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Chapter 3.2 Speleothems
3. ABIOGENIC STROMATOLITE-LIKE STRUCTURES
Chapter 3.2 SPELEOTHEMS John Thrailkill
INTRODUCTION
A variety of crystalline cave deposits (speleothems) resemble stromatolites and related forms in their gross morphology and internal structure. Such common forms as stalagmites (Fig. 1) will often have an internal structure that is at least superficially identical t o described stromatolites. Stalactites seen in thin section (Fig. 2) have a regularity of shape that may suggest a biogenic origin. It is clearly impossible in limited space t o describe all speleothems that may mimic or contribute t o the understanding of stromatolites or other biogenic deposits, and this discussion will center on the two forms that are the most important in this regard: cave popcorn and unattached speleothems. Cave popcorn is also referred t o as cave coral, but the latter term seems t o be more misleading to the uninitiated and should be avoided. Because of its probable biogenic nature and relationship t o the above speleothems, moonmilk will be discussed briefly. The term moonmilk, although a mistranslation from the German, is widely used in the U.S. and Britain to apply t o any soft subaerial speleothem, although much of what is called moonmilk is actually a residue from the destruction of other speleothems, rather than a primary deposit. Because of the implication in the placement of this section that speleothems are of abiogenic origin, an examination of this question would seem t o be in order at the outset. A number of studies have indicated that a varied microflora, including blue-green and red algae, is present in caves (e.g., Claus, 1955; Palik, 1960;Nagy, 1965;Jones, 1965,gives a good review). Although sample location descriptions are often sketchy, it seems likely that algae that are normally photosynthetic do live in complete darkness, and a role of such organisms in speleothem deposition cannot be ruled out. Friedman (1955, 1964) has found filaments of two species of blue-green algae in a cave in Israel to be encrusted with calcium carbonate.
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Fig. 1. Polished sections of stalagmites from (left) Kentucky (collected by D.P. Beiter) and (right) Florida, U.S.A.
Fig. 2. Thin section (cross polarized light) normal to axis of calcite stalactite from Colorado, U.S.A. Width of photograph is 3.5 mm.
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BIOGENIC ASPECTS OF SPELEOTHEM DEPOSITION
It is usually assumed that the principal processes responsible for the deposition of the better-known speleothems (stalactites, stalagmites, flowstone,. and related forms) are abiogenic. The apparent absence uf microorganisms on growing surfaces and the lack of included organic material seem to support this view. Fig. 2 is a photomicrograph of a section normal to the length of a small calcite stalactite in which no included organic material can be seen (there are a few opaque detrital grains). The faint banding near the outer margin and the outline of the central tube is the result of ghost rhombohedral faces. Although the portion of this stalactite outside the central tube has recrystallized, it seems unlikely that such recrystallization could have so effectively excluded organic matter (and left the detrital grains). The author has examined numerous other stalactites and stalagmites which apparently contain no organic matter. At the other extreme, it is nearly certain that some carbonate speleothems are of biogenic origin. The strongest case has been made for soft masses which coat the walls or other speleothems in many caves. These deposits, called moonmilk, have a pasty texture when wet and are powdery when dry. When the original deposit is undisturbed it often has a nodular outer surface, similar to some forms of cave popcorn. A variety of carbonate minerals has been reported from moonmilk and moonmilk-like material (Warwick, 1962;Moore and Nicholas, 1964;Thrailkill, 1971).In thin section, moonmilk is seen to consist largely of very fine carbonate crystals in finely laminated sheets and as loose masses partially filling open pores (Fig. 3). Examination of various occurrences has revealed the presence of fungi (Ulrich, 1938),bacteria, algae, and protozoa (Hoeg, 1946;Caumartin and Renault, 1958;Williams, 1966). Williams (1959)isolated a bacterium from moonmilk which deposited calcium , carbonate in a laboratory experiment. By its very nature, moonmilk is unlikely t o be preserved and, except for the botryoidal appearance of its outer surface in some occurrences, it bears little resemblance to stromatolites. In Carlsbad Caverns, however, there often appears t o be a gradation between moonmilk and cave popcorn. Commonly, the stalagmite deposited at the point of impact of a ceiling drip is surrounded by cave popcorn which grades to moonmilk with distance from the drip. Moonmilk was found growing on cave. popcorn (Fig. 5) and some dry cave popcorn may actually be indurated moonmilk (Thrailkill, 1971). CAVE POPCORN
This form, while seldom the most spectacular, is one of the more common speleothems. It consists of nodules, usually in clusters, developed on bedrock or on other speleothems. The clusters are oriented approximately normal to
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Fig. 3. Thin section (cross polarized light) of calcite moonmilk from Colorado, U.S.A. Width of photograph is 3.5 mm.
