Radiaxial Fibrous Calcite as Early-Burial, Open-System Cement: Isotopic Evidence from Permian of China: ABSTRACT

Abstract

The Nanpanjiang basin of south China occupies about 100,000 km2 in southern Guizhou and eastern Yunnan Provinces and northwestern Guangxi Autonomous Region. The basin contains a thick Paleozoic carbonate sequence overlain by about 3,000 m of Triassic basinal deposits. Permian carbonate rocks comprise a large portion of the Paleozoic strata and form several platforms separated by basins containing dark, thin-bedded limestones, siliceous shales, and cherts. The platform margins are rimmed by sponge or algal reefs. Radiaxial fibrous calcite (RFC) is the most abundant cement in very coarse sponge or algal debris of Upper Permian reef and fore-reef sediments exposed along the western margin of the Nanpanjiang basin. Small volumes of syndepositional cements, interpreted to have been fibrous magnesian calcites and botryoidal aragonite, predate RFC. Coarse, blocky burial calcite postdates RFC. Evidence that RFC was precipitated during sediment deposition was not found. RFC occurs as isopach layers up to 15 mm thick and exhibits white, gray, and black bands about 1 mm wide. The presence of microdolomite inclusions in these cements indicates that they were originally magnesian calcites. ^dgr18O of RFC cements are more positive than any of the earlier or later components of the reef and fore- eef facies. Analyses of successive bands reveals the most positive ^dgr18O near the center of the isopach layers. ^dgr13C of successive bands reveals generally more negative values toward the centers of layers. RFC layers are interpreted to have precipitated during early burial of the platform margin while reef and fore-reef sediments were in communication with seawater. Cement layers recorded isotopic characteristics of seawater as platform-edge sediments subsided through the water column at the basin margin. ^dgr18O of successive bands records cooler water at depth in the basin followed by geothermal warming. ^dgr13C records increased incorporation of light carbon as the platform subsided through the oxygen minimum zone, followed by a return to normal values at depth. These data and interpretations suggest RFC layers precipitated very slowly during time spans commensurate with those of subsiding platforms (millions of years). Isotopic characteristics of RFC may not reflect shallow seawater. Rather, they may reflect burial environments where ^dgr18O is affected by cooler water and ^dgr13C is affected by biologic activity. End_of_Article - Last_Page 261------------