Stephen K Donovan (The Netherlands)
This is the first of six articles that will introduce the geology of the Antillean island of Barbados. It is an expanded and more detailed guide derived from two earlier publications (Donovan & Harper, 2005, 2009). The structure of the guide will include a summary of the geology of the island (in this part) and five, one-day field excursions for the geologically-biased tourist. These excursions will introduce the stratigraphy, structure and geological history of Barbados (Figs. 1 and 2), a small Antillean island shaped like a contorted teardrop, about 34km long by 24km at its widest.
A visitor to the island, who wants to undertake fieldwork, should hire a car. The only other reliable forms of transport are bus and taxi. While cheap, buses tend to stick to the main routes, particularly in the countryside. However, the size of the island means that localities are rarely more than a few kilometres from a bus stop. If money is no object, a taxi driver will be happy to drop you at a site in the morning and collect you at a pre-arranged time.
A glossary of some of the more technical terms (which are highlighted in bold and italics in the text) and some of which will appear in later parts of this guide, appears at the end of this article.
|Getting information on the geology and layout of Barbados:|
|Large-scale road maps are available free or at low cost from many hotels, service stations and bookshops on the island. There are about 1,300km of paved roads in Barbados, so there are many potential ways of getting lost!|
The 1:50,000 geological map of Barbados is published by Poole & Barker (1983).
Geological guidebooks include Anon (1986, 2002), published to coincide with major geological conferences in Barbados, although these are not widely available. The account of the island’s geology by Machel (1999) is available from the Barbados Museum and Historical Society, The Garrison, St. Michael, Barbados (telephone +1-246-427-0201).
The Barbados Ridge, including the island of Barbados, is comprised of more than 4km of Tertiary strata overlying at least 20km of relatively low density sedimentary rocks accumulated on the subduction zone of the South American Plate under-riding the Caribbean Plate (Westbrook et al., 1973). This represents an accretionary prism, indicated by the presence of an arcuate, north-south negative gravity anomaly. In other words, the crust in this region has a low density. This negative anomaly is parallel to the positive gravity anomaly of the Lesser Antilles volcanic arc. The exposed core of the Barbados Ridge consists of deformed turbidites, volcanogenic strata, olistostromic blocks, conglomerates, and possible mudvolcanoes and deltaic deposits. In fact, Barbados is one of the few places in the world where an active accretionary prism is exposed above sea level (Speed, 1994).
With the exception of some volcanic ash bands, the rock succession in Barbados is entirely sedimentary in origin, demonstrating its geological independence from the volcanic arc of the Lesser Antilles. 85% of the exposed rocks are Pleistocene reef limestones; the remaining 15% are Tertiary sedimentary rocks of marine origin, which crop out in a triangular region in the northeast of Barbados in an area called the Scotland District, covering an area of 40km2 (Figs. 2 and 3).
The sedimentary succession of Barbados was originally interpreted as an autochthonous sequence, that is, they were formed in the location where they are presently found (see, for example, Jukes-Browne & Harrison, 1891, 1892; Poole & Barker, 1982, 1983). The oldest known rocks, proved in a borehole, are the so-called P-unit, consisting of black, abundantly fossiliferous marine shales correlated with the Paleocene or lowermost Eocene. The overlying thick, Lower Eocene sequence of the Scotland Group or Beds was interpreted as having a mainly shallow-water, estuarine, deltaic or lagoonal origin (Trechmann, 1925; Poole & Barker, 1983) on the basis of six criteria:
- The fossils found in the Scotland Group are considered to be shallow water, marine and estuarine faunas from sandstones, and shallow water microfossils in shales.
- Presence of in situ bedded gypsum (Morgan Lewis Member), suggesting deposition by precipitation.
- Mud-cracked shales, suggesting drying by the sun.
- Primary red beds, containing oxidised iron compounds,and richly carbonaceous layers (Mount All Member) suggest deposition in a terrestrial setting.
- Connate waters in Scotland Group sandstones are comparatively fresh.
- Although sedimentary structures in sandstones indicate turbidite sedimentation (that is, deposition in deeper water), they may alternately have been formed in a shallow-water environment. However, alternative interpretations have supported a deep-water origin (see below).
