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1.
Time series analysis of the Cenomanian through Aptian interval, in well logs from offshore Adriatic boreholes, facilitates a cycle-by-cycle correlation with measured sections of equivalent age in Umbria, central Italy. The cycle-by-cycle match can be demonstrated between filtered spontaneous potential logs and filtered bed thickness data, but is unequivocal only at wavelengths of greater than 10 metres, which are believed to represent a 2 Myr eccentricity signal; at shorter wavelengths tool resolution becomes a limiting factor. As a result of low sedimentation rates the 100, 41, and 21 kyr Milankovitch signals all appear to be aliased to longer wavelengths in the sonic and gamma ray log records and tend to blend with the 400 kyr signal; this is especially true in the Aptian-Albian interval. Attempts to filter and correlate on the bases of these shorter wavelengths were therefore unsuccessful.
Accepting a 2 Myr duration for each cycle permits an estimation of less than 7 Myr to be made for the duration of the Cenomanian Stage.  相似文献   

2.
A high-resolution, biostratigraphic (calcareous nannofossils, calpionellids), chemostratigraphic (C-isotope) and cyclostratigraphic (magnetic susceptibility) study was performed on the marl–limestone alternations of the Upper Berriasian–Valanginian Orpierre section, deposited in the hemipelagic setting of the Vocontian Basin (SE France). The main aims of this study were to detect orbital forcing, to estimate the duration of the Valanginian “Weissert” episode, and to discuss the palaeoenvironmental implications. Detailed calcareous nannofossil biochronology allowed the recognition of Upper Berriasian–Lower Hauterivian biohorizons. The general trends of the δ13C curve and the major positive C-isotope excursion (amplitude of 1.8‰) recorded at the Orpierre section are very similar to those found in other sections worldwide. Spectral analysis applied on high-resolution magnetic susceptibility (MS) variations coupled with frequency ratio method reveals a strong cyclic pattern related to the Earth's orbital parameters (precession, obliquity and eccentricity). The prominent 405 kyr eccentricity cycle in the MS signal and has been used as a geochronometer to time calibrate the section. The duration of the Weissert episode was estimated as 2.08 Myr. This duration is coherent with those obtained from the Umbria Marche Basin (∼2.3 Myr) and from the Angles section (∼2.14 Myr). Durations of more than 2 Myr suggest that a long perturbation in the dynamic of the global carbon cycle that is not compatible with the rapid and intense volcanic activity. Our cyclostratigraphic study indicates a minimal duration of 4.29 Myr for the Valanginian stage. The comparison between the MS signal at Orpierre and the spectral gamma-ray signal recently obtained in two composite sections of the Vocontian Basin allows to propose a revised duration for the Valanginian stage of 4.695 Myr instead of 5.08 Myr.  相似文献   

3.
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4.
A continuous terrestrial succession was recovered from the Songke-2(SK-2) borehole in the Songliao Basin, Northeastern China. This borehole provides a unique material for further research on the continental paleoclimate during Cretaceous greenhouse period, following a series of achievements of the Songke-1(SK-1) core. In this study, thorium(Th) logging data were chosen as a paleoclimate proxy to conduct a detailed cyclostratigraphic analysis. The Th series varies quasi-periodically; power spectra and evolutionary fast Fourier transformation(FFT) analysis reveal significant cycles in the Quantou(K2 q), Qingshankou(K2 qn), Yaojia(K2 y) and Nenjiang(K2 n) formations. The ratio of cycle wavelengths in these stratigraphic units is approximately 20:5:2:1, corresponding to long orbital eccentricity(405 kyr), short orbital eccentricity(100 kyr), obliquity(37 kyr), and precession cycles(22.5 kyr and 18.4 kyr). The durations of the K2 n, K2 y, K2 qn and K2 q are estimated as 6.97, 1.83, 5.30 and 4.52 Myr, respectively, based on the constructed ～18.62 Myr "floating" astronomical time scale(ATS). Comparison of the durations between the SK-1 s and SK-2 boreholes exhibits a slight difference of 0.06 Myr and 0.459 Myr for K2 qn and K2 y. Nevertheless, our ATS of K2 n supports the chronostratigraphic frame constructed by the CA-ID-TIMS data of the SK-1 s borehole. This new "floating" ATS provides precise numerical ages for stratigraphic boundaries, biozones and geological events in the Songliao Basin, and can serve as a basis for correlation of strata and events between marine and terrestrial systems.  相似文献   

