Bedload composition, transport and modification in rivers of Westland, New Zealand, with implications for the distribution of alluvial pounamu (jade)

Abstract

River bedload surveyed at 50 sites in Westland is dominated by Alpine Schist or Torlesse Greywacke from the Alpine Fault hanging wall, with subordinate Pounamu Ultramafics or footwall-derived Western Province rocks. Tumbling experiments found ultramafics to have the lowest attrition rates, compared with greywacke sandstone and granite (which abrade to produce silt to medium-sand), or incompetent schist (which fragments). Arahura has greater total concentrations (10 3 -10 5 t/km 2 ) and proportions (5-40%) of ultramafic bedload compared with Hokitika and Taramakau catchments (10 1 -10 4 t/km 2 , mostly <10%), matching relative areas of mapped Pounamu Ultramafic bedrock, but enriched relative to absolute areal proportions. Western Province rocks downthrown by the Alpine Fault are under-represented in the bedload. Enriched concentrations of ultramafic bedload decrease rapidly with distance downstream from source rock outcrops, changing near prominent ice-limit moraines. Bedload evolution with transport involves both downstream fining and dilution from tributaries, in a sediment supply regime more strongly influenced by tectonics and the imprint of past glaciation. Treasured New Zealand pounamu (jade) is associated with ultramafic rocks. Chances of discovery vary between catchments, are increased near glacial moraines, and are highest near source-rock outcrops in remote mountain headwaters.

Provenance analysis using Raman spectroscopy of carbonaceous material: A case study in the Southern Alps of New Zealand

“…Once material has entered the river and stream channels as bed load, the frequency of storm events suggests that it is being redistributed on a monthly or bimonthly basis. Cox and Nibourel [] provide a detailed assessment of bed load in nearby rivers of Westland, demonstrating a close relationship between bed load composition and mapped catchment geology in the Southern Alps, albeit with some variation from one catchment to the next.…”

“…where D 0 = initial grain size (mm), D = grain size (mm), L t = travel distance (km), and k = grain size constant (km −1 ). Most studies have focused on quantifying pebble abrasion processes (i.e., constraining k ) using tumbling mills or circular flumes over a century [ Gregory , ; Wentworth , ; Marshall , , ; Krumbein , , ; Kuenen , ; Bradley , ; Bradley et al , ; Brewer , ; Brewer and Lewin , ; Cox and Nibourel , ]. Here we use a similar approach and, in addition, concentrate on the size and distribution of particles produced by pebble abrasion.…”

Erosion of the Southern Alps of New Zealand during the last deglaciation

“…The surface area (~14%) of other schist/semischist (garnet and upper amphibolite and sub-greenschist facies) is significantly smaller than the chlorite and biotite schists. West of the Alpine Fault, the bedrocks comprise metamorphosed sandstones, gneisses and minor granitoids that are covered by glacial gravels (Rattenbury et al, 2010), which are much less eroded than the schists in the Southern Alps and significantly underrepresented in the river bedload (Cox and Nibourel, 2015;Sutherland, 1996).…”

Lithology‐controlled evolution of stream bed sediment and basin‐scale sediment yields in adjacent mountain watersheds, Idaho, USA

“…Cosmogenic nuclide‐based estimates of basin‐scale denudation (von Blanckenburg, ), for example, require assumptions about the mixing of different sediment populations downstream from source to outlet through watersheds (Attal and Lavé, ; Carretier et al ., ). In landscapes with diverse sediment source areas, the downstream evolution of sediment properties provides a tracer of sediment transport processes in a variety of geomorphic settings, including littoral (Perg et al ., ; Garzanti et al ., ), eolian (Jerolmack et al ., ; Garzanti et al ., ), and fluvial (Pizzuto, ; Attal and Lavé, ; Garzanti et al ., ; Cox and Nibourel, ).…”

“…Many of these studies focus on the surface bed material that is more strongly influenced by hydraulic sorting and downstream changes in channel gradient that influence bed armoring. More recently, several authors have shown that variations in sediment supply and abrasion characteristics of different rock types can have a systematic and predictable influence on downstream changes in sediment flux and composition (Pizzuto, 1995;Attal and Lavé, 2006;Sklar et al, 2006;Chatanantavet et al, 2010;O'Connor et al, 2014;Cox and Nibourel, 2015;Menting et al, 2015). Viewed in reverse, the in-stream mixture of lithologies should reveal upstream erosion processes and hillslope-channel coupling within the watershed (Benda et al, 2004;Sklar et al, 2006).…”