Fig. 4. Cave popcorn growing on stalagmite in Carlsbad Caverns, New Mexico, U.S.A. Pencil at bottom is 130 mm long.
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the surface to which they are attached, and although they may depart somewhat from this orientation, they seldom are elongated downward (as a stalactite) or upward (as a stalagmite). Fig. 4 shows a group of cave popcorn clusters on a large stalagmite in which the growth seems to be toward the left side of the photograph and slightly upward. This photograph also illustrates the common phenomenon of cave-popcorn clusters tending to form on one side of a stalactite or stalagmite, or on other surfaces. In some cases, a group of stalagmites will all have cave popcorn developed on the same side. As the term is used here, all cave popcorn is deposited subaerially, i.e., from a thin film of water in an air-filled'cave. Some subaqueous speleothems, such as those lining the walls of deep pools, superficially resemble cave popcorn but are usually coarsely crystalline and lack the branching nodule structure and fine laminae. It should be pointed out that it has been thought by some that the lack of geotropic orientation in cave popcorn indicated a subaqueous origin, but even a casual inspection indicates that it is now being actively deposited in areas that have not been flooded. There is a wide variation in the size of individual nodules, both between localities and, to a lesser extent, within a single cluster (Fig. 5 ) . The largest nodules (which are clearly not stalagmites) have diameters of about 100 mm and the smallest on the order of 2 or 3 mm. There is likewise great variation in nodule shape, from nearly perfectly spherical to contorted masses. The morphology shown in Fig. 6 is quite common, but many individual nodules are more elongate (Fig. 7) and some are cylindrical (Fig. 9). The mineralogy of cave popcorn has been little studied. Most of it consists of calcite and/or aragonite, but hydromagnesite and dolomite occur in cave popcorn from Carlsbad Caverns, New Mexico (Thrailkill, 1968, 1971) as does chalcedonic quartz. Huntite and magnesite have been reported from moonmilk (Pobeguin, 1960) and probably also occur in cave popcorn. In addition, various detrital minerals (quartz, clay, etc.) are commonly present in small amounts. Cave popcorn usually consists of irregularly alternating clear and dark layers of crystalline aragonite or calcite (Figs. 7- 12). Even in highly ellipsoidal or cylindrical forms the layers are commonly nearly hemispherical, with the elongation of the nodule resulting from rapid thinning of the layers away from the axis (Fig. 10). In a few specimens the layering departs significantly from the hemispherical shape (Fig. 11).Reentrants between nodules may have acute angles and the layering may be partially discontinuous between nodules (Fig. 10) and curved concavely (Fig. 11).The former probably represent the joining of two nearby nodules by lateral deposition. The growth of cave popcorn begins often from a smooth layer of flowstone or the surface of a stalactite (Fig. 12) or other speleothem. Fig. 1 2 shows another common phenomenon, early small nodules being covered with smooth flowstone, then later the development of cave popcorn. It should be noted that all cave popcorn examined appears to grow by the addition of surface
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Fig. 5. Cave popcorn growing on flowstone and stalagmites on wall in Carlsbad Caverns, New Mexico, U.S.A. Finest material is moonmilk. Pencil on right is 130 mm long.
Fig. 6. Cave popcorn from Carlsbad Caverns, New Mexico, U.S.A. Mineralogy is calcite and aragonite with small amounts of dolomite.
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Fig. 7 . Polished section of cave popcorn from Colorado, U.S.A.
Fig. 8. Thin section (plane polarized light) of cave-popcorn nodule growing o n flowstone (Kentucky, U.S.A.). Width of photograph is 3.5 mm.
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Fig. 9. Polished section of cave popcorn from Carlsbad Caverns, New Mexico, U.S.A.
Fig. 10. Thin section (plane polarized light) of specimen in Fig. 9. Width of photograph is 3.5 mm.
