AAPG/GSTT HEDBERG CONFERENCE
“Mobile Shale Basins – Genesis, Evolution and Hydrocarbon Systems”
Storm dominated shelf edge deltas in a high accommodation setting; an outcrop example from the Columbus Basin, Trinidad, West Indies
Andrew P Bowman* and Howard D. Johnson
Dept of Earth Science & Engineering, Imperial College of Science Technology & Medicine, University of London, Prince Consort Road, London, SW7 2BP, UK
* Present address: BP Exploration Operating Co Ltd,
The Plio-Pleistocene age
only exposed part of the Columbus Basins sedimentary fill is the Pliocene age
Mayaro Formation which crops out in sea cliffs along Mayaro Bay in south east
Trinidad. The Mayaro formation comprises
approximately 2.5 km of deep-water marine to deltaic sediments deposited in the
hanging wall of the Cedar Grove Growth Fault which bounds the western margin of
The storm-dominated pro-delta facies association comprises silty mudstones containing 1-5 m thick sandstone filled scours and chutes, which become increasingly amalgamated up-section and gradually pass into more tabular bedded silty sandstones. Growth faults with throws of 10-20 m and 3-10 m deep slump scars also occur. Although sediment gravity flow deposits dominate this succession there is abundant evidence for sediment deposition and reworking by storm-waves.
The storm-dominated delta front facies association comprises inter-bedded hummocky-cross stratified sandstones and bioturbated mudstones which pass either gradually or abruptly upwards into tens of metre thick successions dominated by amalgamated swaley cross-stratified fine-grained sandstones. The inter-bedded hummocky facies are similar to classic lower shoreface and offshore transition zone successions. However, the swaley-cross stratified units differ from classic middle to upper shoreface successions through their thickness, aggradational stacking patterns and the presence of 5-10 cm thick silty mudstones within thick sandstone bed sets. These differences reflect high rates of sediment accumulation on a large delta front.
The tide-influenced delta front facies association comprises 2-15 m thick units of discontinuous laminated mudstone inter-bedded with mud-draped cross laminated and wavy to lenticular bedded sandstones. It occurs above the pro-delta facies association, inter-bedded with elements of the storm dominated delta front facies association, and records deposition in a proximal delta front area relatively sheltered from the effects of large storms.
thickest continual exposure of the Mayaro Formation is the ~ 340 m thick MB5
outcrop. It comprises three main units:
(1) a basal ~ 125 m thick upward coarsening pro-delta succession; (2) an ~20 m
thick heterolithic tide dominated delta front succession and (3) an ~ 195 m
thick, high net-to-gross storm dominated delta front succession. It records a single cycle of delta, evolution
from the initiation of deltaic sedimentation at the shelf edge during base
level fall to the abandonment of the shelf edge delta during base level rise. The identification and interpretation of key
stratigraphic surfaces within this succession, and in the subsurface of the
The Mayaro Formation demonstrates that mobile shale withdrawal, growth faulting and resulting high rates of accommodation space creation have a significant impact upon the sedimentary and stratigraphic architecture of shelf edge deltas deposited in such settings.
AAPG Search and Discovery Article #90057©2006 AAPG/GSTT Hedberg Conference, Port of Spain, Trinidad & Tobago