Origin of honeycombs and related weathering forms in Oligocene Macigno Sandstone, Tuscan coast near Livorno, Italy

Abstract

Two types of cavernous-weathering features are exposed in the Oligocene Macigno Sandstone along 5 km of the Tuscan coast south of Livorno, Italy. Honeycomb cells (type 1 features) are typical closely spaced, more or less circular pits of centimetre scale that have been eroded 2 to 6 cm below the general surface of bedding planes or joints. 'Aberrant honeycomb' cells (type 2 features) are highly elongate, polygonal, or irregular flat depressions of decimetre scale surrounded by walls rarely higher than 2 cm, some of which pass into long, free-standing walls or tendrils. Thus, not all type 2 'honeycomb' cells are fully enclosed.We measured the geometry of 551 honeycomb cells and examined various rock properties (microscopic texture and fabric, mineralogy, porosity, permeability, and chemical composition) to isolate factors that control the size, shape, distribution, and pattern of the honeycombs. Our goal was to narrow potential origins of the features and to understand their formation.The ubiquitous occurrence of sea salt in the honeycombs and scanning electron microscope evidence of physical weathering of silicates, especially micas, favours an origin for the honeycombs chiefly by salt weathering. Honeycombs do not form in siltstone, iron-oxide-impregnated sandstone, calcite-cemented concretions, or in case-hardened joints. Thus, salt weathering of type 1 and 2 honeycombs is not effective in very low permeability rocks.We propose for type 1 honeycombs that seawater is drawn into micropores of the sandstone and evolves into selforganized diffusion cells (Turing patterns). Selective evaporation at the stationary nodes of diffusion cells, which form at the same site over time, leads to the precipitation of salt, then grains spall off, and pits are formed. The deepest pits (>40 mm) formed where Turing patterns consistently formed at the same sites. Although the walls are more porous and weathered than the host sandstone, they become selectively case hardened by an unidentified component of low abundance. Initial honeycomb cell shape and gravity locally influenced type 1 honeycomb shapes.We suggest that type 2 honeycombs develop where diffusion-controlled Turing patterns lead to case-hardening along linear trends; gravity and rock fabric are important locally in influencing the orientation of the walls. Only type 2 cells are forming today, suggesting recent environmental changes.Gravity is not a fundamental control on honeycomb shape; in places it is a contributing factor. Pre-existing depressions (quarry tool marks) have strongly influenced honeycomb shape locally.

Biogenic origin of coastal honeycomb weathering

“…The traditional assumption has been that surface hardening is a dominant factor during the formation of cavernous weathering patterns. For example, McBride and Picard (2004) observed algal biofi lms on honeycomb cavity walls, but dismissed the possible signifi cance of . Absorption times for a drop of water applied to the interior surfaces of honeycomb cavities provide evidence of the protective effects of biofi lms.…”

“…Ball (1999) provided an updated overview. McBride and Picard (2004) noted the resemblance of honeycomb weathering patterns to 'Turing cells'. According to the Turing model, the driving force for self-organized patterns is an oscillating balance in diffusion of a general inhibitor and a local reaction.…”

“…Observations from the Pacifi c Northwest resulted in new interpretations of several factors related to coastal honeycomb weathering. Ali Uzun (1998) and McBride and Picard (2004) measured cavity shape to categorize coastal honeycomb weathering patterns. However, the position of honeycomb weathered zones relative to tide level appears to be a more important geomorphic criterion; development of honeycomb cavities in the intertidal zone follows a different pathway from those that form at supratidal elevation.…”

Sculpting of Rocks by Reactive Fluids

“…Tafoni formation (at smaller scales known as honeycomb weathering, Fig. 4.16) has been studied extensively and it is generally believed that growth of salt crystals plays an important role (Bradley et al, 1978;McBride and Picard, 2004). Huinink et al (2004) presented an informative simulation of the growth of a single tafone cavity.…”

The influence of temperature in a capillary imbibition salt weathering simulation test on Mokattam limestone

“…This material is particularly susceptible to salt weathering, which in turn caused important damages in archaeological sites. (1)(2)(3)(4)(5)(6). The mechanism of salt precipitation and consequent weathering forms are controlled by the petrographical characteristics of the limestone, as well as the dominant environmental conditions, e.g.…”