Te Pouhawaiki Volcano and pre-volcanic topography in central Auckland: volcanological and hydrogeological implications

Te Pouhawaiki Volcano in the Auckland Volcanic Field was identified on the basis of a small scoria cone, but whether this cone marked the location of a significant eruption centre has been unknown. Volcanic stratigraphy in th e central Auckland isthmus is complex, with older deposits (possibly entire volcanic centres) obscured by younger deposits. The distribution of lava f lows in the central Auckland isthmus was strongly influenced by the pre-vol canic topography, and is a major control on present-day groundwater flow re gimes. Detailed gravity data from the central Auckland isthmus are used here to mo del the thicknesses of volcanic deposits and hence determine the pre-volcan ic topography. The site of the former Te Pouhawaiki scoria cone is shown to correlate with a distinct positive gravity anomaly (c. 6 muN.kg(-1)) inter preted in terms of a lava-filled depression in the Waitemata surface, surro unded by a tuff ring. This inferred explosive eruption centre is similar in both size and eruption style to a number of others in the Auckland Volcani c Field and suggests that the Te Pouhawaiki scoria cone may have been the s urface manifestation of a substantial eruption centre which also produced p hreatomagmatic deposits and lavas. The gravity model also defines the location and geometry of the paleotopogr aphic divide between the ancestral Waitemata and Manukau River systems, sho wing it to be a complex ridge system. These buried ridges peak at c. 10-20 m depth (60-70 in a.s.l.) with a saddle in an eastern limb of the ridge whi ch may have allowed lava from One Tree Hill Volcano to flow north of this d ivide. The configuration of the pre-volcanic Waitemata surface indicates th at the present-day groundwater flow regime is likely to be complex and dive rgent away from the ridge system, controlled in some areas by narrow paleov alleys. Within the ridge complex, an area in which groundwater flow is like ly to be convergent has been defined which correlates with the location of occasional surface flooding.