A MODEL FOR THE LATE PLIOCENE TECTONICS AND ITS EFFECTS ONQUATERNARY SEDIMENTATION
IN
THE TRINIDAD AREA OF THE EAST VENEZUELAN BASIN
BY
L. TYSON
TRINTOC, POINTE-A-PIERRE
This structural model is based on two concepts:
(i) In the Late Jurassic with the separation of North from South America a rifted margin (Atlantic type) developed
along Northern South America. Some basement related normal faults associated with this rifting can be identified
(indirectly) within the Trinidad area (refer to Fig. 1). These faults are zones of potential weakness, which were exploited during successive stages of
compress ional tectonics.
(ii) A transcurrent fault developed wholly within continental crust usually forms an extensional - strike slip
-compressional triad, because it obeys what may be termed a "law of conservation of continental crust".
In other words, at the trailing end of such a transcurrent fault, extensional features are present, while at the
leading end compressional structures are developed. (Refer to Fig. 2).
In the late Pliocene as a result of oblique collision of the Caribbean and South American Plates three NW-SE directed
transcurrent faults developed - the Warm Springs, Los Bajos and Soldado. Trinidad therefore was subdivided into
three discrete crustal blocks - a Northern, Southern and Southwestern. The Jurassic rift faults became the line
of weakness along which these strike-slip faults terminated by development of compressional structures (basement
reverse faults). In other words three transcurrent - reverse fault couples were developed (refer to Fig. 3):- the Warm Springs Central Range, the Los Bajos - eastern Southern
Range and the Soldado - western Southern Range. The core of the Southern Range Uplift is diapiric and contains
under-compacted clays of late Miocene age Greater horizontal movement southeastwards of the Southern with respect
to the Northern block resulted in divergent wrenching between the two. Literally speaking, a 'hole' developed along
the Warm Springs Fault, and this was followed by block faulting (refer to Fig.
4) leading to the development of a "trough/graben" in the Central Gulf
of Paria.
The southeastward motion of each of these three crustal blocks can also be seen in the weaker compressional anticlines
they produced. These folds generally detach just above the Middle Miocene unconformity boundary i.e. pre-Middle
Miocene sediments are not involved in the folding. However, it is to be noted that this deformation is generally
in proximity to Middle Miocene anticlines. The shallow buckling of sediments perhaps reflects the contrast of ductile
vs. brittle and semi-brittle behaviour
of rocks to deformation. In the Northern Block these shallow detached folds include the Mahaica and Guaico Anticlines;
for the Southwestern Block there is the Soldado Anticline; and in the Southern Block (refer to Fig. 5) there are the La Brea, Vance River - Boodoosingh, Lot One, Fyzabad,
Penal, Rock Dome, Lizard Springs and Palmiste Anticlines. Most of these are diapiric and contain undercompacted
clays of late Miocene age.
The sedimentation of the Late Pliocene and Quaternary are profoundly different (Fig.
7) and are influenced by the tectonic events of the Late Pliocene. In the Quaternary
two major depocenters developed (refer to Fig. 8)
(a) The Central Gulf of Paria trough/graben became the site of significant sedimentation (>10,000') in the Quaternary.
Sediment loading in this oval-shaped depocenter resulted in adjustments along fault blocks. These adjustments are
recorded in the sedimentary record as growth of the Quaternary sediments along some sections of the faults.
(b) The second major depocenter is funnel-shaped and is located offshore south and southeast Trinidad. In contrast
to the Late Tertiary when deltaic sedimentation was spread out all over South Trinidad, in the Quaternary sedimentation
was restricted to this open-ended funnel.
This limited area was quickly infilled, and progradation in the Early Quaternary was so rapid that the delta began
depositing sediments beyond the shelf edge onto the slope. The high depositional rates combined with rapid progradation
over the unstable continental shelf margin (refer to Fig. 9), resulted in the initiation of a major arcuate zone
of listric faulting at the Pliocene shelf edge. The zone of detachment of these faults was in the undercompacted
clays of the Lower Miocene sediments (i.e. below the Middle Miocene unconformity surface). Development of this
zone of listric faults at the old continental shelf margin resulted in instability of sediments backward of it
on the shelf As a result, in typical domino fashion, a series of listric faults developed (detaching in the Lower
Miocene sediments), and propagated as far back as the south east corner of the island. These listric faults were
involved in the partial break-up of anticlines formed both in the Late Pliocene and Middle Miocene.
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