**The origin of gypsum-rich domes and cones in saline playa
environment - the example from White Sands National Monument, New
Mexico**

Anna Szynkiewicz^1\*^, Lisa M. Pratt^1^, Mihaela Gamoclija^2^, David
Bustos^3^

^1^Department of Geological Sciences, Indiana University, 1001E 10^th^
Street, Bloomington, IN 47408

^2^Geophysical Laboratory, Carnegie Institution of Washington, 5251
Broad Branch Road NW

20015 Washington, DC

^3^White Sands National Monument, P.O. Box 1086, Holloman Air Force
Base, NM 88330

^\*^corresponding author

**Introduction**

Continental playas represent saline mudflats where the capillary fringe
intersects the landscape. They show various facies of evaporatic
deposits and processes of solute transport that include surface runoff,
meteoric and/or hydrothermal groundwaters. Modern continental playas
contribute to salt deposits with thicknesses of more than hundred meters
and areas measured in thousands of squares kilometers (Warren, 2006). In
New Mexico, continental playas have developed in the closed-drainage
basins formed by extentional tectonism of the Rio Grande Rift (Ref. )
where sediment deposition has been widely controlled by several abrupt
changes of climate through the late Pleistocene and Holocene (e.g.,
Anderson et 2002).

The saline Alkali Flat from White Sands National Monument is a part of
large continental playa with thick sequences of evaporatic salts, clays
and silts preserved due to regional subsidence of the Tularosa Basin.
The sediments are sulfate-rich that relates to dissolution of sulfate
minerals (anhydrite, gypsum) by meteoric waters from large reservoir of
Permian rocks surrounding the basin (e.g., LeMone, 1987). The intensive
deflation of these sediments occurred at 7,000 and 4,000 years BP
(Langford, 2003) that led to formation of large gypsum dune field in the
eastern side and exposition of lacustrine sediments with dome and
cone-like morphology abundant in sulfate mineralization on the west side
of the Monument.

Here, we present the results of three field campaigns taken in 2007
combined with isotopic analysis (S, H) on gypsum-rich domes and local
groundwater system together with studying of aerial images. The features
of dome and cone-like morphology were detected in Martian surface for
which many terrestrial analogs have been proposed such as cinder cones
(e.g., Wood, 1979), rootless cones (Frey and Jarosewich, 1982), pingos,
mud volcanos and evaporate deposition around geysers and/or springs
(Farrand et al., 2005). As sulfates appear to be a significant part of
rocks and regolith exposed at or near the surface of Mars, we believe
that gypsum-rich domes from White Sands contribute important information
about groundwater seepage in the surface of sulfate-dominated system
leading to characteristic gypsum precipitation. Complex erosion driven
by hydrological, aeolian and tectonic-related processes leave the dome-
and conic-shaped features characterized by different albedo that is
controlled by reflection of sunlight by gypsum crystals occurring on
domes slopes.

**Geological Background**

White Sands National Monument is located in southern New Mexico (Fig.
1). Geographically belongs to the topographically closed basin of
Tularosa. The exact thickness of basin fill is unknown but may vary from
300 to 2,500 m (pers. comm. Dave Love). The pluvial Lake Otero existed
in the northern part of Tularosa Basin during the late-Pleistocene and
left the evaporative sequences with laminated clays and silts, laminated
gypsiferous marls, limestones, and massive silts containing large gypsum
crystals; today, these sediments are exposed in gullies and incised by
at least 20 playa lakes (e.g., Langford, 2003). The largest of the
playas, Lake Lucero, consists of three sub-basins and is located in the
southern part of White Sands National Monument. The aeolian processes
are driven mainly by hydrological changes in groundwater table (e.g.,
Langford, 2003; Kocurek et al., 2006), but vegetation and surface
cementation primarily acts to limit deflation as well (e.g., Schenk and
Fryberger, 1988).

Gypsum-rich domes are located in the northwestern part of White Sands
National Monument (Fig. 1), the area shared by the Monuments with the
White Sands Missile Range. They appear close to the west edge of Alkali
Flat being a part of old, re-exposed lacustrine clay sequence with
reworked sheetwashed alluvium and aeolian sediments with an evaporatic
crust developed on the surface. The underlying clay remains moist, most
if not all of the time; the thickness of clay bed is unknown, however.
Little is known about the age of lacustrine clay sequence with
gypsum-rich domes. Based on radiodating of mammoth tracks found in that
sequence, Spencer et al. (2006) suggest that tracks were made before
30,000 ^14^C yrs B.P. The overlaying sediments containing ostracodes
valves, plant fragments, and charcoal have yielded four radiocarbon
dates ranging from 22,800 to 19,430 ^14^C yrs B.P. (Spencer et al.,
2006).