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Fig. 11. Thin section (plane polarized light) of inner layers of specimen in Fig. 7. Width of photograph is 3.5 mm.
Fig. 12. Thin section (cross polarized light) of cave popcorn (left) growing on wall of stalactite (right) from Kentucky, U.S.A. (collected by D.P.Beiter). Width of photograph is 3.5 mm.
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layers; there is seldom any evidence of disruption of the structure by substantial volume increases below the surface. At present, it is not possible to describe unequivocally the processes responsible for the origin of cave popcorn. As stated earlier, it is clear that it is formed subaerially and it seems equally clear that it forms on surfaces which are covered only with a thin film of water. Its non-geotropic growth pattern is probably due to the fact that only a €ilm of water is present. Stalactites and similar speleothems grow downward because of rapid deposition in the pendant drop; when there is only a film of water growth takes place in directions determined by other factors. It is not uncommon t o see stalactites (carrying a pendant drop) growing from a clump of cave popcorn nodules in areas where the amount of water supplied to the surface has increased. It is immaterial how the water film is supplied to the surface. Moore and Nicholas (1964)believe that the water seeps out from the surface behind the cave popcorn, but the writer’s observations are that the water may flow down from above, be pulled up from below by surface tension, seep out from behind, or be splashed on to the surface from an adjacent drip. Seepage water entering a cave is normally (but not invariably) in equilibrium with a partial pressure of carbon dioxide (Pco,) considerably higher than that of the cave atmosphere and has such a high concentration of calcium, bicarbonate, and possibly magnesium ions that it will become supersaturated with respect to aragonite, calcite, and possibly dolomite and huntite if the Pco, falls t o that of the cave atmosphere or if a significant amount of water evaporation occurs (Thrailkill, 1968, 1970, 1971).Deposition as stalactites and stalagmites occurs where such seepage is relatively concentrated and sufficient evaporation or carbondioxide loss occurs to cause precipitation. Because such precipitation is relatively rapid and the precipitation kinetics for the magnesium-containing carbonates are slow, usually only aragonite and calcite are precipitated in stalactites and stalagmites, with the former more likely when the Mg/Ca ratio is high (Murray, 1954;Thrailkill, 1968,1971). Because of the thinness of the water film on cave popcorn, accurate determination of Pco, or saturation with respect to carbonates have not yet been achieved. It seems likely, however, that in Carlsbad Caverns at least, such water has a Pco, near that of the cave atmosphere and is very nearly saturated with respect to the carbonates. It has been proposed, therefore, that the deposition of cave popcorn is tied to carbon dioxide and/or water vapor escape from the thin film, with fastest deposition occurring where such escape is rapid (Thrailkill, 1965, 1968).Thus deposition will tend to occur where the film is thin, as on nodule tips where the convex curvature is great, and be inhibited where the film is thick, as in reentrants between nodules and on the lower surfaces (where gravitational thickening occurs). Slight irregularities on a surface would produce a local thinning of the film and the budding of a new nodule. Other factors are ventilation, which would also favor deposition
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on nodule tips and inhibit it in reentrants, and water supply (i.e., nodules sometimes seem to grow toward a drip), but since too much water thickens the film (or causes pendant drops to form), this is of doubtful importance. Because aragonite and calcite deposition will increase the Mg/Ca ratio (Thrailkill, 1968), evaporation will increase the Mg concentration, and the water may be in contact with the cave popcorn for a long time, Mg-containing minerals may precipitate. As discussed earlier, algae, bacteria, and other microorganisms have been found inhabiting many cave surfaces. If the above explanation for cave popcorn deposition has any validity, it is obvious that the presence of a microbiota in the water film which utilizes earbon dioxide may have a profound effect on carbonate deposition. That such may be the case is suggested by the fine dark laminae in much cave popcorn (e.g. Figs. 7-12) which, may be organic-rich layers. This might also explain the abundance of hydromagnesite in cave popcorn from Carlsbad Caverns. According to current thermodynamic data, seepage water could reach saturation with respect t o this mineral only at impossibly high Mg/Ca ratios (- 10’) or at Pco, values several orders of magnitude bslow that of the cave atmosphere. It has been suggested by G.W. Moore (personal communication, 1972) that its precipitation is caused by C 0 2 depletion by microorganisms. I t should be emphasized, however, that much cave popcorn (e.g. Figs. 8, 9) contains thick layers of clear calcite or aragonite with no indication of organic involvement. UNATTACHED SPELEOTHEMS
