Saba National Marine Park

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Chapter 3. SITE DESCRIPTION

a - General features of the site

Terrestrial surface under sovereignty, excluding wetlands:

0 sq. km

Wetland surface:

0 ha

Marine surface:

8 sq. km

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The Saba National Marine Park  encompasses 8 sq km of biologically diverse coral reef, seagrass, macro-algal beds, sandy bottom and rocky intertidal habitats.

b - Physical features

Brief description of the main physical characteristics in the area:

The coast of Saba is characterised by steep cliffs meeting the sea with rock boulders at their base. There is an ephemeral beach, three small areas where ‘concrete meets the sea’ and a small gently sloping area at Giles Quarter on the south coast. Consequently, there is little coastal development. The marine environment at the coast comprises of a narrow shelf strewn with large volcanic boulders, lava flows and overhangs most of which have become encrusted with corals, sponges and algae. The boulders become dispersed in some areas and at greater depths, giving way to sandy bottom habitats and patched of seagrass. 'The Pinnacle’s', a sea mount 1km off shore is an environment unique in the region.

Geology:

 

The following description covers the geology of the island of Saba with some specifics on the marine geology.

Saba is the northernmost volcanic island in the Active Arc of the Lesser Antilles with an area of only 13 sq. It is rhomb shaped and is a single volcano measuring 4.6 km east to west and 4.0 km north to south rising to a central peak of Mt. Scenery at 887m. The appearance of the island is that of a deceptively simple stratovolcano, but this is not the case as it has been built up of a large number of Pelean domes with their aprons of coarse pyroclastic deposits that form a distinctive shoulder on the island at about 450 to 500 m a.s.l. Mt. Scenery is a younger cone perched somewhat eccentrically to the north on this foundation of Pelean domes. A prominent sector collapse scar exists on the south-western flanks of the volcano in which the island's administrative capital sits. The upper part of this scar has been buried by the younger deposits of Mt. Scenery and the lower part by the Pelean dome and pyroclastic aprons of Great, Bunker and Paris Hills. The submarine flanks of the island slope away uniformly in all directions, except the west where about 1.3km offshore there is a single parasitic conical submarine Pelean dome rising from depths of 300m to only 23 m below sea level, which forms the Pinnacles dive site.

The island is essentially a complex of andesite Pelean domes with their aprons of block and ash flow deposits, together with a few domes that have produced short thick dome flows that are steeply inclined on the island slopes. From a suite of 96 samples only two were basalt and both came from the oldest volcanic center on the island at Torrens Point. Lavas of basaltic andesite composition are sparse, as are basaltic andesite pyroclastic deposits. However a prominent basaltic andesite lava flow dominates the northeast quadrant of the island with the island airport built on its distal part. Dacite is present mainly as pumice and ash, which together with some andesite reflect Plinian-style activity. Also present in small amounts are semi-vesicular andesite pyroclastic.

The stratigraphy of the island can be subdivided into two parts. The older parts of the island predate the sector collapse scar and the younger deposits that postdate the scar and include Mt Scenery. The rocks of the island have proved too young to be dated by the K-Ar method as they are less than 1 million years old. Recently three 39Ar/40Ar ages were determined for the older parts of the island and yielded ages of 0.42, 0.36 and a third sample that yielded 0.21 and 0.13 million years. The age of the sector collapse scar is estimated at around 100,000 years and younger deposits have been dated by radiocarbon methods.

Saba island has erupted lavas of a single magma series which is medium-K low-Ca calc-alkaline. The compositional range is from 49% SiO2 to 65% SiO2, and the lavas are characterized by an abundance of rounded medium-grained hypabyssal (dike rock) nodules that are more mafic than their host.

Hot springs are present on the northwest and southeast coasts of the island and on the sea floor immediately offshore of these areas. Orange areas of discoloration are present on the flanks of the volcano marking the sites of former fumarolic activity. On the northeast coast immediately underlying the prominent basaltic andesite lava flow, is a layer of yellow sulfur mineralization that has been mined in the past.

For further details see Attachment 4.