**Methods**

*Field sampling*

Three field campaigns were conducted on May, August and October 2007 and
included sampling of selenite crystals from domes for sulfur and
hydrogen isotopic analysis, measurements of strike and dip orientation
of tilted layers of five domes. Samples for sulfur and hydrogen isotope
analysis of dissolved sulfate and water were taken on May 2007 from
surface water of Lake Lucero and 5 piezometers located inside the White
Sand National Monument. Samples of water from the northeastern recharge
zone of Tularosa Basin were taken from Crowford Spring and Three Rivers
Campground.

A spatial occurrence of gypsum-rich domes were presented on aerial
photographs being the digital orthophoto quads (DOQQs) released by Data
Analysis Center (EDAC) at University of New Mexico in 2005.

*Sulfur and hydrogen isotopes*

For sulfur isotope analysis of dissolved sulfate in water, water samples
were filtered, heated to 80-90ºC, acidified with HCl, and supplemented
with 20 ml of 10 % BaCl~2~ solution in order to precipitate BaSO~4~. The
precipitate was washed repeatedly with double-distilled water and
isolated repeatedly by centrifugation as needed to remove chlorine prior
to drying at 110 ºC. Sulfur isotopic compositions were determined using
an EA1110 elemental analyzer coupled to a Finnigan Mat 252 isotope ratio
mass spectrometer via a ConFlo II split interface (Studley et al. 2002).
Isotopic data are reported with respect to VCDT (Vienna Cañon Diablo
Troilite). The calibration was done based on laboratory standards
calibrated relative to IAEA S-1 standard (δ^34^S, -0.30 ‰). Analytical
reproducibility was better than 0.2‰.

Hydrogen isotope composition was analyzed using a Hydrogen Device for
water samples and a TC/EA elemental analser for gypsum crystallization
water, both coupled to a ThermoFinnigan Delta Plus XP mass spectrometer.
Sample of gypsum were freeze-dried overnight to remove contaminants of
water moisture. Isotopic data are reported with respect to VSMOW (Vienna
Standard Mean Ocean Water). The calibration was done based on laboratory
standards calibrated relative to SLAP (δ^2^H= -428.00 ‰) and VSMOW
standards (δ^2^H=0.00 ‰). Analytical reproducibility was better than
+/-2 ‰ for δ^2^H.

**Results and Discussion**

*Domes location and morphological features*

Gypsum-rich domes show a varied size, shape and complex internal
structure. Gypsum precipitation is marked by a variability of selenite
including prismatic, lenticular and irregular crystals. At least three
major morphologic types are recognized in the White Sands National
Monument:

[Type 1]{.underline} - shows a layered internal structure with fine- to
coarse-grained white selenite-rich beds and an amber selenite bed that
is placed at the top of dome (Fig. 2). The amber selenite crystals are
10-20 cm long and preserved in vertical position. Type 1 is usually
within 0.5-2.0 m in height. The white selenite-rich beds are best
exposed in the northern side of each dome and show tilting toward the
south. Both, the white selenite beds and amber selenite from the top
show similar dip under the angle from 20 to 37º.

[Type 2]{.underline} - contains a white selenite breccia imbedded
chaotically with large amber selenite blocks (Fig. 3). Those domes are
the highest up to 3.0 m. Generally, Type 2 represents a disturbed
sequence of Type 1 with poorly exposed white selenite-rich beds inside.
Fragments of blocks with distinctive amber selenite layer are
chaotically placed along the domes slopes. Many domes show a concentric
pattern of bigger selenite growth that surrounding the selenite-rich
beds. Around outer edges and within the rings, the selenite shows
different size and color suggesting many generations of selenite
crystals growth around the domes with no stratigraphic or crystal
orientation. In the case of big domes within Types 1 and 2, more or less
conic shape is preserved that is usually dependent on the position of
large white and amber selenite crystals.

[Type 3]{.underline} - represents flat domes, with concentration of
white coarse-grained selenite in the centre and surrounding big halo,
even up to 8 m in diameter, built of fine-grained selenite, sand and
clay sediments (Fig. 4). The halo parts of flat domes showed darker
color on May and October 2007 in contrast to August 2007 when the
surface temperatures were the highest and all surface of Alkali Flat
dried out.

