龙门山北段志留系宁强组沉积特征和空间相变

Sedimentary facies changes in the Silurian Ningqiang Formation in the northern part of the Longmen Mountains

丘状隆起、链状相连的北东向带状堤礁是龙门山北段地区志留系宁强组的一个突出特点。详细的剖面研究和路线地质追踪表明 :(1)狭长带状的堤礁夹持于南东侧杂色砂 /泥质滨岸沉积带和北西侧陆架泥页岩带之间 ,指示了一个清晰的向西北渐深的古陆架形态和古海岸边缘轮廓 ;(2 )丘状礁灰岩体、层状生物碎屑灰岩体与东南侧陆缘滨岸带的砂体及北西侧的陆架泥岩呈现明显的指状穿插和渐变过渡关系 ,代表了海平面、陆源碎屑输入量频繁变化背景下的沉积响应 ;(3)点状零星分布的礁灰岩体是在面状礁灰岩或生物碎屑灰岩层基础上发育起来的 ;(4)晚志留世 ,杂色砂 /泥质滨岸沉积向北西进积并超覆于礁灰岩之上 ,并最终导致本区隆升成陆 ,则表明加里东构造运动末期发生的上扬子地块隆升、西北缘古陆扩展及海平面的相对下降趋势 。

The hummocky and chainy NE trending barrier reefs occur in large amounts in the Silurian Ningqiang Formation in the northern part of the Longmen Mountains. The detailed study of the cross sections and route geology indicates that: (1) the narrower barrier reefs associations in the Formation intermediate between the motley sandy and muddy littoral sediments in the southeastern part and shelf mudstones and shales in the northwestern part have delineated a well defined deepening northwestward ancient shelf and coastal margin; (2) the interfingering and gradations between hummocky reefal limestones and bedded bioclastic limestones and the sandstones along the littoral zones on the southeastern continental margin and the shelf mudstones in the northwestern part represent the sedimentary responses to sea level changes and terrigenous supply; (3) the point and sporadic reefal limestones are develeped upon the planar reefal limestones or bioclastic limestones, and (4) during the Late Silurian, the motley sandy and muddy littoral sediments prograded northwestwards and overlapped the reefal limestones to give rise to the uplift of the study area to become land. This implies that the uplift of the Upper Yangtze landmass, expansion of the ancient land on the northwestern continental margin and relative sea level falls during the latest Caledonian are the main controlling factors for the evolution of the sedimentary basin.