Mesozoic stratigraphy of the Marokopa area, southwest Auckland, New Zealand

Abstract

The Late Triassic and Jurassic rocks of the Marokopa area, southwest Auckland, form part of the western limb of the Kawhia Syncline. Within the area mapped, the Late Triassic - Early Jurassic Newcastle Group consists of 3100 m of generally fine to very fine sandstone and siltstone with common tuff beds, and occasional coarser sandstones and shellbeds. Faunas are marine, generally sparse to moderate, and dominated by bivalves and brachiopods. The Arawi Shellbed, Ngutunui Formation, Tewharau Formation, Ururoa Shellbed, and Ururoa Formation are recognised. The Middle Jurassic Rengarenga Group marks a change to shallow water, marginal marine, and nonmarine conditions. Lithologies are generally coarse to medium sandstone with abundant fine plant fragments, with occasional pebble-granule conglomerates, tuffs, and shellbeds. Thin coal and plant beds mark the southern limit of nonmarine conditions in the Rengarenga Group. Fully marine conditions return with the Late Jurassic Kirikiri Group. Oraka Sandstone and Ohineruru Formation consist mainly of slightly calcareous, concretionary fine sandstone and sandy siltstone, with a rich molluscan fauna dominated by bivalves. These formations are separated by the 1 m thick Captain King's Shellbed, with a glauconitic sandstone matrix and a distinctive molluscan and brachiopod fauna. The uppermost part of the sequence in the mapped area is the interfingering Kiwi Sandstone and Waikutakuta Formation. Strata generally dip regularly to the east-northeast at between 20 and 45°. Minor folding may be present in the Kairimu valley. The north-south trending Whareorino Fault is part of the Taharoa Fault Zone. A NE-ENE trend is shown by two faults in the Marokopa valley and two in the eastern part of the area. A fault in the Paraohanga valley shows a northwest trend. © 1998 Taylor & Francis Group, LLC.