Physical Modeling of Structures Formed by
Salt Withdrawal: Implications for Deformation
Caused by Salt Dissolution1
Hongxing Ge2 and Martin P. A. Jackson3
ABSTRACT
By creating 15 physical models, we investigated
deformation above subsiding tabular salt, salt walls,
and salt stocks. Dry quartz sand simulated a brittle
sedimentary roof above viscous silicone representing
salt. The modeled diapiric walls had linear planforms
and rectangular, semicircular, triangular, or leaning
cross sectional shapes; the stock was cylindrical.
In models where the source layer (or allochthonous salt sheet) was initially tabular, a gentle,
flat-bottomed syncline bounded by monoclinal flexures formed above a linear zone where the silicone
was locally removed. Above all subsiding diapirs, the
deformed roof was bounded by an inner zone of
steep, convex-upward reverse faults and an outer
zone of normal faults. Above subsiding diapiric
walls, extensional and contractional zones were balanced. Above the subsiding salt stock, conical, concentric fault zones comprised inner reverse faults
and outer normal faults.
©Copyright 1998. The American Association of Petroleum Geologists. All
rights reserved.
1 Manuscript received July 11, 1996; revised manuscript received
February 18, 1997; final acceptance September 11, 1997.
2Bureau of Economic Geology and Department of Geological Sciences,
University of Texas at Austin, Austin, Texas 78713. Present address: Shell
E&P Technology Company, Bellaire Technology Center, P.O. Box 481,
Houston, Texas 77001; e-mail: [email protected]
3Bureau of Economic Geology, University of Texas at Austin, Austin,
Texas 78713.
All models were run at the Applied Geodynamics Laboratory of the
Bureau of Economic Geology, University of Texas at Austin, with financial
support from the following companies: Agip S.p.A., Amoco Production
Company, Anadarko Petroleum Corporation, ARCO Exploration and
Production Technology and Vastar Resources, BP Exploration, Chevron
Petroleum Technology Company, Conoco and Dupont, Exxon Production
Research Company, Louisiana Land and Exploration Company, Marathon Oil
Company, Mobil Research and Development Corporation, Petroleo Brasileiro
S.A., Phillips Petroleum Company, Société Nationale Elf Aquitaine
Production, Statoil, Texaco, and Total Minatome Corporation. The
Department of Geological Sciences and the Geology Foundation at The
University of Texas at Austin and Phillips Petroleum Foundation provided
additional financial support for Hongxing Ge. Sharon Mosher, Bruno
Vendeville, Mike Hudec, William Kilsdonk, Louis Liro, Carl Fiduk, Martha
Withjack, Robert Evans, and Richard Groshong provided invaluable
discussions or comments. The paper was edited by Amanda R. Masterson
and Tucker Hentz. Publication was authorized by the Director, Bureau of
Economic Geology, University of Texas at Austin.
228
Sediments were added both before (prekinematic) and during (synkinematic) salt withdrawal. In
entirely prekinematic roofs, reverse fault zones and
normal fault zones both widened with time.
Reverse faults propagated upward from the corners
of the withdrawing diapirs. New reverse faults
formed in the footwalls of reverse faults, each nearer the center of the deepening roof trough.
Conversely, new normal faults formed successively
outward from the sagging trough. Synkinematic
deposition retarded faulting, but the pattern of
inner reverse and outer normal faults was repeated;
however, reverse faults formed successively outward, whereas normal faults formed inward.
New conceptual models suggest that salt dissolution forms similar structures to those physically
modeled for salt withdrawal. The appropriate physical models resemble natural dissolution structures
above tabular salt. Extension alone above diapirs is
not caused merely by salt withdrawal or dissolution, but by regional extension or active diapirism.
INTRODUCTION
Removal of buried salt by dissolution, withdrawal
(salt flow), or mining causes the overburden to subside and form collapse structures. Dissolution features are the dominant salt-related structures in basins
where salt is still tabular and has not been mobilized
to form salt diapirs or pillows. Dissolution structures
can form hydrocarbon traps (Parker, 1967; Swenson,
1967). Dissolution by undersaturated groundwater is
common along the edges of bedded salt basins
(Anderson, 1981; Jenyon, 1985, 1986, 1988a). Many
depressions on the bottom of the Mediterranean and
Red seas have been attributed to salt dissolution (Ross
and Uchupi, 1973; Schoell et al., 1974).
The rates of salt dissolution are known in only
a few domes (Anderson and Kirkland, 1980).
Dissolution rates vary significantly in different geologic and hydrogeologic environments; for example, compare a low rate of 1 mm/k.y. for bedded
Salado salt in Texas since the end of the Permian
AAPG Bulletin, V. 82, No. 2 (February 1998), P. 228–250.
Ge and Jackson
(Anderson, 1981) with a high rate of 50 m/k.y. for a
Hormuz salt glacier extruding in Iran (Talbot and
Jarvis, 1984). Measurements of salinity around domes
are greatly perturbed by the influx of deep saline
waters up nearby faults, so dissolution rates of salt
domes are uncertain, despite much research connected with nuclear-waste isolation.
The kinematics of salt dissolution is poorly understood. Presumably, salt dissolves gradually inward
from the top and upper sides of the diapirs. Evidence
from differential thickness of the residual cap rock
over salt diapirs (e.g., Jackson and Seni, 1984) corroborates intuition that dissolution is greatest at the crests
of salt diapirs, where salt is rising fastest. Dissolution
tends to diminish down the flanks where groundwater becomes more saturated with depth as meteoric
waters grade into compactional and even thermobaric waters around the base of diapirs. Dissolution thus
tends to advance downward and inward.
Locally, enhanced salt removal forms circular to
subcircular sinkholes that resemble limestone karst
sinkholes. Christiansen (1967, 1971) inferred that
the collapse structures in Saskatchewan comprised
numerous fault blocks bounded by stair-step faults
and that the Crater Lake collapse structure comprised two concentric fault zones, forming an inner
and an outer cylinder. He assumed that the faults
were high-angle normal faults. Lohmann (1972),
however, showed that both high-angle normal and
reverse faults are present in a collapse structure
from the central part of the English Zechstein basin.
Despite many papers on salt dissolution, details
of faulting caused by dissolution are sparse. Highangle, conical normal faults bounding the depression and inward-dipping strata are commonly the
only structures described for many sinkholes
(Anderson, 1981; Autin, 1984; Beck, 1984; Jammal
and Beck, 1985; Mullican, 1988). Limited well
control across the Wink sink in west Texas
(Baumgardner et al., 1982; Johnson, 1986) and limited surface mapping of the Chimney C collapse
structure in southeastern New Mexico (Davies,
1985) preclude any reliable interpretation of the
internal structure. Seismic data of the Crater Lake
collapse structures (Gendzwill and Hajnal, 1971)
and dissolution features on the Flor ida shelf
(Popenoe et al., 1984) show only simple, downwarping structures because the internal structure
of sinkholes is too fine to be resolved seismically.
Grabens above salt diapirs have commonly been
interpreted as forming by salt dissolution (e.g.,
Cater, 1970; Lohmann, 1979; Bacoccoli et al., 1980;
Stokes, 1982; Jenyon, 1984, 1986, 1988b; Doelling,
1985, 1988; Baars and Doelling, 1987; Chenoweth,
1987; Mart and Ross, 1987); however, physical
experiments, theoretical considerations, and reinterpretation of natural examples (Vendeville and
Jackson, 1992a, b; Jackson and Vendeville, 1994;
229
Ge et al., 1995, 1996; Ge, 1996) showed that most
crestal grabens in nature can be explained by (1)
rise of reactive diapirs caused by regional extension, (2) local stretching over active diapirs or in
the crests of buckling or drape antiforms, or (3)
subsidence of diapirs driven by regional extension.
Nevertheless, none of these studies were able to
draw upon modeling to evaluate the structural
effects of withdrawing a diapir—a necessary step
to evaluate whether the role of dissolution in deformation around diapirs has been overestimated.
