首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 609 毫秒
1.
Individual Pennsylvanian clastic sediment intervals above the Seelyville Coal were examined on electric logs from Sullivan County, Indiana, U.S.A. for vertical sequence, interval thickness, and sand content. This information was used to evaluate local variability of this area of the Illinois Basin depositional system.Each clastic interval is composed of a lateral association of vertical sequences of sediments, bounded above and below by a thin association of coal, limestone, black shale, and/or underclay. An “average” constructive clastic unit is characterized by the following thickness parameters: mean , standard deviation (s) = 3.4 m, minimum = 6.1 m, and
Full-size image (<1K)
sand content, as measured by electric log resistivity, is 37%.Clastic sediment intervals characterized as “deep water” sediments tend to be clayrich, have the greatest lateral continuity, are composed of 50% to 90% coarsening upward sequences, are comprised of less than 16% fining upward sequences, and are composed of less than 10% of sequences of facies which are transitional between coarsening and fining upward. Sandstone channels are linear to coarsely dendritic and probably postdate lower portions of the coarsening upward sequences. One example of longshore shoaling between the Springfield and Houchin Creek coals was discovered. This is the youngest stratigraphic evidence of longshore currents in Illinois Basin clastic sediments. This milieu probably represents a distal delta position.Shallow-water sediments are sand-rich, complex, and gradational. They tend to be interfingered and to display poor lateral continuity. Coarsening upward sequences comprise less than 20% of the data sites. Fining upward fluvial sequences are represented by well-developed dendritic map patterns and constitute 20–30% of the sites. Transitional sequences between coarsening and fining upward log profiles are represented by both interfingered and gradational sequences and constitute 20–55% of the sites.Each constructive vertical sequence represents only a portion of the overall progradational deltatic environment.Destructive linear erosional channels are oriented downdip into the basin. The first occurrence in Indiana of the Trivoli Sandstone channel facies, located between the Ditney and West Franklin horizons, was delineated. Destructive channel sands are found commonly between the Houchin Creek and Colchester coals.Clastic subintervals locally began to develop when compacted unit thickness exceeded 18 m.Average regional wedging contributes 0.13 m/km to local sediment interval thickness variability. The average compacted clastic interval thickness has a local range of 19 m owing to local depositional environments. This variation is related closely to the overall clastic ratio of end-member sequences which are situated in close proximity. The average clastic unit varies in thickness of ± 1.4 m owing to the influence of compaction of the underlying Pennsylvanian sediment filling the Mississippian unconformity valleys. Compactional effects between clastic units become negligible as composite interval thickness exceeds 30 m.  相似文献   

2.
We studied upper Albian to Turonian shallow-marine shelf deposits (Ajlun Group) of west central Jordan along a NNE-SSW running transect. The carbonate-dominated succession includes few siliciclastic intercalations, claystones and shales, and can be subdivided into five formations. The Naur, Fuheis and Hummar Formations of upper Albian to upper Cenomanian age represent shallow subtidal to supratidal platform environments. The uppermost Cenomanian to middle Turonian Shueib Formation includes deeper water deposits of the inner/mid-shelf and locally TOC-rich black shales. Shallow-marine platform environments once again dominate the Wadi As Sir Formation (middle-upper Turonian). A new multibiostratigraphic framework is based on ammonites (mainly of the middle Cenomanian rhotomagense Zone to the middle Turonian woollgari Zone) and calcareous nannofossils (biozones CC 9–CC 11), supplemented by benthic and planktonic foraminifers and ostracods. It forms the base of a sequence stratigraphic subdivision, containing eight sedimentary sequences (S1–S8), which are separated by four Cenomanian sequence boundaries (CeJo1–CeJo4) and three Turonian sequence boundaries (TuJo1–TuJo3). This scheme allows the correlation of the platform succession from distal to proximal shelf areas in contrast to previous correlations using lithologic units. Furthermore, comparisons between the platform successions and sequence patterns of west central Jordan and those from neighbouring areas allow to differentiate local, regional, and global controlling factors of platform development within the study area.  相似文献   

