Palaeoceanographic redox environments for the lower Cambrian Hetang Formation in South China: Evidence from pyrite framboids, redox sensitive trace elements, and sponge biota occurrence

The lower Cambrian Hetang Formation, a black shale sequence, contains a stone coal (a flammable, organic-rich mudstone) unit. Abundant pyrite framboids occur in this unit, with the average mean sizes falling in a narrow range from 4.6 to 5.4 μm and the maximum diameters being around 10 μm. The size distribution pattern of the pyrite framboids indicates a euxinic depositional environment. The redox-sensitive trace metals of the stone coal samples reveal a large enrichment of Mo (10–180 times),V (4–40 times), U (10–60 times) and Ni (2–20 times) compared to the average upper continental crust value, consistent with an anoxic environment for their deposition. The redox-sensitive element ratios (Th/U, V/(V + Ni), V/Cr) indicate that the depositional environment for the lower part of the stone coal unit was the most anoxic and euxinic. In contrast, a sponge biota including eleven species of demosponges and hexactinellids and two undetermined forms indicate an oxic or dysoxic environment. To reconcile these two facts, we propose that although an anoxic/euxinic environment predominated during the deposition of the lower Cambrian Hetang Formation black shales, occasional currents may also have brought free oxygen to the bottom water column to allow the growth of the sponges on the sea floor.