Understanding the Physical sedimentology of the Madden Canyon distributary system, New Zealand

Abstract

Gravity-driven flows are one of the most important processes for moving sediments within deep marine environments (Talling et al., 2012). These flows are essential processes that control sediment dispersal in areas which have steep bathymetric gradients, abundant terrigenous sediment, and are located on active tectonic margins (Pouderoux et al., 2012). The Madden Canyon is a non-terrestrially coupled canyon system which is incising the continental slope of the East Coast of the North Island in New Zealand. This study aims to define the stratigraphy and the geomorphology of the area, and in turn characterise the flow behaviour within the system. Sediments from six piston cores located in various parts of the system will be investigated using both quantitative and qualitative methods. These methods include grain size analysis throughout cores, total organic matter values (TOM), carbonate bombs, visual analysis through computerized tomography scans (CT scans) and high-quality imagery, the magnetic susceptibility of sediments along with gamma density, bathymetric data. Chronology is also investigated using tephra analysis and AMS radiocarbon dating, these results used to define bulk sedimentation rates. Watershed analysis calculations using ArcMap 10.5.1 have been used to define the catchment area and flow paths. Three lithofacies have been identified in the Madden Canyon system including (1) silt beds, (2) sandy silt beds and, (3) sand beds. The silt lithofacies has been subdivided in three further facies which include normally graded massive silts (1.1A), laminated normally graded silts (1.1B), and non-graded massive silts (1.2). Lithofacies have been interpreted as non-cohesive stacked turbidites, formed by low-density subcritical and supercritical flows, and are interpreted as being proximal to the source. The steep slopes within the upper parts of the catchment, sediment waves within the upper channel, along with elongated fault related ridges which are perpendicular to the channel influence flowpaths and result in a semi-confined basin where sediments accumulate at the base of the Madden Channel, in the Porangahau Trough. The main flow path is formed by a fourth order channel starting in the Madden Basin and ending in the Porangahau Trough, a potential continuation of this flow path to Akitio Trough is possible also. Sediments are found to consist primarily of Holocene sediments, with older Pleistocene sediments found in areas which are not directly along the flow path. Sedimentation rates in the study area range from 0.2 - 3.9 mm/year, dependent on their location within the system. The main triggering mechanisms for density flows within the area are assumed to be of seismic nature.