Reprinted from The
Implications of
Corrosion Residue from
by
David Herron, Timpanogos Grotto
Getting Started
Although Timpanogos Cave was examined in considerable detail for
my recent masters thesis, and some excellent work has been done on the oddities
of Goshute Cave by Dale Green, most of the other Utah caves have been largely
ignored from a geological point of view. The purpose of this article is to
examine the nature and origin of the corrosion residues of Candlelight and
Blowhole caves, and then discuss their implications. Detailed studies of these
and other Utah caves would likely be very enlightening, but have not yet been
done.
If you are looking for numerous large multimile cave systems,
then Utah is not the best place to look. If you are looking for unusual caves,
however, with an exotic geologic history, then Utah is definitely a good place
to start. The caves of the Guadalupe Mountains or New Mexico (Lechuguilla and
Carlsbad Caverns for example) are well known exotic cave systems, and are
considerably larger than any known caves in Utah. Even so, these caves are
really not much more exotic than many of our favorite caves right here in Utah.
Of the many Utah caves, Candlelight and Blowhole are not only some of our
favorites, they are also some of the most unusual. Other local geologically
interesting caves include Green-Eyed Monster, Gandy Mountain, Nutty
Putty, Hush-Hush, and Goshute Caves.
Alteration at Candlelight
Cave
Few people have been to Candlelight Cave
without noticing the punky and crumbly corroded bedrock. Although this material
is most obvious in Candlelight Cave, similar punky bedrock occurs on the walls
of many of the other nearby caves. I collected several samples of material from
Candlelight Cave years ago, while debating whether to study Timpanogos or
Candlelight Cave for my masters thesis (Perhaps I made the wrong choice...? But
its too late now). Two of the samples I collected were of dissolution residue
from the walls, while another sample was from a large deposit of fine-grained
water-laid sediments.
I took these samples to BYU, where I conducted x-ray
diffraction and preliminary chemical analysis on them. I expected the samples
to be largely silty clays with modest amounts of various metal oxides for
coloring. To my surprise, however, x-ray diffraction showed these samples
to be composed dominantly of crystalline Calcite [ CaC03 ]. This was surprising
because Calcite is the dominant ingredient in Limestone. If Candlelight Cave
was formed by dissolving away the surrounding rock, which was made almost
entirely of Calcite, then why would some of the Calcite be left behind as a
residue on the walls?
I returned to Candlelight to examine the residues and bedrock
alteration more carefully. Compaction testing and careful examination of the
fluffy metal-rich residues showed that nearly all of the original bedrock
had been dissolved away. Similar examination and compaction of the more typical
punky bedrock residues, however, showed that about half of the original rock
had been removed. In one place, this punky alteration was observed extending at
least 8 inches into the surrounding cave wall.
During a recent trip to Blowhole Cave, with Fred Luiszer, Dale
Green, and AI Hinman (see Fred's article elsewhere in this Utah Caver), I
collected a sample of dissolution residue for Fred to analyze. He subsequently
reported that the sample was composed dominantly of Calcite, with only traces
of other things. Although the punky altered bedrock is not as well developed at
Blowhole Cave as it is at Candlelight Cave, the chemistry, texture, and mode of
occurrence appear to be essentially the same.
Mysteries and Answers
It seems reasonable enough that some Calcite grains in the
bedrock might be slightly more or slightly less soluble than the other Calcite
grains. This process allows some grains to dissolve a little faster or a little
slower than others, and often enhances the visibility and relief of fossils on
cave walls. To slightly prefer one grain over another is reasonable and
relatively common. What apparently happened at Candlelight is something else
altogether. To create the punky bedrock at Candlelight Cave, the dissolving
waters would have to pass through up to 8 inches (or more?) of highly porous
bedrock, without any significant dissolution, only to attack and remove
approximately 50 percent of the solid bedrock behind the porous material. This
explanation seemed absurd at the time, but what else could it be? The textures
in the punky rock clearly show that it was once solid bedrock, and continuous
with the surrounding bedrock, but with about half of the material now removed.
The answer to the apparent mystery of preferred dissolution was
the discovery of a bad assumption. The bedrock surrounding Candlelight Cave is
not pure Limestone, but Dolomite or mixed Dolomite and Limestone. For caving
purposes, Dolomite is essentially the same as Limestone, but the chemistry is
somewhat different. Limestone rock is composed mostly of the mineral Calcite,
which is Calcium Carbonate, while Dolomite rock is composed mostly of the
mineral Dolomite, a Calcium Magnesium Carbonate. Dolomite is essentially
Calcite with half of the Calcium being Magnesium instead. The mystery is now
easily explained. If the original rock was Dolomite, which has half Calcium and
half Magnesium, and the altered punky rock is Calcite, with all Calcium and
essentially no Magnesium, then only the Magnesium component of the original
bedrock was removed. Where the water was more corrosive, all of the rock was
dissolved to create a cave passage. Where the water was less corrosive, the
Magnesium component was dissolved and the Calcium component left behind,
creating punky bedrock along the cave walls.