5.
The Upper Permian (Zechstein) slope carbonates in the Roker Formation (Zechstein 2nd‐cycle Carbonate) in North‐east England consist of turbidites interbedded with laminated lime‐mudstone. Studies of turbidite bed thickness and relative proportion of turbidites (percentage turbidites in 20 cm of section) reveal well‐developed cyclicities consisting of thinning‐upward and thickening‐upward packages of turbidite beds. These packages are on four scales, from less than a metre, up to 50 m in thickness. Assuming that the laminae of the hemipelagic background sediment are annual allows the durations of the cycles to be estimated. In addition, counting the number and thickness of turbidite beds in 20 cm of laminated lime‐mudstone, which is approximately equivalent to 1000 years (each lamina is 200 μm), gives the frequencies of the turbidite beds, the average thicknesses and the overall sedimentation rates through the succession for 1000 year time‐slots. Figures obtained are comparable with modern rates of deposition on carbonate slopes. The cyclicity present in the Roker Formation can be shown to include: Milankovitch‐band ca 100 kyr short‐eccentricity, ca 20 kyr precession and ca 10 kyr semi‐precession cycles and sub‐Milankovitch millennial‐scale cycles (0·7 to 4·3 kyr). Eccentricity and precession‐scale cycles are related to ‘highstand‐shedding’ and relative sea‐level change caused by Milankovitch‐band orbital forcing controlling carbonate productivity. The millennial‐scale cycles, which are quasi‐periodic, probably are produced by environmental changes controlled by solar forcing, i.e. variations in solar irradiance, or volcanic activity. Most probable here are fluctuations in carbonate productivity related to aridity–humidity and/or temperature changes. Precession and millennial‐scale cycles are defined most strongly in early transgressive and highstand parts of the larger‐scale short‐eccentricity cycles. The duration of the Roker Formation as a whole can be estimated from the thickness of the laminated lithotype as ca 0·3 Myr.  相似文献   

6.
A high-resolution CaCO3 record in combination with a detailed magnetostratigraphy is presented for the rhythmically bedded marly sequence of the Pliocene Trubi Formation on Sicily (Italy).
Magnetostratigraphic age calibration and subsequent spectral analysis of the variatrions in CaCO3 content yield a remarkably consistent discrepancy between the resulting periodicities and the corresponding quasi-periods of the Earth's orbital cycles. This discrepancy can only be explained by assuming that ages assigned to polarity reversal boundaries in the geomagnetic polarity time-scale lack sufficient precision.
A new polarity time-scale is presented for the major part of the Gilbert and Gauss Chron using the radiometrically dated Gilbert-Gauss Chron boundary at 3.40 Ma as an age reference point and extrapolating an average quasi-period of 21.7 kyr of the precession cycle as the periodicity of small-scale colour annex CaCO3 cycles.  相似文献   

7.
Foreland basin growth strata are ideal recorders of deformation rates and kinematics in tectonically active regions. This study develops a high-resolution chronostratigraphic age model to determine folding rates in the Eocene-Oligocene terrestrial growth strata of the Berga Conglomerate Group, NE Spain. The Berga Conglomerate Group was sampled for rock magnetic, magnetostratigraphic, and magnetic susceptibility (χ) cyclostratigraphy analyses. Analysis of rock magnetic measurements indicate a mixed mineral assemblage with both paramagnetic and ferromagnetic minerals. A new magnetic reversal stratigraphy constrains the time frame of folding and is in agreement with previous interpretations. Time series analysis of χ variations show statistically significant power at expected orbital frequencies and provides precession-scale (20 kyr) temporal resolution. Strain measurements including anisotropy of magnetic susceptibility (AMS) fabrics and bedding plane strain worm burrow distortion are consistent with fixed hinge, flexural folding kinematics. Fault-related folding was modeled using χ cyclostratigraphy timing and strain measurement kinematic constraints. The onset of folding was at 33.85 Ma and the end of deformation is less constrained but is younger than 31.06 Ma. Deformation and sediment accumulation rates are unsteady at 20 kyr time scales but appear artificially steady at polarity chron time scales.  相似文献   

