Black Phytokarst from Hell, Cayman Islands, British West Indies

Home , British West Indies

ROBERT L. FOLK Geology Department, University of Texas at Austin, Austin, Texas 78712 HARRY H. ROBERTS Coastal Studies Institute, Louisiana State University, Baton Rouge, Louisiana 70803 CLYDE H. MOORE Geology Department, Louisiana State University, Baton Rouge, Louisiana 70803

ABSTRACT the whole surface, actively boring and dissolving their way into the rock and are not Phytokarst is a distinctive landform result- simply a surface coating; and the major distinc- ing from a curious type of biologic erosion. tive morphological features—delicately spongy Filamentous algae bore their way into lime- form and random orientation—are produced stone to produce black-coated, jagged pin- by boring plants, not through solution by rain- nacles marked by delicate, lacy dissection that water which simply washes over the surface. lacks any gravitational orientation. Ordinary rainfall-produced karst and littoral karst are LOCATION AND GEOLOGY characterized by flat-bottomed pans and verti- The Cayman Islands (Fig. 1) lie perched cally oriented flutes, thus differing from upon the Cayman Ridge, which extends phytokarst. Algae attack by dissolving calcite westerly toward Honduras from the Sierra preferentially to dolomite. Maestra of eastern Cuba. Grand Cayman (19°20' N.) has 65 in. annual rainfall. The INTRODUCTION fundamental geologic work on the three small An unusually grotesque karst, characterized British islands is by Matley (1926). Each by jagged, spongy pinnacles of black-surfaced island has a nucleus of Bluff Limestone deter- limestone, occurs spectacularly at Hell, Grand mined by Matley to be Oligocene to Miocene Cayman, British West Indies, and other locali- in age based on identification of corals and ties in the Caribbean. This type of inland karst Foraminifera by Vaughan (1926). The Ter- is produced mainly by the attack of boring tiary nucleus is surrounded by a near sea-level filamentous algae and differs markedly from fringe of Pleistocene (Sangamon?) Ironshore more common karst caused by rainwater solu- Formation, produced at a time of slightly tion. Hence, it is designated "phytokarst" higher sea level than at present. (Folk and others, 1971). Petrographic studies of samples of Bluff Phytokarst is herein defined as a landform Limestone at Hell, Spots Bay, Bodden Town, produced by rock solution in which boring and on Cayman Brae show that it is typically plant filaments are the main agent of destruc- a heavily dolomitized biolithite, with corals, tion, and the major morphological features are red and green algae, and Foraminifera. Di- determined by the peculiar nature of this agenesis has been extreme, and the rock re- mode of attack. The process as here described sembles dense snow-white marble; it is ex- is restricted to attack of endolithic algal fila- tremely hard, crystalline, and brilliant white ments on carbonate rocks, but the process can (lighter than N9 on the rock color chart; God- be extended ot other kinds of susceptible rocks, dard and others, 1963). Aragonitic fossils are such as gypsum, and includes effects of other represented only by well-preserved empty types of endolithic plants, such as fungi. The molds. The micrite matrix has been converted process operates from inland to intertidal to 4- to 20-n dolomite. Unusually limpid, 20- to zones; phytokarst as a landform probably 100-/¿-dolomite euhedra line pores on etching occurs only in the more humid tropical cli- appear as tiny water-clear diamond shapes mates. (Fig. 2). The final pore fill occurs as intercon- The key points that distinguish phytokarst nected, poikilotopic calcite in water-clear crys- from ordinary karst are: plant filaments cover tals 1 to 2 mm across. Both limpid dolomite

Geological Society of America Bulletin, v. 84, p. 2351-2360, 12 figs., July 1973 2351

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GRAND CAYMAN

MILES Figure 1. Index map of Cayman Islands (above) and Grand Cayman (Mow). and coarse calcite are the products of fresh- the quarry at Spots Bay (Fig. 5) at Bodden water flushing (Folk and Land, 1972). Town (Fig. 6) and eastward, and near Cayman Kai on the north shore. It is also prevalent on PHYTOKARST IN THE the Bluff Limestone on the neighboring island CAYMAN ISLANDS of Cayman Brae. A small inlier of Bluff Limestone occurs Phytokarst also occurs ubiquitously on the 1 km (0.6 mi) inland in a swamp at the north- Pleistocene Ironshore Formation and con- west corner of Grand Cayman, elevation 0.3 to tributes to case hardening of this aragonitic 1.1 km (2 to 4 ft). Here the limestone has been calcarenite. Phytokarst grades seaward into so fantastically dissected into a grotesque, littoral karst, and also grades landward into razor-sharp spongework of ragged black pin- ordinary rainfall karst (Fig. 7). As there is no nacles set in pools of black, stinking water, sharp boundary between des tructive processes, that the natives appropriately named the the morpholcgic products are also gradational. locality Hell (Figs. 3, 4). The black phytokarst Doran (1954), in his geomorphic study of the in the rear of the Inferno Night Club at Hell is island, mentioned karst briefly but did not dis- the most outstanding example we have seen; cuss algal effects. Warthin (1959) described other good exposures are at the back edge of coastal solution of Ironshore Formation but

