Volume balance of a submarine landslide in the Espírito Santo Basin, offshore Brazil: Quantifying seafloor erosion, sediment accumulation and depletion

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Abstract

Degrees of seafloor erosion, sediment accumulation and depletion are quantified using a 3D seismic volume of a small submarine landslide offshore Brazil. In the study area, the upper boundaries of large remnant blocks — kept in situ during the main instability event — and the interpreted non-failed margins of the landslide comprise a reliable estimate for the position of the seafloor at the time the submarine landslide was generated. Remnant blocks of strata show little internal deformation, particularly at their base, and were kept upright during a principal instability event triggered in response to regional halokinesis and associated overburden faulting. They are laterally bounded by a mixed succession of mass-wasted chaotic strata, rafted blocks and post-slide sub-horizontal units. Despite the thin accumulation of failed strata now visible, the interpreted data indicate that submarine landslides of small areal dimensions can erode substantial volumes of seafloor sediment. Consequently, the more than 300 m of eroded seafloor strata offshore Brazil is of a similar scale to the largest failures recorded on continental margins (e.g. Storegga Slide; Amazon Fan complex; Cape Fear slide). The scale relationships presented in this paper indicate that the magnitude of seafloor erosion experienced during instability events may not be recorded by distal mass-wasted strata, which can be demonstrably thinner than the original volume of failed sediment. Thus, we conclude that the relative expression of accumulated slide strata is not directly related to the original volume of failed material. In addition, small-scale landslides can be responsible for the erosion of thick seafloor deposits, a factor that can potentially cause significant losses of seal capacity in overburden successions due to the sudden release of confining pressures in a vertical direction.

Introduction

Submarine landslides documented on modern continental slopes mostly comprise shallow features with glide planes located a few metres below the seafloor (e.g. Lykousis et al., 2002, Mosher et al., 2004, Minisini et al., 2007). However, a significant portion of submarine landslides are of greater sizes, thicker than the common vertical resolution of Industry data (> 25–30 m), which essentially permits the investigation of geological processes associated with large-scale instability events (Haflidason et al., 2005, Gee et al., 2006, Moscardelli and Wood, 2008). This limitation is particularly evident when locally assessing geological hazards, such as those associated with small-scale slope instabilities, particularly in prospective drilling sites. Consequently, little is still known about: a) scale relationships between the area of sediment failure and the total volume of failed strata deposited during discrete instability events, b) the velocity — and relative inertia — of failed strata for specific types (and sizes) of failure, and c) the frequency of instability phenomena vs. scale of landslides (Urgeles et al., 2007, Hjelstuen et al., 2007). Variations in transport distances and volume balances of failed and accumulated strata are also poorly documented.

Difficulties in defining the pre-failure seafloor morphology and initial region of failure, by solely analysing the volumes of failed strata, are particularly observed when dealing with ancient landslides. In these, headwall areas are commonly not preserved or failed material is transported through large distances, in discrete failure events (e.g. Masson, 1996, Gee et al., 2001). Sparse case-studies in which the volume of failed strata has been thoroughly documented include large slided blocks surveyed offshore Newfoundland (Deptuck et al., 2007), the Storegga slide offshore Norway (Evans et al., 1996, Bouriak et al., 2000, Bünz et al., 2005) and recursive mass-transport deposits in the Gulf of Mexico (Urgeles et al., 2007). Nevertheless, whilst the slided blocks offshore Newfoundland show little internal disaggregation and landslide strata in the Gulf of Mexico spans a great portion of the continental slope, the large-scale Storegga slide was remarkably formed by multiple smaller-scale recursive failure events (Haflidason et al., 2005, Bryn et al., 2005).

