New evidences for sedimentary attributes and timing of the “Macaoyuan conglomerates” on the northern margin of the Yangtze block in southern China

Abstract

Recent field geological investigations and sedimentary facies analysis have disclosed that the “Macaoyuan conglomerates” should be assigned to the dolomitized intraclastic lime grainstones rather than the basal conglomerates as previously reported. The conglomerates display depositionally-erosional conformable contacts with their underlying stratigraphic succession. There is no so-called “angular unconformity” or associated “Shennong movement” between the Macaoyuan Group and Shennongjia Group. The Macaoyuan Group consists primarily of carbonate platform facies association and mixed clastic and carbonate shelf facies association, while the “Macaoyuan conglomerates” are mainly made up of carbonate platform slope-channel gravity flow deposits. The sedimentary structures such as basal scouring structures, slump-convolute structures, imbricate structures, inverse grading bedding and lenticular filling deposits are well developed in the slope-channel gravity flow sedimentary succession, whereas cross bedding, wavy bedding, parallel bedding, ripple bedding, and normal grading bedding are common in the mixed inner shelf and platform-margin shoal sedimentary succession. The SHRIMP and LA-ICP-MS zircon age dating techniques are employed for the synsedimentary tuff sample collected from the upper part and sedimentary detrital zircons from the topmost part of the Baliya Formation in the lower part of the Macaoyuan Group. The synsedimentary tuff sample gives a SHRIMP zircon U–Pb age of 1157±19Ma, which agrees well with the maximum depositional age of 1180 Ma for the sedimentary detrital zircons. It follows that the age of deposition of the Macaoyuan Group should be traced back to the Mesoproterozoic rather than the Neoproterozoic. The ages of the detrital zircons are mainly concentrated in two time intervals: 1.16–1.26Ga and 2.60–2.95Ga. The ca. 2.60Ga as a marked peak age may represent an important period for the crustal accretion and evolution of the Yangtze block.