Tiankeng: Definition and description

Abstract

A tiankeng is a type of very large collapse doline that has evolved by roof collapse over a large cave chamber where a huge mass of breakdown debris has been removed by a substantial cave river. Described first in China, the term tiankeng is recognised as a useful term within the worldwide lexicon of karst.

Full text

CA
VE
AND
KARST
SCIENCE
Vol. 32,
No.2
& 3, 2005
Tr
ansactions
of
the British
Cave
R
esea
rch Association
Tiankeng: definition and description
ZHU Xuewen and Tony WALTHAM
Nottingham
Trent
Universit
y,
UK, tony@
geophotos.co.uk
British
Cave
Re
se
arch Association, 2005
ISSN 1356-1 9 1X
~
In
stitute
of
Kar
st
Geology
,
Chinese
Academy
of
Geological Sciences, Gui lin,
China
SeRA
Abstract: A
tiankeng
is a type
of
very large
co
ll
apse
doline that has
evo
lv
ed
by
roof
collapse
over
a large
cave
chamber
where
a huge mass of breakdown
debr
is has b
ee
n removed by a substantial cave river.
Described first
in
China, the term liankeng is r
ecog
nised as a u
sef
ul
term within the world's lexicon
of
karst.
Key words: karst, doline,
ti
ankeng, C hina, defi nition
(Received December 2005)
Tianke
ngs have been recog
ni
sed
in
China as very large
co
ll
apse
dolines that have both
sig
nificant
sc
ientific value and a lso
considerable
economic
value as s
pect
acular sites within th e country's
expand
in
g tourist industry
(Z
hu,
200
I).
It
is now propo
se
d that the
term should be r
ecogn
ised and accepted as a part of the intemational
karst lexico
n.
The
word tiankeng is a transliteration from two Chin
ese
characters, that roughly mean sky hole
or
heaven p
il
,
or
some
sim
il
ar
variation on that
double
theme.
The
vagaries
of
the pinyin
spe
lling
unf
ortunately mi srepresent the Chi n
ese
sp
oke
n word, and its correct
pronunciation is
mor
e like lienkung.
It is suggested that a
ti
ankeng is a karst landf
or
m that is
distinctive enough to warrant its
ow
n name.
In
broad terms, it is a
co
ll
apse
doline at least 100 m w
id
e and d
eep
, that has formed by
rock fa ilure into a large cave. Howeve
r,
a tiankeng needs to be
defined appropriately and described adequate l
y.
This d
ef
inition must
be morphogenetic,
so
th
at
it takes
accou
nt
of
both the landform's
morphology
(and morphometry), and also its
or
igin a
nd
developmcnt
(
in
both the
geomorp
hological and the hydrological
co
ntexts).
The
exampl es cited to demonstrate features within the following text are
all briefly described
in
the
two
preceding papers in t
hi
s volume, on
tianken
gs
in
China
(by
Zhu and
Chen)
and tiankengs el
sew
h
ere
(by
Walth a m).
TIANKENG
MORPHOLOGY
In
morphological terms, a tiankeng is a
co
llap
se
doline that
is
morc
than 1
00
m deep and wide and has a steep profile wi th vertical cliffs
around a
ll
or
most
of
its perimeter. Dashiwei
Tiankeng
in
Ch
in
a and
Tres
Pueblos
Sink
h
ole
in
Puerto Rico are fine examples.
Tiankeng dimensions
Defining
minimum
dimensions
is necessary, as
size
is a diagnostic
feature
ofa
tiankeng, but the 100 m limit is arbitrary.
The
definition
shou
ld
be interpreted with discrction,
espccia
ll
y
in
those cases
of
very obvious co
ll
apse
dolines that
just
fall short
in
one dimension.
The
diameters (or length and width) can only be measured with a
d
eg
r
ee
of
approximation where th e ed
ges
are degraded and rounded,
as
is
the case on all but a handful
of
very fresh fea
tur
es. Depth is
more eas
il
y measured. However, Chinese
stat
is
ti
cs tend to
ci
te the
maximum
depth from the highest point o n the rim. Where the rim
breaks through a series
of
hills
in
fengcong karst, this figure
is
vcry
different from the minimum depth, from th e lowest point on the rim,
as is
genera
ll
y cited
in
Western statistics.
The
100 m minimum is
norma
ll
y applied to the
maximum
depth, but discretion is required to
ensure
that t
hi
s is trul y indicative
of
any
particular land form's
morphology.