8.
《Gondwana Research》2014,26(4):1348-1356
The Early Toarcian Oceanic Anoxic Event (OAE) in the Early Jurassic Period is associated with a major negative carbon isotope excursion (CIE), mass extinction, marine transgression and global warming. The Toarcian OAE is thought to have been caused by flood basalt magmatism, and may have been a trigger for mass extinction. However, these proposed causes of the Toarcian OAE and associated biotic crisis are not adequately resolved by a precise chronology. The duration of the Toarcian OAE has been estimated to be anywhere from ~ 0.12 to ~ 0.9 Myr, most recently 0.74 to 3.26 Myr from U–Pb dating. The CIE associated with the Toarcian OAE has a similar pattern at numerous localities, and there is evidence that the marine carbon isotope variations recorded astronomical forcing signals. Here we estimate a duration of ~ 620 kyr for the main negative CIE, ~ 860 kyr for the polymorphum zone and > 1.58 Myr for the levisoni zone based on 405-kyr astronomical eccentricity tuning of the marine section at Peniche (Portugal). This 405-kyr tuned series provides a ~ 2.5 Myr continuous high-resolution chronology through the Early Toarcian. There are 6, or possibly 7 short eccentricity cycles in the main CIE interval at Peniche. To confirm this astronomically based estimate, we analyzed three other sections at Yorkshire (UK), Dotternhausen (Germany), and Valdorbia (Italy) from marine carbon isotopic series. These four stratigraphic sections from Early Jurassic western Tethys record the Toarcian OAE with ~ 6 prominent carbon isotope cycles in the CIE that span a 600 ± 100 kyr duration. The Peniche 405-kyr tuned series indicates that the pre- and post-CIE intervals experienced strong precession–eccentricity-forced climate change, whereas the CIE interval is marked by dominant obliquity forcing. These dramatic and abrupt changes in astronomical response in the carbon isotopes point to fundamental shifting in the Early Toarcian paleoclimate system that was directly linked to the global carbon cycle.  相似文献   

9.
The Aptian-Albian 'Scisti a Fucoidi' varicoloured pelagic sediments in central Italy, show a 'couplet' alternation of carbonate-rich/carbonate-poor layers, which are interpreted as the sedimentary expression of precession (frequency 19–23 kyr). Carbonate content, chromatic variation, and planktonic foraminiferal abundance were analysed at a 1-cm spacing for a 10-m interval of the Piobbico core, specifically drilled through this formation. Spectral analysis of these parameters shows a prominent signal equated to the c . 100 kyr cycle of orbital eccentricity at a sedimentation rate of 5 mm kyr−1. The coherency of the spectral response of each parameter suggests that a single mechanism controlled the whole sedimentary record. Detailed study of planktonic foraminiferal distribution of the same section at 1-mm scale resolves the Milankovitch frequencies of 41 kyr and 18 to 23 kyr, equated with the obliquity and precessional cycles. But foraminiferal abundance is not in phase with carbonate content, which was largely controlled by calcareous nannofossils, but peaked at intermediate carbonate values. The proposed model for explaining the discrepancy at the precessional level is that foraminifera thrived at intermediate values of the precession index, when the environment was only moderately fertile but stable, while during highs of the precession index, mixing of the water column increased fertility and caused calcareous nannofossil blooms and restriction of planktonic foraminifera to few and tolerant species. The resulting bimodality of foraminiferal abundance per precessional cycle appears to be recorded in the spectrum by peaks at the 11 and 14 kyr levels. Cross correlation of foraminiferal abundances with the calcium carbonate curve over 1–2 Myr intervals produces discrepant results (apparent phase lags) which we attribute to differences in the response to the fundamental eccentricity cycles.  相似文献   