3.
Integrated sedimentology, mineralogy, geochemistry, and microfossil and macrofossil biostratigraphies of the Maastrichtian–early Paleocene Dakhla Formation of the Western Desert, Egypt, provide improved age resolution, information on the cyclic nature of sediment deposition, and the reconstruction of depositional environments. Age control based on integrated biostratigraphies of planktic foraminifera, calcareous nannofossils and macrofossils yields the following ages for stratigraphic and lithologic sequences. The contact between the Duwi and Dakhla formations marks the Campanian/Maastrichtian boundary (zone CF8a/b boundary) and is dated at about 71 Ma. The age of the Dakhla Formation is estimated to span from 71 Ma at the base to about 63 Ma at the top (zones CF8a–Plc). The Cretaceous/Tertiary (K/T) boundary is within the upper unit of the Kharga Shale Member and marked by a hiatus that spans from 64.5 Ma in the lower Paleocene (base Plc) to at least 65.5 Ma (base CF2, base M. prinsii zones) in the upper Maastrichtian at Gebel Gifata, the type locality of the Dakhla Formation. As a result, the Bir Abu Minqar horizon, deposited between about 64.2 and 64.5 Ma (Plc(l) zone), directly overlies the K/T boundary hiatus. Major hiatuses also span the late Maastrichtian–early Paleocene in sections to the northwest (c. 61.2–65.5 Ma at North El Qasr, c. 61.2–69 Ma at Bir Abu Minqar and c. 61.2–65.5 Ma at Farafra), and reflect increased tectonic activity.During the Maastrichtian–early Paleocene a shallow sea covered the Western Desert of Egypt and the clastic sediment source was derived primarily from tectonic activity of the Gilf El Kebir spur to the southwest of Dakhla and the Bahariya arch. Uplift in the region resulted in major hiatuses in the late Maastrichtian–early Paleocene with increased erosion to the southwest. The area was located near the palaeoequator and experienced warm, wet, tropical to subtropical conditions characterized by low seasonality contrasts and predominantly chemical weathering (high kaolinite and smectite). A change towards perennially more humid conditions with enhanced runoff (increased kaolinite) occurred towards the end of the Maastrichtian and in the early Paleocene with shallow seas fringed by Nypa palm mangroves. Sediment deposition was predominantly cyclic, consisting of alternating sandstone/shale cycles with unfossiliferous shales deposited during sea-level highstands in inner neritic to lagoonal environments characterized by euryhaline, dysaerobic or low oxygen conditions. Fossiliferous calcareous sandstone layers were deposited in well-oxygenated shallow waters during sea-level lowstand periods.  相似文献   

4.
Li  Ji-Jun  Fang  Xiao-Min  Van der Voo  Rob  Zhu  Jun-Jie  Mac Niocaill  Conall  Cao  Ji-Xiu  Zhong  Wei  Chen  Huai-Lu  Wang  Jianli  Wang  Jian-Ming  Zhang  Yie-Chun 《Geologie en Mijnbouw》1997,76(1-2):121-134
A paleomagnetic study of the 510-m-thick Wangjiashan section of Late Miocene and Pliocene terrestrial sediments reveals a fairly complete reversal record with ages from 11 to 1.8 Ma. The magnetostratigraphy of the Dongshanding section, located nearby, reveals a partially overlapping reversal record with ages from 2.2 to 0 Ma, and facilitates correlation of the Wangjiashan section with the global polarity time scale. A new stratigraphic division of the Wangjiashan section replaces the name Linxia formation by five new formation names, based on lithologic variation and mammalian fossil finds. The new formations and their magnetostratigraphically determined ages are: Dongshan Formation (c. 1.75–2.6 Ma), Jishi Fm. (c. 2.6–3.6 Ma), Hewangjia Fm. (4.5–6.0 Ma), Liushu Fm. (6.0–7.6 Ma), and Dongxiang Fm. (7.6–c. 12 Ma). The Neogene stratigraphy and fossil mammals suggest that the nearby part of the Tibetan Plateau experienced a persistent denudation during the Late Miocene and Early Pliocene, but that it was uplifted more rapidly at about 3.6 Ma.  相似文献   

