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

摘要

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 self-organized 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.