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1.
Facies architecture and platform evolution of an early Frasnian reef complex in the northern Canning Basin of north‐western Australia were strongly controlled by syn‐depositional faulting during a phase of basin extension. The margin‐attached Hull platform developed on a fault block of Precambrian basement with accommodation largely generated by movement along the Mount Elma Fault Zone. Recognition of major subaerial exposure and flooding surfaces in the Hull platform (from outcrop and drillcore) has enabled comparison of facies associations within a temporal framework and led to identification of three stages of platform evolution. Stage 1 records initial ramp development on the hangingwall dip slope with predominantly deep subtidal conditions that prevented any cyclic facies arrangements. This stage is characterised by basal siliciclastic deposits and a major deepening‐upward facies pattern that is capped by a sequence boundary towards the footwall (north‐west) and a major flooding surface towards the hangingwall. Stage 2 reflects the bulk of platform aggradation, significant platform growth towards the hangingwall and the development of reef margins and cyclic facies arrangements. Thickening of this stage towards the hangingwall indicates that accommodation was generated by rotation of the fault block and overlying platform. Stage 3 records a major flooding and backstep of the platform margin. The Hull platform illustrates important elements of margin‐attached carbonate platforms in a half‐graben setting, including: (i) prominent, but limited, coarse siliciclastic input that does not have a major detrimental effect on carbonate production near the rift margin in arid to semi‐arid settings; (ii) wedge‐shaped accommodation created by syn‐depositional rotation of fault blocks and tilting of the hangingwall dip slope, resulting in shallow‐water facies and subaerial exposure up‐dip of the rotational axis and deeper water facies down‐dip; and (iii) evolution of a ramp to rimmed shelf, coincident with a sequence boundary–flooding surface, that is accelerated by tilting of the hangingwall dip slope during fault‐block rotation.  相似文献   

2.
Abstract The Infra Krol Formation and overlying Krol Group constitute a thick (< 2 km), carbonate-rich succession of terminal Proterozoic age that crops out in a series of doubly plunging synclines in the Lesser Himalaya of northern India. The rocks include 18 carbonate and siliciclastic facies, which are grouped into eight facies associations: (1) deep subtidal; (2) shallow subtidal; (3) sand shoal; (4) peritidal carbonate complex; (5) lagoonal; (6) peritidal siliciclastic–carbonate; (7) incised valley fill; and (8) karstic fill. The stromatolite-rich, peritidal complex appears to have occupied a location seaward of a broad lagoon, an arrangement reminiscent of many Phanerozoic and Proterozoic platforms. Growth of this complex was accretionary to progradational, in response to changes in siliciclastic influx from the south-eastern side of the lagoon. Metre-scale cycles tend to be laterally discontinuous, and are interpreted as mainly autogenic. Variations in the number of both sets of cycles and component metre-scale cycles across the platform may result from differential subsidence of the interpreted passive margin. Apparently non-cyclic intervals with shallow-water features may indicate facies migration that was limited compared with the dimensions of facies belts. Correlation of these facies associations in a sequence stratigraphic framework suggests that the Infra Krol Formation and Krol Group represent a north- to north-west-facing platform with a morphology that evolved from a siliciclastic ramp, to carbonate ramp, to peritidal rimmed shelf and, finally, to open shelf. This interpretation differs significantly from the published scheme of a basin centred on the Lesser Himalaya, with virtually the entire Infra Krol–Krol succession representing sedimentation in a persistent tidal-flat environment. This study provides a detailed Neoproterozoic depositional history of northern India from rift basin to passive margin, and predicts that genetically related Neoproterozoic deposits, if they are present in the High Himalaya, are composed mainly of slope/basinal facies characterized by fine-grained siliciclastic and detrital carbonate rocks, lithologically different from those of the Lesser Himalaya.  相似文献   