While this process seems to explain the origin of the punky
bedrock, it creates yet another mystery. It is widely known that Calcium
Carbonate (Calcite) is more soluble than Calcium Magnesium Carbonate
(Dolomite), and much more soluble than Magnesium Carbonate (Magnesite). If the
Calcium component is more soluble than either the Magnesium or mixed Calcium-Magnesium
components, why was it left behind as insoluble residue? Why didn't the Calcium
dissolve first, and leave the less soluble Magnesium behind?
Answers Lead To Questions
The solution to this next apparent mystery is again a bad
assumption. Obviously, at the time the cave was dissolved, the Magnesium
component of the bedrock was more soluble than the Calcium component. Every
good geologist and chemist knows that Calcite is more soluble than Dolomite or
Magnesite in relatively fresh water... but who said anything about relatively
fresh water? Since the dissolution of Magnesium was clearly preferred over the
dissolution of Calcium, it seems that the caveforming waters were not
relatively fresh. But under what circumstances would the dissolution of
Magnesium be so strongly preferred?
Candlelight Cave was almost certainly dissolved out by some kind
of non-fresh groundwater... But what kind? Common components of
groundwater (both fresh and otherwise) include Sodium Chloride (Salt), Calcium
and Magnesium Sulfates (Gypsum and Epsom Salt), and Calcium and Magnesium
Carbonates (Calcite and Dolomite). Carbonate-rich water initially seems
likely, since the surrounding area is dominated by carbonate rocks. The
groundwater cannot have been dominantly carbonate-rich, however, because
Calcium Carbonates are more soluble than Magnesium Carbonates. Salty (Sodium
Chloride-Rich) water might seem like a good candidate, since the Great
Salt Lake is very salty, and relatively nearby. Unfortunately, both Magnesium
Chloride and Calcium Chloride are quite soluble. Unless the water was already
loaded with excessive amounts of Calcium, but little or no Magnesium, the
difference in solubility would probably have little effect on cave dissolution.
The third possibility, Sulfate-rich water, is by far the best choice.
Magnesium Sulfate (Epsom Salt) is relatively soluble in water, while Calcium
Sulfate (Gypsum) is much less soluble. Because of this, water with a high
Sulfate content would tend to prefer the dissolution of Magnesium over Calcium.
Of the common types of groundwater, sulfate-rich or sulfate-saturated
groundwater could produce the wallrock alteration and dissolution residues seen
at Candlelight, Blowhole, and other similar Utah caves.
Our mystery is now apparently solved. The corrosion residues and
wallrock alteration at Candlelight Cave were probably produced by the
interaction of Sulfate-rich groundwater dissolving a cave in either
Dolomite or dolomitic bedrock. Under these conditions, dissolution of the
Magnesium component was strongly preferred over the dissolution of Calcium. The
removal could have taken place in either of two modes... (1) Discrete grains of
Dolomite could have been preferentially removed from in-between discrete
grains of Calcite. Or (2) All of the grains, either Dolomite and dolomitic
Calcite, could have dissolved in the groundwater, followed by immediate re-precipitation
of the Calcium and Carbonate as Calcite. Because of the relatively uniform
nature of the punky bedrock and its individual grains, I suspect that Dolomite
grains were dissolved and then re-precipitated as Calcite. Without a more
detailed study, we do not know how the Magnesium removal actually took place.
On and On and On
By now you are probably wondering... If the original mystery is
now solved, why does this article keep going? Unfortunately, even though the
initial questions have been answered, the implications have only begun. For example,
where did the Sulfate-rich water come from? By finding out how the
corrosion residues formed, not only do we learn a lot about how Candlelight
Cave was created, but we can use this information to learn more about other
caves such as Lechuguilla. Did the same thing happen in Lechuguilla Cave, where
Gypsum is abundant, and where similarlooking corrosion residues and punky
bedrock are widespread? Figuring out how the corrosion residues formed at
Candlelight simply opens up more questions.
Lets start with the source of the Sulfate in the water. There
are two possible sources for the Sulfate-rich water, both of which are
reasonably likely. The first possible sulfate source is from deeply buried
gypsum beds, projected to occur in younger rocks beneath the local thrust
fault. If groundwater in the Candlelight area was rising from considerable
depths, it might pass through these gypsum beds and pick up considerable
Sulfate and well as Calcium. Although the gypsum beds are fairly deep, the high
regional heat flow could certainly drive water from such depths by convection.
