Triassic rocks of the Black Forest-Haldon area, Mackenzie Country, South Island, New Zealand

Publisher Website
Google Scholar
Abstract
Tightly folded marine sedimentary rocks, most or all of Triassic age, form the low hills on the north-east shore of Lake Benmore. They are separated from foliated sandstone and phyllite above and to the east by the Black Forest Thrust, which dips gently eastward. The sedimentary sequence in rocks known to be Triassic is, from the base: large lenticular sandstone bodies separated by subordinate finer-grained rocks; smaller sandstone lenses enveloped by siltstone and by thin alternations of sandstone and siltstone which are commonly graded; and homogeneous siltstone with a few graded sandstone beds. The first two associations make up the Haldon Formation; the last is the Spurs Siltstone, The Haldon Formation is interpreted as an accumulation of channel sands and interchannel deposits formed in the upper part of a submarine fan. The Spurs Siltstone was deposited after this portion of the fan became inactive. The Middle Triassic pterioid Daonella and other fossils were found in the uppermost Haldon Formation. The tube fossil Torlessia was found at two localities, one at about the same horizon as Daonella and one at a lower horizon. Sandstone and fine-grained turbidites forming two outlying hills are named the Mount Maggie Formation. Haldon and Spurs Formations (and probably Mt Maggie Formation) are deformed into two interfering sets of folds. The first set consists of basically east-striking and mostly east-facing tight, steeply plunging folds with limbs steeply inclined to the south. The second set consists of a shallowly plunging, north-trending, open anticline to the west and a north-trending, tight synclinal fold belt to the east which is probably also shallowly plunging. Siltstone in the latter contains fracture cleavage and is locally overturned under the Black Forest Thrust.