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1.
Lars Stemmerik 《Sedimentology》2001,48(1):79-97
The carbonate platform of the Upper Permian Wegener Halvø Formation in the Karstryggen area of central East Greenland is an example of a carbonate system with low production rates (2–3 cm kyr–1 ) and differs from most other carbonate platforms by the lack of well-developed highstand progradation. The platform consists of three depositional sequences that formed in response to Kazanian sea-level cycles. Pinning point curves for the subaerial exposure surfaces separating the depositional sequences quantify the amplitude of the relative sea-level fluctuations in the range of 70–140 m. The platform developed on the karstified surface of an older Permian carbonate platform with a topographic relief locally exceeding 70 m. The predepositional relief influenced deposition in all three sequences. Transgressive systems tracts are thin and commonly dominated by condensed siliciclastic deposits in off-platform areas and palaeo-lows. Over palaeotopographic highs they consist of aggrading cementstones. Highstand deposits are limited to palaeotopographic elevated areas and consist of cementstone build-ups along the basin margin, and shallow subtidal to intertidal carbonates and evaporites in the platform area. Elsewhere, carbonate deposition took place during falling sea-level, and thin laterally extensive units of shallow-marine grainstones rest directly on top of deeper marine shales in the two first sequences, whereas thick prograding units of oolitic grainstones form the forced regressive systems tract of the uppermost sequence. 相似文献
2.
Postulated extreme sea-level rise of up to 1-1.5 km with the late Cryogenian Ghaub deglaciation in Namibia is contentious,as is the great rapidity(<104 yr)of the sea-level rise.Such extreme glacioeustatic events,if real,would have been global and affected all continents.In South Australia,up to six glacial advances and retreats during the late Cryogenian Elatina glaciation indicate a fluctuating ice margin.The latter stage of the Elatina glaciation and the immediate post-glacial environment are examined here for evidence of extreme and rapid sea-level rise.In the central Adelaide Rift Complex,diamictite with faceted and striated clasts occurs at the top of the Elatina Formation<1-2 m beneath the early Ediacaran Nuccaleena Formation’cap carbonate’.One hundred kilometres to the south,~30 m of siltstone and sandstone followed by^6 m of clast-poor diamictite with clasts 10+cm long occur between tidal rhythmites and the cap carbonate.Three hundred kilo metres further south,~70 m of siltsto ne,dolo mitic siltstone and minor dolomite separate tidal rhythmites and early Ediacaran strata.Hence the rhythmites were deposited during a high stand(interstadial or interglacial),not during post-glacial sea-level rise.Storm-generated erosional surfaces within tidal rhythmites at Warren Gorge indicate intermittent rhythmite deposition,and water depth and other palaeoenvironmental factors are uncertain,casting doubt on a published estimate of rapid sea-level rise during rhythmite deposition.The lack of late Cryogenian deeply incised valleys and thick valley-fill deposits in South Australia and central Australia argues against extreme sea-level variations.A hiatus occurred between Elatina deglaciation and deposition of the Nuccaleena cap carbonate,and three palaeomagnetic polarity chrons identified in the cap carbonate imply slow deposition spanning 10^5-10^6 yr.This is supported by independent evidence from magnetic chronostratigraphy for Ediacaran strata in South Australia and California,and by stratigraphic and sedimentological arguments for condensed deposition of cap carbonates.It is concluded that neither extreme nor rapid sea-level rise was associated with late Cryogenian deglaciation in South Australia. 相似文献
3.
Eustatic and hydrodynamic controls on the architecture of a deep shelf sand bank (Celtic Sea) 总被引：3，自引：0，他引：3
Jean-Yves Reynaud Bernadette Tessier Jean-Noël Proust Robert Dalrymple Tania Marsset Marc De Batist Jean-François Bourillet & Gilles Lericolais 《Sedimentology》1999,46(4):703-721
The architecture of a tidal sand bank in the south-eastern Celtic Sea was examined using very high-resolution seismic surveys. The bank comprises four depositional units. The lowest unit 1 is characterized by gently dipping (1–8°) strata that strike parallel to the length of the bank. Unit 1 is erosionally overlain by unit 2, which forms the bulk of the bank. This unit consists of stacked sets of downcurrent-dipping (7–12°) master bedding formed by climbing, sinuous-crested tidal dunes that are up to 20 m high. These deposits are locally incised by an anastomosed channel network (unit 3) that may represent a buried swatchway system. The upper part of the bank comprises wave-related deposits that are mainly preserved on the bank flanks (unit 4). The outer bank surface is erosional. The bank is believed to have formed during the last post-glacial sea-level rise. The facies evolution from unit 1 to unit 3 indicates an upward increase in tidal energy, mainly characterized by the thickening of dune cross-bed sets in unit 2. The majority of bank growth is inferred to have occurred in water depths of the order of 60 m. This evolution was controlled by relative sea-level rise, which is likely to have caused an episode of tidal resonance with associated strong tidal currents that were responsible for the incision of the deep, cross-cutting channels of unit 3. The transition to wave-dominated sedimentation in unit 4 is related to the decay of resonance with continued sea-level rise. 相似文献
4.
