Stephen K Donovan (The Netherlands)
Stop 1. Chalky Mount (approximately 59º 33’ 15” W 13º 13’ 55” N; Fig. 1)
The area considered in this part of the guide is outlined in Donovan & Harper (2010, fig. 1d) and Figs. 1 and 2. As with other articles in this series, the starting point is Bridgetown. Those wishing to examine the succession and structure of the Scotland District in considerably more detail than outlined below are referred to Speed (2002). This can be complimented by Patel’s (1995) discussion of the geomorphology. Readers are referred to the glossary in A field guide to Barbados (Part 2): The coastal geology of southeast Barbados
Take Highway H2 northeast from the Bridgetown area. This is the road climbing onto the Upper Coral Rock. Note signs for Harrisons Caves and Welchman Hall Gully, but do not be tempted by their delights just yet (see A field guide to Barbados (Part 6): Central Barbados). Northeast of this area, the scenery changes dramatically, as the road leaves the internally drained plateau of Pleistocene limestones and enters the Scotland District (Fig. 3). Follow the signs to Chalky Mount Potteries on the right and then left. Follow the left fork and park at the end of the road. This is locality 1 of Speed (2002, pp. 19, 26, figs 10, 12).
Despite its name, Chalky Mount isn’t on chalk. It lies within the outcrop of the upper Scotland Formation (Poole & Barker, 1983), that is, the basal complex of Speed and co-workers. This area provides the best exposures of siliciclastic rocks, mainly turbidites, of the basal complex in the Scotland District. The sandstones, siltstones and mudstones within this sedimentary packet, with a total thickness of about 600m, were probably deposited in a basal plane – that is, as an outer, deep sea, sedimentary fan that spread out during the Middle Eocene (Speed et al, 1986, pp. 13-17; Speed, 2002, p. 19).
Walk uphill bearing N70ºE along the track over sandstones dipping to the south. This ridge is part of an anticlinal fold, with the north-dipping limb exposed across the gully to the north (Fig. 4). At the crest of the hill, examine the quartz sandstone succession, interpreted as part of the infill of a channel in an inner sedimentary fan channel fill (Speed, 2002, p. 26). The trace fossils that are so plentiful at some horizons are suggestive of a turbidite succession.
Before leaving the area, you should have a look at what the potters of Chalky Mount are making, using clay quarried locally.
Stop 2. Bissex Hill (approximately 59º 33’ 12” W 13º 12’ 51” N; Fig. 1)
Drive back along the Chalky Mount ridge, past the fork, but turn left at the junction. Follow the road south towards Bissex Hill, then follow the road that curves around the west side of the hill and park near the end.
The rocks of the Bissex Hill nappe, including the Bissex Hill Formation (Poole & Barker, 1983), are structurally higher than the basal complex, but beneath the Oceanic nappes (Fig. 5). They consist of the Eocene Bissex Oceanics overlain by Lower Miocene(?) micrites and the Lower to Middle Miocene Bissex Hill Formation (Speed, 1988, pp. 29:7-29:8; 2002, pp. 34-37). This succession rests on a thrust fault over the Miocene Bissex Intermediate Unit that forms part of the prism cover, that is, sedimentary rocks deposited on the basal complex (Scotland Formation) after it was accreted (Speed, 1994, p. 183).
Although exposures are (at best) poor, boulders of the Lower to Middle Miocene Bissex Hill Formation are scattered at the roadside. These are globigerinid foraminiferal sandstones containing lots of spines of regular echinoids. Other fossils that have been found in this part of the succession include solitary scleractinian corals, fish teeth and isocrinid crinoids (Speed, 1988, p. 29:8; Donovan & Veltkamp, 2001, p. 731; Fig. 6).
Stop 3. Coconut Grove (Joe’s River Formation) (approximately 59º 33’ 3” W 13º 12’ 33” N; Fig. 1)
Return north on the same road, taking the first right turn and then right again. Drive south and southwest, joining the Saddleback Road from the left. Park at the side of the road on the left about 150m past the Parks Road turning on the right. Descend the track to the southeast for a few hundred metres, veering left at a fork. Stop at the top of an exposed slope. This is site 6 of Speed (2002, pp. 55-58).
This hillside (Coconut Grove) represents part of a sedimentary intrusion from below, that is, a dyke-like, diapiric melange derived from mudrocks of the basal complex and is part of the largest surface expression of any Barbadian diapir (Speed, 2002, p. 55). The diapir is emplaced (that is, intruded) into the basal complex and comprises (Speed, 2002, p. 58):
- Matrix. Very coarse and fine-grained sand, clay and organics (probably mainly bitumen).
- Green mudstone granules. Up to tens of millimetres in diameter, mainly pure, less commonly sandy, yet without an identifiable source unit in the basal complex.
- Lithic blocks. 10mm to 25m in diameter, clay ironstone, calcitized radstone and quartz sandstone with a calcite or bitumen cement chert. The latter lacks an identifiable source unit in the basal complex.
