Download RECENT SEDIMENTATION PATTERNS IN TRINIDAD AND TOBAGO

RECENT SEDIMENTATION PATTERNS IN
TRINIDAD AND TOBAGO
by
Derek Hudson
Manager, Natural Resources Division
Institute of Marine Affairs
P.O. Box 3160, Carenage.
INTRODUCTION
From the perspective of bathymetry, the islands of Trinidad and Tobago are situated on the
continental shelf of Eastern Venezuela (Case, 1975). In addition, it is immediately northeast
of the delta system (Delta Amacuro) of the Orinoco River, the northern most of South
America's three major river systems. It is this physiographic relationship which forms the
major influence on recent sedimentation patterns in the marine areas of Trinidad and Tobago.
This paper summarizes these Recent sedimentation patterns as a result of this relationship,
by treating Tobago; the Gulf of Paria, South Coast, East Coast and North Coast of Trinidad
as discrete entities Beach and nearshore sedimentation, localized fluvial inputs, and their
interrelationship, if any, with deltaic, marine platform and basinal deposition are also
examined; in conjunction with prevailing littoral processes and oceanographic conditions.
ORINOCO RIVER SYSTEM
The Orinoco River and its tributaries form a drainage basin which cover an area of
approximately 950,000 km2 . It is bordered by the Andes Mountains to the west and north,
the Guyana Highlands to the east and the Amazon watershed to the south. The river itself is
2,200 km in length. The delta system is a wide triangle of alluvium (van Andel and Postma,
1954), extending for about 400 km along the Atlantic coast from Pedernales (Boca Vagre) on
the Gulf of Paria to Punta Barima on the Boca Grande. The southwestern tip of Trinidad, is
separated from this system by the Serpent's Mouth, is less than 15 km distance away from it.
PREVAILING OCEANOGRAPHIC CONDITIONS
The major current patterns around Trinidad and Tobago result from the effect of the Guiana
Current. Tobago is subjected to the full force of this current. It is predominantly NW-going,
and divides on striking the SE coast of Tobago; one part flowing past the NE at speeds of up
to 2 m/sec, and the other passing between Trinidad and Tobago in a NW direction. Along the
east coast of Trinidad the current is moving toward the NW, while it is W-going along the
north coast. In the Columbus Channel between Venezuela and South Trinidad, the W-going
current attains a speed of 1 m/sec, increasing to 1.5 m/sec as it approaches the Serpent's
Mouth. The current then sets NW as it enters the Gulf of Paria, subsequently turning N and
emerging through Bocas del Dragon. S-going tidal streams which enter Bocas del Dragon is
modified by a N moving current to produce the circulation gyre of the Gulf of Paria (South
America Pilot Vol. 4, 1983).
The predominant wave approach for Tobago and the north and east coasts of Trinidad is from
the east and northeast. Along the south coast of Trinidad wave approach is from the eastsoutheast and southe3st.
RECENT SEDIMENTATICN PATTERNS
Beaches
Composition and textural studies of beach sediments in Trinidad and Tobago indicate
deposition of these sands in close proximity to a local source, e.g. adjacent rock formations
(Hudson et al., 1980; Georges et al., 1983).
Alternating biannual cycles of sediment erosion and accretion are evident on most beaches.
However variations in localized littoral processes result in lack of coincidence of these cycles
between beaches, even along the same coastline.
Tobago
The offshore shelf sediments of Tobago are composed of mixed terrigenous and carbonate
components. Minor amounts of glauconite and gray calcareous grains (organic tests) are also
found associated with these sediments. Studies by Koldewijn (1958) suggest that these
sediments were deposited during the last Pleistocene period of low sea level. An epidote-
hornblende-augite heavy mineral association precludes a nearby, local source for these
sediments, and therefore beach and nearshore sands around Tobago are not transported in
appreciable quantities to offshore areas.
Sediment accumulation of the mineral, carbonate and lithic grains of the nearshore sediments
correlates well with the current patterns. Material is probably transported in a southwesterly
direction along both coastlines of Tobago, and deposited in the lee of the island in the area of
Drew Bank. This seems to compare well with the model for current flow without circulation,
which was developed for islands by Murray et al. (1977), and exemplified by sediment
accumulation patterns in Grand Cayman.
Gulf of Paria
Studies by Van Andel and Postma (1954), van Andel and Sachs (1964) and Milliman et al.
(1982) provide very clear insights to recent sedimentation patterns in the Gulf of Paria.
The substrate of the Gulf is characterized by flat topography and very gentle slopes. The
centre of the western Gulf exhibits a wide depression open to the east with a steep channel in
the middle. Two other deep basins occur in the Gulf, one in the centre and the other off
Pointe-a-Pierre. An elongate deep trough is located south of Guiria. Platforms occur off
Southwestern Trinidad, the Caroni River and at Oropuche Bank. (van Andel and Postma,
1954).
