Geochemical characteristics and development model of transitional source rocks during the continental margin rifting stage, Bonaparte Basin, Australia

The comprehensive analyses of tectonic settings, depositional fillings and organic geochemical characteristics indicated that the source rocks in the Bonaparte Basin were deposited during the main stage of continental margin rifting, and the well-developed river delta sedimentary system was beneficial for terrigenous organic matter input. The episodic rifting of the basin and the differential subsidence of contemporaneous faults provided both plentiful accommodation space and an underfilled depositional environment. Moreover, the pattern of "uplift alternating with sag" was favorable for the development of a semi-restricted depositional environment. The transitional source rocks were deposited in an oxic depositional environment with a large amount of terrigenous input. They mainly consist of carboniferous mudstones and coal-bearing series. Abundant terrigenous plant debris and some marine planktonic debris were observed. The terrigenous materials played a very important role in organic matter accumulation. With shifting sediment sources, the organic matter enrichment was decreasing and the kerogen was increasingly dominated by sapropelic organic matter. The n-alkane distribution was characterized by a back-end biased and bimodal pattern. The (n-C₂₁+n-C₂₂)/(n-C₂₈+n-C₂₉) and αααC₂₇R/αααC₂₉R ratios were increasing, while the Pr/Ph ratio was decreasing. All these variations also indicated that the source organic matter was changing from terrigenous to marine, and the depositional environment was changing from oxic to dysoxic conditions.