Sma
ll
er
depressions formed by
co
ll
apse a re best
described
just
as large collapse dolines. It is g igantism that relatcs
tiankengs to specific conditi
ons
of
f
or
m
at
ion and separates the m
from
other
types
of
collapse dol ines.
A
minimum
volume
co
uld be a
better
way
of defining a
ti
ankeng
by its
morphometry
. H
oweve
r, volume data are rarely ava
il
a
bl
e,
except
as a gross ap
pr
oximatio
n, because wa
ll
profiles
of
do lin es and
tiankengs are not normally measured
in
adequate detail. A perfect
cy
linder
of
the
minimum
depth and diameter has a volume
of
about
800,000 m3; a rea
li
s
ti
c
minimum
for the volume of a tiankeng is
abo
ut
1M
m3
The depth/width ratio
of
a
ti
ankeng
li
es between 0.5 and 2
(0.5<d/w<2.0),
where
the width refers to the maximum width, length
or
diameter
of
the f
ea
ture, m
eas
ur
ed at rim leve
l.
Aga
in
the
se
arbitrary limits should be interpr
ete
d with discretion. Most karstic
depressions wi th depths significantly
grea
t
er
than double their
widths (d/w>2) are best described as hafts, with the genetic
implication that thei r
orig
in
owes
more to dissolution by descending
water than to rock
co
ll
apse.
Some
ti
ankengs have a l
ower
d
ept
h to
width ratio (d/w=0.5-0.2), where they a re e ither significantly
d
eg
rad
ed
or
are multiple features A
de
graded
ti
ankeng has
on
ly
limited re
mnant
s
of
c
li
ff
sections within its profile, as
in
Lu
se
in
New
Britain. A multiple feature is one formed by coalescence
of
a
series
of
co
ll
apses into a large cave passage, as at Qinl
ong
Tiankeng
(C
hongqing); this may therefore be
very
long,
but
its width
and
its
cross-profile remain typical
of
a tiankeng.
Other
large karstic
depressions wi th th
ese
depth/width ratios include large
so
lution
dolines,
some
poljes and genetica
ll
y ambiguo us mega-dolines, but
these lack the collapse
mechanism
that formed a tiankeng.
Any
sha
ll
ower
depression (d/w<0.2) cannot warrant descrip
ti
on as a
tiankeng.
The
ratio
of
the diameters
of
a typical
tiankeng
at
its top (ri m)
and at its bottom (floor) lie between 0.7 and
1.5
(0.7<w/ wb<
1.
5), but
these limits are also arbitrary and
ope
n to interpretation. V
er
tical
perimeter cliffs
in
the
id
eal tiankeng create a ratio
of
exac
tl
y
I.
Various tiankengs
in
China have W/ Wb
in
the range 1.5-2.0, and
Luse,
in
New Britain, has a ratio
of
about 2. Fl
oor
diameter
is
increasingly difficult to define
in
degraded tiankengs
where
apr
ons
of
scree debris merge into a rounded floor profile.
Co
nversely an
incomplete
roof
collap
se
in
what may be regarded as an
immature
tiankeng has w/
wb
<0.7.
Go
l
ondr
in
as
(i
n Mexico) has a la rge
opening
with walls belling o
ut
in
an even lar
ger
sh
af
t,
so
that w/
wb
-0. 15.
Along
with a few oth
er
sites,
Go
l
ondr
inas is best described
as an immature tiankeng. Maoqi Dong,
in
the Leye karst,
co
uld be
regarded as a proto-tiallkeng; its huge shaft is
300
m deep and
200
m
wide but has
only
a sma
ll
sky
li
ght
in
its domed roof, so w/
wb
<0.05
(Fig. 1
).
Tiankeng profiles
A tiankeng is distinguished by having verti cal
or
sub-vertical
wa
ll
s
for much
of
its depth and round most
of
its perimeter.
They
are
a
mong
the clearest
evidence
of
co
ll
apse o
ri
gins, though they
may
be
due either to failure into an underlying cave or to subsequent face
retreat. Vertical cliffs are
dependent
on geological
str
ucture; th
ey
de
velop best
in
strong
and massive limestones, and primarily
where
fractures are close to vertical, which tend to be m
ore
dominant in
beds
of
l
ow
st
ructur
al dip. Limestones dipping at around
45
° tend to
have similarly inclined fractures that
pr
ec
lude
de
velopment
of
steep
tiankeng profiles.
Tianke
ngs with depths greater
tha
n their
widths
tend to be the more s p
ec
tacular, but the k
ey
fact
or
in
the visual
sp
ec
tacle is the
co
mpleteness
of
a ring
of
high cliffs (Fig. 2);
75

0
..
,
__
....
