AAPG/GSTT HEDBERG CONFERENCE

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

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

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, Sunbury Business Park, Chertsey Road, Sunbury-on-Thames, Middlesex, TW16 7LN, UK

The Plio-Pleistocene age Columbus Basin is charcaterised by growth fault bounded depocentres which developed diachronously from west to east above Miocene age overpressured mobile shales.  Since the early Pliocene up to 12 km of marine and deltaic sediment has accumulated in these depocentres, with sediment accumulation rates of 5-10 M/KY exceeding those recognized in other Neogene deltaic systems.  Hence the Columbus Basin represents a global end-member high sediment supply/high accommodation setting and provides unique insights into the impact of rapid basin subsidence and high rates of sediment supply on shelf edge delta stratigraphy.

The 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 Columbus Basin.  Two exposures of the Mayaro Formation, the MB5 and MB8 outcrops, allow detailed analysis of three key facies associations; (1) Storm-influenced pro-delta; (2) Storm-dominated delta front, and (3) Tide-influenced delta front, and a variety of stratigraphic surfaces.

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.

The 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 Columbus Basin is problematic, even when supported by detailed biostratigraphic and palynological data.

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.