Going green: Chloritized tuffs from the Oligocene of Antigua

Trevor A Jackson (Trinidad) and Stephen K Donovan (The Netherlands)

The Caribbean island of Antigua lies towards the northern end of the Lesser Antilles volcanic arc. In this particular region, it is a double arc, diverging northwards. To the west, the arc is a line of islands formed by volcanoes during the Neogene; some are still active. These islands constitute the northern Volcanic Caribbees. To the east are the Limestone Caribbees, islands that were volcanic in origin, but which are older and have long since lapsed into quiescence (Wadge,1 994). Antigua is a Limestone Caribbee.

This article briefly describes the geology of this fascinating island. There is also a glossary of terms at the end, to explain those items whose first appearance in the text is marked out in bold and italics.

Geology of Antigua

Antigua was volcanically active in the Late Oligocene. Indeed, apart from some minor late Quaternary sedimentary deposits, its entire rock record is Late Oligocene (Weiss, 1994). The regional dip is towards the northeast (Fig.1), with the oldest (volcanic) rocks outcropping in the west and south. The rock record of the island is divided into three conformable units:

1. the Basal Volcanic Suite;
2. the Central Plain Group; and
3. the Antigua Formation.

The Basal Volcanic Suite (BVS) outcrops in the mountainous southwest part of Antigua (Fig.1). The BVS is comprised mainly of lava flows and pyroclastic rocks that are predominantly andesitic with lesser amounts of basalt and dacite. Minor limestones in the BVS have yielded fossils, including species known also from the Antigua Formation, indicating that the complete rock succession of Antigua was deposited in a geologically brief period, namely during the Late Oligocene.

The Central Plain Group (CPG) outcrops in a low-lying belt extending from the northwest to southeast of the island (Fig. 1). The CPG consists of mixed siliciclastic and limestone sedimentary rocks of both marine and non-marine origin. The siliciclastic rocks were derived by weathering, erosion and re-deposition of the BVS. The most distinctive rocks of the CPG are silicified, including petrified wood, the national stone of Antigua (Nicholson, 2007, p.94) and cherts preserving abundant freshwater snails (Brown and Pilsbry, 1914).

The Antigua Formation is a sequence of varied limestones, with minor tuffaceous/sandy horizons that are exposed in the north and east of the island (Fig. 1). The differing limestone lithologies indicate contrasting palaeoenvironments of deposition. The macro fossils are varied and, in some sections, silicified. Common taxa include larger benthic foraminifers, scleractinian corals, benthic molluscs, echinoids, crabs and bryozoans. Deeper water environments, high in the succession, are indicated by the presence of crinoid columnals and rarer brachiopods.

Post-magmatic hydrothermal alteration of the rocks of Antigua is very evident throughout its entire stratigraphic sequence. It is responsible for the silicification and chloritization seen in the volcanic and sedimentary rocks of the island.

Chloritized tuffs of Liberta

Chloritization is most evident among the pyroclastic rocks, consisting of agglomerates, lapilli and tuffs of the BVS, particularly along the roadside in the village of Liberta, as well as the surrounding area, where the rocks display a distinctive bright-green colour. Early workers to the island, such as Martin Kaye (1959), had described the tuffs as a part of the Tyrell’s Tuff, a member of the CPG, but more recent work has assigned the tuffs to the BVS (Weiss, 1994). The tuffs are well bedded (Fig. 2) and petrographically contain angular and subangular crystals of compositionally zoned plagioclase feldspar and minor quartz in a matrix of chloritized glass. It is the chlorite in the matrix that accounts for the distinctive and dominant green colour of the rock.

The similarity in the mineralogy of the tuffs and the dacite dome lava that out crops at Sugar Loaf Hill, some 3km to the south of Liberta (Fig. 1), would suggest that the source of the tuffs were genetically associated with silicic eruptions that occurred in that area of the island. The Sugar Loaf dome lava contains phenocrysts of compositionally zoned plagioclase feldspar and quartz in a bluish-grey, finely crystalline groundmass. It is one of the few areas on the island where the lavas were dacitic in composition and contained phenocrysts of quartz.

