A field guide to Barbados (Part 4): Bridgetown and the South Coast
Stephen K Donovan (The Netherlands)
The area considered in this part of the guide is outlined in Donovan & Harper (2010, fig. 1C) and Fig. 1 of this article. As in other articles in this series, the starting point is Bridgetown.
Stop 1: The Barbados Museum
The Barbados Museum and Historical Society was founded in 1933. Its museum occupies St Ann’s Garrison, a nineteenth century British military prison. It is situated in the parish of St Michael, southeast of the central part of Bridgetown, behind the Garrison Savannah racetrack. The museum has displays covering many aspects of Barbadian history and life, including natural history, prehistory and maps. The library is an important research resource, containing 5,000 books, monographs and articles on the culture and natural history of the island. Articles about the island’s natural history, culture and history are published in the annual Journal of the Barbados Museum and Natural History Society.
Stop 2: South Point Lighthouse (59º 31’ 47” W 13º 02’ 38” N; Figs. 1 and 2)
From the Barbados Museum, drive east through Hastings, Worthing and the Dover area. Stay close to the coast by following signs through Miami Beach and Atlantic Shores. Park near South Point Lighthouse.
Two sea cliffs, cut in reefal limestones – one ancient, one modern – are exposed in the South Point area. South Point is within the outcrop area of the Lower Coral Rock (Poole & Barker, 1983). South Point Lighthouse is built on limestones of an Acropora palmata (Lamarck) sedimentary facies, dated as 125,000 years old (Humphrey & Matthews, 1986, p. 97).
Today, the colonial scleractinian coral, A. palmata, is commonly found in shallow water, high-energy reefal environments; it occurs to 16m water depth, but is commonest down to 10m (Wood & Wood, 2000, p. 42; Woodley & Robinson, 1977, p. 22). On the shore, modern sea cliffs are formed of younger limestones, dated as 82,000 years in age (Humphrey & Matthews, 1986, p. 97). Therefore, these cliffs were cut between 125,000 and 82,000 years ago, attesting to the rapid rate of uplift of the island, considered to have been 300mm for each 1,000 years over the past 700,000 years (Humphrey & Matthews, 1986, p. 87).
The limestones forming these modern sea cliffs are in an Acropora cervicornis (Lamarck) sedimentary facies. Today, A. cervicornis is a colonial scleractinian coral, commonly found in shallow water, high-energy reefal environments, down to 40m (Wood & Wood, 2000, p. 42). A notable feature of the limestones in this area is the preservation of scleractinian corals in their original aragonitic state.
This is explained by Humphrey & Matthews (1986, p. 97) by both the low annual rainfall in this area and “… a local high in the Tertiary aquiclude [that] shields the area from subsurface flow of fresh water.” That is, extensive water flow within the limestone, from higher points of the island to the coast, is blocked in this area, due to the limestone being draped over an elevated portion of the underlying Tertiary deposits. This means that water had to take an alternative route when it could not flow uphill. The local high is represented at the surface by the Christ Church Ridge, an E-W elongate hill (Fig. 2).
Stop 3: Foul Bay (59º 26’ 02” W 13º 05’ 58” N; Figs. 1 and 2)
Return along the coast road in a north-westerly direction. At Oistins, turn right and drive northeast to join the ABC Highway. Just north of Oistins, the road climbs the First High Cliff and onto the Middle Coral Rock. Follow the ABC Highway to the Grantley Adams International Airport and then follow minor roads through St Martin’s and Rices. Do not turn left towards Crane, but rather drive straight on, then turn left and park near the coast at Foul Bay.
The limestone cliffs on the coast are part of the Middle Coral Rock (Poole & Barker, 1983). Humphrey & Matthews (1986, pp. 97-98) considered these limestones to be the same age as those in the old sea cliff beneath South Point Lighthouse (Stop 2), but to represent an A. cervicornis sedimentary facies. Although this is an area of low rainfall, there is a high flow of fresh water through the limestones from the area of the St George Valley, between the Christ Church Ridge and the Second High Cliff (Fig. 2). This fresh water dissolved scleractinian corals and other aragonitic fossils, such as gastropods, producing a so-called ‘Swiss cheese’ fabric (Humphrey, 1997, figs 11-14, 11-15). The matrix has re-crystallised to a well-lithified, low magnesium calcite.
Stop 4: Woodbourne Oilfield (approx. 59º 29’ W 13º 06’ N; Figs. 1 and 3)
Return towards the Grantley Adams International Airport. Take the ABC highway west, soon turning left towards Fairview and St Patrick’s. At St Patrick’s, turn right (northeast) on the H6 Highway. A little further on the highway, in the Woodbourne area, the road passes through fields of sugar cane with nodding donkey, unmanned oil wells. If you can, you should stop and examine one of these servants of the oil industry from close quarters.
