。  相似文献   

6.
Peritidal carbonates of the Lower Jurassic (Liassic) Gibraltar Limestone Formation, which form the main mass of the Rock of Gibraltar, are replaced by fine and medium crystalline dolomites. Replacement occurs as massive bedded or laminated dolomites in the lower 100 m of an ≈460‐m‐thick platform succession. The fine crystalline dolomite has δ18Ο values either similar to, or slightly higher than, those expected from Early Jurassic marine dolomite, and δ13C values together with 87Sr/86Sr ratios that overlap with sea‐water values for that time, indicating that the dolomitizing fluid was Early Jurassic sea water. Absence of massive evaporitic minerals and/or evaporite solution‐collapse breccias in these carbonate rocks indicates that the salinity of sea water during dolomitization was below that of gypsum precipitation. The occurrence of peritidal facies, a restricted microbiota and rare gypsum pseudomorphs are also consistent with penesaline conditions (salinity 72–199‰). The medium crystalline dolomite has some δ18Ο and δ13C values and 87Sr/86Sr ratios similar to those of Early Jurassic marine dolomites, which indicates that ambient sea water was again a likely dolomitizing fluid. However, the spread of δ18Ο, δ13C and 87Sr/86Sr values indicates that dolomitization occurred at slightly increased temperatures as a result of shallow (≈500 m) burial or that dolomitization was multistage. These data support the hypothesis that penesaline sea water can produce massive dolomitization in thick peritidal carbonates in the absence of evaporite precipitation. Taking earlier models into consideration, it appears that replacement dolomites can be produced by sea water or modified sea water with a wide range of salinities (normal, penesaline to hypersaline), provided that there is a driving mechanism for fluid migration. The Gibraltar dolomites confirm other reports of significant Early Jurassic dolomitization in the western Tethys carbonate platforms.  相似文献   

7.
Samples of chert nodules, diagenetic carbonates and evaporites (gypsum/anhydrite) collected from the gypsiferous limestones of the Kef Eddour Member (Ypressian‐Priabonian) near Metlaoui and Sehib (Tunisia) show selective silicification with great variety in the silicified by‐products. Based on δ13C values, which support an organic origin for the carbon, carbonates replaced evaporites microbially through bacterial sulphate reduction. Observations and results suggest two scenarios for chert formation that are related to the rate and timing of diagenetic carbonate replacement of the evaporites (anhydrite/gypsum). In the absence of early diagenetic carbonate phases, silica with δ18O values from +25 to +28·6‰ [standard mean ocean water (SMOW)] replaced the outer parts of anhydrite nodules at pH < 9. In contrast, pore‐fluid pH values > 9 in the innermost parts of the anhydrite nodules prevented silica precipitation. The record of this chemical barrier is preserved in the microquartz rims and geode features that formed in the inner parts of the nodules after dissolution of the anhydrite nucleus. The microbial diagenetic replacement of evaporites (bacterial sulphate reduction) by carbonates (calcite, aragonite and dolomite) favoured silica replacement of carbonates rather than evaporites. Silica, with δ18O signature of +21 to +26‰ (SMOW), replaced carbonates on a volume‐for‐volume basis, yielding a more siliceous groundmass, and accounting for 90–95% of the nodules. The relatively higher δ18O values of quartz replacing anhydrite can be explained by a diagenetic fluid in equilibrium with mixed (meteoric/marine) to marine water. The lower δ18O values of the quartz that replaced the diagenetic carbonates are ascribed to flushing by meteoric water in a later diagenetic stage. The silica supply for chert formation could be derived from the reworked bio‐siliceous deposits (diatomites) to the west of the basin [vestiges of an opal‐CT precursor undetectable by X‐ray diffraction (XRD) were revealed by δ29Si magic‐angle‐spinning nuclear magnetic resonance investigations], diagenesis of the extraformational and overlying clay‐rich beds (the host limestones are clay‐poor as shown by XRD measurements), and minor volcanogenic and hydrothermal contributions during early diagenetic stages.  相似文献   

