1
CHAPTER
1:
INTRODUCTION
TO
THE
SUDY
OF
TRAVERTINE
1
–
INTRODUCTION:
THESIS
AIMS
Travertines
are
terrestrial
carbonates
that
are
deposited
exclusively
by
water
issuing
from
hydrothermal
springs
according
to
the
definition
by
Pedley
(1990).
Pleistocene-‐Holocene
hydrothermal
spring
travertines
have
attracted
attention
for
centuries
since
the
Romans
extensively
quarried
the
lapis
tiburtinus
as
a
construction
building
stone.
Travertines
have
been
the
focus
of
numerous
studies
about
Quaternary
geology,
geomorphology,
hydrogeology,
Neogene
tectonics
and
paleoclimatology.
In
the
last
decades
several
travertine
depositing
sites
were
studied
in
detail
from
a
sedimentological
and
carbonate
petrography
point
of
view:
Tivoli
travertines
near
to
Rome
(Chafetz
and
Folk,
1984;
Faccenna
et
al.,
2008),
Mammoth
Hot
Spring
in
Yellowstone
National
Park
(Wyo ming,
USA,
Pentecost,
1990;
Fouke
et
al.,
2000)
and
Pamukkale
(Altunel
and
Hancock,
1993).
There
are
many
open
questions
regarding
the
type
of
precipitation
process
controlling
some
travertine
fabrics,
abiotic
vs.
biologically
(microbial)
mediation.
The
sc ientific
community
working
on
travertines
is
still
divided
on
this
point.
It
is
not
possible
to
clearly
establish
whether
travertine
precipitation
is
a
fully
abiotic
or
microbially
induced
process.
It
is,
however,
first
of
all
important
to
produce
a
detailed
database
of
the
different
fabric
types
and
their
occurrence
in
specific
depositional
settings,
controlled
by
hydrothermal
water
temperature,
velocity
of
flow
and
degree
of
turbulence
to
clearly
determine
the
physical
factors
affecting
travertine
carbona tes.
This
is
what
has
been
attempted
in
this
study.
This
thesis
investigates
the
travertine
deposits
of
the
Tronto
valley,
more
precisely
the
travertine
deposits
in
the
Acquasanta
Terme
area
(Upper
Pleistocene),
about
20
km
from
Ascoli
Piceno,
Southern
Mar che
(Central
Italy).
Different
travertine
units
were
analysed
within
two
quarries
that
show
different
depositional
systems
and
are
placed
at
two
different
topographic
elevations:
Tancredi
quarry
(451m)
and
Cardi
quarry
(335m).
These
elevations
relate
to
different
depositional
phases
and
ages;
in
fact
the
lower
unit
is
the
youngest
due
to
its
position
along
the
fluvial
valley
incision
of
the
Tronto
River.
The
aims
of
the
project
are:
1.
To
analyse,
describe
and
interpret
the
depositional
setting
of
the
several
carbonate
fabrics
of
the
Acquasanta
travertine
proposing
a
general
classification
of
the
carbonate
fabrics.
2
2. To
understand
the
interaction
between
the
precipitation
(calcite
or
arag onite)
and
the
hy-‐
drothermal
water.
3. To
better
comprehend
the
interactions
between
carbonate
fabrics
and
depositional
sy s-‐
tems,
suggesting
the
precipitation
process
and
hydrothermal
water
energy
of
each
deposit
type.
4. To
establish
how
travertine
facies
can
affect
the
formation
of
a
good
reservoir
analysing
the
values
of
porosity
and
permeability.
The
results
from
the
study
of
the
Acquasanta
travertine
deposits
contribute
to
the
knowledge
about
travertine
systems
and
can
be
a
good
example
for
comparisons
with
others
different
deposits
in
Italy
and
worldwide.
The
outcomes
of
this
study
can
help
to
better
understand
the
relationship
between
carbonate
facies,
their
porosity
and
permeability
types
and
depositional
system
geometry.
3
2
–
INTRODUCTION
TO
TRAVERTINE:
WHAT
TRAVERTINE
DEPOSITS
ARE?
The
term
travertine
describes
all
non-‐marine
carbonate
deposits
that
form
as
precipitated
from
hydrothermal
water
(temperature
>
20⁰C)
issuing
from
springs
and
subaerial
vents
with
a
crystalline
primary
fabric
(Ford
and
Pedley,
1996;
Riding,
2002).
