AAPG/GSTT HEDBERG
CONFERENCE
“Mobile
Shale Basins – Genesis, Evolution and Hydrocarbon Systems”
June 4-7, 2006 – Port of Spain, Trinidad & Tobago
Tectonically-driven Plio-Pleistocene structural development of the Columbus Basin,
offshore Trinidad, West Indies
R. Gibson1,
K. Meisling1 and J. Bhajan2
1BP America, Houston,
Texas
2BP
Trinidad & Tobago, Port of Spain, Trinidad
The late Tertiary Columbus
Basin is situated along the
transpressional plate margin between the South American and Caribbean plates,
immediately west of the subduction boundary between the Caribbean and Atlantic
plates (Barbados
accretionary prism). Previous structural
models for this basin have emphasized movement of mobile shale as a primary
control on the observed structural geometries. In this paper, we present an
alternative model in which the observed structures are products of large-scale
tectonic driving forces within a zone of dextral displacement transfer from the
Caribbean-South American plate boundary into the Caribbean-Atlantic subduction
complex. In this model, shale mobilization is only a minor phenomenon, largely
reflecting dewatering of rapidly deposited fine-grained slope deposits.
Major
structures within the 35000+’ thick, Miocene-Pleistocene siliciclastic section
of the Columbus Basin are regionally detached from an
underlying north-dipping Cretaceous-Paleogene passive margin succession.
Although the detachment does not necessarily stay at the same stratigraphic
level everywhere in the basin, there appears to be linkage of the detachment
from the thrust system that bounds the NW side of the basin, through a zone of
extension on the shelf, and basin-ward to the Barbados accretionary prism.
Structures in
the western, shelfal part of the basin include a NW-SE striking extensional
fault system and NE-SW trending contractional folds. 2D restoration of the extensional fault
system shows that in excess of 35km extension occurred, much of it accommodated
on a counter-regional fault system situated near the depositional shelf edge as
it prograded northeastward since middle Pliocene time. Triangular domains of relatively poor seismic
imaging, previously interpreted as masses of mobile shale, are interpreted as
rafted blocks of slope mudstones bounded by (1) relatively young basin-ward
dipping normal faults and (2) faulted segments of older counter-regional normal
faults. Some of the normal faults in this system are folded and rollover
anticlines in the NW part of the basin are locally inverted due to Pleistocene
NW-SE shortening.
Farther NE, on
the present-day slope, E-W oriented dextral strike-slip zones are the dominant
structures and were active during shelf extension. These are spatially
associated with and gradually give way down-slope into NE-SW trending
contractional folds of the Barbados
prism. The folds trend at nearly right angles to the shelf extensional faults
and, thus, cannot be contractional features at the toe of a gravitationally
driven system. 3D restoration illustrates that the strike-slip fault zones in
the slope domain partitioned the counter-regional footwall into large blocks,
thus accommodating the large-scale extension on the shelf and transferring
displacement basin-ward into contractional structures of the Barbados accretionary prism.
We interpret
the Plio-Pleistocene Columbus basin as a thin-skinned pull-apart basin formed
during southeastward stepping of dextral displacement from the Caribbean-South
American plate boundary into the Caribbean-Atlantic subduction complex.
Extensional, strike-slip, and contractional structures in various parts of the Columbus Basin are genetically associated with
one another and accommodate 3D strain in this complex plate corner. Although
small-scale features related to mobile shale (mud volcanoes) are well
documented in the basin, large-scale shale mobility is not required to produce
the observed structural patterns.