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Seismic Geomorphology and Modeling of Deepwater Slope Systems

Rob Gawthorpe, University of Manchester, Manchester, England

Integration of high resolution, shallow-section (Pliocene- Pleistocene) 3-D seismic data and 2-D seismic modeling of outcrop analogs provides a powerful approach to understanding (1) the large-scale sequence-stratigraphic evolution of slope systems, and (2) the detailed character of the main architectural elements of slope depositional systems (e.g., channels and mass transport complexes).The results enhance our understanding of the geometry and evolution of deepmarine reservoirs and geo-hazards along continental margins.

Seismic modeling of large-scale (30-km-long x 500-750 m high) clinoform exposures of Eocene age in Spitsbergen has been used to examine the seismic expression of high frequency, 4th order clinoform sequences, their stratigraphic evolution, and the seismic character of various shelf-edge, slope, and basin-floor reservoir classes. At typical exploration frequencies (30-40 Hz), the synthetic seismic sections allow subdivision of the clinoform units into low-order packages on the basis of changes in the trajectory of the shelf edge. Within the low-order seismic stratigraphic framework, large-scale basin-floor fans can be associated with progradation or basinward-falling shelf-edge trajectory. However, much of the outcrop-scale stratigraphic detail, allowing the definition of high-frequency 4th order sequences and the linkage of reservoir distribution to clinoform geometry, cannot be resolved. At frequencies greater than 70 Hz, sequence stratigraphic division of the clinoforms at a scale similar to the high-frequency outcrop sequences is achievable, and sand-prone incised valleys at the shelf edge and incised slope channels are resolvable.

Shallow-section 3-D seismic data from west Africa and the Nile Delta have been used to investigate individual slope architectural elements (mass transport complexes and slope channels) and their spatial and temporal relationships. Mass transport complexes (MTCs) are a major feature of slope clinoforms and commonly originate from areas where major structures deform the sea floor. They are readily identified by chaotic seismic facies, have high-amplitude smooth basal reflectors, and have an irregular draped upper surface. Many MTCs are composite in nature, with several, discrete chaotic units separated by more continuous seismic reflections, suggesting multiple failure events. Images from the basal surfaces show large-scale striations (>10 m deep and in excess of 10 km long) that appear to record the catastrophic failure and show dramatic evidence for basal erosion. Within proximal areas, giant tabular blocks several kilometers across and a few hundred meters thick may be present, and fully developed debris-flow facies are commonly developed within a few kilometers of the source. Furthermore, volume estimates suggest the MTC deposits commonly exceed the slide scar volume by a factor of two or more, indicating significant seafloor remobilization during emplacement.

Slope channels vary greatly in size and morphology, from small, simple single channels, to large channel complexes that may have been exploited over multiple 4th order sequences. Channels exhibit an extraordinary range of geometries with low and high sinuosities, incision up to several hundred meters, and with or without well-developed levees. Reincision is extremely common and results in poor preservation of the initial stage of channel development. Quantitative analysis of the channels indicates good correlation between sinuosity, incision, and gradient. Channels appear to evolve from high-gradient, low sinuosity systems into highly sinuous, lower gradient systems that are deeply incised. Linear, high amplitude features, and weakly incised furrows identified on the open slope between channels are believed to be the precursors of channels. Evolutionary trends in channel geometry, vertical stacking patterns, and the relationship of the main-slope architectural elements reflect the interplay of local controls (slope gradients, flow properties, structural topography, sediment supply) and regional controls (sea level).

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