3.
A Lower Cretaceous carbonate platform depositional system with a rimmed margin and an adjacent oversteepened slope was analysed in order to determine its depositional architecture and major depositional controls. The platform is made up of coral, rudist, orbitolinid and algal limestones and, in a 12-km dip transect, there is a gradation from lagoon to platform margin, slope and basin environments, each characterized by distinctive sedimentological features and facies associations. The rimmed platform is an aggradational system developed during approximately 4·2 million years of fluctuating relative sea-level rise, and it is bounded by unconformities at its base and top. Internal cyclicity in the construction of the system is evident, mainly in platform interior and slope settings. The seven recognized sequences average 0·6 million years in duration and are related to minor relative sea-level changes. Carbonate deposition occurred in shallow- and deep-water settings during periods of high relative sea level. Reduced rates of sea-level rise led to the development of shallowing upward sequences and, eventually, to the exposure of the shallowest parts of the platform during relative sea-level falls. During low relative sea level, erosion surfaces developed on the slope, and gravitational resedimentation occurred at the toe of slope. Basinwards, resedimented units pinch out over distances of a few hundred metres. Active faults controlled sedimentation at the platform margin, promoting the development of steep slopes (up to 35°) and preventing progradation of the shallow-water platform, despite high sediment production rates. The development of sequences is interpreted to be related to tectonic activity.  相似文献   

4.
Recognition of palaeokarst in the oldest exposed Devonian (Givetian ‐ lower Frasnian) platform successions of the Canning Basin reef complexes has eluded investigators for over forty years. The first evidence for palaeokarst, developed on microbial mud‐mounds in a single stratigraphic horizon, is documented and records an episode of exposure during early carbonate platform development. Surface palaeokarst features are scalloped surfaces, solution pits and a pipe, underlain by fenestral limestone with sediment‐filled fossil moulds and vugs. The platform succession has variably developed metre‐scale cycles which are composed predominantly of shallowing‐upward subtidal facies, with some cycles having fenestral peloidal mudstone caps. Changes in facies type and stratigraphic arrangement up the succession define two deepening‐upward units (～70 and 180 m thick), with the palaeokarst surface representing emergence following rapid shallowing at the top of the lower unit. The stratigraphic position of the palaeokarst between these two units suggests it may represent a sequence boundary. This may have been caused by a low‐magnitude eustatic fall or footwall‐uplift event superimposed on a rapidly subsiding basin margin.  相似文献   

5.
A steep‐margined carbonate platform is developed in the Carboniferous synorogenic foreland basin of northern Spain. Dips of 60–90° produced during Late Carboniferous thrusting enable cross‐sections of a 4‐km‐wide portion of the marginal area of this platform (Las Llacerias outcrop) to be studied in aerial photographs at a seismic scale. Three stratal domains are observed: (1) a horizontal‐bedded platform; (2) a clinoformal‐bedded margin with a relief of up to 500 m; and (3) a low‐angle toe‐of‐slope, where slope beds interfinger with basin sediments. The slope shows well‐bedded sigmoidal clinoforms with depositional dips ranging from 15° to 32°. Based on lithology and stratal patterns, four facies groups have been recognized: (1) a flat‐topped platform, in which thick algal boundstone, skeletal packstone–grainstone and peloidal micrite wackestone with a poorly rhythmic character prevail; (2) the platform margin and upper slope, characterized by microbial boundstone spanning a bathymetric range of ≈150 m measured from the break of slope; (3) a slope, predominantly composed of margin‐derived rudstones and breccias; and (4) a toe‐of‐slope to basin zone, where a cyclic alternation of spiculitic siltstones, packstone to grainstone calciturbidites and rudstone/breccia is visible. Five successive stages of platform development are deduced: (1) Bashkirian: flooding of the pre‐existing Serpukhovian platform giving rise to the nucleation of a low‐angle ramp to the south‐east of the study area with microbial mud‐mound accumulations, and breccias and calciturbidites on the margins; (2) Early Moscovian: an influx of siliciclastic sediment buried part of the platform and reduced the area of carbonate sedimentation; (3) Moscovian: aggradation and progradation of the carbonate system produced an extensive steep‐margined and flat‐topped shallow‐water platform (shelf system); (4) Latest Moscovian–earliest Kasimovian: drowning of the platform; and (5) Kasimovian: covering of the platform by marly calcareous ramp sediments.  相似文献   

