Seismic diffraction imaging to characterise mass‐transport complexes: examples from the Gulf of Cadiz, south west Iberian Margin

Outcrop examples of mass-transport complexes (MTCs) often show a complex internal fabric which reflects disaggregation, deformation and entrainment that occurred during transport and emplacement. This can include intense folding, included blocks of substratum and internal shear zones. Seismic reflection images often cannot properly image this internal fabric as the scale of such structure is usually below the effective resolution. This can limit seismic interpretation to characterising only the overall morphology of the deposits (the top and basal reflectors).Seismic reflections are primarily generated by smooth, laterally continuous interfaces. Discontinuities at or below the scale of the seismic wavelength instead generate seismic diffractions (&#8220;diffraction hyperbolae&#8221; in unmigrated images). Diffractions are often ignored during seismic processing as they are generally lower in amplitude than reflections, though they do not suffer from the same lateral resolution limit as reflections so are potentially sensitive to smaller scale structure. We suggest that the discontinuous internal fabric of MTCs will generate a significant amount of diffraction energy relative to unfailed sediments.The main goal of this study is to use diffraction imaging to image the small-scale, heterogeneous internal fabric of MTCs. We demonstrate this using two high-resolution, multi-channel 2-D marine seismic profiles (3.125 m CMP spacing, 500 m maximum offset) acquired in 2018 and 2019 as part of the INSIGHT project to investigate submarine geohazards in the Gulf of Cadiz. Profile 1 intersects the Marques de Pombal reverse fault and shows a series of stacked MTCs (~1 s TWTT from top to bottom) in the footwall, thought to be related to episodic fault activity. Profile 2 is located in the Portim&#227;o Bank area and contains two large MTCs thought to be related to the mobilisation of a salt diapir. The diffraction imaging method proceeds as i) dip-guided plane-wave destruction to separate reflected and diffracted wavefields; ii) velocity analysis by cascaded constant velocity migrations of the diffraction wavefield; iii) post-stack Kirchhoff time migration of the diffraction wavefield.The unmigrated profiles show that the MTC bodies do generate more internal diffractions than the surrounding unfailed sediments. We also observe large contributions of diffraction energy from the rugose top and base of the MTCs, the rugose top salt interface and from faults within the unfailed sediments. The migrated diffraction images reveal distinct internal structure, thought to represent rafted blocks, ramps and both extensional and compressional faulting. The envelope of the diffraction image is used as an overlay on the conventional reflection image to guide interpretation and highlight potential diffractors. This allows interpretation of thin MTCs and improved delineation of their lateral extent (runout) above conventional reflection images.Diffraction imaging has previously been used to image heterogeneous geology such as fracture networks, channel systems and karst topography. Here we apply the technique to study the internal fabric of MTCs. The resulting images resolve small-scale internal structure that is not well resolved by conventional reflection images. Such structures can be used as kinematic indicators to constrain flow direction and emplacement dynamics, which inform the geohazard potential of future subaqueous mass-movements.

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How deep does sand deposits in the Alentejo basin (Gulf of Cadiz) reach? Evaluating slope stability from bottom-current activities through time

10.5194/egusphere-egu2020-18904 ◽

2020 ◽

Author(s):

Davide Mencaroni ◽

Roger Urgeles ◽

Jonathan Ford ◽

Jaume Llopart ◽

Cristina Sànchez Serra ◽

...

Keyword(s):

Pore Pressure ◽

Sediment Supply ◽

Excess Pore Pressure ◽

Bottom Current ◽

Gulf Of Cadiz ◽

Strait Of Gibraltar ◽

Gulf Of Cádiz ◽

The Stability ◽

Iberian Margin ◽

Excess Pore

Contourite deposits are generated by the interplay between deepwater bottom-currents, sediment supply and seafloor topography. The Gulf of Cadiz, in the Southwest Iberian margin, is a famous example of extensive contourite deposition driven by the Mediterranean Outflow Water (MOW), which exits the Strait of Gibraltar, flows northward following the coastline and distributes the sediments coming from the Guadalquivir and Guadiana rivers. The MOW and related contourite deposits affect the stability of the SW Iberian margin in several ways: on one hand it increases the sedimentation rate, favoring the development of excess pore pressure, while on the other hand, by depositing sand it allows pore water pressure to dissipate, potentially increasing the stability of the slope.In the Gulf of Cadiz, grain size distribution of contourite deposits is influenced by the seafloor morphology, which splits the MOW in different branches, and by the alternation of glacial and interglacial periods that affected the MOW hydrodynamic regimes. Fine clay packages alternates with clean sand formations according to the capacity of transport of the bottom-current in a specific area. Generally speaking, coarser deposits are found in the areas of higher MOW flow energy, such as in the shallower part of the slope or in the area closer to the Strait of Gibraltar, while at higher water depths the sedimentation shifts to progressively finer grain sizes as the MOW gets weaker. Previous works show that at present-day the MOW flows at a maximum depth of 1400 m, while during glacial periods the bottom-current could have reached higher depths.In this study we derived the different maximum depths at which the MOW flowed by analyzing the distribution of sands at different depths along the Alentejo basin slope, in the Northern sector of the Gulf of Cadiz.Here we show how changes in sand distribution along slope, within the stratigraphic units deposited between the Neogene and the present day, are driven by glacial &#8211; interglacial period alternation that influenced the hydrodynamic regime of the MOW.By deriving the depositional history of sand in the Alentejo basin, we are able to correlate directly the influence that climatic cycles had on the MOW activity. Furthermore, by interpreting new multi-channel seismic profiles we have been able to derive a detailed facies characterization of the uppermost part of the Gulf of Cadiz.An accurate definition of sand distribution along slope plays an important role in evaluating the stability of the slope itself, e.g. to understand if the sediments may be subjected to excess pore pressure generation. As sand distribution is a direct function of the bottom-current transport capacity, the ultimate goal of this study is to understand how climate variations can affect the stability of submarine slope by depositing contourite-related sand.

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