5.
内蒙迪彦庙地区早二叠世寿山沟组地层发育良好,岩性相对稳定,变质、变形程度较低,是研究区范围内早二叠世地层层序研究的理想地区。该课题逐层实测了寿山沟组剖面,从剖面的岩性地层人手,着重分析了迪彦庙地区早二叠世的岩性组合特征、基本层序、沉积旋回特征;同时根据古生物化石类型和组合,推断该区域形成的地质时代和沉积环境。研究表明：寿山沟组剖面厚度为9598．98m,根据岩性和粒度的大小,可分3个岩段：一段为滨浅海一浅海陆棚沉积;二段、三段为一套浊流沉积岩系,发育特征性浊流沉积构造,为典型的复理石建造。  相似文献   

6.
Measured lithostratigraphic sections of the classic Permian–Triassic non-marine transitional sequences covering the upper Quanzijie, Wutonggou, Guodikeng and lower Jiucaiyuan Formations at Dalongkou and Lucaogou, Xinjiang Province, China are presented. These measured sections form the framework and reference sections for a range of multi-disciplinary studies of the P–T transition in this large ancient lake basin, including palynostratigraphy, vertebrate biostratigraphy, chemostratigraphy and magnetostratigraphy. The 121 m thick Wutonggou Formation at Dalongkou includes 12 sandstone units ranging in thickness from 0.5 to 10.5 m that represent cyclical coarse terrigenous input to the lake basin during the Late Permian. The rhythmically-bedded, mudstone-dominated Guodikeng Formation is 197 m and 209 m thick on the north and south limbs of the Dalongkou anticline, respectively, and 129 m thick at Lucaogou. Based on limited palynological data, the Permian–Triassic boundary was previously placed approximately 50 m below the top of this formation at Dalongkou. This boundary does not coincide with any mappable lithologic unit, such as the basal sandstones of the overlying Jiucaiyuan Formation, assigned to the Early Triassic. The presence of multiple organic δ13C-isotope excursions, mutant pollen, and multiple algal and conchostracan blooms in this formation, together with Late Permian palynomorphs, suggests that the Guodikeng Formation records multiple climatic perturbation signals representing environmental stress during the late Permian mass extinction interval. The overlap between the vertebrates Dicynodon and Lystrosaurus in the upper part of this formation, and the occurrence of late Permian spores and the latest Permian to earliest Triassic megaspore Otynisporites eotriassicus is consistent with a latest Permian age for at least part of the Guodikeng Formation. Palynostratigrahic placement of the Permian–Triassic boundary in the Junggar Basin remains problematic because key miospore taxa, such as Aratrisporites spp. are not present. Palynomorphs from the Guodikeng are assigned to two assemblages; the youngest, from the upper 100 m of the formation (and the overlying Jiucaiyuan Formation), contains both typical Permian elements and distinctive taxa that elsewhere are known from the Early Triassic of Canada, Greenland, Norway, and Russia. The latter include spores assigned to Pechorosporites disertus, Lundbladispora foveota, Naumovaspora striata, Decussatisporites mulstrigatus and Leptolepidites jonkerii. While the presence of Devonian and Carboniferous spores and Early Permian pollen demonstrate reworking is occurring in the Guodikeng assemblages, the sometimes common occurrence of Scutasporites sp. cf. Scutasporites unicus, and other pollen, suggests that the Late Permian elements are in place, and that the upper assemblage derives from a genuine transitional flora of Early Triassic aspect. In the Junggar Basin, biostratigraphic data and magnetostratigraphic data indicate that the Permian–Triassic boundary (GSSP Level) is in the middle to upper Guodikeng Formation and perhaps as high as the formational contact with the overlying Jiucaiyuan Formation.  相似文献   