6.
基于城口-鄂西海槽西侧地区详细的露头调查、钻井资料分析,结合古生物、古生态以及地层对比,对研究区晚二叠世长兴期碳酸盐台地边缘沉积相及其演化进行详细解剖,得出区内台地边缘骨架礁-凝块石灰泥丘生态-沉积演化对碳酸盐台地边缘构筑具有积极作用的认识。研究表明区内长兴期碳酸盐台地边缘礁滩经历了三个演化阶段:第一阶段,碳酸盐台地边缘坡折带尚未形成,台地边缘礁滩欠发育,且台地前缘斜坡坡度较缓;第二阶段,碳酸盐台地镶边开始形成,发育骨架礁及滩相沉积,垂向上构成进积-加积组合序列,台地前缘斜坡变陡;第三阶段,微生物逐渐占据后生造礁生物生态空间,发育台地边缘凝块石灰泥丘,与上覆台地边缘滩构成垂向加积组合序列,促进了台地边缘正向地貌的发育,使区内台地边缘更加陡峭,台地前缘斜坡发育滑塌角砾岩。造礁生物生态演替及相对海平面变化共同影响并控制长兴期碳酸盐台地边缘构筑过程。  相似文献   

7.
In this study, progradation and the subsequent retrogradation of a late Paleocene isolated carbonate platform (Galala Mountains, Eastern Desert, Egypt) is demonstrated by variations of distinct facies associations from the platform margin in the north to the hemipelagic basin in the south. A combination of a sea-level drop and tectonic uplift at around 59 Ma (calcareous nannofossil biozone NP5) favored the initiation of the carbonate platform. From this time onwards, the facies distribution along the platform–basin transect can be subdivided into five facies belts comprising nine different facies associations. Their internal relationships and specific depositional settings are strongly coupled with the Maastrichtian–Paleocene seafloor topography, which resulted from local tectonic movements. Patch reefs and reef debris were deposited at the platform margin and the horizontally bedded limestones on the upper slope. Slumps and debris flows were stored on the lower slope. In the subhorizontal toe-of-slope facies belt, mass-flow deposits pass into calciturbidites. Further southwards in the basin, only hemipelagic marls were deposited. Between 59 and 56.2 Ma (NP5–NP8), the overall carbonate platform system prograded in several pulses. Distinct changes in facies associations from 56.2 to 55.5 Ma (NP9) resulted from rotational block movements. They led to increased subsidence at the platform margin and a coeval uplift in the toe-of-slope areas. This resulted in the retrogradation of the carbonate platform. Furthermore the patch-reef and reef-debris facies associations were substituted by the larger foraminifera shoal association. The retrogradation is also documented by a significant decrease in slump and debris-flow deposits on the slope and calciturbidites at the toe of slope.  相似文献   

8.
Cenomanian–Turonian strata of the south‐central Pyrenees in northern Spain contain three prograding carbonate sequences that record interactions among tectonics, sea level, environment and sediment fabric in controlling sequence development. Sequence UK‐1 (Lower to Upper Cenomanian) contains distinct lagoonal, back‐margin, margin, slope and basin facies, and was deposited on a broad, flat shelf adjacent to a deep basin. The lack of reef‐constructing organisms resulted in a gently dipping ramp morphology for the margin and slope. Sequence UK‐2 (Upper Cenomanian) contains similar shallow‐water facies belts, but syndepositional tectonic modification of the margin resulted in a steep slope and deposition of carbonate megabreccias. Sequence UK‐3 (Lower to Middle Turonian) records a shift from benthic to pelagic deposition, as the shallow platform was drowned in response to a eustatic sea‐level rise, coupled with increased organic productivity. Sequences UK‐1 to UK‐3 are subdivided into lowstand, transgressive and highstand systems tracts based on stratal geometries and facies distribution patterns. The same lithologies (e.g. megabreccias) commonly occur in more than one systems tract, indicating that: (1) the depositional system responded to more than just sea‐level fluctuations; and (2) similar processes occurred during different times throughout sequence development. These sequences illustrate the complexity of carbonate platform dynamics that influence sequence architecture. Rift tectonics and flexural subsidence played a major role in controlling the location of the platform margin, maintaining a steep slope gradient through syndepositional faulting, enhancing slope instability and erosion, and influencing depositional processes, stratal relationships and lithofacies distribution on the slope. Sea‐level variations (eustatic and relative) strongly influenced the timing of sequence and parasequence boundary formation, controlled changes in accommodation and promoted platform drowning (in conjunction with other factors). Physico‐chemical and climatic conditions were responsible for reducing carbonate production rates and inducing platform drowning. Finally, a mud‐rich sediment fabric affected platform morphology, growth geometries (aggradation vs. progradation) and facies distribution patterns.  相似文献   

