Radiaxial fibrous calcite as early-burial, open-system cement: isotopic evidence from Permian of China
The Nanpanjiang basin of south China occupies about 100,000 km/sup 2/ in southern Guizhou and eastern Yunnan Provinces and northwestern Guangxi Autonomous Region. This basin contains a thick Paleozoic carbonate sequence overlain by about 3000 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 cement,s interpreted to have been fibrous magnesium 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 magnesium calcites. delta/sup 18/O of RFC cements are more positive than any of the earlier or later components of the reef and fore-reef facies. Analyses of successive bands reveals the most positive delta/sup 18/O near the center of the isopach layers. delta/sup 13/C 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.
- Research Organization:
- Geological Survey, Denver, CO
- OSTI ID:
- 6002621
- Report Number(s):
- CONF-850322-
- Journal Information:
- Am. Assoc. Pet. Geol., Bull.; (United States), Vol. 69:2; Conference: American Association of Petroleum Geologists annual meeting, New Orleans, LA, USA, 25 Mar 1985
- Country of Publication:
- United States
- Language:
- English
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