Geology of the Green Valley, Morrisons area, North Otago


Cavaney, Roderick John.


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INTRODUCTION. The area described comprises 4o square miles from Shag Valley Station to Pigroot Creek along r~ain Highway 85 (commonly known as the Pigroot). It lies \v.ithin parts of the National One-mile Series Maps 8135~ S136 (Oamaru), S145, S146 (Moeraki); and in all or part of the following survey districts in the Otago Land District: Waihemo, Highlay, and Swinburn (see Fig.1). 1. Previous work on the area has been mainly of ,a reconnaissance nature. Brown (1938) partly mapped sections of it but the associated m nuscript was never completed. Williamaon (1939) produced a map including some of the northeastern portion and made brief reference to it. Benson (1941) briefly reports the petrography of isolated specimens from Yr.ithin the area, and McKay (1884) made a tr verse through it. ~ ork in surrounding areas includes that by Sang·Ie"en { 1962) in the adjacent Swinburn Survey District, Brown (1955) around Siberia Hill eight miles to the north, Paterson (1941) in the Palmerston district 15 miles to the southeast, and Dodds (1963) in the Budle area 12 miles to the south. The district is a sheep farming one, and contaioo all or part of twelve farms and thirteen homesteads scattered along or close to the Main Highway. Approximately 70 persons live in the area mapped. No towns occur within the boundaries, and the near st town ia Dunback eight miles from Shag Valley Station. Palmerston, 16 miles to the south-east is the main service centre for the district. Fig. I INDEX MAPS TO THE GREEN VALLEY""Y"MORRISONS AREA o 4o I t mdct.5 2. t1ain Highway 85 passes through the area from Palmerston to Ranfurly (16 miles north-east from Pigroot Creek) and on to Alexandra. Side roads in the district are few and short, although an unsealed road runs along Taieri Ridge from Middlemareh on the Maniatoto plain, through Macrae Flat to Dunback. A railway line runs parallel to the Main Highway 85 from the Main Trunk Line at Palmerston to the railhead at Dunback. The c..nnual rainfall is about 22 inches per year at Green Valley, and increases further inland. The vdnter months have a higher average than the summer, and the u permost slopes of the Krutanui anges may be snow covered in winter. The area mapped is in the fault angle depression paralleling the Kakanui Ranges and drained by the Shag or Waihemo River. To the northeast is the upthDown semischist block of the Krutanui Ranges rising to 2500 feet. Along the south-west boundary is the Shag River, falling from 1500 feet near the Brothers, to 500 feet at Shag Valley Station, and flowing in a gorge some 200 feet below the general level of the country. The river opens out into a ide valley with river flats belo Shag Valley Station. Between the river and the Kakanui Ranges are a series of basalt capped hills from 1400 to 1700 feet above sea level and about 500 feet above the general level of the surrounding country. Except to the north-east where l~t grade semischists (Chl.2 subzone) form the up-thrown block of the Kakanui Ranges, the basement is a quartzo-feldspathic-chloritic schist (Chl.3 subzone). Overlying this, where protective basalt caps have preserved them, are a series of Tertiary beds up to 600 feet thick. The lowest of these, described I I as the 'Morrisons Formation'• consists of terrestrial sands at the base, passing into marine sands and silts containing a Bortonian fauna at the top. Overlying this formation, but separat9d by a minor unconformity, is a glauconitic greensand, the Nas~by Greenaand, dated as Duntroonian. This greensand becoroeB ik!Creaaingly calcareous towards the upper parts and passes conform~bly, with no break in deposition, ~. into a Limestone described as the •Green Valley Limestone•. This limestone, wtdch is Waitakian in age, forms promin nt outcrops, esp cially in the Green Va1ley - Happy Valley area. Disconformably overlying t 1e limestone are very thin (less than ten feet thick), localise deposits of terrestrial sands and grits. These represent an erosional period prior to the eruption ot the overlying volcanic rooks. Alkali basalts and dol rite~ of the Vaipiata Formation overly th previously mentioned rocks and are no1h r~ thicker tl~n 200 ~eet. Several different rock types can be recognised but the major division is b t een the lowermost dolerite flow, and the overlying finergrained basalts and basanitic type flows. The two may be se rated by carbor~ceouG sands and muds although the dolerite ia often abs nt. Dykes are common, such as the dolerlte dyke at Trig Fv the dyke in the schist half a mile northwest of Trig H, and the feeder and associated dyke neai' the Brothers. Pleistocene and Recent gravel depoaits, often terraced, are common in the stream beds, and slumping is widespread in the softer Tertiary formations. A table of formations is included (Tabl 1).

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iv, 99 p. : Illus., fold. diagrs., fold. map (in pocket) ; 27 cm. Bibliography: p. 79-85.


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Cavaney, Roderick John., “Geology of the Green Valley, Morrisons area, North Otago ,” Otago Geology Theses, accessed May 22, 2024,

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