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SUNG KWUN CHOUGH 《Sedimentology》2011,58(6):1530-1572
To understand the depositional processes and environmental changes during the initial flooding of the North China Platform, this study focuses on the Lower to Middle Cambrian Zhushadong and Mantou formations in Shandong Province, China. The succession in the Jinan and Laiwu areas comprises mixed carbonate and siliciclastic deposits composed of limestone, dolostone, stromatolite, thrombolite, purple and grey mudstone, and sandstone. A detailed sedimentary facies analysis of seven well‐exposed sections suggests that five facies associations are the result of an intercalation of carbonate and siliciclastic depositional environments, including local alluvial fans, shallowing‐upward carbonate–siliciclastic peritidal cycles, oolite dominant shoals, shoreface and lagoonal environments. These facies associations successively show a transition from an initially inundated tide‐dominated carbonate platform to a wave‐dominated shallow marine environment. In particular, the peritidal sediments were deposited during a large number of depositional cycles. These sediments consist of lime mudstone, dolomite, stromatolite and purple and grey mudstones. These shallowing‐upward cycles generally resulted from carbonate production in response to an increase of accommodation during rising sea‐level. The carbonate production was, however, interrupted by frequent siliciclastic input from the adjacent emergent archipelago. The depositional cycles thus formed under the influence of both autogenetic changes, including sediment supply from the archipelago, and allogenic control of relative sea‐level rise in the carbonate factory. A low‐relief archipelago with an active tidal regime allowed the development of tide‐dominated siliciclastic and carbonate environments on the vast platform. Siliciclastic input to these tidal environments terminated when most of the archipelago became submerged due to a rapid rise in sea‐level. This study provides insights on how a vast Cambrian carbonate platform maintained synchronous sedimentation under a tidal regime, forming distinct cycles of mixed carbonates and siliciclastics as the system kept up with rising relative sea‐level during the early stage of basin development in the North China Platform. 相似文献
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ＩｎｔｒｏｄｕｃｔｉｏｎＴｈｅＳａｎｇｃｈｏｎｇｏｌｄｄｅｐｏｓｉｔｓｏｃｃｕｒａｓｆｉｓｓｕｒｅ-ｆｉｌｌ ｉｎｇｈｙｄｒｏｔｈｅｒｍａｌｖｅｉｎｓｉｎａｎｏｒｔｈｏｓｉｔｅ .Ｔｈｅｃｈａ ｒａｃｔｅｒｉｓｔｉｃｓｏｆｈｙｄｒｏｔｈｅｒｍａｌａｌｔｅｒａｔ 相似文献
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This paper focuses on the formative processes of limestone pseudoconglomerates in the Gushan and Chaomidian Formations (Late Cambrian) of the North China Platform, Shandong Province, China. The Gushan and Chaomidian Formations consist mainly of limestone and shale (marlstone) interlayers, wackestone to packstone, grainstone and microbialite as well as numerous limestone conglomerates. Seventy‐three beds of limestone pseudoconglomerate in the Gushan and Chaomidian Formations were analysed based on clast and matrix compositions, internal fabric, sedimentary structures and bed geometry. These pseudoconglomerates are characterized by oligomictic to polymictic limestone clasts of various shapes (i.e. flat to undulatory disc, blade and sheet), marlstone and/or grainstone matrix and various internal fabrics (i.e. intact, thrusted, edgewise and disorganized), as well as transitional boundaries. Limestone pseudoconglomerates formed as a result of soft‐sediment deformation of carbonate and argillaceous interlayers at a shallow burial depth. Differential early cementation of carbonate and argillaceous sediments provided the requisite conditions for the formation of pseudoconglomerates. Initial deformation (i.e. burial fragmentation, liquefaction and injection) and subsequent mobilization and disruption of fragmented clasts are two important processes for the formation of pseudoconglomerates. Burial fragmentation resulted from mechanical rupture of cohesive carbonate mud, whereas subsequent mobilization of fragmented clasts was due to the injection of fluid materials (liquefied carbonate sand and water‐saturated argillaceous mud) under increased stress. Storm‐wave loading was the most probable deformation mechanism, as an external triggering force. Subsequent re‐orientation and rounding of clasts were probably prolonged under normal compactional stress. Eventually, disrupted clasts, along with matrix materials, were transformed into pseudoconglomerates by progressive lithification. Soft‐sediment deformation is prevalent in alternate layers of limestone and mud(marl)stone and/or grainstone, regardless of their depositional environments. 相似文献
