Geology of the Thomson mountains, Northern Southland, New Zealand


Turnbull, I. M.


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This thesis discusses the geology of an area of weakly metamorphosed sediments in the Thomson Mountains in the province of Northern Southland. The Thomson Mountains include several extensive areas of glacial outwash plains and moraines within a moderately to extremely steep mountainous terrain providing excellent exposures of the basement rocks. An area of some 700 sq. km has been mapped, bounded by major fault systems - the Moonlight Fault Zone and the Livingstone Fault System - in the south-east and west, and by natural topographic boundaries in the north-west and east.
The basement rocks are metasediments, both schistose and unfoliated, assigned to the Caples Terrane; this terrane is here proposed as part of a major tectonostratigraphic unit within the Alpine Assemblage of the Rangitata Geosyncline, and not previously recognised in the South Island. It is proposed to extend the name Morrinsville Facies, first introduced in the North Island to cover this major unit. The implications of this are discussed and a revised nomenclature for Rangitata Sequence rocks is presented.
The Caples Terrane rocks have been divided into four units; these are the Bold Peak, Kays Creek, Upper Peak and Momus lithofacies. In view of the complex stratigraphic and structural relationships between these units, normal formational mapping has proved impractical. Instead, rocks have been assigned to different lithofacies, and are mapped according to those petrographic and sedimentological features that characterise each unit; no stratigraphic sequence is necessarily implied. Within each lithofacies, subordinate lithologic associations are recognised. The Kays Creek lithofacies outlines a large syncline, named the Red Rock Syncline, in the western part of the area. It repeats the Kays Creek and Upper Peak units, but the underlying Bold Peak and Momus rocks, which are completely different in character, appear in equivalent positions across the fold. A second area of Momus lithofacies overlies Upper Peak rocks.
The Bold Peak lithofacies is composed of massive sandstones, thin-bedded sandstones and siltstones, massive dark mudstones and subordinate massive and granule conglomerates. Red and green phyllites and pillow lava- chert associations are minor lithologies. The Bold Peak sequence is interpreted as being a submarine fan deposit, emplaced mainly by fluxoturbidity currents and grain flows; the massive conglomerates probably filled fan channels. Petrographically the Bold Peak lithofacies is characterised by extremely lithic volcarenites. Detritus is almost entirely igneous, dominantly extrusive; basic to intermediate plutonic and hypabyssal rocks are prominent in the conglomerates. Volcanics are mainly andesitic; significant amounts of dacitic material are present. Some sedimentary clasts, including siliceous and calcareous sandstone and Atomodesma limestone, are also recorded.
Kays Creek lithofacies is thought to represent a stratigraphically restricted facies formed within the Bold Peak depositional environment. It is characterised by an exclusively andesitic-volcanic provenance; reworked tuffaceous material is not uncommon. The unit includes massive and thin-bedded sandstones and siltstones and some mudstones, but no conglomerates or primary volcanics have been found.
Upper Peak lithofacies represents a more distal fan deposit overlying the Kays Creek and Bold Peak sediments. It is composed of flysch-like sediments, with abundant evidence for deposition from turbidity currents. There are some thick mudstone sequences; numerous granule conglomerates are thought to represent the extremities of the massive conglomerate-filled fan channels preserved in the Bold Peak lithofacies. Petrographically the unit is more siliceous than Bold Peak and Kays Creek lithofacies, and although volcanic clasts are dominantly andesitic, some more felsitic material is evident. Finer-grained granitic detritus is a minor but significant component.
A sequence of Momus lithofacies rocks underlies Kays Creek lithofacies in the upper Mararoa area; another Momus sequence overlies Upper Peak lithofacies further south at Mt Campbell. Rocks in these two areas are very similar, consisting of alternating brown and green fine to medium-grained siliceous sandstones, with insignificant basic volcanic material, a varied heavy mineral assemblage, and a predominantly dioritic or quartz-diorite provenance. Conglomerates are absent from the lithofacies in this area, as are thick mudstones and primary volcanics. The deposit is interpreted as having been deposited by both turbidity currents and grain flows, which although unusually sand-rich were probably flowing in a fairly distal fan or basin floor environment.