Pedley
(1990)
and
Ford
and
Pedley
(1996)
addressed
carbonates
precipitated
from
non -‐hydrothermal
continental
fresh
water
of
fluvial-‐marsh
origin
and
karstic
springs
as
“Calcareous
Tufa”
or
Tufa
(Fig.
2.1B).
There
are,
however,
differ ent
definitions,
usages
and
meanings
of
the
term
travertine.
Riding
(1991)
considered
high
temperature
as
the
most
important
feature
of
water
depositing
travertines,
which
were
defined
as
"a
product
of
warm
carbonate
springs
where
the
elevated
temperatures,
together
with
the
dissolved
materials
present
in
these
warm
waters,
exclude
most
eukaryotic
organisms".
Pentecost
and
Viles
(1994)
proposed
a
different
nomenclature
for
terrestrial
carbonate
deposits
precipitated
by
water
issuing
from
sub-‐aerial
springs:
they
adopted
the
term
travertine
for
all
such
carbonate
deposits
distinguishing
between
thermogene
travertine
and
meteogene
travertine.
Meteogene
travertine
deposits
are
those
carbonates
precipitated
from
groundwater
with
a
meteoric
carrier
(Pentecost,
2005).
Meteogene
travertine s
are
those
related
to
cold-‐water
spring
(ambient
temperature
<20°C)
in
regions
underlain
by
carbonate
limestone
substrate,
and
consequently
they
are
equivalent
to
the
calcareous
tufa
defined
by
Pedley
(1990).
Thermogene
travertine
deposits
form
by
precipitation
from
hydrothermal
water
(Pentecost,
2005).
Carbon
dioxide
dissolves
in
magmatically
heated
groundwater
and
the
high
concentration
of
CO
2
can
bring
to
dissolution
of
large
volumes
of
limestone
rocks.
Thermogene
travertines
ar e
deposited
when
hydrothermal
water
issues
from
vents.
These
hydrothermal
deposits
have
a
more
localised
distribution
than
the
meteogene
travertine
and
are
often
associated
with
regions
of
recent
volcanism
or
tectonic
activity
(Pentecost,
2005).
Fouke
et
al.
(2000;
2011),
in
more
recent
studies
about
the
Yellowstone
(Wyoming,
USA)
hot-‐
spring
travertine ,
adopted
Pentecost
and
Viles
(1994)
definition
deciding
that
high
temperature
is
not
the
major
diagnostic
feature
but
rather
the
type
of
process
driving
carbonate
precipitation.
In
fact
they
named
travertine
“all
non-‐
marine
carbonate
precipitates
in
or
near
terrestrial
springs,
rivers,
lakes
and
caves”.
In
this
study ,
the
term
travertine
will
be
used
to
address
carbonate
deposited
by
water
issuing
from
hydrothermal
springs
according
to
Pedley
(1990)’s
definition.
4
Fig.
1.1:
A)
Calcareous
Tufa
forming
a
dam
along
a
stream
with
a
small
rim
and
Pool
(bank
of
Tronto
River);
B)
Travertine
deposit
with
a
smooth
slope
(Oliviera
Quarry
near
Rapolano,
Tuscany) .
5
3
-‐
TRAVERTINE
PRECIPITATION
Most
travertines
form
from
the
degassing
of
surfacing
carbon
dioxide-‐rich
groundwaters
containing
>2
mmol
L
–1
(c.
80
ppm)
calcium
(Pentecost,
2005).
A
groundwater
capable
of
depositing
travertine
is
produced
when
dissolved
carbon
dioxide
(‘carbonic
acid’)
attacks
carbonate
rocks
to
form
a
solution
containing
calcium
and
bicarbonate
ions
(‘calcium
bicarbonate’)
(Pentecost,
2005):
CaCO
3
+
CO
2
+
H
2
O
=
Ca
2+
+
2(HCO
3
)
–
Eq.
1
Travertine
deposition
is
the
reverse
of
the
reaction
in
Eq.
1.
Carbon
dioxide
is
lost
from
solution
on
contact
with
the
atmosphere
whose
CO
2
concentration
is
lower
than
that
in
equilibrium
with
the
‘attacking’
groundwater
solution
(Pentecost,
2005).
