GUADALUPE MOUNTAINCAVES: A STUDY OF

SULFURIC ACIDSPELEOGENESISBY ANDREW DIEFFENBACH

WILL MALARKEY

C A L I F O R N I A U N I V E R S I T Y O F P E N N S Y L V A N I AD E P A R T M E N T O F E A R T H S C I E N C E S

[ E A S 3 4 3 - G E O M O R P H O L O G Y ]

.......

Introduction

The Guadalupe Mountains region of southeastern New Mexico

and west Texas is home to over 300 known caves including the

world-famous Lechuguilla Cave and Carlsbad Caverns. The

caves of the Guadalupe Mountains are unique in that the

formation of many of the passages that underlie this region

is not so easily explained through the (relatively) simple

process of carbonic acid dissolution (Hill, 2000). Carbonic

acid is formed as naturally acidic rainwater passes through

the atmosphere and soil, mixing with carbon dioxide to form

a weak acid solution. Limestone, consisting primarily of

calcite (CaCO3), is easily dissolved in the presence of

carbonic acid, forming water-soluble calcium bicarbonate. As

acidic water passes through the slowly-widening fractures

and crevices, the limestone is subjected to further chemical

(and eventually mechanical) weathering, resulting in cave

formation given stable conditions and an adequate breadth of

time. This is the widely-accepted speleogenetic mechanism

responsible for the vast majority of cave formation

worldwide.

.......With carbonic acid karstification, one expects to see an

incontrovertible relationship between cave morphology and

hydrology, but in Guadalupe caves this is not always the

case. Here, many cave passageways have orientations

apparently unrelated to groundwater flow or the above

topography (Hill, 2000). Unlike most other systems, the

caverns, for the most part, are dry, with little actively

flowing water. Passages exist that are uncharacteristically

large, sometimes terminating abruptly without perceivable

reason (Hill, 2000). Additionally, vast deposits of gypsum

have been found, taking the form of both decorative

speleothems (secondary mineral deposits in cave systems) and

massive deposits; prime examples of these include drip-

drilled, tabular gypsum deposits in Carlsbad's famous "Big

Room” (Hill, 2000) and the massive gypsum “glaciers” found

in the depths of Lechuguilla Cave, respectively (Davis,

2000). All of this evidence suggests that carbonic acid

dissolution is not solely responsible for the formation of

these caverns, but that sulfuric acid speleogenesis must

have played a crucial role in their development (Hill,

2000).

Geologic Setting

The Guadalupe

Mountains are situated in

.......the northwestern corner of the Delaware Basin, a component

basin of the well-known Permian Basin of western Texas and

southeastern New Mexico, the most productive petroleum basin

in the United States (Wright, 2011). The caves of this

region have formed in the famous Capitan Reef Complex, a

horseshoe-shaped, massive, fossiliferous limestone and

dolostone reef (see Fig. 1) delineating the perimeter of the

Delaware Basin (Palmer, 2000). At over 200 meters thick, 8

km wide, and approximately 650 km in length, and with

uplift-derived exposures in the Guadalupe Mountains to the

northwest, the Apache Mountains to the southwest, and the

Glass Mountains to the southeast, the Capitan Formation is

an accessible, yet spectacularly well-preserved example of

an ancient, fossilized reef (Hill, 2000).

Geologic History

Although the Permian Basin exists as the marker for the

Permian Period of geologic time (the last period of the

Paleozoic Era, stretching from 299 to 251 mya), the basin

has a history rooted far deeper than its namesake would

suggest, owing its existence to tectonic events stemming as

far back as the late Precambrian (Adams & Keller, 1996). Its

almost impalpably long, complex history is well beyond the

scope of this investigation, and thus won't be covered in

.......detail. Of greater concern are the events tied directly to

the development of the Delaware Basin, one of three

intercratonic, depositional and structural basins (the

others being the Midland Basin to the northeast, and the

Marfa Basin to the southwest) comprising the greater Permian

Basin (Palmer, 2000).

