Significance of the fine drainage pattern for submarine canyon evolution : the Foix Canyon system, northwestern Mediterranean Sea

Abstract

Submarine gullies are relatively small valleys that occur in a variety of submarine slopes. They are very common in continental slopes and in submarine canyon heads and flanks, where they often form tributary networks.

Gullies are smaller than submarine canyons, though there is no standardised size criterion to distinguish between them. Gullies and gully networks have been often viewed as initial steps in the development of larger submarine canyons and more mature drainage networks. The shelf-incising submarine Foix Canyon System (FCS) is

located in the northwestern Mediterranean Sea. Numerous, well-developed and morphologically diverse gullies

carve its two heads and flanks. The objective of this study is to analyse the drainage network of the FCS and decipher the role of gullies in its evolution. Submarine gully and canyon networks were extracted from swath

bathymetry data of 50 m grid size using Geographic Information Systems (GIS). A systematic morphometric analysis was carried out on drainage network of the FCS by using the Horton–Strahler method. Our results show that the FCS is formed by 1660 streams, 1000 km in total length, which can be classified to six stream

orders. To detect relevant morphological changes along valley sections, the drainage density, the stream frequency and the drainage area relief parameters were applied. Furthermore, a branching index (Ib) was developed to

characterise the geometry of the submarine drainage network. In the canyon heads Ib values are ~1.7, which correspond to a dendritic network, whereas Ib in the canyon branches displays values of ~2.2 corresponding to a pinnate one. At a finer scale, we have identified two types of canyon flank gullies, namely “rim gullies” and “toe

gullies”: (1) rim gullies form large, dendritic networks that extend from the canyon thalweg up to the canyon

rim, and (2) toe gullies form smaller pinnate networks restricted to the lower part of the canyon flanks. The formation and development of rim gullies are interpreted as the main process responsible for canyon head growth

and across-flank transport of material from the continental shelf. Toe gullies, on the other hand, are the morphological expression of the rejuvenation of rim gullies and the canyon itself. Axial incision and toe gully formation

are interpreted as the result of bottom-to-top erosive processes acting at different scales, in response to regional and local changes of the factors controlling the system, respectively. This study helps to identify a variety of gullies and the processes they result from and provides a morphometric methodology to characterise submarine drainage networks at fine resolution scales.