Distribution and control of mixed terrigenous-carbonate surficial sediment facies, Wanganui shelf, New Zealand

Shallow-marine, mixed temperate, terrigenous-carbonate sediments are accumulating on the shallow to deep Wanganui shelf(c. 20-110 m water depth). From cluster analysis of textural and compositional parameters, five surficial sediment facies and five subfacies have been defined for the shelf. Facies 1 (Bivalve-beating [gravelly] sand), is a terrigenous-dominated,. shallow shelf facies (<50 m), incorporating Subfacies la (Sand)-a modern felsic sand prism-and Ib (Bivalve-bearing volcaniclastic gravelly sand)-modern to relict andesitic detritus and shell fragments. Facies 2 (Skeletal-dominated sandy gravel) comprises sediments containing >50% CaCO3, and occurs in the shallow to mid shelf region (c. 30-90 m) where: levels of terrigenous input are reduced, and substrates and hydraulic conditions are favourable for carbonate production. Facies 2 comprises Subfacies 2a (Bivalve-dominated sandy gravel), 2b (Bryozoan/bivalve-dominated sandy gravel), and 2c (Bryozoan/bivalve-dominated muddy sandy gravel and gravelly muddy sand). Facies 2 sediments are <c. 800 yr old. Facies 3 (Bivalve-bearing muddy sand) (75-110 m) contains moderate amounts of skeletal material (up to c. 3000 yr old) because of dilution by the terrigenous sediment that characterises Facies 4 (Mud) (85-110 m). The latter constitutes the northern reaches of the Cook Strait Basin mud depocentre and receives sediment from North and South Islands. Facies 5 (Micaceous sand) (95-105 m) is derived chiefly from west coast South Island material swept into Greater Cook Strait by wind-induced and oceanic currents. The overall pattern of surficial sediment facies distribution is determined by the interplay between factors controlling the separate terrigenous and carbonate fractions. These include Wanganui shelf's tectonic setting, the surrounding onland geology, storm-dominated hydraulic regime, bathymetry, and substrate type. The mixed terrigenous-carbonate shelf record affords a modern analogue for interpreting glacio-eustatic cyclothemic facies widely developed in uplifted Pliocene-Pleistocene deposits in onland Wanganui, as well as contributing to facies refinements in the developing model of nontropical shelf carbonate sedimentation.