__
""'-_--J,300
metres
Maoqi
Dong
Figure I. Section through Baidong Tiankeng
and
th
e adjacent skylig/lI
chamber
of
Maoqi Dong which may be regard
ed
as
a proto-tiankeng:
th
e
bedrock projile beneath the debris piles is conjectur
ed.
Huangjing Tiankeng (Guangxi) and Minye (New Britain) are
therefore prime examples.
Degraded
ti
ankengs are those that have lost a significant
proportion
of
their perimeter cliffs.
The
inevitable retreat
of
an
exposed face, by weathering and successive rockfalls, creates
an
apron
of
scree debris at the base
of
the cliff. In an active tiankeng,
rockfall debris is removed by the cave river, so that only modest
scree ramparts
of
debris remain beneath the
cliff
faces, as at
Xia
oz
hai Tiankeng (Chongqing) and Velika Dolina (Sloven ia).
Where the cave river
is
too weak
or
is
lost to a by-pass route,
breakdown debris accumulates until
it
may mask much,
or
all, of the
cliff
faces. Progressive degradation sees the perimeter
cliff
faces
reduced
in
heig
ht
and perhaps also broken
by
gully development.
There
is
an inev
it
able evolution from a tiallkeng like Nare (New
such as Datuo . and then
~~
Figure
2.
The
complete ring
of
vertical
cliffs
of
Huangjing Tiankeng,
Guangxi, seen
in
a velY wide upward
viewji'Oll1
th
ejloor.
76
to a mega-doline such as Garden
of
Eden (Sarawak). There are no
clear boundaries between the three forms (see below).
The
bedrock floor
of
a tiankeng
is
largely
or
wholly masked
by
broken rock debris as an
in
evitable consequence
of
its collapse
origins. This
may
be a single pile
or
ramp
of
breakdown from a cave
roof
failure.
It
is more conunonly a convergent series
of
fans and
aprons
of
debr
is
created by subsequent failures
of
the retreating
perimeter cliffs.
Some
active tiankengs have an exposed cave river
across their floors; others only have an underground river flowing
through their floor debris, and t
hi
s
mayor
not be accessible.
Tiankengs such as Qinlong (Cbongqing) have small
or
underfit
rivers flowing between dominating ramps
of
debris, while most
degraded tiankengs no longer have
any
sign
of
a cave river.
Entrances to cave passages should be a feature
of
any
tiankeng, but
they are commonly obscured behind accumulated rockfall debris;
these should include active inl
et
and outlet caves at floor level , and
may include older passages at higher levels, as at Xiaoyanwan
(Sichuan).
T1ANKENG DEVELOPMENT
A tiankeng
is
a collapse features formed
over
a large underground
cave passage. It may develop by collapse
of
a single large chamber,
as appea
rs
to be the case at Xiaozhai Tiankeng
in
Chongqing.
Alternatively, a tiankeng may form by collapse at the junction of a
series
of
large passages, which includes collapse through
superimposed levels
of
passages that are
of
varied ages, as at
Xiaoyanwan
in
Sichuan. Cave
roof
co
ll
apse normally involves
prolonged
roof
stoping and upward cavity migration. Within this, a
l
ong
sequence
of
progressive failures cause small quantities
of
rock
fall
fTom
the tensile zone beneath the compression arch (which
creates the stab
le
roof
span within the rock spanning the void).
The
ultimate failure through to the ground surface
may
then be a single
event. Alternatively, the surface
fa
ilure may be by multiple events,
with coa
le
scence
of
smaller collapses.
Some
ti
ankengs are elongate
due to collapse along the line
of
very large cave passages; Qinlong
Tiankeng (Chongqing) and Dacaokou (Guizhou) provide fine
examples (Fig. 3). The very large size
of
some tiankengs suggests
that they are multiple
fai
lures; tbey are inordinately larger than
known cave chambers, a
nd
much
of
their volumes
is
accounted
fo
r
by the removal
of
debris from the base by dissolution and erosion.
The rounded shape
of
many tiankengs
is
likely to have evolved
by
cl
iff
retreat (where st
re
ss distribution favours
fai
lure towards a stable
circle).
Most observed tiankengs expose within the limestone one
or
more structural weaknesses, notably sub-vertical faults and
joint
s.
Xiaozhai Tiankeng (Chongqing) provides an example, where the
cave river crossed intersecting, sub-vertical fractures that appear to
be small-displacement
fa
ults (Fi
g.
4).
These
facilitated the processes
of
vertical expansion by
roof
sloping, as we
ll
as lateral expansion by
wall retreat and coalescence
of
collapses. Such structures a
re
however not unique to tiankengs, as nearly a
ll
cave passages a
nd
chambers have developed at some form
of
structural weaknesses.
