Blue Holes: an Inappropriate Moniker for Scientific Discussion of Water-Filled Caves in the Bahamas
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BLUE HOLES: AN INAPPROPRIATE MONIKER FOR SCIENTIFIC DISCUSSION OF WATER-FILLED CAVES IN THE BAHAMAS Stephanie Schwabe* and James L. Carew** *Rob Palmer Blue Holes Foundation, 5 Longitude Lane, Charleston South Carolina, 29401 **Department of Geology and Environmental Geosciences, College of Charleston, Charleston, South Carolina, 29424 ABSTRACT nature of the calcarenites exposed along the frac- ture, the walls of the fractures seem to have col- By 1725, mariners from England had de- lapsed more extensively in localized areas to cre- scribed submarine cave entrances in the Bahamas ate separate small to large openings. These cave as “blue holes,” and in 1843, the term “blue hole” openings, although isolated from each other on first appears on sea charts from the Bahamas. As the surface, are hydrologically connected. the term suggests, what sailors saw from the sur- Whilst the term blue hole has been in the face was a dark blue colour that is in sharp con- literature for nearly three centuries, it is not being trast to the white/aqua blue waters of the shallow suggested that it be eliminated from the general banks. The term, “blue hole”, signified safe an- literature, just from scientific discussion of these chorage for visiting ships, and was not intended to karst features that have been explored, and for be a scientific description of what lies beneath the which the mode of origin can be identified. water surface. The first visual description of these flooded caves was given in 1950 using SCUBA. INTRODUCTION During the late 20th century numerous attempts were made to impose scientific meaning to the Historically, caves in the Bahamas, both term “blue hole”, without taking into account the above and below sea level, were already described varied morphologies and biogeochemistry of these in literature dating as far back as 1725 by an Eng- features. This has resulted in an inappropriate lish naturalist, Mark Catesby (Shaw, 1993). The definition for these caves. It is suggested here underwater marine caves, “blue holes”, were first that caves in the Bahamas should be classified by recorded on sea charts at about 1843. Early on, the nature of their development as follows (1) water-filled cave openings on land were simply horizontal caves, (2) fracture-guided caves, and referred to as “caves” however, during the late (3) vertical caves. Horizontal caves include the 20th century the term, “blue holes” began to be currently subaerial flank margin caves and banana applied broadly to many water-filled holes in the holes formed during past sea-level high stands, Bahamas. and horizontally extensive caves currently below For many years, ideas about the origin of sea level. Pit caves, and black holes fall within blue holes have been mixed with local superstition the vertical cave category. Black holes are and myth. Blue holes are sometimes referred to flooded vertical caves with no known lateral pas- by native Bahamians as “blowing” or “boiling” sages that support a massive bacterial population holes, a phenomenon supposedly created by a giving them their black colour. Black holes form mythical creature called “Lusca”; a belief still only in the interior stable regions of the larger Ba- shared by many Bahamians today. The “blowing” hamian islands, which allows for the development and “sucking” phenomena are largely caused by of distinct geochemical environments enhanced differences in water surface elevation across Ba- by microbial activity. Fracture-guided caves de- hamian platforms generated by tides, local wave velop along fractures that are sub-parallel to the action, and ocean currents (Whitaker and Smart, bank-margins. Because of the non homogenous 1997). 1 The 12th Symposium on the Geology of the Bahamas and other Carbonate Regions It is interesting that dry caves on land in subsurface environments not just as “blue holes,” the Bahamas have been described in recent litera- but with the same detail as has been applied to the ture in great detail and given descriptive names caves currently above sea-level. such as “flank margin caves”, “pit caves”, and “banana holes”. Yet, today, the non-cave-diving CLASSIFICATION OF CAVES scientists studying Bahamian carbonate platforms still refer to all the openings of caves that are Many of the flooded cave openings found flooded or below sea level as “blue holes.” Few on the Bahamian platforms are entrances to some researchers have attempted to define the caves by of the world’s most spectacular underwater cave width, depth, or passage morphology, because few systems. Their openings can be found on dry have ventured into these subsurface terrains. land, and among the shallow creeks, inland lakes, This practice of calling all water-filled and the shallow banks of the Bahamas. The cave entrances, and caves currently below sea caves, which have developed within the Bahamian level, “blue holes” is