In contrast with most published literature, which relate the volume of failed strata with the observed dimensions of a headwall scarp, this paper documents the volume balance of a submarine landslide based on the existence of remnant blocks and marginal areas marking the pre-failure position of the seafloor. This work focuses on a Late Cainozoic submarine landslide generated during a single instability event, concluding on the scale of seafloor erosion, rates of sediment depletion and accumulation expected on small-scale landslides (Fig. 1). In particular, we demonstrate that large volumes of seafloor strata can be eroded in small landslides but, despite this, relatively thin mass-wasted strata are accumulated downslope from the headwall area. Hence, this paper documents:

  • The internal character of an Upper Cainozoic submarine landslide in the South Atlantic;

  • The volume balance of material accumulated and/or depleted during the sliding event;

  • The internal character of mass-wasted strata and their effects on the estimation of transporting distances, and volumes of failed deposits.

The submarine slide of this study differs from case-studies with developed remnant blocks along the Canadian North Atlantic margin (e.g. Shimeld et al., 2003, Deptuck et al., 2007) as it presents undeformed remnant blocks, kept in situ during failure, together with mass-wasted and post-slide strata (Fig. 2). The tops of these remnant blocks are draped by a sub-horizontal reflector comprising a reliable estimate to the position of the palaeo-seafloor prior to failure (Horizon 3, Fig. 2). This character allowed us to investigate the relative volumes of accumulated and depleted material in specific zones of the observed submarine slide. Therefore, this paper documents the ratio between sediment depletion and original volume of strata on a continental slope, giving important hints on the scale of erosion experienced on continental slopes. In the Discussion section, we analyse possible controls on the volume of failed strata and their significance to the interpretation of exhumation and/or depositional rates on continental margins.

Section snippets

Data and methodology

In this work, three-dimensional (3D) seismic data from an area covering 2450 km2 of the continental slope offshore Espírito Santo were used (Fig. 1a). Within the complete seismic volume, we have selected a sub-area in water depths ranging from 1500 m to 2060 m (Fig. 1b). The seismic volume has an inline spacing of 12.5 m and was acquired with a 6 × 5700 m array of streamers within a 12.5 m × 25 m bin grid. Data were sampled in intervals of 2 ms, for a nominal fold of 56. Data processing included

Regional setting

The Espírito Santo Basin comprises a series of rift basins of Late Jurassic–Cretaceous age developed between the Victoria–Trindade High and Abrolhos Bank, Southeast Brazil (Fig. 1a). The sub-area investigated in this paper is located in the middle part of a southeast-dipping continental slope (Figs. 1b and 2). Two-way travel-time (twtt) structure maps in Fig. 3a and b show a simple slope geometry, with deeper parts of basal Horizon 1 roughly coinciding with the axis of the interpreted landslide

Internal character of mass-wasted material

The internal character of the investigated landslide deposits is shown in several twtt-structure and attribute maps in Fig. 3. Maximum magnitude data (also named as reflector strength or envelope amplitude; Brown, 1999) computed within an interval 40–50 ms above the base of failed material shows in great detail the geometry of remnant and rafted blocks (Fig. 3c). Maximum or envelope magnitude is a phase-independent account of amplitude and highlights large amplitude features (independent of

Estimating the true volume of failed material on continental slopes

Principal limitations when interpreting buried landslide deposits often relate to the difficulties in estimating:

  • a)

    Transporting distances of failed strata, particularly when in the presence of distal debrites detached from the headwall scarp, or when this same headwall scarp is either not preserved or is located away from interpreted 3D blocks (Masson, 1996, Gee et al., 2001);

  • b)

    Original volumes of failed material, which may indicate the scale of erosion on a continental margin and, indirectly, the

Conclusions

This paper analyses a submarine landslide deposit in southeast Brazil, concluding that:

  • The relative expression of accumulated landslide strata is not directly related to the original volume of failed (remobilised) material;

  • Topographic confinement imposes a marked zonation in the internal character of landslide units.

  • Small landslides with areal extents of approximately 45 km2 can be responsible for the sudden erosion of thick (more than 300 m) seafloor deposits.

The interpreted landslide differs

Acknowledgements

The authors acknowledge the permission conceded by CGG-Veritas for the publication of this research paper. D. Mosher and EPSL editor Rob D. van der Hilst are acknowledged for their contributions to an earlier version of this manuscript.

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