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1.
Assessment of widely used methods to derive depositional ages from detrital zircon populations 总被引：1，自引：0，他引：1
The calculation of a maximum depositional age(MDA)from a detrital zircon sample can provide insight into a variety of geological problems.However,the impact of sample size and calculation method on the accuracy of a resulting MDA has not been evaluated.We use large populations of synthetic zircon dates(N≈25,000)to analyze the impact of varying sample size(n),measurement uncertainty,and the abundance of neardepositional-age zircons on the accuracy and uncertainty of 9 commonly used MDA calculation methods.Furthermore,a new method,the youngest statistical population is tested.For each method,500 samples of n synthetic dates were drawn from the parent population and MDAs were calculated.The mean and standard deviation of each method ove r the 500 trials at each n-value(50-1000,in increments of 50)were compa red to the known depositional age of the synthetic population and used to compare the methods quantitatively in two simulation scenarios.The first simulation scenario varied the proportion of near-depositional-age grains in the synthetic population.The second scenario varied the uncertainty of the dates used to calculate the MDAs.Increasing sample size initially decreased the mean residual error and standard deviation calculated by each method.At higher n-values(>~300 grains),calculated MDAs changed more slowly and the mean resid ual error increased or decreased depending on the method used.Increasing the p roportion of near-depositional-age grains and lowering measurement uncertainty decreased the number of measurements required for the calculated MDAs to stabilize and decreased the standard deviation in calculated MDAs of the 500 samples.Results of the two simulation scenarios show that the most successful way to increase the accuracy of a calculated M DA is by acquiring a large number of low-uncertainty measurements(300300)approach is used if the calculation of accurate MDAs are key to research goals.Other acquisition method s,such as high-to moderate-precision measurement methods(e.g.,1%-5%,2σ)acquiring low-to moderate-n datasets(50300).Additionally,they are most susceptible to producing erroneous MDAs due to contamination in the field or laboratory,or through disturbances of the youngest zircon’s U-Pb systematics(e.g.,lead loss).More conservative methods that still produce accurate MDAs and are less susceptible to contamination or lead loss include:youngest grain cluster at 1σunce rtainty(YGC 1σ),youngest grain clusterat 2σuncertainty(YGC 2σ),and youngest statistical population(YSP).The ages calculated by these methods may be more useful and appealing when fitting calculated MDAs in to pre-existing chronostratigraphic frameworks,as they are less likely to be younger than the true depositional age.From the results of our numerical models we illustrate what geologic processes(i.e.,tectonic or sedimentary)can be resolved using MDAs derived from strata of different ages. 相似文献
2.
《Sedimentology》2018,65(6):1918-1946
In southern Patagonia, outcrops of the Upper Cretaceous Cerro Toro Formation preserve a >150 km long deep‐water axial channel belt in the Magallanes–Austral Basin, providing a unique opportunity to investigate longitudinal variations in the depositional characteristics of a deep‐water channel system. This study documents sedimentological, stratigraphical and geochronological data from the Cerro Toro Formation in the Argentine sector of the basin. New results are integrated with previous work from the Chilean basin sector to conduct a basin‐scale comparison of the timing of deposition, provenance and lithofacies proportions. The Cerro Toro channel belt includes a nearly 1000 m thick section characterized by high‐density turbidites and mass‐wasting deposits. Two ash beds from the base of the section yield U–Pb zircon ages of 90·4 ± 2 Ma and 88·0 ± 3 Ma, indicating similar initiation ages as documented in the Chilean sector. The U–Pb detrital zircon age spectra from samples in the study area reveal similar provenance trends to samples from the Chilean basin sector, with peak age populations at 310 to 260 Ma, 160 to 135 Ma and 110 to 82 Ma. The maximum depositional age of the channel belt in the Argentine sector is 87·8 ± 1·5 Ma and all new geochronology data corroborate an 86 to 80 Ma depositional age for the main Cerro Toro channel belt. Statistical analyses of 7370 beds from nearly 8000 m of new and previously published stratigraphic sections along the entire outcrop belt suggest progressive variations in the down‐system proportion of lithofacies. In the up‐slope region, lithofacies representing mass wasting processes (for example, debris‐flow and mass‐transport deposits) account for ca 29% of the stratigraphic thickness, as opposed to 5% in the down‐slope region of the channel belt, where turbidity current deposits are more prevalent. The proportion of beds >1 m thick also decreases systematically down slope, particularly for conglomeratic turbidite deposits. This work highlights that: (i) the proportion of thick beds and distribution of lithofacies are key down‐system changes in the stratigraphic fill of this deep‐water channel belt; (ii) detrital zircon trends suggest a relatively well‐mixed longitudinal depositional system; and (iii) geochronology of the main Cerro Toro outcrop belt supports but does not necessitate the model of a single, roughly age‐equivalent, channel system. This study has implications for understanding the downslope variability in depositional processes, stratigraphic architecture and reservoir quality of submarine channel systems. 相似文献
3.
