Showing papers on "Speleothem published in 1990"

Pleistocene climatic change in Southern Australia and its effect on speleothem deposition in some Nullarbor caves

Abstract: Activity ratios of 234U/238U, 230Th/234U, and 230Th/232Th have been determined for calcite, gypsum and halite speleothems from caves of the Nullarbor Plain, mostly in the area N and NW of Mundrabilla Station, for the purpose of U-series dating. All calcite speleothems contain adequate amounts of uranium for dating, but some show an excess of 230Th. Stratigraphic relationships indicate that there were at least three phases of calcium carbonate deposition in the Nullarbor caves. The calcite samples, with one possible exception, have ages in excess of ca. 400000 yrs BP. This suggests that no significant amounts of calcium carbonate deposition have taken place during the last 400ka. At present, active deposition of speleothems is restricted almost entirely to gypsum and halite. The only gypsum speleothem dated was found to have a finite age of ca. 185 ka. Six dates on a small halite speleothem containing insect and arachnid remains indicate that it formed rapidly during Holocene time.

Petrographic and Geochemical Evidence for Origin of Paleospeleothems, New Mexico: Implications for the Application of Fluid Inclusions to Studies of Diagenesis

Abstract: Solution pockets in the Mississippian Lake Valley Formation of New Mexico occur below the sub-Pennsylvanian unconformity. Large masses of columnar calcite spar form within the solution pockets. The calcite shows fine growth banding, stalagmitic and stalaclitic fabrics, coarse detrital inclusions, and truncation of crystal terminations indicating a vadose zone or speleothem origin for the spar. The calcite is twinned and cross-cut by both stylolites and forroan calcite-filled fractures indicating deeper burial after precipitation near the surface. The mean 13C of - 7.29 and 18O of -3.06 PDB of the speleothem calcite defines the light carbon end member of an inverted J-shaped curve of isotopic compositions of pre-Pennsylvanian components in the host limestone. The isotopic relationship between speleothems and pre-Pennsylvanian components suggests a pre-Pennsylvanian age for the speleothems. The isotopic composition and low-magnesium content of speleothem calcite reflects precipitation from meteoric water altered little by stabilization of components of the host rock. Primary fluid inclusions are either one-phase, all liquid which indicate calcite precipitation below about 40-50°C, or two-phase with variable ratios of vapor to liquid. The two-phase inclusions formed by high-temperature reequilibration of low-temperature fluid inclusions, and entrapment of air and water in the vadose zone. The petrographic characteristics and one atmosphere pressure of some of the fluid inclusions suggest precipitation of speleothems in the vadose zone. Some secondary fluid inclusions are two-phase with rather consistent ratios of vapor to liquid. These inclusions indicate a late history of relatively high temperatures. Of the primary fluid inclusions, 74% of the two-phase population have been refilled with brines of deeper burial, and 26% retain fresh water. For the one-phase fluid inclusions, 100% are fresh water. Therefore, the one-phase all-liquid inclusions in partially reequilibrated low-temperature fluid inclusion populations should be viewed as reliable indicators of temperature and salinity of cement precipitation in this and other diagenetic studies.