Giant slump complex structure in Gulf of Guinea according 3d seismic

Sitenkov D.V.

Lomonosov State University, Department of Geology, Moscow

ask.dmithry@gmail.com

Introduction

In the last few decades, a number of petroleum companies has looked over the ocean shelf and the ocean continental slope, where turbidite and avandeltal sediments are studied. Analysis of recent science articles [2,3,4,5] also shows significant complexity in the understanding of the way that different gravitational processes interact, including large-scale slumping. Such processes are clearly manifested near transform margins. For example, the gas field Ormen Lange, Norway, which was discovered in the resent years, is timed to the giant slump complex Storegga Slide Complex [1] near the Atlantic transform margin. This papers goal is to describe the volume (3D) structure of such a geological event, which is located in the North part of Gulf of Guinea.

Tectonic and Geological peculiarities

The landslide complex described in this paper is located in the Atlantic type “Normal” passive margin near San Paul transform fault. This fact predetermines the gradient relief of synrift ocean basement; in addition such disposition provides a relatively high tectonic activity during all geological history. Thousands of square kilometers are subjected to large-scaled sliding events, which are affecting the postalbian marine sediments. Marine succession is presented by hemipilagic sediments and simultaneously turbidities deposition is taking place. Normal sediments are strongly attracted by synrift basement ridges, horst steps, whereas turbidit deposits are quite usual for depressions between them. Tectonic ridges have SW strike; major turbidite channels normally have NS strike. Due to those sediments different rheological behavior, it becomes possible to describe the interior structure of the slump complex. Different seismic packages reply in different ways to the stress field, which is formed because of the gravitational forces. Sandy packages act like brittle layers and have apparent fault events, otherwise clay packages act a different way, and they have a viscous response. This is shown in the seismic field peculiarities.

Landslide structure

Initially, the area is divided into the two principle zones: zone of decompression, typical for “the slump crown”, and the zone of the compression compensation, typical for the proximal part of landslide.

The decompression zone is presented as a classical example of the landslide structure. We can see both rapture and crown faults, the numbers of segments which are separated one from another by the listric faults, and the last aggregate to detachment fault in the bottom of marine succession. On the slices the faults are presented as a polygonal system with the horseshoe-shaped crown fault events. Because of heterogenous compression caused by neighbor slide-block rotation, slump folds can form in the bottom parts of slide blocks. Medium-sized faults are dominated in the upper levels. The antithetic fault system became significant - it adapts rotational deformation.

The compression compensation zone is located in the proximal part of the slide complex. This zone is presented as brittle-elastic deformation in sandy layers and as viscous deformation in clay layers. Also we can see injection clays and thickness anomalies caused by material redistribution. Such behavior is usual for the plastic material as well as for the near detachment rocks. There are few thetic faults, and they are adapting detachment deformation. Also we can see evidence of paleo flows, like mud flow and debris flow, which are complicating the entire structure of paternal slide complex.

The zone of compensation of slide deformation is presented as a very complicated surface, which is takes place in the bottom layers of marine succession (cenoman- turon), which jumps from level to level with the typical geometry “Flat-Rump”. This surface is called slide Detachment Fault. Near this surface we can see grinded rocks; this type of rock forms a quite unique layer, with a thickness of over 100 meters. Rough estimates show, that the horizontal displacement can be 5-7 km, while vertical displacement can reach 500 meters.

The compression compensation zone has importance for the whole system. Brittle deformations and movements that take place in the crown zone are compensating as a viscous deformations in the compression zone.

References:

1. Færseth B., Sætersmoen B.Geometry of a major slump structure in the Storegga slide region offshore western Norway // NORWEGIAN JOURNAL OF GEOLOGY, 2008, vol 88, pp. 1-11

2. Gee M.J.R., Uy H.S., Warren J., Morley C.K., Lambiase J.J. The Brunei slide: A giant submarine landslide on the North West Borneo Margin revealed by 3D seismic data // Marine Geology, 2007, vol 246, pp. 9-23

3. Georgiopoulou A., Wynn B.R., Masson G.D., Frenz M. Linked turbidite–debrite resulting from recent Sahara Slide headwall reactivation // Marine and Petroleum Geology, 2009, vol 26, pp. 2021-2031

4. Schwehr K., Driscoll N., Tauxe L. Origin of continental margin morphology: Submarine-slide or downslope current-controlled bedforms, a rock magnetic approach // Marine Geology, 2007, vol 240, pp. 19-41

5. Wien K., Kolling M., Schulz D.H., Age models for the Cape Blanc Debris Flow and the Mauritania Slide Complex in the Atlantic Ocean off NW Africa // Quaternary Science Reviews, 2007, vol 26, pp. 2558-2573