Determining the controls on flow behaviour, bedform development and stratigraphic architecture from detailed surveys and monitoring of active submarine channels

Abstract

Seafloor-hugging flows, known as turbidity currents, transport sediment from shallow to deep water via submarine channels. These flows carry globally important volumes of sediment, and transport organic carbon, oxygenated waters, nutrients and contaminants that accumulate within submarine channels and at their downslope terminal lobes or submarine fans. The often-powerful nature of turbidity currents poses a significant hazard to critical seafloor infrastructure. Previous studies have largely relied upon the study of ancient deposits or scaled-down measurements of laboratory-scale flows to understand turbidity currents. Several conceptual models exist, but it remains unclear as to whether turbidity currents show a distinct behaviour at different scales, or if a continuum of behaviour exists from small to large events. Recent technological advances allow us to investigate these issues. The advent of Autonomous Underwater Vehicles enables mapping of the seafloor at unprecedented detail, repeat surveys record previously-unseen seascape changes, while Acoustic Doppler Current Profilers record the range of internal structures observed in field-scale turbidity currents for the first time. In this thesis, I use high-resolution data acquired in several modern offshore systems to analyse turbidity current behaviour across various spatial and temporal scales. First, a global analysis of direct velocity measurements of turbidity currents reveals two end-member modes of turbidity current behaviour that range from: i) a sudden peak in velocity that decays exponentially, lasting minutes to hours; to ii) sustained flow that lasts for days. I show that a continuum exists between these flow modes; likely controlled by the proportion of sand or mud within the flow. Second, an extensive (65 x 50 km) and detailed (5 m bin size) seafloor survey offshore East Africa, reveals a variety of bedforms within two deep-sea canyons. Morphometric analysis reveals a continuum from small-scale (10s m wavelength) crescentic bedforms to large-scale (kms wavelength) sediment waves. This continuum is in contrast with a previous global study, but that study did include such high-resolution deep-water data. Previous studies may have missed an intermediate scale of bedform due to the decreasing resolution with increasing water depth-related. Small to medium-scale bedforms may be more common in the deep sea than currently thought. Third, I analyse repeat mapping of an active submarine delta to reveal how turbidity currents build stratigraphy. As a result of the reworking caused by repeated flows, the completeness of the stratigraphy record over three months is found to be 10% on average. The stratigraphic record is dominated by large events. Large slope failures are more likely to be preserved than smaller bedforms, while erosion is dominated by rare, but powerful turbidity currents that can obscure the record of smaller flows.
I conclude that a continuum in turbidity current behaviour exists across various scales of flow, from small fjord channel systems to the largest submarine channels on the planet. The mode of flow is dominantly controlled by the grain size of the sediment available in the system. The continuum in bedform scales reflects both the downstream evolution of turbidity currents, as they expand due to mixing with ambient seawater and entrainment of seafloor sediment, and modifications caused by seafloor morphology. What becomes recorded in stratigraphy does not show a gradual continuum, however, and instead appears to be strongly biased by larger but infrequent events. These findings from modern systems provide new insights to inform the understanding of ancient depositional records and have implications for assessing seafloor hazards and understanding deep-sea sediment transport in general.

More information

Identifiers

Catalogue record

Export record