Fossils or sedimentary structures? Carbonaceous spheroids from the shale of the Cryogenian Nantuo Formation in Shengnongjia area, South China

Abstract

Black shale of the Cryogenian Nantuo Formation, Shennongjia area, South China contains spheroids which co-occur with fossils of macroalgae. These three-dimensional structures occur sparsely as isolated specimens or abundantly as clusters which show different types of boundaries between individuals. They are generally circular on bedding surfaces and elliptical on transecting sections, and range up to several millimeters in diameter. Both petrographic thin sections and scanning electron microscopy (SEM) reveal that these spheroids are mostly organic carbon in composition and exhibit sharp, well-rounded external margins composed of microcrystalline clay minerals. The carbonaceous nature of these spheroids and different elemental abundances between spheroids and matrix are also confirmed by Raman spectroscopy and energy dispersive X-ray spectroscopy (EDS). Organic carbon isotopic values of individual spheroids range from −28.55‰ to −28.83‰, with an average of −28.72‰, which show indistinguishable δ13Corg values with the surrounding sedimentary matrix but exhibit statistically significant difference to those of contemporaneous macroalgae. A range of physical origins for these spheroids—including concretions, fluid escape structures, raindrop impressions, or bubble impressions—are considered and cannot be entirely rejected. Alternatively, biogenic origin is also a possible interpretation from observations of their morphological, mineralogical, geochemical, and sedimentological characteristics. Although their nature remains unresolved, the documentation of these new carbonaceous spheroids from the Nantuo Formation of Shennongjia area adds to a growing list of fossil-like structure records in the Cryogenian Period.