Structure, stratigraphy, and metamorphism in the Dansey Pass area, Otago, New Zealand.
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The Dansey Pass area spans the uplifted fault-block mountains between the flanking tectonic depressions of the Waitaki Valley and the Maniototo Basin, and lies on the north-eastern margin of the Otago schist arc. Basement rock types of these ranges include greywackes and greywacke-derived semischists and schists, with relatively minor phyllites and metavolcanic rocks, and are all included in the Torlesse Supergroup, regardless of metamorphic grade. Direction of younging observations and non-repetition of recognised formations indicate that, locally at least, the Torlesse rocks are about 15,000 m thick, and are regionally overturned. Kaihikuan (Mid-Triassic) faunas are present in the St Mary Range.
Other notable features of the Torlesse rocks are the dominance of greywackes, the general absence of sedimentary structures, the rarity of fossils, and the commonly thick-bedded, massive nature of the greywackes. Although the fossiliferous Mt St Mary rocks are shallow water deposits, a deep water environment is considered more likely for most of the Torlesse Supergroup.
A scheme of textural zoning, modified from the chlorite subzones of Turner (1935) and Hutton and Turner(1936), so as to be more applicable to field mapping, has been developed. Isotects (textural isograds) have been mapped with sufficient accuracy to show clearly crosscutting relationships with stratigraphic horizons, a relationship which requires abandonment of the Haast Schist Group as a formal lithostratigraphic unit. Isotects are sub-parallel to mineralogical isograds, and allow a finer subdivision of the metamorphic sequence than can be achieved by mineralogical zoning.
Metamorphism was preceeded by an early period of deformation, the textural and mineralogical zones being overprinted on a steeply dipping, regionally overturned sequence. Metamorphic grade thus increases in the direction of decreasing age, from prehnite-pumpellyite facies in the west, through pumpellyite-actinolite facies, to greenschist facies in the east. The rocks have been zoned and isograds mapped on the basis of mineral assemblages in metagreywackes; the prehnite disappearance isograd separates the prehnite-pumpellyite facies from the pumpellyite-actinolite facies, and the pumpellyite-clinozoisite isograd marks the onset of greenschist facies conditions. Mineralogical events recorded, in order of increasing metamorphic grade, are:
1. Disappearance of prehnite, growth of actinolite on detrital amphibole, production of additional pumpellyite, and change in composition of chlorite, at or near the prehnite disappearance isograd.
2. Disappearance of detrital potash feldspar, biotite, and garnet, and increase in amount of stilpnomelane and sericite in the pumpellyite-actinolite facies.
3. Disappearance of pumpellyite, accompanied by the production of clinozoisite, additional actinolite and muscovite, and reduction in the amount of chlorite, at or near the pumpellyite-clinozoisite isograd.
New determinative curves are presented for epidote and white micas. Epidote compositions determined from birefringence indicate early formed epidotes are Fe-rich, a later generation is a clinozoisitic variety. Low values of d001 in micas are shown to result from phengitic rather than paragonitic substitution. X-ray and optical data indicates chlorites in metagreywackes are Fe and Al-rich compared with chlorites in metavolcanic rocks. Stilpnomelane is widespread in all metamorphic facies; some probably forms from a reaction involving the breakdown of detrital potash feldspar.
Most rock forming minerals are also found in veins, and are generally compatible with the metamorphic grade of the country rock. Axinite and tourmaline are associated with a thick metavolcanic formation.
The width of the pumpellyite-actinolite facies in the present area appears anomalously wide in terms of current views on geothermal gradients and P-T conditions. Although specific explanations might be applicable to this particular case, indications of anomalously wide metamorphic zones elsewhere in the Otago schist terrane suggest a more general reason is required. A hypothesis of tectonic intercalation of rocks of different temperature during pre- and syn-metamorphic folding, resulting in variations in the geothermal gradient, is tentatively advanced.
In the south the basement rocks (Torlesse Supergroup) are overlain with profound unconformity by the late orogenic Kyeburn Formation, of probably mid to late Cretaceous age. This formation consists of terrestrial breccias, conglomerates, and sandstones ea. 4000 m thick, derived from uplifted fault-blocks to the south and east. Minor acidic tuffs are locally intercalated with the conglomerates. Montmorillonitic and zeolitic alteration of the glassy matrix of these tuffs has occurred and quartz-albite-sericite-chlorite-pumpellyite assemblages (± carbonates and ?stilpnomelane) have been noted in a number of sandstones. Estimated P-T conditions (~100°0, ~1Kb) consistent with stratigraphic data and coal rank seem anomalously low in view of pumpellyite occurrences in metamorphic assemblages elsewhere.
A long period of erosion followed deposition of the Kyeburn Formation as the late Cretaceous-early Tertiary peneplain formed across the area. The peneplain is overlain by the early to mid-Tertiary rocks of the Stranraer Group - a dominantly terrestrial sequence of quartz gravels, sands, clays, and lignite. The Naseby Formation, an intercalated wedge of fossiliferous marine beds, represents the westernmost, landward edge of an early-mid- Tertiary marine transgression, and locally enables separation of a lower and upper non-marine unit, the Pig and Whistle and Wedderburn Formations respectively.