Soil:

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Topography:

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Bathymetry:

The bathymetry of Saba National Marine Park is characterized by near shore dropoffs at Flat Point, Spring Bay, and Corner Point. Within 60 m depth limits, the shelf surrounding Saba is typically 300-500 m wide; at its widest point north of the island, the shelf measures approximately 1,000 m (Attachment 15).

The bathymetry of the Saba Seamounts has been a topic of study since 1991, with accurate maps being produced in 2013 (Attachment 6).

Hydrodynamics:

With Saba’s exposed nature, narrow shelf and surrounding deep water, strong currents and rough seas (1-2 m average wave height) are prevalent around most of the island. Information on ocean currents is limited; there have been no field investigations to date, although there is some generic information available for the region (below). The intensity of the prevailing southeasterly winds makes for rough seas on the southern and eastern coasts of the island for much of the year. From November to March, powerful swells that originate in the North Atlantic impact the leeward western side of the island.

Saba and the neighboring islands are affected by The Antilles Current. The Antilles Current was named in 1876, and flows northward east of the Antilles joining the Florida Current past the outer Bahamas. Its waters are concentrated into a strong northward Jet about 80-100 km wide centered at 400 m (Lee et al., 1996).

Mooring studies have indicated that the Antilles has mean transport speeds of 3.2 Sv northwards in the upper 800m of water. In addition there is deeper flow from the Deep Western Boundary Undercurrent below 800 m carrying 33 ± 10.9 Sv southwards (Lee et al., 1996). The influence of this deep flow results in a large, mean southward transport for the entire water column.

Deeper, colder waters surrounding Saba (between 300m and 500m depth) are through to have some influence on the environment and ecology of the Saba National Marine Park.

Although no tidal data have been recorded for Saba, it may be possible to estimate tidal variations from data recorded on adjacent islands. From 45 tide gauge locations in the Caribbean, concluded that, for the most part, the Caribbean has a micro tidal range of 10-20 cm. Saba is located in an area with a predominantly mixed diurnal tide, with a mean tidal range of 15 cm.

 

Lee, T. N., Johns, W. E., Zantopp, R. & Fillenbaum, E. R. (1996). Moored observations of western boundary current variability and thermohaline circulation 26.5°N in the subtropical North Atlantic. Journal of Physical Oceanography , 962-963.

Volcanic formations:

See Geology, Attachment 4.

Sand dunes:

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Underwater formations:

There are various underwater formations around Saba, making a number of fascinating dive sites each with their own unique character.

The Pinnacles that rise from the ocean floor up to depths of 25m (80ft) were formed by past volcanic activity and are nourished by deep ocean currents. The Pinnacles are covered with corals, sponges and other invertebrates.

The large spires at Wells Bay and Torrens Point form a protected cove, an ideal location for snorkeling or shallow diving. Underwater caves and tunnels are interesting structures that are home to many species.

Man 'O War Shoals and Diamond Rock are guano covers islets that appear to be submerged and semi-submerged extensions of Torrens Point headland. Walls and rocks nearer the shore are covered with colorful sponges, smaller corals and Sea fans.

Around Ladder Bay a natural labyrinth of groove formations and protrusions developed as a result of lava flows.

Just west of the Fort Bay harbour is another unusual geological structure known as Tent Reef. It is an extended rock ledge that starts at only 4 m (13ft) deep but becomes progressively deeper as you head northwest. The ledge is deeply undercut at some points, providing shelter to large fish. It turns into a sheer wall that gradually becomes fragmented and appears as a series of steep coral outcroppings separated by deep sand channels.

Most of Saba's East coast marine environment is dominated by encrusted boulders of volcanic origin. The only true reef (made out of limestone) formations around Saba are found at Greer Gut and Giles Quarter. Diverse species of reef fish and other marine life along with the white sand covering the sea floor provide a very different environment to Saba's other marine sites.

Exposure to the Atlantic on the East side yields the development of hard coral structures more often than soft coral. Close to shore, well-developed Elkhorn coral formations occur despite considerable wave action and exposure to storms.