Elemental geochemical evidence for controlling factors and mechanisms of transitional organic matter accumulation: The upper Permian Longtan Formation black shale in the Lower Yangtze region, South China

The marine-continental transitional shale in the upper Permian Longtan Formation is not only regarded as an excellent source rock, but also one of the crucial layers of shale gas exploration in the Lower Yangtze region, South China. However, controlling factors and mechanisms of transitional organic matter (OM) accumulation in such a setting are not well understood. In this study, the characteristics of total organic carbon (TOC) and elemental geochemistry of 22 rock samples collected from one core and two different types of outcrops are systematically investigated to characterize the paleoenvironmental conditions and OM accumulation mechanisms. Results show that shales developed in a tidal flat-lagoon environment display high TOC contents ranging from 6.16% to 10.10% (average 8.50%), whereas these formed in a deltaic environment exhibit medium-high TOC contents varying from 0.93% to 6.70% (average 3.26%). These data also elucidate that the target shales in the Longtan Formation were deposited in a complex paleoenvironment with strong water-mass restriction that was mainly characterized by warm and humid paleoclimate, high primary productivity, oxic-to-dysoxic conditions, and a high sedimentary rate. Besides, the transitional OM accumulation is not determined by a single factor, but is the result of the mutual configuration and coupling of multiple factors such as paleoclimate, paleoproductivity, paleoredox, and sedimentary rate. All these factors will directly or indirectly affect OM supply or preservation. Finally, the “integrated model” for transitional OM accumulation is proposed. The model stresses two aspects: on one hand, the warm and humid paleoclimate not only facilitates the growth of higher plants, but also accelerates the chemical weathering rate of the parent rocks and increases the input of nutrients to the water column, which is conducive to the blooms of lower aquatic organisms. Both higher plant debris and lower aquatic organisms together provide abundant OM sources. On the other hand, although the oxidized water environment is usually unfavorable for OM preservation, a higher sedimentary rate can greatly shorten OM exposure time in the decomposition region of aerobic bacteria, and a number of organic matters cannot be oxidized or degraded through the rapid burial. These findings also add to our knowledge that despite the oxygenated water environment during transitional shale deposition, TOC contents are not necessarily lower.