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Varve-like parallel laminated and homogeneous muds on natural levees of submarine channels result from the spill-over of the dilute upper parts of channelized turbidity currents. On the levees of the Northwest Atlantic Mid-Ocean Channel, Labrador Sea, laminated turbidite muds show a systematic upward thickness variation of the granular silt laminae and the clay laminae. In many depositional units the former decrease logarithmically upwards in thickness while the latter increase logarithmically. The origin of parallel lamination in current-deposited, fine-grained sediments is interpreted in terms of the ‘burst-and-sweep model’ for the viscous sublayer of turbulent flows. Individual silt laminae of laminated turbidite muds form due to shear sorting during burst-and-sweep events which have a winnowing effect on the clay fraction. In contrast to sand-transporting flows the viscous sublayer of silt- and clay-carrying dilute turbidity currents cannot be permanently in a turbulent state. The presence of clay laminae suggests that the boundary layer relaminarizes periodically. It is speculated that this is caused by pressure waves with negative pressure gradients favouring the suppression of bursts and sweeps. The observed vertical thickness variations suggest that the periods of laminar boundary-layer flow increase in duration as turbulence of the main current decays. The upward thickness decrease of the silt laminae is explained in terms of decreasing supply rates to the boundary layer from the main flow. A multiple bursting-cycle mechanism is favored over a single bursting cycle mechanism for the formation of individual silt laminae by shear sorting. 相似文献
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Depositional processes of the Suwolbong tuff ring, Cheju Island (Korea) 总被引：11，自引：0，他引：11
The Suwolbong pyroclastic sequence in the western part of Cheju Island, Korea, comprises partly preserved rim beds of a Quaternary basaltic tuff ring whose vent lies about 1 km seaward of the present shoreline. The sequence consists of breccia, lapillistone, lapilli tuff and tuff. Eighteen sedimentary facies are established and organized into six lateral facies sequences (LFS) and seven vertical facies sequences (VFS). The LFS 1, 4 and 5 begin with massive lapilli tuff which transforms downcurrent into either planar-bedded (LFS 1), undulatory-bedded (LFS 4) or climbing dune-bedded (LFS 5) (lapilli) tuff units. They are representative of relatively ‘dry’ base surge whose particle concentration decreases downcurrent with a progressive increase in both tractional processes and sorting. The LFS 2 begins with disorganized and massive lapilli tuff and transforms into crudely stratified units downcurrent. It results from relatively ‘wet’ base surge in which sorting is poor due to the cohesion of damp ash. The LFS 3 comprises well-sorted lapilli tuff and stratified tuff further downcurrent, suggestive of deposition from combined fall and surge of relatively ‘dry’ hydroclastic eruption. All seven vertical facies sequences generally comprise two facies units of coarse-grained fines-depleted lapilli tuff and an overlying fine-grained tuff. These sequences are suggestive of deposition from base surge that consists of a turbulent head and a low-concentration tail. Depositional processes in the Suwolbong tuff ring were dominated by a relatively ‘dry’ base surge. The base surge comprises turbulent and high-concentration suspension near the vent whose deposits are generally unstratified due to the lack of tractional transport. As the base surge becomes diluted downcurrent through fallout of clasts and mixing of ambient air, it develops large-scale turbulent eddies and is segregated into coarse-grained bedload and overlying fine-grained suspension forming thinly stratified units. Further downcurrent, the base surge may be either cooled and deflated or pushed up into the air, depending on its temperature. The Suwolbong tuff ring comprises an overall wet-to-dry cycle with several dry-to-wet cycles in it, suggestive of overall decrease in abundance of external water and fluctuation in the rate of magma rise. 相似文献
