Gecun Fm

The base of the formation marked by the occurrence of conglomerate is distinguished from andesite of the unconformably underlying Dawangshan Fm or volcanics of the Niangniangshan Fm in same basin.

The top marked by dark purple silty mudstone is distinguished from tuffaceous sandy conglomerate of the unconformably overlying Pukou Fm.

The formation occurs in Fushan, Hongtushan and Meishan of the Jurong County, Xiaoyoushan of the Gaochun County, the Shengli Village of Nanjing City. It is also exposed in the Funing, Sheyang, Dongtai, Taixian counties, and on the south bank of the Taihu Lake and in the Songjiang County of Shanghai. It varies considerably both in lithology and thickness. In the Jurong basin, it can be divided into five members: The basal member is built up by gray white siltstone interbedded with sandstone, with conglomerate at the base; the lower member gray white sandstone, brown and gray black mudstone interbedded with clayey siltstone; the middle member dark brown and gray green clayey siltstone with gray white sandstone; the upper member dark brown and gray black clayey siltstone interbedded with silty mudstone with gray white sandstone; and the top member dark purple red silty mudstone and clayey siltstone. The formation is 1555-487 m thick. From Yangchong of Jurong to Chejiabian of Dantu, it is divided into three members lithologically. The lower member is equivalent to the basal member in the Jurong basin, but with pyroclastic rocks locally; the middle member purple red and gray yellow calcareous siltstone, fine sandstone and silty mudstone, and also marl, sandy limestone and algal limestone; and the upper member microclastic rocks with conglomerate and gravel-bearing fine sandstone. The formation ranges from 86 to 573 m in thickness. In the Meishan Mine of Lulang of the Jiangning County, it is represented by break red and gray green tuffaceous siltstone, mudstone and gompholite with sediment tuff and sediment breccia tuff. It is 460 m thick as exposed by the borehole in Taixian of the North Jiangsu Plain and 1605 m in the Dongtai City and shows clear rhythms. Marl lenses are seen in the Yanzifan area of the Nanjing City, with a thickness of more than 1054 m. In Wushangang of the Jurong County, the formation contains calcareous bentonite of different thickness of 20 to 30 m. In the Lishui County, the formation contains a large amount of pyroclastic rocks. In the Songjiang County of Shanghai, it is represented by purple red microclastic rocks with several gypsum beds, exceeding 400 m in thickness.