A large underground river
is
an essential feature
of
an active
tiankeng, as it provides the
only
means by which the huge volumes
of missing rock can be removed, either
in
soluti on
or
as clastic
sedimen
t.
This
is
commonly a vadose cave river, but the flow
may
o 200 400 metres
tNorth
1-'
-'----'-'
---'--"
Tiankeng
Figure
3.
The
tiankengs
of
Dacaokou
and
Xiacaokoll, separat
ed
by a natural
bridge.
all
formed on the course
of
the
Yiji
e underground river
in
Guizhou.

Figure 4. StruclUral intelpretation
o/Xiaoz
hai Tianke
ng.
Chongqin
g.
with
the observe
d/ra
clures that may have initiated tiankeng development over the
Difeng cave river a
nd
beneath a doline between three hills
11
'ithin
th
e
/engcong karst.
Th
e fracture traces are simplified;
th
ose along the cave
passages are multiple fracture sets.
and
the SW-NEji'actllres
lila),
hreak illlo
en echelon seri
es.
be phreatic.
The
erosional ability
of
the
cave
river relies on the
supply
of
broken rock that is provided by progressive
roof
stoping
and wall failure. Steeper
hydr
aulic gradients through a breakdown
pile also
enhance
rates
of
di
ssoluti o n and erosion
(Palmer
and
Palmer
, this volume). Large wall undercuts are
common
in
tropical
caves
where
water flow is deflected
around
a pile
of
collapse de bri s;
they contribute to the enl
argement
of
cave
chambers
and tiankengs
alike. Wall
undercutting
is a lso e
nhanc
ed
on
the outside
of
a be nd
in
a
cave
pa
ssage, as
may
be the
case
at Xiaozhai (in Chongqing). Ro
of
stoping and
cave
river transport are synergic
in
tiankeng
development
-which only proceeds when both processes are active
011
a large scale.
The
relationship
of
tiankengs to large underground rivers ties
them
to the
wet
tropical
environment
or
to situations
in
other
climates
where
allogenic rivers are
swallowed
into karst to produce
large cave river passages. This criterion also separates tiankengs
from
other
giant collapse features, including
caprock
co
ll
apses
over
evaporites (as
in
Ca
nada and Russia) and large collapses
over
hydrothermal cavities (as in
Oman),
which do not cia s
if
y as
tiankengs.
The
huge, circular, collapse doline of Sarisarinama,
in
Vene
z
uela
, has
dev
el
oped
in
quart
zite
by
piping
and
cla
stic
sediment
removal on a massive
sca
le. Except for the fact that
it
is
not in
limestone, it would be regarded as a tiankeng. yet there is no
cave
river associated with it. This may be
explained
by an
except
iona
ll
y
long period
of
very
sl
ow
evo
luti
on,
but the site Illay have
implications with
re
sp
ect
to the
pr
ocesses
that develop tiankengs
in
limestone.
Development
in
the vadose zone
Tiankengs
are
deep
open
holes that lie within the vadose zone
of
the
karst. An active tiankeng may h
ave
a
cave
river across its floor (or
through the debris
on
its
fl
oor); this cave
dr
ainage
is
normally
vadose
, but
is
phreatic at
some
site
s.
Vad
ose
conditions also
maximise stress on a cave
chamber
roof, which lacks the
buoyant
support
of
a
comp
letely phreatic chamber. But a
tiankeng
may not be
a
wholly
vadose feature.
Of
the recorded tiankengs,
about
ha
lf
have
no known cave river
flowing
through them or directly associated
with them. However,
half
of
th
ese
ri
ver
-l
ess tiankengs do have
associated cave passages that
ca
rried underground rivers
in
the past,
and some may h
ave
ri
vers that pass unseen through their floor
d
eb
ris.
Some
tiankengs,
in
cluding Xiaoyanwan
in
Sichuan, formed
by collapse into large phreatic passages,
whe
re there is no sign
that
they have
po
si
tively
evo
lved within the vadose zone. Crveno
Jezero
(Croat
ia
) has active
phr
eatic cave drain
age
that is removing the
collapse debris beneath the water-table.