Instrumented moorings were deployed during the winter of 1994–95 at three depths (28, 58 and 101 m) in southern Lake Michigan. Storms during the observation period were not unusually severe, so the processes observed are typical of those that occur during an average winter. Time series observations of water temperature, beam attenuation coefficient (a measure of water transparency) and current velocity show that local resuspension of bottom sediment occurred frequently after the breakdown of the thermocline. Resuspension was most frequent close to the shore but was also observed at the 58 m station. Local resuspension did not occur at the 101 m station, but advection to the site of material resuspended at shallower sites was observed. These observations do not support the hypothesis proposed by previous investigators that local resuspension at depths of 100 m or greater occurs during the unstratified period. It is more likely that fine-grained material resuspended by storm action in intermediate water depths (≈ 30–60 m) is transported into the deeper parts of the lake by the general lake circulation. 相似文献
5.
Fluctuations in sea depth within a magnitude 20–100 m and a duration of 1–10 m.y. are often explained by rapid eustatic changes — so called ‘third-order eustatic events’. Considerable influence of regional tectonics on relative sea-level changes has been demonstrated by many authors, but because of uncertainties in the timing of short events in widely separated regions, the problem of separating tectonic and eustatic factors still remains unsolved. In this paper, a new and simple approach is used to reveal the presence or absence of eustatic events. We consider the St. Petersburg area and North Estonia in the north-eastern region of the East Baltic. From the late Early Cambrian until the middle of the Tremadoc (early Ordovician), deposition was extremely slow and the sea bed remained for a long time in a well defined peritidal zone in a water depth ≤10 m. Under such environmental conditions, a sea-level rise of ≥10 m would result in marked changes in the character of faunas and sedimentation. In the time interval considered here, significant sea deepening in the north-eastern Baltic region occurred only twice, and its magnitude did not exceed 10–20 m. A fall of sea level by ≥10 m would result in complete regression in the peritidal zone. This situation also occurred in region. However, the preservation of a sequence of unconsolidated sands, which is only a few tens of metres thick and includes all the main stratigraphic subdivisions on a regional scale, indicates that the crustal surface reached a very low altitude ≤10–20 m above sea level. These data show that in the late Late Cambrian to the middle of the Tremadoc, over a period of 40 m.y. long, eustatic sea-level changes did not exceed ±10–20 m. This limits the magnitude of several third-order cycles — eustatic events with duration of a few million years, which have been proposed previously for the epoch of the transition from the Cambrian to the Ordovician. In the late Early Cambrian to the Late Cambrian, transgressions and regressions with a magnitude of 50–150 m took place in southern Sweden and Lithuania. Since these phenomena occurred when there were no comparable eustatic sea-level changes, they must be associated with regional tectonic movements. Some were rapid and could be easily misinterpreted as indications of third-order eustatic changes. It is probable that some of the other eustatic events that have been proposed for the Phanerozoic were actually not of eustatic but of tectonic origin. Such rapid tectonic movements with magnitude of 50–100 m in cratonic areas can be caused by changes in the forces in the lithospheric layer with a laterally variable thickness, and by phase transitions in the mafic lower crust. Depending on the spatial distribution of vertical crustal movements, both these mechanisms could have been operating in the East Baltic and southern Sweden in the Cambrian. 相似文献
6.
