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...karstlands developed the outer band the Lesser Antilles island arc, otherwise known as the Limestone Caribbees, and incorporate Anguilla, Antigua, Barbuda, Grande Terre Guadeloupe and nearby Marie Galante and Desirade. Perhaps the best known Lesser Antilles karst is that on Grande Terre, Guadeloupe, where the dry valleys were the object of a classic study by Lasserre (1954). Further south, the karst of Barbados has been studied in some detail (Fermor 1972; Day 1983) and that of Trinidad and Tobago has been the subject of preliminary studies (Day and Chenoweth 2004). The karst of Antigua is an important element of that island's landscape, but has been the subject of only cursory previous research (Day 1978a 1986).
Small carbonate island karst has been an important focus of recent studies in karst geomorphology, and it has been shown to differ intrinsically from that on larger islands (Mylroie and Vacher 1999; Mylroie and Jenson 2002; Vacher and Mylroie 2002; Mylroie 2004). Such distinctive island karst develops where young carbonate rocks on small islands or coasts have not been buried beyond the range of meteoric diagenesis and interact with fresh and saline groundwater that is affected by glacio-eustatic and tectonic changes in sea level. This diageneti-cally immature karst is termed eogenetic and its hydrogeology is defined by the Carbonate Island Karst Model (CIKM) (Mylroie and Vacher 1999; Vacher and Mylroie 2002; Mylroie 2004). Four types of carbonate islands (simple, carbonate cover, composite and complex) are recognized within the CIKM on the basis of a spectrum of carbonate/non-carbonate complexity. Much of the Lesser Antilles karst is adjacent to volcanic islands of the inner island arc and overlies older volcanic or crystalline basement rocks, thus providing a number of potential test cases of the universality of the CIKM.
Geology, soils and climate
The karst in Antigua is developed primarily on the Antigua Formation, which is composed mostly of limestones and marls with beds of calcareous mudstones and clays, plus small intrusions and localized lenses of siliceous reworked volcanic debris (Martin-Kaye 1959; Weiss 1994). Its age has been disputed, but it has been assigned to the Oligocene by Warneford (1949), Martin-Kaye (1959) and Weiss (1994). There are also limestones within the basal volcanic suite and in the Central Plain Group, but these are not extensive and are only locally karstified (Weiss 1994).
The stratigraphic succession on Antigua is the most complete in the Limestone Caribbees, illustrating declining episodic volcanism and intermittent but increasing marine transgression throughout the Oligocene (Weiss 1994). Rich coral communities formed as biostromes and patch reefs fringing or on a bank barrier in the lower part of the Antigua Formation, but the upper part is dominated by deeper water (30-100 m), island slope deposits of mostly fine-grained limestones (Martin-Kaye 1959; Weiss 1994). The Formation attains a thickness of at least 550 m and dips regionally towards the northeast at an angle of less than 15[degrees] (Figure 1), although local exposures suggest that the dips are more depositional than structural.
Limestones within the Antigua Formation are variable, both in terms of purity and mechanical strength. Petrologically, some micrites and biosparites are present, but the limestones are dominantly biomicrites, often highly fossiliferous and containing small amounts of quartz, smectite clays and feldspars (Day 1978a; Weiss 1994). A series of insoluble residue determinations (Day 1978a) showed a range of carbonate contents from 62.0% to 96.2% (mean 82.0%), and a mean acid insoluble residue content of 18.1% (s = 10.65, n = 10). Weiss (1994) characterized acid insoluble residues within the biomicrites as less than 10%, although noting that individual beds contained up to 40% of impurities. Acid insoluble residues in the reef limestones and the biosparites are generally low, but those in the marly limestones may approach 50% (Weiss 1994).
Surface compressive strength values, determined using a Schmidt Test Hammer (Day and Goudie 1977; Day 1980 1982), vary from 12 to 36 (mean 33.3) for fresh sections and from 27 to 41 (mean 37.8) for indurated surfaces. These variations in strength reflect both the conspicuous rhythmic sequences of island-slope limestone beds in the upper part of the Antigua Formation (Weiss 1994), and considerable variability in diagenetic cementation and surface case-hardening.
