Provenance of fluvial terrace sediments within the Waipaoa sedimentary system and their importance to New Zealand source-to-sink studies

Abstract

Late Pleistocene fluvial terrace deposits within the Waipaoa River system of North Island, New Zealand provide some insight into the fluvial history of the region that can be potentially translated to the marine sedimentary record. Pebble counts and sand detrital modes from terrace remnants suggest significant compositional differences between these terraces and the modern stream deposits. The terrace deposits are enriched in sandstone lithics (sand and gravel), as well as monomineralic quartz and feldspar (sand). In contrast, the modern Waipaoa River is dominated by mudstone fragments, mainly products of gullying in the Waipaoa River headwaters region. Both the terrace and modern river sand show a compositional dependency on grain size, with increasing lithic components in the coarse sand fractions. Carbonate lithic content and proportions of lithic types in the modern river and terrace sediments can be related to the distribution of calcareous geological units across the region. For example, the calcareous lithics (sand and gravel) in the Mangatu River, a major tributary of the Waipaoa, are derived from deep-water facies (allochthon) that dominate the catchment. These carbonate facies are petrographically distinct from the marl (calcareous mudstone) that dominates elsewhere in the Waipaoa River system, such as the Pliocene units in the Waimata River catchment. The combined petrologic data sets from the modern rivers and terraces in the Waipaoa River system provide a model to interpret the provenance of sandy deposits in the offshore marine realm.