Cave pearls are rounded unattached speleothems which are occasionally found in caves. They have drawn attention because of their attractiveness and the analogy of their supposed origin with that of pearls. They often occur in small basins under active ceiling drips, and those that have been sectioned usually are found t o consist of layers of calcite or aragonite surrounding a nucleus of some type (Fig. 13). The term cave pearl is usually reserved for specimens that are approximately spherical and whose outer surface is smooth enough to be reflective when wet. There is a second type of unattached speleothem, termed a pool accretion (Thrailkill, 1963) which, although more common than cave pearls, has attracted much less attention. It is either crudely spherical or ellipsoidal (Fig. 14) and has a rough outer surface. In thin or polished section, pool accretions are porous and have a crude concentric layering (Fig. 15). Fig. 16 shows two other features common t o most pool accretions: scattered calcite crystals tending to be arranged radially or dendritically, and wavy laminae. Several cave pearls which have been sectioned by the writer have the internal structure of a pool accretion, with a few thin laminae on the outside. In the unattached forms examined by the writer it is the presence of these laminae
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Fig. 13. Thin section (cross polarized light) of cave pearl from Colorado, U.S.A. Clear calcite layer surrounding microcline crystal nucleus (upper left) is covered by finely laminated outer layers. Width of photograph 3.5 mm.
Fig. 14. Sectioned pool accretion from Kentucky, U.S.A. (collected by G. O’Dell).
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Fig. 15. Polished section of specimen in Fig. 14.
Fig. 16. Thin section (cross polarized light) of pool accretion from Colorado, U.S.A.Width of photograph is 3.5 mm.
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(usually alternating dark and clear) that gives some of them the “polished” appearance of cave pearls, although many cave pearls commonly contain layers of radial calcite crystals more than 1 mm thick (Fig. 13). It is commonly supposed that cave pearls are unattached because they are rotated by*fallingwater, but it is clear that this is not the case in numerous occurrences (Thrailkill, 1963;Moore and Nicholas, 1964).The smooth surface layers of most cave pearls are so similar in appearance to those’of cave popcorn (compare Figs. 10 and 13) that it seems likely that they may both be deposited under similar conditions. It is also unlikely that pool accretions are unattached because of agitation. The ellipsoidal specimen in Figs. 14 and 15 is 9 cm in diameter but the layering is nearly concentric with the center. Deposition must have occurred simultaneously on the bottom and top. All of the pool accretions known to the writer are found either in shallow pools or in positions*where they could have been washed from these pools, and it is likely that they are formed when in contact with standing water. The mechanism responsible for their formation is unknown, but the resemblance of their internal structure (Fig. 16) t o that of moonmilk (Fig. 3)suggests some biogenic processes are involved. CONCLUSIONS
A comparison of these speleothems with illustrations in papers discussing stromatolites reveals many similarities. Some examples are Fig. 9 (this paper) with figure 31 in Walter (1972a)or figure 20 in Hofmann (1969a);Fig. 12 (this paper) with plate 7-1in Monty (1967);and Fig. 15 (this paper) with plate 2F in Logan et al. (1964). Some tentative criteria which may be used to distinguish speleothems from stromatolites are: (1)association with other speleothems such as stalactites; (2)scale, since it is doubtful that cave popcorn nodules attain diameters greater than about 100 mm (although stalagmites may be much larger); (3)presence of nodules or layers in nodules which are apparently abiogenic, but not due t o much later recrystallization (although these may be found in stromatolites as well); (4)presence of initially deposited or early diagenetic magnesium minerals such as dolomite, huntite, or hydromagnesite; ( 5 ) indication that most of the deposition was due to precipitation from solution rather than entrapment of detritus; (6)non-vertical orientation; and (7)lack of filaments or other biogenic structures. Criterion 1 may be useful for distinguishing pool accretions from unattached stromatolites (oncolites), but because it is distinctly possible that such pool accretions are stromatolites deposited in a cave environment, other criteria which might be suggested are likely to stem largely from our ignorance of the origin of these speleothems. The same may be true of many occurrences of cave popcorn as well.
