New evidence for the age of the Macaoyuan Group on the northern margin of the Yangtze block, South China—implications for stratigraphic correlation and palaeogeographic framework

The Macaoyuan Group is an important stratigraphic unit of the Precambrian strata in South China, so its sedimentary characteristics and ages are of great importance for the stratigraphic division and the correlation and reconstruction of the sedimentary facies and paleogeographic framework of the northern margin of the Yangtze block. It is generally accepted that the Maocaoyuan Group comprising the lower Baliya Formation and the upper Huoshaojian Formation on the northern margin of the Yangtze block consists of a succession of the Neoproterozoic molasse formation unconformably overlying the Mesoproterozoic Shennongjia Group. The SHRIMP U-Pb zircon age of the synsedimentary tuff from the upper part of the Baliya Formation is dated at 1165Ma±14Ma. This dating result shows that the Macaoyuan Group should be traced back to the Mesoproterozoic rather than previously-considered Neoproterozoic strata. Field investigation and sedimentary facies analysis demonstrate that there are slump-convolute structures and normal graded bedding formed by the gravity flow in the Maocaoyuan Group, in which the strata are mainly made up of a succession of slope carbonate gravity flow deposits, instead of molasse formation. In combination with previous research results, the authors hold that the Macaoyuan Group is not comparable with the Neoproterozoic rift successions in South China both in sedimentary characteristics and in ages, and the Macaoyuan Group in the Shennongjia area should be assigned to one of the basement strata beneath the Neoproterozoic rift succession in South China.