The Stranraer Group is overlain by piedmont gravels of the Plio-Pleistocene Maori Bottom Formation, which is in turn overlain by late Quaternary terrace gravels.
Four main periods of folding are recognised in the basement rocks. The oldest is pre-metamorphic, and resulted in large-scale overturning. Syn-metamorphic folding (Phase II) was accompanied by the development of foliation and tectonite fabrics. These folds are typically tight, steeply plunging or reclined structures in bedding. Post-metamorphic folding (Phase III) of the foliation is restricted to the outer margin of the Otago schist terrane (the north-east part of the present area). Phase IV resulted in steeply plunging warps of the foliation.
The age of the Torlesse rocks in the present area is known only at Mt St Mary, where they are Mid-Triassic, but elsewhere they may be as young as Jurassic. Accepting an early Cretaceous metamorphic climax at Dansey Pass (Harper and Landis, 1967) a provisional chronology is:- Phase I occurred in early to mid-Jurassic times and was followed by the late Jurassic-early Cretaceous Phase II event. Phase II was accompanied by the early stages of metamorphism, but preceeded the early Cretaceous metamorphic climax. Post-metamorphic folding, accompanied by the formation of fault-block mountains and deposition of the mid-Cretaceous Kyeburn Formation marked the closing stages of the Rangitata orogeny and preceeded the essentially quiescent conditions which prevailed during late Cretaceous-early Tertiary peneplanation and mid-Tertiary sedimentation. The late Tertiary saw the commencement of the Kaikoura orogeny, with renewed movements on older fractures and the initiation of many new faults as the present day faultblock mountains began to rise and a new cycle of orogenic gravels derived from the growing fault scarps commenced to accumulate in the fault-angle depressions.
Faulting, assoc1ated with Phase III folding, occurred on both north-east and north-west trending faults prior to peneplanation. Post-peneplanation faulting has disrupted the peneplain surface to initiate the present fault-block topography. Locally severe folding of the covering strata, mainly around the margins of the tectonic depressions, accompanied this period of faulting. Aspects of faulting such as the steep dips of the fault planes, the rejuvenation of older faults, commonly with reversal of the sense of movement, and the high angles of intersection between pairs of apparently contemporaneous faults suggest they may be the high level crustal expression of essentially reversible types of movement at depth.
The only mineral of economic interest is gold, which occurs in the covering strata of the Maniototo Depression and was probably derived from schists to the south.
Relatively high rates of erosion are associated with the crushed rock of fault zones. Types of mass movement are related to the shape of rock fragments and thus to textural grade.
Other notable features of the Torlesse rocks are the dominance of greywackes, the general absence of sedimentary structures, the rarity of fossils, and the commonly thick-bedded, massive nature of the greywackes. Although the fossiliferous Mt St Mary rocks are shallow water deposits, a deep water environment is considered more likely for most of the Torlesse Supergroup.
A scheme of textural zoning, modified from the chlorite subzones of Turner (1935) and Hutton and Turner(1936), so as to be more applicable to field mapping, has been developed. Isotects (textural isograds) have been mapped with sufficient accuracy to show clearly crosscutting relationships with stratigraphic horizons, a relationship which requires abandonment of the Haast Schist Group as a formal lithostratigraphic unit. Isotects are sub-parallel to mineralogical isograds, and allow a finer subdivision of the metamorphic sequence than can be achieved by mineralogical zoning.
Metamorphism was preceeded by an early period of deformation, the textural and mineralogical zones being overprinted on a steeply dipping, regionally overturned sequence. Metamorphic grade thus increases in the direction of decreasing age, from prehnite-pumpellyite facies in the west, through pumpellyite-actinolite facies, to greenschist facies in the east. The rocks have been zoned and isograds mapped on the basis of mineral assemblages in metagreywackes; the prehnite disappearance isograd separates the prehnite-pumpellyite facies from the pumpellyite-actinolite facies, and the pumpellyite-clinozoisite isograd marks the onset of greenschist facies conditions. Mineralogical events recorded, in order of increasing metamorphic grade, are:
1. Disappearance of prehnite, growth of actinolite on detrital amphibole, production of additional pumpellyite, and change in composition of chlorite, at or near the prehnite disappearance isograd.
2. Disappearance of detrital potash feldspar, biotite, and garnet, and increase in amount of stilpnomelane and sericite in the pumpellyite-actinolite facies.
3. Disappearance of pumpellyite, accompanied by the production of clinozoisite, additional actinolite and muscovite, and reduction in the amount of chlorite, at or near the pumpellyite-clinozoisite isograd.
New determinative curves are presented for epidote and white micas. Epidote compositions determined from birefringence indicate early formed epidotes are Fe-rich, a later generation is a clinozoisitic variety. Low values of d001 in micas are shown to result from phengitic rather than paragonitic substitution. X-ray and optical data indicates chlorites in metagreywackes are Fe and Al-rich compared with chlorites in metavolcanic rocks. Stilpnomelane is widespread in all metamorphic facies; some probably forms from a reaction involving the breakdown of detrital potash feldspar.
