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Smith ScholarWorks Geosciences: Faculty Publications Geosciences 1987 Coral Reef to Eolianite Transition in the Pleistocene Rocks of Great Inagua, Bahamas Brian White Smith College H. Allen Curran Smith College, [email protected] Follow this and additional works at: https://scholarworks.smith.edu/geo_facpubs Part of the Geology Commons Recommended Citation White, Brian and Curran, H. Allen, "Coral Reef to Eolianite Transition in the Pleistocene Rocks of Great Inagua, Bahamas" (1987). Geosciences: Faculty Publications, Smith College, Northampton, MA. https://scholarworks.smith.edu/geo_facpubs/99 This Conference Proceeding has been accepted for inclusion in Geosciences: Faculty Publications by an authorized administrator of Smith ScholarWorks. For more information, please contact [email protected] Proceedings of the 3rd Symposium on the GEOLOGY of the BAHAMAS June1986 edited by · H. Allen Curran CCFL Bahamian Field Station CORAL REEF TO EOLIANITE TRANSITION IN THE PLEISTOCENE ROCKS OF GREAT INAGUA, BAHAMAS Brian White & H. Allen Curran Department of Geology Smith College Northampton, Massachusetts 01063 il REPRINTED FROM: H. Allen Curran (ed.), 1987, Proceedings of the J rd Symposium on the Geology of the Bahamas: San Salvador, CCFL Bahamian Field Station, p. 165-179. Cover photo: Diploria strigosa, the common brain coral, preserved In growth position at the Cockburn Town fossil coral reef site (Sagamon age) on San Salvador Island. Photo by Al Curran. CORAL REEF TO EOLIANITE TRANSITION IN THE PLEISTOCENE ROCKS OF GREAT IN AGUA ISLAND, BAHAMAS Brian White and H. Allen Curran Department of Geology Smith College Northampton, Massachusetts 01063 ABSTRACT finer-grained and better sorted eolianites with prominent rhizomorphs. In places, these Great Inagua Island, the southernmost of are cl~arly part of parabolic-like, lobate the chain of Bahamian islands, lies approxi­ dunes with east-west axes. mately 100 km northeast of the eastern tip The presence of fossil corals and subtidal of Cuba. This paper reports the results of calcarenites in modern intertidal and reconnaissance geologic field work conducted supratidal exposures demonstrates a former on Great Inagua Island in January, 1985, and high stand of sea level, most likely of of more detailed observations made in Sangamon age, The shallowing-upward January, 1986, near Matthew Town, between sequence from subtidal marine to dunal South West Point and Devil's Point. environments resulted from sea regression. Excellent exposures of a fossil coral reef Lateral facies changes along this part of the occur along the coast in the vicinity of coast of Inagua indicate progressive growth Devil's Point, and scattered exposures of in of the island during the Late Pleistocene situ fossil corals, especially Montastrea owing to emergence, spit extension, and annularis and Diploria strigosa, are present migration of wind-blown dunes. farther south along the coast to just north of Matthew Town. Beds containing the fossil INTRODUCTION corals interfinger with and are overlain by medium to coarse, shelly, oolitic calcarenites Great Inagua is the southern-most island of shallow subtidal ongm. The subtidal in the Bahamas Archipelago (Index Map 1) facies rocks contain well-developed trough and lies approximately 100 km northeast of cross beds, probably formed by tidal and the eastern end of Cuba and about the same longshore currents, and steeply dipping, distance north of the northwest coast of planar tabular cross beds, likely formed Haiti. With an area of 1,544 sq. km, Inagua during storms, perhaps as washovers. is the third largest Bahamian island, In places, trace fossils are prominent in exceeded in area only by Abaco and Andros. these shallow marine, carbonate sands, It is approximately 75 km long in an including the shafts and tunnels of Ophio­ east-west direction and from 20 to 35 km mor pha sp. and short, vertical burrows from north to south. Matthew Town, located assignable to Skolithos linearis. The beds in the southwest of the island, has an that contain trace fossils probably formed average annual rainfall of approximately 60 seaward of the swash zone where the sandy cm with a pronounced rainy season from bottom was stable enough to support lined September to December arid a lesser one in burrow systems. May and June. Thunderstorms forming over The vertical sequence· continues upward the island are blown westward by the with better sorted, somewhat finer-grained, prevailing easterly trade winds, making the skeletal, oolitic calcarenites that formed as western .end of the island wetter than the overstepping beach sands. These rocks are eastern parts. Inagua lies on the western laminated with long, low-angle cross beds Atlantic hurricane track, and has an dipping towards the west along much of the approximately 10% chance of being struck coast. However, south of Matthew Town by a hurricane in any given year. Mean some of the beach bedding dips east and monthly temperatures at Matthew Town appears to represent an ancient, narrow sand range from a low of 24°C in February to a spit with both east and west facing beaches. high of 28°C in July (weather data from Upward there is a transition to even anon., 1985). 