caa a22 4500 469045604 CHVBK 20180323132823.0 cr unu---uuuuu 170328e19921201xx s 000 0 eng 10.1007/BF02536804 doi (NATIONALLICENCE)springer-10.1007/BF02536804 Geister Jörn Geologisches Institut, Universität, Baltzerstr. 1, CH-3012, Bern aut Modern reef development and cenozoic evolution of an oceanic island/reef complex: Isla de Providencia (Western Caribbean sea, Colombia) [Elektronische Daten] [Jörn Geister] Summary: Providencia Island in the SW Carbbean is 4.5 to 8.5 km across (including Sta. Catalina Island). In contrast to nearby San Andrés, which is an elevated Tertiary atoll, Providencia is formed by an extinct Miocene volcano. This lies far off the Middle American mainland, and therefore its geological history is somewhat unique among other western Caribbean islands. The submarine basement of Providencia rises with steep to vertical slopes from an ocean sea floor of approximately 2,000 m depth. The island itself is rugged with peaks reaching up to more than 360 m above present sea-level. It is surrounded by a wide carbonate insular shelf protected towards the N,E and SE by the second largest barrier reef (after that of Belize) of the Caribbean Sea. The entire reef complex forms a carbonate shelf, which consists of a 32 km long windward bank-barrier reef with lagoonal environments in its lee, dotted with patch reefs and minor fringing reefs. Seaward of the barrier there is a wide fore-reef terrace dropping off to the upper island slope. In contrast, the leeward shelf, lagoon and coastal areas are unprotected to the open sea by major coral reefs and submarine showls, though minor reef structures resembling relics of a former barrier reef are present. Hence, the leeward environments are exposed against storm waves approaching from the West. The submarine topography of the insular shelf is characterized by several lagoonal basins, up to 14 m deep, which may be partly of karstic origin. At water depths of 2-6m wide areas are occupied by extensive shallow lagoonal terraces. At least, two stream gullies continue from the island as submarine channels onto the insular shelf. A submerged elongate ridge in shelf-margin position is situated at more than 25 m of depth and may be a drowned shelf-edge barrier reef. These observations and the presence of submerged terraces indicate that the contemporary submarine topography of the modern reef complex has geomorphologic features inherited during lowered Pleistocene sea-level stands. A major part of the barrier reef is formed by a wide belt consisting of numerous patch reefs, mostly of the pinnacle type, which rise from the sea floor at −6 to −8 m reaching to the low-tide level. Such discontinuous parts of the barrier reef may be from 100 up to 1,000 m wide. The continuous segments of the barrier reef are around 100 m wide and display well-developed groove-and-spur systems. Locally, segments of a continuous barrier are also present in front of the discontinuous reef belt. The reef crests and upper forereef are overgrown by luxuriantMillepora alcicornis with local patches ofAcropora palmata. Otherwise, the latter species is found mainly in front or behind the crest. Near the NW end of the barrier the outer margin of the reef flat is marked by a true algal ridge. The lagoonal patch reefs vary in shape, size and outline, and their crests are normally below 1 or 2 m water depth. They are characterized by thickets ofAcropora cervicornis in the E and by dense growths ofPorites furcata in the SE of the island. Calm-water associations withMontastraea annularis prevail at deeper and/or calmer lagoonal sites. Crests of unprotected shallow leeward reefs (‘Lawrance Reef', ‘Pearstick Bar') show dense growth ofAcropora palmata. Providencia itself is a volcanic island formed by pre-Miocene(?) to Miocene lava flows, pyroclastics and epiclastic deposits. All of its pre-Miocene(?) to possibly very early Miocene effusives are of the rhyolitic type and seem to be submarine. Subsequent subaerial eruptions in Early to Upper Miocene time yielded large masses of basaltic to andesitic lavas and pyroclastics. As calculated from the dips of their volcanic bedding planes, the former rim of the central crater area may have emerged more than 1,000 m above the present sea-level in Upper Miocene time. Since then, the relief of the island gradually diminished by subaerial erosion aquiring its present aspect of a deeply eroded volcanic cone. The crater diametre was about 1 km. Intercalations of carbonates within the younger series of volcanic deposits reveal lagoonal deposits near its base and carbonate slope deposits interfingering with the higher volcanic strata. The lagoonal deposits are horizontally bedded tuffaceous carbonates, which yielded a soft-bottom coral fauna, burrowing echinoids and pelecypods signalling the first direct evidence of an early carbonate island shelf. The seaward-sloping higher strata contain massive and branching reef corals and large isolated oyster valves. The fossils suggest an Early to possibly Middle Miocene age of the carbonates. The regional tectonic pattern of the Western Caribbean deep-sea floor is notable for its conspicuous fracture zones. It appears that the primitive submarine volcano of Providencia initiated in early Tertiary time along a tectonic fracture line paralleling the NNE trending San Andrés Trough to its E. It is assumed that the outflows of submarine(?) lavas along a fissure