Geochemical characteristics of black shales from Triassic turbidites, Peninsular Malaysia: Implications for their origin and tectonic setting

Abstract

Abstract Field observations indicate that shale intervals of Triassic turbidites from the Semanggol and Semantan formations in the Peninsular Malaysia occur as thick-bedded dark grey to black shales overlying conglomerate, pebbly sandstone, thick-bedded sandstone, and sandstone-shale interbeds. This sequence characterises a submarine fan depositional setting where the conglomerate, sand and sand/shale intercalations represent an inner and middle fan setting in a shallow marine environment, whereas the black shales were deposited in a deeper marine environment of the outer fan. The redox-sensitive trace-metal inventory (V, Ni, Cr) of the black shales is dominated by clastic input and the depositional environment is likely oxic/suboxic. TOC values are highly variable (3.2 ± 3.3 wt%) and derive from washed-in allochthonous organic material. The Semantan Formation black shales have features of a felsic source rock, while black shales from the Semanggol Formation were inherited from more intermediate rocks as constrained by their Al2O3/TiO2 ratios. Similar Al2O3/SiO2 ratios as well as CIA values suggest wet paleoclimatic conditions and intensive weathering of source rocks during the Late Triassic deposition of both shales. The tectonic setting deduced from immobile-trace-metal ratios is a passive margin for the Semanggol black shales and a fore-arc-basin setting for the Semantan black shales. The inconsistency between the interpreted geological and geochemical tectonic settings, particularly for the Semanggol black shales, is due to the recycled nature of the sediment sources that were deposited in a different tectonic setting. The clay fractions of both formations comprise of abundant illite and kaolinite, with some smectite in few samples from the Semantan Formation. Based on illite contents (38–68%) and IC values (0.45–0.62° Δ2θ), the studied black shales are in the late diagenesis stage. The organic matter in the Semanggol and Semantan black shales is characterized by kerogen Type III proposing a terrestrial origin of the organic matter, which was transported along with the clastic sediments by turbidity currents.