During the early Paleozoic -- specifically from the

Cambrian through Mississippian -- the area that is now the

Permian Basin was inundated, existing as a broad, marine

basin in which thick sequences of clastics and carbonates

were deposited (Hill, J., 1972). This unified, ancestral

basin, known as the Tobosa Basin, consisted mainly of

organic-, clay-rich shale and shelf-type carbonates (Hill,

J., 1972). The western border of this basin was defined by

the Diablo Arch, while the eastern side was bordered by the

Texas Arch. Later, during the Carboniferous Period, the

Hercynian Orogeny occurred as Gondwana collided with the

North American Craton, giving rise to the Ouachita-Marathon

fold-thrust belt as what are now South America and northwest

Africa glommed onto the now-Gulf Coast Region of the United

States (Hill, J., 1972). This collision caused massive,

wide-scale deformation of the Tobosa Basin, resulting in

forced-folds over top of steep-angle, normal faults occuring

in the Paleozoic basement level (Rodgers, 2006). Basin

differentiation resulted from folding and faulting, dividing

the Tobosa Basin into two rapidly-subsiding lobes: the

.......western Delaware Basin and the eastern Midland Basin (Hills,

J., 1972). Separating these lobes was a geographical high

known as the Central Basin Platform.

The Delaware Basin continued subsiding until the

Guadalupian (255-251 mya), allowing for relatively rapid

deposition of interbedded carbonates, siliclastics, and

evaporites about the margins of the basin, forming the

Capitan Reef Complex (Hills, J., 1972). Meanwhile, deeper,

basinal deposition also occurred as vast amounts of

siliclastics were deposited, resulting in the Bell Canyon

Formation (Hill, 2000.) Contemporaneously-deposited back-

reef formations include the Seven Rivers, Yates, and Tansill

(Hill, 2000).

The original model of this area worked around the theory

that this reef encompassed the entire basin, effectively

dividing the deep-marine center from the shallow, back-reef

lagoon, and that the basin was supplied with sea water via

the Hovey Channel to the south (Hill, 2000). It is now

suggested that a west-to-east flowing channel, known as the

Diablo Channel, supplied sea water to the basin (Hill,

2000). This new theory would seem to explain the absence of

the Capitan Reef from the outcrop at El Capitan (located at

the southern end of the Guadalupe Mountains) to the

northwestern Apache Mountains (Hill, 2000). This would also

account for observed disparities in deposition and

dolomitization; the Capitan Formation in the Guadalupe

.......Mountains is more expansive and has far greater

dolomitization than the Glass Mountains in the South, which,

by this model, would have been located further from the sea

water source (Hill, 2000). Were this the case, waters in the

Guadalupe region could have been fully-saturated with

respect to calcite, explaining the limited dolomitization of

the Capitan Formation in this area (Einsele, 1992).

Conversely, in the Glass Mountains region, waters could have

become less calcite-saturated, yet still maintained

saturation levels with respect to dolomite, allowing for

greater dolomitization (Einsele, 1992). Regardless of the

source, this dolomite anisotropy has left the Guadalupe

Mountains graded; the lower Capitan is slightly dolomitized,

while the upper Capitan is almost pure limestone (Hill,

1996). It is in these less dolomitized, far more soluble

reaches of the formation that the vast majority of the caves

began to develop as fissures in the Ochoan, occurring

between 251-250 mya (Hill, 2000).

In the late Permian, there is evidence that the sea water

channel was cut off (Hill, 2000), severely limiting inflow

and likely resulting in the extensive evaporite deposition

(Einsele, 1992) responsible for the anhydrite-rich Rustler

and Castile Formations. Further dolomitization occurred as a

result of the closing off of this channel, limiting the

input of sea water, and eventually allowing for the

transition towards a freshwater-dominated basin (Hill,

.......2000). The Guadalupe Mountains first became exposed as the

western-flank of the Delaware basin tilted eastward,

augmenting an existent joint and fracture system that

occurred as a result of differential sedimentation in the

fore-reef and back-reef regions (Hill, 2000). For the time,

the region was mostly stable, creating favorable conditions

for diagenesis. Due to limestone dissolution by meteoric

water in the phreatic regions of the telogenetic zone,

previously unconnected joint and fracture systems began

meshing, resulting in a vast, spongiform network (Hill,

2000). This region remained mostly tectonically-stable until

the Late Cretaceous when the Laramide Orogeny began

(Lundberg et al., 2000).

During the Laramide Orogeny, the Farallon Plate subducted

beneath North America at shallow angle. This created fold-

thrust belts as far as 1,500 km into the interior of the

continent and uplifted the western reaches of the greater

Permian Basin to present-day elevations. Although it has not

yet been proven, it is theorized that this period of uplift

exacerbated the

dissolution of the

spongiform networks by

exposing the area to

more groundwater

interaction,

effectively

.......accelerating the then-slowing process (Hill, 2000).