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In order to understand the mechanism(s) of gold precipitation in the anorthosite- hosted Sangchon gold deposits in the Hadong area, Korea, chemical speciation and reaction path calculations were accomplished by geochemical modeling.The modeling consisted of three- step procedures: reaction with anorthosite, then the simple cooling of the reacted fluid,and finally the boiling of metalliferous fluid. The principal vein minerals of the Sangchon deposits consist of quartz, sericite,kaolinite, pyrite, galena, chalcopyrite, sphalerite and acanthite. The sulfide mineralization is typically zoned from pyrite (preferentially at vein margins) to galena and sphalerite (toward vein center). Electrum is intimately associated with pyrite + chalcopyrite and sphalerite. By comparing the results of modeling with the observed mineral assemblages and paragenesis,the most appropriate evolution path of ore fluids was suggested as follow: reaction of a single fluid with anorthosite at 300℃,then the isobaric cooling of the fluid at temperatures from 2500° to 100℃3 , and then the boiling and cooling of the fluid due to the decrease of pressure and temperature. Calculations also show that all of the observed alteration minerals formed due to fluid - anorthosite interaction at early period, whereas most of sulfides and electrum were precipitated mainly due to cooling.The abundance of gold in veins depends critically on the ratio of total base metals plus iron to sulfide in the aqueous phase,because gold is transported as Au(HS)2- whose solubility is very sensitive to the sulfide activity. Our results of geochemical modeling generally fit to the observed mineral assemblages and mineral composition, indicating the usefulness of numerical simulation for elucidating the genesis of gold deposits. 相似文献
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The Hunghae Formation, SE Korea: Miocene debris aprons in a back-arc intraslope basin 总被引：1，自引：0，他引：1
ABSTRACT During early to middle Miocene times a sudden opening of the Ulleung (Tsushima) back-arc basin in the East Sea (Sea of Japan) led to the development of intraslope basins along the rifted southwestern margin (southeast Korea). Abrupt subsidence resulted in the deposition of the 200 m thick Hunghae Formation (middle Miocene), a sand/mudstone sequence that can be divided into five facies. Facies I (sand and mudstone couplet) and II (coarse sand) are turbiditic in origin, as evidenced by massive, graded, crudely-layered and parallel-laminated sand beds. Facies III (homogeneous mudstone) is characterized by various lignite and plant fragments, clastic and biogenic grains that are randomly oriented, suggestive of hemipelagic deposition. Facies IV (chaotic deposit) is characterized by the disruption of beds, the presence of isolated siltstone blocks (or balls) and large clasts in the muddy matrix, indicative of retrogressive rockfall and slide/slump. Facies V (conglomerate) is of debris flow origin, as evidenced by clast- and matrix-supported features, floating large clasts and absence of traction structures. Individual facies are organized into two types of facies association: (1) homogeneous mudstone (facies III) associated randomly with the rest (facies I, II, IV and V), indicative of hemipelagic and episodic sediment-gravity flow processes, respectively; (2) conglomerate (facies V), coarse sand (facies II) and sand/mudstone couplet (facies I), representing the flow transformation from debris flow to high- and low-concentration turbidity currents. These facies associations are similar in many respects to modern and ancient debris (or slope) aprons found elsewhere. Numerous isolated slide/slump blocks, wedged conglomerates with armoured mudstone balls, discontinuous lignite-containing sand/mudstone beds, chaotic structure and growth faults suggest that the deposition occurred on a steep slope (intraslope basin) off coalescing fan-deltas, mainly by unchannellized sediment-gravity flows. Ancient deposits with irregular facies sequences can be viewed as debris-apron systems, which provide alternatives to submarine-fan models in many clastic basins with a line rather than point source. 相似文献
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本文利用新不列颠岛两个地震在十二个基本台站所记录到的瑞利波,计算了我国不同地区的相速度.着重讨论鉴别和对比不同台站记录中同一震相的方法.在大部分台站的记录中,见到有周期约为35秒的相位,和后面的位相比较,它的周期较大,振幅较小.波形的对比对初步鉴别震相有很大的帮助.详细的震相对比是根据周期随距离变化的规律和各震相到时的规律.两个地震所得的相速度很符合.计算的结果表明:利用三台计算相速度时,如果射线的路程差别较大,海洋路程的校正是不能忽略的.因为有关我国地壳构造的资料还很少,我们所得的瑞利波相速度只能与普瑞司修正后非洲大陆的相速度理论相比较,由此得到我国不同地区的地壳厚度.这样所得的厚度,虽然不能视为最后的结果,但是它们仍然表现与主要地质单元之间有密切的联系. 相似文献