The
sources
of
underground
carbon
dioxide
capable
of
dissolving
carbonate
rocks
(hence
termed
the
carrier
CO
2
or
‘carrier’)
are
manifold.
In
calcareous
tufa
where
the
release
of
carbon
dioxide
occurs
to
the
atmosphere,
additional
CO
2
loss
frequently
occurs
through
the
photosynthesis
of
aquatic
plants
and
evaporation
(Pentecost,
2005).
A
few
travertines
are
formed
by
the
reaction
between
atmospheric
carbon
dioxide
and
hyperalkaline
groundwater
(Eq.
2):
Ca(OH)
2
+
CO
2
=
CaCO
3
+
H
2
O
Eq.
2
These
groundwaters
most
frequently
occur
in
regions
undergoing
serpentinization
(O’Neil
and
Barnes,
1971)
or
those
in
contact
with
natural
or
industrially
produced
calcium
hydroxide.
Travertines
arising
from
Eq.
2,
related
to
CO
2
ingassing
rather
than
outgassing,
are
widely
distributed
but
uncommon.
Another
process
may
be
described
as
groundwater
alkalisation
and
is
observed
when
groundwater
rich
in
calcium
mixes
with
alkaline
surface
water.
Hydroxyl
ions
in
the
lake
water
react
with
bicarbonate
(HCO
3
-‐
)
to
form
carbonate
(CO
3
2–
)
followed
by
precipitation
of
calcium
carbonate
(Eq.
3)
(Pentecost,
2005).
Ca
(HCO
3
)
2
+
OH
–
=
CaCO
3
+
HCO
3
–
+
H
2
O
Eq.
3
This
reaction
is
mainly
confined
to
a
class
of
alkaline
and
saline
lakes
(such
as
Mono
Lake,
California
and
Pyramid
Lake,
Nevada)
where
the
OH
–
concentration
is
elevated
as
a
result
of
6
geochemical
processes
(Pentecost,
2005).
Another
travertine
process
is
the
‘common
ion
effect’.
The
best
known
example
is
related
by
the
reaction
of
groundwater
infiltrating
e vaporites
that
become
saturated
with
gypsum
or
anhydrite
(CaSO
4
)
(Pentecost,
2005).
Gypsum-‐saturated
waters
contain
high
concentrations
of
CaSO
4
,
about
2
g
L
-‐1
.
When
mixed
with
a
Ca
bicarbonate
water,
Ca
is
sufficiently
elevated
to
exceed
the
solubility
pr oduct
of
calcite
and
precipitation
follows
(Pentecost,
2005).
3.1
-‐
ARAGONITE
VS.
CALCITE
PRECIPITATION
The
controls
on
the
type
of
mineralogy
precipitated,
whether
calcite
or
aragonite,
are
poorly
understood.
There
are
several
hypotheses
about
the
processes
influencing
carbonate
mineralogy
precipitation
(Jones
and
Renaut,
2010)
such
as:
• Water
temperature;
• Growth
inhibitors;
• CO
2
degassing
and
saturation
levels;
• Microbial
growth.
Obviously,
these
factors
may
operate
simultaneously
making
it
difficult
to
understand
what
the
most
important
component
is
that
influences
the
type
of
precipitation.
3.1.1
-‐
WATER
TEMPERATURE
Aragonite
precipitation
has
commonly
been
attributed
to
precipitation
from
water
with
temperatures
of
>40-‐45°C
(e.g.,
Moore,
1956;
Siegel,
1965;
Folk,
1994).
Kitano
(1962a)
instead,
suggested
that
aragonite
would
form
only
if
the
temperature
is
>60°C.
Calcite
precipitates
directly
from
waters
with
temperatures
of
>90°C
in
Kenya
(Jones
and
Renaut,
1995)
and
New
Zealand
(Jones
and
Renaut,
1996),
therefore
the
assumption
that
aragonite
is
favoured
by
temperatures
of
>40-‐45°C
does
not
always
apply.
The
agitation
of
the
water,
irrespective
of
its
chemical
composition
and
temperature,
will
increase
the
rate
of
CO
2
exsolution
that,
in
turn,
may
affect
the
CaCO
3
saturation
of
the
water
(Kitano,
1962a).
These
considerations
indicate
that
the
temperature/polymorph
relationship
may
only
apply
in
situations
where
the
waters
are
not
strongly
agitated
(rare
in
many
springs)
and
do
not
contain
the
concentrations
of
ions
or
trace
elements
that
appear
to
influence
precipitation
of
the
polymorphs
(Jones
and
Renaut,
2010).
