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991.
992.
We investigate the seismic structure of the western Philippine Sea using two sets of seismological observations: ScS reverberations, which provide the layering framework for a regional upper mantle model, and observations of frequency-dependent phase delays for direct S waves, surface-reflected phases (sS, SS, sSS), and surface waves (R1, G1), which constrain the velocity and anisotropy structure within the layers. The combined data set, comprising 17 discontinuity amplitudes and layer travel times from the ScS-reverberation stack and more than 1000 frequency-dependent phase delays, was inverted for a path-averaged, radially anisotropic model. Mineralogical estimates of the bulk sound velocity and density are incorporated as complementary constraints. The final model, PHB3, is characterized by a 11.5-km thick crust, an anisotropic lid bounded by a sharp negative G discontinuity at 89 km, an anisotropic low-velocity layer extending to 166 km, a subjacent high-gradient region, and transition-zone discontinuities at depths of 408 km, 520 km and 664 km. The lid is slower than in a comparable model for the Tonga–Hawaii corridor (PA5), but also significantly thicker, requiring a compositional variation between the two regions. We explore the hypothesis that the thickness of the oceanic lid is controlled by the melting depth at the spreading centers during crust formation, and that the thicker crust and lid in the Philippine Sea results from deeper melting owing to a higher potential temperature and perhaps a higher water content in the upper mantle. 相似文献
993.
994.
995.
The occurrence of organic-walled dinoflagellate cysts (dinocysts) in the Diest Formation, a largely decalcified succession with a poor fossil content, and in the adjacent strata of Lower Miocene and Lower Pliocene formations, allowed a biostratigraphic evaluation of these deposits and an assessment of the hiatus between the lithostratigraphic units. The Diest Formation was deposited during Tortonian – Messinian times. Dinocyst biozones defined in the North Sea region and the U.S.A. East Coast are recognised within the Diest Formation, although environmental factors seem to have influenced the presence of some key zonal species in the shallow-marine deposits of northern Belgium. The two members of the Diest Formation studied, i.e., the Dessel Sands and the Diest Sands, appear to be strongly diachronous. The depocentre was located in the Campine area during the early Tortonian and shifted to the area north of Antwerp during late Tortonian to Messinian times. The age assessment provides a correlation of the sequence boundaries of Haq et al. (1987) at the top of the Diest Formation with SB 5.5. 相似文献
996.
The Late Coniacian, shallow-marine Bad Heart Formation of the Western Canada foreland basin is very unusual in that it contains economically significant ooidal ironstone. Deposition of shallow-water and iron-rich facies appears to have been localized over the crest and flanks of a subtle intrabasinal arch, in part interpreted as a forebulge and partly attributed to reactivation of the long-lived Peace River Arch. The formation comprises two upward-shoaling allomembers, typically 5–10 m thick, that are bounded by regionally mappable ravinement surfaces. The lower unit, allomember 1, grades up from laminated mudstone to bioturbated silty sandstone, which is abruptly overlain by bioturbated ooidal silty sandstone grading into an almost clastic-free ooidal ironstone up to 7 m thick. Ooidal ironstone was concentrated into NW- to SE-trending ridges, kilometres wide and tens of kilometres long. Ironstone formation appears to have been promoted by: (a) drowning of the arch, which progressively curtailed sediment supply; and (b) enhanced reworking over the shallowly submerged arch and over a fault-bounded block that underwent episodic vertical movement of 10–20 m during Bad Heart deposition. Allomember 2 also shoals upwards from mudstone to bioturbated and laminated silty sandstone but lacks ooids, apparently reflecting a rejuvenated supply of detrital sediment from the arch. The marine ravinement surface above allomember 2 is a Skolithos firmground, above which is developed a regional blanket of ooidal sediment. In the east, ooids are dispersed in a bioturbated silty sandstone with abundant evidence of repeated reworking and early siderite and phosphate cements. Westwards, this facies grades, over about 40 km, into almost clastic-free ooidal ironstone about 5 m thick; the lateral facies change may reflect progressive clastic starvation distal to a low-relief source area. The two allomembers are interpreted to reflect eustatic oscillations of about 10 m, superimposed on episodic tectonic warping and block-faulting events. The development of ooidal ironstone immediately above initial marine flooding surfaces indicates a close relationship to marine transgression, reflecting sediment-starved conditions. Ironstone does not appear to be related to either sequence boundaries or maximum flooding surfaces. The Bad Heart Formation is blanketed by marine mudstone deposited in response to major flexural subsidence and rejuvenation of clastic sources in the Cordillera to the SW. 相似文献
997.
