鄂尔多斯盆地西南部三叠纪末抬升剥蚀事件及热年代学记录

Uplift and exhumation events and thermochronological constraints at the end of Triassic in southwestern Ordos Basin

三叠纪末期大型鄂尔多斯盆地遭受了中生代成盆以来首次较大规模的抬升剥蚀,显著改造了中晚三叠世延长期盆地面貌,并控制了侏罗纪早期沉积格局和油藏分布,对盆地演化及矿产资源分布产生了重要影响。本文利用地质及大量钻井资料揭示了该期构造事件对盆地的剥蚀改造特征,盆地及周邻地区磷灰石裂变径迹年代学记录并约束了此次构造抬升的时限与过程;综合周邻区域构造研究成果,探讨了其发育的动力学背景。结果表明,鄂尔多斯盆地三叠纪末期的剥蚀具西南强、东北弱的特点,西南部大范围内延长组地层残缺不全,剥蚀量最大可达1000余米;前侏罗纪沉积古地貌总体呈西南高、北东低的特点;其抬升时间始于205~190Ma,西南部稍早于盆地腹部,抬升速率大于1℃/Myr,可持续至中侏罗世(约160Ma)。该期抬升剥蚀事件范围可涉及至盆地西南缘更广阔的区域,与同期秦岭造山带内出现的快速抬升冷却事件具有较好的时空耦合关系,是对秦岭造山带区域构造环境转变的响应和纪录。该研究丰富和发展了三叠纪末期构造事件在华北克拉通的影响,对该区油气、煤炭资源的进一步勘探和评价提供了新的思路,具有一定的现实意义。

The regional sedimentary hiatus of large sedimentary basins records the important events of basin and tectonic environment evolution,which is an important basis and time scale for basin-range response comparison,and often has significance for tectonic dynamics and petroleum geology study.Ordos Basin is one of the most important large energy basins,located in the western North China Plate.At the end of Triassic,Ordos Basin had suffered its first large-scale sedimentary hiatus and exhumation since the basin was formed in Mesozoic,which significantly reformed the paleogeomorphology of Yanchang Formation in the Middle-Late Triassic,controlled the sedimentary pattern and reservoir distribution in the Early Jurassic,and had important impact on the basin evolution and distribution of mineral resources.Based on geological characteristics and a large number of drilling data,as well as the apatite fission track thermochronological data,this paper focuses on the sedimentary hiatus at the southwest of Ordos Basin to determine the temporal and spatial characteristics of its uplift and denudation,furthermore,it also discusses the geodynamic environment of its development in combination with the regional structural study.The results indicate that the uplift and denudation of this area at the end of Triassic is strong in the southwest and weak in the northeast,and the stratum of Yanchang Formation in the southwest is incomplete in a larger range,with a denudation thickness up to more than 1000m.The sedimentary paleogeomorphology of pre-Jurassic is generally characterized by a highland in the southwest and a low depression in the northeast.The time of uplift starts from 205~190Ma and last until Middle Jurassic(about 160Ma),with a cooling rate greater than 1℃/Myr.The uplift and denudation events can involve a wider area in southwestern margin of Ordos Basin.It had a good spatio-temporal coupling relationship with the rapid uplift and cooling events occurring in the Qinling orogenic belt at the same period,and it is a response and record to the regional geodynamic environment transformation of the Qinling orogenic belt.This study enriches and develops the influence of tectonic events at the end of Triassic on the North China Craton,and has certain practical significance for the new ideas for further exploration and evaluation of hydrocarbon and coal resources in this area.