Jamaica’s geodiversity (Part 2): highlights from the Neogene

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 in Issue 31 of Deposits, 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’).

Figure 1
Fig. 1. Cave survey and selected passage cross-sections (A-A’ to G-G’) of the Wait-a-Bit Cave, parish of Trelawny, Jamaica (after Miller & Donovan, 1996, text-fig. 2). The thick dashed line to the west of E’, and south of F’ and G’, marks the edge of the limestone overhang from the northwest.

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.

Figure 2
Fig. 2. Oyster bed at Farquhar’s Beach, near Round Hill, parish of Clarendon. (A) A younger SKD, 1987 vintage, standing in front of the oyster bed, which dips to the right. The bottom of the bed corresponds to the bottom of SKD’s shorts. (B) Crassostrea virginica (Gmelin) in situ, that is, preserved in life position. The curved adult shells are upright and encrusted by juvenile oysters. Hammer head (bottom), about 160mm long. (C) A large, recumbent C. virginica that has lost most of one valve. Scale in cm.

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).

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