HYDROGRAPHIC AND MARINE GEOLOGICAL SURVEYS OF LOS IROS BAY, SOUTHERN TRINIDAD
S. Bachew, P. Joseph and D. Hudson
Natural Resources Programme. Institute of Marine Affairs
P.O. Box 3160. Carenage. Trinidad. West Indies
INTRODUCTION
Los Iros Bay is located on the southern coast of Trinidad, between Erin and Taparo Points at approximately 100
4' North latitude and 610 39' West longitude (Figure 1). Based on the coastal classification
of Trinidad (Georges, 1982), it is defined as an open sea beach, backed by a cliffed coast which experiences an
extremely high rate of coastal erosion, averaging 0.65m/yr, for the years 1957-1971.
A marine survey was carried out at Los Iros in October 1981 by the Geology Laboratory-Natural Resources Programme,
Institute of Marine Affairs. The objectives of this survey were to provide hydrographic and marine geological data
to engineering consultants, for purposes of obtaining design criteria for shore protection structures to combat
the erosion problem. These preventative measures are in response to the importance of Los Iros as a major recreational
facility in Southern Trinidad. These structures are to be designed and constructed by the Drainage Division, Ministry
of Works, Maintenance and Drainage. The following is a brief outline of the results of these surveys.
COASTAL GEOLOGY
According to the work of Suter (1960), Kugler (1959) and field checks by Deane (1974), the major geological formation
outcropping along the Los Iros coast is mud flows, with numerous vents, of Recent age (Figure 1).
These mud flows are formed due to the presence of trapped hydrocarbons under supra-normal pressures in underlying
rocks (Kugler 1965) The mud flows are underlain by boulder and pebble beds in a clay matrix, which are of the Lower
Cruse formation of Miocene age. These conglomeratic beds are exposed as l0m cliffs at Taparo Point.
METHODOLOGY
Hydrographic Survey
Hydrographic surveying is defined as the surveying of a water area, which survey includes measurements of tides,
currents, gravity and determination of the physical and chemical properties of water (Hydrographic Manual, 4th
Ed. 1976). Hydrographic measurements taken during the Los Iros survey include bathymetry, waves and currents.
Bathymetry was determined using "the radial track survey method", whereby depths along radials of 100
intervals from shore-based theologize stations were recorded by a Raytheon DE-719B portable fathometer. Three theodolite
stations (X, Y and Z, Figure 1) were established to ensure proper visual coverage of the bay.
Horizontal control was achieved by having the survey boat follow radial tracks directed by one theodolite and two-way
radio communication. The intersecting angles to position the boat were obtained from the other two theodolites,
which shoot angles at 30-second intervals, at which time, depth fixes were marked on the fathometer. These depths
were corrected to lowest astronomical tide, superimposed on the plotted positions and contoured to give a bathymetric
profile (Figure 2).
Wave heights were measured at offshore stations by running the fathometer at its highest recording rate of 10cm/mm.
At nearshore stations the highest and lowest water levels were observed on a measured staff. Wave periods were
calculated by counting waves over a five-minute period passing the bow of the survey vessel.
Based on the wave data collected, wave refraction~ diagrams were constructed using the orthogonal method (Shore
Protection Manual Vol. 2, 1977).
The speed and direction of longshore currents were determined at beach stations by timing a weighted bottle over
a measured distance. Direction was observed with the hand bearing compass. At offshore stations, a buoy attached
to a measured line and timed over a specific distance, determined surface current speeds. Direction was again observed
with the hand-bearing compass. A current cross attached to the buoy at varying depths measured the currents at
three depths. General Oceanic current meters, positioned at suitable locations, film recorded the speed and direction
of oceanic currents at pre-arranged time intervals.
Marine Geological Survey
Marine geological surveying is defined as the study of an area of the earth under water, with respect to its origin,
structure, composition and history, and the nature of the processes which have given rise to its present state.
Marine geological analyses during the Los Iros survey were restricted to surface sediment sampling.
Approximately 500 gms sediment samples were collected, using the Wildco one-litre grab sampler
and oven dried at 950 C for 12 hours. One hundred grammes sediment were then dry sieved with a Soil Test sieve
shaker, and its textural parameters calculated using the formulae derived by Folk and Ward (1957). Compositional
estimates of the sediments were also determined by examination under a Bausch and Lomb binocular microscope.