Such waters would prefer Magnesium dissolution not only because of the high
sulfate content, but also due to the very high Calcium to Magnesium ratio in
the water from dissolving gypsum. While we are thinking about gypsum beds,
isn't Lechuguilla Cave pretty close to an awful lot of gypsum beds? In fact,
doesn't Lechuguilla Cave contain an awful lot of gypsum? Hmmmmm. It makes you
think there might have been a lot of sulfate in the water when Lechuguilla was
forming.
Returning to Candlelight Cave, the second possible sulfate
source is oxidized ore fluids from or related to the nearby Tintic Mining
District. This mining district produced huge quantities of high-grade
metal-sulfide ores. The ore fluids had an awful lot of sulfide in them as
well. If combined with oxygen-rich surface water, the Sulfide in these
fluids would oxidize to Sulfate, producing Sulfuric Acid. This acid would then
rapidly and easily eat caves into the surrounding carbonate rock. Oxidation of
sulfide-rich water from the Tintic Mining District could then explain not
only the sulfate-rich groundwater, leading to the punky bedrock and
dissolution residues, but could also explain the origin of the cave itself. So
which is the actual source of the sulfate in question? Is it from dissolution
of buried gypsum beds, or oxidation of sulfide-rich fluids related to the
Tintic Mining District? Perhaps the correct answer is both. There was more than
the typical amount of sulfur in igneous rocks of the Tintic Mountains, which is
directly related to the type of ore deposits created. Some geologists speculate
that this extra sulfur was added when the magmas encountered and consumed large
amounts of gypsum from deeply buried evaporite beds.
Curious-er and
Curious-er
If sulfate-rich waters, similar to those of the Tintic
Mining District, are responsible for the alteration we see at Candlelight Cave,
is there additional evidence for such a relationship? Yes. Otherwise I would
not have asked the question. In the Tintic mines, the miners often knew when
they were getting close to a big orebody because of the alteration halos that
surrounded the ore. Immediately adjacent to the large ore bodies were obvious
areas of iron staining, and stringers of low grade ore. Extending much farther
away, however, was often a much larger alteration halo. This alteration
sometimes extended for hundreds of feet from the larger orebodies. It consisted
of what appeared to be solid dolomite, with veins and fossils and even the
typical dolomite color, except that instead of being hard, the rock was soft,
porous, and crumbly. It occurred to some degree almost anywhere that the large
ores cut through Dolomite. The old-time miners called this alteration
halo "Sanded Dolomite". The Sanded Dolomite was so soft it could be
excavated without explosives, and sometimes even with bare hands. Does this
sound the least bit familiar? If you have been to Candlelight Cave, you have
seen sanded dolomite on the cave walls. People who studied the geology of the
Tintic Mines also reported them to be full of caves. Several of the smaller ore
deposits were even described as being caves, which were first dissolved and
then later back-filled with layered oxide ores. Although the mine shaft
which hit Candlelight Cave did not encounter valuable ores, there are a few
small low-grade mineral deposits in the cave. The miners who sunk
Candlelight's mine shaft didn't find any ore, but they probably didn't sink
that shaft in a random spot either. They encountered significant alteration, in
the form of a large cave and lots of sanded dolomite. They simply failed to
discover any significant ore. If Candlelight Cave really is a distant part of
the ores and alteration of the Tintic Mining District, perhaps there are
similar caves to be found elsewhere?
Been to Lechuguilla Cave?
Are there other implications or ideas to pursue? What about the
punky bedrock and dissolution residues in various areas of Lechuguilla Cave?
These have been argued many times as features and residue from a process
referred to as Condensation Corrosion. According to the popular (but incorrect)
condensation corrosion theory, humid cave air circulates and rises from lower
warmer cave levels to higher cooler cave levels. No problems here. As the warm
air rises to cooler areas of the cave, it cools and allows water to condense on
the cave walls. Fine by me, but Dale Green disagrees with this part. This
condensation water is typically somewhat acidic and therefore attacks the
bedrock it condenses on. No argument here. This acidic water then wicks its way
down through porous punky bedrock to attack and dissolve the solid bedrock
behind it. Beep. Eeeent. Wrong. No way Jose. After time, this process
supposedly develops a punky sandy surface coating of altered bedrock over the
underlying unaltered bedrock, which continues to grow deeper into the wall with
time. This theory has the same problem as the original mystery at Candlelight.