Faulting above salt domes has been the subject
of many experiments, but most studies simulated
faulting above actively rising diapirs (Link, 1930;
Parker and McDowell, 1951, 1955; Currie, 1956;
Withjack and Scheiner, 1982; Lemon, 1985; Brewer
and Groshong, 1993; Davison et al., 1993; SchultzEla et al., 1993). Only the physical models of Parker
and McDowell (1955) simulated the effects of salt
dissolution, which was simulated by (1) evaporation of dry ice or (2) removing a rigid plug below
the overburden. The overburden collapsed into the
void by means of high-angle normal and reverse
faults. However, Parker and McDowell’s (1955)
investigation of this topic was not systematic, and
no geologic examples were compared.
Our paper provides guidelines for interpreting
collapse structures induced by withdrawal and,
probably, dissolution and differentiating them from
subsidence features formed by other processes. We
systematically investigated the influence of diapir
shapes on progressive deformation induced by
withdrawing salt, with and without deposition during deformation. Because salt dissolution itself was
not simulated owing to technical obstacles, we
developed conceptual kinematic models of saltdissolution structures based on the incremental
structural evolution of the salt-withdrawal models
and on a range of contrasting assumptions. Finally,
we compared our model results with geologic
structures. The results show that collapse structures can be distinguished from grabens formed by
regional extension or by local arching by the presence of an inner zone of contraction.
The experiments should help in the interpretation of both structural geometries and geologic processes from seismic profiles. Our models are applicable only to removal of salt from entire diapirs or
tabular bodies where the overburden deforms predominantly by brittle faulting. We do not address
sinkholes or chimney structures formed by only
local dissolution within a much larger body of salt.
EXPERIMENTAL METHODS
We used two kinds of modeling materials: dry
quartz sand simulating brittle sedimentary rocks,
230
Salt Withdrawal/Dissolution Models
Table 1. Parameters of Selected Experiments on Salt Withdrawal*
Diapir
Shape
(Apical Angle)
Model
Number
266**
271**
250**
251**
252**
256**
259†
258**
261†
254**
255**
262**
260†
257**
268†
Tabular
Tabular
Rectangular wall
Rectangular wall
Rectangular wall
Rectangular wall
Rectangular wall
Semicircular wall
Semicircular wall
Triangular wall (60)
Triangular wall (90)
Triangular wall (90)
Triangular wall (90)
Cylindrical stock
Leaning wall
Diapir
Height (cm)
2.3
1.9
3.7
2.0
1.0
3.7
4.0
5.0
3.7
3.6
3.7
3.7
4.5
4.4
4.8
Ho/Hs
Duration
(hr)
Withdrawal
Rate
(cm/hr)
Aggradation
Rate
(cm/hr)
1.3
2.1
1.1
2.0
4.0
1.1
0.2
0.8
0.22
1.1
1.1
1.1
0.18
0.91
0.17
118.0
18.0
6.5
7.0
3.0
86.0
93.0
68.0
235.0
25.0
16.2
141.0
263.0
47.0
179.0
0.021
0.111
0.692
0.286
0.333
0.047
0.043
0.074
0.021
0.056
0.120
0.035
0.019
0.106
0.027
–
–
–
–
–
–
0.034
–
0.008
–
–
–
0.008
–
0.007
*Ho/Hs = (thickness of prekinematic roof)/(diapir height). Withdrawal rate = (diapir height loss)/(duration). Aggradation rate = (total thickness of synkinematic
units)/(duration).
**Experiment with only prekinematic sedimentation.
†Experiment with prekinematic and synkinematic sedimentation.
Fixed wall
Fixed wall
Rigid
perforated
base
Contaner
Drain
holes
Container
Prekinematic roof sand layers
Adjoining sand layer
0
10 cm
Silicone diapir
Figure 1—Cross section of experimental apparatus and
a model before deformation. Silicone diapirs of various
shapes were built using rigid molds. An adjoining sand
layer encased the diapirs and had a thickness similar to
the height of the diapir.
and SGM36, which is a transparent silicone polymer,
simulating viscous rock salt. Dry quartz sand has a
frictional-plastic behavior and deforms by slip along
narrow shear zones, representing fault planes
(Vendeville et al., 1987; Mandl, 1988; Krantz, 1991).
The sand has negligible cohesion, an internal friction angle of between 25 and 30°, and a density of
1500–1700 kg/m 3 (Vendeville et al., 1995). Its
mechanical properties are similar to those of sedimentar y rocks that deform by Mohr-Coulomb
behavior in the upper continental crust (Byerlee,
1978; Vendeville et al., 1987; Weijermars et al.,
1993). In contrast, SGM36 silicone is a near-perfect
Newtonian fluid with a dynamic shear viscosity
ranging from 2.5 × 104 Pa·s at a strain rate of 3 ×
10–1/s to 3.3 × 104 Pa·s at a strain rate of 2 × 10–3/s
at room temperature (Weijermars, 1986; Vendeville
and Jackson, 1992a; Weijermars et al., 1993), and a
density of 950–980 kg/m 3 . The silicone spreads
under its own weight. In the interest of brevity and
geologic relevance, we refer to this rock-salt analog
as “salt” throughout the rest of this paper.
All experiments were done in a normal gravity
field. The brittle response of the diapir roof was
scaled to gravity, based on principles in Hubbert
(1937), Ramberg (1967), Vendeville et al. (1987),
and Weijermars et al. (1993). Based on a length
ratio of 10–5 (1 cm in the model = 1 km in nature),
a density ratio of 0.7 for the overburden, a stress
ratio of 7 × 10–6 for the overburden, and a viscosity
ratio of 3 × 10–14 for the salt, the time ratio is 4 ×
10–9. This ratio allows the equivalent durations in
nature to be calculated (from Table 1) to range
from 0.2 to 7 m.y. Because salt viscosity varies naturally over at least two orders of magnitude (van
Keken et al., 1993), any durations or rates calculated by scaling are likely to be accurate only to within two orders of magnitude. Nevertheless, assuming a representative natural salt viscosity of 10 18
Pa·s allows the natural rates of aggradation and salt
Ge and Jackson
Early (252)
Int (266)
Prek
Int (251)
Tabular
Adv (271)
Adv (250, 256)
Parallel
Synk—Adv (259)
Prek—Adv (258)
MODELS
Semicircular
Int (261)
Synk
Salt wall
Adv (268)
Convergent
60°—Prek—Int (254)
Int (255)
Triangular
231
Figure 2—Dendrogram of
systematic experiments
investigating structures
formed by salt withdrawal and
dissolution. Sketches show
idealized predeformational
profiles of diapirs (black).
Parallel diapirs have vertical
flanks, whereas convergent
diapirs narrow upward. Prek =
prekinematic experiment; Synk =
synkinematic experiment; Early,
Int, Adv = early, intermediate, and
advanced stages of deformation,
respectively. Model numbers are
in parentheses.
Prek
Diapiric
90°
Leaning—Synk (268)
Adv (262)
Synk—Adv (260)
Salt stock—Prek—Adv (257)
withdrawal to be estimated. The experimental
range of aggradation rates of 7 × 10–3 to 3 × 10–2
cm/hr (Table 1) is equivalent to 3 × 102 and 1 × 103
m/m.y. in nature; those rates are comparable to natural sedimentation. The range of experimental salt
withdrawal rates, which is 2 × 10 –2 to 7 × 10 –1
cm/hr, is equivalent to 8 × 102 to 3 × 104 m/m.y. in
nature. These rates of withdrawal seem geologically
reasonable, but geologic rates of salt dissolution are
uncertain (as previously discussed), so the realism
of the withdrawal ratio is difficult to evaluate.
Nevertheless, this ratio is essentially irrelevant to
the topic being investigated because the brittle
response of an overburden with Mohr-Coulomb
behavior is time-independent. The salt viscosity is
realistically scaled to the most important variables
of gravity and aggradation rate. Accordingly, variable rates of salt withdrawal in nature should not
materially affect the structural style. Our results are
thus applicable to scales of 100–104 m for sedimentary roofs that deform by faulting with negligible
bedding-plane slip.