10.
High resolution(939 samples)total organic carbon content(TOC)analyses were conducted on the Shuanghe Section of^152.6 m in the Changning area,Sichuan Basin.The sampling section was divided into two units considering the distinct-different deposit environment and sediments accumulation rate.The lower part(Unit 1)and the peer part(Unit 2)with high resolution sample spacing(0.08–0.4 m)enables the identification of the precession cycle in two sedimentary sequences with distinct different sedimentary accumulation rate.MTM Power spectral analyses on untuned TOC series reveals significant peaks exceeding above the 95%confidence level and shows that both Unit 1 and Unit 2 have recorded Milankovitch cycles of 405 kyr long eccentricity,short eccentricity,obliquity and precession.The floating astronomical time scale(ATS)was constructed on the Shuanghe Section in the Early Silurian(~439.673–444.681 Ma),and which was calibrated by 405 kyr long eccentricity cycles.The total duration of the Wufeng and Longmaxi shales is 5.01 Myr.The floating ATS used for estimating the duration of the graptolite zones and each stage in the study interval.Finally,we postulated two models that could verify the linkage between orbital cycle and organic accumulation.To make sure whether productivity or preservation is the main factor that under long eccentricity control,the phase correlation between the obtained filtered signal and the theoretical orbital solution should be made clear in the further research.  相似文献   

11.
An integrated study of the ammonites, inoceramid bivalves, planktonic foraminifera, calcareous nannofossils, geochemistry, stable carbon isotopes, and cyclostratigraphy is provided for the upper Middle to upper Upper Albian sucession exposed in the Col de Palluel section east of Rosans in Hautes-Alpes, France. The Albian-Cenomanian boundary interval described by Gale et al. at Mont Risou is re-examined, a total thickness of 370 m of the Marnes Bleues Formation. Zonal schemes based on ammonites, inoceramid bivalves, planktonic foraminifera, and calcareous nannofossils are integrated with the stable carbon isotope curve and key lithostratigraphic markers to provide a sequence of more than 70 events in the uppermost Middle Albian to basal Cenomanian interval. Time series analysis of the Al2O3 content of the 500 m Albian sequence present in the Col de Palluel and Risou sections reveals the presence of the 20 kyr precession, 40 kyr tilt, 100 kyr short eccentricity, and 406 kyr long eccentricity cycles. Correlation using planktonic foraminiferan and nannofossil data provide a link between the Col de Palluel and Risou sections and the Italian sequence at Gubbio, and in the Piobbico core. This provides a basis for the extension of the orbital time scale of Grippo et al. to the sequence. It reveals a major break in the Col de Palluel succession at the top of the distinctive marker bed known as the Petite Vérole that may represent as much as 2 Ma. It also provides a basis for the estimation of the length of the Albian Stage at 4.12 Ma, 0.8 Ma for the early Albian, 2.84 Ma for the Middle Albian, and 3.68 Ma for the late Albian substages.  相似文献   

12.
曹海洋  王华  赵睿 《地球科学》2017,42(12):2299-2311
在旋回地层学研究中,高分辨率地球化学数据作为一重要的替代指标,其一般获取方式（ICP-MS或WD-XRF）较耗费时间和经费,而手持X射线衍射仪（ED-XRF）能够高效的解决这一问题,目前将ED-XRF运用在沉积地层研究中,尤其是旋回地层学研究领域中较少.以酒泉盆地青西凹陷早白垩世下沟组深湖相泥岩、白云质泥岩为研究对象,将连续采集的岩心样品通过ED-XRF测试,该测试方法能够保证在不破坏样品的前提下,并在误差允许的范围内的条件下,短时间内（5 min）完成对单一样品的测试.通过测试精确得出24种主量及微量元素的含量,将得出的地球化学特征作为旋回地层学研究的替代指标,并针对K/Al,Ca/Al,Si/Al,Fe/Al,Ti/Al,Mn/Al,Ni/Al,Zn/Al和Pb/Al的比值进行频谱分析.数据表明W2井下沟组一段（SQK1g1）地层旋回性表现出与天文旋回明显的对应关系.沉积地层中的旋回厚度比值为20.25:4.75:1.75:1.00,与米兰科维奇旋回中长偏心率、短偏心率、斜率和岁差的周期比值:400 ka:95 ka:37 ka:20 ka相对应.该地区的沉积记录受天文轨道周期的影响得到了证明,与此同时,为手持X射线衍射仪在旋回地层学中的应用开辟了新领域.   相似文献   

