班公湖—怒江结合带西段沙木罗组底砾岩的新发现及地质意义

摘要

沿班公湖—怒江结合带中西段出露的上侏罗统—下白垩统沙木罗组,创名于革吉县盐湖区南部的沙木罗,定义为—套较稳定环境下形成的晚侏罗统—早白垩世滨浅海相沉积.之后众多学者在结合带不同地区分解出“木嘎岗日群”和“沙木罗组”,但二者地层序列划分一直沿革岩性组合差异或粒序变化特征,未获得角度不整合接触的确凿证据.作者于改则地区沙木罗组底部首次发现底砾岩及下伏木嘎岗日群顶部粘土质风化壳,并据此确定该区沙木罗组与下伏木嘎岗日群为角度不整合接触关系.该底砾岩的发现对研究新特提斯洋构造演化、古地理环境、沙木罗组地层地质时代下限及沉积相具有重要地质意义.%Late Jurassic—Early Cretaceous Shamuluo Formation outcropped along the middle—western segment of the Bangongco—Nujiang Belt.Upper Jurassic shore—shallow marine deposits are formed in relatively stable environments,Shamuluo Formation,was named after Shamuluo area of the southern Yanhu district of Geji country.The Structural Belt has been divided into Mugagangri Group and Shamuluo Formation by many geologists based on in different regions,but the stratigraphic sequence division of the two strata has always been characterized by lithologic assemblage difference or granule change.However,they haven't gotten the solid evidence of angular unconformity.At the bottom of the Shamuluo Formation in the Gengze area,the author discovered for the first time the top clayey weathering crust of the basal conglomerate and the underlying Mugagangri Group,which proved that the unconformitty of the two formations is credible.And then to determine the angle unconformity contact relationship between the Shamuluo Formation and the underlying Mugagangri Group.The discovery of the basal conglomerate is of great geological significance in studying the tectonic evolution of the Neo-Tethys ocean,the paleogeographic environment,the lower age limit of the Shamuluo Formation stratigraphy and the sedimentary facies.