The following is based mainly on the accounts Gordon et al. (1996, pp. 112-113), Speed et al. (1991) and Barker et al. (2002, pp. 101-102, fig. 14). The Woodbourne Oilfield is in the Woodbourne Trough, a NE-SE trending depression in the subterranean surface of the basal complex. It is the main oil producing area of Barbados. It was discovered in 1966 and is currently being exploited by the Barbados National Oil Company. Of over 280 wells that have been drilled, about 90 are flowing or pumping at any time in the Woodbourne area of the parishes of Christ Church and St Philip. Currently, average daily production is about 1,200 barrels of oil a day; total production has been over nine million barrels. Oil is refined at the Mobil Oil refinery at Graves End, in the parish of St Michael.
The source rocks are thought to be organic-rich mudrocks of the basal complex, which have been buried to over 7km, the minimum depth for thermal maturation. This unusually deep burial is because accretionary prisms have low geothermal gradients, about one third of the global average (Speed et al., 1991, p. 338). In this situation, reservoir rocks have been folded and thrust over the quartz sandstones of the basal complex and the overlying Woodbourne Intermediate Unit (Fig. 4). The cap rocks are mudrocks and marlstones(?) of the basal complex and Woodbourne Intermediate Unit, and the relatively undeformed Oceanic Nappes (Fig. 4). The current reservoirs probably evolved during deformation in the late Neogene (Fig. 4).
Stop 5: Chapel Quarry (approx. 59º 29’ 15” W 13º 8’ 8” N; Fig. 1)
Continue on the H6 Highway to Six Cross Roads and then turn northwest (second exit from roundabout) towards St Philip’s Church and Church Village, within site of the Second High Cliff. The Chapel Quarry is to the east of the angle formed between this road and the H4B Highway. Permission from the owners is required if the quarry is to be visited.
The Chapel Quarry produces cut limestone blocks, unlike the Arawak Cement Quarry in northern Barbados (Donovan & Harper, 2011, stop 1). Its limestones are “… well graded, good, soft quality and not with a great deal of cementation or recrystallisation which tend to make the rock too hard” (Gordon et al., 1986, p. 114; Barker et al., 2002, p. 106). Blocks are produced in various sizes and are used in building construction (including foundations), as a decorative facing stone, and for walls and groundsills. Exposure to the atmosphere leads to discolouration due to fungal growth, although blocks may be treated as a preventative measure (Gordon et al., 1986, p. 114).
References
Barker, L., Gordon, J., Brathwaite, A. & Chaderton, N. 2002. A Bajan potpourri. In: Field Guides, 16th Caribbean Geological Conference, June 16th-21st, Barbados, West Indies: 99-116. Government Printing Department, Bridgetown, Barbados.
Donovan, S.K., including a joint contribution with Harper D.A.T. 2005. The geology of Barbados: a field guide. Caribbean Journal of Earth Science, 38 (for 2003): 21-33.
Donovan, S.K. & Harper, D.A.T. 2010. A field guide to Barbados (Part 2): Coastal geology of southeast Barbados. Deposits, 24: 28 – 33.
Donovan, S.K. 2011. A field guide to Barbados (Part 3): northern Barbados. Deposits, 25: 28 – 30.
Gordon, M.J., Johnson, J.D., Payne, P.B. & Mottley, W. 1986. Modern cultural aspects of Barbados’s geology. In: 11th Caribbean Geological Congress Barbados — 1986. Field Guide, Barbados, July 1986: 105-125. Government Printing Department, Bridgetown, Barbados.
Humphrey, J.D. 1997. Geology and hydrogeology of Barbados. In: Vacher, H.L. & Quinn, T.M. (eds), Geology and Hydrogeology of Carbonate Islands: 381-406. Elsevier, Amsterdam.
Humphrey, J.D. & Matthews, R.K. 1986. The Pleistocene coral cap of Barbados. In: 11th Caribbean Geological Congress Barbados — 1986. Field Guide, Barbados, July 1986: 85-105. Government Printing Department, Bridgetown, Barbados.
Mesolella, K.J., Matthews, R.K., Broecker, W.S. & Thurber, D.L. 1969. The astronomical theory of climatic change: Barbados data. Journal of Geology, 77: 250-274.
Poole, E.G. & Barker, L.H. 1983. The Geology of Barbados. 1:50,000 sheet. Directorate of Overseas Surveys and Government of Barbados, St Michael.
Speed, R.C., Barker, L.H. & Payne, P.L.B. 1991. Geologic and hydrocarbon evolution of Barbados. Journal of Petroleum Geology, 14: 323-342.
Wood, E. & Wood, L. 2000. Reef Fishes Corals and Invertebrates of the Caribbean including Bermuda. New Holland Publishers, London: 144 pp.
Woodley, J.D. & Robinson, E. 1977. Third International Symposium on Coral Reefs. Field Guidebook to the Modern and Ancient Reefs of Jamaica. University of Miami, Florida: 33 pp.