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

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.