Abstract:

A chaotically brecciated limestone is found in the Pingliang formation of the Middle Ordovician in Fuping area,Shaanxi province.It occurs as bedded or sheet strata in,and in sharp contact with,the foliated and thinly tabular limestones.The fragments of it are highly varied in size and chaotically distributed,with the giant being 1?7m or larger.This limestone is lack of stratification and sorting,in which floating boulders and blocks are scattered in the matrix of finer clasts.These features show that it is of deep water carbonate sediments of debris.This limestone is dominantly composed of breccias of inner basin source.These clasts,formed by gravity sliding of consolidated or semiconsolidated sedimentary layers on slope of deep water basin,are transported by gravity and have accumulated on the margin of the basin floor.In the Middle Ordovician,the Fuping area was located on the northern margin of the deep water basin,where the maximum water depth was nearly one thousand metres.After the deposition of the Early Ordovician Majiagou formation,this area was subsided to form a deep water basin on the continental margin,by the tension fracturing of the back-arc basin on the northern side of the Qinling and Qilian Mountains.