Stephen K Donovan (The Netherlands) and Trevor A Jackson (Trinidad)
This is the second and concluding part of our introduction to Jamaica’s geodiversity. Here, we are concerned with more Neogene ‘highlights’ dating from the Middle or Late Miocene, about 10mya, when the island became, once again, sub-aerially exposed. The glossary provided in Part 1, as well as the maps (Donovan & Jackson, 2012, figs 1 and 2), are also relevant to this article and first appearance of the relevant terms in the text are highlighted in bold italics. Highlights 1 to 5 were discussed in Part 1 and 6 to 12 are described below.
Highlight 6. Wait-A-Bit Cave
Jamaica is a land of caves and sinkholes (Fincham, 1977). About two thirds of the rocks exposed at the surface of the island are limestones, which are soluble in acidic groundwaters, that is, those that are more or less rich in dissolved CO2. The percolation of these waters ‘excavated’ extensive cave systems throughout Jamaica, mainly by dissolution, since the island was sub-aerially exposed about 10mya (Miller, 2004). Wait-a-Bit Cave, south of Green Town in the parish of Trelawny (Fig. 1), is unusual among these myriad caves for reasons apart from its euphonious name.
The cave is dissolved into limestones of the Stettin Formation of the Yellow Limestone Group (Lower Eocene and deposited about 50mya). Our common perception of a cave is of an opening in a hillside with no ‘backdoor’; that is, if water flows into a cave through the mouth, it commonly seeps away into the surrounding rock, entering narrower conduits in the cave system. In contrast, Wait-a-Bit Cave has a river flowing through it, with both an entrance and an exit (and even a ‘side door’).
It is small river cave with a short, single-conduit, sinuous passage about 20 to 25m long, up to 5m high and 5 to 10m wide (Miller & Donovan, 1996). The entrance is a small, sub-horizontal rift at the base of a 15m high limestone cliff. The cave exit (near to section E-E’ in Fig. 1) is a much larger, rectangular passage, downstream of which is a 15m-high cliff with a prominent overhang more than 10m deep and 8 to 8.5m high. The cave also possess of a secondary, upper side entrance (C-C’) about 6 to 7m above the level of the stream, forming a short, steeply-sloping passage down to the `main’ stream conduit. The rectangular section of Wait-a-Bit Cave (Fig. 1, sections D-D’ and E-E’) is typical of the Yellow Limestone Group, indicating the strong control on form exercised by bedding and joint planes, both of which are well-developed in these rocks.
The cave was probably initiated by a surface stream seeping between two beds of limestone, which then dissolved and cut its way into the rock. Conditions may have been wetter when the cave was first initiated, with a higher water table. Dissolution produced a passage whose shape was strongly controlled by the sub-horizontal beds of limestone.
The succession of the Stettin Formation exposed at Wait-a-Bit Cave can be divided, broadly, into two types of limestone. What were originally deposited as lime-rich muds and silts were later transformed by the formation of massive, well-cemented carbonate nodules. Infaunal organisms in the shelly fauna include burrowing bivalves and the burrowing heart urchin Schizaster hexagonalis Arnold & Clark.
The giant snail Campanile trevorjacksoni Portell & Donovan is found in the same rocks and was probably a shallow-water algal feeder. The presence of ribs of a dugong (sea cow; see Donovan & Jackson 2012, Highlight 4) suggests that the muds at the sediment-water interface were stabilised by seagrasses, upon which it would have fed. The inference that seagrasses were present suggests that this unit was deposited in relatively shallow water, less than 50m and probably no more than 25 to 30m deep.
These nodular limestones are overlain by a series of limestone beds comprised mainly of sand-sized grains. The dominant echinoids in these beds are Eurhodia matleyi (Hawkins) and tiny sand dollars. Eurhodia matleyi would have been, at most, a shallow burrower and may have lived on the sediment surface. One bed is an E. matleyi conglomerate, formed of numerous echinoid tests washed together haphazardly, suggesting that these beds were deposited at least above storm wave base. What were presumably live tests of this species had to be concentrated by an energetic event to make such a deposit. If these echinoids were indeed buried alive, as seems probable, their occurrence in this thin unit would support the inference that they were only weak burrowers and could not make their escape from even such a shallow tomb.
Highlight 7. The oyster beds at Farquhar’s Beach
Farquhar’s Beach is a narrow strip of black sand sandwiched between the Caribbean Sea and the imposing bulk of Round Hill in the south of the parish of Clarendon. Round Hill itself is composed of Miocene limestones of the Newport Formation, White Limestone Group. Resting on the seaward side of this hill are two younger, rather different rock units that are beautifully exposed in cliff sections.