165 The major industry on Inagua is the 0 20 MAN OF WAR BAY ~ extraction of salt from sea water by solar Km evaporation in salt pans, th us taking DEVIL'S ~---1""°'·,e .. ~--~--"'·-,c·.••"· POINT.... ,,/ ... advantage of the hot, dry climate. The island GREAT is also notable for the large Bahamas ~INAGUA 1 National Trust wildlife sanctuary where the ~- _ 21~0 sole remaining nesting area in the Bahamas of the American flamingo (Phoenocopterus /' •, . ruber) is protected. Snit Although Inagua has undoubtedly been ··....< Pans . visited by many geologists, the only recently ' published work on the geology of the island ' is in the form of two abstracts by Mitchell (I 985a,b}. In these Mitchell briefly mentions NORTH the presence of Pleistocene fossil coral reefs ATLANTIC and evidence for shoaling upward to major OCEAN dune sequences. We first visited Great Inagua in January, 1985, during an oceano­ graphic cruise through the southern Bahamas, when we studied the Pleistocene beach and dune facies exposed along the coast from Matthew Town south to the lighthouse. We again visited the island in January, 1986, as part of a scientific expedition to Great Inagua sponsored by the CCFL Bahamian Field Station. At that time we made I detailed studies of the coastal exposures southwest, .west, and northwest of Matthew Town (Fig. I) and preliminary observations of a well-exposed fossil reef that crops out for several kilometers around Devil's Point. This fossil reef was first made known to us by Steve Mitchell (personal communication) .-----~ Road and rediscovered by Jim Carew and John A - • Moaaured Goologlc Soctlon Mylroie on the 1986 expedition. 0 3 FIELD CHARACTERISTICS OF Kllomotor:1 SOUTH WEST POINT THE LIMESTONE FACIES Background Fig. I. Map of the southwestern coast of Great Inagua Island showing locations of We have studied, at least at the measured sections and of field sites men­ reconnaissance level, all of the coastal rock tioned in the text. exposures of the southwestern part of Great Inagua from the northern limit of exposure in Man of War Bay to the lighthouse south of Matthew Town. This paper is based stones include in situ coral reef. coral largely on detailed analysis of three vertical rubble, subtidal shelly sand bottoms both sections, located as shown in Figure 1, and mobile and immobile, beach, and dune. strata in their immediate vicinity. Some Although not all facies are always present at observations and illustrations from the a given locality, those that are exposed Devil's Point fossil coral reef and coastal always represent a vertical shallowing-up­ exposures between the measured sections ward sequence. To avoid repetition, we will also are included. The rocks, apart from thin describe each facies in general and later hematitic paleosols, are entirely limestones. examine the vertical relationships and Depositional facies recorded in the lime- differences between the measured sections. 166 0 0 Coral Facies more mobile and contain many cross beds and no trace fossils. These latter beds are Introduction. In modern coral reefs, named the cross bedded, subtidal sand facies. colonial scleractinian corals commonly form Burrowed, subtidal sand facies. This facies massive rocky structures. Strong waves and is characterized by the presence of trace currents may topple, overturn, or collapse fossils belonging to the ichnogenera these coral structures without actually Ophiomorpha (Fig. 3b) and Skolithos (Fig. moving them any significant distance from 3c). In Section A, a particularly good their place of growth. Coral debris commonly exposure formed by recent excavation (see accumulates in the proximity of such reefs for example Fig. Sa), trace fossils are following breakage of the in place corals and especially well displayed. Sko/ithos linearis transport of the fragments a short distance specimens occur as rather short burrows, up away from their source. These two forms of to 4.2 cm long and averaging 2 mm in coral occurrence can be distinguished in the diameter. The shafts and tunnels of Ophio­ fossil record of Pleistocene coral reefs, and morpha sp. are abundant and form an we use the term coralstone for the former irregular box work pattern. · In most instances and coral rubblestone for the latter type of the burrow walls have been eroded away, occurrence (Curran and White, I 985). and it is the more lithified burrow sediment Coralstone facies. For approximately 7 fill that has been eroded in relief on the km along the north and west-facing shores excavation face. However, some large shafts around Devil's Point, a fossil coral reef is and tunnels, up to 3 cm in diameter, with exposed. Species of the genera Acropora, thick, micritic walls displaying a knobby Dip/aria, Montastrea, and Porites are exterior surface are preserved, confirming an extremely well-preserved, many of them in Ophiomorpha sp.

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