Geomorphological evidence of drowned marine shelves : a review of the offshore data for La Gomera Island (Canary Islands)

Abstract

Bathymetric investigations of the Canary archipelago offshore area have shown that several of the islands present an insular shelf sculptured in the submarine volcano flanks. The non-active and highly eroded island of La Gomera (>20 – 4 Ma) does not present a unique shelf but various wide subhorizontal surfaces (slope < 0.5º) that define a staircase morphology observed in the submarine DEM between 30 and 200 m below the present sea-level (bsl). All of them constitute a regional smooth surface that previous authors have interpreted as only one surface related with turbidity current deposition and erosion as well as hemipelagic processes or the result of marine abrasion. That closer to the island forms an almost continuous ring along the current coast at 30 m bsl with an average width of 350 m. It is better developed on the northwestern area where it presents more than 1.5 km width. Seawards, a shelf break with slopes ranging from 1-2º in the north and east, and 2-5º in the south and west reaches 100 m bsl. At that depth, a wider shelf is observed from the west to the northeast of the island. This second morphology width varies from 3 to 4 km and although in its northwestern area is not completely continuous it is 37 km long. Its shelf break leads to the volcanic apron except in the southwest, northwest and eastern areas where another two shelves (ca. 1 km wide each) can be described at 170 and 200 m bsl. The upper one appears isolated in the northwest and northeast areas whereas the deepest one is located in the northeast and southwest. The depth and morphological features of these submarine terraces suggest the present insular shelf is the result of both the active marine erosion during the Quaternary glacio-eustatic oscillations and the continuum subsidence of the oceanic lithosphere due to the volcanic load. Seismic profiles have proved that this flexure is a function not only of the island’s loading history but also of that of its neighbours. Even though some uplift has been previously suggested as a result of large landslides removing load from the islands (ca. 9 Ma, Tazo and San Marcos) several recent geomorphological features may suggest this subsidence process is still acting. For example, the fact that no morpho-sedimentary record of sea-level highstand have been described in the littoral of the island or the occurrence of hanging valleys that denote an strong retreat of the cliffs that only can be explain for the action by a continue drowning of the littoral that enhance the erosive processes of waves. The description of several insular drowned shelves around La Gomera represents the first evidence of a remarkable subsidence process during the Quaternary in this island that has already been described in other archipelagos such as Hawaii.