Stalactites on tropical cliffs: remnants of breached caves or subaerial tufa deposits?

摘要

Stalactitic deposits of calcium carbonate are often seen on cliffs and in other epigean setting's in the tropics. Since it is generally accepted that the formation of stalactites requires the enclosed atmosphere of caves, such deposits at the land surface are often considered decayed speleothems, remnants of former caves breached by erosion and collapse. However, our sampling OF of stalactites from epigean settings in the Mariana Islands, Western Pacific, has revealed that OF they belong to at least two distinct categories of deposits, which exhibit a superficially similar form and structure but have profoundly different depositional histories and are not necessarily related to former caves. We have termed them exposed stalactites and stalactitic tufa. The former are hard, densely layered, coarsely crystalline calcite deposits that formed as true spelcothems and subsequently became exposed at the land surface. They indeed are indicators of dissolutional landforms and former caves. The latter, however, are subaerial deposits of calcareous tufa, which convincingly mimic spelcothems but actively grow in humid epigean settings. They arc contemporary parts of the land surface environments in which they arc observed and cannot be used to indicate de-roofed, intersected, or otherwise exposed spelean environments.