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1.
The study deals with the depositional environment of Jumara Dome sediments. The Jumara Dome is an important outcrop of Bathonian to Oxfordian sediments amongst the Kachchh Mainland exposures. On the basis of facies analysis three associations have been documented, namely, G-1 consisting of low energy facies comprising of cross-bedded sandstone, massive sandstone, grey shale and thin bedded sandstone, bioclastic — lithoclastic grainstone, bioclastic — lithoclastic packstone, microbioclastic packstone/wackestone, bioturbated laminated wackestone to mudstone and pelagic lime mudstone; G-II consisting of moderate energy facies comprising of laminated sandstone and grapestone or agglutinated grainstone; G-III consisting of high energy facies comprising of interbedded gypsiferous shale and sandstone/siltstone, oolitic grainstone to conglomerate and bioclastic grainstone. The facies associations reflect an ideal shallowing upward sequence representing slope, bioclast bar, lagoon and inner shelf. Presence of wide range of facies indicates that the rocks of the studied area were deposited during the fluctuating sea level, interrupted by the storms, in the shallow marine environment.  相似文献   

2.
对鄂西—湘西北地区多个沉积剖面的地层及沉积相进行了详细分析,结果表明,该区二叠纪栖霞期至茅口初期主要为内克拉通碳酸盐岩缓坡环境,发育内缓坡相、中缓坡相、外缓坡相和盆地相.内缓坡相以厚层至块状生物碎屑石灰岩为主,生物颗粒以绿藻和底栖有孔虫为主,缺乏高能沉积的生物颗粒.中缓坡相以中厚层含生物碎屑颗粒石灰岩以及厚层灰泥石灰岩...  相似文献   

3.
Well‐cuttings, wireline logs and limited core and outcrop data were used to generate a regional, three‐dimensional sequence framework for Upper Mississippian (Chesterian), Greenbrier Group carbonates in the Appalachian foreland basin, West Virginia, USA. The resulting maps were used to document the stratigraphic response of the basin to tectonics and to glacio‐eustasy during the transition into ice‐house conditions. The ramp facies include inner ramp red beds and aeolianites, lagoonal muddy carbonates, mid‐ramp ooid and skeletal grainstone shoal complexes, and outer ramp wackestone–mudstone, that grades downslope into laminated silty lime mudstone. The facies make up fourth‐order sequences, a few metres to over 90 m (300 ft) thick. The sequences are bounded along the ramp margin by lowstand sandstones and calcareous siltstones. On the ramp, sequence boundaries are overlain by thin transgressive siliciclastics and aeolianites, and only a few are calichified. Maximum flooding surfaces on the outer ramp lie beneath deeper water facies that overlie lowstand to transgressive siliciclastic or carbonate units. On the shallow ramp, maximum flooding surfaces overlie siliciclastic‐prone transgressive systems tracts, that are overlain by highstand carbonates with significant grainstone units interlayered with lagoonal lime mudstones. The fourth‐order sequences are the major mappable subsurface units; they are bundled into weak composite sequences which are bounded by red beds. In spite of differential subsidence rates across the foreland basin (1 to 3 cm/k.y. up to 25 cm/k.y.), eustatic sea‐level changes controlled regional sequence development. Thrust‐load induced differential subsidence of fault‐blocks, coupled with in‐plane stress, controlled the rapid basinward thickening of the depositional wedge, whose thickness and facies were influenced by subtle structures such as arches trending at high angles as well as parallel to the margin.  相似文献   