Type 1 suggests that an interior of gypsum-rich domes contain the
remnants of layered lakebeds containing sulfate-rich evaporatic
deposits. The lakebeds with white selenite and distinctive amber
selenite layer in the top shows the example of gypsum crystals that are
associated with evaporation/desiccation of the lake. These kinds of
selenite are usually secondary in origin and grow in the media after the
surrounding sediments are already deposited. Cody and Cody (1986) showed
that different morphology of gypsum results from organic content within
the clay and the temperature of crystallization. This infers a different
depositional condition for a varied in size and shape selenite observed
in domes.

In our opinion, Type 2 is a disturbed sequence of Type 1 due to advanced
weathering. In most of cases, the amber selenite layer is located in the
top of each dome, therefore, faster weathering of less coarse-grained
gypsum-rich sediments leads to the re-orientation of this layer into
lower parts of the domes. In White Sands, the weathering is mainly
controlled by aeolian activity as the relatively high annual
temperatures keep the area very dry. Nevertheless, a periodical flooding
of the playa and further erosion by surface water might also increase
temporally the weathering rates and enhance the downward moving of more
resistant selenite beds from the top. Additionally, a downward moving
seems to be promoted by tilting of domes sediments (Fig. 5). With time,
a complex weathering driven by aeolian and hydrological processes led to
the chaotic orientation of different selenite beds; that is what makes
the complex internal structure inside the biggest domes of Type 2.

Flat domes of Type 3 distinctively differ in size and way of gypsum
occurrence compared to Types 1 and 2. The changes in color within the
halo (Fig. 6) seem to relate to seasonal variation in moisture content
within the lacustrine clay sequence. Generally, water level was higher
on May and October 2007 in the all area of White Sands National
Monument. After the heavy monsoon season on 2006, the centre of a dune
field and southwestern side of Alkali Flat was flooded from late August
2006 to late June 2007. The highest peak of flooding was observed for
January 2007 (is it true David?), then the water level started to drop
down and on May 2007 only ponds with remnant water were present on the
surface. The halo around flat dome presented on Fig. 6 did not show
surface water on May but the halo had more moisture compared to August
when the relatively dry crust was observed and the darker area within
the halo decreased in size. On October, the dark area of halo was the
smallest but on the other hand the central part of the halo contained
more moisture than it was observed on August; this probably relates to
the summer 2007 abundant in precipitation that increased the water table
in the all Monument. We also noted that on October the central part of
this dome was a little higher suggesting its upward movement. Generally,
evaporites in continantal playas grow by displacive crystal growth in
the capilary zone or in immediately underlying brine-saturated muds.
Ongoing growth of capillary salts above the water table pushes the playa
surface up out of the capillary zone into the vadose zone (e.g., Warren,
2006). In our opinion this process is responsible for observed seasonal
changes in the dome height presented on Fig. 6 and the way of gypsum
occurrence within the Type 3. Further south, we found the flat dome that
shows similar in size halo but with the central part pushed much higher
above the surface (Fig. 4B). The close similarity in size and gypsum
precipitation to the flat dome from Fig. 6 suggest more advanced stage
of dome formation in this place.

The concentric patterns of gypsum growth within the Type 3 and circular
features of moisture appearance suggest that Type 3 might relate to
gypsum crystallization from groundwater discharging on the playa
surface/subsurface. Rosen and Warren (1990) presented that gypsum
precipitation in the playa sediments might result from groundwater
saturated with respect to gypsum recharges around the playa margin. They
named it as a groundwater seepage gypsum for which the negative δ^2^H
values closely refer to the δ^2^H values of local groundwater system not
affected by evaporation.

The circular patterns of gypsum growth are very common for Type 2, as
well. Different size, color and way of selenite crystalization in Type 2
imply many generations of gypsum crystals. δ^2^H of white and amber
selenite crystals taken randomly from several domes showed a consistent
narrow range from -90 to -69 ‰ that is shifted toward negative values
compared to δ^2^H value of groundwater (from -69 to -53 ‰) from
piezomieters located inside the Monument. In contrast, the δ^2^H of
surface water of Lake Lucero showed significantly positive values + 43 ‰
on May inferring the advanced evaporation of the playa lake. In fact, it
was the last stage of lake evaporation and on August (or June/July?
David put here your observation) there was no more surface water present
on the surface of Lake Lucero. The groundwater from the piezometer
located on the west shoreline of Lake Lucero showed lower δ^2^H values,
-27, -28, and -29 ‰ on May, August and October, respectively, inferring
that evaporation might affect strongly the isotopic composition of
shallow groundwater in the subsurface of the playa.