13.
Well‐exposed Mesozoic sections of the Bahama‐like Adriatic Platform along the Dalmatian coast (southern Croatia) reveal the detailed stacking patterns of cyclic facies within the rapidly subsiding Late Jurassic (Tithonian) shallow platform‐interior (over 750 m thick, ca 5–6 Myr duration). Facies within parasequences include dasyclad‐oncoid mudstone‐wackestone‐floatstone and skeletal‐peloid wackestone‐packstone (shallow lagoon), intraclast‐peloid packstone and grainstone (shoal), radial‐ooid grainstone (hypersaline shallow subtidal/intertidal shoals and ponds), lime mudstone (restricted lagoon), fenestral carbonates and microbial laminites (tidal flat). Parasequences in the overall transgressive Lower Tithonian sections are 1–4·5 m thick, and dominated by subtidal facies, some of which are capped by very shallow‐water grainstone‐packstone or restricted lime mudstone; laminated tidal caps become common only towards the interior of the platform. Parasequences in the regressive Upper Tithonian are dominated by peritidal facies with distinctive basal oolite units and well‐developed laminate caps. Maximum water depths of facies within parasequences (estimated from stratigraphic distance of the facies to the base of the tidal flat units capping parasequences) were generally <4 m, and facies show strongly overlapping depth ranges suggesting facies mosaics. Parasequences were formed by precessional (20 kyr) orbital forcing and form parasequence sets of 100 and 400 kyr eccentricity bundles. Parasequences are arranged in third‐order sequences that lack significant bounding disconformities, and are evident on accommodation (Fischer) plots of cumulative departure from average cycle thickness plotted against cycle number or stratigraphic position. Modelling suggests that precessional sea‐level changes were small (several metres) as were eccentricity sea‐level changes (or precessional sea‐level changes modulated by eccentricity), supporting a global, hot greenhouse climate for the Late Jurassic (Tithonian) within the overall ‘cool’ mode of the Middle Jurassic to Early Cretaceous.  相似文献   

14.
Two parallel drilling cores, BDP96-1 (200 m), and BDP96-2 (100 m), have been taken from top of the underwater Akademichesky Ridge in Lake Baikal and dated by paleomagnetic techniques. These cores are part of the Baikal Drilling Project (Kuzmin et al., 1997a, b). Diatom analysis of the first 100 m of these sediments reveals that they store a 2.5 Myr high-resolution continuous record of the palaeoclimates of East Siberia. Sediments belonging to warmer climates have a high content of diatom algae frustules, whereas those belonging to global glaciations are diatom-barren. The record of Upper Pleistocene (500 kyr BP) strongly correlates with the oceanic SPECMAP curve and contains distinct 100, 41, 23, and 19 kyr periods, as revealed by Fourier analysis. Diversity of diatom species was small in Pliocene. Abrupt and frequent changes of the climate in Pleistocene resulted in frequent dramatic changes in the diatom communities. The extant dominating endemics, Cyclotella minuta and Aulacoseira baicalensis, became important in Lake Baikal 760 kyr and less than 120 kyr BP, respectively.  相似文献   

15.
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   

16.
In this study, the clay and heavy mineral analysis of ODP Site 911 sediments is used to investigate the sources and transport mechanisms (sea ice and oceanic currents) of sediments in the Arctic Ocean during the Mid to Late Pliocene (3.10–2.78 Myr) and upper Quaternary (800 kyr to the present). The time period between 3.10 and 3.00 Myr is characterized by a decreasing smectite and increasing illite content, which is interpreted as reflecting cooling conditions. At the beginning of the Mid-Pliocene Global Warmth period at ∼3.00 Myr, the smectite content shows an abrupt increase. This change can also be seen as a drop in the amount of kaolinite and TOC. After 3.00 Myr the kaolinite and TOC values start to increase, probably indicating high rates of reworked glacially eroded matter. During the Pleistocene, smectite shows a lower and illite a higher fluctuation level compared with the Pliocene. This might be due to reigning glacial conditions during the Pleistocene, when the freshwater input was much lower than during the Pliocene. During the Pliocene, the fluctuating heavy minerals might reflect changes in freshwater input from the great Siberian rivers, which would have led to changes in the supply of terrigenous material delivered to the shelf by the rivers. The heavy mineral fluctuation also reflects changes in the amount of sea-ice formation, which correlates with climate variations and the freshwater input from the continent. Based on the composition of the clay and heavy mineral groups in this study, the most likely transportation path is the Siberian branch of the Transpolar Drift.  相似文献   

