Download PDF - Speleogenesis

60
NCKRI Special Paper No. 1
Figure 26. Speleological map of Parrano Gorge, Central Italy.
Key: a = low-permeability cover of marl and sandstone; b = cherty
limestone; c = normal fault; d = morphological scarps (from
Galdenzi and Menichetti, 1995).
Pozzo del Merro near Rome, Italy, is the deepest
underwater shaft, presumably formed by rising thermal
CO2 and H2S water. It has been explored by ROV to -392
m underwater and has a total depth of about 500 m,
including the entrance sinkhole. It shows the morphology
of a rising shaft (Figure 27), in contrast with the roughly
cylindrical morphology of Sistema El Zacatón sinkholes in
Mexico (see below) where hydrothermal cavities at depth
are assumed to open to the surface through collapse.
In south Italy, in northern Calabria, hypogenic caves
formed by rising flow are known in isolated limestone
massifs (Triassic through Cretaceous) surrounded by
clastic Pliocene sediments, in the vicinity of the Sangineto
transform fault, a collision zone between the European and
African plates (Galdenzi, 1997). The caves have 3-D
structures, rising morphology and abundant gypsum
deposits. Some caves reach the current water table, with
sulfur-rich water at 40 o C. Despite their chemistry and high
temperature, isotopic signatures of the deep waters
demonstrate a meteoric origin.
Figure 27. Phreatic shaft, Pozzo del Merro, Italy (from Caramana,
2002).
Hypogenic caves of chamber and maze types have
been recently reported in strongly folded Alpine Jurassic
limestones from Provence, France (Audra et al., 2002).
The Adaouste (a 3-D maze) and Champignons (a chamber
with deep rifts) caves are shown to have formed by rising
thermal artesian waters. They display the key elements of
the morphologic suite of rising flow and specific
hydrothermal secondary formations. Complex evolution is
reconstructed for these caves, with major periods of
hypogene speleogenesis assumed to be Miocene (older
than 11 Ma) for Champignons and Upper Tortonian (8.5 to
5.8 Ma) for Adaouste. Chevalley Aven and Serpents Cave
are active thermal-sulfidic caves presently at the water
table stage in the Bauges massif in the northern French
Pre-Alps (Audra et al., 2007b). They have convection
spherical cupolas at the ceiling, active condensationcorrosion
processes, deposition of calcite rims and
replacement gypsum and popcorn. Audra et al. (2007b)
leave an open question as to whether the ceiling pattern is
created by condensation-corrosion in the vadose zone or if