Processes and deposits of submarine sediment density flows within the Moroccan turbidite system, offshore NW Africa

Abstract

Submarine sediment density flows are a major process for transporting sediment from the continental shelf to the deep-ocean. Understanding submarine flow dynamics relies upon analysis of their deposits (beds) because monitoring them directly is difficult. However, it is rare to be able to correlate individual beds for long distances. This limits our understanding to ‘idealized’ models based on field data with limited lateral extent. Validation of these models requires individual beds to be mapped out. Using > 100 shallow sediment cores this thesis correlates individual beds across their depositional extent (over 2000 km), within the Late Quaternary Moroccan Turbidite System, offshore NW Africa. The vertical and spatial distributions of facies and grain size are examined in each bed to understand the dynamics of the parent flows. The height to which deposits drape up topography is used to infer flow thicknesses.

Proximally, synchronous flows passed into the system from multiple disparate entry points. Earthquakes could have triggered these flows. However, it is not possible to determine if these beds were related to earthquakes, highlighting the difficulties faced extending turbidite palaeoseismology beyond the historical earthquake record.

Across the central parts of the system flows are interpreted to have been relatively thin and slow moving, yet able to run out for hundreds of kilometers on slopes of < 0.02º. Current, models cannot explain how this is possible.

Distally, channels develop and connect two basins. Examination of these channels reveals they are purely constructional features. Flows were able to bypass > 100 km3 of sediment through the channel axes without eroding. Channel relief was built and maintained by deposition along the channel margins and no erosion.

The distribution of grain-size breaks is examined within individual beds across the entire system. Grain-size breaks between sand and mud occur almost everywhere. This is attributed to fluid mud layers bypassing intermediate grain sizes down slope. Such a process should (almost) always occur; hence this type of grain-size break should be recognized as a typical feature rather than an exception.

The ability to map out individual beds over such distances provides a rare and valuable opportunity to validate models; developed from laterally restricted outcrops, laboratory experiments and theory. Results from this thesis demonstrate current models are limited and that we still have much to learn about the dynamics of submarine flows and how they transport sediment across vast swathes of the seafloor.

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