Stephen K Donovan (The Netherlands) and Trevor A Jackson (Trinidad)
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 above), but, perhaps more importantly, Fig. 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.
To the east of the metamorphic suite lies another interesting and unique association of rocks. These represent remnants of the early Caribbean ocean floor, which are in faulted contact with the Mt Hibernia Schist. These rocks consist of basalts and intrusive equivalents, together with thin layers of interbedded tuffs, red mudstones and chert called the Bath-Dunrobin Formation. The basalts were extruded in a deep oceanic environment as evidenced by their bulbous and pillowed appearance (Fig. 3A); they are estimated to have extruded at water depths greater than 4km.
Some of the best exposures can be seen in the Plantain Garden River and its tributaries. The most recent work in the Blue Mountains by Hastie et al. (2008) suggests that the basalts of the Bath-Dunrobin Formation were derived from the Caribbean Oceanic Plateau, an unusually thick ocean floor that evolved some 90mya and which currently underlies the Caribbean Sea. Where the oceanic plateau collided with existing land masses, such as existed in Jamaica during the Late Cretaceous to Palaeocene, fragments of it became accreted and juxtaposed to rocks of contrasting types.
Highlight 2. Above Rocks
The granitic rocks of Jamaica are confined mainly to Cretaceous inliers and represent a period of deep seated magmatism, which occurred during the evolution of Jamaica in the Late Cretaceous. There are several exposures of granitic rocks in central and eastern Jamaica but none as well preserved and as well studied as those the outcrop in the Above Rocks Inlier (Fig. 4), where the granite is intruded into older volcanic and sedimentary rocks (Reed, 1966; Ahmad et al., 1987; Jackson & Scott, 1994; Jackson et al., 1998).
The age of the Above Rocks granitic pluton was the subject of debate in the early half of the twentieth century, when it was thought to form part of a Basal Complex of Palaeozoic age (Matley, 1929). However, isotopic dating during the latter half of the century has revealed that these rocks are considerably younger than Palaeozoic, with ages ranging from 67 ±5 to 60.4 ±3.4myrs, that is, about latest Cretaceous to early Palaeocene. The pluton is a sub-volcanic stock, which once fed magma from within the Earth’s crust to an overlying volcano that was part of an emergent volcanic arc in the Late Cretaceous. Based on its mineral composition, the estimated depth of emplacement of the granitic magma was no greater than 10km, with temperatures of between 500 to 700°C.
In turn, the Above Rocks pluton was intruded by a host of igneous dykes of different composition to the host granite. Among these dykes is an unusual suite of igneous rocks described as lamprophyres, which are rare to the Caribbean region, but which signalled a shift in Late Cretaceous magmatism from compressional tectonics to extensional tectonics.
Some of the finest exposures can be seen in the Rio Pedro and its tributaries. Most research on Jamaican granitic rocks has been concentrated within the Rio Pedro drainage basin and near the Zion Hill Bridge at the border of the parishes of St Andrew and St Catherine (Figs. 2 and 4A, B). In the riverbed, there are not only fresh exposures of granite, but a contact metamorphic zone between it and older volcanic rocks.
Highlight 3. The Richmond Formation
The first geological map of any area of the Antillean islands was made in the mid 1820s by a plantation owner and, more notably, geologist Henry Thomas De la Beche (McCartney, 1977). De la Beche published his map of the geology of eastern Jamaica in 1827. This represented a major achievement for one man mapping difficult terrain on his own and shows a distribution of rock types that agrees well with more modern maps of the island. Where De la Beche failed was in the first attempt at intercontinental correlation of rock bodies, because no ‘rules’ for this had been determined at this early stage of geology’s development.
Despite the achievements of William Smith, it was still unknown if lithostratigraphy or biostratigraphy would work over great distances. De la Beche recognised rock types and associations similar to those of Europe and made direct correlations between them. Therefore, the ‘Coal Measures’ of De la Beche’s map – mainly beds of sandstones, siltstones and mudrocks with some rare coals – were correlated directly with the Upper Carboniferous Coal Measures of northern Europe, about 315 to 300myrs old.
What we now recognise is that, except locally, such similarity of rock type may not be indicative of a coeval deposition. Rather, it is a result of like environments of deposition producing similar rock types at different times. Instead, international geological correlation relies on fossils, any given species being limited to rocks deposited over a time range limited to the organisms existence. Fossils in De la Beche’s ‘Coal Measures’ show them to be much younger than originally thought, being formed about 65 to 53myrs ago.
