Unravelling the microfacies signatures of parasequences using computer-optimized similarity matrices     

Guy H. Spence  Annie Arnaud-Vanneau†  Hubert Arnaud†  Maurice E. Tucker‡ 《Sedimentology》2004,51(6):1243-1271

The microfacies of a Lower Cretaceous carbonate drillcore from Oman are characterized using optimizing matrices of Jaccard's similarity coefficients of community. Other than systems tract boundaries, there is no obvious evidence of individual parasequences in the core. However, diagnostic patterns in microfossil distribution identify environmental gradients recording changes in water depth. These gradients are used to define individual parasequences, parasequence sets, stacking patterns and key surfaces. The patterns suggest that deposition was controlled by successive fourth‐ to fifth‐order (high‐frequency) relative sea‐level cycles superimposed on an underlying third‐order relative sea‐level rise. Although the correlation of these depositional subunits to systematic changes in water depth and the rate of carbonate accumulation alone is not incontrovertible proof of such a sea‐level control, concurrent multiorder relative sea‐level cyclicity provides by far the most likely explanation. A microfacies deposited when the water depth was shallowing is characterized by a relay of microfossils with affinities that shallow upwards. Conversely, a microfacies that records a gradual increase in water depth has a relay of microfossils with affinities that deepen upwards. Microfacies characterized by an assemblage of microfossils with similar affinities record deposition when the benthic environmental conditions remained stable, either because of an equilibrium between shallow water carbonate deposition and rising sea level, or in deeper water where sediment composition was relatively insensitive to changes of water depth. Microfacies characterized by mixed affinity assemblages record syndepositional reworking. During periods of embedded multiorder sea‐level changes, individual parasequences within systems tracts are shown to record more complex environmental gradients than simply the repetition of successive shallowing‐up units as traditionally represented in carbonate sequence stratigraphic models. The microfacies of an individual parasequence may shallow up, or may record both deepening‐up and shallowing‐up depositional phases, as well as periods of sedimentation when benthic environments remained stable. Individual parasequence boundaries may be submarine or subaerial unconformities, or be conformable, as part of a predictable stratigraphic pattern related to the temporal position of an individual parasequence within the underlying third‐order cycle of relative sea‐level change. The hitherto ubiquitous use of assemblages to describe carbonate microfacies, coupled with the widespread use of the metre‐scale shallowing‐up template to identify parasequences, may have led to such complexities previously being overlooked.  相似文献   

Prospects for interregional correlations using Wisconsin and Holocene aridity episodes, northern Gulf of Mexico coastal plain   总被引：1，自引：0，他引：1  

Ervin G Otvos 《Quaternary Research》2004,61(1):105-118

Luminescence dating of extensive dune fields and associated eolian sandsheets provided a chronology of recently recognized Pleistocene and early Holocene dry climate episodes in the currently humid warm temperate northern-northeastern Gulf of Mexico region. Scattered parabolic dunes and clusters of intersecting parabolic dunes, along with elongated shore-transverse and shore-parallel dunes, developed. These landforms occur in a 390-km-long and 2- to 3-km-wide, semicontinuous belt in southeast Alabama and northwestern Florida. Dune elevations reach ± 22 m. Sangamon coastal barrier sectors were the primary source of the eolian sand. Deflation was coeval with early Wisconsin to mid-Holocene marine low sea-levels and associated distant shorelines. Early Holocene dune dates were synchronous, with indications of a hypsithermal dry interval in southeast Louisiana, the Yucatan, and the south Atlantic seaboard. Overlapping with dry episodes in Yucatan and the High Plains, Texas dunes and Louisiana and Texas prairie mounds, especially in the southwest Texas coast still dominated by dry climate, suggests intervals of early to late Holocene drought. The dates provide the basis for identifying and correlating Wisconsin, early, and late Holocene climate phases between currently semiarid and humid, coastal and interior areas. They contribute to future studies, including interregional paleoclimate modeling. Although Pleistocene coastal eolian deposition coincided with glaciation in the northern interior and with cooler temperatures of a reduced Gulf of Mexico, Holocene aridity phases may have been related to major variations in the position of high-pressure cells, storm tracks, and branches of the jet stream, and even to prolonged La Niña conditions.  相似文献   