The relatively low values of δ^2^H in all analyzed selenite crystals
confirm that their formation might have taken place in several episodes
of groundwater seepage on the playa surface/subsurface instead of
crystallization from surface water or shallow groundwater undergoing
consequent evaporation on the surface/subsurface of Alkali Flat.

*Sulfur isotopes*

Sulfur isotope measurements on lakebeds exposed in the southern part of
Lake Lucero, large selenite from several domes and sulfate dissolved in
modern groundwater system were carried out. Generally, sulfate from
modern groundwater system in the White Sands area shows the lowest
values of δ^34^S ranging from 10.40 to 11.94 ‰, with mean value of 11.26
‰ on May (Fig. 7). This is consistent with the δ^34^S of dissolved
sulfate observed for the surface water and groundwater of northeastern
recharge zone that show values from 10.55 ‰ to 11.81 ‰ (mean 11.18 ‰) on
May. Water/Acid-soluble sulfate from lakebeds exposed in the southern
gullies of Lake Lucero showed the continuous increase of δ^34^S values
from 11.33 ‰ in the bottom to 14.27 ‰ at the top of profile (Fig. 8).
Selenite from gypsum-rich domes showed δ34S values from 12.12 to 13.79 ‰
(mean 13.07 ‰, n=30). During gypsum precipitation, a small fractionation
of sulfur isotopes takes place, within 0.3 to 0.8 ‰ (Ref. ), and gypsum
is enriched in heavier isotopes relative to sulfate remaining in the
solution (Ref. ). The δ^34^S of the upper part of that profile and
selenite from domes is much more shifted to higher values than it would
be expected from sulfur isotope fractionation. Therefore, it may be
suggested that younger sediments and selenite from domes were formed
from sulfate characterized by higher values of δ^34^S compared to modern
groundwater system in this area.

Bacterial sulfate reduction would have caused the positive excursion of
δ^34^S value in sulfate (Ref. ) but in our opinion this process is/was
negligible in the White Sands area. On May 2007, we detected small
amount of H~2~S in the piezometer located in the western side of Lake
Lucero with negative δ^34^S values (-38.08 ‰) indicating bacterial
sulfate reduction. In spite of this, δ^34^S value of dissolved sulfate
(+10.57 ‰) was in the same range of δ^34^S value noted for groundwater
in the recharge zone showing that sulfate reduction does not influence
on δ^34^S value of sulfate. Generally, lake sediments from southern
outcrops of Lake Lucero contain relatively small amount of organic
carbon (from 0.04 to 0.21, with one clay-rich bed showing 0.37 %); this
probably importantly limits the sulfate reduction rates in White Sands.

The other explanation for observed higher values of δ^34^S is mixing the
sulfate of different origin. The increase of δ^34^S value follows the
general increase of sulfate content observed for the lake sediments. Two
different sequences of Permian evaporites are present in the Rio Grande
rift, the ONE (rember to check the name) and Yeso formations. The ONE
formation is exposed on the surface of the Tularosa Basin. Based on
δ^34^S values of sulfate in waters from the northeastern recharge zone
of the Tularosa Basin, it might be suggested that ONE formation is
characterized by δ^34^S values for sulfate in the range from 10.55 ‰ to
11.81 ‰ (mean 11.18 ‰, n=2). Those values are consistent with δ^34^S
values of sulfate in modern groundwater system of White Sands. The Yeso
formation is older in age and not exposed on the surface of Tularosa
Basin. We analyzed δ^34^S values of sulfate in shallow groundwater
system of Estancia Basin in which Yeso formation is present on the
surface. δ^34^S values of sulfate for Yeso varied from 12.88 to 13.81 ‰
(mean 13.27 ‰, n=3) on May 2007 and well correspond to the range of
δ^34^S values obtained for the youngest sediments occurring in the gully
profile from Lake Lucero outcrops (Fig. 7,8) and selenite from domes of
Type 1 and 2 (from 12.12 to 13.79 ‰; mean 13.07 ‰). After the Last
Maximum Glaciation in North America, the climate had been continuously
changed toward increasing aridity and groundwater recharge had become
dominant in the Tularosa Basin. Therefore, it may be expected that more
water coming from deeper circulation had been entering the basin center
because of limited surface run-off along that time. The general increase
of δ^34^S value in lake sediments may imply that groundwater was in
contact with ONE formation and could have changed temporally sulfate
isotope composition that is characteristic for shallow groundwater and
surface run-off. During a time of sampling for this study, modern
groundwater system in White Sands showed δ^34^S values typical for ONE
formation. This suggests that recharge from shallow groundwater system
and/or surface run-off was dominant during that time.