吉林大阳岔上寒武统凤山组—下奥陶统冶里组层序地层和化学地层研究 总被引:1,自引:1,他引:0
根据岩相序列、沉积、成岩作用特征,将吉林省大阳岔地区晚寒武世凤山期至早奥陶世冶里期层序地层分为6个三级层序,代表了6个三级海侵—海退旋回;与之相应,该时期碳酸盐岩δ13C值变化曲线出现5个低谷,其中凤山期至冶里早期泥晶灰岩δ13C值出现4次幅度较大的上升、下降飘移,而冶里中、晚期泥晶灰岩的δ13C值是在负值范围内波动。δ13C值低谷与层序界面和海退事件基本拟合。牙形类C.intermedius带下部海绿石质凝缩段是寒武纪末—奥陶纪初期最大海泛事件沉积,也是寒武纪与奥陶纪之交等时性的年代地层对比标志。 相似文献
998.
999.
The Frewens sandstone is composed of two elongate tide-influenced sandstone bodies that are positioned directly above and slightly landward of a more wave-influenced lobate sandstone. The 20-km-long, 3-km-wide Frewens sandstone bodies coarsen upwards and fine away from their axes, have gradational bases and margins and have eroded tops abruptly overlain by marine shales. These sandstones are superbly exposed in large cliffs on the banks of the South Fork of the Powder River in central Wyoming, USA. The deposits change upwards from thinly interbedded sandstones and mudstones to metre-thick heterolithic cross-strata and, finally, to metres-thick sandstone-dominated cross-strata. There is abundant evidence for tidal modulation of depositional flows; however, palaeocurrents were strongly ebb-dominated and nearly parallel the trend of sandstone-body elongation. Detailed mapping of stratal geometry and facies across these exposures shows a complex internal architecture. Large-scale bedding units within sandstone bodies are defined by alternations in facies, bed thickness and the abundance of shales. Such bedsets are inclined (5°–15°) in walls oriented parallel to palaeoflow and gradually decrease in dip over hundreds of metres as they extend from the sandstone-dominated deposits higher in a sandstone body to muddier deposits lower in the body. Where viewed perpendicular to palaeoflow, bedsets are 100-metre-wide lenses that shingle off the sandstone-body axis towards its margins. The sandstone bodies are interpreted as sand ridge deposits formed on the shoreface of a tide-influenced river delta. Metres-thick cross-strata in the upper parts of sandstone bodies resemble deposits of bars (sandwaves) formed where tidal currents moved across shallows and the tops of tidal ridges. Heterolithic deposits lower in sandstone bodies record fluctuating currents caused by ebb and flood tides and varying river discharge. Erosion surfaces capping sandstone bodies record tidal ravinement. The tidal ridges were abandoned following transgression and covered with marine mud as waters deepened. 相似文献
1000.
辽西四合屯脊椎动物集群死亡事件:火山爆发的灾变记录 总被引:6,自引:1,他引:5
辽西北票以ConfuciusornisSinosauropteryxZhangheotherium为代表的四合屯脊椎动物组合发现于义县组下部湖相沉积中,时代为早白垩世Barremian中期(J—K界线144 Ma)或Valanginian晚期(J—K界线136 Ma)。化石包括鱼类、两栖类、爬行类、鸟类和哺乳类及无脊椎动物和被子植物,孔子鸟类群和具“羽毛”的小型兽脚类恐龙共生。野外发掘表明,化石完整地保存骨骼硬体及羽毛、食物、胃石、卵等软体及生理组织,确认多次非正常生物集群死亡事件。在四合屯发掘剖面上,含化石正常沉积的湖相页岩与火山喷发事件形成的沉凝灰岩互层,中酸性火山爆发形成的环境突变效应是导致脊椎动物集群死亡的主要原因。 相似文献