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boring organisms such as snails, chitons, ur- chins, sponges, and boring algae, aided by the solvent action of sea water, especially at night when the pH is lower because photosynthesis has stopped (Fremy, 1945; Emery, 1946; Gins- burg, 1953; Revelle and Emery, 1954; Purdy and Kornicker, 1958; Kaye, 1959; Neumann, 1966, 1968; Golubic, 1969; Hodgkin, 1970). This peritidal-destructive zone is characterized by a greener, mossy-feeling algal coat and has numerous small flat-bottomed pools with vertically fluted sides and overhanging lips, continuing down to the waterline nip. Somewhat similar morphologic features are produced by solution of inland carbonate rocks (Fig. 10) as in the well-known karren, la pies, or rillenstein (Cvijic, 1924; Bogli, 1960). Ud- den (1925) described the flat-bottomed solution pans called tinajitas, also having vertically fluted sides and overhanging lips, and attrib- Figure 2. Scantling Electron Micrograph (SEM) of uted them to etching by algae that thrive dolomite that comprises the bulk of the Tertiary Bluff when the pools are filled with water after Limestone, Grand Cayman. Superbly euhedral nature infrequent rains. Solution is caused by standing (and water clarity in light microscope) indicates that this is limpid dolomite, believed to be the result of pools of rainwater or by running, concentrated fresh-water diagenesis. Photo by A. Siedlecka. slope wash to give the characteristic gravitational orientation of the sculpture (Laudermilk did not describe the different inland karst and Woodford, 1932; Smith and Albritton, described in this paper. 1941). Rainfall karst also lacks the black algal coat. Wentworth (1944) has an excellent dis- DISTINCTION BETWEEN cussion of these intergrading processes. PHYTOKARST AND OTHER KINDS Major destruction of inland rocks by the OF LIMESTONE DESTRUCTION attack of boring subaerial terrestrial algae to Stony lacework in the peritidal zone (littoral produce characteristic landforms has apparently karst) is well known on most tropical carbonate gone unrecognized, though organic coatings shores. It is caused mainly by browsing and have been often discussed. Algae are supported

Figure 3. Archetype outcrop of phytokarst at Hell, are about 3 m high (torture of locomotion over phyto- Grand Cayman (stereo pair). Pinnacles in the distance karst prevented more accurate measurement).

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Figure 4. Closer view of the swamp at Hell. Intri- cate, randomly oriented sculpturing is evident.

by occasional dampening by rain, dew, or storm-blown salt spray. Fritsch (1907) observed that algal coats are more abundant with higher temperature and rainfall; that they mainly consist of filamentous blue-green algae; Figure 5. Outstanding example of phytokarst, Spot Bay Quarry. Lack of gravitational orientation and that mucilaginous sheaths prevent desiccation; grotesqueness of dissection is striking. Rock face is 70 and that the black or dark-brown pigment cm high. screens out sunlight and prevents destruction of chlorophyll (see Fremy, 1945). Welwitsch (1868), Pia (1937), Fritsch (1945), and Par- random polyconcave surface of phytokarst re- fenova and Yarilova (1965) noted that algae sembles the surface on "nieves penitentes" colonize rock surfaces; however, they said made of old ablating snow (Wentworth, 1940) nothing about geomorphic effects. Jones (1965) or the ablation surface of a meteorite. Both noted surficial penetration of English lime- phytokarst sculpture and snow sculpture are stones by boring filaments, but the lichens do produced by concavity-fo::ming processes that not produce major landforms. destroy the host starting from the surface and working uniformly in all directions—algae in CHARACTERISTICS OF the one case and sublimation in the other. BLACK PHYTOKARST Other than the gross vertical aspect of the Pinnacles in the phytokarst areas range up pinnacles, which mimic & forest of baroque to about 3 m (10 ft) high, though typically the plague columns, there is no gravitational orien- relief is about 0.6 to 1.5 m (2 to 5 ft) (Figs. 3 tation to the sculpture; they are not direction- through 6). Pinnacles are grotesquely dissected ally fluted as is rainfall karst. For detached to spongelike forms, with nested and touching pieces, it is impossible to tell which way is up concavities ranging from a few inches in diam- (Fig. 8). eter down to 1/50 in., mostly 1/8 to 1/2 in. Advanced dissection leaves a jumbled brec- (Fig. 10). The concavities are bounded by cia of jagged detached pieces 5 to 50 cm (2 to 18 knife-edges; completely penetrating holes are in.) in diameter, riddled with holes and repre- often present in this fretwork (Fig. 5). The senting residual fragments from algal dissec-