ALFONSO BOSELLINI 《Sedimentology》1984,31(1):1-24
Various types of progradation of Triassic carbonate platforms are described from the Dolomites of the Southern Alps. The internal and external geometric relationships are exposed in spectacular natural sections and, moreover, their scale (500–1000 m of thickness) is such that they can be compared with features found in seismic profiles. The different types of progradation are controlled by a number of factors which, normally, interact with each other. These factors include: rate of basinal sedimentation, rate of subsidence, width of the platform, depth of the surrounding basin and eustatic variations of sea-level. Progradation is not a continuous process but episodic. Moments of massive debris input, during which the platform advances, alternate with long periods of negligible progradation, during which basinal sediments accrete and onlap the toe of slope. Upper boundary relationships of the prograding platforms include offlap, toplap and erosional truncation. Lower boundary relationships are horizontal, climbing and descending progradations. A variety of phenomena and circumstances have caused the cessation of progradation of the Triassic platforms. They include volcanism, collapse of margins, drowning (rapid relative rise of sea-level), subaerial exposure (relative fall of sea-level) and, probably, a natural decay of the system. In the Triassic of the Dolomites, two main progradation models can be put forward: in the Ladinian model, progradation took place simultaneously with aggradation (relative rise of sea-level), whereas the characteristic feature of the Carnian model is toplap (relative stillstand of sea-level). 相似文献
7.
为了探讨南海碳酸盐台地的发育过程及控制因素, 采用有机分子化合物指标方法对其进行了研究.有机分子化合物指标BIT(branched isoprenoid tetraether)是沉积物中源自陆源的细菌膜脂支链甘油双烷基链甘油四醚(branched glycerol dialkyl glycerol tetraethers,简称bGDGTs)与主要来自海洋泉古菌中的类异戊二烯GDGTs(isoprenoid GDGTs,简称iGDGTs)的含量之比,在古环境研究中,用来区分沉积物有机质的来源、判断沉积环境.通过对西科1井的数据研究发现,西科1井BIT指数随深度呈现三段式规律性变化,从下到上呈现高-低-高的变化,反映了中中新世以来南海海平面变化及碳酸盐台地生长发育的过程:在中中新世晚期,受全球及区域海平面变化下降影响,西沙地区碳酸盐台地形成礁-滩交互的沉积地层,由于大气淡水的影响,造成BIT指数呈现高值;到晚中新世至上新世,全球及区域海平面出现持续上升,有利于西沙碳酸盐台地的生长发育,使该井沉积环境以礁内泻湖相为主,造成BIT指数呈现低值;在第四纪冰期,全球及区域海平面出现总体下降趋势,西沙碳酸盐台地又频繁暴露于地表,造成BIT指数又呈现高值.研究表明,西沙地区碳酸盐台地受区域相对海平面变化影响更大,说明南海海平面变化既受全球海平面变化的影响,也受南海区域构造沉降的控制. 相似文献
8.
Coastal cliff erosion is a problem in many coastal areas. However, often only very limited data are available to quantify the rates of recession for the development of coastal management strategies. In the soft flysch deposits of the Waitemata Group, Auckland, New Zealand, coastal cliffs are associated with shore platforms. Two models exist for the profile evolution of shore platforms and associated cliffs: the first suggests that an equilibrium profile develops in response to erosive processes, and this profile subsequently migrates landward; the second model suggests that the seaward margin of the shore platform is relatively static, and the profile extends landward through a combination of cliff recession and platform lowering. Physical simulations and field measurements for mudstone and limestone lithologies indicate that the second model is more likely for soft flysch deposits. A eustatic sea-level curve for the Weiti Estuary, Auckland, suggests that up to 7120 ± 70 years have been available for shore platform development since sea level reached the present seaward margins of shore platforms. Shore platform widths were measured using GPS at two sites in Waitemata Group rocks: the North Shore of Auckland; and the southern side of the Tawharanui Peninsula, North Auckland. The long-term cliff recession rates estimated from shore platform widths (1.4 ± 0.1 to 14.3 ± 0.1 mm y− 1) are consistent with the lower end of the average range of cliff top and face recession rates published for Waitemata Group rocks using different methods (11–75 mm y− 1), and in agreement with cliff base recession estimates (3.5 mm y− 1). Shore platform widths were qualitatively related to the rock mass characteristics of the associated cliffs, and therefore platform widths could provide a method of identifying regions of potential hazard. 相似文献
9.