In the early- to middle-Oligocene, regional volcanics

caused widespread heating, creating convective currents

through the spongiform networks and pushing the organic-

rich, basinal sediments into the maturation window (Hill,

2000). After thermal maturation, these hydrocarbons

interacted with Ochoan evaporites in the Castile formation,

producing hydrogen sulfide that would then migrate upwards

into structural and sedimentological traps (see Fig. 2

above); these zones of accumulation consisted of anticlinal

reef structures and the salt- and anhydrite-rich Yates

formation, respectively (Hill, 2000).

During the Miocene, the geothermal gradient in the

Guadalupe Mountains region was pushed to levels of

45-50°C/km by continued volcanism (Barker & Pawlewicz,

1987). Comparatively, the geothermal gradient is now, on

average, 25°C/km. The resulting convective currents helped

to circulate heated water (which naturally had a greater

dissolving capacity), allowing for further calcite

dissolution in the presence of a hot water bath (Hill,

2000). This process created many of the current passageways

that are considered typical in carbonic acid dissolution.

Continued uplift resulted in a lowering of the water table,

causing some of the speleothemic deposition visible in the

caves today (Hill, 2000). At this point, the stage was set

.......for the final, most unique phase of Guadalupe

karstification: sulfuric acid speleogenesis.

Mechanism and Key Evidence

Sulfuric acid karstification is a very recent idea in

speleogenesis, however, recent research has helped to

explain the mysteries of caves in the Guadalupe Mountains

region. Palmer (1991) demonstrated that while caves usually

form from “epigene” (surficial or near surficial) processes

involving infiltrating CO2-rich waters, they can also form

from “hypogene” (deep earth) processes as a result of deep-

sourced H2S-rich water. This has been influential in all

further research, opening up new avenues to be considered.

Hill (1996) went on to define the morphological history of

Guadalupe caves, stating that the first three episodes dealt

with their initial development leading up to the fourth and

final phase: sulfuric acid redox.(Jagnow, 1977) first

postulated that sulfuric acid could be the driving mechanism

for the Guadalupe Mountain caves (especially Carlsbad

Caverns and Lechuguilla Cave). He suggested pyrite leeching

could be the source of the acid. Further studies (Davis,

1979) have discounted this idea based on evidence from

isotopic examinations of the gypsum blocks throughout the

cave system and instead suggested a hydrocarbon source for

H2S. Later sulfur isotope testing performed on gypsum blocks

.......in the Big Room of Carlsbad Caverns confirm a link between

hydrocarbons and the deposits (Hill, 2000). The hydrocarbons

in question were primarily deep oil deposits in the Ramsey

member of the Bell Canyon Formation deeper in the basin

(Hill, 2000). As the Guadalupe Mountains uplifted in the

Miocene to late Tertiary due to the effects of the Laramide

Orogeny (Lundberg et al., 2000), this H2S migrated from the

basin to the reef using fracture zones formed by Basin and

Range normal faulting and graben formation (Hill, 2000). As

it came into contact with oxygenated groundwater at the

water table, sufulric acid (H2SO4) formed and began to carve

the paleokarst formed in the preceding stages of karst

development (Hill, 1996).

H2S+2O2=HSO4−¿+H+¿(1)¿ ¿

Equation 1 represents the mixing of hydrogen sulfide (H2S)

with oxygen provided from meteoric groundwater (Hill, 1995).

This then causes the dissolution of the cave passages and

the deposition of the gypsum blocks according to equation 2

below (Hill).

HSO4−¿+H+¿+CaCO3+H2O=Ca

2+¿+SO42−¿+3 H2O+C O2( 2)¿ ¿

¿ ¿

Because the H2SO4 is much more acidic than the

(comparatively) weaker H2CO3 that had been attacking the

.......carbonate reef complex prior, the dissolution progressed at

much higher rate (10-30x) than the earlier stages,

especially with the increased dissolving capacity due to

high thermal levels (Palmer, 2000). This explains why the

cave networks are at seemingly random orientations and can

have outlyingly large passageways. Even a minor change in

water table levels for a relatively short period of time

could drastically affect the direction of dissolution, which

in turn would have an effect that would be felt for all

future stages of dissolution. These hypotheses are further

supported by the abrupt terminations of the cave passages at

Carlsbad. As the sulfuric acid dissolved the limestone, it

gradually weakened until it was neutralized beyond its

ability to dissolve additional units. At such points, the

caves terminate, which despite stumping scientists and

visitors for many years, has proven to be a decisive bit of

evidence used to formulate the current theory of sulfuric

acid karstification (Hill, 2000). A final point of note

about the sulfur redox system is that throughout the 12Mya-

8kya period, sulfur ions that were not deposited within the

cave system were transported out of the reef complex and

distributed throughout the regional setting (Szynkiewicz et

al., 2012.). Isotopically similar trace sulfur has been

found up to 180 km away in drill core samples (Szynkiewicz

et al.).