We conducted experiments in a rigid box 33 cm
long and 23 cm wide having fixed walls (Figure 1).
We drilled 45 holes, each 5 mm in diameter, in the
central part of a rigid wooden baseboard; the perforated area was 10 cm × 23 cm (the width of the
model). These holes allowed the salt to drain slowly, and were selectively covered by adhesive tape to
change the rate of salt withdrawal.
Each diapir was built on the perforated baseboard (Figure 1), except in models 266 and 271
where the salt was tabular. We designed the model
diapirs to be roughly as high as they were wide
because that is the approximate shape of natural
upright diapirs (unpublished compilation by
Jackson). We then added an adjoining sand layer
(the lowest thick, stippled layer) to encase the
diapir; tabular salt was covered by a uniformly
thick overburden. The model was then covered by
roof layers, which included prekinematic strata
and, in some experiments, synkinematic strata.
Prekinematic layers were added before salt withdrawal; synkinematic layers were added during
withdrawal. Deformation was induced by uncovering the drain holes.
Fifteen experiments systematically investigated
the effects of diapir shape for a variety of sedimentation histories (Figure 2). In models having only
prekinematic roofs (referred to as “prekinematic
models” for brevity), all the 4.0-cm-thick, tabular
roof strata were deposited before deformation started. The ratios of roof thickness (Ho) to salt thickness (Hs) ranged from 0.80 to 4.0 (Table 1). In the
models with synkinematic deposition (referred to
as “synkinematic models” for brevity), a 0.8-cmthick (Ho/Hs = 0.17 to 0.22; Table 1) sequence of
prekinematic layers was added, and then after
deformation began, synkinematic sediments were
added episodically
with a mean regional aggrada•
tion rate A = 0.007–0.034 cm/hr (Table 1).
Synkinematic layers are easily recognized in cross
sections by the lateral variations in their primary
thicknesses. Evolution of surface structures was
recorded by automatic time-lapse photographs.
Additionally, we cut and photographed cross sections at the end of the experiments after the models were strengthened by infusion with water.
232
Salt Withdrawal/Dissolution Models
(a)
Model 271, t = 18.0 hr
Trough
Salt weld
Drained area
(b)
0
Figure 3—Cross section of model
271 showing withdrawal of
tabular salt initially 1.9 cm thick.
(a) True scale, (b) vertically
exaggerated by 3× to highlight
the downward increase in
deformation (maximum true dip
increases downward from 11 to
20° on the left, from 12 to 16° on
the right).
5 cm
No vertical exaggeration
Prekinematic
Salt analog
0
5 cm
Vertical exaggeration × 3
Initial top of salt
In contrast to the process of dissolution, which is
thought to operate inward and downward in a salt
body (see previous discussion), our models drained
from the bottom by withdrawal of salt. Accordingly,
we refer to this process as “salt withdrawal” rather
than “salt dissolution.” In each process, the diapir
diminishes in height as the roof subsides unless salt
is replaced by importation from the source layer,
which we assumed had been exhausted (to reduce
the number of experimental variables). Because of
the variability of natural salt dissolution rates, we
designed models with a 35-fold range of vertical withdrawal rates from 0.02 cm/hr to 0.69 cm/hr (Table
1). The range is arbitrary, but variations in these rates
have little effect on structural style of a deforming
roof (see scaling discussion in this section).
Roofs are commonly brecciated above dissolving
salt (Landes et al., 1945; Davies, 1985; Nieto et al.,
1985). The shear zones in our models are partly
analogs to breccia zones in that the sand becomes
less packed and dilates, as in brecciation.
Because all the sidewalls were rigid and fixed,
the lateral dimensions of all our models remained
constant. Thus, for all the models involving salt
walls, all cross sections are balanced with respect
to the overburden, and all extension is precisely
compensated for by contraction. In contrast, cross
sections through stocks are not balanced in two
dimensions because of structural movement of
overburden into and out of the planes of section.
EXPERIMENTAL RESULTS
Models with Prekinematic Layers Only
In this series of models, all the roof sediments
were deposited before deformation to evaluate the
effects of withdrawal without the additional vari-
able of sedimentation rate. The sequential evolution of structures is illustrated by serial maps of
overhead views (except for model 271), and the
structural style is illustrated by cross sections cut at
the end of each experiment.
Withdrawal of Tabular Salt (Models 266
and 271)
Tabular salt represents either autochthonous bedded salt or an allochthonous sheet of salt. In this set
of experiments, salt was differentially removed from
the entire section, but mostly above the draining
area. Model 266 (not illustrated) was an intermediatestage model; model 271 was an advanced stage. The
resulting structure in each was a gentle syncline
bounded by monoclinal flexures (Figure 3). As salt
withdrew, the bottom of the synclinal overburden
grounded onto the rigid base and flattened into a synformal box fold that became more rounded upward.
Strata in the trough thickened slightly because of
gravity slumping, but no faults were visible.
Withdrawal of Walls Having Rectangular
Cross Sections (Models 250, 251, 252, and 256)
In these models, salt walls were 23 cm long,
10 cm wide, and had rectangular profiles. The
diapir walls were initially 1.0 cm (model 252), 2.0
cm (model 251), and 3.7 cm (model 250) high.
(The sizes and shapes of all modeled diapir s
became slightly modified because diapirs sagged
slightly before burial during model constuction.
Results from these deformed models provide a
spectrum of salt-withdrawal structures at different
evolution stages: taller salt walls subsided more and
deformed more than shorter salt walls. Thus,
model 250 passed through similar stages of deformation shown by models 252 and 251.
Ge and Jackson
(a)
Model 252, t = 3.0 hr
(b)
Figure 4—Maps of structures
in the prekinematic layer
above withdrawing rectangularprofiled salt walls. Initially, the
heights of the walls were (a) 1.0
cm, (b) 2.0 cm, and (c) 3.7 cm.
t = duration of experiments in
hours. Locations of cross
sections 5a–c are shown.
Model 251, t = 7.0 hr
N
N
Trough
Trough
233
5b
5a
(c)
Model 250, t = 6.5 hr
0
10 cm
N
Trough
Normal fault, ticks in hanging wall
5c
Reverse fault, dashed where covered,
triangle in hanging wall
Slump scarp
5a
Location of cross section
and figure number
Figure 4 shows maps of surface structures, and
Figure 5 shows corresponding cross sections. At
an early stage of withdrawal (Figures 4a, 5a), a flatbottomed trough formed in the roof above the rectangular diapir wall. In the trough walls, steeply dipping, convex-upward reverse faults bounded the
trough floor and merged at depth with the vertical
sides of the diapir. Steeply dipping, convex-upward
normal faults formed the outer boundaries of the
trough walls. Little deformation was visible in the
subsiding trough roof.
Figures 4b and 5b illustrate an intermediate stage
of salt withdrawal, and Figures 4c and 5c illustrate an
advanced stage. More advanced withdrawal of salt
deepened the central trough (Figure 5b, c). Inward
gravity slides of the oversteepened trough walls are
visible in cross sections as differentially thinned
(updip extension with respect to the dipping trough
walls) or thickened (downdip contraction) shallow
layers of the roof. Sliding narrowed the subsiding
trough floor and flattened the reverse faults (Figure
5c). The contractional zones widened inward over
time as new thrust faults formed nearer the center.
Conversely, the normal fault zones widened outward
over time as new normal faults formed farther from
the center, maintaining a stable slope for the deepening trough walls.
Model 256 investigated structures formed where
the salt was differentially withdrawn across a wider
wall (Figure 5d). The wall was 13 cm wide instead
of 10 cm wide, but the setup was otherwise similar.
