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BADALINI, GIANLUCA, BEN KNELLER, University of Leeds, and CHARLES D. WINKER, Shell E&P Technology Company

Abstract: Late Pleistocene Trinity-Brazos Turbidite System. Depositional Processes and Architectures in a Ponded Mini-Basin System, Gulf of Mexico Continental Slope

The late Pleistocene Trinity-Brazos turbidite system is exposed intact on the Gulf of Mexico seafloor, at water depths ranging from 150 to 1450 metres. It consists of onlap-fill succession in four mini-basins (Basins I-IV, Fig. 1) linked "fill-and-spill" style. Basins I to III are filled to their spill-point, while Basin IV is still underfilled. The onlap-fill succession in each basin overlies thick hemipelagic deposits that are continuous from basin to basin. The down-dip basins (II-IV) are connected to a shelf margin delta by a partially buried channel system with local levee development. Surface channels are clearly expressed on the bathymetry (Fig. 1). In the up-dip part of Basin I an anastamosing complex of low relief channels is recognised. Further down-dip they coalesce to form a single channel which becomes increasingly incised down-current. No levees are observed. In the distal part of Basin I additional tributaries merging with the main channel can be identified. In Basin II the main channel (eastern channel) is shallow up-dip and then becomes increasingly incised and develops a levee complex which shows the typical gull-wing cross section. It becomes less incised down-dip, and then turns abruptly to the south-east to traverse the fault separating Basins II and III, across which it is deeply incised, developing levees in Basin III. The channel continues in the up-dip portion of Basin IV, where it develops a levee only on the western side. A second channel (western channel) connects Basin II directly to Basin IV. It has a NW-SE orientation and is apparently disconnected from the eastern channel in Basin II and from the shelf margin.

Each onlap-fill succession consists of alternating chaotic/transparent units (interpreted as a variety of sediment gravity flow deposits: debris flows, slumps, muddy and sandy turbidites) and layered units (interpreted as turbidite fans). In Basins II, III and IV the final stage of filling is represented by a channel levee complex. Mass flows represented by chaotic/transparent units were supplied both via the channel systems and from the basin margins. The former are tabular, have erosional bases (at least locally) and irregular tops, basinwide extents, and represent multiple depositional events. Locally high amplitude, low frequency, discontinuous reflections can be recognised within the mass flow. Their seismic continuity increases distally. These mud-rich units apparently were very fluidal and mobile, and were able to travel over considerable distances (in some cases from the shelf-edge to Basin IV, a distance of approximately 60 km). The locally-sourced chaotic/transparent units are wedge-shaped, have erosional bases and limited extents. Turbidites, represented by layered units, present a great variety of seismic facies and show an increase in reflection continuity downdip (in each basin and more generally across the entire system). They are sand-rich, show onlapping, and generally have tabular geometry. "Run-up" effect is common along the basin margins. The geometry of the individual depositional units is strongly influenced by the topography created by earlier units. Examples of compensation can be recognised at different scales.

The onlap-fill successions are punctuated by (Fig. 2): (a) erosion surfaces (SR, SM in Fig. 2), commonly at the base of mass flow units or "fans," or associated with channels; and (b) packages of moderate to high amplitude, laterally continuous reflections, that can be seismically correlated into the hemipelagic drape of the surrounding slopes (CR, CM in Fig. 2).

The erosion surfaces are interpreted as the result of re-establishment and/or reorganisation of the main clastic sediment delivery system, and may be regarded as high-frequency "sequence boundaries". The continuous high amplitude reflections are associated with shut-off of sand supply and abandonment of turbidite systems, and/or bypass, and are interpreted as mud-prone condensed sections/starvation surfaces. They can be seismically correlated (when not eroded by overlying units) on the basin flanks and used as a correlation tool between basins. Three main channel systems have been mapped in Basin II. Choking or burial of channels at least locally produced reorganisation and diversion of the dispersal pathway from one basin to the next. The position of channels remained more or less steady through time (fixed entry point effect?), though remarkable differences in terms of width and depth of incisions have been noticed between surface and older channels. Smaller "distributary" channels have been recognised over limited areas and others may be present but are below seismic resolution.

Where the onlap succession is completely developed (Basin II and IV) at least 5 sequences topped by condensed sections can be recognised (Fig. 2). Given that the onlap succession is entirely Wisconsinan in age, and that post-depositional deformation of the basins is slight, the mapped sequences are interpreted as the result of high frequency climatic changes affecting deep marine clastic systems within a single lowstand. This arises the possibility of chronostratigraphic correlation in turbidite systems at scale below the resolution of conventional seismic. 

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