Spherical and ellipsoidal cavities in European sandstones: a product of sinking carbonate dissolution front
Adamovič, Jiří; Mikuláš, Radek; Navrátil, Tomáš
published: Jan 1, 2015
ArtNo. ESP023105901014, Price: 29.00 €
Symmetrical cavities of spherical, ellipsoidal or subtriangular shapes, tens of centimetres to 2–3 m in diameter, have been documented in a number of European sandstone districts extending between Luxembourg and Poland. Although previously thought to be synonymous with true tafoni by some authors, the studied symmetrical cavities share specific features which point to an origin linked with the dissolution of concretionary precursors. Field evidence includes their spatial association with carbonate-cemented concretions and the presence of a variety of convex, convexo-concave and concave forms depending on the degree of dissolution of carbonate cement. The presence of enclaves of loose sand in sandstone – sand nests – and the absence of karst features in outcrops suggest that carbonate dissolution took place largely under the surface, while remodelling of the solutional cavities into more complex forms including caves is governed by present-day weathering processes. A clear vertical zoning in the intensity of carbonate dissolution was observed, with a well-defined transition from completely dissolved carbonate above (fully developed symmetrical cavities) to undissolved carbonate below. This roughly horizontal carbonate dissolution front visible in outcrops is interpreted as a result of infiltration of acidic meteoritic waters and their neutralization by carbonate contained in the bedrock. The established neutralizing front progresses downward as long as the infiltrating waters retain their capacity for carbonate leaching; this may be enhanced by, e.g., their enrichment in CO2 due to elevated soil pCO2 during interglacial periods. Downward progress of the neutralizing front is probably still active in the carbonate-rich Luxembourg Sandstone but was inhibited elsewhere due to reduction of the recharge area (Góry Stołowe Mts or Szczyrzyc area) or protection by a loess cover in the Last Glacial (Kokořín area). Given the wide geographic range and lithological variation of the studied sites, the dissolution of carbonate-cemented concretions can be considered the most common initiation process for symmetrical cavities in sandstones of the humid temperate zone.