Progradational shoreface tongues preserve a near-complete depositional record of relative sea-level highstands, falls and lowstands. Two distinct styles of progradational shoreface tongue are examined in an extensive outcrop and subsurface dataset from Late Cretaceous strata of the Book Cliffs area, Utah, representing (i) highstand through attached lowstand progradation and (ii) highstand through detached lowstand progradation. Using this dataset, key geometrical attributes of the shoreface tongues and their internal facies architecture are identified and quantified that enable the reconstruction of relative sea-level fall history. For example, attached, wave-dominated lowstand shoreface deposits record a slow (0.2– 0.3 mm yr–1 ), low-magnitude (> 14 m) relative sea-level fall punctuated by minor rises. Detached, weakly wave-influenced lowstand shoreface deposits record a more rapid (0.4–0.5 mm yr–1 ), high-magnitude (> 45 m) relative sea-level fall synchronous with a marked change in sediment delivery and depositional process regime at the shoreline. 相似文献
10.
宜昌地区下奥陶统主要为一套台地相碳酸盐沉积,间夹页岩,其中的特征沉积为发育有较多的生物礁。造礁生物主要有海绵类、瓶筐石类、有柄棘皮类、苔藓虫及蓝细菌和Pulchrilamina等。在下奥陶统含礁层系中识别出四种生物相：(1)壳类生物相：发育有Tritoechia-Pelmatozans群落和Tritoechia-Pomatotrema群落;(2)礁生物相：包括Batostoma群落、Calathium--Archaeoscyphia群落、Pelmatozoans--Batostoma群落、Archeoscyphia群落和Calathium--Cyanobacteria群落;(3)静水生物相：发育有Acanthograptus--Dendrogptus群落和Yichangopora群落;(4)异地生物相：产有异地埋藏群,即Nanorthis--Psilocephlina埋藏群。含礁层系存在四个海平面变化旋回,生物礁的发育明显受海平面的升降速率与容纳空间增长速率的控制。生物礁多形成于高海平面时期。由于本区的海平面在上升过程中存在多个次一级旋回,致使生物礁具有数量多、分布广以及厚度薄和规模小等特点。造礁群落存在着演替和取代两种形式,前者有利于生物礁的发育,后者则表现为礁体的衰亡。 相似文献
11.
Analyses of high resolution, seismic reflection profiles and surface sediment samples indicate that the Cat Island shelf is presently in an incipiently drowned state. This small carbonate bank is characterized by a thin (<4 m), coarse-grained, relict sediment cover, along with limited reef development, and a relatively deep (20–30 m) margin indicating that it has been unable to ‘keep-up’ with Holocene sea-level rise. Early flooding at relatively high rates of sea-level rise (4 m kyr-1, 5–8 × 103 yr BP) in conjunction with small bank size and relatively low elevation, led to a reduced rate of carbonate accumulation and incipient drowning. The shelf edge currently lies beneath the zone of maximum carbonate production and exposes the interior shelf to open marine conditions which may result in permanent drowning if it is unable to ‘catch-up’ with continued sea-level rise. Sediment facies patterns are largely oriented perpendicular or oblique to the shelf edge and appear to be controlled by shelf circulation patterns focused by bank-margin reentrants. In comparison with most of the northern Bahamas, the Cat Island shelf was flooded earlier and at relatively higher rates of Holocene sea-level rise which led to selective drowning, implying that carbonate platforms need not drown synchronously over widespread areas as commonly thought. The potential rock record of this incipient drowning event would be a thin, open-marine sand sheet of highly degraded cryptocrystalline and aggregate grains associated with poorly developed reefs. 相似文献
12.
Renato Pereira Lopes Francisco Sekiguchi Buchmann 《Journal of South American Earth Sciences》2011,31(1):17-27
Fossils of terrestrial mammals preserved in submarine environment have been recorded in several places around the world. In Brazil such fossils are rather abundant in the southernmost portion of the coast, associated to fossiliferous concentrations at depths up to 10 m. Here is presented a review of such occurrences and the first record of fossils in deeper areas of the continental shelf. The fossils encompass several groups of both extinct and extant mammals, and exhibit several distinct taphonomic features, related to the marine environment. Those from the inner continental shelf are removed and transported from the submarine deposits to the coast during storm events, thus forming large konzentrat-lagerstätte on the beach, called “Concheiros”. The only fossils from deeper zones of the shelf known so far are a portion of a skull, a left humerus and of a femur of Toxodon sp. and a lower right molar of a Stegomastodon waringi, all collected by fishermen at depths around 20 m. The presence of fossils at great depths and distances from the present coastline, without signs of abrasion and far from areas of fluvial discharges does indicate that these remains have not been transported from the continent to the shelf, but have been preserved directly on the area that today correspond to the continental shelf. These remains indicate the existence of large fossiliferous deposits that have developed during periods of sea-level lowstand (glacial maxima) and have been submerged and reworked by the sea-level rise at the end of the last glaciation. 相似文献
13.