Any
large cave
cha
mber
may
collapse to form a tiankeng, as
l
ong
as through-drainage removes enough
of
the breakdown material
to create a deep open hole. Furtherm
ore
, virtually a
ll
caves and
cave
chambe
rs have
so
me phreatic origins that were developed prior to
their drainage. Vad
ose
Illodifica
ti
on
of
a large,
open
, de
-wa
ter
ed
shaft
or
co
llapse doline
is
in
evita
bl
e, but va
dose
erosion may not be
the
dom
in
ant proc
ess
that forms a
ti
a
l1ken
g. Lago Azul, in Brazil,
has the dimensio ns
of
a tiankeng, but is cl
ear
ly phreatic because it is
fu
ll
to the brim with water. Eventua
ll
y.
regional rejuvenation will
cause
its drainage, and it will then have a morphology very
similar
to
that
of
Go
londrinas,
in
Mexi
co
.
This
may
be an extreme case of
early
phreatic
enlargement
-that will be unrecognisable once the
feature is in vadose zone and is modified by vadose erosion and wall
collapse. Lago Azul may
warrant
description as a proto-tiankeng.
The
same
app
li
es to the very
deep
ce
note
of
Zacaton,
in
Mexico, but
this may partly
owe
its size to corrosive waters rising from a
volcanic source rich
in
with hydrogen sulphide.
These
sites
questi
ons
whether,
or
to what extent, vadose devel
opment
should be
an essential criterion
of
a tiankeng.
It is debatable to what extent
so
lution dolines have contributed to
the
development
of
tiankengs.
The
gr
owth
of
a doline would redu
ce
roof
thickn
ess
over
an underlying
cave
chambe
r, thereby hastening
the eventual co
ll
apse, and it wou
ld
al
so
direct
mor
e drainage into
the
site, thereby
enhancing
l
oca
lised rock
di
ssolution. A solution doline
could be
ex
pected to form
on
the
same
structural weaknesses that
guide the
chamber
dev
elopment
undemeath
it.
The
cave
of
Zhucaojing
in the Xingwen karst
of
Sichuan
(Wa
ltham, this volume)
has d
ee
p soluti on dolines above its large chambers, and a
ppear
s to
be a c lear
examp
le
of
imminent
tiankeng
development.
[n
contrast,
some
tiankengs
in
China
br
eac
h steep hillsides
where
no large
so
lution dolines are likely to have
developed
.
Many
tiankengs
in
China
appear
to be
comp
letely independent
of
the
surface
morphology
,
in
that their walls cut through dolines and
conical hills alike (Fig. 5).
Howev
er, cutting into the cones is
enhanced
by the wall retreat that
expands
the tiankengs.
At
many
sites, th e distribution
of
the
cone
remnants
cut
by the perimeter walls
does leave space for the tiankeng to have originated at a central
so
lution doline with
in
the fengcon g terrain (Fig. 4). It appears that
so
lution
al
dolines have contributed to the
development
of
many
tiankengs, but they were not essential features; the evidence was
destroyed
by
the
collapses that eventually formed the tiankengs.
Most
of
the tiankengs
in
southem
China
lie within the terrains
of
mature fengcong with high local re
li
ef
that are
so
typical
of
the karst
region.
In
New Brita
in
and
Puerto
Rico, tiankengs l
ie
in
cone karst
with much less local re
li
ef, and those
elsewhere
li
e in karst terrains
Figllre 5. Remains
of
t
wo
/engcong karst hills truncated by expansion
of
Dashiwei Tiankeng
in
th
e Leye karst 0/ Guangxi, seen from the flank 0/ the
third hill that has been truncat
ed
by collapse.
77

erosional tianken '
vertical walls Table 1. Comparisons between the
va
rious
fea
tll
res
of
erosional and
col/apse tiankengs.
waterfall ullies
tiankeng contem
orar
with
cave
river
vadose
alluvium and breakdown
river bed to cave outlet
in
valley or depression,
fed b surface stream
f
ew
since late Pleistocene
that are not fengcong.
Though
deeply dissected fengcong m
ay
be the
ideal environment for
ti
a
nken
gs, it does not appear to
be
a criterion
f
or
their development.
E.rosional tiankengs
A sub-class
of
ti
ankeng, the erosional type, has been recognised
within China as
di
stinct from the more co
mm
on co
ll
apse type
(Tab
le
I
).
An erosional tiankeng develops at the site
of
a
si
nking a
ll
ogenic
stream,
as
di
stinct from collapse tiankengs
th
at are independ ent
of
su
rf
ace drainage.
The
erosional type therefore has a cave stream
dr
aining from it, but not into
it.
Qingkou Tiankeng
in
Chongq
in
g is
therefore an
exce
ll
ent example (F ig. 6).
The
relative roles
of
co
ll
ap
se
and waterfall erosion appear to
vary wi dely between sites.