Chapter 3.2 SPELEOTHEMS John Thrailkill
INTRODUCTION
A variety of crystalline cave deposits (speleothems) resemble stromatolites and related forms in their gross morphology and internal structure. Such common forms as stalagmites (Fig. 1) will often have an internal structure that is at least superficially identical t o described stromatolites. Stalactites seen in thin section (Fig. 2) have a regularity of shape that may suggest a biogenic origin. It is clearly impossible in limited space t o describe all speleothems that may mimic or contribute t o the understanding of stromatolites or other biogenic deposits, and this discussion will center on the two forms that are the most important in this regard: cave popcorn and unattached speleothems. Cave popcorn is also referred t o as cave coral, but the latter term seems t o be more misleading to the uninitiated and should be avoided. Because of its probable biogenic nature and relationship t o the above speleothems, moonmilk will be discussed briefly. The term moonmilk, although a mistranslation from the German, is widely used in the U.S. and Britain to apply t o any soft subaerial speleothem, although much of what is called moonmilk is actually a residue from the destruction of other speleothems, rather than a primary deposit. Because of the implication in the placement of this section that speleothems are of abiogenic origin, an examination of this question would seem t o be in order at the outset. A number of studies have indicated that a varied microflora, including blue-green and red algae, is present in caves (e.g., Claus, 1955; Palik, 1960;Nagy, 1965;Jones, 1965,gives a good review). Although sample location descriptions are often sketchy, it seems likely that algae that are normally photosynthetic do live in complete darkness, and a role of such organisms in speleothem deposition cannot be ruled out. Friedman (1955, 1964) has found filaments of two species of blue-green algae in a cave in Israel to be encrusted with calcium carbonate.
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Fig. 1. Polished sections of stalagmites from (left) Kentucky (collected by D.P. Beiter) and (right) Florida, U.S.A.
Fig. 2. Thin section (cross polarized light) normal to axis of calcite stalactite from Colorado, U.S.A. Width of photograph is 3.5 mm.
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BIOGENIC ASPECTS OF SPELEOTHEM DEPOSITION
It is usually assumed that the principal processes responsible for the deposition of the better-known speleothems (stalactites, stalagmites, flowstone,. and related forms) are abiogenic. The apparent absence uf microorganisms on growing surfaces and the lack of included organic material seem to support this view. Fig. 2 is a photomicrograph of a section normal to the length of a small calcite stalactite in which no included organic material can be seen (there are a few opaque detrital grains). The faint banding near the outer margin and the outline of the central tube is the result of ghost rhombohedral faces. Although the portion of this stalactite outside the central tube has recrystallized, it seems unlikely that such recrystallization could have so effectively excluded organic matter (and left the detrital grains). The author has examined numerous other stalactites and stalagmites which apparently contain no organic matter. At the other extreme, it is nearly certain that some carbonate speleothems are of biogenic origin. The strongest case has been made for soft masses which coat the walls or other speleothems in many caves. These deposits, called moonmilk, have a pasty texture when wet and are powdery when dry. When the original deposit is undisturbed it often has a nodular outer surface, similar to some forms of cave popcorn. A variety of carbonate minerals has been reported from moonmilk and moonmilk-like material (Warwick, 1962;Moore and Nicholas, 1964;Thrailkill, 1971).In thin section, moonmilk is seen to consist largely of very fine carbonate crystals in finely laminated sheets and as loose masses partially filling open pores (Fig. 3). Examination of various occurrences has revealed the presence of fungi (Ulrich, 1938),bacteria, algae, and protozoa (Hoeg, 1946;Caumartin and Renault, 1958;Williams, 1966). Williams (1959)isolated a bacterium from moonmilk which deposited calcium , carbonate in a laboratory experiment. By its very nature, moonmilk is unlikely t o be preserved and, except for the botryoidal appearance of its outer surface in some occurrences, it bears little resemblance to stromatolites. In Carlsbad Caverns, however, there often appears t o be a gradation between moonmilk and cave popcorn. Commonly, the stalagmite deposited at the point of impact of a ceiling drip is surrounded by cave popcorn which grades to moonmilk with distance from the drip. Moonmilk was found growing on cave. popcorn (Fig. 5) and some dry cave popcorn may actually be indurated moonmilk (Thrailkill, 1971). CAVE POPCORN
This form, while seldom the most spectacular, is one of the more common speleothems. It consists of nodules, usually in clusters, developed on bedrock or on other speleothems. The clusters are oriented approximately normal to
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Fig. 3. Thin section (cross polarized light) of calcite moonmilk from Colorado, U.S.A. Width of photograph is 3.5 mm.