Soils are dominantly calcareous (up to 74% calcium carbonate), shallow (less than 0.3 m deep), extremely well-drained and stony grey-black rendzinas or mollisols. These are young, dark, calcimorphic soils, derived from weathering of the limestone and/or the build up of humus (Hill 1966; Watts 1987). Hill (1966) recognized two sub-types: (1) generally shallow calcareous clay soils in the north, which are productive in the deeper phases over the softer marls and, despite the high clay content, have good structure and high base saturation, with a pH of around 8.2; (2) a mix of shallow and deeper calcareous soils, mostly in the east, where the drier climate restricts productivity.
Natural woodland vegetation and the native fauna were decimated during the colonial period, and the limestone area is now characterized by a savanna-like grass and scrub vegetation with localized woodland (Beard 1948; Harris 1960; Loveless 1960; Caribbean Conservation Association (CCA) 1991; Lindsay and Horwith 1997). The dry scrub forests are dominated by Acacia spp. and Prosopis chilensis which were introduced and have become naturalized (FAO 2003). The karst area was used extensively for growing sugar cane and cotton until the second half of the twentieth century (Barker 1981; CCA 1991). Some areas of coastal mangrove persist within the karst, and ecosystem variety in the limestone area is enhanced by the presence of caves and by the seasonal drainage channels, streams, potholes and ponds (FAO 2003). Bats are the only native terrestrial mammal, with seven species documented (CCA 1991).
Antigua currently has a semi-arid tropical climate that is characterized by extreme rainfall uncertainty. Mean annual temperature in Antigua is 28[degrees]C and mean annual rainfall is 1110 mm (CCA 1991), with a wide range from 648 mm in 1930 to 1868 mm in 1899 (Martin-Kaye 1959). Rainfall totals generally increase with elevation, and most of the karst belt, which is the driest part of the country, receives less than 900 mm of rainfall annually. A marked dry season extends from January to April, with a wet season from about August to November. Evapotranspiration rates are relatively high, averaging from 87 mm/month in November to 143 mm/month in March, and annual potential evapotranspiration exceeds effective precipitation (McMillan 1985), with an average annual effective precipitation to evapotranspiration ratio of about 0.57 (UNCCD 2005).
Meteorological drought, defined by OAS (2001a) as when yearly rainfall falls below 80% of the annual average, is a recurrent problem for Antigua. Such meteorological drought is in turn associated with agricultural drought, where extended dryness reduces moisture in the soil to the point that plants and crops are adversely affected, and hydrological drought, where the reduction of water levels in wells, surface reservoirs and dams severely affect livestock, or where water must be rationed to households, hotels and other sectors.
[FIGURE 1 OMITTED]
Drought occurs every 5-10 years (Martin-Kaye 1956; Lewis 1984; CCA 1991; FAO 2004), with perhaps the most notable examples in 1930, 1948, between 1964 and 1969 and in 1983-84. Drought risk is generally highest within the karst belt (OAS 2001a). Conversely, heavy rains and flooding may also be ephemeral problems, particularly during hurricanes such as Hugo in 1989 and Luis in 1995. Hurricane damage may also disrupt water supplies.
Karst development in Antigua has occurred throughout a time of considerable paleoclimatic variability. Progressive global cooling culminated in Quaternary glaciations by about 1.6 m per year and glacial-interglacial fluctuations affected the Caribbean, with glacial climates cooling by up to 5[degrees]C, with increased aridity, and sea levels declining by 100 m or more (Graham 2003). These glacial periods would generally have impeded karst development, although carbonates which are now below sea level would have been exposed to subaerial karstification. The impacts of contemporary convergent, compressional and collisional tectonic activity on the Antigua Formation limestones have not been investigated in detail, but they are likely to have overprinted the effects of glacioeustacy (Multer et al. 1986).
Karst landforms
Karst landforms are developed where the Antigua Formation crops out over about 110 [km.sup.2] (nearly 40% of the total land area) in the northeast of the island (Figure 1). 'Karst development affected the Antigua Formation deposits in a major way, but limestone volumes in the Basal Volcanic Suite and the Central Plain Group are not large enough to yield a karst topography' (Weiss 1994, 26). Broadly, the karst area is an undulating limestone upland, which rises fairly steeply from the adjacent Central Plain to elevations over 100 m. There are no permanent streams or rivers. The dominant features are enclosed...
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