The Geology of Providencia
The Colombian island cluster of Providencia (latitude 13 degrees 21 minutes N., longitude 80 degrees 22 minutes E.), the adjacent Santa Catalina island and four small islands inside a broad lagoon with a massive fringing coral reef, are all part of a deeply dissected Miocene stratovolcano. The islands have a combined area of 18 km2 and a population of less than 2000.. Providencia island is elongated N30 degrees E and measures 6.5 by 4 km and rises to 360m above sea level. Santa Catalina island lies immediately to the north with a narrow shallow channel that can be waded and is similarly elongated measuring 1.5 by 1.25 km. A 12 km long arcuate fringing reef is best developed on the windward or eastern side of the island and has a front around 1.7km offshore. The Providencia stratovolcano lies astride the Lower Nicaraguan Rise in the western Caribbean . Seismic refraction profiles across the Lower Nicaraguan Rise suggest that there may be 11 associated submarine volcanoes scattered across the Lower Nicaraguan Rise. Other islands and reef s astride the Nicaraguen Rise are coral and young limestone only (see map).
Providencia is a little-known, little-visited island in the western Caribbean midway between Costa Rica and Jamaica . Access is by boat and air from San Andreas island 60km to the SSW. Santa Catalina is famous as the base used by Sir Henry Morgan for his raid on Panama and a fort and cannon remain there.
The highest hills on Providencia and Santa Catalina are remnants of a carapace of felsic lava flows and domes. On Providencia they form a narrow belt extending WNW-ESE across the center of the island, and on Santa Catalina the unit forms a NW dipping cap. Remnants are also present as small cays inside the eastern lagoon. Beneath and comprising the southern third and northern half of Providencia is a black breccia unit comprising vitrophyric lava clasts, mainly of felsic composition but with minor inclusions of mafic clasts. Felsic lava flows are intercalated with the breccias. Most of the breccias are coarse-grained, lithified, clast-supported and ash-poor containing blocks up to 5m in diameter. Such breccias are typical of near-vent pyroclastic avalanche deposits similar for example to those exposed on the superstructure of Mt. Pelee , Martinique . Such breccias are distinct from the ash-rich matrix-supported block and ash flow deposits typical of the lower flanks of Lesser Antillean volcanoes. The breccias of Providencia island (with a diameter of only 4km) are almost entirely near-vent avalanche facies formed during Pelean-style volcanic activity. Their more distal ash-rich block and ash flow deposits must be buried beneath the lagoon and reefs around the island. These youngest rocks comprise a calc-alkaline suite of minor andesite, dacite and rhyolite lavas ranging in silica content from 61.20 to 77.38%.
The basaltic clasts within the felsic breccias are derived from an underlying foundation of the island dominantly composed of mildly alkali olivine basalt together with minor hawaiite and trachyandesite with a silica range from 47.22 to 59.00% The basaltic unit is best exposed on the extreme SW tip of the island as a SE dipping unit including the black headland of Punta Negra. The latter comprises an 8m thick megacryst-bearing basalt lava with underlying flow foot breccia of blocks with thick chilled margins caused by the flow advancing into the sea. The flow is overlain by 12m of lithified base surge deposits showing pinch and swell and ripple drift structures. The tuffs contain abundant algal balls, shell fragments (intact shells are rare), and solitary coral fragments that were all ejected as lithic inclusions during phreatomagmatic explosions. Additionally there are layers of accretionary lapilli penetrated by worm burrows of more than 1cm diameter.. Further south, the base surge deposits are overlain by 20m of hyaloclastite comprising tongues and wedges of megacryst-bearing basalt contained in bright orange palagonitized tachylite. The succession is cut across by several basaltic and one felsic dike and records the phreatomagmatic emergence of the Providencia stratovolcano. The basaltic lavas are again exposed at sea level on the extreme northern tip of Providencia island.
Whereas Providencia is deeply dissected, there is evidence of a rejuvenation of volcanism following this deep erosion. At Alligator Point and on the southwest coast and also on the northeast coast opposite Crab Cay, white felsic ignimbrite and pumice tuff are exposed. These represent Plinian-style pumiceous volcanic activity that is younger than the main bulk of the stratovolcano. In the northeast part of the island to the east of the airstrip, the ignimbrites occur as a lithified valley-fill deposit which is cut by faulting. At Alligator Point two water-stratified tuffs are intercalated with raised coraliferous limestone. These tuffs have a combined thickness in excess of 9m and comprise white devitrified rhyolitic clasts identical to those in the northeastern outcrops. These pumice tuffs are now exposed at up to 25m above present sea level and raised reef limestone has been found up to 100m above the present sea level on the southwest coast, suggesting a late uplift of the island.
The Providencia stratovolcano has evolved in three stages. First a submarine foundation of alkali olivine basalts formed that underwent an emergent stage to produce subaerial basalt flows now exposed around sea level. A second stage comprised subaerial Pelean-type pyroclastic activity and rhyolite lava dome activity to produce a calc-alkaline suite of minor andesite, dacite and rhyolite lavas. Finally following uplift and erosion there was a final episode of felsic Plinian activity resulting in some valley-fill ignimbrite deposits. K/Ar ages on the older basalts have yielded 14.1 and 7.4 Ma (Ma is a notation for million years) whereas the calc alkaline suite has yielded a preliminary K/Ar age of 4.0 Ma. No ignimbrite material suitable for dating has been found. The very wide spread of the K/Ar ages raises some doubt about a lifespan of ten million years for a single stratovolcano, but perhaps the dominantly submarine basaltic foundation of the island, together with other submarine banks in the area, represent an older episode of volcanism than the younger calc-alkaline carapace found only on Providencia island.
The tectonic setting of Providencia as well as its bimodal magmatic suite, with characteristic Sr, Ns and Pb mantle isotopic ratios, are quite different from the unimodal suites of Central American and Lesser Antillean subduction related suites.
The bimodal suite of Providencia is regarded as a within-plate rift-related suite that has its parallel in the bimodal suites of the East African Rift Zone. It seems to be rift- related volcanism within the Lower Nicaraguan Rise.