Mineralogical and geochemical imprints to determine the provenance, depositional history and tectonic settings of Triassic turbidites in the Semanggol and Semantan Basins, Peninsular Malaysia

Abstract

• Semanggol and Semantan formations are part of Triassic successions in SE Asia. • Triassic turbidites exemplify inner to outer fan in a deep-marine fan system. • Semanggol turbidites were sourced from felsic rocks and represent passive margin. • Semantan rocks have felsic-intermediate origin and show active-continental margin. • Based on redox-sensitive metals, depositional conditions fluctuate from anoxic-oxic. • Type-III kerogen, mature-overmature proposing terrestrial input of organic matter. The present study provides systematic mineralogical, inorganic geochemical and Rock-Eval analyses of turbidite deposits of the Semanggol and Semantan formations to investigate their provenance, depositional environment and tectonic setting as well as type, origin and maturation of organic matter in black shale. These deposits are extensively distributed in Peninsular Malaysia and are observed in the field as conglomerate, pebbly sandstone, thick-bedded sandstone, interbedded sandstone and shale, laminated black shale, and tuffaceous sandstone. These sedimentary sequences are predominantly deep-marine and occasional shallow-marine deposits characterizing a deep-marine fan system. Based on Al 2 O 3 /TiO 2 ratios, more felsic rocks for the Semanggol Formation and felsic-intermediate rocks for the Semantan Formation are proposed as sources for their detrital fractions and this interpretation is also substantiated by the binary plot of Zr vs Ti. V/(V + Ni), V/Ni and Ni/Co analyses indicate anoxic/oxic conditions. Ternary diagram of immobile- trace-elements inferred passive margin for Semanggol turbidites and an active-continental margin for Semantan turbidites. The dissimilarity between geochemically and geologically derived tectonic settings, mainly for the Semanggol turbidites, is because of changing tectonic setting and recycled nature of sediment sources that were deposited in diverse tectonic settings. The black shale clay fractions from both formations dominantly contain illite and kaolininte with abundant traces of quartz. The illite content varies from 43 to 56% and illite crystallinity index (IC) from 0.2° to 0.4° Δ2θ (average: 0.28° Δ2θ), showing studied black shale fall in anchizone. Rock-Eval pyrolysis reveals Type-III kerogens, suggesting a terrestrial source of organic matter in black shale of both formations.