Sandstone xenoliths in the pungarehu formation, western taranaki, new zealand: Implications for petroleum exploration

Abstract

Rare quartz sandstone xenoliths occur in andesitic volcaniclastics of the Pungarehu Formation in western Taranaki. The Pungarehu Formation covers more than 250 km2, and is a massive flow breccia deposit of gravitational collapse origin, deposited about 23 kyr ago from Egmont volcano. The xenoliths are preserved about 15 km from Egmont vent and the largest observed was over 350 m3 in volume. The sandstones are white, friable, medium- to coarse-grained quartz arenites almost certainly derived from the Eocene to Lower Oligocene Kapuni Group, which is now at a depth greater than 3.5 km in this region. Diagenesis of the sandstones has included precipitation of quartz as syntaxial overgrowths and the development of secondary porosity. Traces of dried oil are present. The xenoliths were apparently carried to the surface by rising magma, and heated to an estimated 500 -1050 oC. Dehydrated halloysite, cristobalite and sillimanite and/or mullite formed by thermal metamorphism. Friability of the sandstones has resulted from the fracturing of previously interlocked quartz grains during transport. The xenoliths indicate the presence of potential hydrocarbon reservoir sandstones with secondary porosity in an area more than 20 km from the nearest oil exploration well. In addition they show that large blocks of country rock may be rafted to the surface by rising magma, and that the gravity flow depositing the encompassing breccia must have occurred in association with or soon after magmatic rafting, before erosion destroyed the sandstone blocks on the surface near the vent. © 1985 Journal of the Royal Society of New Zealand.