ONSHORE-OFFSHORE CORRELATION, EAST COAST TRINIDAD
G.Rohr AMOCO
Port of Spain
An onshore reference section is described from outcrops and wells east of the Cedar Grove Fault between Galeota
and Radix Points. The section comprises the Palmiste; Mayaro and Gros Morne Formations of Carr-Brown & Frampton
(1979). Correlative stratigraphic units such as Forest and Cruse Formations and three members (Trinity Hills Sand;
St. Hillaire Silt and Gros Morne Sand) of the Moruga Formation are also discussed. Offshore equivalents of the
Mayaro Fm are proposed. The stratigraphic relationship between the Mayaro Fm as defined herein and the three member
Mayaro Fm of Kugler 's usage has not been investigated. The Palmiste Formation can be recognized only in a restricted
area of both the onshore and offshore and the name should not be applied beyond the Galeota Ridge where equivalent
beds are of a sand rich deltaic facies.
An offshore reference section is described from wells in the Mora Field area where four formations are recognized.
These are informally designated '"A" series; "B" series; " C" series and "D"
series in descending order. Unconformities separate each series in Mora Field and throughout the Galeota Ridge
area. They can be correlated throughout the Columbus Basin and are recognisable by their paleobathymetric sequence
(i.e. regressive or transgressive in the relative sense).
The "D" series is equivalent to the Gros Morne Fm of Carr-Brown & Frampton onshore and the N through
P sands of Galeota Field (Trintopec 's designation) Numerous well-to-well correlations support this. In the Galeota
Ridge area upper members are successively truncated by erosion while off the Ridge it has not been reached. Only
one well (CF-5) in the
Mora Field area penetrated the upper members of the "D" series. The top of the series is recognized
by the first occurrence of a deep-water form of Cyclammina.
The "C" series is a regressive sequence recognised in Mora Field by the first occurrence of Saccamina
spp. By correlation it is equivalent to the C and H sands of Galeota Field and the 19 through 22 sands in Poui
Field. Well-to-well correlation from Mora Field to Galeota Ridge wells and the Palmiste-2 well establishes its
stratigraphic position as overlying the
Gros Morne and overlain by the Palmiste Fm. Thus it is considered equivalent to the Mayaro Fm. which is a less
marine and more sand rich proximal facies compared with the "C" series offshore.
The "B" series is a transgressive sequence unconformably overlying the "C" series in Mora
Field. It includes non-marine channel sands in its lower part and marine beds above. The top of this sequence
by the first occurrence of Miliamina (1) - and it is correlative with the 9 through 12 sands in Poui. It is
thought to be late Pliocene in age and therefore younger that the Palmiste. Structural uplift of the Galeota Ridge
during Late Pliocene and Pleistocene is revealed by stratigraphic thinning and erosional truncation. The presence
of non-marine beds in the lower part of the "B" series with marine equivalents to the southeast suggests
that emergent conditions prevailed northwest of Mora Field during this time and therefore the non-deltaic marine
Palmiste cannot be equivalent to the "B" series. Deltaic deposition south and east of the Galeota Ridge
was continuous and some part of this sequence will necessarily be equivalent to the Palmiste though in a quite
different facies. The most likely candidates would be the 13 through 17 sands in Poui but no direct correlations
are possible.
The "A" series is a transgressive marine sequence of Pleistocene age which has no onshore equivalents.
It is part of the very thick Pleistocene deltaic sequence which characterizes the Columbus Basin farther offshore.
The stratigraphic relationships described above lead to the following conclusions regarding paleogeography: (a)
the Gros Morne deltaic sands are distributed across a broad arcuate area covering most of southern Trinidad and
extending offshore as far as Teak Field but probably not as far as Samaan Field. (b) Epirogenic movements following
Mayaro deposition shifted the deltaic depocenter progressively southeastward. (c) The combination of delta lobe
shifting and the emergence of the Galeota Ridge in the Late Pliocene provided a barrier to deltaic sedimentation
northwest of the Ridge.
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