RESULTS
Hydrographic Survey
A contoured bathymetric map (Figure 2) of Los Iros Bay indicates a certain degree of parallelism
between the coastline and nearshore contours. The even and widely spaced contours are indicative of uniform and
gentle slopes, with maximum and minimum gradients of 50 33' and 00 25', respectively, averaging 00 59'.
The coastline at Los Iros is exposed to a two pronged wave attack, with predominant directions from the ESE and
SE. Wind-induced waves constitute the major component of wave attack, with the frequency of ground swells increasing
from January to April. Measured wave-heights and periods for the survey period are shown in Table 1.
Table 1: Measured values of wave-height and period,
Los Iros Bay, October 1981
Wave Height (m) Wave Period (s)
Maximum 0.8 8.0
Minimum 0.1 1.0
Mean 0.3 4.0
Significant Value 0.5 5.0
A wave refraction diagram drawn with the predominant wave direction from the ESE (Figure 3A),
and a wave-period of 5.0 secs, demonstrates a more concentrated wave attack on the western section of the coastline
compared to the eastern section Another wave refraction map (Fig 3B) with wave approach from
the SE, and a wave-period of 5.0 secs indicates and increased wave attack on the eastern portion of the bay. However,
the ESE wave approach predominates at Los Iros, hence the coastline to the west of the bay is subjected to a more
concentrated wave attack compared to the east.
As suggested by the wave refraction diagrams (Figures 3A and 3B), wave crests approach the
Los Iros coastline at shallow angles of 100~120. The exception being the eastern corner of the bay between groins
3 and 4 and the headland. This is indicative of a stronger longshore component in eastern Los Iros Bay as confirmed
by the measured values outlined in Table 2. These longshore currents move towards the SW.
LOCATION Average Measured Values (m/mm)
Between Groin 4 and Point Z 26
Between Groins 4 and 3 39
Between Groins 3 and 2 15
At Groin 2 5
Between Groins 2 and 1 16
At Groin 1 6
Between Groin 1 and Erin Pt. 7
Current measurements recorded using the film recording current meters are indicative of a dominant E-W flow, especially
during rising tide. During falling tide, a persistent SW component prevails. Hence a complete reversal of current
direction does not occur at Los Iros. Currents flow at average velocities of 21.4cm/sec during high tide and 18.7cm/sec
at low tide. Spot measurements of surface and bottom currents at offshore stations agree with the current meter
recordings.
Marine Geological Survey
Texturally, surface sediments in Los Iros Bay vary in particle size from 1.77 (.30mm) to 3.82 (0.07mm), averaging
3.22 (.11mm) in diameter. That is, they are generally of a very fine size, ranging from fine sand bordering the
beach to muddy sand in deeper bay areas, with portions of slightly gravelly to gravelly muddy sand. Beach sediments
are of a fine sand size, with the exceptions being the western portion of the beach which grades from a muddy to
slightly gravelly sand, and the area bordering the car park which is a coarse sandy gravel (Figure
1). The eastern sector of the bay consists of well sorted sediments, in contrast to the west where the sediments
are poorly to moderately sorted. Moderately well sorted sediments are characteristic of the beach area. An exception
is the area adjacent to the car park which is of a poorly sorted nature.
Los Iros Bay sediments are comprised primarily of quartz grains (15%-91%, averaging 79%), with minor amounts of
mollusc and sedimentary rock fragments. Quartz grains also predominate within the beach sediment (59%-79%, averaging
73%), with subordinate amounts of sedimentary rock fragments. However, in eroded portions of foreshore beach faces,
mollusc particles are highly abundant (71%-91%), and are redistributed to the beach sediment in greater amounts
(1%-19%, averaging 8%), compared to the main bay.
DISCUSSION
Nature and source of Sediments
Sediment size within the bay is generally related to depth with fine sand occurring at depths of 1m or less, and
muddy sand predominating in deeper bay areas (Figures 1 and 2). The exceptions
to the latter being the areas of slightly gravelly to gravelly muddy sand where molluscan shells are deposited
in situ (Figure 1) or differentially sorted by wave action. An occurrence of fine sand southwest
of Taparo Point is probably the result of the tailing off of homogeneous suspended sediment masses, transported
around Taparo Point to the eastern portion of the shallow bay and beach areas. Muddy to fine sand predominates
on the beach, except in areas where eroded anthropogenic features e.g. the car park, result in the occurrence of
sandy gravel or slightly gravelly sand.