4.
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   

5.
The 150–160 m thick lowermost Silurian (Rhuddanian) Becscie and Merrimack formations of Anticosti Island, Canada, represent continuous deposition on a shallow, open marine carbonate ramp. Several rock types are identified: (a) laminated and homogenous mudstone; (b) laminated and homogenous packstone; (c) argillaceous mudstone and packstone; (d) calcareous shale; (e) laminated calcisiltite; (f) medium- to fine-grained grainstone; and (g) bio/intraclastic rudstone. These rock types are arranged into five distinct lithofacies: (LF1) calcareous mudstone-shale; (LF2) laminated-homogenous mudstone; (LF3) calcareous grainstone-shale; (LF4) laminated mudstone-grainstone; and (LF5) laminated calcisiltite-grainstone. The sequence reflects deposition on a low-energy, muddy, carbonate to argillaceous ramp subject to short-lived, episodic high-energy storms. These events produced fining-upwards storm units 5–80 cm thick, or tempestites, consisting of: a sharp scoured base overlain by intra/bioclastic rudstone grading upwards into medium-grained grainstone, finely laminated calcisiltite and mudstone, or shale. These are interbedded with low-energy, fairweather mudstones and calcareous shales. Deposition progressed from a carbonate mud-dominated ramp in the Becscie Formation to an argillaceous mud-dominated ramp in the Merrimack Formation. Lateral tempestite proximality trends and lithofacies distribution indicate that the Anticosti Basin deepened to the south-east into the Iapetus Ocean and shallowed towards a SW—NE-orientated shoreline to the north-west. Vertical tempestite proximality trends and lithofacies changes identify third-order eustatic sea-level changes. After an initial deepening at the base of the formation, a shallowing-deepening event dominated the sequence. Several higher order fluctuations, defined by lithofacies and tempestite proximality trends, are superimposed on these changes. The fluctuations identified with the aid of tempestite proximality trends are of an order of magnitude higher than those identified by either lithofacies or palaeontological methods.  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.
Upper Cambrian carbonates in western Maryland are comprised of platform facies (Conococheague Limestone) west of South Mountain and basin facies (Frederick Limestone) east of South Mountain. Conocheague platform carbonates contain interbedded non-cyclic and cyclic facies. Non-cyclic facies consist of cross-stratified grainstones, thrombolitic bioherms, and graded, thin-bedded dolostones. These were deposited in shallow, subtidal shelf lagoons. Cyclic facies are composed of repeated sequences of cross-stratified grainstone; ribbon-rock; wavy, prism-cracked laminite; and planar laminated dolostone. The cyclic facies are shallowing-upward cycles produced by lateral progradation of tidal flats over shallow, nearshore subtidal environments. Cyclic and non-cyclic facies are interbedded in the Conococheague in a layer cake fashion, but no higher-order cyclicity can be found. The Frederick Limestone is dominated by monotonously thick sequences of graded, thin-bedded limestones, interbedded with massive peloidal grainstones and beds of breccia up to 10 m thick in the lower Frederick. The breccias contain transported megaclasts of Epiphyton-Girvanella boundstones. The basal Frederick was deposited in a slope-to-basinal setting east of a rimmed shelf. An Epiphyton-Girvanella marginal reef along the shelf edge was the source of the blocks in the breccias. The upper Frederick Limestone formed on a carbonate ramp.  相似文献   

8.

The Upper Cambrian Owen Conglomerate of the West Coast Range, western Tasmania, comprises two upward‐fining successions of coarse‐grained siliciclastic rocks that exhibit a characteristic wedge‐shaped fill controlled by the basin‐margin fault system. Stratigraphy is defined by the informally named basal lower conglomerate member, middle sandstone member, middle conglomerate member and upper sandstone member. The lower conglomerate member has a gradational basal contact with underlying volcaniclastics of the Tyndall Group,while the upper sandstone member is largely conformable with overlying Gordon Group marine clastics and carbonates. The lower conglomerate member predominantly comprises high flow regime, coarse‐grained, alluvial‐slope channel successions, with prolonged channel bedload transport exhibited by the association of channel‐scour structures with upward‐fining packages of pebble, cobble and boulder conglomerate and sandstone, with abundant large‐scale cross‐beds derived from accretion in low‐sinuosity, multiply active braided‐channel complexes. While the dipslope of the basin is predominantly drained by west‐directed palaeoflow, intrabasinal faulting in the southern region of the basin led to stream capture and the subsequent development of axial through drainage patterns in the lower conglomerate member. The middle sandstone member is characterised by continued sandy alluvial slope deposition in the southern half of the basin, with pronounced west‐directed and local axial through drainage palaeoflow networks operating at the time. The middle sandstone member basin deepens considerably towards the north, where coarse‐grained alluvial‐slope deposits are replaced by coarse‐grained turbidites of thick submarine‐fan complexes. The middle conglomerate member comprises thickly bedded, coarse‐grained pebble and cobble conglomerate, deposited by a high flow regime fluvial system that focused deposition into a northern basin depocentre. An influx of volcanic detritus entered the middle conglomerate member basin via spatially restricted footwall‐derived fans on the western basin margin. Fluvial systems continued to operate during deposition of the upper sandstone member in the north of the basin, facilitated by multiply active, high flow regime channels, comprising thick, vertically stacked and upward‐fining, coarse‐grained conglomerate and sandstone deposits. The upper sandstone member in the south of the basin is characterised by extensive braid‐delta and fine‐grained nearshore deposits, with abundant bioturbation and pronounced bimodal palaeocurrent trends associated with tidal and nearshore reworking. An increase in base‐level in the Middle Ordovician culminated in marine transgression and subsequent deposition of Gordon Group clastics and carbonates.  相似文献   