Richet Rémy Chazottes Véronique Cabioch Guy Frank Norbert S. Burr George 《Quaternary Science Reviews》2011,30(19-20):2827-2838
Coral reef growth and development depend on several environmental factors, including tectonic and climatic parameters and local ecological drivers. Reef growth is especially sensitive to sea-level variations. Paleo-water depth reconstructions are essential tools used to determine reef growth patterns during different periods of reef growth. Assemblages of corals and/or coralline algae have been commonly used in such paleodepth reconstructions. This study shows that using microendolith ichnocoenoses can sometimes provide better accuracy than traditional coralgal analyses, particularly in the depth-range 0–10 m where coralgal assemblages usually show broad distribution ranges. Holocene and Pleistocene cores from two barrier reef sites on the west coast of Grande Terre in New Caledonia are examined here. Holocene reef development at these sites feature examples of microendolith ichnocoenoses that document rapid environmental changes and small sea-level variations of about 2–5 m in amplitude, and record these changes with more accuracy than coral and coralline algae assemblages which are highly dependant on the hydrodynamic energy of the setting. During the Pleistocene, which was less chronologically constrained, the microendolith ichnocoenoses also reflect paleo-water depths and reef-growth patterns at different periods of reef history. 相似文献
14.
The first Holocene relative sea-level curve from the middle part of Hardangerfjorden, western Norway
ANDERS ROMUNDSET ØYSTEIN S. LOHNE JAN MANGERUD JOHN INGE SVENDSEN 《Boreas: An International Journal of Quaternary Research》2010,39(1):87-104
Romundset, A., Lohne, Ø. S., Mangerud, J. & Svendsen, J. I. 2009: The first Holocene relative sea-level curve from the middle part of Hardangerfjorden, western Norway. Boreas , 10.1111/j.1502-3885.2009.00108.x. ISSN 0300-9483.
The first relative sea-level (RSL) curve from the mid-Hardangerfjorden area covering the entire Holocene is presented. The curve is based on a series of AMS14 C dates on terrestrial plant macrofossils across the isolation level in each of five lakes located between 3.5 and 74.5 m a.s.l. During the first 1200 years, the RSL fell very rapidly from the marine limit at 98 m a.s.l. to 33 m a.s.l., i.e. at a rate of 5.4 cm yr−1 . The emergence rate then slowed considerably and was close to standstill 8000–6500 cal. yr BP. However, an emergence of 16.5 m has taken place during the past 6000 years. Radiocarbon dates of terrestrial plant macrofossils from the basal strata in a lake above the marine limit and mollusc shells from glaciomarine silt in the isolation basins yielded a mean age for the local ice-margin retreat of 11 300 cal. yr BP. This verifies that Hardangerfjorden was glaciated during the Younger Dryas – an interpretation that has recently been disputed. The ice margin retreated at a rate of about 300 m yr−1 from the position of the Younger Dryas moraine to this site some 60 km further into the fjord. 相似文献
The first relative sea-level (RSL) curve from the mid-Hardangerfjorden area covering the entire Holocene is presented. The curve is based on a series of AMS
15.
Two types of pseudotachylytes are observed in the Balmuccia peridotite of the Ivrea zone (Southern Alps, Italy). A-type pseudotachylytes correspond to previously studied occurrences and were formed under temperatures comprising between 550 and 900 °C and pressures comprising between 0.6 and 1.2 GPa. These conditions were met in the Ivrea crust between 350 and 270 Ma, suggesting that A-type pseudotachylytes were formed during Variscan tectonics or Permian transtensional tectonics. B-type pseudotachylytes post-date A-type pseudotachylytes. Textural characteristics of B-type veins suggest a formation in the upper continental crust, at depths of about 5–10 km or less. Petrological constraints indicate that the exhumation of the Ivrea crust at such shallow depths was achieved later than c. 70 Ma, thus providing a maximum age of 70 Ma for B-type veins. Pseudotachylytes appear as markers of the poly-orogenic evolution of the Alpine belt. 相似文献
16.
