Less volcanic debris was supplied to the geosyncline in later Permian and Mesozoic times. The younger Permian strata (Maitai Group) include limestone formations, but the Mesozoic rocks are almost wholly greywackes, argillites, and conglomerates, with occasional beds of red and green volcanic rocks that were erupted into the geosyncline. (The Triassic rocks are known as the Gore and Balfour Series, and the Jurassic as the Herangi, Kawhia, and Oteke Series; the geosynclinal Cretaceous rocks constitute the Taitai, Clarence, and Raukumara Series.) The land from which these thick deposits of sediments were derived is thought to have lain to the west and south of present New Zealand, the reason being that the strata of the New Zealand Geosyncline in the west (marginal facies) are better stratified than those in the east — they are more fossiliferous and of less massive total thickness. This suggests that they accumulated near shore, in the less rapidly sinking margins of the geosyncline. The massive, poorly fossiliferous greywacke deposits (axial facies), typical of the Southern Alps, for example, are thought to be those of the central, rapidly sinking zone of the geosyncline.
The New Zealand geosyncline had its maximum extent in the late Paleozoic and Triassic. It may then have been continuous from as far north as New Caledonia to a long way south of present New Zealand.
Late in the Jurassic there began one of New Zealand's major episodes of crustal compression and mountain building, the Rangitata Orogeny. The sediments of much of the geosyncline were complexly folded, faulted, and raised to form a mountainous land perhaps several times the size of present New Zealand. Geosynclinal sedimentation persisted into the Cretaceous only in the east, where Marlborough and the eastern North Island as far as East Cape are today. The Haast Schists are thought to have been formed by the metamorphism of the thickest sediments of the geosyncline: a gradual transition over a distance of miles from unaltered greywacke through crushed and fissile greywacke to chlorite schist can be observed in many places in Otago and the Alps.
The main structures that were imposed on the thick greywacke and argillite strata of the axial facies are not easy to determine, for the rocks are hard to differentiate lithologically and they contain few fossil marker beds except occasional layers crowded with the Triassic shell Monotis richmondiana. Huge, tightly folded anticlines and synclines have been observed in these rocks in the bare mountains of the Southern Alps: in most other exposures, too, these rocks are seen to be complexly folded. Mapping of the fossiliferous, clearly bedded strata of the marginal facies is easier: major structures recognised in them include the Kawhia Syncline of south-west Auckland, the Nelson Syncline east of Nelson city, and the Southland Syncline, which extends from the Alpine Fault north of Milford Sound across Southland and Otago. Recent study of the Otago Schists suggests that during their formation in the Rangitata Orogeny they were folded into a number of large, nappelike folds which were thrust eastwards.
Vast and mountainous though it must surely have been, the New Zealand land mass raised in the Rangitata Orogeny did not endure long after the Mesozoic. Indeed, rivers and waves had worn it down sufficiently by the late Cretaceous for the sea to begin slowly to spread over it from the east. This gradual submergence continued steadily during the early Cenozoic, becoming almost complete in the early Oligocene.
Late Cretaceous marine sediments (Mata Series), deposited early in this transgression, are preserved in North Auckland (where they make up much of the area of “young sedimentary rocks”) and on the eastern side of New Zealand from East Cape to Otago. In contrast to the thick, geosynclinal sediments of the earlier Cretaceous, those of the Mata Series are predominantly types that accumulate slowly on broad coastal shelves adjacent to lowlands where sluggish rivers bring only fine sediments to the sea. Dark, flinty argillites, white argillites, sulphurous sandstones, bentonitic mudstones, and flint beds are the common late Cretaceous sediments which were deposited adjacent to the land mass that was by then tectonically stable.
Late Cretaceous coal, excellent for metallurgical coke, is mined in the Paparoa mine at Greymouth; poorer Cretaceous coal has also been worked at Puponga coalfields near the base of Farewell Spit and at Kaitangata in Otago. The Cretaceous coal seams were formed from thick masses of vegetation that grew in swamps on the old land surface before the sea advanced over those areas.