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Figure 6. White phytokarst, Bodden Town. Black curvilinear sculpture is still remarkable. coat has been removed by burning brush, but grotesque

tion, as would happen if one tore a huge sponge into small pieces and piled them up. Surface color ranges from N3 to N4, but in contrast with the dazzlingly white limestone the surface looks black (Fig. 8). When exam- ined with a highpower lens, the surface is covered with black algal dots with tiny loose crystals of carbonate scattered over the surface, ready to be washed away. The surface algal dots are Gloeocapsa alpina, a coccoid blue- green algae. In other specimens, algal filaments are exposed at the surface like perpendicular black whiskers, neatly shaved off at the level of the white limestone. Although surfaces exposed to intensive sunlight are black, shaded surfaces on the under- sides of fallen blocks are typically weak olive green (5 GY 4/2) with more viable algae. Rocks have blackest surfaces in completely unshaded areas (Hell, Spot Bay Quarry) and have a paler and greener tint in areas shaded by brush. Broken rocks (Fig. 9) reveal algal filaments Figure 7. Ordinary rainfall-solution karst, George- penetrating 0.1 to 0.2 mm from the surface town. Note vertically oriented flutes and white color of (though a few go deeper than 1 mm). Filaments surface. Wallet is 10 cm long.

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within the rocks are 5 to 10 ju thick but some resemble clusters of tiny stubby fingers; they are khaki green in contrast to the black surface- dwelling algae. Several species of filamentous blue-green algae are present. The algae may bore in order to escape bright sunlight, to obtain metabolic carbon, or to have a damp tunnel to better hold capillary water (see Carriker and others, 1969). Algae dissolve their way into carbonate rock by secreting acid, shown by the fact that the cavities have the shapes of calcite crystal faces and cleavage directions (Jones, 1965; Golubic, 1969). SCANNING ELECTRON MICROSCOPY Figure 8. Broken piece of phytokarst from Hell. For bored rinds, we gold-shadowed the un- White color of mother carbonate rock (Bluff Forma- treated rocks, including the actual algal fila- tion) contrasts sharply with black algal coating. ments; stage rotation produced stereo photo- graphs. Fresh Bluff Formation from Hell shows removed algal filaments with H202 to look at remarkably euhedral dolomite crystals (Fig. unobstructed borings. Algae dissolve out rough 2), probably the result of fresh-water dolomiti- tunnels about twice ths diameter of the fila- zation. ments; calcite is removed first, forming melted- Algal filaments are about 5 ¡i wide with a appearing surfaces, while resistant dolomite scaly surface (Fig. 11). In some specimens we rhombs stick out (Fig. 12). In other specimens,

Figure 9. Phytokarst from Hell. Tiny algal threads white carbonate rock, eating in from the black algal coat and destroying the

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PHYTOKARST i NORMAL KARST SURFACE COLOR 3 lacU j Light

DISSECTION Intn'c&te, ? Simple, 3 pokujy C Flutect

ORIENTATION Ati«.Mt j âV-Avitati'ow»/

AL6AL COAT Vtry Heavy £ Vitry LyAt

EROSIVE AGENT ßoriry Algae I II Soluti,

Figure 10. Distinguishing features of phytokarst, compared with ordinary rainfall-produced karst.

filaments have bored neat holes right through dolomite (?) rhombs. These pictures indicate that algal acid attacks calcite more easily than dolomite. On weath- ered rock surfaces, algae preferentially occupy and dissolve sites where calcite is exposed, even intimately where calcite fills the cell spaces among dolomitized coral septa.

WHITE PHYTOKARST At some places on Grand Cayman the typical grotesque, unoriented solution microtopogra- phy diagnostic of phytokarst occurs without the black algal coat; rock surfaces are white (Fig. 5). In these atypical localities a three-fold sequence occurred: (1) phytokarst had been developed by subaerial endolithic algae in the usual way; (2) the phytokarst had been buried by recent sediment; for example, soil or beach sand washed over it by hurricane tides; and (3) the phytokarst had been exhumed by man or by later natural erosion of the sand covering. During burial, algae died and their traces were Figure 11. SEM photo, algal filaments within rock removed, presumably by bacteria. Thus the at Hell. Filaments are about 5 m thick.

