川东北地区上三叠统须家河组物源分析

Provenance analysis of Upper Triassic Xujiahe Formation in northeastern Sichuan Basin

根据地质剖面野外考察、重点样品的重矿物含量及碎屑锆石U-Pb年龄测定,结合区域构造背景、周缘地质体年龄、地层展布特征及古水流等资料,对川东北地区上三叠统须家河组的物源进行了分析。研究结果表明:(1)川东北地区上三叠统须家河组沉积期的物源来自北东和南东2个方向,盆地腹部的七里乡地区须家河组沉积物是由东北部山前带的大巴山地质公园和东南部山前带的复平镇黑天池汇入,从山前带到腹部,沉积物粒度由粗变细,ZTR指数由小变大。(2)研究区须家河组沉积物主要来自北东方向的秦岭造山带和南东方向的江南造山带,碎屑锆石年龄总体呈现出210~282 Ma,400~500 Ma,650~850 Ma,1500~2000 Ma,2265~2600 Ma共5个年龄区间,峰值分别为~213 Ma,~427 Ma,~725 Ma,~1694 Ma,~2352 Ma,须家河组沉积年龄应不早于~213 Ma。(3)~213 Ma与勉略缝合带形成相对应,主要源自南秦岭造山带;~427 Ma与商丹洋俯冲关闭相对应,主要源自北秦岭造山带;~725 Ma与Rodinia超大陆裂解有关,江南造山带西段最可能成为该年龄段的物源;~1694 Ma年龄段锆石可能形成于Columbia超大陆拼合、裂解过程,该年龄段锆石主要来自华北板块南缘和江南造山带西段等外源区;含量较少的~2352 Ma的锆石来自再旋回物源。(4)研究区须家河组储层主要发育于北东和南东两大物源供给体系下的辫状河三角洲—滨浅湖沉积体系中,2个方向的物源控制着储层岩石类型和孔隙发育情况,使其具有较强的区域性。

Based on field investigation of geological profiles,determination of heavy mineral content and detri tal zircon U-Pb ages of key samples,combined with regional tectonic setting,age of peripheral geological bodies,stratigraphic distribution and palaeocurrent,the provenance of Upper Triassic Xujiahe Formation in northeastern Sichuan Basin was analyzed.The results show that:(1)The provenance of the sediments during the deposition period of Upper Triassic Xujiahe Formation in northeastern Sichuan Basin was derived from the north-east and southeast directions.The sediments of Xujiahe Formation in Qilixiang area were sourced from the Daba Mountain Geological Park in the northeastern piedmont zone and Heitianchi of Fuping Town in the southeastern piedmont zone,and subsequently deposited in the basin.During transportation from the piedmont zone to the ba-sin,the sediment grain size underwent a change from coarse to fine,accompanied by an increase in the ZTR in-dex.(2)The sediments of Upper Triassic Xujiahe Formation in the study area are mainly derived from the Qin-ling orogenic belt in northeast and the Jiangnan orogenic belt in southeast.The ages of detrital zircons are 210-282 Ma,400-500 Ma,650-850 Ma,1500-2000 Ma,and 2265-2600 Ma,with peak values of~213 Ma,~427 Ma,~725 Ma,~1694 Ma,~2352 Ma,respectively.The sedimentary age of Xujiahe Formation should be no earlier than~213 Ma.(3)~213 Ma corresponds to the formation of Mian-Lue suture zone,and the provenance of this stage is mainly from the south Qinling orogenic belt.~427 Ma corresponds to the subduc-tion and closure of Shangdan Ocean,and the provenance of this stage mainly originates from the north Qinling orogenic belt.~725 Ma should be related to the breakup of Rodinia supercontinent,and the western Jiangnan orogenic belt is the most likely source of this age.Zircons of~1694 Ma age may have been formed during the amalgamation and fragmentation of the Columbia supercontinent,and the zircons of this age are mainly derived from the southern margin of the North China Plate and the western section of the Jiangnan orogenic belt.The zir-cons of~2352 Ma account for a small proportion and are derived from retrocyclic provenance.(4)The reser-voirs of Xujiahe Formation in the study area are mainly developed in the braided river delta-shore-shallow lake depositional system under the northeast and southeast provenance supply systems,and the provenance in the two directions controls the rock types and pore development of the reservoirs,making it a strong regional character.