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UTAH - TIMPANOGOS GROTTO OF THE N.S.S.
CANDLELIGHT CAVE GEOLOGY

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Reprinted from The Utah Caver Volume 10, Number 5, October 1998
Implications of Corrosion Residue from Candlelight Cave
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 & 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 Seven Rivers, Yates, and Tansil Formations. These back-reef rocks are dominantly Dolomite. Hmmmm. That would mean that the upper levels of Lechuguilla Cave are developed in Dolomite bedrock. Didn't the punky bedrock tend to occur mostly in the upper levels of Lechuguilla? Maybe Candlelight and Lechuguilla are more similar than we thought?

Well, just because Lechuguilla Cave has a little Dolomite doesn't mean we can apply the Sanded Dolomite model. The model requires the caveforming waters to be high in Sulfate... Is there any evidence that Lechuguilla waters had high Sulfate? If you have been to Lechuguilla or Carlsbad, and know that Gypsum is made of Calcium Sulfate, then this question is a joke. There was so much Sulfate in the cave water, at one time, that Gypsum was precipitated onto the cave floors in locally thick and extensive beds. There was much more sulfate in the water, in some places anyway, than could ever stay dissolved. Lech has no lack of gypsum deposits.

HOW 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 Candlelight Cave. Not only would it be interesting, exciting, and revealing, but they would get to do a lot of caving and still calf it work. Who knows... We/they might even have to visit Lechuguilla?

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