GEOLOGICAL FIELD TRIP TO THE TOCO DISTRICT
by Winston M. Ali.
On Saturday 25 June 1983, a group of fifteen geologists gathered at the Valencia Junction to begin a trip to the
Toco District in the North-East corner of Trinidad. In spite of the dense tropical rain-forest cover of the Northern
Range, excellent road cuttings and superb coastal sections offer a wealth of accessible exposures.
Although it was not possible to visit all the formations in the area comprising the Northern Range Complex, a
representative sequence of Igneous, Metamorphic and Sedimentary rocks were studied.
The geological map and cross-section (Fig.1)
summarises the General Geology of the Toco District while Fig. 2 outlines the stratigraphy. The stops which are located on the geological map (Nos. 1-7) are briefly
described below. In planning the trip and in compiling this guide, reference was widely drawn on previous work
by Barr (1968) and Ferrell and Kuarsingh (1979).
Stop 1 - Bateau Point
Balandra Grits - Grande Riviere Fm
Well exposed coarse to conglomeratic quartzitic grits and coarse sandstones interbedded with graphitic schists.
The beds are steeply dipping to the south at angles of 600
and in places even vertical The strike varies between N660
- 750E. At the Southern limit of Bateau Point a
tight parasitic fold on the Balandra Anticline is seen. The fold axis is vertical and trend in a direction N660E. The competent and incompetent beds have been deformed in concentric
and similar styles respectively, with tension gashes having developed in the anticlinical core of the fold.
Stop 2 - Petite L'Anse
Low-Grade Schists - Grande Riviere Fm
Reddish-brown, weathered, low-grade schists with thin quartzitic interbeds framing a prominent headland along
the coastline on the Northern flank of the Balandra Anticline. In hand-specimen the rock is laminated (1''-2"
thick) having a bedding schistosity with a well-defined crenulation cleavage which is almost perpendicular to the
bedding (Figure 3). However, the original
sedimentary structure of the rock is still largely preserved. The mineral association of quartz-muscovite-chlorite
(sericite) as noted by Barr 1963 are characteristic of the chlorite zone or the lowest grade of green-schist metamorphism
(see Figure 4).
Stop 3 - Guayamara Bay
Parasitic Folds - Grande Riviere Fm
This stop affords an excellent view of a series of tight, parasitic folds (cover photo) associated with the Balandra
Anticline. Once again, a well-developed crenulation is apparent and is seen as a convergent cleavage fan.
At this locality, a (?) Holocene raised beach of coarse quartz grits is seen to rest unconformably on the Grande
Riviere Fm (? Jurassic/Lower Cret-aceous Age). The raised beach now stands at approximately 25' - 30' above present
sea-level.
Stop 4 - Salibia Road
Non-Metamorphosed Sediments -Galera Fm
The Galera Formation at this locality consists of a non-metamorphosed, prominent, coarse-grained, quartz sandstone
with beds up to 6 feet thick and having thin (2') shale interbeds. In most cases, the beds are graded where the
coarse fraction of the sandstone can be seen to progressively pass into the shaly material. These graded units
may be up to 10 feet thick with dips of approximately 360
the North-East.
Stop 5 - Sans Souci Point
Volcanics - Sans Souci Fm
The volcanic rocks consists of a series of basaltic tuffs, agglomerates and lava flows. In hand specimen they
are dull grey-green in colour, generally fine to medium grained and highly weathered. In places the
volcanic mass is sheared resulting in veins which are now filled by calcite, chlorite and sericite. The original
mineral composition of the rock has been identified by Barr (1963) as consisting of opaque feldspars, dark-green
to black pyroxene (augite), homblende, quartz and chlorite.
Stop 6 - Toco Bay
"Cherry Cake" Conglomerate - ?Toco Fm
This conglomerate consists of dark red,elongated pebbles (up to 2" long) set in a ground-mass of pure quartz
which is well-indurated and possibly metamorphosed. The red pebbles which are of a clay-ironstone composition
(Ferret op. cit.) display a preferred orientation with a well-developed imbrication. It is possible that the
"cherry-cake" conglomerate is in a faulted or tectonic relationship with the Toco Formation, the contact
between the two not being readily recognised at the surface.
Stop 7 - Galera Point
Galera Grits - Galera Fm
At this point the Galera Formation consists of a well-indurated quartzitic grit unlike those seen at Stop 4 in
texture. Abundant calcitic and quartz veins are noted in the rock with two systematic sets of joints being present.
Ji (?Older) strike at N600E with a dip of 600 to the NNW. J2 (?Younger) strike at N100W with a dip of 660 to the WSW.
The field trip terminated at Stop 7 where the Venezuelan-Trinidadian branch of the Coastal Ranges plunges eastward
into the Atlantic Ocean.
References:Barr, K.W., 1963 - The Geology of the Toco District, Trinidad, West Indies. Pt. 1. Overseas Geol. &
Mineral Resources V.9 No. I pp.1-29.
Barr, K.W., 1968 Excursion No.2. Eastern Northern Range and Toco District. Trans. 4th Carib. Geol. Conf Port.of-Spain.
1965 pp. 430-432.
Ferrell, A.D. and Kuarsingh, H., 1979 - Field Trip 8- Eastern Northern Range. Field Guide,die 4th Latin Am..
Geol. Cong. Port of~Spain. 1979.
Turner, F.J., 1968 - Metamorphic Petrology - Mineralogical and Field Aspects. International Series in The Earth
and Planetary Science. McGraw-Hill Book Co.
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