The upper unit, the Farquhar’s Beach red beds, is geologically young, being less than 100,000 years old, but several metres thick nonetheless. It is comprised of a red matrix containing white fragments, looking like some strange pudding. These are pebbles and cobbles of ancient white limestone derived from the physical erosion of Round Hill. The red sandy matrix enclosing these clasts includes fossils of terrestrial organisms like land snails and casts of plant roots. The red colouration is also indicative of terrestrial conditions – an oxidising environment, which ‘rusted’ the iron in the sediment. Only at the very bottom of these beds is there evidence of a marine influence, such as corals and clam borings (Donovan et al., 2010), but the overlying bulk is interpreted as a series of alluvial deposits in a more or less arid, terrestrial setting (Donovan & Miller, 1999).
However, the underlying beds are even more intriguing. These are the Mio-Pliocene Round Hill beds of the August Town Formation. This is a sequence of (mainly) brown-to-straw-coloured sandstones and related rock types deposited less than 10mya. In the geologically short time since these beds were deposited, they have been lithified, uplifted, faulted and tilted; some beds, which were originally horizontal, are now vertical. These were then planed off by erosive processes to produce the surface on which the Farquhar’s Beach red beds were then deposited. This angular unconformity is particularly impressive, not least because it is geologically ‘young’, but also because it is so accessible, being exposed over at least 1.5km of coastline.
Therefore, the Round Hill beds present a set of geological structures that provide a dynamic set of features, which are easy to study in coastal section. They have provided an excellent focus for undergraduate geology fieldwork classes from the University of the West Indies at Mona for many years. They also yield a diverse array of fossils, including those of the most impressive fossiliferous deposit in Jamaica – the Farquhar’s Beach oyster bed (Fig. 2A).
The extant mangrove oyster, Crassostrea rhizophorae (Guilding), is well known to the palates of Jamaicans. It is a small oyster, rarely exceeding 100mm in height. Contrast this with Crassostrea virginica (Gmelin), the fossil denizen of the Farquhar’s Beach oyster bed, which may be over 400mm high (Fig. 2B, C) (Littlewood & Donovan, 1988). Although still living as close as the Gulf Coast of the USA, C. virginica is no longer a member of the Jamaican biota. However, as a Jamaican fossil, it is without peer, forming a bed 3.3m thick of giant oysters (Fig. 2).
This extraordinary accumulation includes many specimens preserved in life position, with the shells elevated and juveniles growing on the adults (Fig. 2B). Such exceptional fidelity to the life habit of the oyster could only be the result of preservation in situ. Therefore, the Farquhar’s Beach oyster bed is an association of oysters preserved in their natural habitat that was subsequently fossilised and then uplifted, rather than some storm, current or other hydrodynamic disturbance accumulating these dead shells. The most probable interpretation is that the bed is an oyster bank that grew in an ancient river channel, which maintained a marine influence.
The other notable feature of this bed is the low diversity of fossil organisms. Crassostrea virginica is found associated with few shells of other species, mainly balanid barnacles, and some borings. It either found or made this environment particularly well suited to its needs, whereas few other fossil invertebrate species are present. However, this may be an artefact of preservation, at least in part. Many, perhaps most organisms that live in association with modern C. virginica either lack a mineralised skeleton (for example, acidians or ‘sea squirts’) or have delicate hard parts that soon break down following death (such as ophiuroids, including ‘brittle stars’). In contrast, the large, robust shells of C. virginica were easily preserved.
Highlight 8. The Bowden shell beds
If the international geological community were polled to name the best known deposit in the rock record of the Antilles, it would probably vote for the Upper Pliocene Bowden shell beds in the parish of St Thomas in southeast Jamaica. The shell beds were discovered by Lucas Barrett, the first director of the first geological survey of Jamaica, in 1859. It is actually four beds, totalling about 5m in thickness, which are commonly overgrown and unimpressive to the visitor (Fig. 3A). However, look around for a loose piece of fossiliferous rock and crack it open.
You will see straight away the two features that have made these beds famous – they are extraordinarily fossiliferous and preservation is commonly exquisite. Other deposits of the same age (such as the Hopegate Formation; see Highlight 10) in Jamaica are fossiliferous, but many specimens are preserved as moulds, with the shell itself having dissolved away, with only an impression preserved. However, at Bowden, shells are preserved ‘in the round’ and are quite beautiful.