9.
The Upper Cretaceous (Cenomanian-Maastrichtian) of the south-central Pyrenees shows five carbonate platform sequences where the major parameters are tectonism, relative sea-level fluctuations and inherited depositional profile. Depositional geometries and basin analysis permit an understanding of the depositional history.Five depositional sequences have been recognized: (1) The Santa Fe sequence (Middle-Upper Cenomanian), a ramp to a skeletal rimmed shelf with an escarpment bypass margin. The lower boundary is an angular unconformity and the upper one records a sea-level drop. The platform location of the margin was determined by a listric normal fault. (2) An abrupt sea-level rise drowned the former platform. The Congost sequence (Turonian-Lower Coniacian), a distally steepened ramp with erosional distal deep slope. The depositional model was largely controlled by pre-existing basin morphology. Cessation of platform development was due to a relative sea-level drop. (3) The Sant Corneli sequence (Upper Coniacian-Lower Santonian), a mixed terrigenous-skeletal homoclinal ramp with upright margin, deep slope and dysaerobic basin. The slope results from the backstepping by 24 km of the previous margin and gentle basin tilting. The platform margin remained more or less at the same position, and relief between platform and slope increased indicating continuous relative sea-level rise. The upper boundary is an angular unconformity at the platform margin produced by an abrupt sea-level rise and platform drowning, and by listric normal faulting. (4) The Vallcarga sequence (Upper Santonian-Campanian), a distal-steepened skeletal homoclinal ramp, erosional escarpment and turbidite basin, which corresponds to the Mesozoic maximum marine expansion. A listric normal fault created two depositional areas: a more or less flat footwall block with a north-northwest prograding carbonate ramp.  相似文献   

10.
土库曼斯坦阿姆河盆地卡洛夫—牛津阶沉积相特征   总被引：2，自引：0，他引：2       下载免费PDF全文
以岩心描述、薄片和古生物鉴定为依据,结合测井相和地震相特征和区域构造-沉积背景,对土库曼斯坦阿姆河盆地卡洛夫—牛津阶沉积相特征进行研究,确定该地层单元属于碳酸盐台地沉积体系,识别出蒸发台地、局限台地、开阔台地、台地边缘礁滩、前缘缓斜坡、盆地等6种沉积相类型。以连井沉积相剖面对比、沉积相平面分布特征和纵向演化分析为基础,进一步揭示出该盆地卡洛夫—牛津阶经历了台地前缘缓斜坡、台地边缘礁滩、开阔台地、局限台地、蒸发台地等交替发育构成多期次海进—海退旋回演化和"开放性宽缓型镶边缓斜坡台地"沉积模式。  相似文献   

11.
中东地区主力产油层系Shuaiba组的勘探及研究逐渐以地层岩性圈闭为主,笔者通过总结Bab盆地阿普特阶Shuaiba组沉积相类型,多油田地层格架及空间展布特征的对比,为系统恢复Shuaiba组沉积演化过程及模式提供地质依据。研究表明,阿普特阶Shuaiba组共发育开阔台地、台地边缘、台缘斜坡及盆地相4种沉积相单元和6种亚相类型,各相带单元受控于四个三级层序且沿Bab盆地周缘呈带状展布。开阔台地一侧,主要沉积了第二个三级层序对应的Lower Shuaiba地层;台地边缘往Bab台内盆地区域,则以第三和第四个三级层序对应的Upper Shuaiba地层为主。Shuaiba组生物礁、滩建造集中发育在第二个三级层序的高位域阶段,滩间海、缓坡及盆地相受控于海进和低位体系域。阿普特阶Shuaiba台地边缘生物建隆、台内浅滩在空间上相互叠置并成带展布,符合弱镶边型缓坡台地沉积模式,该类型台地具备深水陆棚性质的非常宽缓的斜坡,且缓坡往盆地一侧发育一个镶边台地边缘。  相似文献   