7
3.1.2
-‐
GROWTH
INHIBITORS
Various
studies
showed
that
the
presence
of
some
chemical
elements
in
the
water
could
inhibit
or
promote
the
precipitation
of
calcite
and/or
aragonite.
Kitano
(1962b)
discovered
that
the
Alkali-‐Chlorides
migh t
inhibit
aragonite
precipitation,
while
Sr
and
Mg
can
increase
the
presence
of
aragonite.
The
aragonite
precipitation
is
related
to
the
Mg/Ca
ratio
of
the
water,
in
fact
Kitano
(1962 b)
synthesized
the
aragonite
in
laboratory
by
adding
MgCl
2
in
the
solution.
Folk
(1994)
suggested
that
aragonite
will
precipitate
from
any
water
that
has
an
Mg/Ca
ratio
>
2:1,
irrespective
of
water
temperature.
In
springs
with
very
high
Mg/Ca
ratios
(e.g.,
>
20),
Mg-‐
carbonates
such
as
hydromagnesite
and
nesquehonite
may
precipitate
around
the
vent
(Stamatakis
et
al.,
2007).
3.1.3
-‐
CO
2
DEGASSING
AND
SATURATION
LEVELS
Laboratory
experiments
and
interpretations
of
natural
precipitates
have
shown
that
changes
in
supersaturation
caused
by
CO
2
degassing
and/or
evaporation
commonly
influence
calcite
or
aragonite
precipitation
(Branner,
1901;
Holland
et
al.,
1964;
Folk,
1974;
Ishigami
and
Suzuki,
1977;
Cabrol
and
Coudray,
1982;
Chafetz
et
al.,
1991).
Rapid
degassing
of
CO
2
from
spring
waters
with
high
pCO
2
can
produce
a
fluid
strongly
supersaturated
with
respect
to
CaCO
3
,
leading
to
precipitation
around
the
vent.
Turbulent
flow
increases
CO
2
degassing
and
the
saturation
levels,
thereby
promoting
CaCO
3
precipitation
(Jones
and
Renaut,
2010).
Aragonite
precipitation,
as
opposed
to
calcite,
has
generally
been
associated
with
waters
that
have
attained
very
high
levels
of
supersaturation
with
respect
to
CaCO
3
(Kitano,
1962a;
Holland
et
al.,
1964;
White
and
Gundy,
1974;
Ishigami
and
Suzuki,
1977;
Arno'rsson,
1989).
3.1.4
-‐
MICROBIAL
GROWTH
Buczynski
and
Chafetz
(1991)
suggested
that
microbial
polymers
might
induce
CaCO
3
polymorph
forms,
irrespective
of
water
temperature
and
composition,
because
they
may
inhibit
the
transfer
of
ions
to
developing
nucleation
centres.
Similarly,
Guo
and
Riding
(1992)
showed
that
aragonitic
laminae
in
the
Rapolano
Terme
travertines
in
Central
Italy
formed
from
waters
with
a
temperature
>45°C
because
of
associated
microbial
activity.
In
Pleistocene
travertine
deposits
exposed
in
quarries
at
Rapolano
Terme
(Tuscany),
Folk
e t
al.
(1985)
interpreted
the
large
amount
of
regular ly
laminated
travertines
as
deposits
controlled
by
8
microbial
activity:
laminae
were
formed
during
diurnal
changes
in
precipitation
related
to
variations
in
the
activity
of
photosynthetic
bacteria.
Folk
et
al.
(1985)
identified
evidences
of
photosynthetic
activity
also
in
the
travertine
deposits
of
Mammoth
Hot
Springs,
Yellowstone
National
Park,
Wyoming.
Unfortunately,
there
are
only
a
few
studies
that
allow
the
understanding
of
the
interactions
between
the
aragonite
precipitation
and
the
microbial
growth.
9
4
-‐
TRAVERTINE
FACIES
There
are
many
studies
regarding
the
travertine
facies,
and
many
authors
elaborated
classifica-‐
tions
of
the
carbonate
fabric
types
precipitated
within
travertine
depositional
systems.