8.
The Berriasian Rupelo Formation of the W Cameros Basin consists of a 2–200 m thickness of marginal and open lacustrine carbonate and associated deposits. Open lacustrine facies contain a non-marine biota with abundant charophytes (both stems and gyrogonites), ostracods, gastropods and rare vertebrates. Carbonate production was mainly biogenic. The associated marginal lacustrine (‘palustrine’) facies show strong indications of subaerial exposure and exhibit a wide variety of pedogenic fabrics. Silicified evaporites found near to the top of the sequence reflect a short hypersaline phase in the lake history. The succession was laid down in a low gradient, shallow lake complex characterized by wide fluctuations of the shoreline. Carbon and oxygen stable isotope analyses from the carbonates show non-marine values with ranges of δ13 from ? 7 to ? 11‰and δ18 from ? 3 to ? 7.5‰. Differences in the isotopic composition of open lacustrine carbonates are consistent with sedimentary evidence of variation in organic productivity within the lake. Analyses from the entire sample suite plot on a linear trend; isotopic compositions become lighter with increasing evidence of pedogenic modification. This suggests progressive vadose zone diagenesis and influence of meteoric waters rich in soil-derived CO2. The stable isotope data thus support evidence from petrography and facies relations that ‘palustrine’limestones form through pedogenic modification of lake carbonates.  相似文献   

9.
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10.
This paper concerns the evaporite units, depositional systems, cyclicity, diagenetic products and anhydritization patterns of the Calatayud Basin (nonmarine, Miocene, central Spain). In outcrop, the sulphate minerals of these shallow lacustrine evaporites consist of primary and secondary gypsum, the latter originating from the replacement of anhydrite and glauberite. In the evaporative systems of this basin, gypsiferous marshes of low salinity can be distinguished from central, saline lakes of higher salinity. In the gypsiferous marsh facies, the dominant, massive, bioturbated gypsum was partly replaced by synsedimentary chert nodules and siliceous crusts. In the saline lake facies, either cycles of gypsiferous lutite‐laminated gypsarenite or irregular alternations of laminated gypsum, nodular and banded glauberite, thenardite and nodular anhydrite precipitated. Early replacement of part of the glauberite by anhydrite also occurred. Episodes of subaerial exposure are represented by: (1) pedogenic carbonates (with nodular magnesite) and gypsiferous crusts composed of poikilitic crystals; and (2) nodular anhydrite, which formed in a sabkha. Additionally, meganodular anhydrite occurs, which presumably precipitated from ascending, highly saline solutions. The timing of anhydritization was mainly controlled by the salinity of the pore solutions, and occurred from the onset of deposition to moderate burial. Locally, a thick (>200 m) sequence of gypsum cycles developed, which was probably controlled by climatic variation. A trend of upward‐decreasing salinity is deduced from the base to the top of the evaporite succession.  相似文献   

11.
塔里木盆地早白垩世沉积相特征与古地理   总被引：4，自引：0，他引：4       下载免费PDF全文
贾进华 《古地理学报》2009,11(2):167-176
根据大量的钻井和露头资料,对塔里木盆地早白垩世沉积相特征和古地理进行了研究。塔里木盆地下白垩统主要发育冲积扇-扇三角洲、辫状河三角洲、氧化宽浅湖泊等3种沉积体系,包括陆相冲（洪）积扇、扇三角洲、辫状河三角洲、滨浅湖等沉积相,分布于下白垩统不同层位。塔里木盆地早白垩世主要包括塔东北和塔西南两大沉积区,总体为干旱氧化的内陆拗陷分割盆地。在塔东北沉积大区,早白垩世早期卡普沙良群沉积物受周缘4大物源区控制,为多物源的氧化宽浅湖盆环境,古地势呈“南高北低、东高西低”,盆地边缘以冲（洪）积扇、扇三角洲、辫状河三角洲相为主,盆地内部为滨浅湖亚相。早白垩世晚期巴什基奇克组沉积时期,塔里木盆地周缘进入一个新的构造活动期,古气候更加炎热干旱,氧化宽浅湖盆消失。在塔东北沉积大区,盆地受周缘4个主要物源区影响,以广泛分布的冲积扇-辫状河三角洲沉积体系占主体,盆地内部普遍为辫状河三角洲前缘-滨浅湖亚相沉积。在塔西南沉积大区,早白垩世克孜勒苏群沉积时期古地势呈“东高西低”,受北部喀什北山前和西南部古昆仑山2个物源区的控制,沉积物沿古昆仑山前呈狭长的条带状分布,沉积厚度自西向东减薄,主要是一套冲积扇-扇三角洲相和滨浅湖亚相沉积。  相似文献   

