THE FORMATION AND SIGNIFICANCE OF CORALLINE SILICA SPELEOTHEMS IN THE SYDNEY BASIN, SOUTHEASTERN AUSTRALIA

Abstract

The vast majority of speleothems are composed of calcium carbonate, but this paper focuses on a suite of less-commonly reported speleothems, those formed almost entirely of silica, and on a quartzose rather than carbonate base rock. On the quartz sandstones of the Sydney region of southeastern Australia, small branched coralline silica stalactites, up to 10 mm in diameter and 80 mm in length, frequently are found. These stalactites have not been described in detail before and are clear evidence of the solution of silica (including quartz) from within the sandstones, and its transport and redeposition at the rock surface. Detailed analysis using SEM, XRD, and thin-section techniques revealed layered composition of both amorphous opal-A and crypto-crystalline chalcedony. Calcite forms no part of these stalactites. The internal structure also shows that they do not form in a manner similar to calcite stalactites. While the growth rate of these silica stalactites remains unknown, it is believed to be quite slow. But they clearly are not relict features and are forming under present-day temperate-climatic conditions, not the tropical conditions demanded by some authors for silica stalactites elsewhere. Further detailed study of these and other silica speleothems is continuing, as they have the potential to be used as important records of palaeoclimatic and land-use change. [Key words: silica karst, speleothem, coralline silica, silica stalactite, opal-A, chalcedony, quartz, silica solution.]