Some examples of palaeokarst in Sardinia

Abstract

The Lower Cambrian limestones and dolostones of Iglesiente (Southwest Sardinia, Italy) are known to host important active and fossil karst phenomena. Besides active and still accessible cave systems, often characterised by important speleothem formations and rare cave minerals (e.g. hydrozincite, hemimorphite etc.) palaeokarsts have been encountered in underground excavations or in quarries. Several of the important mineral deposits are in fact infillings of great karst cavities, such as at Barega or Mount San Giovanni. During various geological surveying campaigns several different palaeokarst deposits have been recognised and studied. Among these some deserve further attention and are here described in detail. On the top of Mount San Giovanni (Gonnesa), a short mine drift at 300 m a.s.l. has intercepted a decametre-sized cavity filled with pyroclastic sediments. Other similar but smaller cavities have been discovered on the western slope of the mountain. These are composed of rhyolitic ignimbrites and epiclastites of unknown age. These volcanoclastics may be related to the volcanism that characterised this part of the Island during Oligo-Miocene, or, in alternative, may be products of the Permian volcanic cycle. In any case, their preservation on the top of the mountain testifies the extremely low denudation rates of these carbonates at least in this part of Sardinia. Some 40 metres higher, in the same area, another sinkhole filled with red detritic sandy sediments has been exposed on a quarry face. Also these sediments may be of Oligocene or Permian-Triassic age and are probably genetically connected to the aforementioned palaeokarsts. Of completely different nature is the palaeokarst discovered in a limestone quarry at San Luigi (Buggerru), 15 km North of Mount San Giovanni. Here several pockets and fractures are filled with fine and laminated yellowish-brownish sands. The almost vertical layering of these sediments is concordant with the closeby contact between Lower Cambrian ceroid limestones and dolostones. This would indicate their sedimentation before their dislocation occurred in Middle Ordovician ("Sardinian Phase", Auct.). If this is true, these karst sediments are the among the oldest found in Iglesiente. Further detailed investigations on these palaeokarsts are under way and will give further insight into the karst evolution of this complicated area.

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Full text

Geophysical Research Abstracts
Vol. 12, EGU2010-3436, 2010
EGU General Assembly 2010
© Author(s) 2010
Some examples of palaeokarst in Sardinia
Jo De Waele (1), Sandro Pasci (2), and Laura Sanna (3)
(1) Italian Institute of Speleology, University of Bologna, Via Zamboni 67 - 40127 Bologna, ITALY (jo.dewaele@unibo.it),
(2) Consulting geologist, Project Carg Autonomous Region of Sardinia, Cagliari, ITALY (sanpas@tiscali.it), (3) GEO,
Department of Earth Sciences, Bergen University. Allegaten 41, N-5007, Bergen, Norway (speleokikers@tiscali.it)
The Lower Cambrian limestones and dolostones of Iglesiente (Southwest Sardinia, Italy) are known to host im-
portant active and fossil karst phenomena. Besides active and still accessible cave systems, often characterised by
important speleothem formations and rare cave minerals (e.g. hydrozincite, hemimorphite etc.) palaeokarsts have
been encountered in underground excavations or in quarries. Several of the important mineral deposits are in fact
infillings of great karst cavities, such as at Barega or Mount San Giovanni.
During various geological surveying campaigns several different palaeokarst deposits have been recognised and
studied. Among these some deserve further attention and are here described in detail.
On the top of Mount San Giovanni (Gonnesa), a short mine drift at 300 m a.s.l. has intercepted a decametre-sized
cavity filled with pyroclastic sediments. Other similar but smaller cavities have been discovered on the western
slope of the mountain. These are composed of rhyolitic ignimbrites and epiclastites of unknown age. These vol-
canoclastics may be related to the volcanism that characterised this part of the Island during Oligo-Miocene, or,
in alternative, may be products of the Permian volcanic cycle. In any case, their preservation on the top of the
mountain testifies the extremely low denudation rates of these carbonates at least in this part of Sardinia.
Some 40 metres higher, in the same area, another sinkhole filled with red detritic sandy sediments has been exposed
on a quarry face. Also these sediments may be of Oligocene or Permian-Triassic age and are probably genetically
connected to the aforementioned palaeokarsts.
Of completely different nature is the palaeokarst discovered in a limestone quarry at San Luigi (Buggerru), 15 km
North of Mount San Giovanni. Here several pockets and fractures are filled with fine and laminated yellowish-
brownish sands. The almost vertical layering of these sediments is concordant with the closeby contact between
Lower Cambrian ceroid limestones and dolostones. This would indicate their sedimentation before their disloca-
tion occurred in Middle Ordovician (“Sardinian Phase”, Auct.). If this is true, these karst sediments are the among
the oldest found in Iglesiente.
Further detailed investigations on these palaeokarsts are under way and will give further insight into the karst
evolution of this complicated area.