9.
The depositional history of the Dinantian on the Derbyshire Dome can be divided into three phases: (1) pre-Holkerian: onlap of an irregular basement surface by evaporite and carbonate sediments, (2) Holkerian to Asbian: sedimentation on a carbonate shelf formed by the merging of early Dinantian depocentres following burial of the basement topography, and (3) Brigantian: formation of intrashelf basins and the development of a carbonate ramp on part of the pre-existing shelf. A model of the basement structure underlying the Derbyshire Dome is presented to explain the location of the Brigantian intrashelf basins and carbonate ramp. The basement consists of two main tilted fault blocks separated by a smaller tilt block. Movement on faults bounding the tilt blocks caused the development of intrashelf basins. The basin margins were controlled by structures which developed in the cover sediments. The carbonate ramp present during the late Brigantian developed in response to an eastward tilting of the basement.  相似文献   

10.
The Middle Triassic–Lower Cretaceous (pre-Late Albian) succession of Arif El-Naga anticline comprises various distinctive facies and environments that are connected with eustatic relative sea-level changes, local/regional tectonism, variable sediment influx and base-level changes. It displays six unconformity-bounded depositional sequences. The Triassic deposits are divided into a lower clastic facies (early Middle Triassic sequence) and an upper carbonate unit (late Middle- and latest Middle/early Late Triassic sequences). The early Middle Triassic sequence consists of sandstone with shale/mudstone interbeds that formed under variable regimes, ranging from braided fluvial, lower shoreface to beach foreshore. The marine part of this sequence marks retrogradational and progradational parasequences of transgressive- and highstand systems tract deposits respectively. Deposition has taken place under warm semi-arid climate and a steady supply of clastics. The late Middle- and latest Middle/early Late Triassic sequences are carbonate facies developed on an extensive shallow marine shelf under dry-warm climate. The late Middle Triassic sequence includes retrogradational shallow subtidal oyster rudstone and progradational lower intertidal lime-mudstone parasequences that define the transgressive- and highstand systems tracts respectively. It terminates with upper intertidal oncolitic packstone with bored upper surface. The next latest Middle/early Late Triassic sequence is marked by lime-mudstone, packstone/grainstone and algal stromatolitic bindstone with minor shale/mudstone. These lower intertidal/shallow subtidal deposits of a transgressive-systems tract are followed upward by progradational highstand lower intertidal lime-mudstone deposits. The overlying Jurassic deposits encompass two different sequences. The Lower Jurassic sequence is made up of intercalating lower intertidal lime-mudstone and wave-dominated beach foreshore sandstone which formed during a short period of rising sea-level with a relative increase in clastic supply. The Middle-Upper Jurassic sequence is represented by cycles of cross-bedded sandstone topped with thin mudstone that accumulated by northerly flowing braided-streams accompanying regional uplift of the Arabo–Nubian shield. It is succeeded by another regressive fluvial sequence of Early Cretaceous age due to a major eustatic sea-level fall. The Lower Cretaceous sequence is dominated by sandy braided-river deposits with minor overbank fines and basal debris flow conglomerate.  相似文献   

