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71.
华北地层大区寒武纪早期地层统一划分与对比问题 总被引:1,自引:0,他引:1
基于华北地层大区数百条寒武纪早期地层剖面与安徽淮南地区基准层序(2组2亚组15段1个亚段)和带序(6个三叶虫带)的确切对比,编制了新的沉积分区图;依据有无猴家山组和馒头组下亚组是否完整,划分出5个沉积区;讨论了各区与基准层序-带序的对比;明确了各地方性岩石地层单位(五山组、李官组、碱厂组、辛集组、朱砂洞组、苏峪口组、五道淌组、昌平组、府君山组、老庄户组、水洞组、黑沟组等)的层位归属(具体至段),确认与寒武纪早期地层有关的3个沉积间断(皖北上升、豫陕上升和燕辽上升),归属于重新厘定的蓟县运动的3个期次. 相似文献
72.
通过对保留在沉积物中与水动力条件有密切联系的原生沉积构造的观察和研究,分析了淮北煤田芦岭井田二叠纪含煤岩系属于以河流作用为主的建设性三角洲沉积环境,阐明了根据原生沉积构造序列解释沉积环境的基本方法. 相似文献
73.
云南鹤庆盆地近1Ma来的孢粉植物群很好地揭示了本区的环境演化过程。本文利用植物生态位知识,通过模糊数学方法对鹤庆(QH)孔孢粉序列所记录的古气候信息进行重塑,建立了近1Ma来的年均温、年降水量及年较差序列,并将本区气候演化分为4个气候旋回,32个气候期。该序列可与黄土-古土壤序列及海洋δ18O阶段对比,但冷暖波动的幅度及降温时段持续的长短有其特色,表明本区气候既受全球气候变化的控制,又受青藏高原阶段隆升的制约;近1Ma来气温的波动幅度呈递增趋势,0.30MaB.P.尤其显著;冷、暖段间年均温的变化幅度由第一气候旋回的7℃增大到第三气候旋回的9℃(按4次平滑值计算),而样点的实际最大差值由8℃增至16℃;最强烈的降温发生在第二气候旋回中的0.45~0.14MaB.P.时间段,这与玉龙雪山冰川活动规模最大相一致;降温时年降水量及年较差值增大,证实近1Ma来,西南季风区的气候组构与东南季风区不同。 相似文献
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76.
The Upper Cretaceous Juniper Ridge Conglomerate (JRC) near Coalinga, California, provides a rare, high-quality exposure of a submarine channel to overbank transition. The facies architecture of the JRC comprises a thick, predominantly mudstone sequence overlain by a channellized conglomerate package. Conglomeratic bounding surfaces truncate successions of interbedded turbiditic sandstones and mudstones both vertically and laterally. Thick-bedded, massive sandstones are interbedded with conglomerates. Facies architecture, palaeocurrent indicators, slump features, sandstone percentages and sandstone bed thickness trends lead to the interpretation that these elements comprise channel and overbank facies. A vertical sequence with conglomerate at the base, followed by thick-bedded sandstone, and capped by interbedded turbiditic sandstone and mudstone form a fining-upward lithofacies association that is interpreted as a single channel-fill/overbank system. Three similar lithofacies associations can be related to autocyclic processes of thalweg migration and submarine fan aggradation or to allocyclically driven changes in sediment calibre. 相似文献
77.
Spatial variation of overbank aggradation rate and its influence on avulsion frequency 总被引:4,自引:0,他引:4
Abstract River avulsions are commonly considered to be driven by the aggradation and growth of alluvial ridges, and the associated increase in cross‐valley slope relative to either the down‐channel slope or the down‐valley slope (the latter is termed the slope ratio in the present paper). Therefore, spatial patterns of overbank aggradation rate over stratigraphically relevant time scales are critical in avulsion‐dominated models of alluvial architecture. Detailed evidence on centennial‐ to millennial‐scale floodplain deposition has, to date, been largely unavailable. New data on such long‐term overbank aggradation rates from the Rhine–Meuse and Mississippi deltas demonstrate that the rate of decrease of overbank deposition away from the channel belt is much larger than has been supposed hitherto, and can be similar to observations for single overbank floods. This leads to more rapid growth of alluvial ridges and more rapid increase in slope ratios, potentially resulting in increased avulsion frequencies. A revised input parameter for overbank aggradation rate was used in a three‐dimensional model of alluvial architecture to study its effect on avulsion frequency. Realistic patterns of avulsion and interavulsion periods (≈1000 years) were simulated with input data from the Holocene Rhine River, with avulsions occurring when the slope ratio is in the range 3–5. However, caution should be practised with respect to uncritical use of these numbers in different settings. Evidence from the two study areas suggests that the avulsion threshold cannot be represented by one single value, irrespective of whether critical slope ratios are used, as in the present study, or superelevation as has been proposed by other investigators. 相似文献
78.