Sediments are better sorted in the eastern section of the bay, compared to the west. This is due to the prevailing
EW current direction, continued massive erosion at Erin Point, and the occurrence of molluscan deposits in the
western sector.
The most conspicuous detrital constituent occurring in the sediments of Los Iros are quartz grains, which are derived
from the adjacent mudflows, and the sedimentary sequences of the Lower and Upper Cruse formations to the west of
the bay. The source of the sedimentary rock fragments (lithic grains) are the boulder and pebble beds which underlie
the mudflows. The presence of shell particles within the area is due to the presence of environmentally suitable
molluscan ecological habitats on the Erin-Moruga platform. Molluscan grains may also be derived from dead shell
assemblages deposited at, or near, the present sediment surface. The presence of these dead communities is probably
due to some catastrophic event, which occurred according to radiocarbon dating, approximately 700 years ago (Van
Andel and Postma, l954).
Coastal Erosion and Related Sediment Dynamics
The extent of Los Iros Bay is generally controlled by its two bordering headlands (Deane, 1974), which on erosion
result in the retreat of the contained coastline. This retreat is more prevalent to the west, where the soft, clayey
material of Erin Point is more susceptible to massive cliff erosion. To the east, Taparo Point has stabilized in
recent years, due to a surrounding mantle of naturally deposited boulders, which has inhibited cliff recession
(Rothfuchs and Gerrard, 1982). An additional factor is the predominant ESE wave attack, whose refraction map (Figure 3B) is indicative of a more concentrated wave attack to the west. The sediments of Los
Iros Bay are predominantly derived from eroded mudflows to the east, and to a lesser extent by its bordering outcrops.
These sediments from the east are transported by wave diffraction and longshore current movement around Taparo
Point, and deposited between Pt. Z and groin 4 (Figure 2). A relatively strong longshore component
in Eastern Los Iros Bay, results in a piling up of this deposited sediment to the windward of groin 4. Hence of
the four groins built groin 4 is the only structure which has retarded erosion effectively. Weak longshore currents
along the remainder of Los Iros Beach, has resulted in the ineffectiveness of groins 1 to 3 with respect to the
erosion problem. Wave generally impinge on the central and western coastline of Los Iros at shallow angles, a fact
which results in an onshore-offshore movement of sediment compared to lateral transport Deposited sediments at
Pt. Z are reactivate (by wave action and transported by the prevailing east-west current direction through and
west of Los Iros Bay. During calm periods when long swells are generated, wave action transports inland some of
the sediments which are travelling in the prevailing currents, and deposits 'them on the beach giving rise to an
aceretionary period in which the onshore movement predominates. During other periods, short, steep waves are generated,
resulting in beach erosion. The sediments which are moved offshore are caught up in the prevailing east-west current
movements, and lost to the system, a depletionary period in which the offshore movement predominates. The positioning
of the present groins, perpendicular to wave approach produces rip currents, which induce the seaward movement
of material. The smooth bottom topography and parallel bathymetric contours are also indicative of an onshore-offshore
movement of sediment. Owing to continuous erosion at Los Iros. It is fair to assume that the deflationary period
prevails, for the sediments lost to the bay through current transport are far greater than that deposited on the
beach during calm periods.
REFERENCES
DEANE, C. (1974) Los lros / Erin Report, 26pp.
FOLK, R.L. & WARD. W.C. (1957) Brazos River Bar: a study in the significance of grain size parameters. Jour.
Sedimentary Petrology, Vol.29 ~ (I): p87-97
GEORGES, C. (1982) - A coastal classification for Trinidad. IMA Research Report IMAIS/83, Institute of Marine Affairs,
Trinidad, 1lpp.
HYDROGRAPHIC MANUAL, 4th Ed. (1976). - US Dept. of commerce, National Oceanic and Atmospheric Administration, National
Ocean Survey
KUGLER, H. C. (1959). - Geological Map and cross-sections of Trinidad (scale 1/l 00,000): E. Stanford Ltd., Long
Acre, London, WC2.
