Geoscience Reference
In-Depth Information
14
Speleothems and tufas in arid areas
Dans les champs de l'observation, le hasard ne favorise que
les esprits prepares.
In the field of observation, chance favours the prepared mind.
Louis Pasteur (1822-1895)
Lecture
, 1854
14.1 Introduction
The term
speleothem
is a hybrid word derived from the two Greek words for 'cave' and
'deposit', which is slightly misleading, since not all cave deposits are speleothems, but
all speleothems are cave deposits. For the sake of simplicity, we define speleothems
as secondary mineral deposits formed within caves and usually consisting of calcite
or aragonite (calcium carbonate: CaCO
3
) but sometimes made up of gypsum (cal-
cium sulfate: CaSO
4
). Speleothems thus comprise the well-known dripstone deposits
termed
stalactites
(growing down from the roof of the cave) and
stalagmites
(growing
up from the floor of the cave, as well as more extensive
flowstone
sheets and curtains
(
Figure 14.1
). Other types of cave deposits include material that has been blown or
washed in, as well as cave breccias formed from roof collapses.
Two other common forms of secondary CaCO
3
deposit are
travertine
and
tufa
,
which some authors include as speleothems but which are best kept separate. In his
monograph
Travertine
, Pentecost (
2005
,p.3)defines
travertine
as: 'a chemically-
precipitated continental limestone formed around seepages, springs and along streams
and rivers, occasionally in lakes and consisting of calcite or aragonite, of low to
moderate intercrystalline porosity and often high . . . framework porosity within a
vadose or occasionally shallow phreatic environment'. (The
vadose
zone in caves and
bedrock refers to the zone above the regional watertable, while the
phreatic
zone lies
beneath the watertable.)
In the third edition of their
Glossary of Geology
, Bates and Jackson (
1987
, p. 705)
define
tufa
as 'a chemical sedimentary rock composed of calciumcarbonate, formed by
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