17.
Oceanic crust production and climate during the last 100 Myr   总被引：2，自引：0，他引：2  
In order to evaluate the possible influence of oceanic crust production on climatic changes during the past 100Myr variations in total oceanic crust for this period including production at mid-ocean ridges, oceanic plateaus, and back-arc basins were calculated using the most recent and accurate time-scales. The rates presented here differ from those of Larson (1991a, b) on Cenozoic fluctuations and show that (1) maximum production values occurred during the Cenomanian, Palaeocene, and late Oligocene-early Miocene and (2) minimum values occurred in Campanian-Maastrichtian, late Eocene, and middle Miocene. Significantly, variations of oceanic crust production correspond with variations in the δ18O of deep-water benthic foraminifera: maximum values of oceanic crust production correspond with minimum values of δ18O, and minimum production values with maximum values of δ18O. This latter synchronism suggests that changes in land-sea relationships and atmospheric CO2 related to major fluctuations in oceanic crust production were the main cause of mid-Cretaceous warming and Late Cretaceous cooling, and of climatic quasi-cycles having a periodicity of 33–38 million years over the last 100 Myr. This is the first report showing variations of ocean crust production synchronized with the Cenozoic climate changes.  相似文献   

18.
The Kaskapau and Cardium Formations span Late Cenomanian to Early Coniacian time and were deposited on a low‐gradient foredeep ramp. The studied portion of the Kaskapau Formation spans ca 3·5 Myr and forms a mudstone‐dominated wedge thinning from 700 to <50 m from SW to NE over ca 300 km. In contrast, the Cardium Formation spans about 2·1 Myr, is about 100 m thick, sandstone‐rich and broadly tabular. The Kaskapau and Cardium Formations are divided, respectively, into 28 and nine allomembers, each bounded by marine flooding surfaces. Kaskapau allomembers 1 to 7 show about 200 km of offlap from the forebulge, accompanied by progradation of thin sandstones from the eroded forebulge crest. In contrast, Kaskapau allomembers 8 to 28 and Cardium allomembers C1 to C9 show overall onlap onto the forebulge of about 350 km, and contain no forebulge‐derived sandstones. This broad pattern is interpreted as recording a latest Cenomanian pulse of tectonic loading which led to shoreline back‐step in the proximal foredeep and coeval uplift of the forebulge, leading to erosion. The advance of the sediment wedge after Kaskapau allomember 7 is attributed primarily to the isostatic effect of a distributed sediment load; the advance of the orogenic wedge had a subordinate effect on subsidence of the forebulge. For Kaskapau allomembers 1 to 6, isopachs trend north to south, suggesting a load directly to the west; allomembers 7 to 28 show an abrupt rotation of isopachs to NW–SE, suggesting that the load shifted several hundred kilometres to the south. This re‐orientation might be related to a change from an approximately orthogonal to a dextral transpressive stress regime. Within the longer‐term offlap–onlap cycle recorded by the Kaskapau and Cardium Formations, individual allomembers are grouped into packages reflecting higher‐frequency onlap–offlap cycles, each spanning ca 0·5 to 0·7 Myr. Offlap from the forebulge tends to be accompanied by more pronounced transgression in the foredeep, whereas onlap onto the forebulge is accompanied by progradation of tongues of shoreface sandstone. This relationship suggests that changes in deformation rate in the orogenic wedge modulated proximal subsidence rate, enhancing or suppressing shoreline progradation, and also causing subtle uplift or subsidence of the forebulge region. Wedge‐shaped allomembers in the Kaskapau Formation contain shoreface sandstone and conglomerate that prograded, respectively, <40 and <25 km from the preserved basin margin; progradation of coarse clastics was limited by rapid flexural subsidence. Tabular allomembers of the Cardium Formation imply a low flexural subsidence rate and contain sandy and conglomeratic shoreface deposits that prograded up to ca 180 km from the preserved basin margin. This relationship suggests that low rates of flexural subsidence promoted steeper alluvial gradients, more vigorous gravel transport and more extensive shoreface progradation. Overall, observed stratal geometry and facies distribution is explained readily in terms of current elastic flexural models. Most shoreface sandstones in the proximal foredeep show evidence of forced regression. Eustasy provides the most plausible explanation for relative sea‐level rise–fall cycles on the 125 kyr allomember timescale. Geometric relationships suggest eustatic oscillations of about 10 m. Forced regressive shoreface development was suppressed during Kaskapau allomembers 1 to 10 when the rate of flexural subsidence was at its highest.  相似文献   