formed an elongated submarine volcanic ridge in fairly shallow water. By Miocene time, subsidence compensated by shallow water carbonate sedimentation and upgrowth of coral reefs lead to the formation of a carbonate platform, most likely of the coral bank or atoll type. It probably showed already the approximate outline and size as the present insular shelf. A second period of volcanic activity in Early to Late Miocene yielded basaltic to andesitic lavas and pyroclastics during several major subaerial eruptions. These formed five conspicuous volcanic tongues, which radiate to the sea from a crater area in the centre of the island. The early eruptive phase was probably contemporaneous with the formation of several additional shallow submarine volcanos in the Western Caribbean. They appear equally bound to fracture lines on the sea-floor. These volcanic structures are deeply submerged today and capped by thick limestone deposits forming the remaining atolls and islands of the archipelago. Of these, only nearby San Andrés was uplifted in latest Tertiary times thus revealing today its Miocene reef and lagoonal deposits. But, in contrast to Providencia, in none of these was there a second period of eruptive activity in late Tertiary times to form a long-living emergent volcanic build-up. Quaternary sea-level oscillations are indicated by subaerial and submarine terraces cut into coastal limestone by advancing sea cliffs. There is a relic of an erosional terrace at +50-60 m in the Miocene limestone, probably of Early or Middle Pleistocene age. The wide fore-reef terrace with its outer margin at depths around −20 to −40 m indicates a prolonged low sea-level stand of pre-Sangamonian age. A fossil fringing reef terrace of Sangamonian age, reaches a maximum elevation of about +3 m above present sea-level. The fossil coral associations of this reef indicate an environment fairly protected from major waves. Thus, it may be assumed that the contemporary outer reef barrier protecting the island coast reached more than 3 m above the present sea-level. In addition there is evidence that the coral associations of the fossil reef lived in water depths possibly near 10 m. The very shallow terraces situated in front of the active limestone cliffs and around certain patch reefs were formed by planation towards the end of the Holocene transgression. Size and shape of the island changed periodically during Pleistocene sea-level fluctuations. Due to the high and relatively steep island relief, the Pleistocene high sea-level recorded by the '+50-60 m—Terrace' would not have submerged more than about 35% of the present land surface area. With the exception of the flooding of the coastal low-lands and some deeper valleys and the formation of smaller satellite islands by temporarily isolating some of the higher headlands, the configuration of Providencia did not undergo any essential change. By contrast, during low stands (−25 to −120 m) that followed the great Sangamonian transgression until the Wisconsinian stage, the total of the extensive island shelf was almost permanently emergent for a period of more than 100,000 years. Geomorphologically, the reef complex appeared as an elongated limestone table mountain bounded by sheer cliffs which rose more than 100 m above sea-level. During this period the emergent insular shelf formed an extended northern prolongation of the original volcanic island. The entire island measured some 30 km in length in Wisconsinian times, and its surface area totalled roughly 12 times that of the present island. From the evidence above we may draw the preliminary conclusion that the existence of an insular shelf can be traced back at least to Miocene time. The contemporary shelf morphology is the product of a complex history of sea-level oscillations accompanied by terracing at different levels, renewed reef growth and erosion. Of this history, at present, only a few evolutionary stages may be recognized. Volcanic activity did not contribute to the geomorphologic evolution of the island and shelf in post-Miocene time. The shelf was last exposed to subaerial weathering during the sea-level lowering that accompanied the late Wisconsin glaciation. It appears that since reflooding in the early Holocene some 5,000 years ago, renewed reef growth and sedimentation have only partly concealed or modified the pre-existing shelf topography. Institut für Paläontologie, Universität Erlangen, 1992 Ecology nationallicence Oceanic reefs nationallicence Barrier reef nationallicence Coral reefs nationallicence Island evolution nationallicence Palaeogeography nationallicence Volcanism nationallicence Isla de Providencia (Colombia) nationallicence Caribbean sea nationallicence Tertiary (Miocene) nationallicence Pleistocene nationallicence Holocene nationallicence Facies Springer-Verlag 27/1(1992-12-01), 1-69 0172-9179 27:1<1 1992 27 10347 https://doi.org/10.1007/BF02536804 text/html Onlinezugriff via DOI 1 research-article jats NATIONALLICENCE 856 40 https://doi.org/10.1007/BF02536804 text/html Onlinezugriff via DOI NATIONALLICENCE 100 1- Geister Jörn Geologisches Institut, Universität, Baltzerstr. 1, CH-3012, Bern aut NATIONALLICENCE 773 0- Facies Springer-Verlag 27/1(1992-12-01), 1-69 0172-9179 27:1<1 1992 27 10347 Metadata rights reserved Springer special CC-BY-NC licence nationallicence BK010053 XK010053 XK010000 NATIONALLICENCE NATIONALLICENCE NL-springer