- Fossils. Mainly foraminifera and benthic molluscs (Kugler et al., 1984), and trace fossils such as Ophiomorpha isp. This fauna is considered indicative of an ancient, cold seep community (Gill et al., 2005). Cold seeps are areas of sea floor where geochemical energy sources are released, such as methane and/or hydrogen sulphide.
Stop 4. East Coast road near Barclays Park
Turn your vehicle around and return the way you came until the fork to the east of Bissex Hill, where the right fork is taken east towards the coast. Turn left (northeast then northwest) along the east coast road. Drive past Barclays Park and park by the monument on the right, which commemorates the opening of the road by HM Queen Elizabeth II (Fig. 1). The sequence exposed on the opposite side of the road includes several impressive sedimentary structures within a vertically orientated sequence in the basal complex. This provides evidence both for the original horizontal orientation of the rocks and the palaeoenvironment.
The most impressive sedimentary structures in this sequence are spherical diagenetic nodules up to about one metre in diameter (Fig. 7). A thin conglomerate bed, exposed about 100 to 200m below the section with giant nodules, have yielded clasts bored by clionid sponges (= the trace fossil Entobia isp.). Trechmann’s (1925) estimate of the age of the basal complex (=Scotland Beds) relied on fossil molluscs collected from conglomerates such as this one, although this road section is relatively new. Trechmann probably visited the localities that Hill (1899, p. 176) used in comparing the age of the Scotland Beds with the Palaeogene Richmond Formation of Jamaica (Donovan, 2003). The only other macrofossils in this part of the succession are indeterminate carbonised plant fragments preserved parallel to bedding in sandstone units.
Stop 5. Oil seep
From the conglomerate bed at Stop 4, walk over the low slopes to the south, slowly diverging from the highway. Careful searching will reveal an oil seep at the surface – a rare surface expression of the hydrocarbon resources of the island (Fig. 8).
Stop 6. Limestone ‘mushrooms’ at Bathsheba (approximately 59º 31’ 12” W 13º 12’ 35” N; Fig. 1)
Drive southeast, back past Barclays Park and Cattlewash. Turn off the highway to Bathsheba and park near the coast, in view of the large fallen limestone boulders. Then walk down to the beach.
These limestone boulders, as big as a house, are presumably derived from the face of the Hackleton’s Cliffs to the southwest. A cliff on a rocky coastline presents one face to the sea and will develop a sea level or intertidal notch at its base. The tidal range in the Caribbean is less than one metre, which is reflected by the height of the notches in these fallen boulders. However, unlike a cliff, these boulders present all sides to the sea and have developed a notch through 360º, giving rise to their distinctive shapes, like a group of top-heavy limestone mushrooms.
The notch is typically deep, with the well-lithified limestone blocks not collapsing until the ‘stalk’ of the mushroom is slender. The notch is not actually formed by the erosive action of the sea, but rather by the feeding action of herbivorous invertebrates scraping algae from the limestone. Typical eroders are the radulae of chitons and gastropods, and the Aristotle’s lantern of regular echinoids (Bromley, 1975). At low tide, it is apparent that the notch is floored by a horizontal platform that is the remnant of the base of the boulder (Wilder, 1999, photograph on pp. 254-255).
Based on his observations of blocks such as these, Trechmann (1937, pp. 347-348) included an interesting explanation of how some exotic blocks (‘klippen’ or dropstones) may become incorporated in deeper water deposits adjacent to such oceanic island settings by means of their downslope movement under the influence of gravity. Bennett (et al., 1996), in a paper in tune with Trechmann’s ideas, provided a review of the principal mechanisms by which such clasts are transported under the influence of gravity.
Stop 7. Bath Cliff (approximately 59º 29’ 46” W 13º 11’ 42” N; Fig. 1)
Drive back to the East Coast Road and continue southeast. After about 6km, take the turn on the left to Bath Beach. Turn right at the bottom and park. Walk a short distance along the coast to the Bath Cliffs. This is site 8 of Speed (2002, pp. 62-68).
The Bath Cliff section forms part of an Oceanic nappe. It is impressive to look at, albeit steep and difficult to access, as it provides a particularly complete section across the Eocene/Oligocene boundary interval (Saunders et al., 1984) in an area of structural complexity (Torrini et al., 1985).
It is one of the most complete and well-studied exposures throughout this part of the geological succession in the tropical western Atlantic. It is a rare example of a continuous section in deep water lithofacies across the Eocene/Oligocene boundary, making it particularly important for international correlation. It consists of a sequence of “… pelagic rocks (indurated oozes, pelagites) that vary from totally noncalcareous to highly calcareous, the end members being radiolarian indurated oozes … and nannofossil marly chalks” (Saunders et al., 1984, pp. 391, 393) associated with interbedded volcanic ashes.
Rate of sedimentation was estimated as 27.1m every million years (Saunders et al., 1984, p. 394), which is considered to be untypically high when compared with deposits laid down at the same time in deep water settings. The ancient depth of the sea is estimated to have been between 2,800 and 4,800m.
Walk further along the beach towards the southeast end of the bay. A petroleum seep occurs at beach level, just below the embankment of the closed railway line.
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