Sedimentary gravity flow scars

The morphology of the broad depressions is also consistent with that of cross-shore turbidity flow scars developing from a shoreward-convex head and removing patches of surficial sediment from the nearshore into the mid shelf. The full saturation of the unconsolidated sediments and the momentum induced by the material set in motion should allow the flow to run for several hundreds of meters despite the gentle slope (1-3º) of the seabed. Lowe (1976) indicated that natural laminar liquefied flows of fine-grained sand will generally re-sediment after moving a kilometre or less, which would agree with the length of the broad depressions identified off Pedro Miguel.

The presence of various heads and overlap between neighbouring depressions further suggests that the scars originate from multiple events. Where two depressions intersect and display a vertical offset, the relative age interpretation is opposed to the one drawn under the long-shore hydrodynamic forcing hypothesis. In the case of gravity flows, more recent and powerful gravity flow will have carved further into previous ones (e.g., in profile d-d’ in Figure 4-B).

The concentration of coarse sediments within the scars could be explained by that fraction re- sedimenting first, as the turbulent energy associated with the slide decreased and became insufficient to keep it in suspension. Alternatively, existing sub-surficial coarse layers could play a critical role in the facilitation of the mass movements by undermining the foundation of the surface layers and providing a detachment plain that was subsequently exposed.

Substantial sediment transport occurring along the channel, which is indicated by the sediment waves present to the SE of the depressions, could be responsible for erasing turbidite deposits at the downslope end of the scars.

5.1.2.1. Gravity flow origin

Four potential triggers are suggested for the initiation of these sedimentary flows: (i) freshwater surges percolating through the sub-surficial coarse layer; (ii) seaward down-welling currents induced by large swells or onshore winds; (iii) seismically-induced liquefaction (e.g., Lowe, 1976; De Groot et al., 2006) or (iv) hyperpycnal flows (Mulder et al., 2003; Parsons et al., 2001). The first possibility implies freshwater from onshore watercourses or aquifers partially percolating through layers of higher permeability underneath the seafloor (e.g., coarse sub-surficial layer). Such events could be promoted by intense rainfall events (a characteristic of the Azorean climate) overcharging onshore groundwater pathways and reservoirs (for a review of seabed fluid flows see Judd & Hovland, 2007). When emerging through the thin overlying fine sand blanket, the lower density waters could fluidise the unconsolidated surface and trigger small turbidity flows down the shelf surface.

An alternative hypothesis of seaward down-welling currents induced by large swells is inspired by the mechanism proposed by Cacchione et al. (1984). In this case, the detachment of surficial horizons and subsequent turbidity currents would be induced by the pressure created by cross- shelf near-bed currents. The area is regularly impinged by large N/NW swells coming through the northern entrance of the passage either directly or by diffraction around the Ribeirinha headland. Less frequently, the area can also be affected by large swells from S/SW coming in through the passage’s southern entrance.

Since the study area is also subject to moderate seismic activity, it is also possible that these underwater mass movements are triggered by co-seismic fault movements. The most recent strong earthquake was in 9th_{July 1998 (Richter magnitude = 5.8; MMI=VIII; Costa Nunes et al.,}

1999), with an epicentre 12 km to the NNE of the study area. The whole north-eastern sector of Faial was significantly affected, particularly along the Pedro Miguel graben faults, which trend in a direction identical to the broad depressions and respective branches. Although coated by a sediment wedge, these faults extend beneath the neighbouring seafloor (some of them recover surficial expression in the middle of the passage) and their movement may have initiated turbidity flows in the overlying sediments.

Finally, it should be mentioned that hyperpycnal flows produced by the neighbouring water courses during intense rainfall events could be responsible for the near-bed cross-shore currents promoting the slides. The most likely source for such phenomena is the neighbouring watercourse of Ribeira de Pedro Miguel, which represents the second largest watershed in Faial, totalling 1,500 ha. As shown in Figure 10, the broad depressions are approximately in line with the on-shelf steepest paths that would be followed by gravity-driven flows starting at the mouths of current water courses.

Figure 10: On-shelf steepest paths initiated at the mouths of current water courses. Note how most of them direct to the CSZ field.

5.2. Sorted bedforms

The sloping CSZ found in the nearshore of the Pedro Miguel headland are comparable to the “sorted bedforms” reported by Swift and Freedland (1978), Thieler et al. (1998, 2001), Schwab et al. (2000), Murray & Thieler (2004) or Ferrini & Flood (2005). Similarly, the coarse beds are found on narrow steep faces (interpreted as the updrift flanks) of low amplitude transverse bedforms featuring long gently sloping stoss flanks composed of finer sediment.

The concentration of this pattern in the shoaling convex area less than 30 m depth off a rocky promontory (Figure 4) indicates the mechanism is likely affected by local bathymetry (Ferrini & Flood, 2005). The lateral restriction created by the coastal protrusion and the elevated convex topography off Pedro Miguel headland is thought to steer and intensify long-shore currents and increase bottom stresses (Cacchione & Drake, 1990). The trend and sense of asymmetry of crests (with NW-facing coarse faces) suggest forcing currents moving towards SE with ebb tides steered around the headland representing their most likely and frequent driver. Alternatively, the currents could be associated with strong swells piling up large volumes of water against the headland and venting the set-up water through nearbed downwelling-induced flows (Cacchione, 2005). Swells generating currents fitting the transverse bedforms trend would probably be those impinge on the area through the northern entrance (directly or by refraction around the Ribeirinha headland). Finally, the higher density of smaller CSZ observed farther from the headland could be a material occurrence of the steady-state narrower coarse domains expected according to the model developed by Murray & Thieler (2004) for current reversal conditions. Being farther from the buffering effect of the shoreline and shallow seabed, the area in question may be subject to a stronger influence of the periodic current reversals generated by the tides which would generate sorted bedforms with smaller wavelengths.