Some
large holes
in
karst are
just
grea
tl
y
e
nl
arged solutional sh
af
ts, typically with a
hi
gh depth/width ratio (d/
w>5).
There
mu
st
be some elements
of
collapse, shaft coalescence
and/or wall retreat to enlarge these to the dimensions
of
an erosional
tiankeng, as a s ingle waterfall shaft either retreats into a s inuous
canyon or be
ll
s out to no more than about 20 m
wi
de. Simi l
ar
ly
strea
m erosion has contributed to
th
e erosion
of
many
co
ll
apse
tiankengs. Xi
aoz
h
ai
Tiankeng (Chongqing) has three waterfalls
d
esce
nding its walls in wet weatber, but these are
fe
d
by
immed iate
run-off
from the limesto
ne
.
So
me erosional
ti
ankengs are la rge caprock dolines, as they h
ave
formed by collapse within underlying limestone that has propagated
to the ground surface through a cover
of
in
soluble rock.
These
the refore ga
th
er
sign
ifi
cant a
ll
oge
ni
c
dr
a
in
age
off
the surrounding
cover. Others only gather allogenic input from adjacent outcrops
of
cove
r rock.
Furthennore,
it
is
open to de bate whethe r many o
th
er
tiankengs,
espec
ia
ll
y
in
th
e Leye karst
in
Guangxi, collected
a
ll
ogenic drainage before retreat
of
the margin
of
the
cover
rocks.
The
separ
at
ion
of
erosional
ti
ankengs from co
ll
apse tiankengs
therefore appears to be dubious.
It
is better to regard both as end
members o f a spectrum o f conditions, where collapse and disso
lu
tion
vary
in
their rela
ti
ve
imp
ortance. Both processes contribute to th e
development
of
all
ti
ankengs, which
may
therefore be described as
polygenetic.
Degraded tiankengs
An
in
ev
it
able process
in
ti
ankeng evolution is wall retreat, but thi s
o
nl
y
contr
ibutes to e
nl
argement while a rive r is remov
in
g the
breakdown debris, as
is
conspicuous at Kukumbu
in
New Britain.
When
th
e rate
of
debris accumulation
excee
ds
th
e rate
of
removal, a
tiankeng starts to degrade, as
ever
larger fa ns and aprons
of
scr
ee
accumulate at the base
of
the wa
ll
. This rockfall debris from the
wa
lls is added to debri s remaining from the earlier
roof
failure. A
degraded tiankeng has its perimeter c
liff
s progressively buried by the
aprons
of
scree, while the rim is be
in
g lowered at the same time, and
the depth/width r
at
io therefore decreases (d/w<0.5). Ultimately, trees
gr
ow
to m
ask
the rock debris.
Of
the 74 recorded tiankengs,
at
least
12 are properly described as degraded (Zhu and
Che
n, this volume,
Table
I; Waltham, t
hi
s volume, Table I).
There
is
a cl
ear
ly re
cog
ni
sable evolution
of
a
ti
ankeng into a
degraded state, and
bey
ond. Ult
im
ate ly, a tiankeng d
eg
rades so
much that almost a
ll
its perimeter cliffs are lost, and
it
is the n
rega
rd
ed as a mega-doline.
Many
very large dolines could have
78
Figl/re 6. One of the watel
/a
lls that cascade into Qingkall
Ti
ankeng
(Chongqing) . which all dra
in
olltthrollgh a lar
ge
stream cm
"e
.
originated a tiankengs, and it m
ay
be questioned as to h
ow
many
deep dolines wi
th
in very mature,
hi
gh-re
li
ef, fengcong karst
or
iginated as tiankengs. However, it may be unhelpf
ul
to describe
these large dolines as heavily degraded tiankengs, as this b
lu
rs the
conce
pt
of
the tiankeng. Si mil a
rl
y, a tiankeng could be regarded as a
sub-type,
or
a stage
in
the evolution
of
, a mega-doline, though such
would not be the
or
ig
in
of
a
ll
mega-dolines.
The
age
of
tiankengs requires furth
er
investigation. Ages
of
the
large tiankengs
in
the Nakanai karst on
New
Britain were estim ated
at
200-300 ka , but this was only based on the overall rate
of
surface
l
owe
ring
(Maire
, 1981). Most tiankengs
in
China are relatively
youthful features, as
they
truncate the topography
of
solution dolines
and co
ni
cal hills within the fengcong karst. However, such eviden
ce
only dates the surface collapse that created the
ope
n tiankeng, and
solutional and stop
in
g development of the underground
cave
rn
must
have taken l
onge
r.