辽东半岛盖州市梁屯地区古元古代大理岩的碎屑锆石U--Pb定年及地质意义 总被引：1，自引：0，他引：1
通过地层对比、锆石LA--ICP--MS U--Pb定年及岩石地球化学研究限定胶—辽—吉活动带北东段辽东半岛盖州市梁屯地区一套碳酸盐岩建造的原岩形成时代、物质来源和地层归属等。其炭质方解大理岩锆石阴极发光(CL)图像显示碎屑锆石核部大都具有振荡环带,且其Th/U比值较高(Th/U=0. 15～2. 04),表明其为岩浆来源成因。锆石U--Pb年代学定年结果显示存在两个主要的年龄峰值,主要年龄峰值为新太古代晚期(2 560～2 500 Ma),次要峰值为古元古代中晚期(2 040～2 000 Ma),其沉积时限应晚于2. 0 Ga,与古元古代辽河岩群沉积时代一致。此外,该年龄与区域上的太古代基底和大量出露的辽吉花岗岩的年龄相一致,表明炭质方解大理岩物质来源于太古代基底和辽吉花岗岩。并通过岩相学、岩石地球化学、年代学和区域对比等研究,确认梁屯地区分布的这套富含炭质的大理岩建造(原岩为沉积石灰岩)可与辽河群高家峪岩组对比,而非前人认为的大石桥岩组。 相似文献
4.
5.
U–Pb detrital zircon ages are reported from Puncoviscana Formation (late Neoproterozoic–Early Cambrian) and Mesón Group (Late Cambrian) greywackes of northwest Argentina, to constrain provenance and depositional environment.The new data are combined with previously-published detrital zircon ages, to show that Puncoviscana Formation age patterns contain two broad groups: late Mesoproterozoic–early Neoproterozoic (1150–850 Ma) and late Neoproterozoic–Early Cambrian (650–520 Ma); with their relative proportions varying inversely with youngest component age. The 1150–850 Ma age components are dominant in greywackes with oldest late Neoproterozoic components > 600 Ma. The former diminish considerably when late Neoproteozoic components become dominant and younger, to 520 Ma. A northernmost greywacke sample from Purmamarca, Jujuy, is distinctive: whilst its zircon age pattern partly resembles other Puncoviscana Formation samples, it contains no Cambrian–late Neoproterozoic ages, the youngest ages being early Neoproterozoic. This may reflect an early, Neoproterozoic, passive-margin depocentre for the Formation, or an older (early Neoproterozoic) succession within it, which may predate the Brasiliano orogeny in Brazil. The youngest age components, c. 520 Ma, in a greywacke from Rancagua (Cachi, Salta province), dominate an almost unimodal pattern suggestive of contemporary volcanic sources at a late Early Cambrian depocentre. Detrital zircon age patterns of the Mesón Group (Lizoite Formation) have major Cambrian–latest Neoproterozoic components resembling those of the Puncoviscana Formation, but its Mesoproterozoic component is diminished, and there are no significant age components of this age. Small youngest components at c. 500 Ma suggest a maximum Late Cambrian stratigraphic age. The Puncoviscana Formation detrital zircon patterns suggest a provenance in a continental hinterland having a stabilised, extensive late Mesoproterozoic orogen (with minor Paleoproterozoic and Archean precursors), and a more variable late Neoproterozoic orogen containing an evolving sequence of less extensive subcomponents. A direct relationship with the Brazilian Shield is suggested; with sediment supplies originating within active-margin orogens of the interior and collisional orogens at the suture between African and South American cratons, but ultimate deposition in passive-margin environments of western Gondwanaland. 相似文献
6.
