15Earth-surface processes operate across erosionally dominated landscapes and 16 deliver sediment to depositional systems that can be preserved over a range of timescales. 17

No abstract

The processes within deep-sea sedimentrouting systems are diffi cult to directly monitor. Therefore, we rely on other means to decipher the sequence and relative magnitude of the events related to erosion, sediment bypass, and deposition within channels that crosscut the seascape, and in particular, continental slopes. In this analysis, we examine the nature of slope channel fi ll in outcrop (Cretaceous Tres Pasos Formation, southern Chile) in order to evaluate the geological evidence of the full channel cycle, from inception to terminal infi ll with sediment, and we attempt to provide insight into the enigmatic deep-sea processes that are critical for a comprehensive understanding of Earth surface dynamics. In the stratigraphic record, slope channel fi lls are typically represented by sandstoneor conglomerate-dominated deposits that defi ne channelform sedimentary bodies tens of meters thick and hundreds of meters across. Despite the prevalence of coarse-grained sediment, key information is recorded in the fi ne-grained deposits locally preserved within the channelform bodies, as well as a breadth of scours or internal channelform stratal surfaces. These characteristics preserve the record of protracted sedimentary bypass and erosion. In many instances, the life of a slope channel is dominated by sedimentary bypass, and the stratigraphic record is biased by the products of shorter-lived channel fi lling and abandonment.

Crescent-shaped bedforms with wavelengths from 20 to 80 m, amplitudes to 2.5 m, and concave down-canyon crests occur in the axial channel of Monterey Canyon (offshore California, USA) in water depths from 11 to more than 350 m. The existence of these features may be an important new clue as to how sediment moves through submarine canyons. Three complementary studies were initiated in 2007 to understand the origin and evolution of these bedforms. (1) Vibracoring. Three transects of closely spaced remotely operated vehicle-collected vibracores were obtained across these bedforms. The seafl oor underneath these features is composed of gravity-fl ow deposits. (2) Acoustic array. Three boulder-sized concrete monuments containing acoustic beacons were buried just below the surface of the canyon fl oor in ~290 m water depth and their locations were redetermined on 17 subsequent occasions. Although the beacons became more deeply buried >0.6 m below the seafl oor, they still could be tracked acoustically. Over a 26-month period the position of 1 or more of the beacons moved down-canyon during at least 6 discrete transport events for a total displacement of 994-1676 m. The movement and burial of the monuments suggest that the seabed was mobilized to >1 m depth during gravity-fl ow events. (3) Autonomous underwater vehicle (AUV) repeat mapping. AUV-acquired high-resolution multibeam mapping , and CHIRP (compressed highintensity radar pulse) subbottom profi ling surveys of the seafl oor in the active channel were repeated four times in the fi rst half of 2007. In addition, the movement of large instrument frames deployed in 2001-2003 within the axis of Monterey Canyon in the area now known to be associated with the crescent-shaped bedforms is documented.The fate of the frames has helped elucidate the frequency, transport potential, and processes occurring within the axis of Monterey Canyon associated with these bedforms. The crescent-shaped bedforms appear to be produced during brief gravity-fl ow events that occur multiple times each year, commonly coincident with times of large signifi cant wave heights. Whether the bedforms are generated by erosion associated with cyclic steps in turbidity fl ows or internal deformation associated with slumping during gravity-fl ow events remains unclear.