The Cenozoic Lower Rhine Basin – rifting, sedimentation, and cyclic stratigraphy     

A. Schäfer  T. Utescher  M. Klett  M. Valdivia-Manchego 《International Journal of Earth Sciences》2005,94(4):621-639

In the Cenozoic, the Lower Rhine Basin formed as a rift at the southeastern terminus of the Dutch German Central Graben, while the Rhenish Massif was uplifted. The study focusses on the marginal marine and fluvial fill of the Lower Rhine Basin. A basin model is developed. Support for this study was given by extensive industry outcrop and well data, by new stratigraphical and sedimentological observations. The ingression and subsequent regression of the Cenozoic North Sea is analysed using the concept of base level cyclicity. As the geohistory of the basin was complex, a subsidence curve is constructed. Furthermore, an attempt is made to trace the simultaneous uplift of the Rhenish Massif.  相似文献   

A Multi-cycle Climatic Fluctuation Record of the Last Interglacial Period:Typical Stratigraphic Section in the Salawusu River Valley on the Ordos Plateau,China   总被引：5，自引：0，他引：5  

LIBaosheng DavidDianZHANG WENXiaohao DONGYuxiang ZHUYizhi JINHeling 《《地质学报》英文版》2005,79(3):398-404

The Last Interglacial Period strata in the Milanggouwan section in the Salawusu River valley on the Ordos Plateau, China, have 8.5 sedimentary cycles composed alternately of eolian dune sands, fluvio-lacustrine facies and paleosols. Based on comprehensive analyses on the distribution of magnetic susceptibility and CaCO3 and paleo-ecology indicated by fossils in the region, it is considered that the sedimentation cycles resulted from dry-cold and warm-humid climate fluctuations. Magnetic susceptibility values and CaCO3 contents in stratigraphic sectors Ⅰ, Ⅲ, Ⅴ and Ⅱ, Ⅳ basically respectively present peaks and low vales, and the former three can in time correlate with MIS5a, MIS5c and MIS5e successively and the latter two with MIS5b and MIS5d. In addition, some horizons of eolian dune sands and the low vales of their magnetic susceptibility and CaCO3 are also correlated with 6 periods of cooling events indicated by the higher content of foraminifer Neogloboquadrina pachyderma (S.) documented in the V29-191 drill in the North Atlantic and the cold events recorded by δ^18O in the ice cores in GRIP, especially with 9 periods of dust events in Chinese Loess Plateau.  相似文献   

An unusual fluvial to tidal transition in the mesoarchean Moodies Group, South Africa: A response to high tidal range and active tectonics     

Kenneth A. Eriksson  Edward L. Simpson  Wulf Mueller 《Sedimentary Geology》2006,190(1-4):13-24

The Moodies Group in the Dycedale Syncline, Barberton Greenstone Belt consists of a 100–130 m-thick upward-fining succession that exhibits a transition from fluvial to tide-modified sedimentation. A basal, 10–30 m-thick conglomerate–sandstone interval of alluvial origin is overlain by stacked upward-fining, decimeter- to meter-scale cycles within which three facies are recognized: 1) conglomerate lag; 2) cross-bedded sandstone; and 3) interlaminated sandstone–siltstone and mudstone. Within the cycles, the abundance of mudstone drapes increases upwards. Structureless conglomerates and cross-beds lacking mudstone drapes record braided-alluvial processes. In contrast, cross-beds with mudstone drapes and interlaminated sandstone–siltstone and mudstone are products of flows modified by various tidal beats. Sand and/or silt transport took place during the ebb and flood stages and mudstone accumulated during slack water phases. Alternating thick–thin laminations reflect dominant and subordinate, twice-daily tides. Thicker groupings of foresets and thicker intervals of vertically stacked sandstone–siltstone and mudstone laminations are interpreted as spring tide deposits whereas thinner groupings of foresets and thinner laminations record neap tides. Desiccated mudstone drapes on foresets indicate that bedforms rarely were locally exposed during some portion of the tidal cycle. Abundant exposure structures in the interlaminated sandstone–siltstone and mudstone facies indicate that the cycles are upward shoaling. The stacked upward-fining cycles are attributed to alternating subaerial exposure and fluvial influx followed by marine inundation, probably related to absolute sea level fluctuations. Lack of high-order vegetation on the Archean landscape promoted rapid lateral migration or avulsion of tidally influenced fluvial channels.

