COASTAL TERRACE TECTONIC GEOMORPHOLOGY, TRINIDAD, WEST INDIES

Geomorphic features in northern Trinidad reflect westward sinking and tilting into the active Gulf-of-Paria pull-apart basin, which results from a right step in the transforms that take up the ~20mm/yr of eastern movement of the Caribbean Plate relative to South America. Raised coastal terrace deposits have been mapped at ~15 meters above modern sea level along the northeastern coast and along the northern coast but no further west than Blanchisseuse along the north coast. The purpose of our study is to determine if the raised terraces in the east are marine in origin and related to sinking in the west. We are testing two possible hypotheses: (1) as the western end of the island sinks, the eastern end rises in a seesaw-like effect, and (2) as the western end of the island sinks, the eastern end remains fixed as to sea level changes. We studied terrace deposits at 11 locations near Toco (the northeastern corner of Trinidad), and 4 locations near Blanchisseuse (~40 km west of Toco). We measured and described stratigraphic sections, collected samples, did grain size, grain shape, XRD analyses, and optically stimulated luminescence (OSL) dating.

We found the terrace deposits range in thickness from 60 to 280 cm and are a mixture of sand and gravel (mean grain size from 5 sieved samples ranged from -0.84 to -0.23f.). Fifty pebbles were randomly selected and measured from one outcrop, plotted on a Zingg diagram, and clustered as rounded. XRD showed that fine sieve fractions (4.00 to 4.25f) consisted almost entirely of quartz (silt or cement ?). Three OSL ages obtained from 5 samples (two incalculable ages) ranged from 40 to 138 ka; two fall near global sea level high stands.

Our preliminary sedimentary analysis (sedimentological similarities to modern beaches) and the field mapping (coastal locations) support that these terraces are marine in origin. The OSL ages suggest that they could be high-stand deposits. These preliminary data support the idea that as the western side of Trinidad sinks into the Gulf-of-Paria, the eastern side probably remains fixed as sea level changes.