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 Black Phytokarst from Hell, Cayman Islands, British West Indies ABSTRACT the whole surface, actively boring and dis- solving 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/7/2351/3433327/i0016-7606-84-7-2351.pdf by guest on 24 September 2021 2352 FOLK AND OTHERS GRAND CAYMAN f 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/7/2351/3433327/i0016-7606-84-7-2351.pdf by guest on 24 September 2021 BLACK PHYTOKARST FROM HELL, B.W.I. 2353 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 character- ized 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 solu- tion 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 gravita- tional 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). Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/7/2351/3433327/i0016-7606-84-7-2351.pdf by guest on 24 September 2021 2352 FOLK AND OTHERS 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) ob- served 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.