川西北地区上三叠统卡尼阶古喀斯特的发现及研究意义

摘要

在川西北地区绵竹汉旺、安县雎水剖面上三叠统卡尼阶海绵礁(丘)灰岩顶部发现古喀斯特;野外露头可见明显的花斑状构造以及蜂窝状溶孔,显微镜下可见泥晶化颗粒,喀斯特角砾以及喀斯特溶孔充填物,其中包括泥-亮晶方解石,生物碎屑以及陆源石英颗粒;根据露头岩性和显微薄片特征判断其属于早成岩期喀斯特;喀斯特成单层发育,之上覆盖化石丰富的黄色或黑色页岩,其中的菊石化石鉴定表明其为上卡尼阶底部沉积.该喀斯特是重要的等时界面,且可作为该地区上下卡尼阶的界线.喀斯特之上的岩性在汉旺和雎水有差异:在绵竹汉旺青岩沟剖面,喀斯特灰岩之上覆盖有黑色粉砂质页岩与生屑灰岩(如层状海绵灰岩、海百合灰岩),在汉旺观音崖剖面,喀斯特面之上发育海绵丘灰岩及页岩,在安县雎水剖面,古喀斯特发育在滚落的海绵巨砾之上.综合研究表明,喀斯特的形成与早期龙门山的隆升以及卡尼期湿润气候事件在发生时间上非常相近,推测其成因与龙门山的隆升以及卡尼期湿润气候事件有关.%A karst surface is first found at the top of the Upper Triassic Carnian sponge limestone at Hanwang sections in Mianzhu and Jushui section in Anxian, northwestern Sichuan Province, China.Piebald structures and honeycomb dissolved pores are obvious in the karst outcrops, and karst breccia, filling material (e.g., micritic-sparry calcite and terrigenous quartz grains) in the dissolved pores and micritization granules are discovered with microscope.According to the microscopic characteristics and the lithology, the eogenetic karst is determined to be a single layer, overlied yellow and black shale containing abundant marine fossils.The identification of ammonoids in the shales shows that the karst is an important isochronous surface regarded as the boundary of the upper and lower Carnian.The strata above the karst are different between the Hanwang sections and Jushui section, i.e., the overlying rocks above the karst in the Qingyangou section, Hanwang are black silty shale and bioclastic limestone (e.g.lamellated sponge limestone and crinoidal limestone) while in Guanyinya section, Hanwang, the overlying rocks above the karst are shale and moundy sponge limestone.However, karst caves are developed at the top of sponge boulders in the Jushui section.Comprehensive research implies that the formation time of this karst is close to the occurrence of the Longmen Mountain uplift and the Carnian Pluvial Event, it is thus inferred that the karst was caused by the joint effect of uplift of Longmen Mountain and Carnian paleoclimatic event.