Cavernous weathering in sandstone: lessons to be learned from natural exposure

Abstract

Abstract The most common small-scale weathering features on sandstone landscapes are caverns, widely known as ‘tafoni’. Caverning is by no means confined to one climatic regime; it is, however, especially well-developed in salt-rich environments, such as arid or coastal areas. Caverns are convergent forms created by a variety of different weathering mechanisms; salt may play a role in directly producing mechanical breakdown, or indirectly promoting chemical decay. This study investigates the occurrence of tafoni on the Aztec sandstone in The Valley of Fire, Nevada and the Navajo sandstone in Zion National Park, Utah, USA. These Jurassic sandstones are similar: porous, fine-grained, quartzitic. The weathering features hich characterize them provide a marked contrast. Caverns are frequently developed in the Aztec sandstone; the Navajo sandstone is sparsely caverned, displaying sheer cliffs with arched hollows due to block collapse. The role of microclimatic variations in accelerated weathering in caverns is postulated, on the basis of field observations. Scanning electron microscope analysis suggests that an important control on tafoni occurrence is the lithology and structure of the rock.