Although sequence stratigraphic concepts have been applied extensively to coarse-grained siliciclastic deposits in nearshore environments, high-resolution sequence stratigraphic analysis has not been widely applied to mudstone-dominated sedimentary successions deposited in more distal hemipelagic to pelagic settings. To examine how sequence stratigraphic frameworks can be derived from the facies variability of mudstone-dominated successions, the Tununk Shale Member of the Mancos Shale Formation in south-central Utah (USA) was examined in detail through a combination of sedimentological, stratigraphic and petrographic methods. The Tununk Shale accumulated on a storm-dominated shelf during the second-order Greenhorn sea-level cycle. During this eustatic event, the depositional environment of the Tununk Shale shifted laterally from distal middle shelf to outer shelf, then from an outer shelf to an inner shelf environment. At least 49 parasequences can be identified within the Tununk Shale. Each parasequence shows a coarsening-upward trend via upward increases in silt and sand content, thickness and lateral continuity of laminae/beds, and abundance of storm-generated sedimentary structures. Variations in bioturbation styles within parasequences are complex, although abrupt changes in bioturbation intensity or diversity commonly occur across parasequence boundaries (i.e. flooding surfaces). Due to changes in depositional environments, dominant sediment supply and bioturbation characteristics, parasequence styles in the Tununk Shale show considerable variability. Based on parasequence stacking patterns, eleven system tracts, four depositional sequences and key sequence stratigraphic surfaces can be identified. The high-resolution sequence stratigraphic framework of the Tununk Shale reveals a hierarchy of stratal cyclicity. Application of sequence stratigraphic concepts to this thick mudstone-dominated succession provides important insights into the underlying causes of heterogeneity in these rocks over multiple thickness scales (millimetre-scale to metre-scale). The detailed sedimentological characterization of parasequences, system tracts and depositional sequences in the Tununk Shale provides conceptual approaches that can aid the development of high-resolution sequence stratigraphic frameworks in other ancient shelf mudstone successions. 相似文献
7.
详细描述了四川名山建山大邑砾岩实测剖面,讨论了大邑砾岩的岩性组合特征及沉积环境。认为大邑砾岩主要为一套位于风化壳之上的河流-冲积扇沉积,自下而上由细变粗,属于物源区总体不断上升过程中的近源沉积。通过大邑砾岩中的孢粉组合,讨论了其地质时代。该剖面孢粉组合以水龙骨科孢子为主,年代地层归属于新近系上新统。 相似文献
8.
《地学前缘(英文版)》2023,14(1):101477
New and compiled detrital zircon U–Pb ages from the southern Neoproterozoic–Cambrian Ribeira Belt, SE Brazil, demonstrate Laurentian affinity of the Embu Terrane which is statistically distinct from the adjoining Apiaí and São Roque terranes with cratonic affinity (e.g., São Francisco Craton). Zircon provenance results indicate that the type-area of the Embu Terrane is dominated by detrital zircon age modes at ca. 1200 Ma, 1400 Ma, and 1800 Ma, with maximum depositional age of ca. 1000 Ma. In contrast, the Apiaí and São Roque terranes are dominated by Paleoproterozoic detrital zircon ages (ca. 2200–2000 Ma age dominant component), with maximum depositional ages of ca. 1400 Ma and 1750 Ma, respectively. Multidimensional scaling (MDS) analysis of non-parametric similarity measurements on zircon age populations indicates for the first time that the Embu Terrane encompass two statistically distinct detrital zircon age spectra, which is also reflected in the metamorphic zircon age record. The statistical characterization of the Embu Terrane through populational metrics allow a quantitative comparison with surrounding tectonic domains and rock samples classified such as Embu-type. Our results clearly highlight the distinction between the statistically differentiated Embu Terrane from the Apiaí and São Roque terranes, supporting an allochthonous interpretation. In addition, we demonstrate that rocks samples previously classified as Embu-type are significantly dissimilar to the definition of Embu Terrane, failing to support alternative tectonic models (e.g., intracontinental evolution). Detrital zircon age spectra reveal that the Apiaí and São Roque terranes have similar zircon provenance to domains sourced from the São Francisco Craton, whereas detrital zircon populations from the Embu Terrane have greater affinity with SW Laurentia basins (and their inferred sediment sources), consistent with previous findings. Therefore, we interpret the Embu Terrane as a Rodinia descendant developed along the active margin of the SW Laurentia that collided with the Ribeira Belt during early Neoproterozoic (810–760 Ma). 相似文献
9.