These rocks are now called the Richmond Formation (Pickerill & Donovan, 1991; Pickerill et al., 1992) and were deposited in a range of shallow-to deep-water environments around the emergent massif of the Blue Mountains (Fig. 5B). Indeed, there are coals in some of the shallower water deposits, but these are rare compared with the sandstone and mudrock sequences originally deposited in deep water. Apart from the grandeur presented by these thick successions of sedimentary rocks – which are reminiscent of some sort of gigantic layer cake (Fig. 5A) – these rocks also contain locally abundant evidence of the spoor of ancient organisms, but (commonly) not their skeletons.
The Richmond Formation is notable for preserving a broad diversity of burrows and trails, and rare borings, made in the sediment at the time when it was the sea floor and had yet to be turned to sedimentary rock. Most of these trace fossils were the products of deep sea organisms, which lacked any hard, mineralised skeleton and have left no other fossil record. Further, some trace fossils are known to be produced by organisms with a mineralised skeleton, such as a group of echinoids called heart urchins. Although these remain unknown as skeletons from the Richmond Formation, their trace fossils are well preserved (Donovan et al., 2005).
Highlight 4. Pezosiren portelli and the Jamaican rhinoceros
The Jamaican record of fossil vertebrates is poor. There are terrestrial cave deposits, mostly dated as less than 100,000 years old, which yield bones of hutia (a rodent), bats, birds, lizards and frogs. Older rocks have yielded only sparse remains of (mainly) fishes. The one very notable exception is in the Eocene Guys Hill Formation, Yellow Limestone Group (about 50myrs old), at Seven Rivers in the parish of St James. Seven Rivers has yielded a fauna of aquatic vertebrates, such as fishes and a turtle, together with rarer terrestrial elements. The deposit is undoubtedly marine, so terrestrial organisms were probably washed in as carcasses. However, two notable elements, one from each environmental setting, are jewels in the crown of Jamaican vertebrate palaeontology (Donovan et al., 2007).
In 1855, Richard Owen of the British Museum described a bizarre skull, which had been sent to him from Jamaica, as Prorastomus sirenoides Owen. It was obviously a sea cow, related to living manatees and dugongs, but, unlike any other known species at the time, it had a straight snout, rather than one turned downwards for feeding on sea grasses. It was regarded as primitive, but further specimens of related sea cows were not forthcoming until the 1990s.
Excavations over the past 20 or more years have yielded a near-complete skeleton of a new and closely related species, Pezosiren portelli Domning, 2001 (Fig. 6A).
This is named after Roger Portell of the Florida Museum of Natural History in Gainesville. He found this deposit despite originally being misled by the geological map of this part of St James, which showed these rocks to be Upper Cretaceous; they are actually Eocene. Unlike other extant and fossil sea cows, Pe. portelli had hind legs. Therefore, it is an intermediate between its fully terrestrial ancestors and its fully aquatic descendants. That it was amphibious is demonstrated by features like its dense, heavy ribs, which would have acted in a manner analogous to a diver’s weight belt, and its retracted nostrils on top of the skull, allowing it to breathe while largely submerged. Like Pr. sirenoides, its snout was straight. Both species were probably amphibious, although most of the post-cranial skeleton of Owen’s species remains unknown.
Equally significant was the discovery of a jaw bone of a rhinoceros at Seven Rivers (Fig. 6B), again by Roger Portell (Domning et al., 1997). It comes from a time when Jamaica was close to the Yucatan Peninsula and the rhinoceros, Hyrachyus sp., could presumably have walked most or all of the way from North America to Jamaica. That rhinos are unknown from any younger rocks on the island is explained by Jamaica’s total submergence from about 40 to 10mya. By the time the island was again sub-aerially exposed, it had moved further to the east and Yucatan was too far away for its terrestrial fauna to walk to Jamaica once more. Therefore, colonisers of the newly-emergent island would have to swim, float or fly to get there. Jamaica’s modern terrestrial fauna is the product of colonisation over broad expanses of water during the past 10myrs.
The Seven Rivers site has also yielded diverse fossil marine invertebrates. Most particularly, it is one of many localities in the Yellow Limestone Group to yield the giant snail, Campanile trevorjacksoni Portell & Donovan, 2008 (Fig. 6C). This is a true monster, probably reaching at least 440mm in length in complete, mature individuals. Related species were diverse and widespread at this time. The Jamaican species has been known since the 1820s, when De la Beche illustrated a specimen and noted its similarities to giant snails from the London Clay and coeval deposits in France. This was the first time that fossils were used to correlate rock units between continents, although De la Beche had his doubts about the results and preferred to use similarities of rock sequences (see above).