The sh
eer
size
of
many tiankengs suggests a very
long
hi
story to allow time for erosional removal
of
the hu ge volumes
of
mi
ss
in
g rock.

CLASSJFICA
TlON
OF TlANKENGS
Tiankengs
ma
y be classified as a very large sub-type
of
collapse
doline
(Wa
ltha m et
ai
, 2005).
So
me may be
cl
assified as
cap
rock
tiankengs, and the
se
would include
mo
st erosional tiankengs
if
that
sub-type is accepted. Th
ey
are distinguished from collapse and
caprock do lines
by
their very large size.
It
is appropriate to use a
thr
ee
-f
o
ld
sub-division of
ti
ankengs that
is already applied
in
China
(Z
hu
and
Chen, this vo lum
e)-
• Very la
rg
e tiankengs are
more
than
500
m
in
diameter and depth.
• Large tiankengs are
300-500
m
in
diamete r and depth.
• Normal
ti
ankengs are 1
00
-
300
m
in
diamet
er
and depth.
•
Sma
ll
er
features a re large
co
ll
apse
(or
caprock) do
li
nes.
These
limiting
dimensions
should be interpreted loosely
and
with discretion. A large
tiankeng
should h
ave
its depth and length
greater than about
300
m, though its width
may
r
eac
h rather less than
that figure.
Maximum
dim
ensions
of
a
ti
ankeng with very irr
eg
ul
ar
shape m
ay
give a false impr
ess
i
on
of
its size;
Xi
ashiyuan and
Zhongshiyuan
, in Chongqing, both have large maximum dimensions
that belie their r
ela
tively sha
ll
ow
but inclined structure
in
a slope
that fo llows the limestone dip.
The
depth
wo
uld sensibly be
mea
sured from the mean level
of
the rim, and not from th e
hi
ghest
point (as is cited
in
Chinese
li
terature), but the mean
fi
gure is rarely
available.
In
the
ory
, the depth o f a
ti
ankeng
sho
uld be meas
ur
ed to
the
base
of
the debris pile over its rock floor, but this d
ept
h cannot
be determined at
some
sites.
It could be m
ore
precise to sub-di v
id
e
ti
anke
ngs on the basis
of
their vol
um
es.
The
disadvantage
of
this is that volumes are not
so
eas
ily
or
frequently measured w ith any
degree
of
accuracy
(most
of
the
fi
g
ur
es quoted
in
the tables
in
this volume a re very roug h
estimates from basic maps).
The
minimum
volumes fo r
Il(
;Jnllal,
large and very large tiankengs are about I M, 10M, and 70M mO.
Within some karst r
egio
ns
in
China,
ti
ankengs have
become
pa
rt
of
the local culture.
Some
features so d
esc
ribed have depths and
diameters
of
50-1
00
m, and are d
escr
ibed as sma
ll
tiankengs, thoug h
in
oth
er
parts
of
the
wor
ld
these would be described on ly as l
arge
co
ll
apse dolines. Because a
ti
ank
eng
is defined as a large co
ll
apse
do lin e, a
sma
ll
ti
ankeng would be a "sma
ll
large
co
ll
apse doline",
and this is
un
acceptable
in
a w
id
ely used term.
To
inc
lu
de featu res
with a
ll
dimensions less than 1
00
m detracts fr
om
the value
of
the
tiankeng
as a di stinctive feature.
There
are 75 recorded tiankengs
worldwide
, including
49
in
Ch
in
a.
Of
these, o
nl
y 3 are very l
arge
, and are a
ll
in
China.
Then
the re are
16
large tiankengs,
of
w
hi
ch 9 are
in
China.
The
oth
cr
56
are n
orma
l tiankengs,
of
which 37 are in China.
There
are also many
hundreds
of
large
co
llapse dolines, both within and outside China.
Tiankengs
do not lend th
emse
lves readi ly to a D
avis
ian sequenc e
of
evo
luti
on
, due to the
in
stantaneous cave
roof
co
llapse by which
th
ey
are formed. A m
at
ure
ti
ankeng is disting
ui
shed by its almost
co
mpl
ete
p
erimeter
c
li
ffs that are close to vertical (Fi
g.
