The Upper Cretaceous-Upper Eocene Belaga Formation is characterized by thick deep-water sediments that were accumulated as one of the biggest ancient submarine fans covering a time interval of 30 Ma. Although the Belaga Formation is the extensively investigated formation in the Rajang Group, its source and tectonic setting are still arguable. This study utilizes, for the first time, detailed mineralogical and geochemical investigations on sediments from the Belga Formation to examine its origin. The Belaga Formation is subdivided into five members that are made of seven lithofacies dominated by mudstone. Clay fractions from the mudstones of the five members are composed of illite (40�60 wt ), kaolinite (30�45 wt ) and chlorite (5�15 wt ). Al2O3/TiO2 ratios and calculated SiO2 (wt. ) as well as Zr�TiO2, Al2O3�TiO2, SiO2�Al2O3/TiO2 and Cecn-La/Ybcn plots suggested felsic to intermediate igneous source rocks for all members probably the Lower-Upper Cretaceous granites and tonalities of the Schwaner Mountains situated southwest of the studied area. The abundance of kaolinite, Sr/Cu ratios, C-values, Sr/Cu ratios, Sr/Cu versus Ga/Rb plot and high Chemical Index Alteration (CIA) indicate warm and humid paleoclimate throughout the formation of the Belaga Formation. Geochemical proxies from the major oxides (SiO2�K2O/Na2O, K2O/Na2O�SiO2/Al2O3 binary plots and CaO�Na2O�K2O ternary plot), trace elements (Sc/Cr�La/Y discrimination diagram) and uniform REE patterns with distinct Eu negative anomalies propose a mature passive margin tectonic setting for the five members of the Belaga Formation. Facies analysis indicate typical turbidite sequences in all members. Domination of mudstone and the passive margin tectonic setting suggest a channel-levee complex without lobes submarine fan model (Type III) for these turbidites. Stability in source area composition, paleoclimate, palaeogeography and tectonic setting of Sundaland during the Late Cretaceous-Late Eocene (~30 Ma) indicated from sedimentology and geochemical proxies is consistent with the global paleogeographic and paleoclimatic models for this area during this period. Due to their geological and economic importance as potential hydrocarbon source in the region an all over the world, results of this study provide new insights for the architecture and sedimentology of turbidites. © 2021 Elsevier Ltd