The
first
studies
about
travertine
petrography
focused
on
the
travertine
quarries
of
central
Italy
were
performed
by
Chafetz
and
Folk
(1984),
followed
by
Guo
and
Riding
(1998),
who
recognized
seven
main
travertine
lithotypes:
• crystalline
crust;
• shrub;
• paper-‐thin
raft;
• coated
bubble;
• pisoid;
• lithoclast;
• reed.
4.1
–
CRYSTALLINE
CRUST
This
facies
is
very
common
in
the
travertine
and
is
associated
with
high-‐energy
environment
(Smooth
Slope
and
Terraced
Slope;
Guo
and
Riding,
1998).
Jones
and
Kahle
(1986)
described
this
facies
as
single
crystal
dendrite s
with
multiple
level
of
branching.
Crystalline
dendrite s
are
divided
in
two
different
classes:
crystallographic
and
non-‐crystallographic
(Fig.
1.2;
Keith
and
Padden,
1964).
Fig.
1.2:
Diagram
representation
of
crystalline
dendrites
(after
Jones
and
Renaut,
1995)
10
Crystallographic
dendrite
has
a
definite
and
regular
orientation
of
the
branches,
while
non -‐
crystallographic
dendrite
has
a
complex
morphology
and
orientation
(Fig.
1.2;
Keith
and
Padden,
1986).
Non-‐crystallographic
dendrite
has
two
different
fabrics:
a)
feather
type,
characterized
by
complex
branches
and,
usually
it
has
a
pinnate
form,
and
b)
scandulitic
type,
which
has
branches
formed
by
"plate
crystal"
that
are
stacked
en
echelon
(Fig.
1.2)
(Jones
and
Renaut,
1995).
The
same
type
of
flowing
water
forms
feather
and
scandulitic
crystals
but
the
growth
setting
is
different.
The
feather
dendrites
grow
in
water
where
the
water
flow
is
constant
and
with
high
energy,
instead
the
scandulitic
dendrites
are
formed
in
particular
microenvironment s
dominated
by
intermittent
flow
over
a
terrace,
dam
or
sloping
mound
surface
(Jones
and
Renaut,
1995).
The
crystalline
dendrites
also
correspond
to
“ray
crystals”
by
Folk
et
al.
(1985)
and
“feather
dendrite”
or
“crystalline
crust”
by
Guo
and
Riding
(1992,
1998).
4.2
–
SHRUB
Kitano
(1963)
used
the
term
“shrub”
for
the
first
time
and
subsequently
it
was
adopted
by
Chafetz
and
Folk
(198 4)
to
describe
travertine
in
the
Tivoli
area
(Central
Italy).
The
shrubs
are
characterized
by
an
upward
expanding
growth
and
consists
of
peloids,
micritic
clots
and
microsparite
organised
in
a
tree-‐like
centimetre
size
form.
Typically
shrubs
grow
in
terrace
pools
and
depression
systems
or
sub-‐horizontal
layers
(Guo
and
Riding,
1998),
therefore
they
are
typical
micro-‐fabric
of
low
energy
environments
(Guo
and
Riding,
1998).
Chafetz
and
Folk
(198 4)
suggested
that
bacteria
play
an
important
role
to
shrub
precipitation.
Chafetz
and
Guidry
(1999)
distinguished
three
kinds
of
shrub:
bacteria l,
crystal
and
ray
crystal.
The
bacterial
shrub
name
is
due
to
fully
bacterially
precipitation
and
the
fabric
consists
of
micrite
and
microsparite.
In
contrast
crystal
and
ray
crystal
shrubs
consist
of
sparite
crystals
and
are
due
to
abiotic
precipitation
(Chafetz
and
Guidry,
1999).
Pentecost
(1990)
proposed
an
abiotic
origin
for
all
the
shrub
formation.
4.3
–
PAPER-‐THIN
RAFT
Paper-‐thin
rafts
are
thin
crystalline
layers
precipitated
at
the
water
surface
(Guo
and
Riding,
1998).
This
fabric
is
localized
on
the
floors
of
small
stagnant
pools
where
they
sink
after
precipitation
on
the
surface
and
it
is
a
typical
de posit
of
low-‐energy
environment s
(Guo
and
Riding,
1998).
The
raft
remains
intact
only
when
there
is
not
a
strong
turbulence
of
the
water
because
the
desiccation
promotes
a
good
deposition
on
sub-‐horizontal
layers
of
shrub
or
micritic
deposit.
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