New Zealand Journal of Geology and Geophysics abstracts
Mesozoic stratigraphy of the Marokopa area, southwest Auckland,
New Zealand
D. A. B. MacFARLAN
Geology Department
The University of Auckland
Private Bag 92 019
Auckland, New Zealand*
*Present address: Geosearch Independent Geological Consultants, P.O. Box 3360,
Richmond, Nelson, 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 45deg..
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.
Keywords Triassic; Jurassic; Marokopa; Herangi Range; Kawhia
Syncline; Murihiku Supergroup; lithostratigraphy; biostratigraphy; bivalves;
brachiopods
New Zealand Journal of Geology and Geophysics, 1998, Vol. 41: 297-310
0028-8306/98/4103-0297 $7.00/0 (c) The Royal Society of New Zealand
1998
PDF file of entire paper: medium quality (1870K); (scanned from paper original: notes about this process)
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