Revision of the Cretaceous–Paleogene stratigraphic framework, facies architecture and provenance of the Xigaze forearc basin along the Yarlung Zangbo suture zone

Abstract

New field investigation, structural analysis, review of biostratigraphy, stratigraphic framework, facies architecture, provenance, and flow direction of the Cretaceous–Paleogene sedimentary strata in southern Tibet resulted in reinterpretation of the evolution of the Xigaze forearc basin (XFB). The XFB is comprised by flysch-dominant Xigaze Group (consisting of Ngamring, Chongdoi, and Sangzugang Formations) and of the shallow marine Cuojiangding Group (Padana, Qubeiya, Quxia, and Gyalaze Formations), with their ages herein revised as the Albian–Coniacian and the Santonian–Ypresian respectively. The deep-sea flysch represented by the Ngamring Formation constitutes the early sedimentary fill of the XFB. It was subdivided into five megasequences MS1–MS5 in order: the early submarine fan facies stage, channel submarine fan stage, the mature submarine fan stage, the highstand submarine fan stage, and outer fan-pelagic stage. The paleocurrent flows were eastwards and westwards during the early and late stages of basin development and northwards and southwards during its middle stage, indicating typical forearc basin sedimentation with longitudinal supply and lateral filling. Provenance studies point to multiple sources, with andesitic debris sourced from the Jurassic–Lower Cretaceous Yeba Formation and Sangri Group formed in the volcanic island arc chains in the southern Gangdese. The four periods of the XFB development are: 1) the formation of the XFB basement by the Xigaze ophiolite, and completed before the Aptian. The coeval deposits on the shelf are the Sangzugang Formation carbonate; 2) the earliest XFB deposits of the earliest Aptian age (ca. 125/120Ma) represented by the Chongdoi Formation, related to the residual forearc; 3) composite forearc developed, represented by the late Albian–late Cenomanian flysch megasequences (MS1–MS2) and the Turonian–Coniacian flysch megasequences (MS3–MS5), respectively; and 4) the final stage of the XFB development started the terminal forearc stage represented by the late Late Cretaceous–Early Paleogene shallow marine Cuojiangding Group and terminated during middle Ypresian, when the Gyalaze Formation was deposited.