川西北地区中三叠统顶部溶蚀型喀斯特特征及地质意义

Characteristics and geological significance of corrosion karst at the top of Middle Triassic in northwestern Sichuan Basin

中三叠统顶部溶蚀型喀斯特在什邡金河剖面及四川盆地大部分地区已有报道。在近期的野外工作中,在川西北地区绵竹汉旺观音崖、江油黄莲桥和江油马鞍塘剖面天井山组近顶部、顶部也识别出这一古特提斯面,其形成与强烈的构造活动或全球海平面下降造成的岩层长时间的暴露有关。其表现形式主要为:(1)暴露溶蚀间断沉积面;(2)溶沟、溶缝及其内的碳酸盐、陆源碎屑充填物;(3)暴露期黏土层;(4)喀斯特角砾。其中暴露溶蚀间断沉积面上的沉积构造以鸟眼、层状孔洞构造最为典型,在绵竹汉旺观音崖剖面最为发育,孔、洞充填物主要为亮晶方解石和白云石;溶沟、溶缝及其内的碳酸盐、陆源碎屑充填物在江油马鞍塘、汉旺观音崖、什邡金河等剖面均有发育,这些溶缝、溶沟一般近于竖直发育,宽度及深度不一,被碳酸盐角砾、亮晶方解石、白云石或陆源黏土等所充填;暴露期黏土层主要见于江油黄莲桥剖面,共发育2套,厚5~20 cm,与铁质矿物共生,其上下发育深灰色潟湖沉积;喀斯特角砾主要见于汉旺观音崖剖面,喀斯特角砾最大可达15×20cm大小,呈棱角状充填于富含泥质和有机质的暗色碳酸盐基质中。天井山组近顶部溶蚀型喀斯特的发现可为区域地层等时对比提供依据,与其有关的古岩溶作用可在天井山组(或雷口坡组)顶部碳酸盐岩中形成以溶蚀孔、洞、缝为主要储集空间的古岩溶型储集层。

The corrosion karst at the top of the Middle Triassic was widely found at the Jinhe section in Shifang and in most parts of the Sichuan Basin. With the recent field studies,we identified this karstsurface in the Triassic sections at Hanwang Guanyinya in Mianzhu,Huanglianqiao and Ma＇antang in Jiangyou along the northwest margin of the Sichuan Basin. The karst yielded in top Tianjingshan Formation and its origin is thought to be related to strong tectonic activity and global sea-level fall that resulted in longtime exposure of the Middle Triassic. The main forms of the karst are as follows：（1） exposed erosion and discontinuous sedimentary surfaces,（2） solution channels,solution fractures and associated carbonate and terrigenous clastic fillings,（3） exposed clay layer,and（4） karst breccia. The exposed erosion and discontinuous sedimentary surfaces are most typical of birdeye structures or stromatactis,and at the Hanwang Guanyinya section in Mianzhu,they are most developed. The fillings of solution channels and fractures are mainly carbonate and terrigenous debris,commonly developed at the Ma＇antang section in Jiangyou,Hanwang Guanyinya section in Mianzhu and Jinhe section in Shifang. The related dissolution joints and dissolution ditches are nearly vertical,with different widths and depths,and are filled with carbonate breccia,sparite,dolomite or terrigenous clay. The exposure soil is mainly confined to Huanglianqiao section in Jiangyou where two sets of exposure soils,about 5 ~ 20 cm thick each,contain iron minerals and the overlying and underlying intervals are dark gray limestone deposited in a lagoon paleoenvironment. The karst breccia is most common at Hanwang Guanyinya section in Mianzhu,with a maximum breccia size of up to 15×20 cm. The angular karst breccia is surrounded by dark carbonate matrix rich in mud and organic matter. The discovery of the corrosion-type karst at or near the top of the Tianjingshan Formation can provide the basis for regional stratigraphic isochronous correlation. In addition,the paleokarst oil/gas accumulations can be formed in the carbonate rocks at the upper Tianjingshan and Leikoupo Formations with dissolution pores and fractures as the main storage space.