Origin of the Furongian limestone breccias in the North China Platform

Abstract

Limestone breccias are a common phenomenon in the Cambrian successions worldwide. They bear important geological implications that have attracted geologists for several decades. There are, however, still controversies on their origins, especially those of the breccias with abundant vertically orientated clasts. The Furongian (upper Cambrian) Chaomidian Formation of the North China Platform contains numerous levels of limestone breccias and conglomerates that provide an excellent example to look into their formative processes. These breccias and conglomerates have been the focus of study and discussion since the 1980s, but yet there is still no consensus with respect to their geneses. Recently, Van Loon and others argued that the vertically orientated clasts of the breccias developed by a number of simultaneous “fountains” on the paleo-seafloor; the “fountains” formed by upward-directed fluidized flows originated from the sediment underlying the brecciated limestones. While the novel “fountain” hypothesis is not impossible, based on field evidences and theoretical considerations, however, it is most likely that the vertically orientated clasts resulted from their re-orientation by upward flow of thixotropically liquidized, uncemented argillaceous sediment that was interbedded with brecciated limestone fragments. Besides, the deformation processes most likely took place under shallow burial.