Because salt drained fastest in the center above
holes in the perforated base, subsidence was
nonuniform. Although the boundary faults merged
downward into the salt body rather than along the
diapir contacts as in models 250, 251, and 252, the
overall structures remained similar. Residual cusps
of salt (C in Figure 5d) at the edges of the subsiding
trough floor were overlain by both normal and
reverse fault zones, unlike salt cusps formed by
234
Salt Withdrawal/Dissolution Models
West
East
(a) Model 252, rectangular wall, Hs = 1.0 cm
(b) Model 251, rectangular wall, Hs = 2.0 cm
TW
TW
TF
(c) Model 250, rectangular wall, Hs = 3.7 cm
EZ
EZ
CZ
CZ
TF
(d) Model 256, rectangular wall, Hs = 3.7 cm
C
C
Drained area
Prekinematic
Salt
0
analog
Adjoining
Initial top of salt
5 cm
Figure 5—Cross sections of deformed models (a) 252,
(b) 251, (c) 250, and (d) 256. See Figure 4 for the locations of (a)–(c). C = cusp of residual salt, CZ = contractional zone, EZ = extensional zone, Ησ = initial height of
salt wall, TF = trough floor, TW = trough wall.
extensional subsidence of salt structures, which are
overlain only by normal fault zones (Vendeville and
Jackson, 1992b).
Withdrawal of Wall Having Semicircular
Cross Section (Model 258)
The evolution of structures above a subsiding
diapiric wall having a semicircular profile is shown
in Figure 6. The trough floor, bounded by inner
reverse-fault zones, became narrower over time.
The outer limit of the trough wall was marked by
normal faults that shifted outward over time. Fault
blocks in the trough wall were tilted and sliced by
many small normal faults (not shown) as the subsiding structure widened. The cross section (Figure 7)
shows a wide, synclinal trough. The lowermost
prekinematic roof in the trough was deformed into
a rough inverted image of the initial shape of the
semicircular diapir; that is, a semicircular trough
formed in the roof above a subsided semicircular
diapir. This inverted symmetr y was degraded
upward by faulting, thinning of strata in the trough
walls, and thickening of strata in the trough. The
base of the syncline sank and welded onto the flat
base as the salt was completely withdrawn. Both
boundary normal and reverse faults merged downward into the wall contacts of the withdrawn diapir
and had a convex-upward profile.
Withdrawal of Walls Having Triangular
Cross Sections (Models 254, 255, and 262)
The triangular-profiled walls initially were 23 cm
long and 3.7–4.5 cm high (Ho/Hs = 1.1; Table 1).
Models 254 and 255 initially had apical angles of 60
and 90°, respectively, and were deformed to an
intermediate stage. Model 262 had an apical angle
of 90° and was deformed to an advanced stage
(Figure 2). Overhead views and cross sections
showed similar structural patterns in all models,
suggesting that the structural style above a vanishing triangular diapir is largely independent of the
diapir’s apical angle. Only models 255 and 262 are
discussed here, and model 254 is not reproduced
to avoid repetition.
Figure 8 shows maps of evolving structures
above a subsiding triangular diapir in model 262.
Figure 9a and b are cross sections at the end of the
experiment. As in model 258, the trough floor narrowed over time as the inner contractional zones
widened inward and the outer extensional zones
widened outward (Figure 8). The trough floor contained no visible reverse faults at depth. The roof
was deformed into a syncline, whose profile varied
from trough-shape to v-shape, a rough, inverted
image of the originally triangular diapir.
A cross section of deformed model 255 (Figure
9c), in which salt was not completely withdrawn,
provides insights into the generation of intermediate structures. Each new reverse fault formed when
a new part of the roof grounded as the diapiric limits shrank. The new reverse fault then propagated
upward and toward the center of the subsiding
trough. In both models, the boundar y faults
extended downward into the lower apices of the
shrinking diapirs. All faults were convex upward.
Ge and Jackson
Figure 6—Maps of model 258
showing structural evolution
of prekinematic layer above a
withdrawing salt wall having a
semicircular profile. Location of
cross section and photograph
shown in Figure 7 are given in (c).
(b) Model 258, t = 40 hr
(a) Model 258, t = 20 hr
235
Trough
N
N
Trough
(c) Model 258, t = 60 hr
N
0
10 cm
Normal fault, ticks in hanging wall
Reverse fault, dashed where
covered, triangle in hanging wall
7
Slump scarp
Cross section
Withdrawal of Salt Stock Having Rectangular
Cross Section (Model 257)
Initially, the cylindrical diapiric stock had a circular planform and was 4.4 cm high (H o /H s =
0.91; Table 1) and 10 cm in diameter. During early
stages of deformation, a shallow, subcircular,
sunken f loor bounded b y a conical scarp of
slumped sand first formed above the subsiding
stock (Figure 10a). We interpret the inner boundar y of this scarp as the reverse fault visible in
cross section (Figure 11a). Next, conical reverse
faults (obscured in the map by slumps) and normal faults formed two concentric zones. As in previous models, an outer extensional zone surrounded an inner contractional zone (Figure 10b).
Intermediate strata tilted inward. At an advanced
stage (Figure 10c), subsidence of the floor caused
the fault zones to widen at the expense of the narrowing floor.
As expected in an axisymmetric model, structures in cross sections of model 257 varied significantly depending on their locations. The central
section (Figure 11a) shows a structure similar to
those above walls having rectangular profiles
(Figure 5). The structure in Figure 11b, cut near the
edge of the stock, is complicated by out-of-plane
slip of faults. Synclinal eye-shape patterns result
from a planar section across a funnel-shape structure sliced by reverse faults dipping away from the
reader. A simple cross section beyond the rim of
the stock (Figure 11c) intersected only the conical
extensional zone.
Models with Synkinematic Sedimentation
In this series of experiments (models 259, 260,
261, and 268; Figures 12–17), we investigated the
236
(a)
Salt Withdrawal/Dissolution Models
fault were much narrower than the corresponding
prekinematic models at advanced stages (Figure 4b,
c), but similar to an early stage (Figure 4a). As in
the models with only prekinematic layers (Figure
5), faults in synkinematic models were concentrated along the edges of the central trough (Figure
12c). The inner reverse faults sequentially shifted
outward, whereas the outer normal faults shifted
inward, which is opposite to the evolution in the
prekinematic models.
Model 258
Drained area
(b)
Model 258, semicircular wall, Hs = 5 cm
C
C
Salt weld
0
5 cm
Figure 7—(a) Photograph and (b) tracing of a cross section of deformed model 258. See Figure 5 for key and
Figure 6 for location. Ησ = initial height of salt wall.
influence of synkinematic deposition on the saltwithdrawal structures. In experiments, a 0.8-cmthick prekinematic unit (lowest four layers above the
diapir; H o /H s = 0.17 to 0.22; Table 1) was added
above the prebuilt diapirs, which had various profiles
before being deformed. During subsidence, synkinematic layers were added episodically. Each synkinematic layer was leveled to a regional datum (marked
by the evenly thick layers outside the subsiding
diapir area) and completely filled in the structural
relief. Thus, gravity slumping was not significant
along the base of the trough walls. The mean aggradation rate relative to the regional datum ranged from
0.007 to 0.034 cm/hr (Table 1). Aggradation above
the subsiding diapirs was faster, but varied laterally.
Withdrawal of Salt Wall Having Rectangular
Cross Section (Model 259)
The diapiric wall initially was 4 cm high (Ho/Hs =
0.2; Table 1) and had a rectangular profile.• Six synkinematic layers were added over 93 hr ( A = 0.034
cm/hr; Table 1). Figure 12a and 12b are maps of
evolving subsiding diapir s at 40.3 hr (before
deposition of layer S2 in Figure 12c) and 67.3 hr
(before layer S4 in Figure 12c). The trough walls
between the outer normal fault and inner reverse
Withdrawal of Salt Wall Having Semicircular
Cross Section (Model 261)
This experiment examined the influence of synkinematic deposition on structural style above a subsiding salt wall. The wall initially had a semicircular
profile and was 3.7 cm high (Ho/Hs = 0.22; Table 1)
and 23 cm long. •Seven layers were added episodically over 239 hr (A = 0.008 cm/hr; Table 1). A slight
variation in initial height along the strike of the salt
wall caused differential loading and subsidence during the experiment. Map views (Figure 13a, b), cross
sections (Figure 13c, d), and thickness variations of
synkinematic sediments (Figure 14) record the migration of the depocenters. Thickened overburden
(Figures 13c, 14) caused fast salt withdrawal and created a major depocenter in the north (Figure 13a).
