Organic matter preservation conditions in the third member of the Shahejie Formation (Dongpu Depression, China)

The third member of the Paleogene Shahejie Formation (Es3) in the Dongpu Depression, China, was studied to interpret its paleoenvironment through a series of trace element proxies, to reveal paleosalinity conditions, to elucidate on the hydrological conditions of the paleolake using carbon and oxygen stable isotopes, and to investigate the effects of paleoredox conditions on organic matter preservation. Most of the redox-sensitive proxies, such as U/Th, V/Cr, Ni/Co, Uauth, (Cu + Mo)/Zn, Cu/Zn, Th/U, and V/Sc, indicate that the Es3 was deposited under predominantly oxic and oxic/suboxic conditions, while V/(V + Ni) and 8U and previous studies suggested that the formation was deposited under mainly anoxic conditions. Oxygen levels agreed with the trace element ratios and showed a decrease from the lower to the upper part of the Es3 and an increase in paleoproductivity in the same direction. Salt rocks interbedded within the formation were shown to have overall lower oxygen levels than their neighboring organic-rich layers, black shale, and mudstone intervals, thus have likely altered the paleoredox proxies to oxygen consumption. Although Sr/Ba values point to a saline lake environment, the relative fluctuations seen suggest that the above ratio is not an appropriate indicator of paleosalinity in saltbearing strata and that the variations in paleoredox conditions in the formation are not a function of paleosalinity. Based on the negative 813C and 818O stable isotopic values, it is inferred that the shales in the Es3 were deposited in a hydrologically open lake environment. However, the presence of palynomorphs such as dinoflagellate cysts that are typically found in middle to outer neritic and open marine environments reinforces the assumption of a sea level transgression and inundation of the lake basin and mixing with freshwater algae and rare terrigenous phytoclasts such as cuticles, wood fragments, and suberin. The higher paleoproductivity and lower oxygen levels in the upper Es3 have contributed to greater OM preservation conditions in the absence of any variations in salinity or freshwater influx. The study also demonstrates that many of the paleoredox proxies and thresholds established are not valid to properly characterize certain formations deposited in lacustrine lake paleoenvironments such as the Shahejie, thus their applicability in certain situations must be re-evaluated.

Automated summary

This study investigates the paleoenvironment and hydrological conditions of the Paleogene Shahejie Formation (Es3) in the Dongpu Depression, China, using trace element proxies and carbon and oxygen stable isotopes. The results show that the formation was mainly deposited under oxic and oxic/suboxic conditions, with anoxic conditions in certain areas. There is a decrease in oxygen levels and an increase in paleoproductivity from the lower to the upper part of the Es3. The presence of salt rocks affects the paleoredox proxies, and the Sr/Ba values are not a reliable indicator of paleosalinity in salt-bearing strata. The study suggests a hydrologically open lake environment with sea level transgression and freshwater influx in the lake basin.