Although there is evidence of chloritization throughout the BVS, the scale at which it occurred in the tuffs at Liberta and environs is very noticeable. Fink (1971) described the existence of a quarry that was once located a short distance north of the Moravian Church (Fig. 3)

During our brief visit, we were unable to locate any major quarry in the area, although hand specimens were easily collected locally (Fig. 4). The attractiveness of the stone has prompted villagers in the area to use the blocks as decorative stone. Here, the stone can be seen cladding the outer walls of houses and buildings. The incongruity between the sparse exposure (Fig. 2) and common usage is stark (Fig. 3).

Acknowledgements

We gratefully acknowledge the support for our field work in Antigua during May 2013 that was provided by National Geographic Society grant #GEFNE55-12.

References

Brown, A.P. and Pilsbry, H.A. 1914. Fresh-water molluscs of the Oligocene of Antigua. Proceedings of the Academy of Natural Sciences of Philadelphia, 66: 209-213.

Nicholson, D.V. 2007. Heritage Treasures of Antigua and Barbuda. Museum of Antigua and Barbuda, St. John’s: 114pp.

Fink, L.K., Jr. 1971. In Donnelly, T.W. (ed.), Guide book to the Caribbean Island-Arc System. American Geological Institute, Washington, D.C.

Martin-Kaye, P.H.A. 1959. Reports on the Geology of the Leeward and British Virgin Islands (Antigua, Barbuda, Sombrero, Anguilla, St.Kitts, Nevis, Redonda, Montserrat, the Virgin Islands). Voice Publishing Co., St. Lucia, 117pp.

Wadge, G. 1994. The Lesser Antilles. In Donovan, S.K. and Jackson, T.A. (eds), Caribbean Geology: An Introduction: 167-177. University of the West Indies Publishers’ Association, Kingston.

Weiss, M.P. 1994. Oligocene limestones of Antigua, West Indies: Neptune succeeds Vulcan. Caribbean Journal of Science, 30: 1-29.

Glossary of terms
Agglomerates: pyroclastic rocks mainly made up of fragments that are more than 2cm across (see below).
Andesite: a fine-grained intermediate igneous (volcanic) rock containing the feldspars (see below) oligoclase or andesine.
Benthic: the benthic zone is the ecological region at the lowest level of a body of water such as an ocean or a lake, including the sediment surface and some sub-surface layers.
Chert: a siliceous sedimentary rock. It commonly forms by precipitation of silica from solution (see silicification below) in a pre-existing sedimentary rock, such as a limestone.
Chloritization: the introduction of, production of, replacement by, or conversion into chlorite (see below).
Chlorite: a disparate group of layer-lattice minerals with a distinctive green colouration.
Dacite: also known as rhyolite, this is a fine-grained to glassy volcanic rock.
Feldspars: these are the most significant single group of silicate minerals that form rocks.
Groundmass: a finer-grained material making up the main body of a rock, in which larger units appear. Lapilli: fragments of igneous rocks of between 4 and 32mm in size.
Phenocrysts: relatively large crystals found in a finer-grained ground mass (see above).
Plagioclase: minerals within the feldspar family (see above), made up of sodium and calcium.
Pyroclastic rocks: rocks made up of fragments of volcanic material, which have been ejected into the atmosphere by explosive (volcanic) activity.
Silicic eruptions: large-scale explosive volcanic eruptions (rather than basaltic eruptions, which are mostly effusive).
Silicification: the process by which silica is introduced into non-siliceous rocks, either by the filling of spaces between pores or by replacement (for example, of calcite).
Siliclastic: sedimentary rocks consisting of fragments of silicic components (mostly quartz, feldspars and heavy minerals).
Tuff: originally unconsolidated material consisting of volcanic ash, which consolidates to form a rock.