Tafoni weathering is an azonal process: Examples from Antarctica, Sardinia and Australia

Highlights

• Tafoni occur with azonal conditions like salt, aridity, high evaporation/sublimation.

• Tafoni can develop and persist for many thousands, if not millions, of years.

• High daily temperature amplitudes can be another commonality behind tafoni formation.

Abstract

Cavernous weathering (tafoni) is described from coarse-grained granitic bedrock in Northern Victoria Land (Antarctica), northern Sardinia (Italy) and southwestern Australia. It also occurs elsewhere in the world and on other coarse-grained lithologies. The cause of this unusual in-situ flaking and granular disintegration is problematic. Azonal conditions appear to be: (1) the presence of salt, (2) aridity and (3) high evaporation and sublimation rates. Zonal (site-specific) considerations are: (1) snow may transport salts far inland in Antarctica and (2) epilithic and endolithic lichen growth may assist biological weathering in Sardinia and Antarctica. In stable landscapes of great antiquity, such as southwestern Australia, tafoni-like forms appear to have developed over millions of years; they suggest that salts in solution move slowly and continuously towards the surface where they evaporate. Combinations of these conditions result in tafoni in these different environments.

Introduction

Cavernously-weathered forms (tafoni) are well developed on coarse-grained monzogranite in the cold, arid permafrost environment of Terra Nova Bay (TNB) in Northern Victoria Land, Antarctica. We suggested initially that granular disintegration was caused by the apparent susceptibility of feldspar minerals to saline cryogenic conditions (French and Guglielmin, 2000, French and Guglielmin, 2002). Unfortunately, the cryogenic explanation is not universally applicable because similar phenomena occur in regions where frost action is minimal and permafrost is absent. Subsequent studies in Antarctica suggested that lichens and biochemical weathering processes are also involved (Guglielmin et al., 2005, Guglielmin et al., 2011). Accordingly, the authors undertook reconnaissance studies in Sardinia, adjacent to the type area where tafoni were first described, and southwestern Australia where a stable granitic landscape of some antiquity can be observed.

Our observations suggest the possible azonal and zonal (site-specific) conditions that permit similar weathering morphologies to develop in such widely different climatic environments. Discussion focusses upon the age and rate at which tafoni develops and the persistence of tafoni in old landscapes not subject to coastal influences.

Tafoni weathering was first described from the Mediterranean region (Tuckett, 1884). The accepted definition of tafoni (sgl: tafone) is that of cavities of ‘measureable size’ with arch-shaped entrances, concave inner walls, overhanging margins (visors) and fairly smooth, gently-sloping debris-covered floors (Turkington and Phillips, 2004). Tafoni are known to occur in many of the arid and semi-arid regions of the world (e.g., see Twidale, 1976; Ollier, 1984; Campbell, 1999; Migon, 2006; Viles, 2011; Goudie, 2013) but obviously also in the traditional Mediterranean sites (i.e. Reusch, 1882; Brandmeier et al., 2011). Tafoni have also been described from many parts of Antarctica (see Calkin and Cailleux, 1962; Campbell and Claridge, 1987; Conca and Astor, 1987; Matsuoka, 1995; French and Guglielmin, 2002; Andrè and Hall, 2005; Guglielmin et al., 2005; Strini et al., 2008). Only a few examples related to more temperate and relatively humid climate as in Finland (Kejonen et al., 1988), South Korea (Matsukura and Tanaka, 2000) or South Africa (Grab et al., 2011). The major exception may be the case of the tafoni reported in Honk Kong (Wilhelmy, 1964) in tropical conditions with precipitation that exceeds 2000 mm per year and with a MAAT of more than 20 °C.

Tafoni occur on different kinds of substrate, generally on granular rocks such as sandstone, granite, tuff and conglomerate (I.e. Dragovich, 1967; Mellor et al., 1997; Matsukura and Tanaka, 2000; Una Alvarez, 2004; Guglielmin et al., 2005; Strini et al., 2008; Mol and Viles, 2012) and to a lesser extent on other kinds of rock such as limestone (Smith, 1978; Norwick and Dexter, 2002), dolerite (Conca and Astor, 1987) and various metamorphics (Reusch, 1882; Kejonen et al., 1988). Here, we will concentrate only granitoid rocks (acidous intrusive rocks as granite, granodiorite, diorite etc.; Table 1).

Although their formation is still debated several processes related to the climate forcing were suggested as the driving processes behind tafoni formation and growth from wind action (i.e. Hall, 1989; Gillies et al., 2009), the moisture regime (i.e. Mellor et al., 1997), or the thermal stress (Strini et al., 2008). Many of the geomorphic processes evocated to explain the formation and growth of the tafoni are related to the salt action (both mechanical and/or chemical) that can act differentially within the rock (i.e. Mellor et al., 1997; Brandmeier et al., 2011). Salt action in the tafoni formation needs a least an arid period during the year (Brandmeier et al., 2011). It is also true that biological action (cryptoendolitic, epilitic lichens, algae and or bacteria) can have a fundamental role especially in the case hardening formation (i.e. Souza-Egipsy et al., 2004; Viles and Goudie, 2004; Guglielmin et al., 2005, Guglielmin et al., 2011).

There is relatively little information upon growth rates. Values that range between 2.9 and 220 mm/Ka have been suggested (Brandmeier et al., 2011; Paradise, 2013) although Norwick and Dexter (2002) found very old tafoni on sandstone outcrops of Arizona suggesting a slower weathering rate ranging between (0.84 and 2 mm/Ka) while more recently, in Antarctica, Ponti et al. (2021) substantially confirm weathering rates on granites ranging between 11 and 238 mm/Ka. According to Matsuoka and Matsuoka (1991), the rate of deepening of the hollows is thought to be an exponential function of time.