12.
The Vempalle Formation of the Proterozoic Cuddapah basin has a well developed sequence of carbonate rocks, which are interbedded with shales, siltstones and chert. The stromatolitic carbonates are conspicuous at many places but the oolitic carbonates are less prominent and are present only in some areas. All the carbonates are pervasively dolomitized. Petrographic examination of these carbonates revealed that they are predominantly made up of fine grained micrite with patchy development of sparite and chert/quartz. The stromatolitic carbonates show distinct banding of alternate carbonate and cherty layers. The latter are rich in organic matter indicating prevalence of profuse biogenic activity. The oolitic carbonates comprise of ooids showing both concentric and radial patterns and made up of carbonate/chert and cemented by micro/mega quartz or carbonate itself. Diagenetic and post depositional features are reflected in cementation, recrystallization, compaction, stylolite formation and silicification processes. Various stages of cementing material are observed. Secondary vein fillings of carbonate or quartz traverse the carbonate/cherty groundmass. Intraclasts present suggest occasional erosional destruction of associated sediments, short lived transport and local redeposition. Accessory silicate minerals represent terrigenous influx during deposition. Dolomitization of the carbonates was fabric retentive and early diagenetic. The environmental conditions were characterised by low energy, within a shallow water zone, in occasional higher energy events and turbulence. The carbonates appear to have been deposited on a shallow water ramp within a tidal regime.  相似文献   

13.
A thick sedimentary sequence comprising fluvial, lacustrine and volcano-sedimentary rocks is present in the Neogene Beypazari Basin, central Anatolia. These units display considerable lateral facies variation and interfinger with alkaline volcanic rocks along the north-eastern margin of the basin. The uppermost Miocene Kirmir Formation contains numerous evaporite horizons. The evaporite sequence is up to 250 m thick and may be divided into four lithofacies. In ascending stratigraphical order these are: (1) gypsiferous claystone facies, (2) thenardite-glauberite facies, (3) laminar gypsum facies and (4) crystalline gypsum facies. These facies interfinger with one another laterally along a section from the margins to central parts of the basin. The lithological and sedimentological features of the Kirmir Formation indicate fluvial, saline playa mudflat, hypersaline ephemeral playa lake and very shallow subaqueous playa lake depositional environments, which probably were influenced by alternating semi-arid and evaporative conditions.  相似文献   

14.
A. Sáez  L. Cabrera 《Sedimentology》2002,49(5):1073-1094
ABSTRACT A small, closed, lacustrine system developed during the restraining overstep stages of the Oligocene As Pontes strike‐slip basin (Spain). The increase in basin accommodation and the headward spread of the drainage, which increased the water input, triggered a change from shallow, holomictic to deeper, meromictic conditions. The lower, shallow, lacustrine assemblage consists of mudstone–carbonate cycles recording lacustrine–palustrine ramp deposition in a saline lake. High Sr content in some early diagenetic calcites suggests that aragonite and calcite made up the primary carbonate muds. Early dolomitization took place together with widespread pedogenic activity. The upper, deep, freshwater, lacustrine assemblage includes bundles of carbonate–clay rhythmites and fine‐grained turbidite beds. Primary calcite and diagenetic siderite make up the carbonate laminae. The Mg content of the primary carbonates records variations in Mg/Ca ratios in lacustrine waters. δ18O and δ13C covariance trends in calcite reinforce closed drainage conditions. δ18O data indicate that the lake system changed rapidly from short‐lived isotopically light periods (i.e. from seasonal to pluriannual) to longer steady‐state periods of heavier δ18O (i.e. from pluriannual to millennial). The small δ13C changes in the covariant trends were caused by dilute inflow, changing the contributions of dissolved organic carbon in the system and/or internal variations in lacustrine organic productivity and recycling. In both shallow and deep carbonate facies, sulphate reduction and methanogenesis may account, respectively, for the larger negative and positive δ13C shifts recorded in the early diagenetic carbonates (calcite, dolomite and siderite). The lacustrine system was very susceptible to high‐frequency, climatically forced water balance variations. These climatic oscillations interfered with the low‐frequency tectonic and morphological changes in the basin catchment. This resulted in the superposition of high‐order depositional, mineralogical and geochemical cycles and rhythms on the lower order lacustrine infill sequence.  相似文献   