Structurally the area is dominated by north-trending sub-horizontal folds in bedding with vertical to steeply east-dipping axial planes. The Red Rock Syncline is the largest of these; some other macroscopic structures have been mapped, but mesoscopic folds are not common. In Bold Peak lithofacies, several completely inverted folds have been mapped. One, named the Ripper Anticline, has been traced for 8 km, has a wavelength of 1- 2 km, and is synformal. All these folds are products of one phase of deformation (Phase 1); this event was long-continued and metamorphism probably coincided with its closing stages. Phase 1 structures in non-schistose rocks are transitional into the structures associated with foliation in schistose rocks; apart from having steeper plunges, the metamorphic structures are of similar orientation.
Associated with Phase 1 folding was the development of zones of extremely disrupted rock along axial planes, and in some places on limbs, of major folds; these zones are interpreted as tectonic slides. Such zones of deformed rock are not found in the coarser grained Bold Peak lithofacies; there, the tectonic slides are discrete fracture surfaces unaccompanied by penetrative features. Tectonic slide zones are characterised by a penetrative cleavage in fine-grained lithologies, broken and boudinaged folds, transposed bedding, and in several areas, by macroscopic isoclinal folds which have been disrupted by axial plane faults. There is a marked coincidence between the zones of tectonic sliding (which range from a few metres to several kilometres wide and are up to 20 km long) and zones of lawsonite-bearing rocks. It is suggested that such tectonic slide zones may be much more common in the deformed low grade rocks of the Alpine Assemblage than has previously been realised.
Some observations on the distribution and habit of metamorphic minerals are given, and a study of densities of Caples Terrane rocks has been made. Predictable increases in density with increasing volcanogenic content of sandstones and with increasing textural grade have been noted; density is less affected by metamorphic grade and lawsonite-bearing rocks are not significantly more dense than those without lawsonite. A facies series from prehnite-pumpellyite through lawsonite-albite-chlorite to pumpellyite-actinolite facies is represented. Three main metamorphic zones are recognised, each characterised by one of the key minerals prehnite, lawsonite and actinolite; the pumpellyite-actinolite facies has been divided into two zones, the lower-grade of which contains only pumpellyite as a diagnostic phase. Texturally most of the Caples Terrane rocks are within textural zone 1; there is a gradational transition into zone 2A and 2B schists in the eastern part of the Bold Peak lithofacies.
The first recorded occurrence in New Zealand of axinite as a metamorphic mineral, rather than as a vein phase, is also reported; an outcrop of axinitic volcanogenic siltstone occurs within textural zone 2B and pumpellyite-actinolite facies on Mt Nicholas.
The area is cut by several large north-east trending faults assigned to the Moonlight Tectonic Zone; these faults record a variety of movements, dominated by dextral strike-separation. The amounts of net slip are not known. On two of these faults, the Moonlight Fault Zone and the Holm Creek Fault, tiny inliers of Oligocene marine sediments of the Bobs Cove Beds are preserved, indicating a post-Oligocene history for at least part of the Moonlight Tectonic Zone in the area. (The geology of these inliers is discussed in an appendix). Two Quaternary fault traces are also mapped.
The Quaternary geology has been investigated in detail, and the deposits and landforms associated with the several advances recorded in each valley system are described and discussed. A sequence of several small Holocene moraines in the numerous cirques in the area is also described.
In conclusion, the part of the Caples Terrane in the history of the Ragitata Geosyncline is outlined. It is suggested that the Caples is a volcanogenic trench deposit within an arc - trench system, with the volcanic arc being represented by the Brook Street Terrane, the arc-trench gap by the Hokonui Terrane, and the trench by the Caples (Morrinsville Facies). The quartzofeldspathic Torlesse-type rocks of the Alpine Assemblage were either brought into contact with the Morrinsville Facies by subsequent plate movements, or were deposited on the eastern side of a basin of which the Caples Terrane represents part of the western margin.

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2 v. (xviii,462[11],39 leaves) : illus., front., maps (6 fold. col. in pocket) ; 31 cm.


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Turnbull, I. M., “Geology of the Thomson mountains, Northern Southland, New Zealand,” Otago Geology Theses, accessed May 22, 2024,

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