7.
Late Quaternary glacier fluctuations in the Macha Khola valley (Gorkha Himal, Nepal) were reconstructed using relative and absolute dating techniques. Our results indicate that younger moraine complexes were left by Late Holocene (<1.7 cal. ka BP), mid-Holocene (ca 3 cal. ka BP), and Lateglacial (ca 13 cal. ka BP) ice advances. Older Late Quaternary glacier advances occurred during Marine Oxygen Isotope Stages (MIS) 2 and 3–4. No relics of Middle or Early Pleistocene glaciations could be found. During MIS 3–4, glaciers advanced down to an altitude of at least 2150 m a.s.l., corresponding to an ELA depression of approximately 1300 m. At about 3500 m a.s.l., the MIS 2 Macha Khola glacier reached almost the thickness of the former MIS 3–4 glacier and retreated some time before 17.9 cal. ka BP. The Lateglacial glacier advanced again several times to altitudes between 2450 and 3400 m a.s.l. The mid-Holocene glaciers extended much farther down-valley than the Late Holocene ones. Dendrochronological data of Abies spectabilis suggested several periods of unfavourable growth conditions especially at the beginning of the 19th (1820) and 20th (1905) centuries.  相似文献   

8.
A stratigraphic section may be divided into lithologic units which in turn may be divided into beds. This paper gives a mathematical formulation of stratigraphic sections that takes these two levels into account and uses bed properties to yield the thickness and number of beds in lithologic units. The model is a semiMarkov chain in which the succession of lithologic bed types forms a Markov chain and is an independent random variable. The model is tested against stratigraphic data obtained from micrologs. There is close agreement between the observed and calculated thicknesses of lithologic units. Tests for the degree of agreement between observed and calculated numbers of beds in lithologic units are hampered by inability to observe thin beds on micrologs. Some implications of this limitation to stratigraphic analysis are noted.  相似文献   

9.
A total of 165 samples were collected from the Qom Formation investigated in a stratigraphic section north of Deh Namak, in Central Iran. From these, 35 genera and 47 species of benthic foraminifera were identified. The age of the studied section is Early Miocene (Aquitanian to Early Burdigalian) based on the occurrence of Borelis melo curdica, Meandropsina anahensis, Meandropsina iranica, Elphidium sp. 14, Peneroplis farsensis, and Triloculina tricarinata. The thickness of the Qom Formation is 401 m of which 161.2 m is early Burdigalian in age. Foraminiferal assemblages in the Deh Namak section are referable to the Borelis melo group-Meandropsina iranica Assemblage Zone and Miogypsinoides-Archaias-Valvulinid Assemblage Zone of [Adams, T.D., Bourgeois, F., 1967. Asmari biostratigraphy. Iranian Oil Operating Companies, Geological and Exploration Division, Report1074 (unpublished) 1–37.] described originally from the Asmari Formation.  相似文献   

10.
The Zambales ophiolite is the major source of chromite ore in the Philippines. The chromitites are concordant cumulates and are associated with distinct chromitite-bearing sequences within the mantle peridotites. Refractory and metallurgical chromite deposits are spatially separated and related to different lithologic associations, which crystallized from different parental magmas. — Refractory chromite ores (30–44 wt% Cr2O3; 20–30 wt% Al2O3) are linked with the peridotite-troctolite-olivine gabbro lineage. Two main types were found: (1) Al-rich refractory ores associated with harzburgites and feldspathic periodotites and (2) more Cr-rich varieties associated with lherzolites. — Metallurgical chromite ores (45–53 wt% Cr2O3; 12–18 wt% Al2O3) are linked with the peridotite-pyroxenite-norite lineage. Two main types were also found: (1) Cr-rich metallurgical ores associated with orthopyroxenites and (2) more Al-rich varieties related to clinopyroxenites. — The chemical composition of chromite within the deposits varies depending on the chromite/silicate ratios of the ore types and grades continuously into accessory chrome spinels in the wall-rock peridotites. — The geochemistry of accessory chrome spinels in various peridotites and mafic cumulates depends on the mineralogical composition and the stratigraphic position of their host rocks.New address: BEB Erdgas und Erdöl GmbH, Riethorst 12, D-3000 Hannover 51The terms chrome-spinels and chromite are used as follows: 1. Chrome-spinel is only used for those occuring as accessory minerals in various ultramafic and mafic rocks (= accessory chrome-spinels). Their chemical composition has been determined only by microprobe analysis. — 2. Chromite is used for ore and ore deposits (=chromitites); the chemical composition has been determined by wet chemistry (AAS) or by microprobe analysis  相似文献   