The overlying Oceanic Group consists of organic-rich sedimentary rocks with planktonic microfossils indicating deposition in at least 1km of water. The evidence of microfossils indicates apparently continuous sedimentation from the Middle Eocene to the Middle Miocene (Poole & Barker, 1983). Pale grey, siliceous, radiolarian-rich mudstones are interbedded with whitish-grey, Globigerina-rich marls. (Globigerina are foraminiferans, which live as marine plankton.) Beds of dark grey volcanic ash, up to 30cm thick, are common in the lower part of the sequence, presumably transported east by wind from subaerial eruptions in the Lesser Antilles.
The Bissex Hill Formation rests unconformably on the Oceanic Group and consists of pale yellow, sandy limestones and marls rich in planktonic microfossils. A basal conglomerate includes pebbles derived from the Scotland and Oceanic groups. The Bissex Hill Formation contains early Miocene planktonic microfossils similar to those of the Conset Marl of the Oceanic Group, suggesting either a similar environment or reworking.
The Joe’s River Beds are interpreted as mud diapirs, comprised of dark grey to black, structureless, slickensided and oily clays. These are associated with fragments of coarser-grained, siliciclastic rocks and limestones, along with olistostromic ‘rafts’ of the Scotland and Oceanic groups.
Three principal Pleistocene reef terraces are separated by two prominent cliff lines (Schellman & Radtke, 2004). However, multiple reef trends, which generally get older towards the centre of the island, have been recognised. The oldest terrace is topographically the highest (Fig. 2).
Upper Coral Rock Up to 700,000 years old
~~~~~~~~~~~~~~~~~~~~ Second High Cliff ~~~~~~~~~~~~~~~~~~~~
Middle Coral Rock 127,000-484,000 years old
~~~~~~~~~~~~~~~~~~~~ First High Cliff ~~~~~~~~~~~~~~~~~~~~
Lower Coral Rock Less than 127,000 years old
Extensive studies have led to a detailed understanding of the palaeoecology of these ancient reefs (in which the dominant fossils are hermatypic scleractinian corals) and also of their age. Although solution structures such as caves are present, that karst features are not more prominent is probably due mainly to the geologically brief period for which the island has been subaerially exposed (Fig. 4).
Soils are redder in colour and more clay-rich on older terraces. This has been demonstrated to be due to derivation, not just from insoluble limestone remnants, but also from volcanic ash from the Lesser Antilles and wind-blown dust from the Sahara (Muhs, 2001).
Speed and co-workers (Speed & Larue, 1982; Speed et al., 1986, 1991; Speed, 1988, 1994, 2002; Torrini et al., 1985; Torrini, 1988) have reinterpreted the units of deformed Tertiary rock in the Scotland District. They believe they show a structural complexity indicating that they are considerably displaced in respect to each other, that is, they are allochthonous (Figs. 5 and 6).
Therefore, it is necessary to recognise these separate structural units (‘packets’) to determine their true relationships. Only the overlying Pleistocene reefs are autochthonous (Fig. 1).
Speed (1988) suggested a seven phase evolution of Barbados, before the deposition of the autochthonous Pleistocene reefs:
- The accretion of a basal complex by the end of the Eocene. This is an amalgam of fault-bounded packets of sedimentary rocks, at least 4.5km thick and probably accreted by scraping sediment from the Lesser Antilles Accretionary Prism in the late Eocene. It consists of deep-water siliciclastic sedimentary rocks of turbiditic origin (=Scotland Group) and hemipelagic, radiolarian-rich clays.
- The deposition of three sedimentary facies of the ‘Oceanic Group’ occurred from the middle Eocene onwards:
- Facies I (Oceanic Nappes): these were deposited in the forearc basin in the Eocene to Middle Miocene, receiving no input from rocks eroded from the land, but including debris from the outer parts of a volcanic arc;
- Facies II (Bissex Oceanics): Middle to Late Eocene in age. Lithologically similar to Facies I, but with minor beds of quartz sandstones and channelized volcaniclastics; and
- Facies III (Cattlewash Oceanics): forming a diapiric melange near the area called Cattlewash (=Joe’s River Beds).