Recent sediments in the Gulf of Paria are deposited on a Pleistocene surface. This surface is
marked by low relief with wide valleys and gentle undulations. Recent deposition has tended
to eliminate the irregularities of Pleistocene topography. Sediment accumulations are greatest
in the northern areas of the Gulf of Paria and the depressions of the Eastern Gulf. Pleistocene
highs e.g. Northern Caroni Platform are characterized by minimal sedimentation. van Andel
and Postma (1954) tentatively calculated depositional rates of 60-100 cm/100 years for the
delta zone, 12 cm - 30 cm/100 years for Central Gulf of Paria and 20 cm/100 yrs for the
platform area off Southwestern Trinidad. van Andel and Sachs (1964) recorded 3 regionally
correlatable erosional surfaces separated by sediments as a result of deposition of Gulf of
Paria sand and silts during the Holocene transgression.
Compositionally, a large part of the Gulf is covered by honey-coloured Orinoco Sands. Schist
fragments occur along the Paria Peninsula and Bocas del Dragon. On the platform areas of
southwestern Trinidad well-rounded quartz sands with rock fragments (Soldado Sands) are
found. North and eastward on the Soldado Platform and Oropuche Bank, the Soldado Sands
are replaced by Glauconitic Sands, which are marked by authigenic glauconite. Shoreward of
this area the Glauconitic sands are replaced by pink-yellow quartz sands (Oropuche Sands).
In several areas limonitic pellets and aggregates are conspicuous. Ovoid faecal pellets are
found in the water of the Northern Caroni Platform. Clay minerals are characterized by illite,
montmorillonite and kaolinite. Dead shell beds are observed in the eastern half of the Gulf of
Paria (van Andel and Postma, 1954).
Heavy mineral assemblages are characterized by 0 (Orinoco)-hornblende and -zircon
associations in the Orinoco delta sands. The platform areas of Southwestern Trinidad are
marked by T (Trinidad)-epidote, T-zircon and T-garnet associations. In the northern Gulf of
Paria, 4 assemblages are recognized. These associations are P (Paria)-chloritoid, Ptourmaline, P-hornblende and P-epidote.
The heavy mineral associations indicate that the Orinoco Sands are derived from the Orinoco
Drainage Basin, or from the Guyana Shield through westward movement of longshore
currents. The latter source is transported through the main mouth of the Orinoco River. Rock
particles in the northern Gulf of Paria are deposited by rivers from the adjacent mountainous
areas. The source of the platform sediments around Southwestern Trinidad are the Tertiary
and Pleistocene deposits in this area, including sea-floor outcroppings. These sediments are
supplied by cliff and headland recession through wave erosion (van Andel and Postma,
1954). Studies by Milliman et al. (1982) indicate that about 20% of the finer material in the
Gulf is derived from the Orinoco, with the remainder from the Amazon. Localized effects of
fluvial input to nearshore areas are exemplified by seasonal changes in surficial sedimentary
fades patterns (Hudson et al., 1985).
South Coast, Trinidad
The Southwestern section of this gently southward. This smooth coastline, the Erin-Moruga
Platform, slopes topography is interrupted by several mud volcanoes. Towards the east, the
platform thins out considerably on reaching the Guayaguayare area, where a coral/algal
hardground exists.
Compositionally the sediments are comprised of Glauconitic Sands, which are replaced
shoreward by quartz-rich Oropuche Sands. Faecal pellets occur in the shallow areas of
therein-Moruga Platform, along with extensive accumulations of shell beds. Orinoco Sands
have been observed on the margin of the platform, with minor mud accumulations in the
upper areas (van Andel and Postma, 1954). T-garnet association is the major heavy mineral
assemblage deposited in the area.
Sediments in this area are derived from the adjacent rock formations, with extensive
reworking by prevailing wave activity. The coastline is characterized by areas of erosion and
deposition usually associated with each other locally, through longshore transportation.
Orinoco Sands are derived from the delta system through current transportation, while the
Orinoco and Amazon River Systems are the source of the muds. Sedimentation beyond the
shelf break is probably minimal as a result of current scour by the prevailing W-going Guiana
Current.
East Coast, Trinidad
The shelf edge along this coast lies at about 80 m depth. The shelf/slope break is not very
distinct. Topographic highs inhabited by coral colonies are the dominating physiographic
features in this area. Examples are Emerald Shoal, Delaware Bank, Darien Rock and
Manzanilla Bank (Koldewijn, 1958).
Sediments in the southern part of this area are composed of sandy calcareous grains and
muds. A hard dark gray clay with minimal carbonate content occurs along the shelf edge.