11.
The Bowland Basin (northern England) contains a series of carbonates and terrigenous mudstones deposited during the Ivorian to early Brigantian. Two regional depositional environments are indicated by facies and facies associations. Wackestone/packstone and calcarenite facies indicate deposition in a carbonate ramp environment, while lime mudstone/wackestone, calcarenite and limestone breccia/conglomerate facies, often extensively slumped, represent a carbonate slope environment. Stratigraphic relations suggest that the depositional environment evolved from a ramp into a slope through the Dinantian. Two main sediment sources are indicated by the sequence; an extra-basinal terrigenous mud source and a supply of carbonate from the margins of the basin. Deposition from suspension and from sediment gravity flows, in situ production and remobilization of sediment during sedimentary sliding were important processes operating within the basin. Periods of enhanced tectonic activity in the late Chadian to early Arundian and late Asbian to early Brigantian are indicated by basin-wide horizons of sedimentary slide and mass flow deposits. Both intervals were marked by a decline in carbonate production resulting from inundation and uplift/emergence. The first of these intervals separates deposition on a seafloor with gentle topography (carbonate ramp) from a situation where major lateral thickness and facies variations were present and deposition took place in a carbonate slope environment. The second interval marks the end of major carbonate deposition within the Bowland Basin and the onset of regional terrigenous sedimentation.  相似文献   

12.
North Carnarvon盆地是澳大利亚最主要的产油气盆地,也是世界上主要的富气盆地之一.三叠纪North Carnarvon盆地属于克拉通边缘坳陷,地层厚度巨大且平面分布广.中三叠世晚期澳大利亚西北陆架发生强制性海退,造成North Carnarvon盆地沉积格局发生转变,沉积环境由滨浅海突变为海陆过渡环境,从而沉积了两套差异性明显的地层,即Locker shale 和Mungaroo组.本文从宏观和微观、有机与无机入手,阐述了Locker shale 和Mungaroo组的古地理背景、沉积充填演化及其相关的烃源岩与有机质显微组分发育特征.宏观上,Mungaroo组发育大型三角洲,中粗粒分支流水道砂岩与暗色泥岩频繁互层,其中近端三角洲平原泥岩厚度较薄,薄煤层只局部发育,陆源有机质含量较高,平均TOC含量为1.59%;远端三角洲平原泥岩厚度较大,薄煤层广泛发育,陆源有机质含量高,TOC含量最高,达4.11%;三角洲前缘泥岩厚度较大,陆源有机质含量低,TOC含量低,为1.05%.Locker Shale以滨浅海相沉积为主,局部发育小型三角洲,其中开阔浅海亚相烃源岩厚度较大,前三角洲、滨浅海、开阔浅海三个亚相TOC含量相当,平均值为1.16%.微观上,Mungaroo组由近端平原亚相—远端平原亚相—前缘亚相壳质组含量逐渐增多,镜质组先增加后下降,惰质组先减少后增加;远端三角洲平原镜质组含量最高.Locker shale由前三角洲相—浅海相壳质组增加,镜质组和惰质组均减少.研究区三叠系烃源岩及有机质显微组分的分布规律与特提斯南缘特殊的古地理、季风洪水的古气候背景密切相关,Mungaroo组的远端三角洲平原为最有利的烃源岩发育环境. North Carnarvon 盆地三叠系岩相古地理格局与沉积充填演化规律体现了温室气候效应下阵发性水流主导的沉积特色,为类似地质背景下油气资源评价提供了新的思路.  相似文献   