- Deposition of a (so far) unrecognised older prism cover (that is, those sedimentary rocks deposited on the basal complex after it had accreted), and a younger prism cover including the T and Woodbourne Intermediate Unit.
- Tectonic transport of Oceanic Facies II in the Middle Miocene or earlier, deformation occurring before being covered by the Bissex Hill Formation.
- Mud diapirism in the Late Miocene and later, with debris of diapiric origin occurring in the Kingsley unit of the younger prism cover.
- Thrusting of Oceanics:
- Facies III (Cattlewash) thrusted over the Late Miocene Kingsley Unit;
- Facies II (Bissex) thrusted above the Miocene Bissex Intermediate Unit; and
- Facies I (Oceanic Nappes) thrusted above the Miocene Woodborne Intermediate Unit.
- Uplift of the structural high of the accretionary prism, centred at or near Barbados, in the Early Miocene, possibly due to a decrease in the rate of subduction (Westbrook et al., 1973).
This interpretation of a complexly deformed, allochthonous Tertiary succession overlain by autochthonous Pleistocene reef limestones is now accepted as the correct model for the geological evolution of the island.
My research on the geology and palaeontology of Barbados has been supported by the National Geographic Society grant #5722-96, The University of the West Indies, Mona, and the Nationaal Natuurhistorisch Museum, Leiden. Thanks also go to the many geologists, who have discussed the complexities of the geology of Barbados with me over the years, notably David Harper, Trevor Jackson, John Saunders, the late Bob Speed and Rudolf Torrini.
The following glossary explains some of the more specialised terms referred to in this and the succeeding five field guides to Barbados.
|Glossary of selected terms|
|accretionary prism:||The sediment scraped from a descending tectonic plate as it is subducted. Accretionary prisms show complex patterns of deformation, as in the Tertiary rocks of the Scotland District.|
|allochthonous:||An allochthonous sedimentary succession has been moved by tectonic forces after it was deposited, for example, a thrust-faulted nappe.|
|autochthonous:||An autochthonous sedimentary succession has not been moved by tectonic forces since it was deposited. The Pleistocene coral cap of Barbados is an autochthonous succession, resting on the allochthonous Tertiary deposits of the island.|
|arcuate:||Curved in an arc.|
|basal complex:||An outmoded idea that islands in the Caribbean and elsewhere were formed on top of foundered continents.|
|cap rock:||An impermeable sedimentary rock, such as a mudrock or evaporite, through which fluids cannot migrate. Cap rocks overlie traps formed by reservoir rocks and in a position that prevents further migration of petroleum.|
|carbonaceous layer:||A bed rich in carbon, probably containing ample fossilised plants or plant debris.|
|Caribbean plate:||The crustal plate on which most of the Caribbean islands are situated.|
|channelized volcaniclastics:||Tuffaceous extrusive rocks preserved in a channel. The channel most probably formed by the erosive action of tuffs at the time of eruption, although they could also infill a dry river valley.|
|cold seep:||An area of the sea floor where geochemical energy sources, such as hydrogen sulphide, other reduced sulphur compounds and/or methane, are being released onto the sea floor. Cold seep communities rely on geochemical rather than photosynthetic energy; the primary producers are chemosynthetic bacteria.|
|connate waters:||Water trapped in the pores of a sediment at the time of deposition.|
|diapiric melange:||See diapirism.|
|diapirism:||An intrusive body, either igneous or sedimentary (muds, evaporites) in origin, which pierces and deforms the overlying beds into domed structures. Mud diapirs of the diapiric melange in the Scotland District deformed the basal complex, but, in turn, originated within the lower parts of the complex.|
|dripstone:||Calcitic deposits produced by precipitation from dripping water, rich in dissolved calcium carbonate; usually formed in caves within limestones.|
|foraminifera:||Protistan (= unicellular) microfossils, with a skeleton composed of calcite, agglutinated particles or secreted organic matter (tectin). Foraminifera include both planktic (for example, Globigerina) and benthic taxa (for example, Amphistegina).|
|forearc basin:||The basin on the oceanward side of a volcanic arc, that is, on the side where subduction is occurring.|
