Morphology of the Faial Island shelf (Azores): The interplay between volcanic, erosional, depositional, tectonic and mass‐wasting processes

Abstract

[1] The extents of volcanic island shelves result from surf erosion, which enlarges them, and volcanic progradation, which reduces them. However, mass-wasting, tectonics and sediment deposition also contribute to their morphology. In order to assess the relative significance of these various processes, we have mapped in detail Faial Island's shelf in the Azores archipelago based on interpretation of geophysical and geological data. The nearshore substrates of the island, down to 30-50 m depth, are rocky and covered by volcaniclastic boulder deposits formed by surf action on now-submerged lava flows. Below those depths, sandy and gravel volcaniclastic beds dominate, building clinoforms up to the shelf edge. In some sectors of the coast, Copyright 2012 by the American Geophysical Union 1 of 30 prograding lava has narrowed the shelf, but, in contrast to nearby Pico Island, we find fewer submarineemplaced lavas on the shelf. In this island, we interpret the distance between the coastline and the shelf edge as almost entirely a result of a straightforward competition between surf erosion and lava progradation, in which erosion dominates. Therefore shelf width can be used as a proxy for coastline age as well as for wave energy exposure. The stratigraphy of shelf deposits in boomer seismic data is examined in detail to assess the roles of different sediment sources, accommodation space and wave exposure in creating these deposits. We also show evidence of mass-wasting at the shelf edge and discuss the possible origins of slope instability. Finally, we discuss the contributing role of tectonics for the development of the shelf.

Bathy-morphological setting of Terceira Island (Azores) after the FAIVI cruise

“…Boudon, Le Friant, Komorowski, Deplus, & Semet, 2007;Coombs, White, & Scholl, 2007;Mitchell, Masson, Watts, Gee, & Urgeles, 2002;Moore, Normark, & Holcomb, 1994;Oehler, Lénat, & Labazuy, 2007;Romagnoli, Casalbore, Chiocci, & Bosman, 2009a;Romagnoli, Kokelaar, Casalbore, & Chiocci, 2009b), these features are subordinate in the offshore areas. They are mainly represented by small and medium-scale slope failures affecting the outer edge of the insular shelf, similar to that observed on nearby Faial Island (Quartau et al, 2012).…”

“…The latter are very common features developed in the offshore areas of volcanic islands, where they have been interpreted as littoral deposits accumulated below the present-day wave-base level on steep coasts by means of storm-induced downwelling currents that transport sediment offshore (e.g. Chiocci & Romagnoli, 2004;Quartau et al, 2012).…”

Coastal evolution on volcanic oceanic islands: A complex interplay between volcanism, erosion, sedimentation, sea-level change and biogenic production

“…The resulting sediment dynamics, with strong cross-and along-shore currents, means that sediments constantly shift along and across the shelf. In fact, in all these settings, sediments (especially fine ones like small gravel, sand or silt) tend to have a short residence time nearshore since high storm waves effectively remove coastal sediments and transport them offshore (Tsutsui et al, 1987;Quartau et al, 2012;Romine and Fletcher, 2013;Meireles et al, 2013). These sediments are typically deposited further offshore to form clinoform bodies on the deeper parts of the shelf (Chiocci and Romagnoli, 2004;Quartau et al, 2012).…”

“…In fact, in all these settings, sediments (especially fine ones like small gravel, sand or silt) tend to have a short residence time nearshore since high storm waves effectively remove coastal sediments and transport them offshore (Tsutsui et al, 1987;Quartau et al, 2012;Romine and Fletcher, 2013;Meireles et al, 2013). These sediments are typically deposited further offshore to form clinoform bodies on the deeper parts of the shelf (Chiocci and Romagnoli, 2004;Quartau et al, 2012). The accumulation of sediments within the island shelf is, however, transient, since most sediments end up being transported over the shelf break to the surrounding seafloor; sediment accumulations on the steep and narrow insular shelves are very prone to submarine mass wasting, producing turbidity currents that contribute to the formation of deep-sea volcaniclastic aprons (Menard, 1956(Menard, , 1983Schmincke et al, 1995;Schneider et al, 1998;Ávila et al, 2008a;Carey et al, 2011).…”

“…The accumulation of sediments within the island shelf is, however, transient, since most sediments end up being transported over the shelf break to the surrounding seafloor; sediment accumulations on the steep and narrow insular shelves are very prone to submarine mass wasting, producing turbidity currents that contribute to the formation of deep-sea volcaniclastic aprons (Menard, 1956(Menard, , 1983Schmincke et al, 1995;Schneider et al, 1998;Ávila et al, 2008a;Carey et al, 2011). The transport of sediments over the shelf break is generally episodic (seasonal or occasional), occurring mostly during major storms (Tsutsui et al, 1987;Saint-Ange et al, 2011) or as result of earthquakes, and are more frequent during sea level lowstands when island shelves are reduced in size (Ávila et al, 2008a;Quartau et al, 2012). Transport is greatly enhanced when submarine canyons cut through the shelf break Sisavath et al, 2011;Romine and Fletcher, 2013).…”