19.
Based on detailed analyses of facies evolution and stacking pattern of Berriasian carbonate-dominated sections in the Swiss Jura Mountains, the Swiss Ultrahelvetic and the French Vocontian Trough, a high-resolution platform-to-basin correlation is proposed. Biostratigraphical tie points are furnished by ammonites, dinoflagellates, calpionellids, and ostracod-charophyte assemblages. The hierarchical stacking of small-scale depositional sequences reflects Milankovitch cyclicity: sequences corresponding to the 20-, 100-, and, locally, 400-ka orbital cycles can be identified. Elementary (20 ka) sequences on the platform generally consist of one bed of shallow subtidal to intertidal, high- or low-energy carbonate facies, whereas on the slope and in the basin they are commonly developed as limestone-marl couplets. These elementary sequences group into small-scale composite sequences reflecting the first orbital eccentricity cycle (100 ka), which in turn build up large-scale (3rd-order) composite sequences. One 3rd-order sequence has been analysed in detail: according to the cyclostratigraphic interpretation, it took ?2 Myr to form, which is in accordance with the duration of the corresponding ammonite subzones. Sequence-stratigraphic and cyclostratigraphic platform-to-basin correlation shows that for about 900 ka the platform was exposed or only partly flooded, whereas on the slope and in the basin, lowstand deposits with channel fills and slumps accumulated. With rising sea level, accommodation space gradually increased on the platform and a thickening-upward sequential pattern with transgressive facies developed, while in the basin the facies still had lowstand characteristics with thick and nodular limestone beds. This situation lasted about 700 ka. The following 300 ka were characterized by sediment starvation and increased bioturbation on the platform, and by more marly, transgressive sediments on the slope and in the basin. The maximum-flooding phase is more or less isochronous on the platform and in the basin, although the surface with the best-developed maximum-flooding features may be displaced by one or two small-scale composite sequences because of superimposed high-frequency sea-level fluctuations, and/or local variations in substrate morphology and sediment distribution. Third-order highstand conditions prevailed for only about 100 ka. The combination of cyclostratigraphy and high-resolution sequence stratigraphy, constrained by good biostratigraphy, is thus a powerful tool for detailed stratigraphical correlation over long distances and from one sedimentary environment to another.  相似文献   

20.
青藏高原中部尼玛盆地是一个形成于班公湖-怒江缝合带之上的新生代陆相裂谷盆地。交通不便和勘探程度较低等因素导致其高精度沉积旋回特征和地层学一直鲜有研究。尼1井是盆地内第一口地质探井,通过对该井古近系牛堡组三段的自然伽马测井曲线进行频谱分析,证实了天文轨道周期对盆地内湖相地层的沉积具有显著影响。滤波分析显示牛堡组三段保存了41个短偏心率(～100 kyr)周期,76个斜率(～54 kyr)周期和178个岁差(～23 kyr)周期,平均沉积速率为92.03m/Ma,沉积时限～4.1 Myr。此外,结合Fischer图解与总有机碳含量的变化趋势,确定牛堡组三段湖平面经历了1次长周期的升-降变化过程。综合分析,认为尼玛盆地优质烃源岩的形成主要受天文旋回周期影响下的湖平面变化所控制。  相似文献   

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