Characteristics and formation mechanism of carbonate-related silica in late Ediacaran, South China

Abstract

The development of silica has played an important role in both the preservation of Ediacaran macroscopic life and the destruction of reservoir porosity and permeability. Extensive cherts of the late Ediacaran in South China were widely developed, typically in the 4th member of the Dengying Formation (refer to as D4 Formation below). Nevertheless, the characteristics and genesis of these cherts in the D4 Formation remain poorly described and constrained. Here, we have performed a careful petrographic description of siliceous rocks in the D4 Formation and geochemical modeling. It was found that (1) the silica is widely filled in primary matrix pores in the form of early-stage cement; (2) siliceous replacement of carbonate rocks can be occasionally observed under microscope, and residual carbonate inclusions exist in the cherts; (3) regular “adatom islands” structure was observed on the silica surface under scanning electron microscopy (SEM), and no luminescence was observed under cathodoluminescence (CL). The above phenomena indicate that the silica was formed in a low-temperature environment, and the solution precipitating silica was supposed to be several orders of magnitude supersaturated with respect to amorphous silica (SiO₂(am)) to replace carbonates and form the “adatom islands”. Therefore, directly nucleating silica at the ambient solution seems impossible because the aqueous silica concentration (SiO2(aq)) of late Ediacaran seawater is around 2 mmol/kg and is not supersaturated enough. Further geochemical modeling in a solution condition with organic matter (Catechol) shows that the formation and destruction of the organosilicon complex can lead to the supersaturation of SiO₂(aq) concentration with respect to SiO₂(am), and the decrease of pH value can facilitate the destruction of organosilicon. Combined with the widely developed microbialite in the D4 Formation, a comprehensive mechanism with microbial mediation for silica formation is presented here. First, organic matter on the surface of microorganisms was capable of being compounded with aqueous silica (SiO₂(aq)) in seawater, forming abundant organosilicon complexes. Later, the early degradation of organic matter in early diagenesis leads to the destruction of the organosilicon complex and the release of a large amount of SiO₂(aq) in the pore solution, a closed system, which led to the supersaturation with respect to SiO₂(am) and promotes the nucleation and growth of SiO₂(am). The degradation of organic matter also leads to the decrease of pH, which further promotes the destruction of organosilicon and leads to an unsaturated solution with respect to carbonate.