Geology of the Tuapeka group, Bull creek area, South Otago, New Zealand.
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The southern most coastal exposure of Tuapeka Group rocks is in the Bull Creek area, south of Milton, south Otago.
Four mapping units were separated because of sedimentological or structural differences.
Massive homogeneous sandstones and siltstones are (from four point counted thin sections) dacitic feldspathic psammites. Related rocks include thick bedded sandstones, rhythmicly bedded siltstones and argillites with fossil worms, colour banded argillites and bedded radiolarian cherts.
Black chert nodules from one area were studied geochemically. 2000 ppm tungsten and about 500 ppm native silver occur in these iron and manganese rich radiolarian chert cobbles.
The rocks are metamorphosed to upper prehnite - pumpellyite to lowest pumpellyite - actinolite facies; they belong to textural zones 1 and 2A (after Bishop, l972A).
Despite the low degree of metamorphism, the rocks are often structurally complex. Six phases of deformation have been identified. 1) Extensive soft sediment deformation caused sedimentary melange in some isolated areas. 2) Isoclinal folding and shearing disrupted the area. Because of the double deformation of the sedimentary melange areas, granules to blocks of the various lithologies are strewn around in an argillite matrix. This is mapped as a broken formation. 3) A deformation overturned the sequence. 4) Normal faulting occurred. 5) Low angle shear zones with well developed long quartz fibers formed. 6) The area was crossed by joints and veins. The placement of what is here labelled 3 and 5 are not definite; they each may have occurred one earlier. The principal quadratic extensions determined from deformed radiolaria are 1.34, 1.10 and 0.68. Most of the elongation probably occurred during phase 2.
The geologic history and possible plate tectonic associations are discussed.
Four mapping units were separated because of sedimentological or structural differences.
Massive homogeneous sandstones and siltstones are (from four point counted thin sections) dacitic feldspathic psammites. Related rocks include thick bedded sandstones, rhythmicly bedded siltstones and argillites with fossil worms, colour banded argillites and bedded radiolarian cherts.
Black chert nodules from one area were studied geochemically. 2000 ppm tungsten and about 500 ppm native silver occur in these iron and manganese rich radiolarian chert cobbles.
The rocks are metamorphosed to upper prehnite - pumpellyite to lowest pumpellyite - actinolite facies; they belong to textural zones 1 and 2A (after Bishop, l972A).
Despite the low degree of metamorphism, the rocks are often structurally complex. Six phases of deformation have been identified. 1) Extensive soft sediment deformation caused sedimentary melange in some isolated areas. 2) Isoclinal folding and shearing disrupted the area. Because of the double deformation of the sedimentary melange areas, granules to blocks of the various lithologies are strewn around in an argillite matrix. This is mapped as a broken formation. 3) A deformation overturned the sequence. 4) Normal faulting occurred. 5) Low angle shear zones with well developed long quartz fibers formed. 6) The area was crossed by joints and veins. The placement of what is here labelled 3 and 5 are not definite; they each may have occurred one earlier. The principal quadratic extensions determined from deformed radiolaria are 1.34, 1.10 and 0.68. Most of the elongation probably occurred during phase 2.
The geologic history and possible plate tectonic associations are discussed.
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91 leaves : illus. map (in pocket) ; 30 cm.
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1977Bejnar
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POLYGON ((170.188408172110627 -46.053471126147116,170.211224774378337 -46.055459410881326,170.213191938413445 -46.084322600848004,170.209514818354336 -46.109713914512632,170.191020076296269 -46.150220670148563,170.144117716737156 -46.189160250394401,170.087512766991637 -46.209163460003133,170.075888375549596 -46.19553069616822,170.136481771809571 -46.151343063713625,170.173886612152529 -46.116791800585304,170.18570949240285 -46.086363526347618,170.187455077563243 -46.065497665836297,170.188408172110627 -46.053471126147116))
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Bejnar, Tor John Stark., “Geology of the Tuapeka group, Bull creek area, South Otago, New Zealand.,” Otago Geology Theses, accessed April 14, 2026, https://theses.otagogeology.org.nz/items/show/98.