11.
Manganiferous quartz-mica schists (4 m in stratigraphic thickness) overlie epidote amphibolite in the Chiroro River area, Hidaka Mountains, Hokkaido. The schist layers have a considerable range of A/F ratios and bulk oxidation ratios which vary from 21.5 to 100. Manganese contents are from 4 to 30 times higher than that of the average shale with 0.09% MnO. The schists are essentially quartz-white mica-biotite-Mn garnet-tourmaline-±epidote-magnetite assemblages. A highly oxidized layer (5–8 cm thick) 95 cm above the epidote amphibolite contact is characterized by viridine-piemontite-spessartine-Mn white mica-Mn tourmaline-Ti-Mn haematite indicative of both high initial manganese content and very high f O2 conditions of recrystallization.Viridine contains up to 17 mol% Mn3+SiO5 and coexists with piemontite with between 13.6 and 15.4 wt% Mn2O3. Mn-poor-Fe-rich (Ps32) epidote occurs in the less oxidized schist enclosing the viridine-piemontite bearing seam. Garnets vary widely in composition with end member variations (mol%) of Spess22.9–80.5; And0.2–11.7; Alm1.1–57.1; Pyr2.0–12.2; Gross7.0–49.0. The more manganiferous garnets occur in rocks with higher oxidation ratios while almandiferous varieties occur in schists with low oxidation ratios. Biotite ranges from green to red-brown varieties (increasing Ti and Fe) with Mg/ (Mg+Fe) ratios varying from 56 to 48. Ten to fifteen percent octahedral R2+ is replaced by Al indicating a trend towards eastonite-siderophyllite. The white micas deviate only slightly from dioctahedral stoichiometry but have up to 25% of octahedral sites occupied by Fe, Mg and to a lesser extent Mn and Ti as R2+ Si4+2Al3+ and in highly oxidized rocks as (Fe,Mn)3+Al3+. The white mica in the highly oxidized viridine-piemontite schist is pale pinkishorange, exhibits reverse pleochroism, and has between 0.30 and 0.43 wt% Mn2O3.There is a close comparison, both in terms of stratigraphic thickness and Fe-Mn variation, between the Chiroro schist sequence and many oceanic cores so that the bulk chemistry and mineralogy of the pelitic schists is largely an extension of the original Eh-pH conditions of hemipelagic sedimentation and post-depositional adjustments during diagenesis. The thin viridine-piemontite bearing schist is correlated with an oxidized, Fe-Mn rich layer commonly found in present day oceanic cores. The viridine presumably formed by reaction of original ferro-manganese microgranules and clay minerals. Halmrolytic alteration of the underlying metabasalt resulted in leaching of Mn and Fe (in particular) into the overlying sediments and the formation of concentrations of haematite — manganese oxide — Mn garnet along the schist-epidote amphibolite contact.Estimation of the P-T conditions of metamorphism from the phase relations and compositions in the epidote amphibolite associated with the manganiferous schist gives T °C = 530560 and a minimum P fluid > 3 kb which corresponds to the epidote amphibolite facies of Barrovian-type terrains.This paper is dedicated to Professor Kenzo Yagi on the occasion of his retirement from the Chair of Mineralogy, Department of Geology and Mineralogy, Hokkaido University, Sapporo, Japan  相似文献   