|fore reef:||The seaward side of a reef, sloping into deeper water from the reef crest.|
|geochronology:||The subdivision of geological time into intervals of known duration.|
|geothermal gradient:||The sequential difference in temperature of the Earth’s crust with increasing depth.|
|grainstone:||A grain-supported, clastic, mud-free limestone.|
|gravity anomaly:||The difference between measured gravity and that expected based on theoretical determinations. A negative gravity anomaly, as occurs in Barbados, is a lower measurement than would otherwise be predicted. This can be explained by the island resting on a thick pile of ‘light’ Cenozoic sediments and sedimentary rocks.|
|hemipelagic:||Hemipelagic sedimentary sequences are deposited on the continental shelf and continental slope.|
|hermatypic:||A reef-building organism, such as certain colonial species of scleractinian corals.|
|isochronous:||Occurring or formed at the same time.|
|karst:||Topography associated with limestone (or other soluble rock such as evaporites), generally in regions of at least moderate rainfall and produced by solution, which occurs mainly in the subsurface.|
|Mountain Uplift Theory:||CT Trechmann’s hypothesis that the uplift of mountains was driven by the gravitational pull of the moon.|
|mud diapirs:||See diapirism.|
|mud volcanoes:||A mud diapir that has breached the ground surface and is extruding mud to form a structure of positive relief.|
|nappe:||A large-scale, allochthonous rock unit formed by thrust faulting, for example, the Oceanics of Barbados.|
|olistostrome:||An allochthonous sedimentary mélange formed by chaotic submarine slides or debris flows of blocks into deeper water.|
|olistostromic blocks:||See olistostrome.|
|orogenic activity:||Mountain-building processes.|
|packet:||A fault-bounded tectonostratigraphic unit consisting of more or less deformed rocks, but with a definable internal stratigraphy.|
|packstone:||A grain-supported, clastic limestone with some interstitial lime mud.|
|palaeobathymetry:||The depth of deposition of an ancient, commonly sedimentary rock unit, determined using uniformitarian principals and preferably using multiple criteria, such as the data made available from sedimentology, body fossils and trace fossils.|
|radiolarians:||Siliceous protistan (= unicellular) microfossils, commonly with complex and ornate skeletons.|
|reservoir rock:||An incompressible, porous, permeable sedimentary rock in which oil and/or gas can accumulate following migration from a source rock. A reservoir rock of suitable stratigraphic or structural geometry, for example, an anticline capped by an impermeable mudrock, forms a hydrocarbon trap in which petroleum may accumulate.|
|siliciclastic:||A sedimentary rock formed from mechanically-derived fragments of pre-existing, silica-rich rocks. Examples of siliciclastic rocks include mudrocks, siltstones, sandstones and conglomerates.|
|slickensides:||Striated surfaces on a fault plane, gouged out as fault moves. Therefore, a slickenside indicates the direction in which a fault has moved.|
|source rock:||Sedimentary rocks in which hydrocarbons are generated. Source rocks are invariably fine-grained sedimentary rocks (=mudstones), with low permeability and porosity, that were deposited under low energy conditions and which contain a large proportion of organic matter.|
|South American plate:||The crustal plate on which the continent of South America and adjacent floor of the western Atlantic Ocean are situated.|
|subduction zone:||The elongate and arcuate region where one tectonic plate is sliding under the edge of a second plate, forming an oceanic trench.|
|tectonostratigraphy:||The stratigraphy of tectonic units (packets) in each of which may be a definable sedimentary succession.|
|terrace:||A flat-lying area of land in a region of stepped topography, with a steep descending slope on its more seawards margin and a steep ascending slope on its more landwards margin (these steep slopes may be cliffs or ancient cliff lines).|
|trace fossils:||Sedimentary structures produced by the actions of organisms.|
|turbidites:||Sedimentary deposits formed by settling of a suspended sediment load derived by rapid, downslope transport, usually formed in deep water.|
|volcanogenic strata:||Sedimentary strata derived from eruption of particulate volcanic rocks, such as ash and tuff.|
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