试论伸展不整合及保山地块二叠系–三叠系界线特征

Research on Extensional Unconformity and Characteristics of P-T Boundary in Baoshan Block

摘要: 通过1:5万金鸡村等幅区域地质调查，在保山金鸡地区三叠系上统大水塘组中发现了大规模的钙质浊积岩(大多改造成震积岩)、南梳坝组中发现了碎屑浊积岩和大规模的滑塌(带)堆积。结合永德一带牛喝塘组中双峰式火山岩及其中的碱性花岗岩侵入体，笔者认为这些是有成因联系的灾变事件地层，进一步认为保山地块在二叠纪–三叠纪时处于拉伸背景，强烈的拉伸一方面造就了永德–黑山裂谷(盆地环境)，堆积了巨厚的沉积物，裂谷中心在永德一带，P-T沉积是连续的。另一方面，强烈的拉张坳陷使其容纳沉积物的空间迅速增大，诱发保山金鸡地区海水向永德地区发生“海退”，缺失晚二叠世龙潭期–长兴期沉积，造成三叠系中下统喜鹊林组印度期沉积物超覆于二叠系沙子坡组茅口期沉积物之上，由于在拉张背景下沉积物的基底地形发生巨大改变，同时伴随着盆地(裂谷)形成，从而形成了伸展不整合。

Abstract: Extensional unconformmity is special geology phenomenon. The predecessors thought that P-T boundary of Baoshan block was conformmable contact-parallel and unconformmable contact. Lar- ge-scale calcareous turbidites (most of them have been modified into seismites) in the Dashuitang Formation, clastic turbidites and large-scale slumping accumulations in the Nanshuba Formation of Upper Triassic have been discovered in Jinji area, Baoshan. Combined with the bimodal volcanic rocks from Niuhetang Formation of Upper Triassic and the alkaline granitoid intrusions among them in Yongde area by 1:50,000 regional geological survey of Jinji village site, this paper thought that all these geological phenomena were catastrophic event stratigraphy with genetic connection. It is believed that Baoshan block in the tensile background during Permian-Triassic. Strong tension created Yongde-Heishan rift valley (basin environment), and accumulated thicker sediments. The center of the rift valley lies in Yongde area and the P-T deposition was continuous. On the other hand, strong tension and depression made the spaces of accommodating sediments increased rapidly, forced the seawater of Baoshan Jinji occurred “marine regression” to Yongde area, the late Permian Leping-Changxing deposition was deleted. And then caused the sediments from India age of mid-lower Triassic Xiquelin Formation covered directly on the sediments from Maokou age of Permian Shazipo Formation, which formed the typical extensional unconformity. It was formed that base relief of the sediments occurred enormous changes under stretching background. Meanwhile, with the formation of the basin (rift valley), which formed extensional unconformity. It was different from the unconformity because of vertical movement and intracontinental orogeny, but it is a new type of unconformity that represents the sequence missing, the time intervals of upper and lower stratigraphic gap were long, and there was very great difference for sedimentary environment. In addition, the rift valley formed by gravitation-tension effect, its sedimentation was obviously different from the sedimentation of the passive continental margin. Therefore, extensional unconformity has been found and studied out were very important for the development of geology.