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Figure 12. SEM stereo pair. Algal filaments were believed to represent calcite, which is selectively at- removed by H2O2. Observe three-dimensional view of tacked by the algae in preference to protruding dolo- the tunnel eaten out by algal threads penetrating the mite rhombs, which are here little affected. mother rock. Rounded, corroded-appearing forms are

black coat disappeared, but the typical sculp- limestones in northeast New Guinea with ture remained, and tiny empty algal borings every external appearance of phytokarst. can still be seen in the rock. Phytokarst can It is an interesting fact that the fantastic also lose its black coat (confirmed experi- cone or towerkarst occurs only in humid mentally in a fireplace) by native brush tropical areas with rich vegetation such as in burning. Yunnan and North Viet Nam (Silar, 1960), New Guinea (Williams, 1972), Cuba, Jamaica, SIGNIFICANCE Puerto Rico (Sweeting, 1958; Panos and Important algal rock sculpture is either rare Stelcl, 1966), and similar climatic regions. or has gone unrecognized in tropical karst. These are precisely the areas where boring Such authorities as Verstappen (1964), New blue-green algae should flou rish. Perhaps there Guinea; Doerr and Hoy (1957), Caribbean; is a link between the vigor of boring aigae and Lotschert (1959), Cuba; Sweeting (1958), the occurrence of this most spectacular type of Jamaica; and Silar (1960), Yunnan and North karst landscape. It would be indeed interesting Viet Nam, do not specifically mention boring if such humble and inconspicuous creatures as algae as a rock-destructive process, though they boring algae were responsible for some of the note that in general vegetation (through humic world's most curious landforms, which were acids, and so forth) plays an important part in the inspiration for much great Chinese land- karst formation. Small (1930) showed photo- scape art. graphs of intricate honeycomb rock in the Florida Keys which looks similar to, and may ACKNOWLEDGMENTS actually be, phytokarst. He mentioned nothing The Organization of Tropical Studies fi- about a surface algal coat, however, and attrib- nanced the 1971 geology course during which uted the intricate sculpture to "atmospheric this discovery was made; we :hank the students agencies" and humic acids. K. W. Crook for discussion. James F. Quinlan, (McMaster (1972, oral comm.) observed black-coated University) provided some references, and

