H.—34

82

Maruia Series. —The oldest Tertiary rocks are conglomerates and breccia-conglomerates, consisting of angular and subangular fragments of greywacke, argillite, schist, and quartz strongly cemented in a gritty matrix. The conglomerate is roughly stratified, and bands of carbonaceous shale and sandstone are sparingly interbedded. These rocks, which are about 150 ft. thick, outcrop on and occupy a small area a little to the west of the divide between the Matiri and Wangapeka rivers. They closely resemble the Hawk's Crag breccia of the adjacent Buller-Mokihinui Subdivision, and probably accumulated under similar conditions at the base of the Tertiary sequence. In Trent Creek, a tributary of the Matiri from Trent Peak, the basal Tertiaries, arkositic grits and sandstones with bands of granitic conglomerate, rest on granite not appreciably weathered and with a surface parallel to the bedding-planes. These strata, which, are about 35 ft. thick, contain a 2 in. seam of sub-bituminous coal, and grade upward into massive blue to dark-grey mottled mudstone, in places almost black with carbonaceous material. About 1,750 ft. of these beds are exposed along Trent Creek, their dip averaging 45° slightly north of west. A boulder of fossiliferous mudstone with a matrix comparable only with a carbonaceous mudstone of the basal Tertiaries of Trent Creek was found in that creek. It contained numerous Turritella of a type that ranges from the Eocene to the Lower Miocene. The same species was obtained from a similar carbonaceous mudstone outcropping on the track from Matiri Lake westward to the Matiri Tops, where gently dipping mudstone rests on granite. The thin impure coal-seams contained in grits and sandstones, outcropping in the bed of the Maruia near its junction with the Buller, are probably contemporaneous with the lowest beds of the Trent Creek section. Massive and banded sandstones and mudstones, in places concretionary, overlie. The sandstone at the bridge-site on the Maruia, about 40 chains from the Buller, contains Cucullaea and a massive fossils not characteristic of any definite horizon ; the Turritella found in Trent Creek is not present. A basal granitic conglomerate containing a coal-seam outcrops about 10 chains downstream from Glen Cairn Creek, and the sequence northward from this point along the Maruia resembles that farther down-stream, except that there is no fossiliferous band. The total thickness of the Maruia Series in this locality is about 2,000 ft. The coal in Frying-pan Creek, and the conglomerates and grits outcropping 20 chains below the junction of this stream with the west branch of the Owen River, are referred to the Maruia Series. The rocks are here much faulted, and the thickness of the beds and even the sequence are not clear. Matiri Series. —The Matiri Series consists chiefly of sandstones and mudstones. There are minoi conglomerate bands, and the mudstones in places are calcareous and tend to grade into limestone. In Trent Creek the basal beds of the Matiri Series are gritty sandstones, and fine-grained conglomerates containing pebbles of the Maruia beds up to 3 in. across, as well as smaller pebbles of granite and quartz. They accord in strike and dip with the Maruia beds, but the conglomerate is believed to mark a minor break. Overlying the grits and conglomerates, which are 6 ft. thick, are light blue-grey calcareous mudstones 4,000 ft. thick. These, though banded in places, are for the most part massive and at horizons respectively about 2,000 ft., and 3,500 ft. from their base grade into impure limestones. Above the lower limestone is massive mudstone 700 ft. thick, followed by als ft. band of sandy mudstone showing the effects of subaqueous gliding. Banded sandstones overlie the upper limestone. The total thickness of the Matiri beds in this section is about 5,500 ft. The Matiri Tops are formed of gently undulating Maruia and Matiri beds, some 2,500 ft. thick. The highest layers, which form the top of the Haystack, are bryozoan limestones, the equivalent of the lower limestone of the Trent Creek section. Near Fern Flat the calcareous sandstones, grits, and limestones of the Sphinx Rock plateau (north-east corner of Maruia Survey District) overlie the Maruia mudstone. The 7,500 ft. of strata exposed east of Doughboy Stream are probably the equivalent of the Matiri beds of the Trent Creek section. The calcareous rocks outcropping on the ridge east of the Doughboy valley are the same set of strata as the argillaceous limestone exposed near where the Doughboy track joins the main road. Thick massive mudstone, showing slight banding in places, overlies this limestone and grades upward into banded sandstone containing calcareous layers which outcrop 40 chains north-west of the bridge over the Matakitaki near Murchison. The same calcareous layers are exposed at several points west of the Matakitaki, and again nine miles to the south 5 chains south-west of the suspension bridge over the Matakitaki near Ten-mile Creek. The banded sandstones outcropping along the Matiri valley east of the river are on the strike of the rocks of the Doughboy section, and are probably their continuation north of the Buller. The limestone on the hillside about 30 chains east of the junction of Valley Creek with the Matiri is probably of the same horizon as the lower limestone of the Doughboy section. It is somewhat broken by faulting and underlies mudstone about 3,000 ft. thick. West of the Matiri strongly calcareous mudstone crops out near All Nations Point (immediately west of the Buller-Matiri junction), and again about six miles to the north, at a point in the bed of the Matiri 60 chains up-stream from Sandstone Creek. In the section from Maggie Creek west down the Owen River the calcareous Matiri mudstones grade into impure limestone bands which form the ridge between Sandstone Creek and the middle Owen, and extend south of that river toward Owen Junction. About 7,000 ft. of Matiri strata are exposed in this locality. The Matiri beds also outcrop in the headwaters of Murchison Creek (Nuggety Creek), and in the Grassy and Upper Mangles valleys, where they are about 6,500 ft. thick. Though calcareous throughout in this locality, no distinct limestone bands were observed. Mangles Series. —A group of banded sandstones and mudstones outcropping up-stream from the Blackwater almost to the upper end of the Mangles Gorge forms the Mangles Series. In the valley of the Upper Mangles they are about 6,000 ft. thick. In this locality their contact with the Matiri mudstones, underlying in parallel position, was not observed, but in Taylor and Johnson creeks, joining the Matakitaki from the west, a thin conglomerate containing sandstone pebbles separates