Big Stories Told by Little Things

BIG STORIES TOLD BY LITTLE THINGS:

A RETROSPECTIVE ON FIFTEEN YEARS OF MICROPALAEONTOLOGICAL RESEARCH

 

Brent Wilson

Petroleum Geoscience Programme, Department of Chemical Engineering,

The University of the West Indies, St. Augustine, Trinidad and Tobago

 

I arrived in Trinidad and Tobago in 1998 to find a thriving industrial micropalaeontological community, but little academic palaeoecological and palaeoenvironmental research being either conducted or published.  The main models in use then were (a) Batjes’ biofacies system for the Forest and Cruse Formations, and (b) depth distributions as recorded in the Gulf of Mexico, nearly 2000 km away. On joining the Petroleum Geoscience Programme of the University of the West Indies in 2002, I elected to deepen our knowledge of the Neogene foraminiferal palaeoecology of Trinidad and adjacent areas.  This talk covers three aspects of my work.

The Orinoco Plume is a hyperpycnal lens of nutrient rich water that surrounds Trinidad and Tobago, extends NW across the Caribbean Sea, and expands and contracts between the rainy and dry seasons. It has been in existence since the Late Miocene. The nutrient flux from it is apparently responsible for biogenic gas off northern Trinidad. The plume’s structure is complex, it entraining watermasses derived from the Amazon and Orinoco Rivers that meet along sharply defined fronts both SE and NW of Trinidad. Nutrient concentration along these fronts induces biofacies at an angle to bathymetric contours. This limits the use of benthic foraminiferal biofacies as depth indicators around Trinidad, but presents the prospect of our being able to assess the distribution of the plume and fronts throughout the Neogene.

The Brasso Formation is a succession of predominantly claystones and limestones of Early to Middle Miocene age in the Central Range of Trinidad. It was long thought to be deposited at monotonous neritic depths. However, dense sampling and micropalaeontological analysis revealed that is was laid down during two transgressive-regressive cycles tectonically induced by the SE motion of the proto-Northern Range allochthon. Palaeodepths during these cycles ranged between middle bathyal (~600 m) and shallow neritic (~30 m water depth). The transgressions are presumed to have been due to tectonic loading creating accommodation space, the regressions occurring from filling of this space by erosion of subaerial parts of the allochthon. The first cycle ended during planktonic foraminiferal Zone N10, with the transgressive part occurring during late N8 – early N9. The second regression spanned Zones N11 to N13, with the preceding transgression during N10. Such large cycles, not yet fully integrated into plate tectonic models, are not found in the overlying Manzanilla Formation. This indicates that tectonic induction of transgressive-regressive cycles was slighter during Manzanilla times and implies that there was a change in tectonic style between the uppermost Brasso and the Manzanilla, with a transition to primarily lateral (west – east) motion of the proto-Northern Range allochthon

The Nariva Formation was long thought to be older than the Catapsydrax stainforthi Zone (N6) and to underlay the Brasso Formation. Identifying outcrops using lithostratigraphy rather than biostratigraphy, however, sections have been found ranging in age between the mid Oligocene and Middle Miocene (N12, Globorotalia fohsi robusta Zone). Samples yielding planktonic foraminifera cluster in age around N10–N11, although precise correlation with the second of the Brasso transgressions has yet to be proven. Samples with entirely agglutinated foraminiferal faunas appear to have been influenced by hot spring activity, but are of Brasso aspect. A preliminary model for the Cunapo-Brasso-Nariva-Cipero complex (here regarded as a potential Cipero Group) will be presented.

 

 

 

 

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