However, the Jamaican giant snail had to wait until 2008 to be formally named, although it was known to be a species of Campanile for many years. This is because almost all specimens are preserved as internal moulds, the shell having dissolved away. Without the external features of the shell, comparison could not be made with other species of Campanile. It was not until Roger Portell found impressions of the external surface of the shell (an external mould, albeit not from Seven Rivers) that he was able to determine its unique morphology by taking rubber casts.
Highlight 5. Low Layton volcanics
The last recorded episode of volcanic activity on Jamaica occurred approximately 10mya (Wadge, 1982; Jackson & Smith, 1982). At that time, a large portion of the current island was below sea level. Evidence of this event can be seen at Low Layton, located along the north coast near to Hope Bay (Fig. 2), where a sequence of alternating pillow lavas, pillow breccias and hyaloclastics outcrop (Fig. 7). These rocks have a basaltic composition, with a texture that is generally fine-grained and vesicular. The breccias are interbedded with lavas and contain angular fragments of basalt in a yellow-brown matrix of altered volcanic glass. The presence of abundant glass in these rocks is due to the rapid cooling of the basaltic magma coming into contact with seawater. Based on the average vesicle size of these basalts, it is estimated that they erupted in water depths of about 100m (Roobol, 1972).
The Low Layton volcanic rocks form part of an east-west ridge about 2.5km in length, which intersects the north coast and outcrop in an area of no more than 3km2, with the total volume of volcanic rocks above sea-level being about 90 million cubic metres. These volcanic rocks occur within a succession of foraminiferal limestones of the Lower Coastal Group that were deposited early in the Late Miocene. This agrees well with a potassium-argon radiometric date of 9.5 ±0.5myrs for the volcanic rocks. The eruption of the Low Layton basalts coincided with major changes in sea-floor spreading that were taking place in the Cayman Trench to the north of the island.
SKD thanks Niko Korenhof (NCB – Naturalis, Leiden, The Netherlands) for scanning his original colour slides, which are reproduced as certain of the figures in these articles. Prof Grenville Draper (Florida International university, Miami) kindly provided the image in Fig. 3B.
|Accretion: The growth of continents or islands by combination with other, smaller land areas.|
|Amphibolite facies: An assemblage of metamorphic minerals that indicate conditions of moderate to high pressures (greater than 3kb) and temperatures in the range of 450 to 700°C.|
|Chitons: A group of molluscs, elliptical in outline and commonly found on hard substrates, with a shell composed of a series of eight valves that give them the common name of ‘coat-of-mail’ shells.|
|Contact metamorphic zone: mineral alteration zone produced as a result of intrusion of magma that has come into contact with older surrounding rock.|
|Elephant tusk shells: A group of molluscs, the Scaphopoda, with an elongate, tubular shell, open at both ends, which, in many species, superficially resembles the tusk of an elephant.|
|Granodiorite: A light-coloured, coarse-grained plutonic rock that contains abundant quartz and feldspar, and where plagioclase feldspar exceeds orthoclase feldspar. The mineral grains of gabbros, granites and granodiroites are big enough to see with the naked eye.|
|Greenschist: Metamorphic rocks (schists) containing green minerals such as epidote, chlorite or actinolite.|
|Hyaloclastics: Glassy volcaniclastic rocks produced by the reaction between magma and water or ice.|
|Lamprophyre: An igneous rock that contains large crystals of dark-coloured minerals, known as phenocrysts (pronounced feno-crysts) in a groundmass of light and dark minerals.|
|Parasitic fold: A fold that is on the limb or hinge of a larger fold.|
|Pillow lavas and pillow breccias: Pillow lavas are produced when basalt lava is erupted in an aqueous environment. The rapid cooling results in a bulbous shaped lava, which can sometimes become fragmented to form pillow breccias.|
|Potassium-argon radiometric dating: A method used in the determination of the absolute age of a mineral or a rock by measuring the amount of radiogenic argon 40 to potassium 40 and applying a known radioactive decay constant.|
|Serpentinite: A dark coloured rock containing abundant serpentine minerals, such as antigorite, chrysotile and lizardite.|
|Stock: An igneous intrusive body that is less than 104lm2 in surface exposure.|
|The parts of this article comprise:|
|Jamaica’s geodiversity (Part 1): Introduction and some older highlights (Cretaceous to Miocene)|
|Jamaica’s geodiversity (Part 2): Highlights from the Neogene|
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