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I. Upper Sub-schist Belt
Distribution. The rocks south-east of a line about three miles north-west of the main divide are mapped as Upper Sub-schist. This belt extends north and south for many miles roughly parallel to the main divide and east for an unknown distance beyond it. It was examined in detail only in the Trent Gorge and the adjoining area (fig. 4).
Content. With the possible exception of the limestone and volcanics, all the beds are typical geosynclinal sediments. They were rapidly eroded from an elevated area, quickly transported without much weathering, and then rapidly deposited. Again, with the exception of the limestone and volcanics, they show no mappable large-scale changes in lithology, the same limited range of lithologies being repeated again and again in a short distance. Irrespective of metamorphism, they differ considerably from the most rapidly deposited Tertiary sediments known in New Zealand. In the bulk they consist of medium to coarse sandstone mostly in three to six foot bands with interbedded thinner bands of mudstone and siltstone. More conspicuous are the conglomerate and grit bands. These are common in the river gravels, but were found in place only at the lower end of the Trent River Gorge, where they are at least 300 feet thick. The component pebbles—indurated sandstone and mudstone and quartz (or quartzite)—range up to half an inch through. The fossiliferous beds have the greatest interest. Near Mt. Monotis they consist of well-bedded dark siltstones which break slightly more readily along than across the bedding planes. In Confirmation Rill the loose fossiliferous blocks are very similar to those near Mt. Monotis, but perhaps slightly more massive. These rocks show no trace of slaty cleavage, though equally as fine grained as those in the next metamorphic belt to the west. When exposed, they slowly break down into conchoidal fragments, breaking along fractures which are largely independent of joint or bedding planes. Bell and Fraser (1906, p. 45) described a rock east of Browning Pass that may be from the southern continuation of this belt as “an exceedingly hard black mudstone, which, devoid of
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lamination and breaking with a splintery fracture, often resembles a compact fine-grained basaltic rock.” Quartz veins are absent or extremely rare in the mudstones and siltstones, but are not uncommon in the sandstones. Volcanics were found in place on the east side of Monotis Gulch and in the stream that drains the large slip on the west side of Trent River. At Monotis Gulch the volcanics comprise 50 feet of hard, light green rock—probably an altered tuff. A boulder of tuff at this place contained a limestone band, a few inches thick. The following thicker section crops out in the stream below the slip on the opposite side of Trent Valley. It may represent the same band on the opposite side of a syncline.
| Feet | |
| Well-bedded sandstone and siltstone (greywacke and argillite) | 1,000 |
| Basic lava, tuff and tuffaceous sandstone | 500 |
| Limestone slightly tuffaceous | 50 |
| Gritty tuffaceous limestone | 10 |
| Tuffaceous sandstone | 200 |
| Dark mudstone with many quartz veins | 100 |
| Tuffaceous sandstone | 300 |
| Sandstone and siltstone | 500 |
McKay (1881) discovered abundant Monotis at Okuku River (north Canterbury) in limestone associated with similar volcanics, but no definite fossils were found in the limestone at Trent River. Three miles north of Confirmation Rill on the main divide one of us (F. W. M.) found similar volcanics, and in the upper Taramakau Valley within three miles of Harpers Pass, McKay (1893, p. 135) found “cherts, diorite sandstone, diabase ash beds, often highly calcareous and frequently associated with red jasperoidal slates.” Neither of these localities was re-examined.
In New Zealand, limestone is closely associated with volcanics in sequences of many different ages. The association is too direct to be accidental, but the controlling mechanism is uncertain.
Fossil Content. The Monotis shells, which are confined to this belt, have already been described. Other possible fossils are flattened tubes of sandstone found in mudstone boulders in the lower part of the Trent Gorge. These tubes may represent worm casts. The tubes are gently curved, up to three inches in length, and elliptical in cross-section, the larger diameter—along the bedding planes—being a quarter of an inch and the lesser an eighth of an inch. With these tubes and apparently closely related to them, are fine radiating marks partly defined by pyrite. Their origin is uncertain. These fossils are distinct from the well-known annellid Torlessia mackayi.
Thickness. As the structure is complex, the thickness of beds in this belt is uncertain. According to the cross-section (fig. 3), which gives the simplest interpretation of the field evidence, the total thickness within the mapped area is 8,000 feet. The cross-section indicates that the stratigraphic interval between the Monotis locality at Mt. Monotis and that at Confirmation Rill is about 5,000 feet, but the proved thickness of Monotis-bearing beds is much less—only 500 feet—the stratigraphic distance between the first and second collections at Mt. Monotis.