Meganodular limestone of the Pagoda Formation: A time-specific carbonate facies in the Upper Ordovician of South China.

The decimeter-scale 3-D network structure of the Upper Ordovician Pagoda Formation in South China has been an enigmatic sedimentary feature and a case of non-uniformitarian carbonate depositional system, without modern analog. The network bands are developed pervasively throughout the entire Pagoda Formation, superficially resembling desiccation or syneresis cracks, especially in bedding-plane view, with decimeter-scale sub-polygonal to irregularly sinuous patterns. Detailed petrographic analysis, however, shows that the crack-like network bands have the same carbonate matrix, skeletal grains, and overall fabrics as the rock they surround. In this study, we demonstrate that the network structure is not generated by shrinkage cracks but is rather a time-specific lithofacies. It originated from prolonged growth of nodular carbonate, controlled by the convergence of several geological and paleoclimatic factors, including a) a protracted sea-level highstand lasting for about 5 Myr, b) an interval of tectonic quiescence favoring the dominance of carbonate production, with minimal siliciclastic input, and c) location of the South China paleoplate in the hurricane-free paleoequatorial zone devoid of severe storm disturbances between the fairweather and maximum storm wave bases (normally 15–120 m depths), favoring long-term, uninterrupted growth of CaCO 3 meganodules on a mid-shelf substrate. This study corroborates recent paleogeographic reconstructions that position the South China paleoplate on the Equator during the Late Ordovician. [ABSTRACT FROM AUTHOR]