C. W. Kwon S.-I. Park J. Park J.-H. Oh B. C. Kim 《Australian Journal of Earth Sciences》2013,60(6):865-880
Sedimentological characteristics and zircon provenance dating of the Babulu Formation in the Fohorem area, Timor-Leste, provide new insights into depositional process, detailed sedimentary environment and the distribution of source rocks in the provenance. Detrital zircon sensitive high-resolution ion microprobe (SHRIMP) U–Pb ages range from Neoarchean to Triassic, with the main age pulses being Paleozoic to Triassic. In addition, the maximum deposition ages based on the youngest major age peak (ca 256–238 Ma) of zircon grains indicate that the basal sedimentation of the Babulu Formation occurred after the early Upper Triassic. The formation consists predominantly of mudstone with minor sandstone, limestone and conglomerate that were deposited in a deep marine environment. These deposits are composed of six lithofacies that can be grouped into three facies associations (FAs) based on the constituent lithofacies and bedding features: basin plain deposits (FA I), distal fringe lobe deposits (FA II) and medial to distal lobe deposits (FA III). The predominance of mudstone (FA I) together with intervening thin-bedded sandstones (FA II) suggest that the paleodepositional environment was a low energy setting with slightly basin-ward input of the distal part of the depositional lobes. Discrete and abrupt occurrences of thick-bedded sandstone (FA III) within the FA I mudstone suggests that sandstone originated from a collapse of upslope sediments rather than a progressive progradation of deltaic turbidites. This combined petrological and geochronological study demonstrates that the Babulu Formation in the Fohorem area of the Timor-Leste was initiated as a submarine lobe system in a relatively deep marine environment during the Upper Triassic and represents the extension of the Gondwana Sequence at the Australian margin. 相似文献
10.
从准层序到米级旋回——层序地层学与旋回地层学相互交融的纽带 总被引：3，自引：0，他引：3
层序地层学的基本单位是层序。准层序和准层序组构成了层序的基本结构单元,它们在层序内有规律的空间叠置样式是识别体系域的客观标准。准层序被定义为以海泛面为界的向上变浅序列,但是准层序是一个比较松散的概念,存在问题是对其成因机制研究不足。米级旋回作为旋回地层学的基本单位,定义为数厘米至数米厚的向上变浅的岩相序列,是高频率海平面升降变化的产物,其形成时限为相应的米兰柯维奇天文旋回周期。作为旋回地层学的基本单位,米级旋回以其明确的沉积学涵义和有规律的天文成因机制,是对层序地层学概念体系中的"准层序"的良好补充,对其进行深入研究具有重要的理论意义和应用价值。从准层序到米级旋回这一概念体系的演变,代表了层序地层学与旋回地层学相互交融的纽带。最后介绍旋回地层学的最新进展亚米兰柯维奇旋回,它对精确数字定年具有重大意义。 相似文献
11.
上扬子区的震旦系包括陡山沱组和灯影组，前者包含较多的黑色页岩，而后者则以发育大套白云岩为特征。从岩相序列到米级旋回，从沉积相序列到三级层序，可将浅水台地背景的震旦系划分为5个三级层序。这些三级层序可以进一步归为一个二级层序。以各个主要剖面的层序划分为基础，依据地层记录中的两种相变面和两种穿时性，可以建立研究区震旦系的层序地层格架。其中包含较多的深水黑色页岩系的陡山沱组，反映了大冰期之后的海侵作用的特点；与寒武系的变化特征不同，自北西而南东沉积环境由浅变深，该沉积背景中沉积了原生沉积组构保存较为完好的灯影组白云岩，这种变化可能反映了前寒武纪末期特殊的白云石化作用特征，同时也代表了许多有待于进一步研究的沉积学问题。从陡山沱组黑色页岩系到灯影组白云岩，不但构成了一个更大级别的总体向上变浅的沉积相序列，还反映了一个与三级和二级海平面变化过程相响应的缓坡型碳酸盐台地的生长发育过程，该过程较为清楚地显示在系列层序地层格架栅状图和岩相古地理图之中。 相似文献
12.
