Characteristics and causes of submarine channel abandonment

Deep marine clastic systems are the largest depositional systems on the face of the planet, and host significant records of earth history as well as economic resources. Yet understanding the dynamic evolution of these systems continues to pose significant challenges. A key process in the development of these deep sea systems is the initiation, development and abandonment of submarine channels –which may occur in single or repeated cycles. Understanding such cycles is important as submarine channels are key conduits for sediment transport, and prediction of their sedimentary fill – type and distribution of lithologies, together with what is bypassed – is a key challenge in developing both academic and applied interpretations.

Channels are commonly incisional higher on slope and generally become aggradational (i.e., bounded by levees) lower on the slope, and on basin floors, although in practice, complicated transitions may occur at any one location. To better understand the evolution of channel networks, it becomes necessary to differentiate the effects of internally-driven (autogenic) vs. externally-driven (allogenic forcings) as channels undergo cycles of incision, bypass, backfill and re-erosion before eventually passing some critical threshold leading to channel abandonment. In fact abandoned channels themselves are relatively understudied. Yet their presence may be indicative of the gross dynamics of a system and they are also important from an economic perspective because of their stratigraphic trapping potential. Factors associated with channel abandonment include:

- upstream avulsion and downstream segment abandonment – principally in aggradational settings;

- upstream sediment delivery system reorganisation – e.g., delta lobe switching, sea level-induced effects on long shore drift, stream capture in distributory systems etc.;

- cyclical changes in sediment input in terms of grain size and flux, linked to sea level.

The principal project goal is to apply a combined fieldwork and databasing approach to describe and classify different depositional motifs of abandoned submarine channels, based on the mode of channel abandonment and the mechanisms of mud deposition within the channel, and to distinguish the principal controls on these processes through a database interrogation approach.

Topic description
Possible field areas include outcrops of sand-rich and mud-rich canyon and channel fills of the Castissent Group in the South-Central Pyrenees and of the Tertiary Piedmont basin fill of the Nortwestern Italy. Outcrop data collection will include detailed sedimentological facies analysis, mapping and correlation of key surfaces and construction of three-dimensional models using photogrammetric techniques. The aim will be to produce a detailed characterisation of canyons and channels in terms of their fill variability and their relative position within the larger turbidite system (e.g. relative proximality). This characterisation and interpretation will be integrated into the tectonic and sea level history of the study area to look for associations between controlling factors and channel characteristics.

Original field data will be integrated with a compilation of literature-derived datasets that describe the multi-scale sedimentary architecture of deep-marine successions containing mud-prone channel fills, which are currently underrepresented in the literature – particularly that describing outcrops. All the sedimentological data will be loaded onto the TRG Deep-Marine Architecture Knowledge Store (DMAKS) database, together with information on the type of depositional setting and controlling factors of each case-study depositional system. The database will be applied to undertake a synthesis of sedimentological data in facies models of muddy channel fills, which will summarise the state of the knowledge. Also, a database-assisted compound analysis of multiple case studies will enable the identification of potential controls on channel abandonment, which can be applied to assess the possibility of predicting stratigraphic trapping. It is hoped that a complimentary study using seismic data can also be conducted.