12.
近年来,我国海相碳酸盐岩油气勘探正处于大发现期,其中四川盆地龙王庙组气藏的突破,使得寒武系碳酸盐岩储层越来越获得研究者的重视。洗象池群作为寒武系有利储集层段目前还处于勘探初期,地质研究程度相对滞后。为此,基于前人的研究成果,通过综合分析露头、钻井、地震和测井等资料,研究了四川盆地及邻区洗象池群沉积相、沉积体系模式,编制岩相古地理图,建立洗象池群“三滩夹两凹”沉积模式,指出了该区下一步天然气勘探的方向。研究结果表明:(1)四川盆地洗象池群呈现碳酸盐岩镶边台地的古地理格局,发育蒸发台地、局限台地、半局限台地、台地边缘、斜坡盆地等5种沉积相;(2)建立了洗象池群“三滩夹两凹”沉积模式,其中一条为台地边缘颗粒滩,分布于该盆地东缘,另外两条为台内颗粒滩,分布于局限台地相的东西两侧,分属局限台地相和半局限台地相;(3)台内颗粒滩发育主要受古地貌及高频海平面升降变化所控;台地边缘颗粒滩主要分布于盆地的东缘,发育受沉积古地貌坡折控制。结论认为:台内颗粒滩和台地边缘滩储层质量好,是天然气勘探的重要领域;洗象池群“三滩夹两凹”沉积模式和颗粒滩带分布的新认识,对古老地层的沉积体系研究具有重要的理论意义。  相似文献   

13.
An early Paleozoic inter‐platform, platform, slope and shelf succession comprised of five Cambrian to mid‐Ordovician seismic sequences, has been identified in the southwestern part of the Tarim Basin. A previously unknown, elongate shelf succession that extends from east to west transects the Taxinan area of the southwestern Tarim Basin. It was flanked by carbonate platforms and platform margins, and by peri‐platform margin slopes on its north and south sides. The Markit platform margin and slope, located along the northern side of the shelf, extended nearly linearly west to east with some headlands and embayments towards the east. The Tangnan platform margin and slope, located along the southern side of the shelf, extended southwest to northeast and was more sinuous with pronounced northwest‐directed headlands flanked by embayments. Differentiation of these settings began in earliest Cambrian time during continental breakup. Initially, broad gentle slopes flanked an open shelf seaway in Cambrian time. In Early Ordovician time, the Markit and Tangnan platform margins became more distinct and rimmed, and were flanked by slightly steeper, but still very gentle slopes. During their deposition, these platform margins and their adjoining slopes exhibited some lateral migration, with a slight overall widening of the open shelf seaway between them from Cambrian to Ordovician time. The presence of these platform margin and slope successions in the southwestern area of the Tarim basin provides another focus for petroleum exploration and discovery of additional carbonate reef‐shoal reservoirs in the Markit and Tangnan areas. The adjoining deep‐water shelf shale and mudstone shelf succession itself is an additional prospective petroleum source rock in close proximity to these potential reef‐shoal reservoirs.  相似文献   