尽管还存在一些不同的认识和争论,陆生植被对河流沉积作用的影响,得到了越来越多的地质事实的支持,主要表现在以煤或泥炭的形式保存下来的碳质物质对河堤的保护,从而促进了曲流河与网状河的发育。四川盆地上三叠统须家河组为一套含有煤系地层的冲积序列为主的地层,属于具有前陆盆地性质的磨拉石序列,成为研究植被发育条件下河流序列的典型代表。重庆永川普安剖面的须家河组是以河流相沉积为主的须家河组的典型代表,包括6个正式的岩性段,在其中一段、三段和五段为含煤细粒碎屑岩组成的低能河流相地层,二段、四段和六段由砂质底荷载高能河流相沉积构成,岩石地层划分及其所反映出的旋回性成为层序地层的识别和划分的基础,从而将须家河组划分成3个河流相沉积层序。两个主要的特征赋予了普安剖面须家河组河流相层序序列重要意义：（1）总体向上变粗的而且与现行的河流相层序模式形成巨大差异的河流相沉积序列组成的冲积构架;（2）明显受到植被发育影响的高能底荷载河道相砂岩。基于植被发育对河流沉积作用的影响,对普安剖面须家河组的河流相沉积序列的观察与研究,对长期以来须家河组沉积相分析存在的较大争议将提供重要的线索和思考途径。 相似文献
13.
《地学前缘(英文版)》2016,(5)
This study is focused on sedimentary environments, facies distribution, and sequence stratigraphy. The facies and sequence stratigraphic analyses of the Bahram Formation(middleelate Devonian) in southcentral Iran are based on two measured stratigraphic sections in the southern Tabas block. The Bahram Formation overlies red sandstones Padeha Formation in sections Hutk and Sardar and is overlain by Carboniferous carbonate deposits of Hutk Formation paraconformably, with a thickness of 354 and386 m respectively. Mixed siliciclastic and carbonate sediments are present in this succession. The field observations and laboratory studies were used to identify 14 micro/petrofacies, which can be grouped into 5 depositional environments: shore, tidal flat, lagoon, shoal and shallow open marine. A mixed carbonate-detrital shallow shelf is suggested for the depositional environment of the Bahram Formation which deepens to the east(Sardar section) and thins in southern locations(Hutk section). Three 3rdorder cyclic siliciclastic and carbonate sequences in the Bahram Formation and one sequence shared with the overlying joint with Hutk Formation are identified, on the basis of shallowing upward patterns in the micro/pertofacies. 相似文献
14.
Stratigraphic and structural interpretative studies have been carried out within the paralic sequence of Agbada Formation in the Niger Delta Basin. The method of study involved integrating sequence stratigraphic techniques with well logs, biostratigraphy, and 3D regional seismic data obtained from several producing fields in the Coastal Swamp depobelt of the onshore Niger Delta Basin. This was targeted at establishing regional sequence stratigraphic and structural framework from well log correlation and seismic mapping so as to better define hydrocarbon leads. Well log correlation using stratigraphic bounding surfaces such as maximum flooding surfaces (MFSs) and sequence boundaries (SBs) indicates that the stratigraphic package was deposited within the Middle to Late Miocene (8.5 to 12.1 Ma) age. Recognized depositional sequences revealed differential thickening and thinning of the associated system tracts (lowstand system tract (LST), transgressive system tract (TST), and highstand system tract) (HST)). This observed stratal behavior has been attributed to the influence of syndepositional tectonic structures that characterize the Coastal Swamp depobelt, which lies within the extensional zone. Flattening of MFSs at various ages indicates observable basinward shift of the depositional center. The genetic units of LST and HST serve as good hydrocarbon reservoir whereas HST and TST constitutes mainly the source and seal packages rocks in the area. Structural interpretation revealed the occurrence of simple/faulted rollover anticline, collapsed crest, regional hanging wall and footwall, horst block, and subdetachment fault structural styles that constitute the key hydrocarbon entrapment mechanisms. Structural top maps show fault-dependent closures which dominate the extensional zone, which could form possible hydrocarbon leads and prospects that should be targeted during exploration studies for development. 相似文献
15.