西藏层序地层研究进展 总被引:2,自引:0,他引:2
迄今为止,西藏区内所开展的层序地层分析主要涉及到中生代的二级或三级层序。基于露头剖面的沉积相、古生物、磁性地层以及地质事件的综合研究,探讨了三叠纪、侏罗纪和白垩纪的层序地层模式及其形成机制。现今所取得的进展主要表现在3个方面:①初步建立了区内中生代层序地层年代格架;②不同时代、不同类型盆地中的层序数量、结构与层序类型具有较大的差异;③层序地层与磁性地层的结合研究,取得了重要的成果。未来区内的层序地层研究应在3个方面展开;①高分辨率层序地层研究,树立株罗纪-第三纪海平面变化曲线;②探索层序地层填图新方法;③多岛弧造山模式和多机制的隆升模式等大陆动力学问题也是层序地层所遇到的新挑战。 相似文献
79.
南秦岭横丹浊积岩系是一套巨厚的浊流沉积,以向南或南东倾的单斜构造产出。由下而上,该沉积层序包括深水盆地、深水浊积扇和斜坡水道3个相序。相应地,沉积物粒度变粗,厚度变大,火山质组分含量增加,凝灰层大量发育,表明横丹浊积岩系为活动型浊积岩;其古水流方向为NNW—NNE向,物源区为南侧的碧口火山岩系。另外,横丹浊积岩系内还见石英岩、重结晶大理岩成分的砾石,说明其物源还包括被动陆缘环境的沉积物。相序、组构、沉积特征和物源区综合分析表明,横丹浊积岩系为弧前盆地充填物。构筑这一弧前盆地的动力学机制是洋壳板块向南俯冲于扬子板块被动陆缘之下,时代可能晚于中晚泥盆世。 相似文献
80.
Sub-wavebase cross-bedded grainstones on a distally steepened carbonate ramp, Upper Miocene, Menorca, Spain 总被引:1,自引:0,他引:1
Cross‐bedded grainstones on carbonate ramps and shelves are commonly related to the locus of major wave energy absorption such as shorelines, shoals or shelf breaks. In contrast, on the Early Tortonian carbonate platform of Menorca (Balearic Islands), coarse‐grained, cross‐bedded grainstones are found at a distance from the palaeoshoreline where they were deposited below the wavebase. Excellent exposures along continuous outcrops on the sea cliffs of Menorca reveal the depositional profile and three‐dimensional distribution of the different facies belts of the Tortonian ramp depositional system. Basinward from the palaeoshoreline, fan deltas and beach deposits pass into 5‐km‐wide gently dipping bioturbated dolopackstone (inner and middle ramp), then into 12–20°‐dipping dolograinstone/rudstone clinobeds (ramp slope) and, finally, into subhorizontal fine‐grained basinal dolowackestone to dolopackstone (outer ramp). In this Miocene example, coarse‐grained grainstones exist in five different settings other than beach deposits: (1) on the middle ramp, where cross‐bedded grainstones were deposited by currents roughly parallel to the shoreline at 40–70 m estimated water depth and are interbedded with gently dipping bioturbated dolomitized packstones; (2) on the upper slope, where clinobeds are composed mostly of in situ rhodoliths and red‐algae fragments; (3) on the lower slope, as small‐scale bedforms (small three‐dimensional subaqueous dunes) migrating parallel to the slope; (4) at the transition between the lower slope and the outer ramp, where mollusc‐rich and rhodolithic rudstones and grainstones, interbedded in dolomitized laminated wackestones containing abundant planktonic foraminifera, infill slide/slump scars as upslope‐backstepping bodies (backsets); (5) at the toe of the slope, where coarse skeletal grainstones indicate bedform migration parallel to the platform margin, induced by currents at more than 150 m estimated water depth. This Late Miocene example also illustrates how changes in intrabasinal environmental conditions (nutrients and/or temperature) may produce changes in stratal patterns and facies architecture if they affect the biological system. Two depositional sequences compose the Miocene platform on Menorca, where a reef‐rimmed platform prograded onto an earlier distally steepened ramp. The transition from the ramp to the reef‐rimmed platform was effected by an increase in accommodation space caused by ecological changes, promoting a shift from a grain‐ to a framework‐producing biota. 相似文献