Geology of the Waihaorunga Valley, South Canterbury

Middlemiss, Sasha, 1974-

Towards the east, the Waihaorunga valley (South Canterbury) is underlain and fault bound by greywacke and argillite flysch sequences of the Torlesse Terrane (Torlesse). Towards the Carnpbell Hills in the west, outcrops of massive weathered greywacke predominate. The greywacke can be classified following Bishop's textural zone groups as TZI with minor TZII. Mineralogically, the Torlesse belongs to the actinolite - pumpellyite facies. Red siliceous argillites, and conglomerates containing granite clasts are found as cobbles and boulders in streams draining the Campbell Hills in the vicinity of Hursts Road, and are derived from Ford's (1994) field of study. Quartz:feldspar:lithics plots constructed to estimate the likely age of the Torlesse in this area following MacKinnon (1983) corresponded with MacKinnon's lower Upper Triassic Q:F:L ratios. In the central southern section of the valley (i.e. - south of Waihaorunga Back Road) the Torlesse exhibits pre-Tertiary steep folding and in places is overturned. The base of the Tertiary sedimentary sequence forms an angular unconformity with the underlying Torlesse. The Taratu Formation consisting of carbonaceous grey clay and lignite outcrops at Quambys Bridge (J40/GR409048) and in the south of the field area at (I40/GR273036). The Taratu is approximately lOm thick. Unconformably overlying this is the Kauru Formation considered by Maxwell (1999) to be Waipawan - ?Heretaungan in age. The thickness of the Kauru is poorly constrained, but is thought to be up to 30m(+) thick. The Kauru can be divided into 7 categories based on field observations. Upstream from Quarnbys Bridge (J40/GR408045), a band of Kauru outcrops as an Asterocyclina rich, limonitic sediment containing clasts of quartz and greywacke ("Asterocyclina limestone" member) overlain by sub-horizontal, brown Kauru sands ("friable glauconitic quartz sands" member). Limonitic quartzose blocks resting on the erosional surface of the Torlesse in the central part of the field area are lag deposits of Gage's Raupo Concretionary Sandstone Member. Maxwell classified horizons of greensands, glauconitic quartz sands, with shell beds, and concretions at J40/GR378075 as Kauru Formation ("brown carbonaceous quartz sands, greensands and jarositic siltstone" member). Fossil record form (J40/f 211) for this locality gives an upper Bortonian or Kaiatan age. Here, the Kauru is dipping -50"E towards the South Branch of the Waihao River fault (SBWRf) and the dip decreases westward along this outcrop to sub-horizontal bedding. Outcropping above an angular unconforrnity (J40/GR319055) is the "carbonaceous sands and silt" member. The base of the Kauru exposed immediately upstream of the back bridge to Pentland Hills station at J40/GR359112 consists of several horizons of fossiliferous conglomerates (containing greywacke) and sands ("fossiliferous quartz conglomerate" member). The top section of the Kauru is exposed in the stream bed at J40/GR355114 where sub-horizontal, fossiliferous dark grey/blue sands (Five Forks Glauconitic Sand member) are overlain by clean quartz sands of the Opawa Sandstone (the shallow water lateral equivalent of the Bortonian Waihao Greensand). The Opawa Sandstone is -60m thick, and is defined as clean, friable, micaceous quartz sands (plus rare glauconite) with fossiliferous concretions often containing rounded quartz pebbles. The sands are often massive, but also exhibit horizontal bedding and cross bedding. In Waihaorunga valley the Waihao Greensand and Opawa Sandstone seem to grade into each other and are not easily distinguishable, therefore they are considered as one formation here. Resting on the Waihao Greensand/Opawa Sandstone is the Otiake Group (comprising the Kokoamu Greensand and Otekaike Limestone). The Kokoamu Greensand is a richly fossiliferous, glauconitic sand (<5m thick) which grades up into the Otekaike Limestone (seen at J40/GR347103). Lack of exposure restricts accurate thickness estimations, however sinkholes and springs are used to estimate the upper and lower limits where there is no outcrop. The Otekaike Limestone is estimated to be approximately 55m thick. This limestone is a fossiliferous, bioturbated, karst and commonly glauconitic unit. In outcrop it is massive to bedded, and poorly to well cemented. At Bell's Quarry (I40/GR292044) rare mang~nese is found in association with the limestone. The macro fossil assemblage suggests an age range from Dhntroonian to Waitakian. The Mount Harris Formation overlies the Otekaike Limestone at J40/GR333087. Macro fossils from this new fossil locality suggest an Altonian age. In the Waihaorunga valley the Mt Harris Formation varies from a silty marl - (Waitoura Marl Member- at J40/GR333087), to an infaulted massive silty sand at J40/GR377084 (siltstone member). The forarninifera collected from the siltstone member suggest a Waitakian age (or younger). It is likely that this -10m thick silty sand also belongs to the Mt Harris Formation. A blue/grey fossiliferous sand unit of unconstrained thickness outcrops in a fault bound depression at J40/GR376047. Maxwell (1999) has classified this as the Southburn Sand, and fossils indicate an Altonian age. Quaternary gravels and loess form a veneer over much of the Waihaorunga valley. At Quambys Bridge, the gravels are up to approximately 4m thick. Gravels (a few metres thick) can be found on the upthrown side of the SBWR fault. On the downthrown side of the SBWR fault the gravels are up to -lOm thick at J40/GR374093, where they unconformably overlie the Waihao Greensand/Opawa Sandstone. There are numerous fluvial terraces (especially in the north-east of the field area) where the river has cut down through its own deposits and underlying lithology (Waihao Greensand/Opawa Sandstone). In the southern part of the SBWR terraces expose Torlesse, Kauru Formation, and Waihao Greensand/Opawa Sandstone. A veneer of loose Torlesse cobbles and boulders are found on the sloping limestone and sandstone hills of the west, while river deposits were observed in at least one valley draining the Campbell Hills.

x, 133 leaves : col. ill. (some folded), maps (some col., some folded) ; 30 cm.

1999Middlemiss

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