Characterization and origin of tafoni on conglomerate slopes in Huoshizhai National Geopark of the western Loess Plateau

Tafoni are globally developed on cliffy slopes, and many of them are favorable places for the preservation of stone historical relics. However, the characteristics and formation processes of tafoni in the Loess Plateau are yet to be understood. This paper studied the features of the tafoni on conglomerate slopes in Huoshizhai National Geopark of Ningxia Hui Autonomous Region and discussed its formation processes by field investigation and morphometry, in-situ relative humidity (RH) measurement, salt chemistry and X-ray fluorescence spectrometer (XRF) experiments of 24 samples. The bedrock of the tafoni is dominated by reddish fluvial conglomerates of the Lower Cretaceous Heshangpu Formation with abundant chemically unstable components including feldspars, lithic fragments, and calcite cements. The RH values vary from 5% to 100%, but the backwalls of the tafoni have higher RH values than outer surfaces. The more moisture on the backwalls is possibly generated by water influx from the rock interior, resulting in more salt precipitation on the backwalls. As a result, the backwalls have been subject to predominant salt weathering. The dominant salts involved in salt weathering are probably derived from the dissolution of the salt interbeds in the basin, although the chemical dissolution of the unstable components such as feldspars, lithic fragments, and calcite cements might have produced small amounts of salts. The salt types dominantly include nitrates, sulfates, and halite. In the progression of tafoni, the moisture maintenance on the backwalls gives rise to the accretion of salts, which in turn enhance the weathering rates of the backwalls. As a result, the volumes of the tafoni have become enlarged owing to inward growth and coalescence of adjacent smaller ones.