The Bowden shell beds are best known for their molluscs. There are over 600 species of molluscs known from the shell beds (Fig. 3B, C), but many other groups are represented by multiple species. Wendell P Woodring’s (1925, 1928) monograph of the clams, elephant tusk shells and snails numbers almost 800 pages, and other molluscs have been described subsequently, including chitons (Donovan et al., 1998) and the mineralised jaw elements of squids. However, to these shells need to be added calcareous algae, foraminiferans, scleractinian corals, bryozoans, crabs, barnacles, mantis shrimps, sea eggs (sea urchins), starfishes, brittle stars, bony fish ear bones (otoliths) and trace fossils, such as predatory boreholes in some shells. The total diversity may be over 800 species (Donovan, 1998).
The nature of the fossil diversity of the Bowden shell beds indicates the history of its formation. Consider the disharmonious association of fossil snails as the best evidence. The shell beds preserve land snails (Goodfriend, 1993), shallow and deep water marine snails, and planktonic snails typical of the open ocean (Janssen, 1998). These could not and did not live together. The Bowden shell beds preserve a suite of fossils derived from disparate environments and brought together by physical processes. It was formed by sediment mass flows – essentially, submarine landslides – collapsing from shallow into deeper water and taking a mixture of live and dead shells with it (Donovan, 2002). The probable depth of deposition was greater than 100 to 200m, that is, comparable with the slightly younger Manchioneal Formation (see below), although the latter was a very different environment.
Highlight 9. Manchioneal Formation at Christmas River
Christmas River, in the parish of Portland, is an idyllic spot on Jamaica’s east coast. The narrow beach is flanked on either side of the bay by tall cliffs of white limestone. These rocks are the Early Pleistocene Manchioneal Formation (about one million years old), which is also exposed next to the main road at Manchioneal, further to the south. These deposits were formed in an ancient reef environment. It is usual to associate modern reefs with shallow water environments and an abundant biota.
However, reefs also extend into deeper water offshore, an environment referred to as the ‘fore reef’. The only convenient way to examine a modern fore reef is by research submersible or scuba. However, at Christmas River and Manchioneal, the ancient fore reef has been uplifted, so that its secrets can be determined with relative ease and without recourse to a submarine.
The limestones in the cliffs on the south side at Christmas River were deposited in over 150m of seawater, as indicated by two groups of fossils that are otherwise rare in the Jamaican fossil record – stalked crinoids and brachiopods (Harper et al., 1995; Donovan, 1995; Donovan & Harper, 2001; Harper & Donovan, 2007). Both of these groups were highly successful during the Palaeozoic and are familiar fossils in rocks of this age, but, since then, have had a reduced diversity and are mainly found in deep water today. Christmas River is the best site for fossil crinoids in Jamaica, but, even then, only four species are known and only from fragments of the stalk.
Brachiopods (Fig. 4) also have a fossil record in Jamaica and the Antilles that is poor, although better than that of the crinoids. And again, Christmas River and nearby Manchioneal are the best localities to find fossil brachiopods in Jamaica.
Highlight 10. Falmouth Formation at East Rio Bueno Harbour
A strip of low-lying sedimentary rocks exposed mainly along the central north coast, but also in other parts of the island, represents a suite of ancient lithofacies representing sub-environments that formed part of a Late Pleistocene reef system. These beds are commonly a well-exposed hard limestone, for example, close to sea level at Discovery Bay. Cracking open a fresh face can reveal abundant fossil molluscs, mainly marine snails and clams (Donovan & Littlewood, 1993), which provide vital clues to the exact environment by comparison with modern reefs. These beds are part of the Falmouth Formation, which is a raised reef about 125,000 years old.
One notable, yet atypical, site in the Falmouth Formation is exposed on the east side of Rio Bueno Harbour in the parish of St Ann. Here, the beds are only weakly lithified, that is, they are not turned to hard rock. The three dimensional coral framework is prominent (Fig. 5A) and a range of fossil organisms can be collected from the interstitial sediment. It is a wonderful site for collecting fossils and, in particular, sediment samples can be examined under the microscope for minute organic remains, including fragments of groups that disarticulated easily after death, like echinoids (Gordon & Donovan, 1992), chitons and ophiuroids. The probable environment is uncertain and has variously been interpreted as either a patch reef in a back reef lagoon or part of the shallow water fore reef.
