北喜马拉雅地区下白垩统海底扇沉积环境

北喜马拉雅地区早白垩世沉积以碎屑岩为主，海底扇沉积十分发育。根据沉积岩的矿物成分、结构、构造和产状特征，可将这些海底扇分为6个亚相。根据亚相在空间的排列组合所指示的沉积环境，海底扇的发展过程可划分为萌芽、青春、成熟和消亡四个阶段。在早白垩世早、中期海底扇处于萌芽阶段和青春阶段，沉积岩的砂/?泥比值高，砂岩的矿物成分和结构多样，反映出海岸平原和大陆架较窄，海底坡度较大，从早白垩世开始沉积环境经历了由陆棚向大陆斜坡转移的过程，海平面升高，构造性质主要为水平拉张、裂陷。早白垩世晚期海底扇处于成熟阶段，海岸平原和大陆架宽度加大，砂/?泥比值降低，沉积物以泥质组分为主，富含菱铁矿、钙质结核，少见菊石等生物化石，相变缓慢，水体低能，属于缓倾斜、无明显坡折带的陆缘，反映了北喜马拉雅区的最大海侵事件。由于印度洋扩张、印度板块向北漂移，在早白垩世晚期北喜马拉雅被动陆缘已趋成熟，本区处于大陆斜坡下部强还原的深海-半深海环境。晚白垩世总体上属于海退，海底扇处于消亡阶段，其岩石由泥岩类向砂岩类直至砾岩类演化。白垩纪沉积盆地则相应经历了由陆棚→拉张断陷盆地→陆坡→深海盆地的演变。

SEDIMENTARY ENVIRONMENTS OF SUBMARINE FAN OF THE LOWER CRETACEOUS IN NORTHERN HIMALAYAS

The Lower Cretaceous in northern Himalayas mainly consists of clastic rocks. The submarine fan was well developed in this area. According to the minerals, textures, structures and occurrence of sedimentary rocks, the submarine fan can be divided into six subfacies. The subfacies and their arrangements and combinations indicate various sedimentary environments. The evolution of the submarine fan can be divided into the embryo stage, young stage, peak stage, and waning stage. The embryo and young stages were developed during the Early Age and Middle Age of the Early Cretaceous. The sand/mud ratio of sedimentary rocks is higher than any of other stages. The sedimentary rocks are mainly sandstones with different minerals and textures. It means that the coastal plain and shelf was narrow and the sea floor was steep. The sedimentary environment was changed from shelf to slope. Major tectonic movements were extension, split and subsidence. During the Late Age of the Early Cretaceous, the submarine fans were in peak stage. Coastal plain and shelves were wider than before. Sedimentary rocks are black shale and fine-grained elastics with large thickness. There are many siderite and calcic nodules and a few ammonite fossils in black shales. The ratio of sand/mud of sedimentary rocks is lower than any of other stages. The sedimentary facies changed slowly in this area. The sedimentary conditions belong to low energy, gentle slope without obvious slope breaks of continental margin. These characteristics show the significant transgression events in northern Himalayas. As a result of the Indian Ocean spreading and the Indian Plate moving to the north during the Early Cretaceous, the passive continental margin was well developed. The sedimentary environment was transformed into bathyal and abyssal environments. Due to the regression, the submarine fans evolved into waning stage in the Late Cretaceous and the sedimentary rocks changed from shales to coarse sandstones even conglomerates. The sedimentary basins developed stage by stage from shelf to extension basin and to slope, and at last to deep sea basin during the Cretaceous.