.......G ypsum Features

.......Caves of the Guadalupe

Mountains region are famous for

many things: the enormous rooms

that open up in front of you,

seemingly at random, the cave

passages that terminate

abruptly, and perhaps most

intriguing, the massive deposits of gypsum that litter the

caverns and passages. As referenced above, these blocks of

gypsum were particularly troublesome for researchers to

explain. They can be up to 10m thick, and rough estimates

indicate there are thousands of tons of gypsum desposits

within Lechuguilla Cave alone (Davis 2000). It was these

blocks of gypsum deposited during the dissolution of the

limestone materials that were isotopically determined to be

the product of H2S sourced from hydrocarbon deposits in the

Bell Canyon Formation (Hill, 2000). The gypsum spelothems

are highly distinctive within the Guadalupe cave systems,

often taking the form of highly decorative features such as

drip-drilled gypsum sheets

(see Fig. 4) on the cave wall surfaces. These are sometimes

.......as much as 3-6 inches thick, even after exposure to air and

chemical weathering. In addition to the sheets, large

“chandelier” features are also found within Lechuguilla

Caves (see Fig. 3). These are truly massive, with the

largest being some 18m in length with branching sub-members

radiating out up to a meter (Davis). Visitors to the cave

often walk away feeling truly intimidated by these

structures, and they remain their most vivid recollection

years after visiting. A final example of the more unique

features present within the Guadalupe cave systems are the

zenithal tube holes found within Carlsbad Caverns,

particularly the Hall of the White

Giant. These are very unique cigar

shaped vertical tubes in the

ceiling formed from sulfuric acid

dissolution. The dissolution is

localized and preferentially

vertical because the exsolved H2S

gas became trapped within micro-

depressions in the ceiling

(Calaforra et al, 2011). These then

swirl and expand under pressure, at

least some of which can be attributed to the convective

cycle within the volcanically heated fluid bath (Barker and

Pawlewicz, 1987). Gypsum is often found within these tubes,

both as a powdery residue and as sheeting on the inner

.......surfaces. While small in diameter (most are a few

centimeters or less), they can grow to be several decimeters

in vertical length (Calaforra et al., 2011.). They occur in

both clusters and as solitary forms adorning portions of the

Carlsbad Caverns system. These and other features exemplify

the unique and complex geomorphological history of the basin

and its subsequent reef complex.

Conclusion

The uniqueness of certain features in the Guadalupe

Mountain Caves -- and the Capitan Reef Complex itself, for

that matter – is a prime example of how what seem at first

to be small-scale, scientific problems can eventually cause

a large-scale rift in professional opinion throughout the

scientific community. In geology, when a problem presents

itself, there is an answer to that call independent of

whether or not there are any immediate, monetary gains to be

had. In this case, an abstruse, geomorphologic history not

so easily given up by the rocks was intensely investigated,

and, through good science and multi-disciplinary

collaboration, was eventually accounted for. How easy it

would have been to simply state that the cave systems of

Guadalupe Mountains were a product of carbonic acid

dissolution and nothing else, yet that was not the route

that was chosen. And as a result, concomitant problems have

.......been created, giving rise to many questions that will

undoubtedly be fielded by the next generation of

geoscientists if for no other reason than uncovering the

truth.

Works Cited

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Davis, Donald G. "Extraordinary Features of Lechuguilla Cave, Guadalupe Mountains, New Mexico." Journal of Cave and Karst Studies. Vol. 62(2). (2000): 147-157.

Palmer, Arthur N. "Hydrothermal Interpretation of Cave Patterns in the Guadalupe Mountains, New Mexico." Journal of Cave and Karst Studies. Vol. 62(2). (2000): 91-108.

Szynkiewicz, Anna, B. Talon Newton, Timmons Stacy S., and David M. Borrok. "The Sources and Budget for Dissolved Sulfate in a Fractured Carbonate Aquifer, Southern Sacremento Mountains, New Mexico, USA." Applied Geochemistry. Vol. 27(8). (2012): 1451-1462.

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.......Thesis. (1977): 197.

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