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Mixed Tidal-Wave Processes in a Growth-Fault Controlled Outer Shelf Conduit Near the Pliocene Orinoco Shelf-Edge


Outcrops of the Pliocene Orinoco Delta along the southeast coast of Trinidad Island display an incised conduit complex cutting into outer-shelf delta lobes (Mayaro Fm.), part of the very thick Neogene continental-margin prism. Along this east coast succession, with the strata younging towards the north, there is an observed abrupt facies change across a main erosional surface. Below this surface, the deposits are shallow water facies generated by storm-wave processes (wave-dominated delta-front deposits) whereas the infill above the erosional surface shows a vertical facies change to silty mudstones and sediment gravity flows and eventually an upper interval of channelized tidal deposits. There are at least two significant erosional surfaces representing the edges of two relatively deep channelized conduits down-cutting into distal delta-front deposits. The conduits and their composite infill are some 100 m thick, as the top part of the infill shows features of sinuous delta-plain channels with a sandy thalweg passing up to tidal cycles on gently sloping point bars. This suggests that the conduits were part of a large channel system incised across the shelf, most likely driven by relatively high river discharge and sediment supply during the falling stage of sea level.

The complexity of the channel incision is increased due to the delta-front location occurring within a known listric regional growth fault compartment. The depositional system developed in response to gravitational instability, formation of the growth faults, and generated rollover anticlines which have positive relief towards the basin. Additionally, the only Mayaro stratigraphic level that can be seen to be close to the paleo-shelf edge occurs some 500 meters lower in the Mayaro stratigraphy. During the subsequent 500m of aggradation, with significant growth faulting and overall eastward progradation of the sedimentary system, the shelf edge would have moved basinwards by more than 10 km, so the incisions are firmly located on the outer shelf. The example of large channels conduits on the Orinoco shelf is similar to many other large delta systems, where sediments delivered past the shelf edge is clearly transported via large channels on the outer shelf.