Fig. 4. Cave popcorn growing on stalagmite in Carlsbad Caverns, New Mexico, U.S.A. Pencil at bottom is 130 mm long.
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the surface to which they are attached, and although they may depart somewhat from this orientation, they seldom are elongated downward (as a stalactite) or upward (as a stalagmite). Fig. 4 shows a group of cave popcorn clusters on a large stalagmite in which the growth seems to be toward the left side of the photograph and slightly upward. This photograph also illustrates the common phenomenon of cave-popcorn clusters tending to form on one side of a stalactite or stalagmite, or on other surfaces. In some cases, a group of stalagmites will all have cave popcorn developed on the same side. As the term is used here, all cave popcorn is deposited subaerially, i.e., from a thin film of water in an air-filled'cave. Some subaqueous speleothems, such as those lining the walls of deep pools, superficially resemble cave popcorn but are usually coarsely crystalline and lack the branching nodule structure and fine laminae. It should be pointed out that it has been thought by some that the lack of geotropic orientation in cave popcorn indicated a subaqueous origin, but even a casual inspection indicates that it is now being actively deposited in areas that have not been flooded. There is a wide variation in the size of individual nodules, both between localities and, to a lesser extent, within a single cluster (Fig. 5 ) . The largest nodules (which are clearly not stalagmites) have diameters of about 100 mm and the smallest on the order of 2 or 3 mm. There is likewise great variation in nodule shape, from nearly perfectly spherical to contorted masses. The morphology shown in Fig. 6 is quite common, but many individual nodules are more elongate (Fig. 7) and some are cylindrical (Fig. 9). The mineralogy of cave popcorn has been little studied. Most of it consists of calcite and/or aragonite, but hydromagnesite and dolomite occur in cave popcorn from Carlsbad Caverns, New Mexico (Thrailkill, 1968, 1971) as does chalcedonic quartz. Huntite and magnesite have been reported from moonmilk (Pobeguin, 1960) and probably also occur in cave popcorn. In addition, various detrital minerals (quartz, clay, etc.) are commonly present in small amounts. Cave popcorn usually consists of irregularly alternating clear and dark layers of crystalline aragonite or calcite (Figs. 7- 12). Even in highly ellipsoidal or cylindrical forms the layers are commonly nearly hemispherical, with the elongation of the nodule resulting from rapid thinning of the layers away from the axis (Fig. 10). In a few specimens the layering departs significantly from the hemispherical shape (Fig. 11).Reentrants between nodules may have acute angles and the layering may be partially discontinuous between nodules (Fig. 10) and curved concavely (Fig. 11).The former probably represent the joining of two nearby nodules by lateral deposition. The growth of cave popcorn begins often from a smooth layer of flowstone or the surface of a stalactite (Fig. 12) or other speleothem. Fig. 1 2 shows another common phenomenon, early small nodules being covered with smooth flowstone, then later the development of cave popcorn. It should be noted that all cave popcorn examined appears to grow by the addition of surface
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Fig. 5. Cave popcorn growing on flowstone and stalagmites on wall in Carlsbad Caverns, New Mexico, U.S.A. Finest material is moonmilk. Pencil on right is 130 mm long.
Fig. 6. Cave popcorn from Carlsbad Caverns, New Mexico, U.S.A. Mineralogy is calcite and aragonite with small amounts of dolomite.
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Fig. 7 . Polished section of cave popcorn from Colorado, U.S.A.
Fig. 8. Thin section (plane polarized light) of cave-popcorn nodule growing o n flowstone (Kentucky, U.S.A.). Width of photograph is 3.5 mm.
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Fig. 9. Polished section of cave popcorn from Carlsbad Caverns, New Mexico, U.S.A.
Fig. 10. Thin section (plane polarized light) of specimen in Fig. 9. Width of photograph is 3.5 mm.