*Domes Crossing Alignment*

Aerial photograph reveals that domes of type 1 and 2 are aligned along
two cross-cutting trends (Fig. 9). Trend 1 extends NNE-SSW and contains
four single domes. Trend 2 is located NW-SE with seven domes, five of
them are gathered in a close proximity; we called them a Family Group.
The Family Group contains many small domes (0.2-0.3 in height), barely
visible on aerial photograph, that in most of cases represent transition
between Type 1 and 2. Because of small size, Type 3 is not visible on
aerial photographs, and usually those domes occur outside the Trend 1
and 2. However, similar to big domes, they are more or less linearly
spread out across the Alkali Flat and are located in a close proximity
to main crossing alignment observed for Type 1 and 2. Only one flat dome
presented on Fig. 6 is within the main alignment, located about 1 m
south from the first dome of Trend 1 (Fig. 9).

Generally, big domes show significant albedo differences relative to
dark background of Alkali Flat (Fig. 9). It results mainly from high
reflection of sunlight by selenite crystals that are placed around the
domes. These selenites come from erosion of amber selenite layers being
in the top of the dome and/or from selenite that was precipitating
around the domes due to groundwater seepage (Fig. 3). The low albedo of
Alkali Flat results from higher moisture and bigger content of fine
grained material such as clays, fine gypsum precipitates and dust.

The evident alignment of Type 1 and 2 suggests that groundwater seepage
may relate to local fractures conducting more water beneath the Alkali
Flat. The western margin of the Tularosa Basin is, in fact, developed
along the San Andres fault, the main Pleistocene tectonic dislocation in
this area. Therefore, the domes alignment could be related to the local
fractures originated from regional tectonic activity in the Tularosa
Basin. Generally, little is known about development of fault system of
this area because of limited access for the past 50 years relating to
the military activity (Ref. ). In Fig. 1 we presented the localization
of major faults that closely relate to domes appearance on the surface
of Alkali Flats and orientation of strike and dip measured for tilted
layers occurring in five domes. There is no offset in the adjacent lake
beds exposed on gullies of the west edge of Alkali Flats that would
correspond to the two-crossing domes alignment and the orientation of
tilted layers in domes. This indicates that the fractures recorded by
domes alignment were developed before deposition of sediments overlying
the lacustrine clay sequence or the fractures relate to other
synsedimentary processes that are not directly associated with the
extentional tectonism of the Tularosa Basin.

It was shown, that aeolian activity in White Sands is dependent on the
groundwater table and the moisture content in the evaporatic sediments.
Because all domes are located on the surface of Alkali Flat that
underwent an extensive deflation during Holocene, the groundwater
seepage in the places where we see domes today could have prevented from
their erosion during the final Holocen deflation. This might have
resulted from better cementation by large selenite and bigger moisture
content. The Type 3 suggests that groundwater seepage on the surface of
Alkali Flat continuous until today.

**Conclusions**

**Acknowledgments**

**References**

Anderson, R.Y., Allen, B.D., Menking, K.M., 2002. Geomorphic expression
of abrupt climate change in southwestern North America and the glacial
termination. Quat. Res. 57, 371-381.

Cody, R.D., Cody, A.M., 1987. Gypsum nucleation and crystal morphology
in analog saline terrestrial environments. J. Sediment. Petrol. 58,
247-255.

Farrand, W.H., Gaddis, L.R., Keszthelyi, L., 2005. Pitted cones and
domes on Mars: observations in Acidalia Planitia and Cydonia Mensae
using MOC, THEMIS, and TES data. J. Geophys. Res. 110, E05005.

Frey, H.M., Jarosewich, M., 1982. Subkilometer Martian volcanoes:
properties and possible terrestrial analogs. J. Geophys. Res. 87,
9867-9879.

Kocurek, G., Carr, M., Ewing, R., Havholm, K.G., Nagar, Y.C., Singhvi,
A.K., 2006. White Sands Dune Field, New Mexico: Age, dune dynamics and
recent accumulations. Sediment. Geol. ....

Lemone, D.V., 1987. White Sands National Monument. Geological Society of
America Contennial Field Guide -- Rocky Mountain Section, 451-454.

Rosen, M.R., Warren, J.K., 1990. The origin and significance of
groundwater-seepage gypsum from Bristol Dry Lake, California, USA.
Sedimentol. 37, 983-996.