15.
微生物碳酸盐岩是当前沉积学的研究热点之一,但关于古近系湖相微生物碳酸盐岩的报道还非常少。为探讨山东平邑盆地古近系官庄组中段湖相微生物碳酸盐岩的形成机理,笔者进行了野外实测以及室内偏光显微镜和扫描电镜观察。研究结果认为：（1）研究区微生物碳酸盐岩主要有核形石、叠层石和凝块石;（2）在核形石、叠层石和凝块石内部均发现了球状微生物化石、席状体和凝块状泥晶等微生物成因的显微组构,表明了它们形成过程中共同的生长、粘结和钙化等微生物作用;（3）微生物作用对环境变化比较敏感,水体较小、变化频繁的湖泊环境,对微生物碳酸盐岩结构和形态的发育有很大影响;核形石、叠层石和凝块石的形成环境大致呈现出水动力依次减弱、陆源碎屑物质依次减少、水深依次加大的规律;（4）提出了研究区微生物碳酸盐岩的成因模式,认为这3种微生物碳酸盐岩正是以微生物活动为共同基础,在不同的环境条件下形成的。  相似文献   

16.
The Lower Miocene Clews Formation at Alvord Mountain, Mojave Desert, California, comprises an upward coarsening sequence of synextensional continental deposits of variable thickness; this sequence pinches out to the west against the Alvord Mountain pre-Tertiary basement complex and thickens eastward to c.300 m over a distance of 7 km. Two stages of sedimentation are recognized in the formation. During the initial stage of sedimentation, the depocentre was delineated by a lacustrine system that was bounded to the west by small, sheetflood-dominated alluvial fans and to the north by a southerly flowing fluvial braidplain. Lacustrine mudstone, siltstone and carbonate suggest an initially closed basin. Conglomerate to the west represents small, sheetflood-dominated fans. Igneous clasts and a south-east palaeoflow direction suggest a proximal, low-relief source in the western Alvord Mountain area. Pebbly sandstone of the southward-prograding braidplain has a metasedimentary provenance in the Paradise Range to the north. The second stage of deposition was dominated by coarse conglomerate and breccia. West to southwest palaeotransport indicators and a distinctive metaigneous petrofacies indicate a provenance in the Cronese Hills, 8 km to the east. These strata reflect rapid westward progradation of sheetflood- and debris flow-dominated alluvial fans that advanced across the axial braidplain and lacustrine system. Basin development is interpreted to have been controlled by regional NE-directed extension on a detachment fault associated with the central Mojave metamorphic core complex. 30 km to the west. The Alvord Mountain - Cronese Hills region was initially transported as a single hangingwall block on the E-dipping detachment. Initial lacustrine sedimentation reflects the development of a flexural or sag basin in the hangingwall. Subsequent westward progradation of alluvial fans out of the Cronese Hills is believed to record the propagation of a NW-striking, SW-dipping normal fault antithetic to the low-angle E-dipping detachment fault.  相似文献   

17.
Carbon and oxygen stable isotopic composition of Cenozoic lacustrine carbonates from the intramontane Qaidam Basin yields cycles of variable length and shows several distinct events driven by tectonics and climate changes. From Eocene to Oligocene, the over-all trend in the δ13C composition of lacustrine carbonates shows a shift toward higher values, possibly related to higher proportions of dissolved inorganic carbon transported to the lake or lower input of soil derived CO2. At the same time, the δ18O composition of lacustrine carbonates is decreasing in accordance with the global cooling trend and northwards drifting of the whole region. During the Miocene, distinct isotopic events can be recognized, although their interpretation and linkage to a certain tectonic event remains difficult. These events may be related to uplift in the Himalayas, to the strongest phase of uplift in the Altyn Mountains, to pronounced subsidence of the Qaidam Basin or to the expansion of C4 plants on land. Generally cold, highly evaporative conditions can be deduced from enrichment of δ18O isotopic compositions during Pliocene and Quaternary times.  相似文献   