12.
Mathematical correlation of geologic sections from mechanical well logs depends on comparison of frequency components derived from the recorded amplitude variations of the well logs as they respond to changes in lithology. Techniques have been developed that compensate for changes in thickness of units within lithologic sequences and for moderate numbers of missing and truncated beds. However, accurate correlation depends on time stratigraphic units being composed of relatively consistent sequences of rock lithologic units. Correlation is uncertain in areas where time stratigraphic sequences exhibit considerable facies variation. Visual correlation experiences similar problems but the human eye can sort out facies changes more easily and may be better than mechanical correlation in problem areas. Both mechanical and visual correlation can be enhanced by the application of relatively simple transforms to the basic log data. They are applied by computer to digital logs and are designed to magnify specific diagnostics of the more continuous beds while subduing distracting events and noise. Both mechanical and visual correlation are improved by the use of transformed logs; moreover, the digital logs permit optically pleasing displays that maintain the overall advantage of trained geologists over mechanical intervention.  相似文献   

13.
This paper discusses the occurrence of 28,30-dinor-17α,18α,21β-hopane (bisnorhopane) in stratigraphically, fairly well preserved Viking Group sections from wells in local depressions in the North Viking Graben Area. The results show the presence of high relative amounts of bisnorhopane in the “Syn-rift sections”, whilst the “Post-rift sections” contain little or no bisnorhopane. Since most exploration wells are drilled on structural highs, primarily penetrating the “Post-rift Draupne”, this may explain why many analyzed source rock sections in this area contain little bisnorhopane.As a correlation of Draupne sections using the vertical, relative bisnorhopane distributions, it is suggested to be a potential stratigraphic marker for the area, indicating the presence of “Syn-rift Draupne” sediments.The relative bisnorhopane amounts follow a logarithmic reduction with depth and thermal maturity. The bisnorhopane signal is nearly extinguished at 3700 m depth at a maturity of Ro = 0.9–1.0%.  相似文献   

14.
The Precambrian Aksu blueschist is located in the northwestern margin of the Tarim Block, NW China. In recent decades, many studies were carried out with focus on the metamorphic age. However, a complete understanding of the evolution of the Tarim Block requires the cooling history of the Precambrian metamorphic rocks and the time–temperature paths as determined by low-temperature thermochronometry. Therefore, apatite fission track (AFT) technique was applied on the Precambrian Aksu blueschist to reveal the thermo–tectonic evolution of the north Tarim basement. All of the six blueschist samples analysed in this study yielded AFT ages spanning 107.5–62.5 Ma, much younger than the blueschist facies metamorphic age of Neoproterozoic, and confined track lengths are between 10.46 and 12.12 µm. Based on regional stratigraphic sequences, the AFT thermal history modeling as well as previous chronological results, the thermo–tectonic evolution of the Aksu blueschist can be roughly reconstructed with four stages: (1) the Precambrain Aksu blueschist exhumed to the surface soon after its formation. Erosion during the Early Sinian is indicated by the lack of sedimentation until the Late Sinian; (2) the Late Sinian strata are continuous, while the Middle–Upper Silurian and the Lower–Middle Carboniferous strata are absent. The total thickness of the Late Sinian and Paleozoic strata probably reached 10,000 m and resulted in the total annealing and thermal resetting of AFT ages; (3) the AFT ages in the Cretaceous are related with the widespread uplift in Tian Shan and its adjacent regions that restarted the AFT clock during the Late Mesozoic. These reflect a distant effect of the collision of the Lhasa terrane with Eurasia in the Late Jurassic–Early Cretaceous; and (4) sediments of Cenozoic are documented in the Aksu area. The Aksu blueschist was heated to partial annealing zone with the overlying Cenozoic sediments. During Miocene time, the Aksu blueschist was re-exhumed which was probably a distant response to the ongoing India–Eurasia convergence.  相似文献   

