13	Tobago Field Trip P. Farfan (Amoco)

Dr. Arthur Snoke (University of Wyoming) has been working in Tobago over the last 2 years on a new geological map of the island. A field trip to Tobago was led by Dr. Snoke, in association with geologists from the Ministry of Energy, on July 9 - 10. 1958 (Figure 7).
On arrival in Tobago we were given a brief review by Dr. Snoke in which he outlined his findings and explained the objectives and agenda for the trip.
He demonstrated a favourable correlation between his geological interpretations and available gravimetric data. He showed us his geological map which is the result of numerous field seasons of research, with the assistance of students, and it is significantly different from Maxwell's (1948) map. He has also successfully dated rocks radiometrically from various formations and has discovered a previously unknown fossil locality which has yielded a good ammonite fauna, a first for Tobago. Dr. Snoke's work represents
a significant contribution to understanding Tobago's geology. He is
currently trying to locate funding for the publication of his map and an explanatory text.

ROCKY POINT: The rocks at the southern end of Back Bay were described as porphyritic rhyodacite lava flows containing pillow lavas (Plate 1) amygdaloidal quartz and veins of red jasper which are typical of the Hawk's Bill Formation of Maxwell (1948). Dr. Snoke now correlates these with the Bacolet Formation and has reclassified the rocks here as jasper-bearing silicified andesites.

ORANGE HILL ROAD QUARRY: The recent excavations in the quarry face have unearthed ammonites in the tuffs of the Merchiston/Bacolet formations (see cover plates). At the time of the field trip these had not been thoroughly classified or dated but are expected to be of Upper Cretaceous age. The ammonites are well preserved, small (about 2-3 cm in diameter) and as yet only found in a single layer in the tuffs.

ROCKLEY BAY: In the beach cliffs southwest of Scarborough, the Rockley Bay Formation consists of brown clays, marls and beds containing Balanus tininabulum, Ostrea, Pecten. Cycinieris and other molluscs. Normal faults cut these clays, developing slickenslides and drag.
Saunders and Muller-Merz (1955) dated these sediments as Pliocene on the basis of their micro fauna.

MINISTER POINT: Fine-grained basalt dykes have injected the crudely stratified andesite tuffs and agglomerates of the Bacolet Formation on the south side of the point. Around the point the tuffaccous facies of the Bacolet Formation appeared to be in fault contact with the agglomerates.

THE COURLAND RIVER AT MASON
HALL: Inside the igneous complex, at this locality, there are layered diorites arid tonalites which have been intruded by felsic and basaltic dykes.
MOUNT DILLON FORMATION: The road from Plymouth to Parlatuvier cuts through the metamorphosed volcaniclastics of the Mount Dillon formation.
Near Castara, at the big landslide, the phyllitic schists of the Parlatuvier Formation are very graphitic (Plate 2). This area is believed to be part of a major shear zone which transects the island.

ENGLISHMAN'S BAY: Tight multi-phase folding in the Parlatuvier Formation is well exposed on the north side of the bay.

DOCTOR'S RIVER: In the stream bed of this river there is an outcrop which Dr. Snoke considers to be the Rosetta stone of Tobago geology because of the cross cutting relationship between the various suites of dyke swarms which inject the volcanic complex here.

TYRELL BAY: Large (1 cm) porphyroblasts of hornblende in the greenschist facies rocks exposed on the north side of the Bay have been dated as Upper Cretaceous. This proves that the metamorphism was coincident with the intrusion of the ultramafic complex.

LOUIS D'OR RIVER: The outcrops along the banks of the rivet demonstrate the variety of ultramafic cumulates included in the complex. Some of these were hornblende diorites with phenocrysts 12-15" long, giving the rock a pegmatitic appearance, coarse pyroxenites, serpentinites and fine-grained dunites/periodotites.
Further upstream, in a tributary of the river, we traversed a suite of volcaniclastics which had been thermally metamorphosed from greenschist through to amphibolite grade facies. In the river we saw the contact between the country rock and the ultramafics responsible for the thermal metamorphism.
Acknowledgements:
The production of this summary would not have been possible without the assistance of K. Rowley, D. Hudson and H. Innis.

REFERENCES

Maxwell, J.C., 1948, (Geology of Tobago, British West Indies: Bull, (Geol. Soc. America, V.59, p.801-854.

Saunders, J.B. and Muller-Merz, I., 1985, The age of the Rockly Bay Formation, Tobago:Trans. 4th Latin American Geological Congress, Port of Spain, 1979, V.1, p.339344.

Editor's Note: Recent correspondence from Dr. Snoke has indicated that the ammonites described above at the Orange Hill Road Quarry are unquestionably Albian in age.

THE GEOLOGICAL SOCIETY OF TRINIDAD & TOBAGO (1976) PAGE THIRTEEN
Plate 7: Pillow lavas in the Hawk's Bill Formation, Rocky Point, Tobago.

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.

Comments or Questions?