Subaerial geomorphology has become one of the most rapidly expanding and exciting fields of the Earth Sciences in recent years, driven in part by the need to understand and quantify interactions between climate, erosion and tectonics. Progress has been enabled by new techniques for measuring erosion, transport and deposition rates and availability of high quality topographic data. Computer-based models of landscape evolution have become increasingly realistic at representing the Earth's surface as they encapsulate improvements in understanding of stream-bed and hillslope sediment erosion and transport laws.
Motivated by a sense that Marine Geology will also increasingly adopt a more generalist and quantitative approach to understanding topographic evolution and the development of stratigraphy, we were invited by Niels Hovius and Colin Stark to run a special geomorphology session at the EGU meeting in Vienna in 2005, as a way to encourage interaction among subaerial geomorphologists and marine geologists. The call attracted seventeen abstracts covering many aspects of submarine erosion and morphology, and including data collected from a wide variety of areas: off northeast and west Iberia, west Ireland, Italy, South America, Mexico, west Africa, New Zealand, Brunei and the USA.
Most of the presentations concerned canyon morphology and its creation by erosion by sedimentary flows and slope instability. Straub and Mohrig 14 described the evolution of channels revealed in industry 3D seismic data where they cross a break in slope caused by a major slope failure, demonstrating the importance of pre-existing topography in guiding the directions of—and deposition along—the channels. Draganits et al. 5 presented field evidence for erosion by tools in dense sedimentary flows that have left giant 4-m-wide grooves in the base of greywacke deposits. Krastel et al. 10 talked about a west African canyon system with terraces indicating varied turbidity current throughput over time, possibly originating from variations in pelagic deposition in the upper slope related to varied ocean upwelling. Vizcaino et al. 17 described canyons off SW Iberia interpreted as carved by sedimentary flows resulting from seismically triggered slope failures, also described elsewhere in this area 7 . Estrada et al. 6 presented channels developed below debris flow deposits suggested to represent debris flows transforming to turbidity currents. d 18 O stratigraphy in cores by Georgiopoulou et al. 8 constrain depths of erosion (in the given case minimal) due to turbidity currents. Urgeles et al. 16 contrasted canyon relief in the NE Iberian margin interpreted as due to differences in base level for sedimentary flows traversing the canyons and sediment supply to canyon heads. Alvarez 1 presented evidence for tectonic control on development of a canyon off SW Mexico. Several presentations showed high-resolution data on slope failures 2-4,15,17 . Geotechnical measurements on cores and slope stability modelling by Joanne et al. 9 illustrated the point that canyon wall sediments are commonly stable, suggesting that an external agent such as an earthquake or a now absent transient effect such as overpressure or creep may be required to explain the abundant slope failures often observed in canyon walls. Mitchell 13 showed that USA Atlantic slope-confined canyon systems can have upward-concavity and converging channel topography at confluences that are remarkably similar to characteristics of subaerial channels driven by runoff, and, by adapting stream-power laws of fluvial geomorphology, showed how comparable behaviour might arise from turbidity current erosion.
In subaerial geomorphology, diffusive processes are known to act in landscapes mantled by regolith because of down-slope creep of soil. Two presentations addressed a comparable submarine diffusion. McAdoo and Simpson 11 looked at the rounding of geologically recent landslide scarps to show that geometrically it can be approximated by a linear diffusion equation. As many of the submarine slides have been dated, this work promises to quantify the variability in the diffusion constant which represents the rate of topographic smoothing due to diffusion. The other presentation 12 showed other examples of diffusive-like terrain and discussed various sedimentary-oceanographic mechanisms causing topographic diffusion and its variability. It is unclear the extent to which sediment creep is responsible for diffusion in submarine environments, rather it may more likely arise from bedload movements of sand under oscillating currents, from biological activity, and from variations in current stress affecting deposition and erosion of fine-grained sediment. Although the mechanisms are less well understood and quantified than in subaerial geomorphology, there nevertheless does appear to be diffusion of sorts acting in some submarine landscapes. Only one presentation as far as we remember addressed the issue of erosion by oceanographic currents 2 , but understanding how the rate of such erosion arises from current shear will be needed as part of broader efforts to model slope evolution.
Overall, the presentations illustrated the large quantities of morphological and associated core and oceanographic data now becoming available and the basic knowledge arising from them. Along with improving techniques for measuring erosion, stratigraphic control, slope stability and theoretical modelling, it seems to us that the session did a good job of illustrating the great potential of quantitative submarine geomorphology to advance in the future.
1 Alvarez, R., Bahia de Banderas, Mexico: evidence of on going submarine erosion induced by faulting.
2 Antobreh, A.A., et al. , Sedimentation processes on the slope and rise offshore Uruguay inferred from reconnaissance high resolution seismic reflection survey.
3 Budillon, F., et al. , Morphology and surface geology of the Augusta Bay (Eastern Ionian Sea): results of geophysical surveys.
4 Casas, D., et al. , Submarine erosion on the Prestige sinking area (SW Galicia Bank).
5 Draganits, E., et al. , Giant striations at the base of submarine landslides (Late Proterezoic, NW Himalaya): implications for their formation.
6 Estrada, F., et al. , Large-scale mass-flows in the Magdalena turbidite system.
7 Gafeira, J., et al. , Morphostructural mapping of the Marques de Pombal fault area (SW Portuguese margin).
8 Georgiopoulou, A., et al. , Debris flow and turbidite deposits off northwest Africa.
9 Joanne, C., et al. , Measurements of mechanical parameters on sediment cores collected in the Matakaoa avalanches system, South Kermadec subduction, New Zealand.
10 Krastel, S., et al. , Sediment transport within canyons and open-slope systems off Mauritania.
11 McAdoo, B.G., & G. Simpson, Morphometric dating of submarine landslide scarps.
12 Mitchell, NC, & JM Huthnance, Diffusive tendencies in continental slope bathymetry.
13 Mitchell, NC, Form of submarine erosion from confluences in Atlantic USA continental slope canyons.
14 Straub, K. & D. Mohrig, Control of mass-failure events on evolution of a submarine channel network, offshore Brunei Darussalam.
15 Unnithan, V., et al. , Rockall Bank mass flow: evidence for mass wasting episodes, west of Ireland.
16 Urgeles, R., et al. , Submarine erosion: insights from the NE Iberian margin.
17 Vizcaino, A., et al. , Active tectonic and sedimentary processes along the Sao Vicente Canyon (SW Iberian Margin): high-resolution imaging.
Full abstracts and author lists are published as Geophysical Research Abstracts, Volume 7, 2005, ISSN: 1029-7006.
PDF version of this report