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Stjepko Golubic (Boston University) and U.S. Geol. Survey Prof. Paper 317-B, 140 p. Harold Bold (University of Texas) identified Laudermilk, J. D., and Woodford, A. O., 1932, Concerning Rillensteine: Am. Jour. Sci., v. the algae. 223, p. 135-139. Lötschert, Wilhelm, 1959, Vegetation der West-Kubanischen Kegelkarstberge: Frank- REFERENCES CITED furt, Umschau, h. 3, p. 85-88. Bogli, Alfred, 1960, Kalklosung und Karren- Matley, C. A., 1926, The geology of the Cayman bildung: Zeitschr. Geomorphologie, Supp. bd. Islands (British West Indies): Geol. Soc. 2, p. 4-21. London Quart. Jour., v. 82, p. 352-387. Carriker, M. R., Smith, E. M., and Wilce, R. T., Neumann, A. Conrad, 1966, Observations on eds., 1969, Penetration of calcium carbonate coastal erosion in Bermuda and measurements substrates by lower plants and invertebrates: of the boring rate of the sponge, Cliona lampa: Am. Zool., symp., v. 9, p. 629-1020. Limnology and Oceanography, v. 11, p. 92- Cvijic, Jovan, 1924, The evolution of lapies: A 108. study in karst physiography: Geog. Rev., v. 1968, Biological erosion of limestone coasts, 14, p. 26-49. N.Y., in Fairbridge, R., ed., Encyclopedia of Doerr, Arthur H., and Hoy, Don R„ 1957, Karst geomorphology: New York, Reinhold Book landscapes of Cuba, Puerto Rico, and Ja- Co., p. 75-81. maica: Sci. Monthly, v. 85, p. 178-187. Panos, Vladimir, and Stelcl, Otakar, 1966, Vyvoj Doran, Edwin, Jr., 1954, Land forms of Grand Izolovanych Vrchu na Kube (Development of Cayman Island, British West Indies: Texas isolated hills in Cuba): Ceskoslovensky Kras, Jour. Sci., v. 6, p. 360-376. v. 18, p. 7-22 (English and Czech test). Emery, K. O., 1946, Marine solution basins: Jour. Parfenova, E. I., and Yarilova, E. A., 1965, Miner- Geology, v. 54, p. 209-228. alogical investigations in soil science (trans, Folk, Robert L., and Land, Lynton S., 1972, by A. Gourevitch and N. Kaner, Israel Prog. Mg/Ca vs salinity diagram: A frame of refer- Sei. Trans.): Akad. Nauk. SSSR Isdatel'stavo ence for crystallization of calcite, aragonite (1962), 178 p. and dolomite: Geol. Soc. America, Abs. with Pia, Julius, 1937, Die Kalklösende Thallophyten: Programs (Ann. Mtg.), v. 4, no. 7, p. 508. Stuttgart, Archiv Hydrobiologie Beihefte, v. Folk, Robert L., Roberts, Harry H., and Moore, 31, p. 264-328, p. 342-398. Clyde H„ 1971, Black phytokarst from Hell Purdy, Edward G., and Kornicker, Louis S., 1958, [abs.]: Geol. Soc. America, Abs. with Pro- Algal disintegration of Bahamian limestone grams (Ann. Mtg.), v. 3, no. 7, p. 569-570. crusts: Jour. Geology, v. 66, p. 96-99. Fremy, P., 1945, Contribution a la Physiologie des Revelle, Roger, and Emery, K. O., 1954, Chemical Thallophytes marins perforant et cariant les erosion of beach rock and exposed reef rock roches calcaires et les coquilles: Monaco, (Marshall Islands): U.S. Geol. Survey Prof. Annales l'lnst. Oceanog., v. 22, p. 107-144. Paper 260-T, p. 699-709. Fritsch, F. E., 1907, The role of algal growth in Silar, Jan, 1960, Kuzelovy Kras v Jizni Cine a ve the colonization of new ground and in the Vietnamske Demokraticke Republice (Conical determination of scenery: Geog. Jour., v. 30, karst in South China and in Vietnamese p. 531-548. Democratic Republic): Ceskoslovensky Kras, 1935-1945, The structure and reproduction v. 13, p. 147-162. of the algae: Cambridge, Cambridge Univ. Small, John K., 1930, Vegetation and erosion on Press, 2 v., 315 p. the Everglade Keys: Sci. Monthly, v. 30, p. Ginsburg, Robert N., 1953, Intertidal erosion on 33-49. the Florida Keys: Marine Sci. Gulf and Smith, J. F., Jr., and Albritton, C. C., Jr., 1941, Caribbean Bull., v. 3, p. 55-69. Solution effects on limestone as a function of Goddard, E. N., (Chm.), 1963, Rock-color chart: slope: Geol. Soc. America Bull., v. 52, p. Geol. Soc. America. 61-78. Golubic, Stjepko, 1969, Distribution, taxonomy, Sweeting, Marjorie, 1958, The karstlands of Ja- and boring patterns of marine endolithic maica: Geog. Jour., v. 129, p. 184-199. algae: Am. Zool., v. 9, p. 747-751. Udden, J. A., 1925, Etched potholes: Austin, Texas Hodgkin, E. P., 1970, Geomorphology and bio- Univ. Bull., v. 2509, 9 p. logical erosion of limestone coasts in Malaysia: Vaughan, T. Wayland, 1926, Species of Lepidocy- Malaysia Geol. Soc. Bull., v. 3, p. 27-51. clina and Carpentaria from the Cayman Islands Jones, R. J., 1965, Aspects of the biological weather- and their geological significance: Geol. Mag., ing of limestone pavement: Geol. Assoc. v. 82, p. 388-400. London Proc., v. 76, p. 421-434. Verstappen, H. Th., 1964, Karst morphology of Kaye, Clifford A., 1959, Shoreline features and the Star Mountains (central New Guinea) and Quaternary shoreline changes in Puerto Rico: its relation to lithology and climate: Zeitschr.

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Geomorphologie, v. 8, p. 40-49. Sci., v. 238, p. 112-116. Warthin, A. S., 1959, Ironshore in some West 1944, Potholes, pits, and pans: Subaerial and Indian Islands: New York Acad. Sci. Trans., marine: Jour. Geology, v. 52, p. 117-130. ser. 2, v. 21, p. 649-652. Williams, Paul W., 1972, Morphometric analysis Welwitsch, Friedrich, 1868, The Piedras Negras of polygonal karst in New Guinea: Geol. of Pungo Andongo in Angola: London, Jour. Soc. America Bull., v. 83, p. 761-796. Travel and Nat. Hist., v. 1, p. 22-26. Wentworth, Chester K., 1940, Ablation of snow MANUSCRIPT RECEIVED BY THE SOCIETY JANUARY under the vertical sun of Hawaii: Am. Jour. 26, 1973

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