Geology of the Central Bryneira Range.


Ritchie, Duncan David.


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Sedimentary rocks of upper Permian Bryneira Group exposed in the Central Bryneira Range, Pyke Valley have been mapped and described in detail. The five lowermost Bryneira Group formations outcropping in this region form a distinctive 2.4 km thick sedimentary pile, disconformably underlain to the east by Humboldt Group volcanic rocks, and in faulted contact with Eocene-Oligocene Tertiary sediments, and Alabaster Group volcanic rocks to the west. The Bryneira sediments are interpreted as having been deposited in an arc-trench gap basin by a variety of offshore sedimentological processes.
The basal unit, Upukerora Formation, consists of a 25-50 m sequence of coarse breccias which were eroded off a nearby submarine volcanic scarp. This unit passes abruptly upward into a 1 km thickness of turbidity-current deposited Howden Limestone (Trabeculatia prism limestone), with distinctive interbeds of green volcanogenic sandstone. The overlying, more quartzo-feldspathic sandstones and siltstones of Annear Formation (260 m), were deposited partly from sediment-carrying bottom contour currents (derived from a more silicic terrane to the south), and partly from downslope turbidity deposition. Both the Howden and Annear Formations are divided into two members on lithological grounds (BH1 and BH2), (BA1 and BA2). Annear Formation grades upward into an 1180 m thickness of Key Summit Volcanogenic Sandstone, which is overlain by 100 m of grey thinly laminated siltstones of Tapara Formation, most of which is interpreted as contourites.
Low-grade Prehnite-Pumpellyite to Lawsonite-AlbiteChlorite facies metamorphic mineral assemblages are developed representing regional-burial metamorphism in the mid-late Mesozoic. A complete range of lithologies~ from siltstones to boulder conglomerates, is found within the Annick Group Tertiary beds which are derived from volcanic rocks of Alabaster Group and plutonic rocks perhaps of the McKay Intrusives.
Structurally, the majority of Bryneira Group sediments are overturned, forming the eastern limb of the near isolc1inal Key Summit Syncline which appears to plunge both to the north and to the south from a 'high' in the Bryneira Range. The Hollyford Fault Zone downfaults most of the west limb, forming a graben which provided a basin for Mid-Tertiary sedimentation.

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76 leaves : ill. (some col.), 2 col. maps ; 30 cm.


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Ritchie, Duncan David., “Geology of the Central Bryneira Range.,” Otago Geology Theses, accessed May 22, 2024,

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