Sand-size carbonates and terrigenous deposits predominate throughout the rest of the area,
with a coarse calcareous content around the coral formations. Authigenic glauconite is also
observed in these sediments, with the major heavy mineral association being epidote-zircon
(Koldewijn, 1958).
The heavy mineral association indicates that seaward transportation of coastal material is
very minimal. Sands produced by erosion or brought in by rivers remain within the nearshore
depositional system. Studies by Koldewijn (1958) suggest that sands in this region were
brought in by a tributary of the Orinoco during a Pleistocene sea level low stand. These sands
are being reworked by wave and current action at present.
North Coast, Trinidad
The northwestern region of the North Coast is characterized by a shallow platform, which is
bordered by the 40 m contour. The central part is a wide depression with a maximum depth of
170 m.
Sediments in this area are marked by sandy calcareous muds in the east, grading into
terrigenous muds to the west. Very fine-grained pyrite has also been identified in these
deposits. The heavy mineral assemblage is primarily epidote-zircon, and augite with
hypersthene (Koldewijn, 1958).
Like the East coast, the heavy mineral associations suggests minimal transportation of sand
to offshore marine areas from the coastline. Sediments in this region are derived from
reworking and transportation of east coast deposits by the prevailing Guiana current, and to a
lesser extent by localized submarine volcanic mounds. This current also transports mud-size
material to this area from the Orinoco and Amazon River Systems, via the East coast and the
Gulf of Paria.
PROPOSED STUDIES
In order to better understand Recent sedimentary phenomena in the marine areas of Trinidad
and Tobago, complement the information summarized above, and set this phenomena in a
geologic framework which can be applied more directly by Oil Companies exploiting our
hydrocarbon marine resources, the geologists of the Institute of Marine Affairs have recently
developed a project to meet these needs. The project "Recent sedimentation and geological
mapping study in the Petroleum exploration and Producing areas in Trinidad" proposes to
provide sediment data, which would assist oil and gas well site installations; produce local
models of shallow marine and deltaic deposition, and develop regional maps of Recent
sediment distribution, stratigraphy and marine bedrock geology.
Based on the ambitious objectives outlined, it is hoped that the necessary support would be
forthcoming from the Oil Companies and Government Agencies concerned, such that this
venture would be a success.
REFERENCES
Case J.E., 1975, Geologic Framework of the Caribbean, In Geology, Geophysics and
Resources of the Caribbean: Report of the IWE Workshop on Geology and Marine
Geophysics of the Caribbean Region and its Resources, Kingston, Jamaica, p. 3-26.
Georges, C., Greenidge, B., and Hudson, D., (in press) Beach Sediments of Trinidad, West
Indies. Transactions of the 10th Caribbean Geological Conference, Cartagena, Colombia,
August 1983.
Hudson, D. et al., 1986, Beach Sediments of Tobago, West Indies. Transactions of the 9th
Caribbean Geological Conference, Santo Domingo, Dominican Republic, p. 139-149.
Hudson, D., Bachew, S., and Williams, E., 1985, Sedimentological and Geophysical Studies
in the Oropouche Bank Area, Gulf of Paria, Trinidad. Transactions of the 1st Geological
Conference of the Geological Society of Trinidad and Tobago, Port of Spain, p. 67-77.
Koldewijn, B.W., 1958, Sediments of the Paria-Trinidad Shelf: Reports of the Orinoco Shelf
Expedition, Vol III Mouton and Co. 'S-Gavenhage. 109 p
Milliman et al., 1982, Depositional Patterns of Modern Orinoco/Amazon Muds on the Northern
Venezuelan Shelf. Journal of Marine Research, V. 40, No. 3, p. 643-657.
Murray, S.P., et al., 1977, Nearshore Current Fields around Coral Islands: Control on
Sediment Accumulation and Reef Growth. Proc. 3rd Int. Coral Reef Symposium, Vol. 2,
Geology, Miami, Florida, p. 54-59.
South America Pilot, Vol. 4 1983, North coast of South America, Cabo Orange to Punta Tirbi
including Trinidad and Tobago, Rio Orinoco and Nederlandse Antillen. Hydrographer of the
Navy, Second Edition.
van Andel TJ., and Postma, R., 1954, Recent Sediments of the Gulf of Paria:
Reports of the Orinoco Shelf Expedition, Vol 1. North-Holland Publishing Company,
Amsterdam, 240 p.
van Andel, TJ., and Sachs, P.L., 1964, Sedimentation in the Gulf of Paria during the
Holocene Transgression; a Subsurface Acoustic Reflection Study. Journal of Marine
Research, V. 22, No. 1, p. 30-50.