The depth of this depocenter increased over time
until the deposition of layer S4 (Figures 13c, 14), and
then decreased owing to depletion of salt.
Conversely, a minor depocenter formed in the south
at early stages, which gradually deepened with time
(Figures 13, 14) until the salt was depleted. Both the
depocenters and extensional and contractional fault
zones young southward.
As in model 259 (Figure 12), map views of
model 261 show narrow fault zones bounding a flat
central trough; little gravity slumping occurred
(Figure 13a, b). The prekinematic layer was
deformed into a rounded synformal profile that is
the inverted shape of the initially antiformal diapir.
The synformal shape was gradually lost upward in
the synkinematic layers. Again, the active inner
reverse faults successively shifted outward, whereas the active outer normal faults shifted inward
(Figure 13c, d). Residual cusps of salt formed
below the faults at the edges of the roof. Again, the
cusps are flanked by reverse faults diagnostic of salt
withdrawal, unlike cusps created by diapir fall during regional extension, which are flanked only by
normal faults (Vendeville and Jackson, 1992b)
Withdrawal of Salt Wall Having Triangular
Cross Section (Model 260)
The initial triangular salt wall had an apical angle
of 90° and was 4.5 cm high (Ho/Hs = 0.18; Table 1).
Ge and Jackson
(a) Model 262, t = 17 hr
Figure 8—Maps of model 262
showing structural evolution of
the prekinematic layer above a
withdrawing salt wall having a
triangular profile. See Figure 4
for key; location of cross section
and photograph in Figure 9 are
given in (d).
(b) Model 262, t = 24 hr
N
237
N
Trough
(c) Model 262, t = 48 hr
(d) Model 262, t = 135 hr
N
N
9b
0
10 cm
Five 0.4-cm-thick
synkinematic layers were added
•
over 263 hr ( A = 0.008 cm/hr; Table 1). Maps in
Figure 15a and b have markedly fewer faults than in
the prekinematic counterparts (Figure 8), although
the basic structural zonations remain similar: inner
contractional reverse faults and outer extensional
normal fault zones bounded a central trough. The
west limb of the prekinematic roof tilted and subsided along extensional faults, whereas the east
limb was thrust over it and was shortened by
reverse faults. Some older faults in the prekinematic layer propagated directly upward into young
synkinematic strata. Some normal faults rotated and
became contractional faults (faults EC, Figure 15)
during syndepositional deformation.
The prekinematic layer was deformed into a
v-shaped syncline that is the approximate inverted
image of the originally triangular diapir. Again, the
active outer normal faults shifted inward with time.
Although the active inner reverse faults generally
shifted outward (left limb in Figure 15c), they were
not restricted to this sequence. The syncline
became less angular upward.
Withdrawal of Leaning Salt Wall (Model 268)
•
The constructed salt wall (H o /H s = 0.17; A =
0.007 cm/hr; Table 1) initially had a semicircular
cross section as in models 258 (Figure 7) and 261
(Figure 13c, d); however, leaning and sagging during
preparation created an eastern upward-diverging
flank (Figure 16a), which provided unexpected
insights into salt withdrawal (Figures 16, 17).
The arcuate diapir top (Figure 16a, b) quickly
flattened during early withdrawal and remained
gently tilted (Figure 16c, e). Flattening probably
238
(a)
Salt Withdrawal/Dissolution Models
(a) Model, 257, t = 2.3 hr
Model 262
N
(b)
Model 262, triangular wall, Hs = 3.7 cm
0
10 cm
(b) Model 257, t = 18.8 hr
N
Salt weld
(c)
Model 255, triangular wall, Hs = 3.7 cm
Drained area
0
5 cm
Figure 9—(a) Photograph and (b) tracing of a cross section of deformed model 262. See Figure 5 for key and Figure 8 for location. (c) Cross section of deformed model
255 showing the structural style at an intermediate
stage of deformation. Η σ = initial height of the salt
wall.
was caused by a combination of (1) maximum
withdrawal rates in the diapir’s axis because of
boundary drag and (2) maximum loading by synkinematic strata in the diapir’s axis. As salt was
withdrawn from the inclined diapir, the subsiding
roof was forced to shift to the west, tracking the
vanishing diapir. That shift encouraged contraction
in the west and extension in the east.
On the western convergent flank (where the
diapir narrowed upward), reverse faults formed an
inner contractional zone, and normal faults formed
an outer narrow extensional zone. The oldest normal
fault on the westernmost boundary locally rotated,
however, and became a reverse fault (EC) during
later syndepositional deformation (Figure 16b–f).
(c) Model 257, t = 46.8 hr
Sections
11c
11b
11a
N
Figure 10—Maps of model 257 showing the growth of
conical, concentric structures in the prekinematic layer
above a withdrawing, flat-topped, salt stock. Location of
cross sections shown in Figure 11 is given in (c). See Figure 4 for key.
Ge and Jackson
West
East
Model 257
(a)
(b)
Drained area
(c)
Drained area
239
17). Differential salt withdrawal inverted the early
depocenter D1 at the level S3–S4 into a subtle anticline. An inner reverse fault was reactivated first into
a normal fault, and then into a reverse fault (CEC,
Figure 17) during this episode of salt withdrawal.
In summary, the basic structural zonation in
these synkinematic models was similar to their
prekinematic counterparts. The prekinematic layers subsided to form profiles roughly similar to the
inverted initial profiles of the diapirs. The subsiding
diapirs were bounded by convex-upward normal
and reverse faults forming an inner contractional
zone and an outer extensional zone; however, the
synkinematic fault zones were much narrower than
in prekinematic models. The episodic deposition
continually decreased the slope of the trough walls,
preventing repeated slumping and inward folding
of reverse faults. The growth sequence of the
reverse faults was generally from the center outward, which is opposite to the sequence in models
having only a prekinematic roof. Conversely, active
normal faults generally shifted inward in the synkinematic roofs.
COMPARISON OF PHYSICAL MODELS
WITH CONCEPTUAL MODELS OF
SALT-DISSOLUTION STRUCTURES
0
5 cm
Figure 11—Serial cross sections of deformed model 257
showing the structures at (a) center, (b) edge, and (c)
outside of the withdrawing salt stock. See Figure 5 for
key and Figure 10 for locations.
On the eastern divergent flank (where the diapir
widened upward), a more complex structure
formed. Unlike the other models, the outer stairstepped normal faults did not root to the shoulder of
the diapir, but formed a wide extensional zone
(Figure 16b); nor did the inner reverse faults initially
root to the shoulder of the diapir. Later, the inner
reverse fault emplaced a thin slab of hanging-wall
salt over the central roof (Figure 16c, d). The innermost normal and reverse faults then became inactive
as a new reverse fault formed along the trough wall
(Figure 16c–e). The three salt projections in Figure
16f are residual structures, not dikelike injections.
Along strike of the salt wall, salt above the
divergent flank was trapped between the rapidly
subsiding center (D1, Figure 17) and the diapir
edge. The trapped salt was later withdrawn after
the central salt was depleted, resulting in a new
depocenter starting with the deposition of layer
S4 (D2, Figure 17). A similar flanking depocenter
(D3) is also visible along the west flank (Figures 16f,
In the introduction, we concluded that as a diapir
dissolves, salt is probably removed from the crest
downward; however, our experiments drained salt
from the base of the salt structures. How applicable,
then, are our models to interpreting salt-dissolution
collapse structures? Can salt dissolution be distinguished from salt withdrawal by structural criteria?