13.
The Neoproterozoic Purana succession in the eastern part of Chattisgarh basin around Sarangarh has been classified into a conglomerate-sandstone-shale dominated proximal assemblage, and a lithographic limestone-shale dominated distal assemblage. The proximal assemblage constitutes the Chandarpur Group, and unconformably overlies the Archean crystalline basement complex. The Chandarpur succession has been classified into three formations that were deposited in fan-fan delta, deep water prodelta and storm- tide dominated prograding shelf environments. The distal assemblage, the Raipur Group, conformably overlies the Chandarpur Group, and may be subdivided into two shale-dominated formations separated by a limestone-dominated formation. The limestone sequence, the Sarangarh Limestone, comprises a lower member of mixed carbonate-siliciclastic succession deposited in a storm dominated shallow water platform, and an upper member of pelagic limestone that grades upward into a deep water shale, the Gunderdehi Shale. The rapid transition from shallow water platformal succession to deep-water pelagic limestone and shale points to abrupt deepening of the basin and drowning of the craton. The peak of transgression is represented by a persistent horizon of black limestone, a product of basin wide anoxia. Disposition of facies belts in proximal and distal assemblages and palaeocurrent directions measured from different facies belts point to a north-northwesterly palaeoslope of the basin. Signatures of intense storm and tidal currents in different litho-units collectively point to an open marine circulation condition. It has been inferred that the basin was connected to a major seaway that skirted the northern and north-western margin of the craton. Development of thick fan-delta sequence at the base of the succession, occurrence of felsic welded tuff within the Gunderdehi Shale, thick sandstone-mudstone cyclothems in the Chandarpur Group, and abrupt drowning of the carbonate platform leading to pelagic sedimentation collectively point to major tectonic control on basin evolution. The basin developed as a cratonic rift and evolved into a deeply subsiding one, without any major stratigraphic hiatus, through episodic tectonic pulses.  相似文献   

14.
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   

15.
Abstract The Kyokpori Formation (Cretaceous), south‐west Korea, represents a small‐scale lacustrine strike‐slip basin and consists of an ≈ 290 m thick siliciclastic succession with abundant volcaniclasts. The succession can be organized into eight facies associations representing distinctive depositional environments: (I) subaqueous talus; (II) delta plain; (III) steep‐gradient large‐scale delta slope; (IV) base of delta slope to prodelta; (V) small‐scale nested Gilbert‐type delta; (VI) small‐scale delta‐lobe system; (VII) subaqueous fan; and (VIII) basin plain. Facies associations I, III and IV together constitute a large‐scale steep‐sloped delta system. Correlation of the sedimentary succession indicates that the formation comprises two depositional sequences: the lower coarsening‐ to fining‐upward succession (up to 215 m thick) and the upper fining‐upward succession (up to 75 m thick). Based on facies distribution, architecture and correlation of depositional sequences, three stages of basin evolution are reconstructed. Stage 1 is represented by thick coarse‐grained deposits in the lower succession that form subaqueous breccia talus and steep‐sloped gravelly delta systems along the northern and southern basin margins, respectively, and a sandy subaqueous fan system inside the basin, abutting against a basement high. This asymmetric facies distribution suggests a half‐graben structure for the basin, and the thick accumulation of coarse‐grained deposits most likely reflects rapid subsidence of the basin floor during the transtensional opening of the basin. Stage 2 is marked by sandy black shale deposits in the upper part of the lower succession. The black shale is readily correlated across the basin margins, indicating a basinwide transgression probably resulting from large‐scale dip slip suppressing the lateral slip component on basin‐bounding faults. Stage 3 is characterized by gravelly delta‐lobe deposits in the upper succession that are smaller in dimension and located more basinward than the deposits of marginal systems of the lower succession. This lakeward shift of depocentre suggests a loss of accommodation in the basin margins and quiescence of fault movements. This basin evolution model suggests that the rate of dip‐slip displacement on basin‐margin faults can be regarded as the prime control for determining stacking patterns of such basin fills. The resultant basinwide fining‐upward sequences deviate from the coarsening‐upward cycles of other transtensional basins and reveal the variety of stratigraphic architecture in strike‐slip basins controlled by the changes in relative sense and magnitude of fault movements at the basin margins.  相似文献   