The Lower Jurassic Mashabba Formation crops out in the core of the doubly plunging Al-Maghara anticline, North Sinai, Egypt. It represents a marine to terrestrial succession deposited within a rift basin associated with the opening of the Neotethys. Despite being one of the best and the only exposed Lower Jurassic strata in Egypt, its sedimentological and sequence stratigraphic framework has not been addressed yet. The formation is subdivided informally into a lower and upper member with different depositional settings and sequence stratigraphic framework. The sedimentary facies of the lower member include shallow-marine, fluvial, tidal flat and incised valley fill deposits. In contrast, the upper member consists of strata with limited lateral extension including fossiliferous lagoonal limestones alternating with burrowed deltaic sandstones. The lower member contains three incomplete sequences (SQ1-SQ3). The depositional framework shows transgressive middle shoreface to offshore transition deposits sharply overlain by forced regressive upper shoreface sandstones (SQ1), lowstand fluvial to transgressive tidal flat and shallow subtidal sandy limestones (SQ2), and lowstand to transgressive incised valley fills and shallow subtidal sandy limestones (SQ3). In contrast, the upper member consists of eight coarsening-up depositional cycles bounded by marine flooding surfaces. The cycles are classified as carbonate-dominated, siliciclastic-dominated, and mixed siliciclastic-carbonate. The strata record rapid changes in accommodation space. The unpredictable facies stacking pattern, the remarkable rapid facies changes, and chaotic stratigraphic architecture suggest an interplay between allogenic and autogenic processes. Particularly syndepositional tectonic pulses and occasional eustatic sea-level changes controlled the rate and trends of accommodation space, the shoreline morphology, the amount and direction of siliciclastic sediment input and rapid switching and abandonment of delta systems. 相似文献
16.
甘肃北山北部双沟山地区发现泥盆系雀儿山群地层 总被引：2，自引：0，他引：2
在《红宝石幅》区域地质调查过程中,发现在甘肃北山双沟山地区分布着一套灰绿色中性、中酸性火山熔岩及其凝灰岩地层,通过岩石地层、生物化石及地层年代等方面的研究,经区域地层对比划归雀儿山群,形成时代确定为早泥盆世。该群的确定对恢复哈萨克斯坦板块南缘区域地质演化历史有重要意义。 相似文献
17.
自四川盆地西北部(川西北)泥盆系观雾山组油气勘探取得重要进展以来，观雾山组的沉积储层研究受到了广泛重视。然而，由于观雾山组地层划分方案的变动和地层的穿时性，导致目前对观雾山组的时代分布和充填过程认识不清，从而影响了沉积古地理格局的恢复。本文以四川广元西北乡、潜溪河剖面泥盆系观雾山组牙形石生物地层研究为基础，结合区域古生物地层资料，认为江油马角坝以北的观雾山组绝大多数沉积于晚泥盆世早—中弗拉期，且地层沉积时间自南西向北东逐渐变晚。龙门山北段观雾山组具有自西向东、自南向北地层厚度减薄和地层时代变新的特征，表明来自扬子地台西部的古特提斯洋海水向东侵入龙门山地区，因地形差异和渐进式海侵形成了自南西向北东的超覆沉积。由于岩相分异，超覆沉积形成的观雾山组与金宝石组在区域上既表现为上下叠置关系，也表现为同期异相的相变关系。该研究不仅对约束川西北地区观雾山组地层时代提供了可靠的古生物证据，而且对认识龙门山地区泥盆系的充填过程、沉积古地理格局均具有重要意义。 相似文献
18.