We addressed these questions by developing conceptual models of salt-dissolution structures above
vanishing salt walls having various cross sectional
profiles. We mentally adjusted the observed evolution of physically modeled salt-withdrawal structures to emulate salt vanishing by dissolution of
progressively lower salt rather than by withdrawal
of progressively higher salt.
We conceptually examined diapirs with parallel,
upward-diverging and upward-converging sides to
illustrate some expected similarities and differences between salt dissolution and withdrawal.
The effects of synkinematic deposition are omitted
to clarify these differences. Initially, we assumed
that the overburden roof deforms solely by visible
faulting and that as the upper part of a diapir is uniformly and systematically dissolved downward, no
salt rises to take its place, but then we changed
these assumptions.
In parallel and divergent diapirs (Figures 18a,
19a), the roof of the diapirs subsides into a void
created by withdrawal or dissolution. Because of
240
Salt Withdrawal/Dissolution Models
(a) Model 259, t = 40.3 hr
Figure 12—Maps of model 259
(a) before layer S2 was added
and (b) before layer S4 was added
showing structural evolution in
synkinematic layers above a
withdrawing diapir having a
rectangular profile. (c) Cross
section of the deformed model.
S1–S6 = number of synkinematic
layers.
(b) Model 259, t = 67.3 hr
N
N
Trough
Trough
12c
0
10 cm
(c)
West
Model 259, t = 93.0 hr
East
Synkinematic
S6
S5
Prekinematic
S4
Adjoining
S3
Salt analog
S2
Initial top of salt
S1
0
5 cm
Drained area
the parallel or diverging sides and flat crest, salt withdrawal at the base of the diapirs always causes the
diapir crest to subside. Diapir crests also are likely to
subside by dissolution before the diapir f lanks
because the saturation of groundwater tends to
increase with depth. If both (model) withdrawal and
(natural) dissolution tend to lower the crest of a
diapir, we expect that structures resulting from dissolution would be similar to those in our models (Figure
5; east flank in Figures 16 and 17). Both boundary
normal and reverse faults root to the edges of the salt
diapirs (Figures 18, 19). Normal faults (1′) form in the
outer zone, and reverse faults (1) form in the inner
zone. Further removal of salt deepens the trough,
widening the extensional zone outward to create
new normal faults (2′), and widening the contractional zone inward to create new reverse faults (2).
A diapir having sides converging upward to a
point or an arc should behave differently (Figure 20).
Our models (Figures 7, 9) show that during salt withdrawal, the apex of the diapir is the last to be
removed. The extensional outer zones (marked by
normal faults 1′ and 2′, Figure 20) widen outward
through time, whereas the contractional inner zone
(characterized by reverse faults 1–4) widens inward
(Figure 20b, c). In contrast, during salt dissolution,
the apex is the first to be removed. That causes both
the extensional and contractional zones to widen
outward (Figure 20b′, c′). The normal fault 1′ (at
stage c′) becomes offset by young reverse faults
(e.g., 3) as new boundary faults form (e.g., 2′).
Normal faults, therefore, would not be limited to
outer zones of pure extension, such as in the saltwithdrawal structures in Figure 20b and c, but could
coexist with reverse faults. Despite these different
kinematics, the basic structural zonation produced
by salt dissolution and withdrawal at advanced
stages are predicted to be similar (but not identical).
Hence, with the simplifying assumptions, our models would be applicable to both processes.
In the preceding discussion we assumed that as
salt dissolved from the crest of the structure, the
roof subsided to fill the potential void; however,
salt pressurized by the weight of the overburden
on the adjoining source layer possibly would rise
from below to fill the potential dissolution cavity.
For example, in Oakwood dome (East Texas
basin), stylolitic cleavage, pressure shadows, shear
fractures, and buckled halite-filled extension fractures are present at the base of the cap rock (Dix
and Jackson, 1982). These features indicate that
anhydrite sand was dissolved out of the salt crest
Ge and Jackson
(a) Model 261, t = 18.5 hr
Figure 13—Maps of model 261
showing structural evolution in
synkinematic layers above a
withdrawing diapir having a
semicircular profile (a) before
layer S2 was added and (b) before
layer S8 was added. See Figure 4
for key. Both depocenters and
accommodating fault zones
migrated southward with time.
(c, d) Cross sections of the
deformed model showing (c) the
older depocenter in the north and
(d) the younger depocenter in the
south. The prekinematic layer is
intruded by a small diapir in (d).
The oldest synkinematic layer S1
was arched by the rising diapir at
an early stage in (d). The reverse
faults shifted outward, whereas
the normal faults shifted inward.
The older faults became inactive
and were buried under
synkinematic sediments. The
left reverse faults in (c) are
blind, so they do not appear in
map view. S1–S8 = numbers of
synkinematic layers.
(b) Model 261, t = 213.0 hr
N
N
Trough
13c
Trough
13d
West
(c)
0
East
5 cm
Model 261, t = 235.0 hr
S7
S5
S3
Synkinematic
S1
241
Prekinematic
Adjoining
(d)
Salt analog
S8
Initial top of salt
S6
S4
0
5 cm
S2
Drained area
6
North
South
5
Growth ratio
and repeatedly accreted against the base of the cap
rock by episodic rise of the salt stock, which closed
the ephemeral dissolution cavity.
If pressurized salt rises into the dissolution cavity, then the structural evolution will proceed along
one of three paths. (1) If the surrounding source
layer is still thick enough to supply salt to the diapir
fast enough to offset the dissolution rate, the diapir
conserves volume and the roof does not subside.
(2) If salt supply exceeds salt dissolution, the diapir
rises actively and raises its roof above regional elevation (Schultz-Ela et al., 1993). (3) If the diapir
loses volume and sags, subsidence is probably
greatest at the base of the diapir because the
weight of overlying rocks is greatest there. If so,
the kinematics is expected to be equivalent to that
of salt withdrawal, and the structures similar to
those of the models.
In summary, in flat-topped diapirs having vertical
to upward-diverging sides, or in starved diapirs of any
ideal shape, the kinematics of roof deformation
4
3
2
1
S1
S3
S5
S7
Synkinematic layer number
Figure 14—Plot of growth ratios vs. synkinematic layer
numbers showing the differential subsidence in the northern and southern parts of the withdrawing diapir in Figure
13. Growth ratio = maximum thickness of unit/regional
thickness of unit.
242
Salt Withdrawal/Dissolution Models
(a) Model 260, t = 41.5 hr
(b) Model 260, t = 65.5 hr
N
N
15c
Trough
Trough
15d
West
(c)
Model 260, t = 263.0 hr
East
0
Figure 15—(a, b) Maps of model
260 showing the structural
evolution of synkinematic
sediments above a withdrawing
salt wall having a triangular
profile. See Figure 4 for key.
(c, d) Cross sections of the
deformed model. The apex of
the deformed triangular diapir
shifted to the east because of
extension in the west limb and
contraction in the east limb in
the prekinematic layer. EC =
faults that rotated from earlystage extensional to late-stage
contractional.
10 cm
EC
Synkinematic
EC
Prekinematic
Adjoining
Salt analog
(d)
EC
Initial top of salt
Salt weld
0
5 cm
Drained area
above dissolving diapirs should be similar to that of
our salt-withdrawal models. In diapirs with upwardconverging sides, whose salt does not rise into the
dissolution cavity, the sequence of faulting could be
reversed from that in our models, but the resulting
geometry would be broadly similar.
The experimental results have some parallels
with the physical models of Sanford (1959) and
Horsfield (1977) on cover deformation above a
basement block draped above regional elevation.
Our models differed in two respects: (1) They had
a diapiric roof downthrown below regional elevation; and (2) our diapirs were viscous, rather than
being rigid or elastically f lexible country rock.
Nevertheless, despite these fundamental differences, our experiments show that very similar
cover structures can form on the borders of uplifted basement blocks or sagging diapirs.
