Eastern Otago Research Articles

The greenschist facies in part of eastern Otago, New Zealand

Rocks of the greenschist facies in eastern Otago, New Zealand, have been investigated in an area some thirteen to sixteen kilometers wide and sixty-five kilometers long extending northeastwards approximately normal to the boundary of the schist with lower grade rocks. Quartzo-feldspathic schists predominate but greenschists and metacherts occur sporadically throughout the area. At the southwestern edge of the area schists are in the chlorite zone, slightly above the high-grade limit of pumpellyite. Metamorphic grade increases toward the northeast into the biotite zone which occupies about half the terrane studied and is believed to be everywhere little advanced in metamorphic grade past that of the biotite isograd. Some 130 mineral specimens have been partially analysed with the electron probe. Results derived from these data as well as other mineralogical investigation are as follows: Albite contains a maximum of 1% anorthite plus orthoclase in epidote-bearing rocks from all parts of the area.

Microscopic fabric of a refolded schist from Eastern Otago, New Zealand

Abstract The mesoscopic structure of a twice-folded schist from Central Otago is described, together with patterns of orientation of muscovite, quartz, and clinozoisite. Crystallisation of the first two outlasted the final episode of folding; clinozoisite on the other hand ceased crystallising before the second episode. Patterns of preferred orientation of muscovite and of quartz show the influence of both episodes.

The geology of the Mt Stroker area, Eastern Otago

Abstract Tertiary rocks occurring as several small outliers resting on schist, south-east of Middlemarch, are mapped and separated into two formations. The Summer Hills Formation, probably Miocene in age, is composed of indurated and friable quartz sandstones and conglomerates. It contains material from the underlying Otago schist, but also orthoclase and p.erthitic microcline from a plutonic terrain. It rests on a slightly dissected schist erosion surface and is overlain, apparently conformably, by basanitic volcanic rocks of the Waipiata Formation, of late Miocene or early Pliocene age. The basanites are chemically and mineralogically similar to the majority of volcanic rocks peripheral to the central Dunedin complex. They contain gabbroic, pyroxenitic, and peridotitic fragments, and also partly assimilated quartz xenoliths. Low-angle, north-west-striking thrust faults and high-angle, north-east-striking faults occur in the area. The latter are apparently related to Otago basin-and-range structures. An early erosion surface is correlated with Cotton's Cretaceous peneplain; a later erosion surface was formed after the volcanic activity but before initiation of the present youthful drainage,

The geology of the Mt Stoker area, Eastern Otago

Abstract The area mapped, south-east of Middlemarch, is underlain by schist but also includes several small Tertiary outliers of basanitic volcanic rocks resting on a thin formation of quartz sands and conglomerates. In this paper the lithology, mineralogy, and structure of the schist are described. The following conclusions are drawn: (1) bedding in a wide range of thicknesses is recognisable in the schist, (2) the area lies within a biotite and possibly garnet zone of metamorphism, (3) retrogressive metamorphism occurred and appears to have been accompanied by a deformational phase, and (4) there were at least two phases of deformation during the metamorphic period, the first possibly having produced large-scale recumbent folds.

Age relations of the Dunedin volcanic complex and some paleogeographic implications—Part II

Abstract An attempt is made to determine the duration of the Dunedin volcanicity which was shown in Part I (Coombs, White & Hamilton, 1960) to have been initiated during Waiauan or early Tongaporutuan times. Paleobotanical data offer potential indicators of age since carbonaceous beds are widely distributed at various horizons within the volcanic sequence but present microfloral evidence is indecisive. It indicates an age from Waiauan to Opoitian for material deposited during the intervals between both the First and Second, and Second and Third Major Eruptive Phases. From the occurrence of Third Phase phonolite boulders in Late Pleistocene soliflual deposits, the deep weathering of some of these phonolites and the general lack of constructional landforms in the Dunedin district, it is suggested that volcanic activity was complete not later than early Pleistocene and probably earlier. If world-wide correlations on paleomagnetic data prove to be valid, then the lavas of the latter part of the Second Major Eruptive Phase having anomalous or reversed magnetisation may be assigned to a Late Miocene - basal Pliocene period of reversal recognised in France. The normal magnetisation of the Third Major Eruptive Phase lavas may likewise indicate that volcanicity ceased before a period of reversals in mid-Pliocene times. Earlier concepts of a Late Tertiary peneplain in Eastern Otago require modification. Although a pre-volcanic erosion surface of low relief occurs in the western part of the Dunedin region, the conformable relationship between Waiauan sediments and volcanics in the lower Harbour area rules out the possibility of a peneplain there.

The alignment of fold axes in the Jurassic of south-eastern Otago and Southern Southland

Summary In southern Southland and south-east Otago on the southern limb of the Southland Syncline each axial trace of several minor folds is sharply flexured southward to strike north-south. As normal faulting in the area has had no effect on the axial traces of folds, except in the immediate vicinity of faults, and as late-Cretaceous to Recent folding has had little or no effect on the axial traces of folds, subsequent folding and faulting does not satisfactorily explain the flexuring. A working hypothesis of a regional flexure affecting the axial trace of the Southland Syncline and of the Otago Schist Anticlinorium is proposed. In Eastern Otago the strike of bedding in subschists and greywackes of the Tuapeka Series and of the “b” lineation of the Otago Schists supports the hypothesis, but not unequivocally so. Field confirmation of a regional swing of strike of bedding and lineation from south-easterly inland to south-south-easterly at the coast is necessary before a regional flexure is proven. It is conjectural whether the proposed regional flexure represents a permanent swing to the south of the post-Hokonui structures or whether it denotes a local regional buckling in the south-easterly trend. The alternatives should be considered in paleogeographic reconstructions of the New Zealand region.

The Basic Igneous Rocks of Eastern Otago and Their Tectonic Environment [Parts I and II]. W. N. Benson

Previous articleNext article No AccessReviewsThe Basic Igneous Rocks of Eastern Otago and Their Tectonic Environment [Parts I and II]. W. N. Benson C. A. CottonC. A. Cotton Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Journal of Geology Volume 52, Number 1Jan., 1944 Article DOIhttps://doi.org/10.1086/625193 Views: 3Total views on this site PDF download Crossref reports no articles citing this article.

Structural petrology of the schists of eastern Otago, New Zealand

Structural petrology of the schists of eastern Otago, New Zealand

Structural petrology of the schists of eastern Otago, New Zealand

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Structural Petrology of the Schists of Eastern Otago, New Zealand

Structural Petrology of the Schists of Eastern Otago, New Zealand