Outer-shelf conduit within growth-fault compartment Pliocene Orinoco Delta

Abstract

Prominent outcrops of the Pliocene Orinoco Delta along the southeast coast of Trinidad Island display a well-known, very thick Icehouse succession of storm-wave dominated delta lobes, as part of a 10–12 km thick Neogene continental-margin prism. There is a previously discovered, steep incision or conduit (400 m wide) with south and north-bounding erosion surfaces, cutting deeply into the Mayaro Formation shelf-edge delta deposits. The ca. 100 m-thick infill of the conduit shows an initial muddy succession passing upwards to very coarse-grained debrites, rotated and slumped blocks, and structureless sandstones that pass upward to a stacked series of sandy to muddy channel fills. The channels show spectacular tidal rhythmites, probably developed on the point bars of subaqueous tidal channels. As one piece of the topset of the shelf-edge delta clinoform, this conduit displays features of the linkage hub between the landward on-shelf delta systems and the basinward slope deposits.New fieldwork and a re-examination of the incision-infill reveal new details and suggest three possible origins of the conduit: ① the first is a no river feeder, shelf-edge collapse scar, extending onto the upper slope and infilled by deeper to shallower deposits. This shelf-edge cutting canyon interpretation is supported by the presence of hardground ichnology on the steep walls of the incision and the abundance of chaotic blocks and debrites in the lower two-thirds of the incision infill. ② The second is a shelf channel or valley linking fluvial channels to on-shelf and outer shelf deltas. This river-connected, subaqueous shelf valley origin, is supported by the upward-coarsening nature of the infill succession and especially the occurrence of shallow-water tidal channels in the upper third of the infill. ③ The third is a canyon developed mainly during transgression or highstand with a collapsed canyon-head infill and an overlying muddy tidal-channel system that had only limited sediment bypass to the deepwater slope and basin floor. This case represents both the shallow-water and the deeper water features of the infill succession, and is supported by similar large shelf-edge collapse features seen elsewhere in the Cruse and Moruga Formations of the paleo-Orinoco margin. This case, unrelated to river supply, also benefits from slight rotational shelf-edge uplift to trigger collapse.The known Cedar Grove growth fault was active and located some 4 km to the west from the present coast on the southeast corner of Trinidad Island. The growth-fault trapped thickened outer shelf to shelf-edge delta-front and delta-plain sediments within the hanging-wall compartment as the fault stretched about 20 km along strike. All the shelf-edge aggradation and its partial collapse were likely triggered by growth-faulting. After some muddy backfilling and coarse-grained block collapse into the invaginated shelf-edge, this scalloped area attracted a belt of subaqueous tidal channels, thus healing the eroded topography.