KUGLER, H. C. (1965). Sedimentary volcanism. Transactions of the 4th Caribbean Geological Conference, Port of Spain,
Trinidad and Tobago, p 11-13
GERRARD, T. and ROTH FUCHS, E.C. (1982) - Los Iros erosion protection, design report. Drainage Division, Ministry
of Works, Maintenance and Drainage, l4pp.
SHORE PROTECTION MANUAL, Vol.; (1977). - US Army coastal Engineering Research Centre, Fort Belvoir, Virginia.
SUTER, H.H. (1960). - The general and economic geology of Trinidad, W.I. HM Stationary Office, London, 2nd Ed.
(revised)
VAN ANDEL, TI and POSTMA, H. (1954). - Recent sediments of the
Gulf of Paria. Reports of the Orinoco Shelf Expedition, Vol. 2, North
Holland Publishing Co., Amsterdam, 240pp.
LARUE, D and R.C. SPEED (1983) - Quartzose Turbidites of the Accretionary complex of Barbados, 1. Chalky Mount
Succession.' Journal of Sed. Petrology, v.53, No.4, p.1337-1352.
SAUNDERS, J.S. (1965) - Field Trip Guide - Barbados. - Proceedings of Fourth Caribbean Geological Conference, Trinidad,
p.443-449.
BARBADOS FIELD TRIP
by
Victor Young-Qn Fazal Hosein
Trinidad Tesoro Petroleum Co Ltd., and Texaco Trinidad nc., Santa Flora,
Trinidad ~
Pointe-a-Pierre, Trinidad W.l.
A three-day field trip to the island of Barbados was held from April 20-23, 1984. Some fifteen participants were
led in the field by Dr. J. Frampton and ~Iessrs. J. Keens-Dumas and
W. Ah. Valuable assistance was also provided by local Barbadian geologists Messrs. P. Payne and L. Barker.
The generalized geology of Barbados is shown in Figure 1 and the stratigraphy is summarized in the charts of Figure
2. Except for a small area on the northeast of the island, almost the entire land surface of Barbados is covered
by a Pleistocene coral cap lying unconformably on an accretionary complex of Tertiary age. The coral cap rises
from sea level to an elevation of 1070 feet in a series of nine stepped terraces. each documenting separate phases
of Pleistocene uplift.
A portion of the accretionarv corn plc-x associated with the Lesser Antilles fore-arc basin is exposed in the ScotTand
District of Northeastern Barbados, where it has probably been a sub-aerial high since Middle J\liocene time. This
prism of Tertiary sediments has been subjected to extreme deformation during several phases of tectonism. The geology
of the Scotland District is illustrated in Figure
3.
The G.S.T.T.'s field trip provided some valuable insights into the island's geology. The various localities visited
in the field are shown in Figure 4., and are described in the following compilation of field notes. Reference is
drawn from the works of Saunders (l965), (1970), and Larue and Speed (1983).
DAY 1
STOP 1
CHAPEL QUARRY - Coral Rock Formation.
This exposure consists of a thick (200 ft.) unit of coarse-grained, white coral rock composed mainly of consolidated
back-reef coral detritus. The unit is of Pleistocene aged Coral Rock Formation. The rock weathers to a dark grey
soil, which forms the growing medium for the island's sugar cane crop. At Chapel Quarrv, the rock is cut into rectangular
construction blocks.
STOP 2
RAGGED POINT-Chalky Mount Type Beds,Upper Scotland Formation
At this location, the coastal section exposes early Mid-Focene turbiditic mid-fan deposits similar to the Chalky
Mount beds. Several fining upward sequences can be discerned consisting generally of coarse gritty sands gradinp
upwards into thin silts and light grey clay beds A high iron content, associated with these beds, account for the
occurrence of secondary iror nodules and banded red-brown and yellow-browr colourations. Strong deformation has
resulted in various E-dripping normal faults and contorted beds dipping about 450 to the N.W.
STOP 3.