16.
The Kaskapau Formation spans Late Cenomanian to Middle Turonian time and was deposited on a low‐gradient, shallow, storm‐dominated muddy ramp. Dense well log control, coupled with exposure on both proximal and distal margins of the basin allows mapping of sedimentary facies over about 35 000 km2. The studied portion of the Kaskapau Formation is a mudstone‐dominated wedge that thins from 700 m in the proximal foredeep to 50 m near the forebulge about 300 km distant. Regional flooding surfaces permit mapping of 28 allomembers, each of which represent an average of ca 125 kyr. More than 200 km from shore, calcareous silty claystone predominates, whereas 100 to 200 km offshore, mudstone and siltstone predominate. From about 30 to 100 km offshore, centimetre‐bedded very fine sandstone and mudstone record along‐shelf (SSE)‐directed storm‐generated geostrophic flows. Five to thirty kilometres from shore, decimetre‐bedded hummocky cross‐stratified fine sandstone and mudstone record strongly oscillatory, wave‐dominated flows whereas some gutter casts indicate shore‐oblique, apparently mostly unidirectional geostrophic flows. Nearshore facies are dominated by swaley cross‐stratified or intensely bioturbated clean fine sandstone, interpreted as recording, respectively, areas strongly and weakly affected by discharge from distributary mouths. Shoreface sandstones grade locally into river‐mouth conglomerates and sandstones, including conglomerate channel‐fills up to 15 m thick. Locally, brackish lagoonal shelly mudstones are present on the extreme western margin of the basin. There is no evidence for clinoform stratification, which indicates that the Kaskapau sea floor had extremely low relief, lacked a shelf‐slope break, and was probably nowhere more than a few tens of metres deep. The absence of clinoforms probably indicates a long‐term balance between rates of accommodation and sediment supply. Mud is interpreted to have been transported >250 km offshore in a sea‐bed nepheloid layer, repeatedly re‐suspended by storms. Fine‐grained sediment accumulated up to a ‘mud accommodation envelope’, perhaps only 20 to 40 m deep. Continuous re‐working of the sea floor by storms ensured that excess sediment was redistributed away from areas that had filled to the ‘accommodation envelope’, being deposited in areas of higher accommodation further down the transport path. The facies distributions and stratal geometry of the Kaskapau shelf strongly suggest that sedimentary facies, especially grain‐size, were related to distance from shore, not to water depth. As a result, the ‘100 to >300 m’ depth interpreted from calcareous claystone facies for the more central parts of the Interior Seaway, might be a significant overestimate.  相似文献   

17.
The Pliocene to possibly Pleistocene uppermost Orubadi and Era Formations, southwest margin of the Papuan Peninsula, are interpreted as having been deposited in alluvial-fan, fan-delta and shallow-marine environments. The alluvial-fan facies consists primarily of lenticular, coarse-grained conglomerate (up to 2 m boulders) and cross-bedded and horizontally laminated sandstone. Conglomerate and sandstone were deposited in shallow fluvial channels and by overbank sheetfloods. The facies also contains thick mudflow diamictite and minor tuff and terrestrial mudstone. The shallow-marine and fan-delta facies, in contrast, consists of heterogeneously interbedded marine and terrestrial mudstone, sandstone, diamictite, conglomerate and limestone. Marine mudstone is calcareous, sandy, bioturbated, and contains marine shells. Limestone is mostly packstone that has a varied, open-marine fauna. Rare coral boundstone is also present. Marine sandstone is burrowed to bioturbated and is hummocky cross-stratified in places. Some marine mudstone contains sandstone pillows formed by loading of unconsolidated sand by storm waves. Other sandstone in the fan-delta facies is cross-bedded, lacks shells and was probably deposited by fluvial processes. Several conglomerate beds in the fan-delta facies are well sorted and imbricated and were also deposited by stream floods. The synorogenic Orubadi and Era Formations were deposited in a foreland basin formed from loading of the Papuan–Aure Fold and Thrust Belt on the edge of the Australian craton. Deformation in the fold and thrust belt was probably related to docking and compression of the Finisterre Terrane–Bismarck Arc against the New Guinea Orogen. The Era Formation interfingers with the reefal Wedge Hill Limestone in which reef facies likely grew on a deforming anticline. Era Formation siliciclastics were sourced from volcanic, metamorphic and sedimentary rocks that were uplifted in the orogen to the northeast. Volcanic sediment was derived mostly from a then-active volcanic arc likely related to southward subduction at the Trobriand Trough.  相似文献   

