Download PDF - Speleogenesis

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NCKRI Special Paper No. 1
Evidence for hydrothermal hydrogeologic and karst
features is abundant from carbonate-composed folded
structures in many regions of Romania, particularly in the
Codru Moma and Bihor Mountains. In the former region,
thermal waters rise through Mesozoic limestones, along
thrust-related faults and basin-limiting faults, from the
underlying dolomites and quartzitic sandstones, producing
thermal springs (Oraseanu and Mather, 2000). In the Bihor
Mountains, Onac (2002) described skarn-hosted and
classical hydrothermal caves, passages, and rooms with
specific mineralization encountered by ore mines.
Individual caves vary in size and total up to 500 m in
length. The skarn-hosted caves are supposed to form
during contact metamorphism of limestones, which
involves decarbonation and release of CO2. Later on, caves
were formed within the overlying limestones by rising
thermal flow, either by retrograde solubility along cooling
paths or by mixing of ascending hydrothermal fluids with
oxygenated waters of a shallow flow system.
The famous Movile Cave in the Miocene oolitic
limestones of the Dobrogea Plateau, the Black Sea area, is
assumed to have an epigenic origin related to the drop in
sea level of the Black Sea during middle Pleistocene
(Lascu, 2004), but the presence of a confined aquifer in the
deeper sections of the carbonate sequence with low-grade
thermal sulfidic waters, and a highly specific assemblage
of cave fauna indicative of an isolated cave environment,
suggest the possibility of a hypogenic origin for this cave.
This is also supported by the presence of a 4 km long maze
cave nearby.
One of the most outstanding examples of deep-seated
hypogene speleogenesis is a giant cavity encountered by
boreholes in Precambrian marbles in the Rodopy
Mountains, Bulgaria (Dubljansky V., 2000). The top of the
cavity is intercepted at 560-800 m below the surface, and
the greatest measured vertical dimension of the cavity is
more than 1340 m (the borehole did not reach the bottom).
The cavity has an estimated volume of 237.6 million m 3
and is filled with thermal waters. The highest measured
water temperature was 129.6 o C at a depth of -1279 m (359
m below sea level). Hydraulic head is several hundred
meters higher than the top of the cavity. Many smaller
cavities of hydrothermal origin are known in this area.
Outstanding hypogenic caves, formed by rising
thermal H2S waters, are known in central and south Italy
(Umbria and Marche regions), in Jurassic and Paleogene
carbonates of the Neogene fold-and-thrust belt (Galdenzi
and Menichetti, 1995). Jurassic carbonates are variable in
initial porosity, with well-developed sedimentary facies in
4-5 m thick cyclothemic sequences. Some hypogenic caves
occur in Pleistocene travertine. They are large 3dimensional
maze systems, in which patterns of basal input
zones and points are recognized through fractures and
vuggy porosity networks at the bottom of the carbonate
sequence where mineralized water rises up from the
Triassic evaporitic beds. Their morphology displays
characteristic features of uprising flow, including rising
shafts, wall and ceiling half-tubes, roof pendants, cupolas
and blind domepits, through network and spongework
passages situated at different levels. These caves contain
massive gypsum deposits, like those found in the caves of
the Guadalupe Mountains, USA. Galdenzi and Menichetti
(1995) noted the remarkable similarity of internal
morphologies in the caves that have different patterns and
local geological contexts. Relict hypogenic caves are
represented by Monte Cucco caves (31.3 km long and 930
m deep 3-D maze system; Figure 24) and Pozzi della Piana
(3.5 km long network maze in two levels within a vertical
range of 25 m); whereas sections in some caves, such as
those in Frasassi Gorge and Acquasanta Terme, are still
active, presently at the water table, with H2S waters rising
from depth. The principal chemical mechanism for
hypogenic speleogenesis in the region is sulfuric acid
dissolution due to mixing of deep and shallow flow
systems.
Figure 23. Plan view and profile of Frasassi caves. Note inclined
cave stories in the profile (from Hose and Macalady, 2006).
Some caves that display quasi-horizontal levels (e.g.
the Frasassi caves; Figure 23) were interpreted to form in
the shallow phreatic zone near the water table, where H2S
oxidation is most intense in the present setting. However,
Galdenzi and Menichetti (1995) have noted difficulties in
explaining the upper levels of the Frasassi caves by
analogy with the present water table/shallow