18.
The Abu Ruweis Formation is composed of carbonates, evaporites, and mudstones, with some locally developed pelletic, oolitic and stromatolitic limestones. The lateral persistence of bedding, the purity of the evaporite rocks, the alternating arrangement of marine carbonates and evaporites indicates periodic deposition in subaqueous conditions (salina). Petrographic investigations, X-ray diffraction analysis as well as chemical analysis have shown that the outcropping evaporite beds are mainly composed of secondary gypsum, with rare anhydrite relics. Five microfacies of gypsum were recognized according to their fabrics: porphyroblastic and granoblastic gypsum showing polarization texture, gypsum pseudomorph after anhydrite laths, and satin spar gypsum. The textures they display indicate a hydration origin of precursor anhydrite, which is in turn rehydrated from primary gypsum. Some of these anhydrites were formed as a result of replacement processes of the carbonate sediments associated with the evaporites, as evidenced from the textural relationships of the carbonate and sulfate minerals. The O18 content ranges from 1.45 to 8.38% PDB and the C13 content ranges from −1.52 to 4.73% PDB. Trace elements analysis has shown that the Abu Ruweis dolomites are rich in strontium (up to 600 ppm), and sodium (up to 835 ppm). The isotope composition and trace elements content, as well as the petrographic characteristics point to a penecontemporaneous hypersaline dolomitization origin for the Abu Ruweis dolomites. The evaporites were deposited during a regressive lowstand systems tract, whereas the carbonates were deposited under shallow water marine conditions during a highstand systems tract. The Abu Ruweis succession represents a relatively stable arid climate within a rapidly subsiding basin. Restricted conditions were provided by the development of beach barriers.  相似文献   

19.
The Cuddapah basin consists of generally well-preserved Palaeoproterozoic–Neoproterozoic sedimentary and associated volcanic rocks. The detailed lithological studies of sedimentary rocks of Vempalle Formation from the narrow linear belt of 15 km, in the southern margin, show the occurrence of layered gypsum in the cherty dolostone–shale intercalated facies, red shale and phosphatic dolostone facies of the Vempalle Formation. The petromineralogical studies reveal that gypsum is in close association with anhydrites. Microscopically, three different types of gypsum and anhydrite are identified, viz., lath-shaped, equant-shaped and anhedral-shaped grains. The equant variety corresponds to a granular gypsum, whereas the anhedral grains of gypsum exist as the granular and fibrous variety as seen in the hand specimen. The presence of gypsum/anhydrite has been confirmed by the petromineralogical, X-ray diffraction and chemical analytical data. The phosphatic dolostone is the host rock for stratabound type of uranium deposit at Tummalapalle, Cuddapah district, Andhra Pradesh, which is one of the most unique types of uranium mineralisation in the world. Abundant pseudomorphs of gypsum and anhydrite relicts and discontinuous gypsum layers within these dolostones, nodules of chert and gypsum indicate the interrelationship between the diagenesis and genesis of uranium mineralisation which indicates the carbonate precipitation in the sulphate-rich hypersaline environments.  相似文献   

20.
The Urrea de Jalón tufa deposits constitute the 20‐ to 50‐m‐thick caprock (0·3 km2) of an isolated mesa. They disconformably overlie horizontal strata of the Tertiary Ebro Basin (NE Spain), which contains a thick succession of lacustrine gypsum and marls, followed by limestones, marls and, locally, fluvial sandstones and mudstones. The tufa deposits show a complex, large‐scale framework of basin‐like structures with centripetal dips that decrease progressively from the base to the top of the tufa succession, and beds that thicken towards the centre of the structure (cumulative wedge‐out systems). These geometries reveal that the tufa deposits were affected by differential synsedimentary subsidence. Distinct onlapping depressions reflect time migration of the subsiding areas. The studied carbonates are composed mostly of low‐Mg calcite, with minor quartz. Some samples have anomalously high contents of Fe, Mn and Ba that may exceed 1% (goethite, haematite and barite are present). Carbonate facies are: (a) macrophyte encrustation deposits; (b) bryophyte build‐ups; (c) oncolite and coated grain rudstones; (d) non‐concentric stromatolite‐like structures; (e) massive or bioturbated biomicrites; and (f) green and grey marls. Facies a and c show a great variety of microbial‐related forms. These facies can be arranged in dm‐ to 2‐m‐thick vertical associations representing: (i) fluvial–paludal sequences with bryophyte growths; (ii) pond‐influenced fluvial sequences; and (iii) lacustrine–palustrine sequences. The Urrea de Jalón tufa deposits formed in a fluvio‐lacustrine environment that received little alluvial sediment supply. Isotope compositions (δ13C and δ18O) reveal meteoric signatures and accord with such a hydrologically open system of fresh waters. The Fe, Mn and Ba contents suggest an additional supply of mineralized waters that could be related to springs. These would have been discharge points in the Ebro Depression of a regional aquifer of the Iberian Ranges. Rising groundwater caused the solution of the underlying evaporites and the synsedimentary subsidence of the tufa deposits.  相似文献   

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