CONTRACTION AS A DIAGNOSTIC SIGN
Underlying the subtle variations in fault sequence
described in the previous section is a robust, diagnostic geometry. Partial or complete leveling of a salt
diapir by dissolution or withdrawal causes the same
basic structural pattern in the roof strata, regardless
of diapir shape. An outer extensional zone is balanced by an inner contractional zone (Figure 21).
This inner contractional zone is particularly significant because its presence distinguishes diapirs that
subside by dissolution from diapirs that subside by
regional extension and lack a contractional zone. Our
experiments indicate that simple crestal grabens
above diapirs are not caused merely by salt withdrawal or dissolution, as is often claimed (see references in the introduction); rather, simple crestal
grabens are produced by regional extension or by
Ge and Jackson
West
East
Model 268
Co
D
iv
e
fla rge
n k nt
nve
flan rgen
t
k
(a)
(b)
t = 0 hr
S1
Figure 16—Restored evolution of model 268 showing
the influence of divergent (east) and convergent
(west) flanks of a leaning, round-topped salt wall on
the structures. Restoration used the software RESTORE ©
(Schultz-Ela, 1992; Schultz-Ela and Duncan, 1994).
Strata were unfolded using vertical shear because of
proximity of reverse and normal faults. Restored
structures were confirmed by sequential overhead
views photographed during the experiment. D3 = latestage flanking depocenter, EC = fault that rotated from
early-stage normal to late-stage reverse fault, S1–S4 =
numbers of synkinematic layers.
t = 3 hr
Model 268, t = 179 hr
West
(c)
East
D1
S2
D3
S1
243
D2
S4
EC
CEC
S3
t = 23 hr
S2
(d)
S1
S3
S2
Drained area
S1
t = 40 hr
0
Synkinematic
Adjoining
Initial top of salt
Prekinematic
Salt analog
Salt weld
5 cm
Figure 17—Cross section of deformed model 268 showing inverted central depocenter (layers S3–S4 in zone
D1) and younger flanking depocenters (D2, D3) caused
by differential salt withdrawal. CEC = initially contractional fault that rotated into an intermediate-stage
extensional fault, and then into a young contractional
fault. S1–S4 = numbers of synkinematic layers.
(e)
S4
S3
S2
S1
t = 51 hr
(f)
D3
S4
S3
S2
S1
Drained area
Synkinematic
Adjoining
Prekinematic
Salt analog
t = 179 hr
0
5 cm
Salt weld
local arching above active diapirs. Roof structures
above diapirs, however, always should be evaluated
in light of the regional tectonic setting. For example,
sporadic contractional structures can arise from
basin inversion or tilting; only if the contractional
zone is restricted to the trough and largely flanked by
an extensional zone is it diagnostic of salt dissolution
or withdrawal. In addition, just because contractional structures are missing does not exclude dissolution completely. If regional extension is very strong,
as it typically is in diapir provinces (Jackson and
Vendeville, 1994), this extension can relieve the
room problem caused by roof subsidence sufficiently
to mask the effects of dissolution.
An additional problem is to recognize the diagnostic contractional zone. Contraction can be recognized in cross section if it is intersected by incised
exposures, if repeated section is found in boreholes,
or if seismic resolution is sufficient to image the
reverse offset of correlatable reflectors; however,
seismic resolution may be inadequate or the seismic
image may be obscured near the surface. Because of
its low position at the base of the unstable trough
walls, the contractional zone is expected to be usually buried by talus, slump blocks, or alluvium.
244
(a)
Salt Withdrawal/Dissolution Models
Initial stage
Salt withdrawn by
stage (b)
(b)
(a)
,,,
,,,
Salt dissolved by
stage (b)
Intermediate stage
1'
1
Salt withdrawn
by stage (b)
(b)
1'
1
,,,,
,,,,
Initial stage
Salt dissolved by
stage (b)
Intermediate stage
1'
1
1'
1
Relict salt removed
by stage (c)
Relict salt removed
by stage (c)
(c)
Advanced stage
2'
1'
(c)
2'
1
1
1'
Advanced stage
2'
1'
2'
1
2
2
1
1'
2 2
Salt weld
Salt weld
Figure 18—Schematic sections showing salt-withdrawal
and salt-dissolution structures for a parallel-sided diapir
overlain by an entirely prekinematic roof, and flanked
by a depleted source layer. Whether salt is dissolved
from the top or withdrawn from the base, the deformation evolves similarly through stage (b) to (c). 1, 2 =
reverse faulting sequence; 1′, 2′ = normal faulting
sequence.
Although outer extensional zones have been
reported across sinkholes (e.g., Christiansen, 1971;
Dunrud and Nevins, 1981; Baumgardner et al.,
1982; Jammal and Beck, 1985; Johnson, 1986;
Mullican, 1988), we would not expect an inner
contractional zone to form because the room problem created by outer extension is relieved by simply draining rock fragments from the collapsed roof
into the underlying cave.
GEOLOGIC EXAMPLES
Salt-related structures in the North Sea Basin
(Lohmann, 1972; Jenyon, 1984, 1986, 1988a),
Saskatchewan basin (De Mille et al., 1964; Smith
and Pullen, 1967), and Williston basin (Anderson
and Hunt, 1964; Parker, 1967; Swenson, 1967)
Figure 19—Schematic sections showing salt-withdrawal
and salt-dissolution structures for a flat-topped, divergentsided diapir overlain by an entirely prekinematic roof
and flanked by a depleted source layer. Whether salt is
dissolved from the top or withdrawn from the base, the
deformation evolves similarly through stage (b) to (c). 1,
2 = reverse faulting sequence; 1′, 2′ = normal faulting
sequence.
resemble the structure of models 266 and 271
(Figure 3), where tabular salt has been locally dissolved (Figure 22). This deduction is based on (1)
the resemblance of these salt structures to our
models, (2) the lack of reactive diapirs that would
be associated with regional extension of this magnitude, and (3) the lack of a plausible tectonic mechanism for causing tabular salt to withdraw; however, without knowing the three-dimensional setting,
these inferences can be only tentative. The base of
the roof in the dissolution trough sank to become a
box-shaped syncline. Monoclinal flexures bound
the trough. No visible faults formed during the dissolution collapse. In the Saskatchewan and
Williston basins, late-stage dissolution of the surrounding salt inverted the synkinematic sediments
in the trough, forming an antiform that trapped
hydrocarbons (e.g., Swenson, 1967). Some mound
structures within dissolution zones in the North
Ge and Jackson
(b)
(c)
Intermediate stage
1'
1 2
2
1'
1
Advanced stage
2' 1'
1
2
2
1' 2'
1
3
4
al
raw
(a)
Initial stage
thd
t wi
Sal
Salt weld
Relict salt removed by
stage (c)
,,,,,
Salt withdrawn
by stage (b)
(b')
Salt dissolved
by stage (b')
Sal
t di
(c')
Intermediate stage
1'
2 2
1'
Advanced stage
2'
1' 4
1
,,,,,
,,,,,
sso
lutio
n
Sea (Jenyon, 1986, 1988a) also might result from
similar processes.
An outcrop in northeast England (Figure 23)
reveals more details of structures attributed to salt
(a')
Undeformed rectangular wall
(b')
Undeformed semicircular wall
(c')
Undeformed triangular wall
0
5 cm
dissolution. Dissolution of the tabular Boulby Halite
caused the overburden marls to partially weld onto
the Billingham anhydrite, forming a gently draped
f lexure similar to that in model 271 (Figure 3).
CZ
EZ
CZ
EZ
Deformed triangular wall
EZ
2'
Salt weld
Deformed semicircular wall
EZ
(c)
1'
3
Deformed rectangular wall
EZ
(b)
22
Figure 20—Schematic
sections of (a) a triangular
diapir deforming into
similar structures by salt
withdrawal (b, c) and salt
dissolution (b′, c′). Each
process causes a different
sequence of faulting, but
the structural style of each
sequence is similar. 1–4 =
reverse faulting sequence;
1′, 2′ = normal faulting
sequence.