ST. MARKS CHURCH-Conset Mans
The road cut at this location exposes a white marl containing a rich planktonic and benthonic foraminiferal fauna
(Globorotalia
barisanensis) of early Mid-Miocene age. Ihis exposure also reveals the presence of large, cream coloured exotic
blocks consisting of a coarser, foram-rich sand. perhaps originating from the slightly older Bissex lull Formation(?).
The Conset Marl at this stop is considered to be the youngest rock found in the Barbados aecretionary complex.
and its top surface represents a major depositional hiatus. This formation also contains well preserved radiolaria
which have been identified as belonging to the Dorcadospyris alata zone of the '\Iiddle Miocene.
STOP 4
CONSET BAY & BATH CLIFF-Conset Mans & Bath Cliff Oceanics.
On the south side of Conset Bay, the foramrich Conset Marls are exposed in fault contact with the Bath Cliff
Oceanic formation. The Bath Cliff beds are high1y deformed resulting in tar-impregnated fractures and warped beds
tilted as steeply as 500 to the West. The Bath Cliff Oceanics are comprised of interbedded siliceous radiolarian
oozes and foraminiferal Marls. Thin. dark-grey volcanic ash beds are interpersed throughout the oceanics, but attain
their highest frequency in the older section of the exposure. These ash beds are perhaps associated with the initiation
of volcanic activity in the Lesser Antilles island arc during the Middle Eocene. The Bath Cliff Oceanics cover
an age span from Middle Locene to Early Miocene.
STOP 5
BISSEX HILL-Bissex Hill Formation
Outcrops at the summit of Bissex Hill reveal a cream coloured , medium grained, indurated formainiferal sandstone,
containing minor yellow-brown grandular inclusions. Thin veins of mineralised calcite are observed and are perhaps
produced by secondary diagenetic processes. The Bissex Hill Formation has been dated as Early Miocene, just slightly
older than the Conset Mans. The possible occurrence exotic blocks of the Bissex Hill formation within the St. Marks
Church Conset Marls would suggest that the tectonic uplift of Bissex Hill was probably initiated in the Middle
Miocene.
STOP 10
STOP 6
SPEIGHTSTOWN-Pleistocene Coral Reef
The road cut here, along Highway 1, exposes a cross-sectional view across a Pleistocene coral reef complex. The
back-reef facies is composed of a framework of huge, consolidated fragments of branching coral, Acropora palmata
fronds, Montastraea annularis and Diploria within a very coarse matrix. This grades laterally into the massive,
finer-textured, indurated reefal core, which passes laterally into the fore-reef facies, composed primarily of
fragments of branching coral, palmata fronds and Acropora cervicornis within a coarse indurated matrix.
DAY 2
STOP 7
MOUNT HILLABY-Oceanic Formation
The summit of Mount Hillaby presents a picturesque view of the Scotland District. The Oceanic Formation outcrops
here and consists of white, foraminiferal marls of Middle Eocene age. The oldest zone of the Oceanics are encountered
at the base of this Mount Hillaby section.
STOP 8
MOUNT ALL-Mount All Member, Upper Scotland Formation
This exposure of the Mount All beds comprises stacked turbiditic deposits of well-graded grits, sandstones and
siltstones. The sequence has been dated as early Eocene. The predominance of sands in the section suggests that
these beds were generally laid down in the proximal outer fan zone of the turbiditic depositional environment.
The sequence is probably made up of several supra fan lobes prograding on top of each other. Very severe deformation
of the beds has resulted in tight folding, complex faulting and almost vertical dips.
STOP 9
THE SADDLEBACK-Joe's River Formation
The Joe's River Formation is seen here as a massive, unbedded black oily clay containing large fragments of the
Scotland Fm. and theclays are unstable and unconsolidated. Erosion due to surface runoff have resulted in the formation
of deep gullies. This unusual deposit is perhaps a mudflow facies which has beenintruded into the surrounding rock.
The presence of Oceanic fragments (?) within the formation would indicate an Early Miocene intrusion.
CHALKY MOUNT-Chaky Mount Member, Upper Scotland Formation
This spectacular coastal exposure is comprised of stacked turbiditic sequences of fine to coarse sands, grits,
thin silty clays and channelized conglomerates near the base of the member. Graded bedding, load casts, scour marks,
convolute laminations and oriented pebbles can be observed. Mid-fan to basin plain deposits are present but the
sandier mid-fan to proximal outer fan facies tend to predominate.