Spencer, G.L., Allen, B.D., Morgan, G.S., Myers, R.G., Love, D.W.,
Bustos, D., 2007. Mammoth footprints from the upper Pleistocene of the
Tularosa Basin, Doña Ana County, New Mexico. In: Cenozoic vertebrate
tracks and traces. Lucas, Spielmann and Lockley (Eds.) New Mexico Museum
of Natural History and Science Bulletein 42, 149-154.

Warren, 2006. Evaporites ...

Wood, C.A., 1979. Monogenetic volcanoes of the terrestrial planets.
Proc. Lunar Planet. Sci. Conf. 10^th^, 2815-2840.

+-----------------------+-----------------------+---------------------+
| Fig. 2. Gypsum-rich   |                       |                     |
| dome showing the      |                       |                     |
| characteristic        |                       |                     |
| morphological         |                       |                     |
| features of Type 1.   |                       |                     |
+-----------------------+-----------------------+---------------------+
| ![](medi              |                       |                     |
| a/image1.jpeg){width= |                       |                     |
| "6.345138888888889in" |                       |                     |
| height="4.2125in"}    |                       |                     |
+-----------------------+-----------------------+---------------------+
|                       |                       |                     |
+-----------------------+-----------------------+---------------------+
|                       |                       |                     |
+-----------------------+-----------------------+---------------------+
| Fig. 3. Gypsum-rich   |                       |                     |
| domes showing the     |                       |                     |
| characteristic        |                       |                     |
| features of Type 2.   |                       |                     |
|                       |                       |                     |
| ![](medi              |                       |                     |
| a/image2.jpeg){width= |                       |                     |
| "5.983333333333333in" |                       |                     |
| height="              |                       |                     |
| 4.007638888888889in"} |                       |                     |
+-----------------------+-----------------------+---------------------+
|                       |                       |                     |
+-----------------------+-----------------------+---------------------+
| ![](medi              |                       |                     |
| a/image3.jpeg){width= |                       |                     |
| "5.982638888888889in" |                       |                     |
| height="3             |                       |                     |
| .9444444444444446in"} |                       |                     |
+-----------------------+-----------------------+---------------------+
|                       |                       |                     |
+-----------------------+-----------------------+---------------------+
| ![](medi              |                       |                     |
| a/image4.jpeg){width= |                       |                     |
| "5.727083333333334in" |                       |                     |
| height="              |                       |                     |
| 3.801388888888889in"} |                       |                     |
+-----------------------+-----------------------+---------------------+
|                       |                       |                     |
+-----------------------+-----------------------+---------------------+
| Fig. 4. The examples  |                       |                     |
| of flat domes within  |                       |                     |
| the Type 3.           |                       |                     |
+-----------------------+-----------------------+---------------------+
| ![](medi              | ![](media             | ![](media/ima       |
| a/image5.jpeg){width= | /image6.jpeg){width=" | ge7.jpeg){width="2. |
| "2.170138888888889in" | 2.1506944444444445in" | 1506944444444445in" |
| height="              | height="1             | height="1.4         |
| 1.448611111111111in"} | .4305555555555556in"} | 361111111111111in"} |
+-----------------------+-----------------------+---------------------+

Fig. 5. The example of advanced erosion of dome of Type 2 showing
downward movement of resistant selenite beds from the top.

![](media/image8.jpeg){width="5.995833333333334in"
height="3.988888888888889in"}

  ---------------------------------------------------- ------------------------------------------------------ ------------------------------------------------------
  Fig. 6. Seasonal variations in moisture content                                                             
  within the halo area of flat dome belonging to Type                                                         
  3.                                                                                                          

  ![](media/image9.jpeg){width="2.209722222222222in"   ![](media/image10.jpeg){width="2.0972222222222223in"   ![](media/image11.jpeg){width="2.0972222222222223in"
  height="1.475in"}                                    height="1.395138888888889in"}                          height="1.395138888888889in"}

  May 2007                                             August 2007                                            October 2007
  ---------------------------------------------------- ------------------------------------------------------ ------------------------------------------------------

Fig. 7. δ^34^S values of gypsum in domes and lakebeds gully outcrops
from Lake Lucero compared to δ^34^S of sulfate in modern groundwater
system.

![](media/image12.wmf)

Fig. 8. δ^34^S value of water/acid soluble sulfate in lakebeds for
section LL-3.

![](media/image13.wmf)

Fig. 9. Aerial photograph showing two-crossing alignment of Type 1 and
2.