The process simply cannot work that way. The problem is that the water would
attack the rock (first and most) where the water first condensed. The water
would condense on the wall surface, not somewhere underneath. The porous fluffy
rock on the wall surface is dominantly Calcite, and being porous has a much
greater surface area than the solid bedrock behind it. There is no reason for
the acidic water to pass through the fluffy porous calcite, only to attack the
similar but solid calcite farther behind. Unlike the Sulfate-rich waters
at Candlelight, condensation waters would essentially be fresh water. A high
Carbon Dioxide content in the cave air would allow the condensed water to be
rather acidic. Such water would dissolve the first carbonate rock it
encountered, not pass through porous carbonates to dissolve other carbonates
elsewhere. The currently popular model for punky bedrock at Lechuguilla is
clearly wrong, but maybe what we see at Candlelight Cave can help us to better
understand Lechuguilla.
Not At Lechuguilla?
Although the alteration at Candlelight looks very much like the
alteration at Lechuguilla, there is a fundamental problem with applying the
same process. Unlike Candlelight Cave, the punky alteration and corrosion
residues of Lechuguilla seem to occur predominantly in the upper levels of the
cave. Why would the Sanded Dolomite process only affect the upper parts of the
cave? It doesn't make any sense. Even worse, many cave geologists can tell you,
with complete certainty, that Lechuguilla Cave is developed in the world-famous
Capitan Reef. This ancient reef, they tell you, is composed entirely of
Limestone, not Dolomite. Without Dolomite, the Sanded Dolomite model cannot
work. Are we stuck? Did the Lechuguilla alteration form by yet another
mysterious process? Maybe yes and maybe no, but either way, there is another
bad assumption at work here. The Capitan Reef is indeed composed of relatively
pure limestone, but not all of Lechuguilla
Cave is developed within the reef rocks. The lower levels of the
cave (the big boreholes) are developed in reef rock, while the upper levels are
often developed in other back-reef carbonates. Above the massive reef
limestones are thick to thin bedded back-reef carbonates of the
Well, just because
Have We Solved Everything?
Well? Have we solved all of the mysteries at Candlelight Cave?
Certainly not. In fact, we still don't know much of anything for certain about
Candlelight, except that it's not a typical cave. In this article, I have only
presented a long-winded arm-waving and rambling theory for some of
the alteration seen at Candlelight. Whether my theory is actually correct
remains to be seen. I have apparently implied that what happened at Candlelight
may also have happened at Lechuguilla. Keep in mind that this is also just
theory and certainly open to debate. A lot of strange things happened at
Lechuguilla, and I am suggesting only that there are several striking
similarities. Lechuguilla Cave has more weird stuff, but it's also about 80
times longer (so its hardly a fair comparison). Maybe we should go and
investigate these similarities firsthand sometime? I bet the park service would
let us, if I write up a better version of this article first.
What about other weird caves in the United States? What about
Wind Cave and Jewel Cave... Aren't they fairly similar to Candlelight and
Lechuguilla? Is it just me, or does there seem to be a correlation between
complex maze caves, sparcoated walls, corroded and punky bedrock, gypsum
deposits, Iron and Manganese sediments and formations, and caves that are dry
and did not contain streams? How do these other exotic caves compare? Clearly
there is a lot of work to be done before we can prove one way or the other what
really happened at Candlelight, Blowhole, and the other exotic caves. Until we
really know, I'll continue to speculate... What we need now is another geology
student to do their masters thesis on Candlelight Cave.
Summary
In summary, punky bedrock alteration at Candlelight and Blowhole
Caves demonstrate dissolution of the caves by distinctly non-fresh
groundwater. This groundwater was most likely heavily loaded with sulfate ions,
and was likely at or near gypsum saturation. This allowed Magnesium to be
preferentially dissolved from Dolomite bedrock on the cave walls. Sulfate in
the water could have been derived either from dissolution of deeply buried
gypsum beds, or from oxidation of sulfide-rich waters related or similar
to the mineralizing fluids of the Tintic Mining District. Similar dolomite
alteration was widely observed in many mines of the Tintic Mining District.
Apparently similar alteration is also observed in other more famous exotic
caves such as Lechuguilla, Wind, and Jewel Caves. At Lechuguilla Cave, similar
conditions to those predicted for Candlelight are known to have existed within
the cave during dissolution (Sulfate-saturated groundwater, Dolomite
bedrock locally).
We need another geology student, who could do their masters
thesis on
Return to the Timpanogos Grotto Website
Copyright © 1998 The
National Speleological
Society
All rights
reserved
Copyright © 2002 Timpanogos Grotto
Maintained by Jon
Jasper
- last updated