1
Relict salt removed by
stage (c')
(a)
4
3
245
CZ
Salt weld
EZ
Figure 21—Cross sections
summarizing the roof
structures induced by
salt-withdrawal of (a, a′)
rectangular, (b, b′)
semicircular, and (c, c′)
triangular diapiric walls.
The left panels show the
diapir shapes before
deformation and the right
panels show the deformed
roof after salt withdrawal.
Extensional zones (EZ) and
contractional zones (CZ)
balance in each section.
246
Salt Withdrawal/Dissolution Models
(a)
1
Two-way traveltime (s)
BT
Salt-dissolution trough
CU
TZ
Zechstein Salt
BZ
0
2 km
(b)
Salt-dissolution
trough
0
Depth (km)
2
Figure 22—(a) Seismic
profile from the North Sea
(after Jenyon, 1986) and
(b) vertically exaggerated
depth section from
Saskatchewan (after
De Mille et al., 1964)
showing structures
induced by dissolving or
withdrawing tabular salt.
Compare with similar
model structure in
Figure 3. BT = base Tertiary,
BT–TZ = Zechstein salt
interval, CU =
unconformity.
1
Cretaceous
Salt weld
Upper Devonian
Jurassic
Prairie Salt
0
10 km
Vertical exaggeration ≈ 15
Mississippian
Flexure created many closely spaced axial-planar
fractures perpendicular to bedding. Fracturing
decreased outside the dissolution zone. On a smaller scale, the Honeycomb layer also grounded and
formed a strikingly similar syncline to that in model
271 (Figure 3). Also in the framed area are what we
interpret as small reverse faults (dotted lines)
bounding an inner contractional zone—perhaps a
small-scale version of contraction in models of
withdrawn diapiric salt (Figures 4–9).
Figure 24 is a possible example of an entirely
vanished, deeply buried diapir. In the absence of
regional extension or shortening, the diapir could
have vanished by dissolution or salt withdrawal,
Figure 23—Outcrop sketch
showing structures induced by
dissolution or withdrawal of
tabular Boulby salt, northeast
England (after Jenyon, 1986;
adapted from Smith, 1985).
Reverse faults in the outlined
area are our interpretation.
Compare with similar model
structure in Figure 3.
Upper & Rotten Marls
Boulby
Halite
Billingham A
nhydrite
Honeycomb
Seaham For
mation
0
30 m
Ge and Jackson
Two-way traveltime (s)
0
247
Figure 24—Seismic
profile showing the
effects of synkinematic
sedimentation during
complete dissolution or
withdrawal of a low-relief
salt diapir (after Nely,
1989; location unknown).
Reverse and normal faults
are our conjectural
interpretations. Compare
with similar model
structure in Figure 15.
1
Base salt
0
depending on the three-dimensional geometry,
which is unknown to us. Both reverse and normal
faults cut the base of the syncline. Convex-up
reverse faults are restricted to the center of the syncline. All the faults die out upward in synkinematic
sediments. We interpret the vanished salt to have
been either a low triangle onlapped by surrounding
strata now apparently downlapping against the
base of salt, or an initially tabular salt that has been
locally dissolved.
This example of what we infer to be an entirely
dissolved low-relief diapir (Figure 24) is the only
one we have found in the literature. Such structures either must be uncommon or have not been
widely recognized. Our modeling may assist in recognizing and interpreting vanished, formerly highrelief salt, diapirs that might be encountered in
petroleum exploration.
SUMMARY
The following observations apply to the roof
structures formed by salt withdrawal or by dissolution of tabular salt (including allochthonous sheets
typical of the Gulf of Mexico) or of high-relief
diapirs. Only observation 9 differentiates between
the effects of dissolution and withdrawal by salt
flow.
(1) Local withdrawal of tabular salt forms a
box-shaped syncline bounded by monoclinal flexures. The box-shaped syncline becomes rounded
upward.
(2) Withdrawal of diapiric walls of any ideal shape
creates a trough having an inner contractional zone
and an outer extensional zone of faults (Figure 21).
The contractional zone contains reverse faults,
synclines, and slump-thickened strata. Conversely,
the extensional zone contains normal faults and
2 km
slump-thinned strata. The normal faults dip toward
the diapir, whereas the reverse faults dip away from
the diapir.
(3) These contrasting zones balance to produce
zero regional lateral displacement. In contrast,
crestal grabens formed by regional extension or
active diapiric arching lack the inner contractional
structures. The diagnostic inner contractional zone
is likely to be partly or entirely obscured by talus or
slump deposits derived from the trough walls or by
introduced alluvium. The origin of the reverse
faults should be carefully evaluated in inverted
basins, where they might have regional, as well as
local, causes.
(4) Conical, concentric rings of inner contraction and outer extension form above a subsiding
salt stock in the absence of regional extension.
(5) All faults steepen downward, having a convexup geometric profile, or are vertical, unlike the dipping planar or listric faults typical of regional extension. Exceptions are the normal faults rooted to the
upward-divergent flanks of diapirs, which have a
shallower dip (Figures 16, 17).
(6) The outermost normal and reverse faults
merge downward to a position corresponding to
the sides of the former salt body. Above subsiding
triangular diapirs, each new reverse fault forms
when a new part of the roof grounds as the diapiric
limits shrink.
(7) The cross sectional profile of prekinematic
strata in the deformed roof is roughly an inverted
image of the original profile of the diapir before it
subsided (Figure 21). Thus, a flat-floored graben
forms above an originally flat-topped subsiding
diapir; a synclinal trough forms above an originally
arch-shaped diapir; and a v-shaped trough forms
above an originally triangular diapir, regardless of
the diapir’s apical angle. The inverted structures
die out upward into synkinematic sediments.
248
Salt Withdrawal/Dissolution Models
(8) If synkinematic deposition accompanies salt
dissolution, most early-formed faults die out
upward, especially those in the central trough. The
active faults may change from normal faults to
reverse faults or vice versa. Normal faults propagate inward, whereas reverse faults propagate outward, the opposite of prekinematic models.
(9) Conceptual models predict that the structural
patterns caused by salt dissolution and salt withdrawal will be broadly similar, although for diapirs
having sides that converge upward, the sequence of
faulting could be reversed; however, the diagnostic
inner contractional zone should be present regardless of diapir shape and whether the diapir was leveled by salt dissolution or withdrawal.
(10) These models suggest that diapir s are
unlikely to diminish much in size, let alone entirely
disappear, by dissolution alone. The apparent
paucity of natural examples of entirely vanished
diapirs supports a tenet of salt tectonics:
Subsidence of diapirs is caused primarily by regional extension or by salt expulsion into neighboring
salt diapirs that are taller, especially if these diapirs
reach the surface (Vendeville and Jackson, 1992b).
(11) Comparative experiments indicate that similar cover structures can form on the borders of
uplifted basement blocks or of sagging diapirs.
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250
Salt Withdrawal/Dissolution Models
ABOUT THE AUTHORS
Hongxing Ge
Hongxing Ge received his B.S.
degree (1985) from Nanjing University, China, M.S. degree (1990)
from Colorado State University, and
Ph.D (1996) from the University of
Texas at Austin. He was employed as
a Postdoctoral Research Fellow at the
Bureau of Economic Geology,
University of Texas at Austin, before
he joined Shell E&P Technology
Company, where he is presently a
structural geologist. His current interests include salt tectonics, tectonic modeling, and petroleum geology.
Martin Jackson
Martin Jackson’s early career
included lunar structures, mineral
exploration, and Precambrian geology. He received his Ph.D. from the
University of Cape Town in 1976
and joined the Bureau of Economic
Geology in 1980, where he directs
the Applied Geodynamics Laboratory funded by a consortium of oil
companies. A recipient of AAPG’s
Sproule Award (with S. J. Seni),
Matson Award, and Dott Award, he lectured in AAPG’s
Structural Geology School, was an AAPG Distinguished
Lecturer, and served six years as associate editor for
AAPG Bulletin and GSA Bulletin.