14.
The Taltheilei, Utsingi, McLean and Blanchet formations form a 175–390 m thick carbonate platform-to-basin succession in the lower part of the PaleoProterozoic Pethei Group, preserved in the eastern arm of Great Slave Lake. Carbonates accumulated along the south-east margin of the Slave Craton within a foredeep formed during the collision of the Slave and Churchill Cratons. The rocks include eight, predominantly microbial, carbonate facies that comprise five facies associations representing (1) shallow-water rimmed shelf, (2) shallow-water open shelf, (3) shallow-water ramp, (4) upper slope and deep ramp, and (5) lower slope and basin plain environments. Microbialite facies grew by organically mediated precipitation of spar and micritic cement and trapping and binding of lime mud. These wholly subtidal facies typically reflect progressive shallowing and changing geometry of the lower Pethei sea floor, from ramp, to open shelf, to shallow rimmed shelf, with associated slope and basin plain deposition. Repeated relative sea-level changes influenced platform growth. This resulted in five shallowing upward packages; each separated by an incipient drowning event of varying magnitude. Antecedent topography and the size of the preceding drowning event strongly influenced the initial growth of each interval. This repeated pattern is attributed to interaction between (a) the inherent tendency of microbial carbonates to aggrade vertically, (b) changing sedimentation rates and (c) readjustments of relative base level. The lower Pethei succession is one of few PaleoProterozoic examples of carbonate platform growth within a foreland basin. It has (1) a low gradient profile, (2) extensive slope and basin plain carbonate production and sedimentation, (3) no ooids, (4) minor terrigenous clastic sediments, and (4) a mobile, submergent shelf rim lacking substantial carbonate sand shoals.  相似文献   

15.
章学刚 《地质与勘探》2023,59(6):1347-1355
为明确四川盆地仪陇-平昌地区长兴组地震相及沉积特征,运用地震岩性学和地震地貌学的综合分析技术,开展井-震结合及礁体三维识别研究,以阐明长兴组生物礁滩储集体的展布特征。研究结果表明,仪陇-平昌地区长兴组主要为四个沉积区带,开阔台地相对应平行-亚平行席状中弱断续地震相、台地边缘相对应平行-亚平行丘状或透镜状中弱地震相、台地前缘斜坡相对应前积席状弱地震相,海盆相对应平行-亚平行席状中强地震相,可细分为台缘礁、台内礁和台内滩亚相;30 Hz振幅调谐体切片揭示海盆岸线为北西向展布,25 Hz调谐体切片对于海盆线西侧的台缘带礁体和台内礁滩体反应明显;生物礁滩体的平面展布特征、发育规模受控于长兴组沉积期西南高、东北低的古地貌形态,台缘礁生长发育的有利区域为台地相带两排北西向凸起带。  相似文献   

16.
During the early Middle Devonian in South China, an extensive carbonate platform was broken up through extension to create a complex pattern of platforms, and interplatform basins. In Givetian and Frasnian carbonate successions, five depositional facies, including peritidal, restricted shallow subtidal, semi‐restricted subtidal, intermediate subtidal and deep subtidal facies, and 18 lithofacies units are recognized from measured sections on three isolated platforms. These deposits are arranged into metre‐scale, upward‐shallowing peritidal and subtidal cycles. Nine third‐order sequences are identified from changes in cycle stacking patterns, vertical facies changes and the stratigraphic distribution of subaerial exposure indicators. These sequences mostly consist of a lower transgressive part and an upper regressive part. Transgressive packages are dominated by thicker‐than‐average subtidal cycles, and regressive packages by thinner‐than‐average peritidal cycles. Sequence boundaries are transitional zones composed of stacked, high‐frequency, thinner‐than‐average cycles with upward‐increasing intensity of subaerial exposure, rather than individual, laterally traceable surfaces. These sequences can be further grouped into catch‐up and keep‐up sequence sets from the long‐term (second‐order) changes in accommodation and vertical facies changes. Catch‐up sequences are characterized by relatively thick cycle packages with a high percentage of intermediate to shallow subtidal facies, and even deep subtidal facies locally within some individual sequences, recording long‐term accommodation gain. Keep‐up sequences are characterized by relatively thin cycle packages with a high percentage of peritidal facies within sequences, recording long‐term accommodation loss. Correlation of long‐term accommodation changes expressed by Fischer plots reveals that during the late Givetian to early Frasnian increased accommodation loss on platforms coincided with increased accommodation gain in interplatform basins. This suggests that movement on faults resulted in the relative uplift of platforms and subsidence of interplatform basins. In the early Frasnian, extensive siliceous deposits in most interplatform basins and megabreccias at basin margins correspond to exposure disconformities on platforms.  相似文献   

