Characteristics and significance of microfossils in carbonate concretions of the Upper Triassic succession, Ordos Basin, China

Nodules are good indicators for the sedimentary burial and diagenetic environments. Indeed, microfossils isolated from carbonate nodules can be better preserved, more abundant, and increasingly likely to preserve delicate and fragile structures than those isolated from surrounding rocks. We studied carbonate nodules collected from the Chang 7 lacustrine shales within the Triassic Yanchang Formation in the Tongchuan area of the Ordos Basin. These nodules occur across a range of different shapes and sizes, but are mostly conical, elliptical, and flat. We performed field observations, thin-section identifications, X-ray diffraction analyses, scanning electron microscopy (SEM), and other approaches to study these nodules. It has been suggested that because microorganisms or faecal particles are highly enriched when they are deposited, these form the basis for the early growth of carbonate nodules. The early diagenetic growth of these structures can therefore be inferred from fine sedimentary layers and small-scale structures scattered within fossil-bearing carbonate nodules.