15.
The copper deposits of the O'okiep District rank amongst the top three S. African producers, with an annual output of 3 million metric tons grading 1.65% Cu on average. They occur in what is now considered the S. extension of the 1200 m. y. Kibaran orogenic belt. Copper mineralization is linked to cross-cutting bodies of "noritoid" RO=0.7126–0.7250) which have been emplaced into the granulite facies metamorphic country rocks 1100 m. y. ago. The results of 150 electron probe analyses show that orthopyroxene in the non-ore-bearing noritoids is En51–52, in the ore-bearing noritoids En60–65. There is a concomitant variation in the composition of phlogopitic mica which carries higher Fe in non-ore-bearing noritoids. The Ticontents of magnetite are below 0.1%; there are significant chromium values. Spinel exsolution lamellae contain up to 14% ZnO. The copper sulphides occur as granular aggregates with silicates, on grain boundaries and on cleavage planes of hypersthene and mica; they also replace altered Fe-Ti-oxides. Pyrite (up to 1.4% Co) and pentlandite (up to 5% Co) are not widespread. The possible derivation of the noritoids from a reservoir of basic magma at depth is considered. A source-bed model, involving generation of noritoid "magma" from Cu-bearing members of the stratigraphic sequence, during the peak of metamorphism (800–1000 °C, 6–8 kb) is tentatively proposed. Stratabound base metal concentrations (Aggeneys, Gamsberg) in lower grade (amphibolite facies) metamorphic terrains to the East have not been mobilized.formerly, Mineralogisch-Petrographisches Institut, Universität Hamburg  相似文献   

16.
We present a reconstructed lithologic column compiled from a series of lacustrine outcrops along a tributary of the Nyang River, a major tributary of the Yarlung-Tsangpo in southeast Tibet. The deposits were preserved between terraces at altitudes of 2950–3100 m asl. The stratigraphic record features at least two sets of coarsening-upward sequences depicting episodic aggradation and progradation of a glacially dammed lake related delta. Recognized facies changes illustrate the evolution cycles of depositional environments from pro-delta, delta front, to delta plain. Radiocarbon and optically stimulated luminescence dates reveal an aging-downward trend in stratigraphic order and provide an approximate timeline for the formation of glacially dammed lakes in late Pleistocene. This result reflects that the Zelunglung Glacier had progressively advanced to block the Yarlung-Tsangpo river and the dam materials had stepwise stacked up to an altitude of 3095 m asl during Marine Oxygen Isotope Stages 4 to 2.  相似文献   

17.
Interpretation of reprocessed seismic reflection profiles reveals three highly coherent, layered, unconformity-bounded sequences that overlie (or are incorporated within) the Proterozoic “granite–rhyolite province” beneath the Paleozoic Illinois basin and extend down into middle crustal depths. The sequences, which are situated in east–central Illinois and west–central Indiana, are bounded by strong, laterally continuous reflectors that are mappable over distances in excess of 200 km and are expressed as broad “basinal” packages that become areally more restricted with depth. Normal-fault reflector offsets progressively disrupt the sequences with depth along their outer margins. We interpret these sequences as being remnants of a Proterozoic rhyolitic caldera complex and/or rift episode related to the original thermal event that produced the granite–rhyolite province. The overall thickness and distribution of the sequences mimic closely those of the overlying Mt. Simon (Late Cambrian) clastic sediments and indicate that an episode of localized subsidence was underway before deposition of the post-Cambrian Illinois basin stratigraphic succession, which is centered farther south over the “New Madrid rift system” (i.e., Reelfoot rift and Rough Creek graben). The present configuration of the Illinois basin was therefore shaped by the cumulative effects of subsidence in two separate regions, the Proterozoic caldera complex and/or rift in east–central Illinois and west–central Indiana and the New Madrid rift system to the south. Filtered isostatic gravity and magnetic intensity data preclude a large mafic igneous component to the crust so that any Proterozoic volcanic or rift episode must not have tapped deeply or significantly into the lower crust or upper mantle during the heating event responsible for the granite–rhyolite.  相似文献   