17.
Upper Callovian to Tithonian (late Jurassic) sediments represent an important hydrocarbon reservoir in the Kopet‐Dagh Basin, NE Iran. These deposits consist mainly of limestone, dolostone, and calcareous mudstone with subordinate siliciclastic interbeds. Detailed field surveys, lithofacies and facies analyses at three outcrop sections were used to investigate the depositional environments and sequence stratigraphy of the Middle to Upper Jurassic interval in the central and western areas of the basin. Vertical and lateral facies changes, sedimentary fabrics and structures, and geometry of carbonate bodies resulted in recognition of various carbonate facies related to tidal flats, back‐barrier lagoon, shelf‐margin/shelf‐margin reef, slope and deep‐marine facies belts. These facies were accompanied by interbedded beach and deep marine siliciclastic petrofacies. Field surveys, facies analysis, parasequences stacking patterns, discontinuity surfaces, and geometries coupled with relative depth variation, led to the recognition of six third‐order depositional sequences. The depositional history of the study areas can be divided into two main phases. These indicate platform evolution from a rimmed‐shelf to a carbonate ramp during the late Callovian–Oxfordian and Kimmeridgian–Tithonian intervals, respectively. Significant lateral and vertical facies and thickness changes, and results obtained from regional correlation of the depositional sequences, can be attributed to the combined effect of antecedent topography and differential subsidence related to local tectonics. Moreover, sea‐level changes must be regarded as a major factor during the late Callovian–Tithonian interval. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

18.
Late Frasnian mounds of the Yunghsien Formation, Guilin, South China, developed as part of the Guilin platform, mostly in reef‐flat and platform margin settings. Microbial mounds in platform margin settings at Hantang, about 10 km west of Guilin, contain Frasnian biota, such as Stachyodes and Kuangxiastraea and, thus, occur below the Frasnian‐Famennian mass extinction boundary. Platform margin facies were dominated by microbes, algae and receptaculitids. Massive corals and stromatoporoids are not common and rarely show reef‐building functions as they did in Givetian time. The margin mounds are composed of brachiopod‐receptaculitid cementstone, and a variety of boundstones that contain Rothpletzella, Renalcis, thrombolite and stromatolite. Other microbial communities include Girvanella, Izhella, Ortonella and Wetheredella. Solenoporoid algae are abundant locally. Zebra structures and neptunian dykes are well‐developed at some intervals. Pervasive early cementation played an important role in lithification of the microbial boundstones and rudstones. Frasnian reefs of many regions of the world were constructed by stromatoporoids and corals, although a shift to calcimicrobe‐dominated frameworks occurred before the Famennian. However, the exact ages of many Frasnian margin outcrops are poorly constrained owing to difficulties dating shallow carbonate facies. The Hantang mounds represent a microbe‐dominated reef‐building community with rare skeletal reef builders, consistent with major Late Devonian changes in reef composition, diversity and guild structure occurring before the end of the Frasnian. A similar transition occurred in the Canning Basin of Western Australia, but coeval successions in North America, Western Europe and the northern Urals are either less well‐known or represent different bathymetric settings. The transition in reef‐building style below the Frasnian‐Famennian boundary is documented here in the two best exposed successions on two continents, which may have been global. Set in the larger context of Late Devonian and Mississippian microbial reef‐building, the Hantang mounds help to demonstrate that controls on microbial reef communities differed from those on larger skeletal reef biota. Calcimicrobes replaced stromatoporoids as major reef builders before the Frasnian‐Famennian extinction event, and increasing stromatoporoid diversity towards the end of the Famennian did not result in a resurgence of skeletal reef frameworks. Calcimicrobes dominated margin facies through the Famennian, but declined near the Devonian‐Carboniferous boundary. Stromatolite and thrombolite facies, which occurred behind the mound margin at Hantang, rose to dominate Mississippian shallow‐water reef frameworks with only a minor resurgence of the important Frasnian calcimicrobe Renalcis in the Visean when well‐skeletonized organisms (corals) also became volumetrically significant frame builders again.  相似文献   