Most rock forming minerals are also found in veins, and are generally compatible with the metamorphic grade of the country rock. Axinite and tourmaline are associated with a thick metavolcanic formation.
The width of the pumpellyite-actinolite facies in the present area appears anomalously wide in terms of current views on geothermal gradients and P-T conditions. Although specific explanations might be applicable to this particular case, indications of anomalously wide metamorphic zones elsewhere in the Otago schist terrane suggest a more general reason is required. A hypothesis of tectonic intercalation of rocks of different temperature during pre- and syn-metamorphic folding, resulting in variations in the geothermal gradient, is tentatively advanced.
In the south the basement rocks (Torlesse Supergroup) are overlain with profound unconformity by the late orogenic Kyeburn Formation, of probably mid to late Cretaceous age. This formation consists of terrestrial breccias, conglomerates, and sandstones ea. 4000 m thick, derived from uplifted fault-blocks to the south and east. Minor acidic tuffs are locally intercalated with the conglomerates. Montmorillonitic and zeolitic alteration of the glassy matrix of these tuffs has occurred and quartz-albite-sericite-chlorite-pumpellyite assemblages (± carbonates and ?stilpnomelane) have been noted in a number of sandstones. Estimated P-T conditions (~100°0, ~1Kb) consistent with stratigraphic data and coal rank seem anomalously low in view of pumpellyite occurrences in metamorphic assemblages elsewhere.
A long period of erosion followed deposition of the Kyeburn Formation as the late Cretaceous-early Tertiary peneplain formed across the area. The peneplain is overlain by the early to mid-Tertiary rocks of the Stranraer Group - a dominantly terrestrial sequence of quartz gravels, sands, clays, and lignite. The Naseby Formation, an intercalated wedge of fossiliferous marine beds, represents the westernmost, landward edge of an early-mid- Tertiary marine transgression, and locally enables separation of a lower and upper non-marine unit, the Pig and Whistle and Wedderburn Formations respectively.
The Stranraer Group is overlain by piedmont gravels of the Plio-Pleistocene Maori Bottom Formation, which is in turn overlain by late Quaternary terrace gravels.
Four main periods of folding are recognised in the basement rocks. The oldest is pre-metamorphic, and resulted in large-scale overturning. Syn-metamorphic folding (Phase II) was accompanied by the development of foliation and tectonite fabrics. These folds are typically tight, steeply plunging or reclined structures in bedding. Post-metamorphic folding (Phase III) of the foliation is restricted to the outer margin of the Otago schist terrane (the north-east part of the present area). Phase IV resulted in steeply plunging warps of the foliation.
The age of the Torlesse rocks in the present area is known only at Mt St Mary, where they are Mid-Triassic, but elsewhere they may be as young as Jurassic. Accepting an early Cretaceous metamorphic climax at Dansey Pass (Harper and Landis, 1967) a provisional chronology is:- Phase I occurred in early to mid-Jurassic times and was followed by the late Jurassic-early Cretaceous Phase II event. Phase II was accompanied by the early stages of metamorphism, but preceeded the early Cretaceous metamorphic climax. Post-metamorphic folding, accompanied by the formation of fault-block mountains and deposition of the mid-Cretaceous Kyeburn Formation marked the closing stages of the Rangitata orogeny and preceeded the essentially quiescent conditions which prevailed during late Cretaceous-early Tertiary peneplanation and mid-Tertiary sedimentation. The late Tertiary saw the commencement of the Kaikoura orogeny, with renewed movements on older fractures and the initiation of many new faults as the present day faultblock mountains began to rise and a new cycle of orogenic gravels derived from the growing fault scarps commenced to accumulate in the fault-angle depressions.
Faulting, assoc1ated with Phase III folding, occurred on both north-east and north-west trending faults prior to peneplanation. Post-peneplanation faulting has disrupted the peneplain surface to initiate the present fault-block topography. Locally severe folding of the covering strata, mainly around the margins of the tectonic depressions, accompanied this period of faulting. Aspects of faulting such as the steep dips of the fault planes, the rejuvenation of older faults, commonly with reversal of the sense of movement, and the high angles of intersection between pairs of apparently contemporaneous faults suggest they may be the high level crustal expression of essentially reversible types of movement at depth.
The only mineral of economic interest is gold, which occurs in the covering strata of the Maniototo Depression and was probably derived from schists to the south.
Relatively high rates of erosion are associated with the crushed rock of fault zones. Types of mass movement are related to the shape of rock fragments and thus to textural grade.
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xiv, 216 pages, 1 legend, 1 block model, structural trends map, geological map
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1970Bishop
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POLYGON ((170.116768521122879 -45.06104702052852,170.130436670225038 -44.727987945469479,170.462932614101419 -44.737007336915944,170.44787973125969 -45.067589956424591,170.116768521122879 -45.06104702052852))
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Bishop, D.G., “Structure, stratigraphy, and metamorphism in the Dansey Pass area, Otago, New Zealand.,” Otago Geology Theses, accessed April 23, 2025, https://theses.otagogeology.org.nz/items/show/42.