CONCRETION FORMATION INFLUENCING MOLAR-TOOTH STRUCTURE IN THE NEOPROTEROZOIC ZHANGQU FORMATION, CHINA

Molar-tooth (MT) structure is an enigmatic carbonate fabric that consists of a wide variety of blebs, spindles, and elongated cracks filled with calcite microspar. In this study, we investigate the origin of ‘circular’ molar-tooth structure (MTS) morphologies from the upper Zhangqu Formation (<930 Ma), Qingyunshan National Geopark, China. MTS observed in the Zhangqu Formation consist of nested polygonal to circular rings joined by multiple septae. We suggest that the key to understanding this unusual MTS lies in its outcrop distribution, wherein MTS appears broadly restricted to meter-scale concretionary bodies. Field observations show a density of MTS within these three-dimensional bodies substantially higher than in their surrounding matrix, and often restricted to the interior of these bodies. Sedimentary layers around the concretionary bodies pinch above and below the concretion boundaries or thicken dramatically at the boundary of the concretion. MTS at this interface also show a distinct change, either arresting at the boundary or curving inward to the main concretionary body. These observations suggest a crack morphology similar to septarian nodules but with fractures filled with equant microspar, characteristic of MTS. We suggest that the formation of MTS-bearing concretions directly reflects a change in the rheology of the substrate, wherein the regions within the concretion were subject to higher carbonate saturation states—likely the result of microbial activity—and differential early cementation resulting in semi-rigid concretionary layers susceptible to gas-produced fracture along and between regions of differential rheologic behavior. Elevated carbonate saturation within concretions then favored rapid infill of fractures by MT microspar. MTS within meter-scale concretions previously had only been reported from the Neoproterozoic Burovaya Formation (Russia). These two examples provide evidence of the importance of substrate rheology on the formation of MTS and may provide a critical constraint on changing the saturation state of oceans in the Neoproterozoic. Future work should seek methodologies to distinguish potential metabolic reactions that may have influenced crack formation and cementation.