19.
Cross‐bedded grainstones on carbonate ramps and shelves are commonly related to the locus of major wave energy absorption such as shorelines, shoals or shelf breaks. In contrast, on the Early Tortonian carbonate platform of Menorca (Balearic Islands), coarse‐grained, cross‐bedded grainstones are found at a distance from the palaeoshoreline where they were deposited below the wavebase. Excellent exposures along continuous outcrops on the sea cliffs of Menorca reveal the depositional profile and three‐dimensional distribution of the different facies belts of the Tortonian ramp depositional system. Basinward from the palaeoshoreline, fan deltas and beach deposits pass into 5‐km‐wide gently dipping bioturbated dolopackstone (inner and middle ramp), then into 12–20°‐dipping dolograinstone/rudstone clinobeds (ramp slope) and, finally, into subhorizontal fine‐grained basinal dolowackestone to dolopackstone (outer ramp). In this Miocene example, coarse‐grained grainstones exist in five different settings other than beach deposits: (1) on the middle ramp, where cross‐bedded grainstones were deposited by currents roughly parallel to the shoreline at 40–70 m estimated water depth and are interbedded with gently dipping bioturbated dolomitized packstones; (2) on the upper slope, where clinobeds are composed mostly of in situ rhodoliths and red‐algae fragments; (3) on the lower slope, as small‐scale bedforms (small three‐dimensional subaqueous dunes) migrating parallel to the slope; (4) at the transition between the lower slope and the outer ramp, where mollusc‐rich and rhodolithic rudstones and grainstones, interbedded in dolomitized laminated wackestones containing abundant planktonic foraminifera, infill slide/slump scars as upslope‐backstepping bodies (backsets); (5) at the toe of the slope, where coarse skeletal grainstones indicate bedform migration parallel to the platform margin, induced by currents at more than 150 m estimated water depth. This Late Miocene example also illustrates how changes in intrabasinal environmental conditions (nutrients and/or temperature) may produce changes in stratal patterns and facies architecture if they affect the biological system. Two depositional sequences compose the Miocene platform on Menorca, where a reef‐rimmed platform prograded onto an earlier distally steepened ramp. The transition from the ramp to the reef‐rimmed platform was effected by an increase in accommodation space caused by ecological changes, promoting a shift from a grain‐ to a framework‐producing biota.  相似文献   

20.
库鲁克塔格地区保存了相对完整的寒武纪-奥陶纪沉积地层,其沉积特征及演化规律的研究,对塔东地区寒武系-奥陶系油气勘探具有重要意义。通过库鲁克塔格地区详细的野外地质调查,结合前人相关研究成果,对该区寒武系-奥陶系沉积体系、沉积模式及演化规律的研究表明,寒武纪时期,库鲁克塔格地区经历了一次大的海侵-海退旋回,南、北两区沉积具有相似性。寒武纪早期的快速海侵导致南、北两区均发育陆棚相-深水盆地相沉积;寒武纪晚期,在逐渐海退的背景下,南、北两区开始出现沉积分异。奥陶纪,经历了新一轮大的海侵-海退旋回,南北两区沉积差异显著。北区从早奥陶世到晚奥陶世,发育台地边缘斜坡相-广海陆棚相-缓斜坡相-台地边缘礁滩相-开阔台地相相序,构成整体向上变浅的碳酸盐岩沉积层序;而南区发育深水盆地相-陆棚斜坡相-浊流盆地相-碎屑陆棚相相序,形成一套巨厚的深水复理石建造。库鲁克塔格地区寒武纪时期发育缓坡型碳酸盐岩台地,因台地不断向南构筑以及断裂活动,导致奥陶纪晚期台地边缘快速变陡,并在经历斜坡相快速堆积填平补齐之后,重新演变为缓坡型碳酸盐岩台地。晚奥陶世,由于周缘构造活动影响,却尔却克山-雅尔当山一带下沉,逐渐向远端变陡缓坡型碳酸盐岩台地演化。  相似文献   

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