生物礁及碳酸盐台地是南海南部重要的油气储层之一,但目前对万安盆地生物礁的识别及碳酸盐台地沉积相带的划分尚不够深入.基于钻井和地震数据对该区的生物礁及碳酸盐台地进行了精细刻画,万安盆地中生物礁及碳酸盐台地的发育可分为4个演化阶段:早中新世时期,碳酸盐台地初始发育,台地规模小、数量少,零星分布于盆地中部;中中新世台地发育进入繁盛阶段,主要分布于北部隆起、中部隆起及隆起周缘的斜坡之上,横向上呈东、西带状展布,此时期的生物礁以台地边缘礁和块状礁为主;至晚中新世时期,碳酸盐台地开始衰退,而生物礁类型全、数量多,包括台地边缘礁、块状礁、塔礁、点礁等;上新世以来,生物礁及碳酸盐台地全面被淹没,盆地内部不再有生物礁及碳酸盐台地的发育.构造作用和相对海平面变化控制了碳酸盐台地的发育演化过程,古近纪基底断裂产生的地形控制了生物礁及碳酸盐台地初始发育位置及后期发育的空间分布,晚中新世以来的快速沉降和相对海平面变化控制了台地的衰退及淹没过程. 相似文献
17.
JOHN S. COMPTON 《Sedimentology》2007,54(1):55-70
Coastal sediment-filled depressions (pans) are one of the few areas that contain Quaternary records of sea-level and palaeoenvironmental change along the western margin of southern Africa. Anichab is a 128 km2 salt-encrusted pan on the hyper-arid southern coast of Namibia with an emergent, well-preserved and in-place mid-Holocene mollusc assemblage. The molluscs are typical of subtidal sands on the sheltered side of offshore islands but include several warm-water species no longer found living along this coast. The Holocene evolution of the pan was largely influenced by changes in sea level and supply of sand along the coast. Calibrated radiocarbon ages of mollusc shells indicate a maximum Holocene sea level of ca 2 m above mean sea level (msl) from 7·0 to 6·3 ka and a return to near present-day sea level by 5·3 ka. The pan surface is 2 m below msl and has been emergent since 4·9 ka from the build up of sandy beaches and coastal dunes. A thin (1–4 cm) halite crust occurs over much of the pan surface but a layer of halite-cemented sand up to 40 cm thick is restricted to the central pan. Gypsum occurs near the subsurface brine interface and is limited by calcium to the edges of the pan. Nodules of calcite-cemented sand are forming in brackish, relatively high alkalinity subsurface waters in the south-east corner of the pan and nodules of aragonite-cemented sand are forming in brines 1 m below the central pan surface. Although modern dolomite has been reported from coastal lagoons of Brazil and Australia, carbonate cements are a minor feature of Anichab Pan and dolomite was restricted to a single reworked nodule most likely of Late Pleistocene age. Therefore, Anichab Pan does not appear to be a modern analogue to extensive, mixed-water dolomite cements found in Upper Pleistocene sediment-filled depressions on the Namibian shelf. 相似文献
18.
江西修水二叠纪·三叠纪界线地层海平面下降的岩石学证据 总被引：6，自引：3，他引：6
在江西修水四都镇东岭村发现了一个连续的、出露完好的二叠纪-三叠纪界线地层剖面。在剖面中有一段厚约2.1m 的地层具有指示海平面下降的岩石组构。这段地层的底部为红色的碳酸盐颗粒岩。红色是颗粒表面的红色铁染,成分为褐铁矿,是出露地表风化形成的。这层红色颗粒岩之上有约2.0 m 的去白云石化灰岩,其中保留了原来粉晶白云石的晶形。去白云石化作用是白云岩受淡水影响而发生的,指示海平面下降事件。这段地层之上是厚1m 的颗粒岩和灰泥岩,代表浅水潮下环境。其上是二叠系-三叠系的界线。这段地层之下的地层中生物丰富多样,并且有复(蜒)、钙藻等大绝灭前的生物。这段地层及其以上的地层中生物种类很少,(蜓)、钙藻等已经消失,表明集群绝灭事件开始于这段地层之下,生物大绝灭的发生与海平面下降事件的发生是基本同时的。由此认为,可能是海平面下降有关的环境剧变事件引发了二叠纪末的集群绝灭事件。 相似文献
19.
