Gravitational, erosional, sedimentary and volcanic processes on the submarine environment of Selvagens Islands (Madeira Archipelago, Portugal)

Abstract

Abstract Volcanic oceanic islands are the emerged portions of large volcanoes that rise abruptly from the ocean floor. Although the geological record of these islands provide some insights into the evolution of their entire volcanic edifices, for a complete picture, it is essential to study the islands submarine pedestals and surrounding settings. In this study we document the submarine morphology of the Selvagens Islands (Madeira Archipelago, Portugal) and relate it with their subaerial evolution, in order to gain a more complete insight on the evolutionary history of these islands. To this purpose, we employed bathymetric surfaces, backscatter mosaics and subbottom profiler data integrated with geological observations of the subaerial part of the islands. Our study reveals the presence of a complex submarine morphology composed mainly of two deeply eroded central volcanoes – the edifices of Selvagem Grande and Selvagem Pequena – and a less eroded saddle-like ridge related to a fissural volcanic system located between them. Other secondary landforms generated by volcanism, erosion, gravitational collapses, and possible faulting activity, are also present. Whilst the building of the two central volcanoes relates to the older volcanic stages of both islands, the fissural system, with a general NE-SW trend, is probably contemporaneous of the younger volcanic stages of Selvagem Grande. The shelf of Selvagem Grande also features the eroded remains of numerous relatively young volcanic cones, seemingly forming several NW-SE and NE-SW lineaments, the same direction observed onshore for the younger volcanic stage of this edifice. The shelves break of both Selvagens was affected by multiple landslide scars and channel headwalls. The channels constitute a large drainage network that fans out in a radial pattern along the flanks of both central edifices. In the deeper parts of this submarine drainage, large sediment wave fields are observed in some locations with wavelengths of 300–1,900\u202fm and 2–45\u202fm in height; these tend to form cyclic steps that change gradually downslope to antidunes, and their origin is attributed to sediment density flows that suffer hydraulic jumps when their channel confinement ends and the slope gradient decreases from ~7° to ~2.5°. Several scour features are also found at 3,300–4,000\u202fm water depth, being 3–20\u202fkm in width, 10–20\u202fkm in length, rectangular or U-shaped in cross-section and 40–100\u202fm deep. They occur around the base of the submarine edifices, where seafloor gradients change to 3.0°–0.5°. Their origin is also related to hydraulic jumps of unconfined sediment density flows, which are able to erode the seafloor and create these features. These morphologies are a testimony to how volcanic, erosional and depositional processes conjugate to shape the flanks of a very long-lived volcanic island system, one of the oldest in the North Atlantic Ocean.