Stratigraphy and volcanology of a submarine apron from an offshore stratovolcano, Waitakere Group, Muriwai, New Zealand

A mid to late Miocene volcanic sequence is well exposed along the west coast of the Northland Peninsula, New Zealand. The Waitakere Group are uplifted submarine apron deposits from offshore arc stratovolcanoes, and outcrop from Kaipara in the north, to Manukau Harbour in the south. The submarine coherent and clastic succession at Muriwai (the Muriwai volcanic succession) is well preserved and records the medial depositional environment of a volcaniclastic apron from one or more submarine to emergent stratovolcanoes. Several major Miocene volcanic centres have been identified offshore from Northland. The present location of Muriwai is closest to the Manukau (WSW) and Kaipara (NE) centres. The stratigraphy is dominated by resedimented volcaniclastics, thinly bedded fme sandstone, coherent lava and pyroclastic fallout. Six lithofacies groups were identified in the Muriwai volcanic succession, and each represents various transport and deposition process. Basal thinly bedded fine sandstone represents slow background sedimentation, prior to the onset of extensive eruptive activity. Scoriaceous and pumiceous facies, volcanic conglomerate facies and coarse granule sandstone facies record resedimentation of more proximal volcaniclastic deposits. Thin pumice and crystal rich facies record water-settled fallout from explosive eruptions. Coherent facies probably represent satellite vent eruptions on the volcano flanks. In general, the facies document rapid submarine deposition in the closing stages of the eruptive history of the source volcano(es). Transport and deposition of the pumice and scoria, conglomerates and granule sands was dominated by water-supported sediment gravity flows. Rounded to subangular volcanic clasts are a significant component of these deposits, which suggests they were sourced from existing deposits that became remobilised on the upper flanks of a volcanic edifice. Uncommon allochthonous pebbles suggest this depositional environment also incorporated a component of non-volcanic sediments, derived from the Northland Allochthon to the north. Pumice and scoria clasts record significant explosive eruptions, whereas coherent bodies and lava clasts record effusive eruptive activity. Bulk rock geochemistry defines the volcanic succession as a medium-K, calc-alkaline suite, a characteristic product of arc volcanism. Lavas and volcanic clasts have a compositional range from basalt to andesite. Representative sampling suggests each clast type corresponds to a distinct bulk rock composition. Lavas are basalt to basaltic andesite, pumice are basaltic andesite, and scoria and hyaloclastite are andesitic. The chemical variations suggest multiple eruptive events and varying degrees of fractionation. Many pumice clasts are compositionally banded, suggesting magma mingling and complexities in the magma chambers of the source stratovolcanoes. Stratigraphic relationships, facies types and comparison with other arc stratovolcanoes suggests that the bathymetry surrounding the source volcanoes once consisted of a series of radial erosional submarine canyons, separated by depositional highs. Towards the final stages of volcanism, mass wasting and edifice degradation disrupted background sedimentation in the marine basin. Resedimentation processes lead to the extension of a volcaniclastic apron, which in-filled submarine canyons at medial distances from the source volcano. The facies within the Muriwai volcanic succession form a thick canyon filling sequence and represents part of a clastic apron that formed in the basin adjacent to one or several volcanic centres.