The association of facies within the 100–130 m-thick upward-fining succession is comparable to Holocene and ancient paleovalley fills characterized by basal alluvial gradational upwards into estuarine facies. However, in the absence of vegetation, the land–ocean interface in the Archean probably consisted of laterally extensive fan or braid deltas rather than point sources of sediment characteristic of most modern rivers. The abrupt up-section change from syntectonic, high-energy, alluvial–fluvial flash flood deposits to tide-influenced sedimentation implies a proximal source that provided sediment to a shoreline influenced by strong tidal action. Possible Holocene analogues are orogenic settings such as the Canterbury Plains of New Zealand, the Indo-Gangetic Plains of India and strike-slip settings such as the Gulf of Aqaba but all three examples lack a direct transition to tidally influenced sedimentation.  相似文献   

图的圈和路剖分问题     

陈丽娟 《南京气象学院学报》2006,29(5):713-717

设G是一个顶点数为n的图，k为任意正整数且k≤n，证明了如果图G中任何一对不相邻顶点的最大度至少为n-k＋1/2，则G能剖分成k个子图Hi,1≤i≤k，其中Hi是圈或路；如果G是2-连通图，σ2（G）=min{dG（x）＋dG（y）x，Y∈y（G），x≠y，d（x，y）=2}≥n-k，G也能剖分成k个子图Hi，1≤i≤k，其中Hi是圈或路。  相似文献   

不同初始饱水状态红砂岩冻融损伤差异性研究   总被引：4，自引：1，他引：4  

王俐  杨春和 《岩土力学》2006,27(10):1772-1776

借助于红砂岩冻融循环CT扫描试验，从扫描层面内不同的感兴趣区域内CT均值数大小及其变化以及CT图像，对4种典型的不同初始饱水状态红砂岩进行冻融循环条件下损伤扩展差异性研究。研究结果表明，对于初始损伤相同的岩石，初始饱水状态将决定冻融循环对其损伤扩展的影响程度。  相似文献   

Density variation of the zebra mussel Dreissena polymorpha in Lake Zürich, from 1976 to 1988     

Hans Burla  Georg Ribi 《Aquatic Sciences - Research Across Boundaries》1998,60(2):145-156

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Western Pacific in glacial cycles:Seasonality in marginal seas and variabilities of Warm Pool   总被引：11，自引：0，他引：11  

Pinxian Wang 《中国科学D辑(英文版)》1998,41(1):35-41

A serics of low-latitude marginal seas, ranging from the southern South China Sea in the north to the Arafura Sea in the south, are located within the Western Pacific Warm Pool. As shown by rnicropaleontological, isotopical and organic geochemical analyses, the sea surface temperatures in the marginal seas at the last glacial maximum were much cooler than those in the open Western Pacific Ocean. The emergence of extensive shelves of the marginal seas at the glacial low sea-level stand and the decrease of surface temperatures in their deeper water parts resulted in a remarkable reduction of the ability of vapor and heat transport to the atmosphere, causing variabilities to the Warm Pool in the glacial cycles. The intensification of winter monsoon at the glacial stages not only led to a decrease of the surface water temperature and hence to an enhanced seasonality, but also carried moisture from the sea to the tropical islands, giving rise to the downward shift of snowline and mountainous vegetation zones there. It may offer a new alternative in solution of the “Tropical Ocean Paleo-temperature Enigma”. Project supported by the National Natural Science Foundation of China (Grant No. 49576286).  相似文献   

东营凹陷牛38井沙三中上段红灰韵律层中的环境信息   总被引：11，自引：0，他引：11  

王慧中  刘青春 《沉积学报》1996,14(1):47-58

通过环境磁学、沉积学、遗迹化石学与地球化学对牛３８井厘米级红灰韵律层的综合研究发现红层中具明显的胶体凝聚现象，其磁化率值、总铁与游离铁含量较高，层内生物扰动作用相对较强，表明气候变化引起的化学沉积是控制韵律旋回的主因，底栖生物的潜穴活动促进了沉积界面处微化学环境的变化。经频谱分析等时序分析法的估算可知该红灰韵律层是受十年级与百年级主周期的控制，可能与太阳黑子或太阳磁场活动等天文周期有关。虽然此类天文周期对地球气候变化的影响能力也许不强，但由于化学沉积对气候变化反应灵敏，而且可能当时湖水化学环境正处于胶体沉积的临界值附近，因而水体化学性质稍有偏离即可导致韵律层的形成。  相似文献