AAPG/GSTT HEDBERG CONFERENCE

“Mobile Shale Basins – Genesis, Evolution and Hydrocarbon Systems”

June 4-7, 2006 – Port of Spain, Trinidad & Tobago

Episodic Slab “Roll-Back” and its Effects on the Caribbean-Atlantic Accretionary Prism: A regional trigger mechanism for the diapirism of Eastern offshore Trinidad and Tobago.

N.G. Evans¹, C.J. van Brummen¹, T.P. Buerkert¹, P. Eisner¹, D. Widdoes¹,

L. Jennings¹, R.K. Sawyer¹, K.Tindale¹ and R. Butler²

¹BHP Billiton Petroleum (Americas) Inc., Houston, USA

²University of Leeds, United Kingdom

Episodically shifted forearc magmatic complexes and adjacent sedimentary basins, significant regional unconformities indicative of major structural readjustment, and sedimentary basins locally encountered on a sporadically advancing foreland propagating thrust system together characterize the Barbados-Trinidad Accretionary prism. All of these features, as they are encountered, are not easily explained by a steady state deformation model of progressive plate subduction and classic plate accretion. Rather, they exhibit clear features of a system that has deformed in pulses: active structural deformation and uplift followed by periods of quiescence and rapid subsidence. Localised shale diapirism is also a key component of the system. This contribution will focus on the eastern offshore of Trinidad, a prolific hydrocarbon province, where understanding of the diapiric belt is generally poor despite recent industry activity, and will provide a discussion of the overprint of the regional crustal-scale tectonics on its development.

Many authors describe the eastward retreat of the Lesser Antilles Trench and the growth of the Caribbean Sea floor at the expense of the Atlantic. The distinct relocation of Caribbean arc volcanism indicates a pulsed history of roll-back, perhaps linked to a pulsed propagation of the southern lithospheric tear fault located close to the northern margin of the South American continent (the STEP model of Govers and Wortel, 2005, EPSL). The tectonic adjustments at the plate boundary are recorded by regional unconformities that bound and explain the distribution of distinct depositional megasequences. Specific observations that support this model in the greater Trinidad area are as follows:

1)   Thick sedimentary accumulations of the Barbados Trough and Columbus Basin are located between the inner fold and thrust belt (the continuation of the Interior Range) and outer fold and thrust belt (present day maximum eastern extent of the Trinidad-Barbados accretionary prism).

2)   Thick accumulations such as these are indicative of the creation of vast amounts of accommodation space (consistent with a subsidence anomaly).

3)   Observed seismic geometries indicate rapid development of the Tobago basin, and time the jump of magmatism from Bonnaire-Aves to the Lesser Antilles.  This large-scale shift in the position of intra arc magmatism is not consistent with progressive subduction and fixed arc location.

4)   The age, distribution and thickness variation in foredeep sedimentation coupled with clearly defined, significant regional unconformities suggest that the thrust front established during the Miocene rapidly shifts to the east.

The sequence consisting of the creation of accommodation space and fill, regional uplift, relocation of volcanism and the shift in the location of the frontal deformation belt is consistent with pulsed eastward rollback of the Atlantic plate.

The foredeep ahead of the Paleogene/Early Neogene Interior Range fold belt of the Eastern Venezuela Basin (EVB) is characterised by a northeast oriented mud diaper belt which continues through Trinidad and swings northwards to Barbados. This mobile shale zone runs roughly parallel to the Lesser Antilles Arc and the continuation of the Interior Range fold belt. The diapiric belt in the Eastern Offshore of Trinidad widens to a maximum of 200 km, north and east of the Columbus Basin. The shale, thought to have been derived from the Miocene Foredeep in front of the Interior Range fold belt, exhibits a lateral variation in composition and, more significantly, thickness. In the EVB, onshore and coastal Trinidad, the foredeep basin is restricted and consequently moveable shale was scarce. Here, little evidence exists of significant diapirism other than in response to backthrusting at the frontal edge of the underlying Paleogene/Early Neogene thrust front. In the eastern offshore of Trinidad where the foredeep basin is more extensive, moveable shale is widely distributed and was initially thicker. The shales have undergone several episodes of movement from Miocene through Plio-Pleistocene times, and complex diapiric structures predominate.  In this region, poor seismic imaging complicates efforts to fully understand the link to regional tectonics.  The number and complexity of diapiric structures, which are genetically related to the volume of shale, configuration of the foredeep basin and overburden load, present an extremely challenging geophysical terrane.

Within a geodynamic model of pulsed slab roll-back, a trigger mechanism exists for the development of the diapiric zone in eastern offshore Trinidad.  It is proposed that the diapiric belt was created in the following manner. The initial production of a foredeep basin in the Miocene accumulated significant thicknesses of ‘mother’ shale. Subsequent creation of a subsidence anomaly by pulsed slab roll-back, as indicated by the sudden eastward relocation of the thrust front to its present day location, resulted in the formation of a deep intra-prism basin (Barbados Trough). Finally, the accumulation of a thick Plio-Pleistocene overburden initiated shale overpressuring, mobilisation, and diapirism.