7). An
imm
at
ur
e
ti
ankeng is
one
with
overhanging
cliffs
due
to incomplcte
co
llap
se
of
the o
ri
ginal cave ch
ambe
r, as a
consequence
, the s
urf
ace
ope
ning is significantly
sma
ll
er that the
fl
oor
area
(w
/ w
b<
0.7). T here
is no benefit
in
r
et
reating further o n the
evo
lution cha
in
by defin
in
g
a youth
ful
tiankeng. A large
cave
chamb
er
with a thin
roof
or a sma
ll
sky
li
ght (or a
fl
oo
ded
cha
mber)
may
eve
ntua
ll
y collapse to
fo
ml a
mat
ure
ti
ankeng, but is bett
er
d
esc
ribed as a proto-tiankeng pri
or
to
its
roof
fai
lu
re; it does not yet have the app
ea
rance
of
a tiankeng. A
degraded tiankeng still retain its largc
dimensions
and many sec
ti
ons
of
perimeter
cli
ff
, but its
fl
oor
is
much
sma
ll
er
than its su
rf
a
ce
ar
ea
(W
/ Wb> 1.5). It has an
excess
of
debris o n its
fl
oor
and
in
ramps up
its perimeter, and has no
cave
river through
it.
It
co
uld be described
as one
of
old age, but description as degraded is more helpful.
There
is no clear borderline between a degraded
tiankeng
and a large
doline
or
me
ga
-dol
in
e.
Immature
Golondrinas
Mature
Dash
iwe
i
Degraded
Lus
e
Figure 7. Comparalive profiles a/ good examples 0/ immature. mature
all
d
degraded lianken
gs.
Tiankeng karst
The
conc
ep
t
of
tiankeng karst has been
co
ns
id
er
ed
within China as a
teml
to describe an
ext
r
eme
ly
ma
ture type
of
karst landsc
ape
that has
matured beyond normal fengcong karst wi th
hi
gh relief.
The
term
could be used to describe the Leye karst
in
Guangxi, China, and
perhaps the Nakanai karst
in
N
ew
Britain, Papua
New
Guin
ea
, both
of
whic h are di stinguished by unusually large
numb
ers
of
tiankengs.
However,
some
mature karst terrains contain j ust a few
ti
ankengs,
not
ably
ju
st two
in
eac
h
of
the karsts
of
Xingwen,
Croat
ia a nd
Mexico, and these question the applicability
of
the term. Tiankeng
karst m
ay
be
purely
descriptive
of
the L
eye
and Nakanai terrains, but
the teml has not yet been sh
own
to have
any
geomorpho
logical
status with reference to karst evolution.
CONCLUSIONS
The
tiankeng is r
ecog
ni
sed as a
si
gnificant k
ar
st la
ndf
or
m.
The key
features
of
a
ti
ankeng
may be s
umm
a
ri
sed as -
• It is a
di
stinctive type
of
very large
co
ll
apse doline.
•
It
is more than 100 m wide and d
ee
p, with a depth/width ratio
that is close to unity.
• It has vertical cliffs round most
of
its perimeter.
• It was formed by
co
ll
apse
of
a large
cave
c
hamb
er
in
to a large
underground rive
r.
•
Surface
lowering within a solution doline
may
have contributed
to this co
ll
apse.
• Tiankeng
development
is largely
in
the vadose zone within
deeply
dissected fengcong karst.
•
Some
tiankengs are partly erod
ed
by sinking
al
logenic streams.
• Immature tiankengs bell out to floor areas that are larger than
their surface
open
in
gs.
• Degraded
tiank
engs have most
of
their perimeter c
li
ffs masked
by rock debris.
These
properties constitute an appropriate defi nition
of
a
tiankeng.
It
is di fficult and unnecessarily pedantic to define ab
so
lute
limits to large
co
ll
apse dolines that might be too sma
ll
or
too
degraded to be regarded as tiankengs,
or
may be
of
a di fferent
genesis.
Tiankeng
is a
usef
ul
add
iti
on to the int
ema
ti
onal lexicon
of
karst terminology.
As
a distinctive landform, a tiankeng fits within
the spectrum
of
dolines and karst depressions that characterise karst
terrains.
Though
ti
ank
eng
research
or
iginated
in
Cbina, the term is
appl i
ca
bl
e
in
karst terra
in
s elsewhere.
This paper orig
in
ated fro m a discussion meeting he
ld
in
Guil
in
at
the end
of
the Tiankeng
In
vestigation Project
in
2005. A
dr
a
ft
text
was circulated to a
ll
delegates, and was approved by a
ll
after
incorporating amendments. Delegates
at
the meeting were Chen
Weihai,
Andy
Eavis, John Gunn, Julia James, Alexand er Klimchouk,
Andrej Kranjc, Liu Zaihua, Art Palm
er
and Will White, and the
editors.
The
pap
er
is
therefore intended to accord internationa l
recogn
iti
on
to the research and work
on
tiankengs that was initiated
and developed at the
In
stitute
of
Karst
Geology
in
China.
79