the equivalent of calling all carbonate platforms, can be laterally and verti- the currently dry caves on land “air holes”. The cally very extensive. Lateral cave passages can reaction to calling dry caves “airholes” would extend to several kilometres, and may range in fetch a smile for its absurdity. It would also be a depth from ten metres to thousands of metres, par- fair assumption that sailors from three hundred ticularly the fracture caves. Mylroie and Carew years ago would have had the same response to (1995, p. 60) stated that, “caves on carbonate plat- the suggestion that the water-filled cave openings forms fall into three main categories: vadose, on land should be labelled “blue holes”. Although phreatic, and fracture caves.” Here we suggest labelling land-based openings of flooded caves as that Bahamian caves can be broadly classified into “blue holes” is inappropriate, the idea that such three general types: (1) horizontal caves, (2) frac- caves have something in common with the off- ture-guided caves, and (3) vertical caves. Hori- shore caves is not wrong. zontal caves mostly begin as voids developed in More than 120 years ago, Agassiz (1884) the phreatic zone, especially in association with described how marine blue holes might have the fresh-water/salt-water mixing zone, and the formed. He assumed that the conditions for cave water table surface. Vertical caves, generally re- development in the aeolian strata were the same ferred to as pit caves (Mylroie and Carew, 1995), processes that formed potholes, boiling holes, ba- are developed primarily in the vadose zone, be- nana holes, caverns, caves, sinks and other such cause of dissolutional processes associated with openings on the shores of islands. He speculated meteoric water descending to the water table. that these features were formed by the action of Over time, features formed by these processes rain percolating through the aeolian rocks and be- may come to intersect one another to produce coming charged with carbonic acid. He believed composite, or hybrid, cave morphologies. Cate- that ocean holes formed under very much the gorization of such composite caves into one of the same conditions when the submerged bank was three proposed end-member cases may be diffi- above the high water mark. cult, but many caves are identifiable as predomi- Today, after about twenty plus years of nantly one type or the other. exploration and scientific study of these water- filled cave environments, results and observations Horizontal Caves appear to support, for the most part, Agassiz’s hy- pothesis that cave development in the Bahamian Horizontal caves, such as the subaerially environment, is similar for both dry and flooded exposed flank margin caves and banana holes caves. The purpose of this paper is to bring to- (Mylroie and Carew, 1990), are minimally devel- gether the science and observations generated oped horizontal caves. The currently flooded, from below the water level together with those predominantly horizontal caves initially formed as made in dry caves, and properly describe these flank margin caves in association with the 2 The 12th Symposium on the Geology of the Bahamas and other Carbonate Regions Figure 1. In the large diagram of Andros Island, on the east side of the island, it is possible to see that the cave openings(Paleoreef symbol) follow the fracture, as the fracture appears onshore and offshore. The smaller diagram in the lower left-hand corner is marked to generally show the direc- tion of the fractures on Andros, Grand Bahama Island, and New Providence Island (arrow). boundaries between two water masses. How- Fracture-Guided Caves ever, they have been subsequently modified by additional dissolution during episodes of subae- Fracture-guided caves have formed rial (vadose) exposure and renewed submer- within fractures that are aligned sub-parallel to gence in the phreatic environment. steep bank margins adjacent to very deep water, such as the Tongue of the Ocean (Fig.1), or The geological nature of the wall rock, (i.e., Agassiz Canyon located immediately offshore eolianites, coral deposits, and cement) also in- of the north side of New Providence Island. fluences the form of passage development, the Two other islands besides New Providence are rate of passage development, and may also in- known to have fracture-guided caves. Grand fluence the sites at which the caves breach to the Bahama has a fracture that runs the length of the surface. island from east to west just inland of the south shore. Andros Island has a north-south 150 km- long fracture along the eastern margin. 3 The 12th Symposium on the Geology of the Bahamas and other Carbonate Regions The circular cave openings (Paleoreef symbol) seen along the Andros fracture are the result of the non-homogenous nature of the rock exposed along the fracture. Because of the in- homogeneties the fracture has collapsed more extensively in localized areas to create separate small to large openings, which appear to be in- dividual caves.