N. P. Lukashina 《Stratigraphy and Geological Correlation》2006,14(3):283-301
Foraminifers from Middle-Upper Jurassic and Upper Cretaceous sediments of the Kaliningrad basin located in the southwestern
part of the East European platform are studied. During the greater part of the Late Mesozoic, the study region represented
a northern margin of a spacious epicontinental sea in the Boreal zoogeographic realm. The analyzed composition and quantitative
distribution of foraminifers, ratio between planktonic and benthic species, ornamentation degree of tests, and their preservation
are used to reconstruct paleogeography and history of eustatic sea-level changes. The upper Callovian through Upper Jurassic
zonation based on distribution of Epistomina species is proposed. Defined foraminiferal assemblages are correlated with coeval assemblages from the East to West European
platforms and North Atlantic 相似文献
20.
The Quilalar Formation and correlative Mary Kathleen Group in the Mount Isa Inlier, Australia, conformably overlie rift-related volcanics and sediments and non-conformably overlie basement rocks. They represent a thermal-relaxation phase of sedimentation between 1780 and 1740 Ma. Facies analysis of the lower siliciclastic member of the Quilalar Formation and the coeval Ballara Quartzite permits discrimination of depositional systems that were restricted areally to either N-S-trending marginal platform or central trough palaeogeographic settings. Four depositional systems, each consisting of several facies, are represented in the lower Quilalar Formation-Ballara Quartzite; these are categorized broadly as storm-dominated shelf (SDS), continental (C), tide-dominated shelf (TDS) and wave-dominated shoreline (WDS). SDS facies consist either of black pyritic mudstone intervals up to 10 m thick, or mudstone and sandstone associated in 6–12-m-thick, coarsening-upward parasequences. Black mudstones are interpreted as condensed sections that developed as a result of slow sedimentation in an outer-shelf setting starved of siliciclastic influx. Vertical transition of facies in parasequences reflects flooding followed by shoaling of different shelf subenvironments; the shoreface contains evidence of subaerial exposure. Continental facies consist of fining-upward parasequences of fluvial origin and tabular, 0·4–4-m-thick, aeolian parasequences. TDS facies are represented by stacked, tabular parasequences between 0·5 and 5 m thick. Vertical arrangement of facies in parasequences reflects flooding and establishment of a tidal shelf followed by shoaling to intertidal conditions. WDS facies are preserved in 0·5–3-m-thick, stacked, tabular parasequences. Vertical transition of facies reflects initial flooding with wave reworking of underlying arenites along a ravinement surface, followed by shoaling from lower shoreface to foreshore conditions. Parasequences are stacked in retrogradational and progradational parasequence sets. Retrogradational sets consist of thin SDS parasequences in the trough, and C, TDS and probably WDS parasequences on the platforms. Thick SDS parasequences in the trough, and TDS, subordinate C and probably WDS parasequences on the platforms make up progradational parasequence sets. Depositional systems are associated in systems tracts that make up 40–140-m-thick sequences bounded by type-2 sequence boundaries that are disconformities. Transgressive systems tracts consist of C, TDS and probably WDS depositional systems on the platforms and the SDS depositional system and suspension mudstone deposits in the trough. The transgressive systems tract is characterized by retrogradational parasequence sets and developed in response to accelerating rates of sea-level rise following lowstand. Condensed-section deposits in the trough, and the thickest TDS parasequences on the platforms reflect maximum rates of sea-level rise and define maximum flooding surfaces. Highstand systems tract deposits are progradational. Early highstand systems tracts are represented by TDS and probably WDS depositional systems on the platforms and suspension mudstone deposits in the trough and reflect decreasing rates of sea-level rise. Later highstand systems tracts consist of the progradational SDS depositional system in the trough and, possibly, thin continental facies on the platforms. This stage of sequence development is related to slow rates of sea-level rise, stillstand and slow rates of fall. Lowstand deposits of shelf-margin systems tracts are not recognized but may be represented by shoreface deposits at the top of progradational SDS parasequence sets. 相似文献