FIELD TRIPS
1. MATURA BAY FIELD TRIP
L. Tyson (TRINTOC)
On January 31, 1987, Dr. John Frampton led a party of 14 geologists to examine outcrops of Plio-Pleistocene rocks
on the Fast Coast of Trinidad (Figure 2). The formations exposed here include the Manzanilla,
Springvale and Talparo, striking perpendicular to the N-S Matura Bay coastline.
Of significance in this area, both from the stratigraphic and the structural viewpoint, is the change in dips of
the strata. The contact between the Talparo (numbered 3, Figure 2) and Springvale (numbered
5,Figure2) Formations is seen at a point midway along the coastal section. This important contact
represents a Late Pliocene/Early Pleistocene tectonic event, resulting in angular unconformity between the two.
Unfortunately there are no rock exposures at this point along the coast to show the actual contact, but by viewing
the Talparo and Springvale beds from a distance, the angular relationship of the two is clearly discernible. Within
the Talparo dips average 300 N, whereas in the underlying Springvale section the dips average 500N
(Plate 1).
The second noteworthy point concerning the dips is the fact that even in one recto-sedimentary unit, represented
here by the Springvale and Manzanilla (numbered 7, Figure 2), there is marked sweetening of
the beds. As mentioned above, dips at the top of the Springvale average 500 N and increase progressively southward
to a maximum of about 700 at Point Paloma, the southern-most 250 dip change within one tectosedimentary
unit is the result of the progressive approach to the Caigual Fault (reverse), just south of the area under investigation.
The oldest rocks examined are in the cliffs at Point Paloma, and constitute part of the Montserrat Member (7b)
of the Manzanilla Formation. These strata are dark green and are really a conglomeratic sandstone with abundant
glauconite and shell fragments.
The Telemaque Member (7a), which here is of the order of a few hundred feet thick, conformably overlies the Montserrat
Member and consists of silty sands displaying rippling and lamination. The Gransaull Member (Sc) of the Springvale
Formation is conformable over the Manzanilla Formation, and varies lithologically from conglomeratic sandstone
to silty sands, clays and lignites. A sandstone is present with day pellets at its base and showing rippling and
lamination near its top, which, together with its generally upward fining appearance, suggest a channel. The angular
unconformity between the Springvale and Talparo has been mentioned, the latter being characteriesd by the fairly
common occurrence of what appears to be mangrove roots or Callianassa burrows. Two lignite beds (+3' thick) are
present, and associated with them are baked clays or porcellanites; the latter were probably formed as a result
of spontaneous combustion of the lignites. Channels similar to those described for the Springvale Formation are
also present. At Point Lobin the laterally persistent, alternating fine sands and silts are interpreted as being
part of a lagoonal environment.
This field trip is relevant to a wide cross-section of geologists, and especially the petroleum geologist,
since commercial oil has been discovered in the Manzanilla Formation in the Gulf of Paria (the North Soldado Field).
The trip allowed one to view first hand the petrophysics of the Manzanilla reservoir as well as the sealing characteristics
of the overlying Springvale Formation. Secondly, the remarkable slipperiness of the clays of the Springvale and
Manzanilla when wet (as indicated by the number of falls of members of the party!), as well as the interbedding
of thick sandstones and clays must be of relevance to the engineering geologist. The sliding of sandstone beds
along wet clay surfaces due to the very low coefficient of friction that exists under such conditions, can perhaps
account for some landslides in areas where Manzanilla and Springvale Formations outcrop, e.g. the landslide present
on the eastern side of the Solomon Hochoy Highway about one mile north of the Clayton Bay exist.
To the structural geologist, the significance of the changes in dip (due to faulting and angular unconformity)
is useful data in analysing the tectonic history of the island. In addition, for the geologist with an interest
in pedology, it is interesting to note that soils developed from the weathering of rocks of the Manzanilla and
Springvale Formations are perhaps the richest in Trinidad. This is so since these rocks contain abundant glauconite
(a silicate mineral containing Fe, Mg, K, and P), which breaks down to form a natural fertiliser. Finally, for
the rock collector the trip was worthwhile as good samples of macrofossils (Cailanassa, Pecten and Anadara), porcellanite,
coal/I ignite and glauconite-bearing, conglomeratic sandstones could have been collected.
The Society is grateful to Dr. Frampton and the 1986-1987 Field Trip Sub-Committee for having organised a very
successful field trip to Matura Bay.
2. ENGINEFRING GEOLOGY FIELD TRIP
L. Seegobin (TRINTOC)
A field trip on Engineering Geology for geologists was led by Ms. Cassandra Rogers and held on Saturday, 16th May
1987. Several areas where engineering geology problems occur were visited, and in some of these areas corrective
methods were seen to be implemented. The need for geologic knowledge in the determination of preventive and corrective
measures was emphasized throughout the trip.
At the Lady Young Road Lookout (Stop 1, Figure 3), the problems experienced by the builders
of the Financial Complex, the Tatil Building, and the W.H. Scott Building, due to subsurface salt water encroachment
were discussed. The planners of these buildings did not fully appreciate the subsurface geology or the position
of the water table in the area before construction. The lack of a subsurface geology map of the Port of Spain area
was suggested as a major reason for this. The problem at the Financial Complex and Tatil Building was effectively
dealt with by sealing off the foundation walls. The lighthouse and the Telco building also experienced problems
caused by the high water table.
The landslides along the Lady Young road were examined, and two types of slides were identified. Opposite the Lady
Young road lookout, the outcrop shows bedding failure along the synclinal axis, resulting in the landslide.
Along the Lady Young road between the Hilton Hotel and Lookout (stop 2) failure along bedding planes is responsible
for the slide. At present a few areas along the Lady Young road have been benched in order to prevent further sliding;
this does not, however, protect the areas which are sliding alongside the road. At the Hilton Hotel car park, wire
netting covering the outcrop bas been effectively used in preventing further sliding.
A slide of major concern was visited at St. Barb's Hill in Laventille (Stop 3). The slope instability in this area
is a result of limestone removal by quarrying at the base of the slope. This limestone is in faulted contact with
phyllites (upslope) of the Grande Riviere formation (the Chancellor beds). With the removal of the limestone in
a geologically unstable area the land
eventually slid downslope. This area has a large low income population, many of whom live within the unstable zone
of the slide (250m long in a down-slope direction and 150m across). At present no remedial action is being undertaken
to prevent further sliding, although several homes have been destroyed.
Two other landslide localities were visited along the Solomon Hochoy Highway. At the Claxton Bay south bound exist
(Stop 4), a landslide within the Talparo Clay was viewed and discussed. This slide occurs along bedding planes,
and is caused by the high montmorillonite contents and high slope angle (>140).
At the Springvale Quarry area (Stop 5), the landslide which partially damaged the northbound lane of the Solomon
Hochoy Highway was examined (Plate 2). This landslide was probably caused by percolation
of water between the fill used in the building of the highway and the bedrock. This created a water cushion on
which the land moved. This landslide has resulted in the destruction of one house and further movement could destroy
several more buildings, including a school.
At the conclusion of the trip all participants agreed that geologic knowledge is needed in the planning of road
and building construction, especially in geologically unstable areas. It is also necessary that the quarry industry
take precautionary measures to prevent the development of potentially hazardous conditions.
Home | About Us | Links To The World | Publications | Upcoming Events |
Feedback | Executive Members | Geology |Teaching Resources| Virtual Field Trip
THE GEOLOGICAL SOCIETY
OF TRINIDAD & TOBAGO
P.O. Box 3524, La Romain, Republic of Trinidad and Tobago, W.I.
