The geology of islands

Islands are attractive places to visit, not just for geologists. Nonetheless, for us, they provide three advantages that favour collecting and research in the Earth Sciences. One of the attractions of an island is its small size in comparison with continents. The corollary of this small size is its relatively long coastline. Assuming that our island is not the mound of sand with a palm tree so loved by cartoonists, a long coastline indicates abundant exposures of rock, commonly well-exposed and accessible.

Second, because of their relatively small size, islands offer a limited possible area of outcrop. The island may be volcanic in origin, so you may have one (or a few) volcanoes and its deposits to map, log and sample, producing a self-contained study. A particular sedimentary deposit may be (probably will be) limited to a single island. If you want to determine the palaeontology or palaeoenvironments of this deposit, the only place it can be studied is on one island. To give one example (among many), the Middle Miocene Grand Bay Formation, exposed on the east coast of Carriacou in the Grenadines, Lesser Antilles, includes the only crinoid-rich deposits in the Caribbean Islands. I had been studying the few Antillean fossil crinoids for ten years until I went to Carriacou and the sum total of specimens I had collected until then could have rested, comfortably, in the palm of one hand. From Carriacou, I collected bags of crinoid-rich bulk sediment samples (Donovan and Veltkamp, 2001; Donovan et al., 2003).

Figure 1
Fig. 1. Outline map of the eastern Caribbean region, showing part of the Greater Antilles (Puerto Rico (PR) and the Virgin Islands (VI)), the Lesser Antilles (LA, including Antigua) and north-eastern South America (Venezuela (VE), Trinidad (Tr) and Tobago) (modified after Donovan & Harper, 1999, fig. 1A).

Third, the geology of islands is commonly unique. The geology of the Isle of Wight is similar to that of the adjacent mainland of the south of England. However, the island status of the Isle of Wight is ephemeral – during times of low sea level, such as during glaciations, the Solent is just a river valley or lagoon, and it may be possible to walk and wade from Portsmouth to Ryde. But not so for an oceanic island, which has no major landmass to which it might be linked, whatever the state of sea level. Jamaica’s origins were as part of a volcanic arc in the mid-Cretaceous, close to what is now the position of Central America. Plate tectonic processes have ‘smeared’ this volcanic arc eastwards. Jamaica became subaerially exposed as it ground past the Yucatan Peninsula in the Paleogene, sinking below the waves for about 40myrs in the mid-Cenozoic to form a Bahamas-like carbonate bank and eventually being uplifted for the past 10myrs or so. This geological history has been deciphered from the Jamaican stratigraphic succession (Draper, 1998). There are aspects that are similar to other Antillean islands, but no other island has a rock record and a geological history quite like Jamaica.

Figure 2
Fig. 2. Geological map of Tobago (after Frost & Snoke, 1989, fig. 2; copied from Jackson & Donovan, 1994, fig. 11.2A).

To illustrate these ideas, I have written thumb-nail sketches about the geology and geological history of two of my favourite islands and research sites. I have worked on the geology of seven Antillean islands – Trinidad, Tobago, Barbados, Carriacou, Antigua, Hispaniola and Jamaica – and each has ample diversity to satisfy any geologist. From these, I choose Tobago and Antigua, islands with strongly contrasting geological histories and much to recommend them to the geological ‘tourist’.

Figure 3
Fig. 3. Aspects of the geology of Tobago (images after Donovan & Jackson, 2010, figs 8, 10, 4, 5 and 7, respectively). (A) Rockly Bay Formation: a roadside locality between the type section and the lateral unconformity (Fig. 4, centre), showing the weakly lithified sandstone beds rich in Megabalanus shells and valves. (B) Rockly Bay Formation: sandstone with balanids and bivalves. The rock is derived from weathered basalt. This site is close to the unconformity (Fig. 4, left). (C) NCSG: Parlatuvier Formation at Englishman’s Bay, showing polyphase deformation features. Lens cap (centre) for scale. (D) TVG: tonalite (light coloured) intruded by dark mafic dykes, near Plymouth. (E) Rockly Bay Formation: cobbles eroded from the type section preserving clusters of Megabalanus shells (Fig. 4, right). Each side of the tape measure is 50mm.

Tobago

Tobago is one of the geologically most diverse of the Caribbean islands (Figs. 1-4). It is small, long and thin, oriented northeast-southwest, and is about 42km long by 12km at its widest. Snoke et al. (2001) interpreted the island as part of an allochthonous terrane (that is, originating in a place other than where it is now found) that forms the easternmost fragment of the Caribbean Mountain System of northern South America. The island can be divided into essentially three geologic provinces: one sedimentary, one igneous and one metamorphic (Jackson et al., 1988; Jackson and Donovan, 1994; Donovan and Jackson, 2010; and Fig. 2). Snoke et al. (2001) divided these provinces into five major units. These are:

  1. Metamorphosed –North Coast Schist Group (NCSG).
  2. Igneous – Tobago Volcanic Group (TVG).
  3. Ultramafic-felsic plutonic suite.
  4. Mafic (that is, containing dark coloured silicate materials) dyke swarm.
  5. Sedimentary – Tertiary and Quaternary sedimentary rocks.

The oldest of these units, the NCSG in the metamorphic province, is comprised of a succession of Early Cretaceous(?) or older, low-to-medium grade metamorphic rocks that crop out in the north-eastern third of the island. The Mesozoic metamorphic and igneous rocks that form the greater part of Tobago represent two stages of oceanic arc growth with an intervening phase of regional metamorphism. Before deformation and metamorphism of the NCSG, basaltic, andesitic and dacitic volcaniclastic rocks and minor lava flows were extruded onto a basement of oceanic crust. This first stage of arc development occurred in either the Late Jurassic or Early Cretaceous. The second stage of arc growth occurred during the mid-Cretaceous, and was preceded by penetrative deformation and lower greenschist facies metamorphism of the NCSG (Fig. 3C).


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 Stephen K Donovan (The Netherlands)


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