Sand volcanoes in the Avon–Heathcote Estuary produced by the 2010–2011 Christchurch Earthquakes: implications for geological preservation and expression

Abstract

Sand volcanoes produced by liquefaction of saturated sands were widespread in the Avon–Heathcote Estuary following individual major events of the Christchurch 2010–2011 Earthquake sequence. Sand volcanoes were composed of fine grey sands and varied in diameter over the range 0.2–5 m, fed by vertical to subvertical feeder pipes of 15–25 cm diameter that cross-cut mud and shell horizons. Tides and wind-driven waves reduced, and at times removed, the profile of surface features; however, subsurface features remained unaffected. The geological signature of these volcanoes, formed in wet sandy tidally influenced settings such as estuaries, would be preservation of straight-sided feeder pipes 15–25 cm in diameter, >30 cm in length that cross-cut undeformed strata with or without expression of the erupted cone. Sediments were not able to be excavated beneath the shallow water table, and all observed feeder conduits occurred as roughly cylindrical pipes rather than elongated planar dykes; this does not preclude dykes being present deeper in the subsurface, however. Coseismic subsidence may enhance the preservation potential of surface features by increasing relative water depth, thereby increasing sedimentary accommodation space, and decreasing the influence of wave action in removing cones.