Very severe structural deformation is evident. The beds are contorted and generally plunge steeply North. Abundant
N-S faults are present but the major movement occurs along E-W trending thrusts or wrenches. The origin of these
turbiditic sequences within the Scotland Formation are still obscure. However, heavy mineral studies indicate a
likely source to be the Venezuela/Trinidad Coastal Range. The Chalky Mount beds are dated as early Middle-Eocene
age.
STOP 11
BAWDEN & RIVER ESTATE-Walkers member Lower Scotland Formation
The Walkers beds outcrop at this stop and consist of an interbedded sequence of cream-coloured, fine grained sandstones
and green-grey clays. Abundant flute casts and sole marks are observed. The sequence is interpreted as distal outer
fan to basin plain turbiditic deposits. The Walkers Member is the oldest unit of the Scotland formation and has
been dated as Eocene/Paleocene (?). Other interesting features to note are the several oil impregnated sand bodies,
the secondary iron colourations and the calcite mineralization along fault planes. Severe deformation of the beds
have resulted in warping and complex faulting.
DAY 3
STOP 12
WOODBOURNE OIL FIELD
Mr. Philip Payne, a geologist employed with the Barbados National Oil Company, explained that the island is presently
producing approximately 1700 BOPD from about ninety wells. The productive reservoirs are sand bodies in the Scotland
Formation. Structural interpretation is extremely difficult because of the complex folding and faulting on the
island. Wells are usually 4000 to 6000 feet deep and can encounter pay zones as much as 350 feet thick.
STOP 13
SPA HILL-Murphy's Beds, Upper Scotland Formation
The Murphy Beds exposed at Spa House consist mainly of fine to coarse-grained, micaceous, sandstones interbedded
with thin layers of silts and clays. These beds are interpreted as turbiditic supra fan deposits; and cross bedding
and convolute bedding are observed. Structural deformation is complex and beds generally dip steeply to the south.
STOP 14
MORGAN LEWIS-Morgan Lewis Member, Lower Scotland Formation
Along a coastal hillside at this stop, the Morgan Lewis beds are seen in fault contact with the Walkers beds. The
Morgan Lewis beds are predominantly dark grey shales with thin, intermittent, cream-coloured, very fine-grained
sands and represent a more basin plain or distal fan turbiditic depositional facies. Secondary gypsum are associated
with these beds, and load casts and flute casts are also observed. Thrusting and folding complicates the relationship
of the Morgan Lewis and Walkers beds along this exposure.
STOP 15
GAY'S COVE-Oceanic Formation
At Gay's Cove the Coral Rock Formation lies unconformably on the gently dipping Oceanic Formation. The oceanics
here are mainly marls interbedded with thin, black ash beds and radiolarian oozes. At the base of this coastal
exposure, a sequence of dark grey silts interbedded with thin, yellow laminations, may represent an exotic block
of the Scotland Formation (?).
The intriguing geology of Barbados continues to arouse the curiosity of the larger geological community and, not
surprisingly, the island will be hosting the next (11th) Caribbean Geological Congress in 1986. It is also interesting
to note that the Joint Oceanographic Institutions is scheduled to drill the third leg of their Ocean Drilling Program,
in Barbados. Their new drill ship, the Sedco/BP471, has been designated to penetrate the toe of the Barbados accretionary
prism and the underlying decollement, in order to "document the tectonic character of the deformation front
and to permit hydrogeological experiments in the thrust zone."
DRILLING & PRODUCTION DATA
APRIL - JUNE 1984
| COMPANY |
| DEVELOPMENT |
| NO. OF WELLS |
TOTAL FOOTAGE |
|
|
| EXPLORATION |
| NO. OF WELLS |
TOTAL FOOTAGE |
|
|
AVERAGE BOPD |
| AMOCO |
|
|
85,485
|
| P.C.O.L. |
|
|
403
|
| TENNECO-OCCIDENTAL |
|
|
|
| TEXACO |
|
|
9,843
|
| TRINMAR |
|
|
38,619
|
| TRINTOC |
|
|
8,547
|
| TRINIDAD TESORO |
|
|
22,804
|
| TOTAL |
|
|
166,061
|
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