Miocene and Pliocene Silicic Tuffs in Marine Sediments of the East Coast Basin, New Zealand

Abstract

Most of the marine sediment infill of the East Coast Basin of New Zealand consists of monotonous sequences of mudstone, siltstone, and fine sandstone which are intercalated with silicic tuffs. Between 24 Ma and 3.5 Ma, at least 830 silicic tuffs were deposited in the East Coast Basin, representing an average frequency of about 1 eruption every 25,000 years. However, this figure is likely to be far too low because of gaps in the marine tuff record caused by breaks in the sedimentary sequences. As well, inefficiency of ash transport mechanisms, and post depositional mixing of volcanic ash with non-volcanic sediment would have caused many of the silicic eruptions to go unrecorded as macroscopic marine tuffs. In the Waiauan of Northeast Mahia Peninsula, conditions were favourable for ash deposition and preservation because nearly 500 tuffs occur, representing a frequency of 1 eruption every 2000 years. This frequency is comparable to that estimated for the last 50,000 years of rhyolitic activity in the Taupo Volcanic Zone, New Zealand. The temporal and spatial distribution of the silicic tuffs is highly localized and there are large differences in the thickness and textures of tuffs within and between sub-basins which make up the East Coast Basin. This is best explained if the majority of the volcanic ash entered the sea from river discharges and was emplaced in the marine basins by sediment gravity flows. Sixteen percent of the tuffs contain sedimentary structures clearly indicative of deposition from sediment gravity flows, and the remaining 84% were probably deposited from dilute clouds of suspended ash associated with sediment gravity flows. Because there were few lithostratigraphic units that were both thin and mappable, the presence of silicic tuffs has been used as a criterion for distinguishing formations within Miocene-Pliocene strata of the East Coast Basin. This study has shown that the first appearance of silicic tuffs in the sedimentary sequences is diachronous by several million years, and that the presence of tuffs is not a useful criterion for distinguishing formations. It is recommended that the Mapiri Formation, which was defined on this basis be discontinued as a lithostratigraphic mapping unit. The thickness of a tuff in the study area does not have regional significance and should not be used as a criterion for correlating tuffs within or between sedimentary basins, Chemical analysis of glass shards, using an electron microprobe, is shown to be the most practical method of characterizing silicic tuffs in the study area for correlation, and the presence of certain shard morphology types and mineral phases can also be used to add confidence to a tuff correlation based on other features. Silicic tephras in deep sea drill core from the Lord Howe Rise northwest of New Zealand, and the Chatham Rise southeast of New Zealand are shown to be chemically similar to tuffs in the East Coast Basin and tentative correlations are made. Rhyolitic glass shards form the largest component of most tuffs, but small amounts of dacitic and andesitic shards also are present. The range in composition of shards could result from compositional gradients within the source magmas, or from the mixing of erupted products from two or more magmas. The components of the tuffs show the affects of hydrodynamic sorting during transport and deposition, and the pyrogenic mineral content is generally low (<1 wt.%) because of preferential removal during transport. Glass shard morphologies indicate at least 21% of the tuffs in one stratigraphic section were derived, at least in part, from phreatomagmatic eruptions. The sub-basins of the East Coast Basin are thought to have developed as accretionary basins within a subduction complex. The most likely source of the silicic ash in the marine basins is the Northland-Coromandel-Kiwitahi region, which was the site of subduction related, calc-alkaline volcanism during the Miocene and Pliocene. The Early Miocene and some of the Middle Miocene tuffs were probably derived from volcanic centres in Northland, and most Middle Miocene, Late Miocene and Pliocene tuffs were probably derived from the Coromandel Volcanic Zone. Silicic volcanic source centres to the east or southeast of the North Island, which had been proposed by earlier workers, are considered unlikely because 1) the accretionary prism and oceanic crust to the east of the North Island are not considered likely tectonic settings for voluminous silicic volcanism, 2) no silicic centres have been identified east of the North Island by any means (piston coring, dredging, seismic surveys, etc.), and 3) the texture and distribution of the marine tuffs in the East Coast Basin can be explained without invoking a source center to the east. The transport history of the ash deposits in the East Coast Basin supports the models showing dextral displacement of eastern North Island relative to western North Island between Miocene and Present.