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Fig. 11. Thin section (plane polarized light) of inner layers of specimen in Fig. 7. Width of photograph is 3.5 mm.
Fig. 12. Thin section (cross polarized light) of cave popcorn (left) growing on wall of stalactite (right) from Kentucky, U.S.A. (collected by D.P.Beiter). Width of photograph is 3.5 mm.
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layers; there is seldom any evidence of disruption of the structure by substantial volume increases below the surface. At present, it is not possible to describe unequivocally the processes responsible for the origin of cave popcorn. As stated earlier, it is clear that it is formed subaerially and it seems equally clear that it forms on surfaces which are covered only with a thin film of water. Its non-geotropic growth pattern is probably due to the fact that only a €ilm of water is present. Stalactites and similar speleothems grow downward because of rapid deposition in the pendant drop; when there is only a film of water growth takes place in directions determined by other factors. It is not uncommon t o see stalactites (carrying a pendant drop) growing from a clump of cave popcorn nodules in areas where the amount of water supplied to the surface has increased. It is immaterial how the water film is supplied to the surface. Moore and Nicholas (1964)believe that the water seeps out from the surface behind the cave popcorn, but the writer’s observations are that the water may flow down from above, be pulled up from below by surface tension, seep out from behind, or be splashed on to the surface from an adjacent drip. Seepage water entering a cave is normally (but not invariably) in equilibrium with a partial pressure of carbon dioxide (Pco,) considerably higher than that of the cave atmosphere and has such a high concentration of calcium, bicarbonate, and possibly magnesium ions that it will become supersaturated with respect to aragonite, calcite, and possibly dolomite and huntite if the Pco, falls t o that of the cave atmosphere or if a significant amount of water evaporation occurs (Thrailkill, 1968, 1970, 1971).Deposition as stalactites and stalagmites occurs where such seepage is relatively concentrated and sufficient evaporation or carbondioxide loss occurs to cause precipitation. Because such precipitation is relatively rapid and the precipitation kinetics for the magnesium-containing carbonates are slow, usually only aragonite and calcite are precipitated in stalactites and stalagmites, with the former more likely when the Mg/Ca ratio is high (Murray, 1954;Thrailkill, 1968,1971). Because of the thinness of the water film on cave popcorn, accurate determination of Pco, or saturation with respect to carbonates have not yet been achieved. It seems likely, however, that in Carlsbad Caverns at least, such water has a Pco, near that of the cave atmosphere and is very nearly saturated with respect to the carbonates. It has been proposed, therefore, that the deposition of cave popcorn is tied to carbon dioxide and/or water vapor escape from the thin film, with fastest deposition occurring where such escape is rapid (Thrailkill, 1965, 1968).Thus deposition will tend to occur where the film is thin, as on nodule tips where the convex curvature is great, and be inhibited where the film is thick, as in reentrants between nodules and on the lower surfaces (where gravitational thickening occurs). Slight irregularities on a surface would produce a local thinning of the film and the budding of a new nodule. Other factors are ventilation, which would also favor deposition
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on nodule tips and inhibit it in reentrants, and water supply (i.e., nodules sometimes seem to grow toward a drip), but since too much water thickens the film (or causes pendant drops to form), this is of doubtful importance. Because aragonite and calcite deposition will increase the Mg/Ca ratio (Thrailkill, 1968), evaporation will increase the Mg concentration, and the water may be in contact with the cave popcorn for a long time, Mg-containing minerals may precipitate. As discussed earlier, algae, bacteria, and other microorganisms have been found inhabiting many cave surfaces. If the above explanation for cave popcorn deposition has any validity, it is obvious that the presence of a microbiota in the water film which utilizes earbon dioxide may have a profound effect on carbonate deposition. That such may be the case is suggested by the fine dark laminae in much cave popcorn (e.g. Figs. 7-12) which, may be organic-rich layers. This might also explain the abundance of hydromagnesite in cave popcorn from Carlsbad Caverns. According to current thermodynamic data, seepage water could reach saturation with respect t o this mineral only at impossibly high Mg/Ca ratios (- 10’) or at Pco, values several orders of magnitude bslow that of the cave atmosphere. It has been suggested by G.W. Moore (personal communication, 1972) that its precipitation is caused by C 0 2 depletion by microorganisms. I t should be emphasized, however, that much cave popcorn (e.g. Figs. 8, 9) contains thick layers of clear calcite or aragonite with no indication of organic involvement. UNATTACHED SPELEOTHEMS
