DIFFERENTIAL WEATHERING OF DIAGENETIC CONCRETIONS AND THE FORMATION OF NEOPROTEROZOIC ANNULATED DISCOIDAL STRUCTURES

Abstract

ABSTRACTAnnulated discoidal structures are common in many Neoproterozoic and particularly Ediacaran successions. Their interpretations, especially their biogenicity, are often contentious. Some of them (e.g., Aspidella and related forms) are demonstrably biological structures and may represent holdfasts of frondose Ediacara-type organisms. Others may represent fluid escape structures or tool marks produced by the rotation of tethered organisms. Here we show that differential chemical weathering of diagenetic dolomite concretions can also produce annulated discoidal structures that could be mistaken as Ediacara-type discoidal fossils. Using transmitted-light and cathodoluminescence microscopy in conjunction with Raman and energy dispersive spectroscopy, we analyzed dolomitized discoidal concretions embedded in a phosphorite matrix from the Ediacaran Miaohe Member of South China. Our observations reveal that pervasive and displacive syn-compactional dolomitization of the concretions and differential compaction of surrounding sediment led to the formation of biconvex or upward-convex primary laminae within the concretions. When exposed along parting surfaces and/or bedding planes, disparities in the weatherability of the warped primary laminae within the concretions resulted in the formation of circular annuli that mimic Neoproterozoic discoidal structures, some of which have been contentiously interpreted as discoidal fossils. These findings emphasize the importance of thin-section petrographic observations in the study of discoidal dubiofossils in order to assess their origin as diagenetic concretions or as discoidal macrofossils.