Microfossil populations in the context of synsedimentary micrite deposition and acicular carbonate precipitation: Mesoproterozoic Gaoyuzhuang Formation, China

Abstract

Silicified stratiform stromatolites of the Mesoproterozoic Gaoyuzhuang Formation (1.4–1.5 Ga), China, contain well-preserved microfossils. The cherts also harbor varied synsedimentary precipitates and void-filling cements, replaced by early diagenetic silica minerals. These precipitates disclose microenvironments characterized by supersaturated solutions in protected, shallow depressions within an intertidal setting. The precipitates provided surfaces for selective microbial settlement, and rigid sediment matrix for microbial growth. Together with early silicification, the frequent precipitation events contributed to preservation of microfossils by reducing sediment compaction and shearing. Fossiliferous cherts display fine, flat and wavy lamination, characterized by an alternation of highly silicified, thin organic-rich layers with thick sediment-rich layers. Organic-rich layers are dominated either by coccoid or by filamentous microfossils, whereas sediment-rich layers contain abundant synsedimentary precipitates, within which the microfossils are preserved in their growth position. Four dominant microfossils Coccostratus dispergens n. gen. et sp., Eoentophysalis belcherensis, Eoschizothrix composita and Siphonophycus inornatum occur contiguously through several tens of laminae, and are identified as main frame-building biological components of Gaoyuzhuang stromatolites. Community composition, microbial density, distribution, orientation and developmental patterns of the frame-building microfossils are closely correlated with the changing depositional events of Gaoyuzhuang cherts, contrasting conditions of sedimentary kinetics with those of sedimentary stasis. Each assemblage of frame-building microfossils responded to sedimentation with different mechanisms to escape burial. High sedimentation rates correlate with scattered colonies of coccoids and with loose webs of predominantly upright filaments. Low sedimentation rates correlate with dense, laterally connected colonies of coccoids and with a change in filament orientation from vertical to horizontal. In multi-trichomous microfossil Eoschizothrix composita, low sedimentation rates are also accompanied with an increase in number of trichomes per filament. The observed morphological variability of the frame-building microfossils is explained by microbial development, reproduction and behavior by interactions between sedimentological and biological controls.