Cave pearls are rounded unattached speleothems which are occasionally found in caves. They have drawn attention because of their attractiveness and the analogy of their supposed origin with that of pearls. They often occur in small basins under active ceiling drips, and those that have been sectioned usually are found t o consist of layers of calcite or aragonite surrounding a nucleus of some type (Fig. 13). The term cave pearl is usually reserved for specimens that are approximately spherical and whose outer surface is smooth enough to be reflective when wet. There is a second type of unattached speleothem, termed a pool accretion (Thrailkill, 1963) which, although more common than cave pearls, has attracted much less attention. It is either crudely spherical or ellipsoidal (Fig. 14) and has a rough outer surface. In thin or polished section, pool accretions are porous and have a crude concentric layering (Fig. 15). Fig. 16 shows two other features common t o most pool accretions: scattered calcite crystals tending to be arranged radially or dendritically, and wavy laminae. Several cave pearls which have been sectioned by the writer have the internal structure of a pool accretion, with a few thin laminae on the outside. In the unattached forms examined by the writer it is the presence of these laminae
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Fig. 13. Thin section (cross polarized light) of cave pearl from Colorado, U.S.A. Clear calcite layer surrounding microcline crystal nucleus (upper left) is covered by finely laminated outer layers. Width of photograph 3.5 mm.
Fig. 14. Sectioned pool accretion from Kentucky, U.S.A. (collected by G. O’Dell).
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Fig. 15. Polished section of specimen in Fig. 14.
Fig. 16. Thin section (cross polarized light) of pool accretion from Colorado, U.S.A.Width of photograph is 3.5 mm.
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(usually alternating dark and clear) that gives some of them the “polished” appearance of cave pearls, although many cave pearls commonly contain layers of radial calcite crystals more than 1 mm thick (Fig. 13). It is commonly supposed that cave pearls are unattached because they are rotated by*fallingwater, but it is clear that this is not the case in numerous occurrences (Thrailkill, 1963;Moore and Nicholas, 1964).The smooth surface layers of most cave pearls are so similar in appearance to those’of cave popcorn (compare Figs. 10 and 13) that it seems likely that they may both be deposited under similar conditions. It is also unlikely that pool accretions are unattached because of agitation. The ellipsoidal specimen in Figs. 14 and 15 is 9 cm in diameter but the layering is nearly concentric with the center. Deposition must have occurred simultaneously on the bottom and top. All of the pool accretions known to the writer are found either in shallow pools or in positions*where they could have been washed from these pools, and it is likely that they are formed when in contact with standing water. The mechanism responsible for their formation is unknown, but the resemblance of their internal structure (Fig. 16) t o that of moonmilk (Fig. 3)suggests some biogenic processes are involved. CONCLUSIONS
A comparison of these speleothems with illustrations in papers discussing stromatolites reveals many similarities. Some examples are Fig. 9 (this paper) with figure 31 in Walter (1972a)or figure 20 in Hofmann (1969a);Fig. 12 (this paper) with plate 7-1in Monty (1967);and Fig. 15 (this paper) with plate 2F in Logan et al. (1964). Some tentative criteria which may be used to distinguish speleothems from stromatolites are: (1)association with other speleothems such as stalactites; (2)scale, since it is doubtful that cave popcorn nodules attain diameters greater than about 100 mm (although stalagmites may be much larger); (3)presence of nodules or layers in nodules which are apparently abiogenic, but not due t o much later recrystallization (although these may be found in stromatolites as well); (4)presence of initially deposited or early diagenetic magnesium minerals such as dolomite, huntite, or hydromagnesite; ( 5 ) indication that most of the deposition was due to precipitation from solution rather than entrapment of detritus; (6)non-vertical orientation; and (7)lack of filaments or other biogenic structures. Criterion 1 may be useful for distinguishing pool accretions from unattached stromatolites (oncolites), but because it is distinctly possible that such pool accretions are stromatolites deposited in a cave environment, other criteria which might be suggested are likely to stem largely from our ignorance of the origin of these speleothems. The same may be true of many occurrences of cave popcorn as well.