On Sunday March 6, 1983, a party of thirteen geologists from the Ministry of Energy and Natural Resources, Trinmar Limited and Trinidad- Tesoro Petroleum Company Limited visited the Guapo/ Point Fortin area to study the sedimentology of the Morne I'Enfer (Pliocene Age) and Cruse (Pliocene- Upper Miocene Age) Formations and exposures of the Los Bajos wrench fault. The field trip was led by Mr. Winston Ali of Trinmar Limited.
The first stop on the trip was at Point Galba to observe the La Brea Antic/me where the Upper Morne I'Enfer formation is exposed. High tide prevented the party from studying the section which is normally exposed along this part of the coast. Large outcrops of asphalt were observed on the beach, a phenomenon associated with the La Brea Antic/me. The breaching of this anticline and subsequent biodegradation of the oil which migrated to the surface, has resulted in one of the Seven Wonders of the New World - the La Brea Pitch Lake - which is situated about 1/2 mile inland from Point Galba. (Figure 1)
The second and third stops were at Guapo Bay to study the sedimentology of Units I - IV of the Upper Morne l'Enfer formation as described by Saunders and Kennedy, 1965. (Figure 2)
Stop two was along the northern part of the bay where Unit IV and the upper part of the unit III are exposed. Unit IV, the youngest unit, is dominantly a silty section 70 to 80 feet thick with a gradual upward transition from a sandy base to a clayey top and finally to a lignite. Unit I V is interpreted to be a tidal fiat deposit at the base gradually becoming swampy towards the top.
Overlying Unit IV are bands of red porcellanite interbedded with lignite and carbonaceous clay. The upper part of the Unit III comprises fine cross-bedded sands and clayey silts.
Stop three was along the southern part of Guapo Bay where exposures of Units I, II and the lower part of Unit Ill were easily accessible. The lower part of Unit III consists of cross-bedded fine sands within which an imbricated, tabular, silt-pebble intraformational conglomerate (30 thick) is present. The overall thickness of Unit II is about 130 feet and is interpreted to be distributary channel deposits.
Unit II consists of about 60 feet of silt with some very fine lenticular sand laminations and is interpreted to be an interdistributary deposit.
Unit I is exposed as near vertical east-west striking beds comprising about 100 feet of alternating fine sands and clayey silts with minor channeling features. This unit is interpreted to have been deposited in a near-shore marine to brackish water environment - an interdistributary bay or coastal lagoon deposit. The very steep dips in this unit are probably related structurally to movement on the Vance River fault.
The afternoon session was spent studying exposures along the right-lateral Los Bajos Wrench Fault in the Point Fortin area. This fault is of obvious economic importance as several prolific oil fields are located along both faces of the fault in this area.
Stop four of the trip was at Harriman Park, a residential area built on a WNW-ESE trending ridge of thick vertical trough cross-bedded sandstones of the Cruse formation. This ridge is the surface expression of the Los Bajos fault in the Point Fortin area. Un fortunately, urbanization has obliterated many of the classic outcrops which were previously available for study.
The fifth stop was at the proposed site of Trintoc's La Fortune's Housing Estate which is situated directly on the westward extension of the 'Los Bajos Fault Ridge'. Here we observed the excellent exposures of trough cross-bedding and channeling features within the highly faulted vertical sandstones of the Cruse formation north of the fault, and the less steep southerly dipping massive fine sands of the Erin formation (Late Pliocene) south of the fault. The Cruse sands are interpreted to be distributary channel deposits while the Erin sands are more non-marine fluviatile deposits. The most striking geological feature of this stop is the fact that the Cruse sands are oil impregnated, as distinguished by a purplish colour, while the Erin sands are 'wet', as seen by the buff white co/our (see cover photo). This occurrence would suggest that migration into the Cruse sands occurred prior to rejuvenated movement along the Los Bajos fault which affected the younger Forest, Morne L'Enfer and Erin formations. However, in other areas adjacent to the Los Bajos fault, Forest and younger sands are known to form oil-bearing reservoirs. C C. Wilson (1965) considers that these reservoirs owe their presence to the movement of the Los Bajos Fault and their oil content depended largely on the degree of impregnation of older reservoirs, e.g. Cruse, Herrera and possibly still older, at the fracture plane of the fault. This idea should certainly be revived and applied to the search for petroleum in the unexplored areas adjacent to the Los Bajos Fault.
The sixth and final stop was at Point Ligoure where the Los Bajos fault extends westward into the Gulf of Paria. Drilling adjacent to the fault in Trinmar's acreage has confirmed the presence of an offshore extension of the prolific Point Fortin fields.
The trip provided participants with the opportunity to become physically acquainted with formations with which they are already mechanically intimate, through hundreds of well logs. It is hoped that the interpreted environments of deposition, made from field observations, would assist the geologist in regional lithostratigraphic mapping in the ongoing quest to win that extra barrel of oil.
On a closing note, it was expected that a field trip held in an area of active exploration and development, by all land-operating oil companies, would have been better attended. We look forward to more meaningful participation on future field trips.
Refs. : Saunders, J. B. and Kennedy,]. F 1965. Sedimentology of a Section in the Upper Morne L' Enfer Formation, Guapo Bay, Trinidad. Fourth Caribbean Geol. Con f. Trans. (1968)
Wilson, C. C 1965. The Los Bajos Fault. Fourth Caribbean Geol Con f. Trans. (1968).