The reef grew on the hard limestones of the Late Pliocene to Early Pleistocene(?) Hopegate Formation and the contact is readily apparent (Fig. 5B). The Hopegate Formation is a comparatively poorly known lithostratigraphic unit, about the same age as the Bowden shell beds, but deposited in shallow water and composed chiefly of dense, coral-rich massive limestones exposed in a band across the north coast of central and western parts of the island. Antillean Pliocene carbonate environments, particularly reefs, are poorly known compared with those of the Miocene and Pleistocene.
Late Pliocene reef deposits are of exceptional importance because the modern Caribbean reef coral fauna emerged intact from a period of elevated extinction at around this time, as did typical Caribbean, soft-bottom molluscan faunas. On a regional scale, the detailed tempo and timing of faunal change differs between molluscs and reef corals, and locally for reef corals between different parts of the reef complex.
Therefore, the raised reef of the Hopegate Formation is an important remnant of a poorly known ancient environment (Donovan & Miller, 1995; TA Stemann, research in progress). It is particularly well exposed as a cliff line between Discovery Bay in the parish of St Ann, and Rio Bueno in the parish of Trelawny, on the north coast of Jamaica, a distance of about 5km, but access is particularly good at East Rio Bueno Harbour. Preservation of fossils is commonly as moulds.
Highlight 11. The Natural Bridge at Riversdale
The Natural Bridge at Riversdale, in the parish of St Catherine, was first illustrated by De la Beche in 1827. Many major features of Jamaica’s scenery were first noted by him and his observations are still relevant. However, the Natural Bridge had to wait until the end of the twentieth century to receive the detailed description and analysis it deserved (Miller & Donovan, 1999). It is one of the most impressive of Jamaican geological phenomena, with its features emphasised by the deep, vertically-sided gorge in which it occurs and the boulders in the stream valley, some as big as a house (Fig. 6).
The Natural Bridge is the result of collapse and solution of the White Limestone Group, and must have formed in a sub-aerial environment. Therefore, it is geologically young, as Jamaica was only exposed sub-aerially about 10mya. And, as a structure formed by erosion and collapse, so one day, it will be lost, crashing down into the gorge.
Highlight 12. Judgement Cliff
Jamaica’s mountainous topography, its situation within an earthquake zone, and a humid tropical climate prone to tropical storms and hurricanes combine to make downslope mass movements – landslides in a broad sense – a common occurrence. Rocks near the surface in Jamaica are commonly more or less fractured, not uncommonly by major geological faults. These fractures fill with water during major storms, acting as added weight to force rocks downslope, while lubricating surfaces between them. Excessive fracturing and precipitation may even engender catastrophic rock movements.
Many of Jamaica’s major landslides occurred in prehistory. The most famous landslide in historic times was at the ominously-named Judgement Cliff in the Yallahs River valley in the parish of St Thomas (Zans, 1959; Maharaj, 1994). More correctly, this feature was formed by a rock avalanche, that is, a rapid downslope movement of bedrock fragments that were shattered as they flowed. Such an avalanche arises from a major rockfall or rockslide.
Judgement Cliff can be seen from the road on the opposite side of the Yallahs River valley, appearing as an impressive scar on the limestone of the hills (Fig. 7). Its avalanche debris stretches out before it. That the rock avalanche occurred in 1692 is undoubted, but exactly when during that year is uncertain. Different reports suggest that it coincided with the Port Royal earthquake (7 June 1692) or, perhaps more likely, during a hurricane on 18 to 19 October 1692. The earthquake may have fractured the limestones of Judgement Cliff and the abundant storm waters of the hurricane acted to fill, overload and lubricate the fractured rocks.
The resulting rock avalanche must have been impressive to behold, unless you were one of the poor souls buried by it. Estimates of the volume of rock moved in this rock avalanche vary between about 130 and 180 million cubic metres of (mainly) limestone. This mass of rock extended about 1.76km (about a mile) across the valley and dammed the Yallahs River. Further disastrous affects accompanied the catastrophic breaching of this natural dam.
However, Judgement Cliff is not alone. It is only one of nine major rock avalanches apparent in this area. The other eight are prehistoric, so leave no record in print or folklore, and they are now overgrown, so their origins and even existence are disguised by Jamaica’s luxuriant vegetation. Where the next rock failure in this area is likely to occur is unknown, but it seems probable that it will happen.
SKD thanks Niko Korenhof (NCB – Naturalis, Leiden, The Netherlands) for scanning his original colour slides, which are reproduced as certain of the figures in this article.
|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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Donovan, S.K. (ed.). 1998. The Pliocene Bowden shell bed, southeast Jamaica. Contributions to Tertiary & Quaternary Geology, 35: 175 pp.
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