Submarine Fan Fringes with Feather-Edged Planform Geometry:
Core Calibration of Turbidite and Linked Debrite Architecture

Project duration: Two years starting from spring 2006

 

Introduction

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The Rhone Fan Project aims to calibrate sediment facies of fan-fringe deposits that display lobate or feather-edged planform geometry. This ‘ground-truthed’ information can be implemented in reservoir models of sandbody architecture, at locations where lobate or feather edged geometries are observed in 3D seismic reflection data.

The project will determine whether this distinctive feather-edged planform geometry is indicative of deposits that contain significant volumes of mud-rich, ungraded, debrite sandstone. The lobate geometry of these submarine deposits is similar to those displayed by subaerial debris and mudflows that undergo abrupt ‘freezing’ due to their rheological properties (i.e. yield strength). We suggest that the feather-edged geometry in submarine settings may also be produced by debris flows. A single core into the terminal lobe of the Rhone Neofan encountered an ungraded sandstone interval with abundant mud clasts (Mear, 1984). Field studies by UK-TAPS (Talling et al., 2004; Amy et al., 2005; Amy and Talling, in press) have shown that this type of deposit is often indicative of submarine debrites with abrupt distal and lateral sandstone pinch-outs.

Previous work has shown that a single flow event can deposit both clean turbidite sandstone and muddy debrite sandstone. Two contrasting models for the relative position of turbidite and debrite sandstone intervals have been proposed. Model 1: A turbidity current decelerates. Sediment concentration becomes progressively more stratified. Near-bed turbulence is suppressed. A positive feedback continues, until a debris flow forms at the base of the flow. The associated debrite is found at the fringes of the deposit (Talling et al., 2004). Overpassing of the debris flow above a dewatering layer of turbidite sandstone may also lead to fringe debrites (Haughton et al., 2003). Model 2: In the second model, a turbidity current locally erodes the sea floor. Down-flow from these areas of local erosion, the flow ‘bulks up’ and transforms into a debris flow. The core of the deposit locally comprises debrite (Talling et al., 2004). Shear mixing with the surrounding sea water generates flanking turbidity currents, whose clean sandstone deposits encase the debrite.

The two models predict opposite (core or fringe) positions for the debrite and the turbdite sandstone intervals. Understanding the spatial arrangement of turbidite and debrite sandstone is crucial for effective reservoir modelling; debrite sandstone typically has much higher mud content than turbidite sandstone, and the two types of sandstone have very different reservoir quality. 

This project will identify if the lobate feather-edged Rhone Neofan deposits are sandy debrites and, if so, determine whether debrites occur in the central part or at the fringes of individual lobes.

Work Package I: Coring Cruise to the Rhone Neofan

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The central part of this project will be a research cruise to the modern Rhone Fan in the Mediterranean Sea (Fig. 1). This is one of the largest active fans in the Mediterranean. A recent avulsion of the main Rhone Fan-Channel, at a break in slope, has generated the most recent deposits, termed the Neofan (Fig. 2). The Neofan is an elongate feature covering an area of ~50 x 25 km. Carbon-14 dating shows that the fan has been active recently (Torres et al., 1997). The 30 kHz side scan sonar image shown in Fig. 2 comes from just beyond the terminus of a shallow channel on the Neofan, on one of the terminal lobes (Kenyon et al., 2003). A single preliminary core from this area encountered ~2 m of medium to coarse sand located a few centimetres below the sea floor (Mear, 1984). This ungraded sandy interval contained mud clasts and may represent a debris flow deposit.

The Rhone Neofan is an ideal location for testing whether a lobate feather-edged planform shape indicates the presence of linked debrites, and for assessing the relative position of linked debrite and turbidite sandstone. The terminal lobes of the Neofan lie at a water depth of ~2400 metres (Fig. 1), ensuring fairly rapid recovery of cores. The compact extent of the selected terminal lobe means that a detailed picture of its internal architecture can be built up during five days of coring. A small number of existing cores, that appear to contain debrites, are available to guide the extended coring program. These previous cores show that the deposits can be successfully penetrated and recovered by shallow coring.

Fig 1: Perspective view of the Rhone Fan, offshore from the Gulf of Lions in the Mediterranean Sea. Isopachs of the Rhone Neofan are superimposed. Bathymetry in metres.

The first stage of the cruise will involve a medium-resolution (30 kHz) sidescan survey of the Neofan terminal lobe shown in Figure 2, and adjacent areas. This will produce data of comparable quality to that shown in Figure 2, and may reveal the presence of other feather-edged terminal lobe fringes in the vicinity (as well as linking them with feeder channels). Then, we will target specific sections of the lobe with high-resolution (100 kHz) sidescan sonar, which produces images at outcrop-scale resolution. A shallow seismic profiler mounted on the sidescan towfish will provide sub-bottom control. These planform and cross-sectional geophysical data will then be processed on board and used to guide the coring phase of the cruise, whereby a series of cores will target key areas to calibrate the geophysical data. Ship-based geo-referencing technology will ensure accurate positioning of the cores in specific environments, e.g. distributary channel, lobe, inter-channel areas etc. Figure 2: (Top) Perspective view of the Rhone Neofan. (Bottom) 30 kHz sidescan sonar image showing ‘feather-edged’ pattern of terminal lobe (TTR2 data, Kenyon et al., 1993). The position of the sidescan image is shown on the upper figure.

The cruise is planned for summer 2006, using the Russian vessel RV Prof Logachev, as part of the European TTR program.

The cruise will provide the following deliverables:

	Accurately geo-referenced medium (30 kHz) and high (100 kHz) resolution side-scan sonar imagery of terminal lobe planform geometry;
	A dense network of shallow seismic lines documenting the three-dimensional architecture of the terminal lobe;
	High resolution bathymetry and sand thickness maps derived from the above datasets;
	A carefully positioned grid of gravity cores that constrain the sandbody architecture of the uppermost lobe deposit, including the frond-like features imaged in Fig. 3.

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© UK-TAPS Group
November 2005