18.
山东沂水地区寒武系张夏组中上部地层的分布，受控于沂沭断裂（Ｆ４和Ｆ３断裂），分为西区、中区和东区。各区岩性、岩相雷同，只是厚度存在明显差异。地层记录旋回性明显，反映高频旋回特点。根据岩性、沉积相序变化及界面特点，将该套地层划分成１个三级、４个四级、１４～１８个五级和６０～８０余个六级旋回层序。各级别旋回层序均由次一级旋回层序向上变浅、变粗、变厚的进积系列组成。其底部均以相应级别的间断－加深面为界。通过高分辨率层序对比，三级、四级旋回层序具有区域对比意义。因受沂沭断裂的影响，区内地层分布并非原始沉积位置所在，由于地方性因素，造成了五级或六级旋回层序数量上的差异。对于三级旋回层序而言，由早期→晚期，随着沉积环境由陆棚盆地→深潮下带→浅潮下带变化，海水由深→浅；沉积速率由小→大；沉积物质由页岩→瘤状灰岩、泥灰岩→鲕粒灰岩→藻灰岩变化。张夏期各地海平面变化趋势相同，均具有高频率振荡特点，为一个复合海平面变化旋回。  相似文献   

19.
滇西勐海西定地区,在大地构造上位于西南“三江”造山带南段昌宁-孟连结合带东侧的双江-澜沧俯冲增生杂岩带内。该区分布有一套晚古生代的浅变质碎屑岩系--南段组(DCn)。该套地层既显示层型面状构造型式,又发育有顺层片理、顺层掩卧褶皱、粘滞型石香肠及顺层韧性剪切带等构造形迹,具明显的分层剪切变形、横向构造置换和顺层固态流变特点,构成较典型的褶叠层构造。本文对其构造变形特征进行了系统分析与总结,结合区域构造背景研究成果认为,南段组褶叠层构造总体形成于昌宁-孟连古特提斯洋闭合后同碰撞造山收缩挤压背景下的伸展剪切机制。同时指出,南段组是经过构造改造重建而形成的构造地层单元,应称其为南段岩组;现今露头剖面所见南段岩组岩性组合及厚度只代表改造后构造地层的岩性组合特征及褶叠层的露头厚度,不能完全代表南段组原沉积地层的韵律组合及沉积地层厚度;应按构造地层单位进行划分、填图和研究。褶叠层构造作为浅变质岩区一种普遍的构造型式,是对岩石变形组合特征的客观认识,其构造型式、形成机制、形成层次以及形成时间的研究对分析造山带构造演化过程和深化大陆动力学机制的认识具有科学意义。  相似文献   

20.
Holocene prograded coastal sequences at Becher/Rockingham, southern Western Australia, contain a detailed record of sea level over the last 6400 yr. Radiocarbon dating and use of a distinct stratigraphic indicator as a sea-level marker permit reconstruction of sea-level history and suggest that the sea was at least 2.5 m above present datum about 6400 yr B.P. before falling to its present level. No evidence was found for eustatic fluctuations of the scale proposed by R. W. Fairbridge [1961, in “Physics and Chemistry of the Earth” (L. H. Ahrens, F. Press, K. Rankema, and S. K. Runcorn, Eds.), Vol. 4, pp. 99–185, Pergamon, Oxford]. The sea-level record preserved on this coast can be explained by hydro-isostasy, tectonism, or eustasy, acting individually or in concert. Without a fixed reference point or analogous data from other locations, a firm conclusion on which mechanism(s) has(have) operated could not be reached. Published sea-level data from this and other coasts are often insufficiently detailed to compare with this study. Application of the techniques of this study to analogous sedimentary sequences elsewhere will provide data of comparable accuracy that would contribute to a more precise understanding of relative sea-level movements in the late Quaternary.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号