The Geology of Cascade Creek, Eglinton Valley.
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The area round Cascade Creek from the Livingstone Fault to the Te Anau - Milford road has been mapped. Previous work on the Permian sequence in Otago - Southland and Nelson is examined in some detail. Group names applied to the rock formations of the area are; Humboldt, Brook Street and Maitai. Reasons for this nomenclature are given in Appendix 1.
A section of the Eglinton Volcanics along Cascade Creek is described, lithologies present include quartz keratophyre, andesite and volcanogenic sediments. An infaulted sliver of Murihiku Supergroup may be represented by one outcrop in the creek. The north end of the southern section of the Livingstone Volcanics occurs one mile south of Cascade Creek. These rocks are briefly described and classified as epidiorites.
The Red Mountain Ultramafics consist of sheared serpentinite, blocky serpentinite and tectonic inclusions. X-ray analysis of the sheared serpentinite suggests that all three serpentine polymorphs are present, viz., antigorite, crysotile and lizardite. Two types of tectonic inclusions are recognised, those with dioritic composition, and those metasomatised to rodingite. X-ray analysis of hydrogrossular from five rodingites suggests a low degree of hydration and a formation temperature (630 - 750°C) outside the stability field of serpentine - assumed to be 500°C. Chlorite from inclusion margins is moderately aluminous and rich in magnesium relative to iron. It is not known whether the ultramafics were laid down before the Livingstone Volcanics or subsequently intruded along the Livingstone Fault.
The steeply dipping sediments exposed between the Eglinton and Livingstone Volcanics are referred to as the Maitai Group. Volcanic breccias, sandstones and argillites underlying the Howden Limestone, and previously included in the Livingstone Volcanics, are placed in the newly defined Cascade Formation. The type section is situated half a mile south of the triple forks in Cascade Creek. Atomodesma - bearing Annear Sandstone overlies the Howden Limestone and is overlain by the massive, unfossiliferous, well sorted Key Summit Sandstone. Most of the Maitai sediment is volcanically derived.
Alkali analyses of the two volcanic suites show a higher K20:Na20 ratio in the Eglinton Volcanics. These results are considered to support Challis' theory that the two belts were genetically distinct.
The Key Summit Syncline axis is mapped as a faulted axis along the contact separating the Annear and Key Summit Formations The Livingstone and Hollyford Faults are steeply dipping features approximately one mile apart east of Lake Gunn. The strike directions of two other near vertical faults make an angle of approximately 30° with the Livingstone Fault. They may be part of a conjugate set between the Livingstone and Hollyford faults. Active, or very recent scarps near the top of, and parallel to ridge crests, may be the result of gravitational adjustment in mountains whose slopes have been oversteepened by ice movement.
All rocks in the area have undergone regional metamorphism. The Maitai sediments contain lawsonite without prehnite and are classified as lawsonite- albite facies. The Eglinton Volcanics are assigned to the prehnite - pumpellyite facies, and the Livingstone Volcanics to the higher grade pumpellyite-actinolite schist facies.
A brief geological history is given. The width of Maitai sediments exposed in Cascade Creek is probably the narrowest anywhere along the marginal syncline, and this area may have the greatest depth of erosion in the syncline.
A section of the Eglinton Volcanics along Cascade Creek is described, lithologies present include quartz keratophyre, andesite and volcanogenic sediments. An infaulted sliver of Murihiku Supergroup may be represented by one outcrop in the creek. The north end of the southern section of the Livingstone Volcanics occurs one mile south of Cascade Creek. These rocks are briefly described and classified as epidiorites.
The Red Mountain Ultramafics consist of sheared serpentinite, blocky serpentinite and tectonic inclusions. X-ray analysis of the sheared serpentinite suggests that all three serpentine polymorphs are present, viz., antigorite, crysotile and lizardite. Two types of tectonic inclusions are recognised, those with dioritic composition, and those metasomatised to rodingite. X-ray analysis of hydrogrossular from five rodingites suggests a low degree of hydration and a formation temperature (630 - 750°C) outside the stability field of serpentine - assumed to be 500°C. Chlorite from inclusion margins is moderately aluminous and rich in magnesium relative to iron. It is not known whether the ultramafics were laid down before the Livingstone Volcanics or subsequently intruded along the Livingstone Fault.
The steeply dipping sediments exposed between the Eglinton and Livingstone Volcanics are referred to as the Maitai Group. Volcanic breccias, sandstones and argillites underlying the Howden Limestone, and previously included in the Livingstone Volcanics, are placed in the newly defined Cascade Formation. The type section is situated half a mile south of the triple forks in Cascade Creek. Atomodesma - bearing Annear Sandstone overlies the Howden Limestone and is overlain by the massive, unfossiliferous, well sorted Key Summit Sandstone. Most of the Maitai sediment is volcanically derived.
Alkali analyses of the two volcanic suites show a higher K20:Na20 ratio in the Eglinton Volcanics. These results are considered to support Challis' theory that the two belts were genetically distinct.
The Key Summit Syncline axis is mapped as a faulted axis along the contact separating the Annear and Key Summit Formations The Livingstone and Hollyford Faults are steeply dipping features approximately one mile apart east of Lake Gunn. The strike directions of two other near vertical faults make an angle of approximately 30° with the Livingstone Fault. They may be part of a conjugate set between the Livingstone and Hollyford faults. Active, or very recent scarps near the top of, and parallel to ridge crests, may be the result of gravitational adjustment in mountains whose slopes have been oversteepened by ice movement.
All rocks in the area have undergone regional metamorphism. The Maitai sediments contain lawsonite without prehnite and are classified as lawsonite- albite facies. The Eglinton Volcanics are assigned to the prehnite - pumpellyite facies, and the Livingstone Volcanics to the higher grade pumpellyite-actinolite schist facies.
A brief geological history is given. The width of Maitai sediments exposed in Cascade Creek is probably the narrowest anywhere along the marginal syncline, and this area may have the greatest depth of erosion in the syncline.
Thesis description:
vi. 69 p. ill, photos, map (folded in pocket); 27 cm.
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1969Williams
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POLYGON ((168.119659543000012 -44.939279575999933,168.119527070000117 -44.940937253999948,168.067110221000121 -44.939093817999947,168.061880999000095 -44.938901906999945,168.061910163000107 -44.938632691999942,168.062106685000117 -44.936838016999957,168.062831060000121 -44.930161535999957,168.06608299200002 -44.925179793999973,168.072297500000104 -44.915654509999968,168.085080416000096 -44.896054763999985,168.093130446000032 -44.883703814999933,168.095969731000082 -44.879343938999966,168.124377071000026 -44.880687299999977,168.120627166000077 -44.92726734799993,168.119659543000012 -44.939279575999933))
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Williams, JG, “The Geology of Cascade Creek, Eglinton Valley.,” Otago Geology Theses, accessed April 14, 2026, https://theses.otagogeology.org.nz/items/show/41.