With a length of only about 240km, Jamaica cannot be considered a large island. It is also relatively ‘young’ geologically, the oldest rocks being only about 140myrs old. This might sound old enough, but contrast it with, for example, rocks in the islands of the Scottish Outer Hebrides, which are about 2,000myrs old. But Jamaica is nevertheless noteworthy in having a rich diversity of rock types and geological features, and it is rightly known for its high biodiversity, both on land and in the surrounding seas. To give one example, the 500 or more species of extant land snails make Jamaica a biodiversity ‘hot spot’ for these familiar molluscs. However, Jamaica should similarly be recognised as a geodiversity hot spot, with a range of geological and physiographic features, strata and fossils that make it an unusually fruitful focus for earth sciences research.
We could support our bold assertion by a detailed exposition with tabulation of principal features and comparison with similar-sized islands elsewhere, although such an approach would perhaps be better suited to a dry research journal. The potential for producing such a long, boring discursion is large and we intend to avoid the temptation to do so. Rather, we want to illustrate Jamaica’s geodiversity by reference to a dozen key features. These are available for inspection to anyone who is interested and which we will describe in two articles in Deposits. The choice of these features is personal – other geologists would choose a different suite of examples, although we would hope that certain of ours would be worthy of any list. For example, the aberrant rudist clams of the Jamaican Cretaceous are bizarre, diverse, often huge and of interest internationally (Mitchell, 2011). They are only omitted here because they deserve a detailed palaeontological exposition devoted to them alone. However, this is a personal listing and dissidents will, we trust, at least enjoy the selection even if they disagree with it.
It is not our intention to preface the chosen 12 with a detailed discussion of the geological history of Jamaica; such accounts are readily available elsewhere (for example, Draper, 1987a; Donovan, 1993, 2010; Robinson, 1994). But we do want to emphasise the importance of the history of the study of Jamaica’s geology. Our choice of features has been made with half an eye on the past and how the geology of the island was unravelled by a determined band of researchers, working alone or in groups, since the early nineteenth century. Further, part of the geodiversity of Jamaica is its ancient biodiversity; the island has a rich fossil record, one that deserves celebration.
The arrangement of our 12 sites is broadly from oldest to youngest. We include a simplified geological map as support for the main text (Fig. 1), but, perhaps more importantly, Figure 2 shows the distribution of our 12 sites chosen with the budding geotourist to Jamaica in mind. There is a brief glossary at the end of this article, applicable to both parts, which explains some of the more important technical terms used in the text (the first appearance of which appear in bold italics in the text). Those who need explanation of the geological timescale are referred to the numerous relevant websites. More relevant information for those planning to visit Jamaica can be found in Donovan et al. (1995).
Highlight 1. The Blue Mountains
The most mountainous area in Jamaica is the Blue Mountains, with the highest point being the Blue Mountain Peak at an elevation of 2,270m. The geology of this area was first explored in the mid 1820s by Henry (later Sir Henry) Thomas De la Beche, who produced the first geological map of Jamaica in his publication entitled the “Remarks on the geology of Jamaica” (1827). A more detailed geological survey of Jamaica, including the Blue Mountains, followed in the latter half of the century, when Lucas Barrett and James Sawkins in 1859 were appointed to the West Indian Geological Survey based in Jamaica. Their report on the geology of Jamaica, with an accompanying geological map, was finally published in 1869 by Sawkins after the death of Barrett in 1862. It was the work of Barrett (1860) and Sawkins that established the correct age of the rocks that make up the Blue Mountains. They realised that the oldest rocks were not Palaeozoic, as suggested by De la Beche, but were much younger Cretaceous rock. Since the publication of their seminal work, there has been a continuation of geological work in the Blue Mountains producing more detailed geological maps of the area over the past century.
The geology of the Blue Mountains is complex, especially the Cretaceous inlier, which makes up much of the region and where some of the oldest rocks in Jamaica are exposed (Wadge et al., 1982; Draper, 1987b). Recently, Hastie et al. (2010) attempted to simplify the geology by dividing the Blue Mountains into three contrasting suites of rocks. The oldest is in the south-western section of the inlier, where a suite of regionally metamorphosed rocks and minor serpentinites crop out. A second suite of rocks, which consist of basalts and gabbros, and deep water sedimentary rocks, are exposed in the south-eastern portion of the inlier (see below). To the north, there is third suite, comprised of volcanic arc-related rocks of lava flows, volcaniclastics and shallow-water sedimentary rocks.
The metamorphic deposits are the oldest in Jamaica and occur within fault-bounded blocks, where the rocks are divided into two groups: the Westphalia Schist and the Mt Hibernia Schist. The Westphalia Schist is in the upper amphibolite facies whereas the Mt Hibernia Schist is both blueschist and greenschist facies. The term ‘facies’ applies to certain pressure and temperature (P-T) conditions under which the rocks have been subjected during metamorphism, and the particular mineral assemblage that is associated with each of these suites of conditions. According to Abbott and Bandy (2008), the mineralogical changes in metamorphic facies, particularly the blueschist to greenschist transition in the Mt Hibernia Schist, are among the best examples of such metamorphism seen in the world. Because of the rugged topography of the Blue Mountains, there are few easily accessible outcrops with the exception of the Mahogany Vale area, where there are roadside exposures of Westphalia Schist, and in the western arm of the Morant River near Serge Island (Fig. 3B), where Mt Hibernia Schist is exposed. On the basis of the mineralogy and the mineral assemblages, these rocks are interpreted as having been buried deep in the Earth’s crust to temperatures estimated to be as high as 600°C and pressures of 7.5kbars before uplift to their present elevation above sea level. This took place in two phases: a major Late Cretaceous-Palaeocene uplift followed by a final late-Miocene event, representing the final emergence of the island of Jamaica.