Geology of the Auckland urban area : sheet R11. Scale 1:50 000 / Les Kermode.

"Sheet R11, scale 1:50 000."

Folded coloured map (84 x 128 cm.) in pocket.

Geological map sheet R11 Auckland covers much of New Zealand's largest urban area. Scoria cones, lava fields, and tuff deposits are obvious and distinctive landscape features. These are underlain by several, older, more widespread, sedimentary lithologies. The oldest rocks are the "greywacke" basement of Late Triassic to Late Jurassic age, and are divided into the Waipapa Group, outcropping in the east, and Murihiku Supergroup, which does not outcrop, but is inferred to underlie the western half of the area. The boundary between them probably coincides with the NNW-trending Junction Magnetic Anomaly. Thin Eocene coal measures of Te Kuiti Group (subsurface occurrence only) overlie basement rocks in the southeast. Almost the whole area is underlain by Waitemata Group strata of Early Miocene age that were deposited in a deep marine basin between two sub-parallel volcanic arcs (West Northland Volcanic Arc and Coromandel Volcanic Zone). In the eastern districts, basement greywacke is overlain, with marked unconformity, by breccia, conglomerate, sandstone, and bioclastic limestone (Kawau Subgroup). Elsewhere, flysch (alternating sandstone and mudstone) of Warkworth Subgroup (East Coast Bays Formation) dominates. In the west, volcaniclastic sediments of the contemporaneous Waitakere Group interfinger with East Coast Bays Formation. Over much of the area, the Waitemata Group strata are unconformably overlain by a wide range of relatively thin, alluvial and shallow marine sediments of Late Pliocene to Holocene age (Kaawa Formation, Kaihu Group, Tauranga Group). Many of these deposits contain pumiceous material derived from Taupo Volcanic Zone. In the southwest, the northern part of the Awhitu peninsula consists of consolidated and drifting, ferriferous, dune sands. The rocks of Auckland Volcanic Field are commonly of basaltic and basanitic composition. Although volcanism probably commenced more than 150 000 years ago, the latest eruption (Rangitoto) was only about 600 years ago, and future volcanic activity is a potential geological hazard. The main structural trend in the greywacke basement is NNW, and most of the major post-Miocene faults trend NNW and ENE. The alignments of volcanic vents in Auckland probably reflect the fault patterns within the underlying greywacke. Important mineral resources in the mapped area include basalt (for building and roading aggregate), groundwater (for auxiliary supplies), and clay (for bricks and pottery).