The Attic–Cycladic Crystalline Belt in the central Aegean region represents a major tectono‐stratigraphic unit of the Hellenides. The essential geological, magmatic and tectono‐metamorphic features are well documented. Unresolved questions concern the time of sediment accumulation and litho‐ and/or tectono‐stratigraphic relationships across the study area. In order to address this issue we have studied siliciclastic metasedimentary rocks from Andros Island, northern Cyclades. The sampling strategy aimed at covering the complete age range recorded by the Andros metamorphic succession. Detrital zircon U–Pb dating of nine samples indicates maximum depositional ages of c. 260 Ma for the topmost part of the metamorphic succession and of c. 160–140 Ma for rock sequences below a prominent serpentinite belt that is interpreted to outline a major tectonic contact. These age constraints are in accordance with interpretations suggesting that the metamorphic rocks of Andros represent different tectonic subunits (Makrotantalon Unit and Lower Unit) that are separated by a thrust fault. Modification of the internal structure of the Lower Unit by tectonic stacking can currently not be ascertained. The new data for the Lower Unit corroborate the importance of Late Jurassic–Early Cretaceous sediment accumulation for the larger study area. In contrast to some of the neighbouring islands, no evidence for transfer of Late Cretaceous (c. 80 Ma) material into the Andros sedimentary environment was found. The most striking feature of the zircon populations of the Lower Unit is a remarkable age cluster at 250–200 Ma that documents the importance of Triassic igneous sediment sources. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
19.
碎屑锆石年代学不但能够限定地层沉积开始的最大时限,还能为示踪沉积物源区提供关键信息。中国西南部的松潘-甘孜褶皱带广泛出露一套巨厚的三叠纪复理石沉积,其物源区和可能存在的同期抬升与剥蚀历史并未得到很好约束。本文获得的松潘-甘孜褶皱带南部雅江地区上三叠统四套地层（由老至新分别为侏倭组、新都桥组、两河口组和雅江组）5件砂岩样品的碎屑锆石U-Pb年龄和锆石Hf同位素数据表明,最年轻锆石年龄指示侏倭组从~229Ma后开始沉积,新都桥组则从~223Ma后开始沉积。碎屑锆石年龄频谱图显示四套地层都具有中奥陶世-早泥盆世（465~398Ma）和中二叠世-晚三叠世（271~225Ma）的年龄峰。除两河口组外的其他三套地层还具有较强的古元古代（1.90~1.86Ga）和新元古代（872~712Ma）的年龄峰。锆石Hf同位素显示松潘-甘孜褶皱带南部上三叠统小于300Ma的锆石颗粒主要来自峨眉山大火成岩省和义敦岩浆弧。本文物源区示踪结果表明,华南板块和义敦地体可能为松潘-甘孜褶皱带南部地层的主要物源区。晚三叠世由于周缘地体的强烈汇聚,松潘-甘孜褶皱带在小于~18Myr的时间内经历了快速的隆升和剥蚀作用,剥蚀产生的碎屑物质被搬运至四川盆地的西缘再沉积。 相似文献
20.
Whole-rock chemistry and precise U – Pb zircon chronology have been used to determine the provenance of Archean greenschist-facies siliciclastic sedimentary rocks of the Diemals Formation in the Marda – Diemals area of the central Yilgarn Craton, Western Australia. Field evidence shows that these siliciclastic rocks are, at least in part, derived from uplift and erosion of underlying greenstones, and this is borne out by the similar La/Sc, Cr/Th and REE chemistry of Diemals Formation siltstones and some sandstones to mafic volcanic rocks of the underlying greenstones. The higher Cr/V and lower Y/Ni of some siltstones is consistent with input from ultramafic and mafic rocks. Diemals Formation sandstones and siltstones cannot be separated in terms of ratios such as Zr/La, and siliciclastic rock chemistry reflects provenance rather than the effects of transport and depositional processes, such as sorting. Chemistry does not support input to Diemals Formation sedimentary rocks from the Marda volcanic complex despite both units being close to each other, and having overlapping maximum depositional and crystallisation ages, respectively. Instead, it is likely that detritus for the two units was deposited in adjacent, physically discrete basins. Some Diemals Formation sandstones are geochemically similar to felsic rocks intruding the underlying greenstone succession, with higher La/Sc and lower Cr/Th, and LREE-enriched patterns with negative Eu anomalies. Support for a genetic relationship is shown by the overlap in the maximum depositional age of these sandstones with the crystallisation age of the geochemically identical Pigeon Rocks Monzogranite. Combined whole-rock chemistry and precise U – Pb zircon chronology indicates that Diemals Formation sedimentary rocks were in large part derived from the underlying mafic volcanic rocks, with progressive unroofing of this succession leading to erosion of felsic intrusive rocks, now represented by sandstones found at various levels in the Diemals Formation. 相似文献