18.
川西北早志留世陆源碎屑──碳酸盐混积缓坡   总被引：10，自引：6，他引：10  
研究区位于扬子地块西北缘，由一套巨厚的页岩、泥岩层夹生物礁及透镜状和不规则层状风暴生屑灰岩、瘤状灰岩组成。根据区内特征的岩石类型及其组合、分布和相应的化石生态，作者认为本区兰多维列期至早温洛克期时为一典型的陆源碎屑－－碳酸盐混积均斜缓坡，并且从浅到深划分为滨岸、浅缓坡、深缓坡及盆地等亚环境。滨岸区位于潮间带，以潮坪碳酸盐岩为代表；浅缓坡位于浪基面至风暴浪基面之间，岩性组合为Ａ、Ｂ类风暴岩，Ａ、Ｂ类瘤状灰岩、生物礁灰岩及粘结岩；深缓坡位于风暴浪基面至最大风暴浪基面之间，岩性以Ｄ、Ｅ类风暴岩、Ｃ类瘤状灰岩及泥页岩组合为特征；盆地处于最大风暴浪基面之下，为黑色泥页岩沉积。结合早志留世时全球冰川作用，文中讨论了海平面变化的原因及对环境的影响。  相似文献   

19.
The Jubaila Formation (Upper Jurassic) in central Saudi Arabia has been divided into lower, middle, and upper parts purely on lithologic grounds. Each part consists of a major lower unit of lime mudstone and a minor upper unit of grainstone. This persistent change in the limestone facies is interpreted as a reflection of repeated shoaling up in the depositional shelf environment. It is a normal marine carbonate sequence that varies in thickness from 85 to 126 m. In the Hanifa Formation, the lowermost brown ledges in the section comprise a series of coarsening upward sequences which generally terminate in a fossiliferous/peloidal packstone and grainstone and subordinately lime mudstone facies. The middle slope member is yellow, blocky weathered shale and marl. Above this slope member are several thick beds of brown-coated fossiliferous wackestone, packstone, and grainstone with the association of lime mudstone in certain levels. These are fairly resistant ledges due to the occurrence of stromatoporoids. Dedolomitization occurs in the Jubaila Formation in various textural forms which include composite calcite rhombohedra, zonal dedolomitization, regeneration of predolomitization fabric of the limestone, and coarsely crystalline calcite mosaics with or without ferric oxide rhombic zones. Rhombohedral pores commonly occur in intimate association with dolomite, possibly resulting from the leaching of calcitized dolomite rhombohedra. The regional dedolomitization was most likely brought about by calcium sulfate solutions reacting with dolomites. The source of sulfate solutions is the dissolved anhydrite deposits of the Arab–Hith Formations, sometime before their erosion, and it takes place at or near an exposed surface. The Hanifa Formation shows various diagenetic features. These include dolomitization, dedolomitization, micritization, cementation, and recrystallization. Most of the examined samples of the Hanifa carbonates are dolomitized and subsequently dedolomitized as evidenced by the presence of iron-coated dolomite rhombs partially or completely calcitized. Dolomite also occurs in the lime mudstone, wackestone, packstone, and grainstone facies, while leaching of wackestone and packstone and dedolomitization of dolomite and dolomitic limestone followed by recrystallization are common processes.  相似文献   

20.
湘中下石炭统风暴岩的研究   总被引：5，自引：0，他引：5       下载免费PDF全文
邵龙义 《地质科学》1993,28(4):336-346
本文在对湘中各地发现的下石炭统风暴岩的一般特征的讨论的基础上,区分出浅缓坡近源型、过渡带远源型以及深缓坡末梢型等风暴岩类型,并认为这些风暴岩的发育受当时全球古地理控制。早石炭世北方大陆和南方大陆之间存在的古地中海,造成了赤道附近飓风的形成条件。当时华南板块位于赤道以南接近赤道的地方,必然形成了大量风暴沉积。  相似文献   

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