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    The geology of the lower Pohangina Valley, Manawatu, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Earth Science at Massey University, Palmerston North, New Zealand
    (Massey University, 2015) Rees, Callum
    The geology of the lower Pohangina Valley has been mapped at a scale of 1:30,000. This has involved the reclassification of formations described by Carter (1972) in northern Pohangina and correlation of stratigraphy and stratigraphic nomeclamenture to both the Whanganui and Dannevirke subdivisions (Fleming, 1953; Lillie, 1953). Pohangina geology has been linked to the cyclostratigraphy preserved within the Whanganui Basin (Naish, et al, 1998; e.t.c.) via tephrochronology and biostratigraphy, allowing basin wide correlation. The Kaimatira Pumice Sand Formation (Fleming, 1953) is recognised within the study area by the identification of the Potaka Tephra, allowing an upper boundary to be placed upon the Takapari Formation of Carter (1972). Kai Iwi Group sediments are mapped near the axis of the Pohangina Fauted Monocline allowing correlation to the Beehive Creek, Cullings Gully and Finnis Road sections to the west of the study area (Brackley,1999; Townsend, 1993; Seward, 1976; Manning, 1988; and MacPherson, 1985). Balanced geological cross sections have been constructed and used as aids in structural interpretation. The Pohangina Faulted Monocline (Marden, 1984), a major regional structure, is controlled at depth by a shallow reverse fault. This underlying reverse fault is correlated to the Raukawa Fault of Rich (1959), which outcrops at the western end of the Manawatu Gorge. The Pohangina Faulted Monocline is tentatively correlated with another monoclinal flexure to the south of the Manawatu Gorge, also interpreted to be related to and controlled by the Ruakawa Fault (Rich, 1959). When the eastward dipping Ruakawa Fault is at greater depth, westward dipping normal faults are found and are interpreted as antithetic faults splaying off from the underlying thrust fault. The Pohangina Fault is mapped as an active normal fault, displacing an Ohakean terrace on the western side of the Pohangina River. The potentially active Whareroa Fault (Ower, 1943) is inferred to cross the Manawatu Saddle area trending SE – NW as a contact fault between Torlesse greywacke and Plio-Pleistocene sediments. The thrust faulting in the area has resulted in the intense deformation and uplift of Torlesse bedrock contemporaneous with drag-tilting and folding of the Plio-Pleistocene sediments. Erosion within the study area has exposed Takapari Formation beds dipping at up to 70° to the west. Steep dips are traced SW-NE across the landscape, interpreted as representing the axis of the Pohangina Faulted Monocline and also allowing links to be made between areas of exposed faulting mapped by Rich (1959), Ower (1943) and in this study. Lignite and tephra beds within the Takapari Formation are associated with deposition in an estuarine environment on a coastal plain bordering the Whanganui Basin, during Early Castlecliffian time. Geochemical analyses are used to identify eight tephras, which are used for both stratigraphic control and paleogeographic interpretation. During Early Nukumaruan time an influx of gravel within the south eastern Whanganui Basin is associated with the formation of a prograding Gilbert-type fan delta within the Cg Member of the Konewa Formation. The gravels are interpreted as being derived from exposed greywacke in the vicinity of the present day northern Ruahine Ranges, 40 to 70 km north of Pohangina. Distance from source is calculated from clast size within the conglomerate and together with mineralogy provide evidence of provenance. Biostratigraphic and lithostratigraphic changes are used as evidence to support wider interpretations involving paleogeography and the geological history of the lower North Island. Depositional environments are interpreted using facies analysis, tephrochronology, grain size analysis, and biostratigraphy. Detailed stratigraphic logs are compiled and interpreted in terms of depositional history and sequence stratigraphy. Marker horizons and bio-events allow correlation of stratigraphy to the Whanganui Basin cyclostratigraphy and marine oxygen isotope record. This information is then used to build an overall regional geological history of the area, including understanding basin development, paleogeography, provenance and depositional history.
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    Erosion and land use in the Pohangina region : a study using GIS and remote sensing : a thesis presented in partial fulfilment of the requirement for the degree of Master of Applied Science in Soil Science, Massey University
    (Massey University, 1999) Miri, Seyed Abbas
    In this study a combined remote sensing and GIS approach, using aerial photographs, a SPOT satellite image and a digital elevation model was employed to extract hillslope units and watershed boundary maps. The acquired data were also used to investigate relationships between topographical features (slope angles and slope aspects) and soil slip erosion and land management practices in the Pohangina region. The procedures were first developed on a representative area. It was typical of the district in term of the climate, topography, soils, geology and land management practices. These methods were then used to identify those areas most susceptible to soil slip erosion in the Pohangina region. A raster GIS and image processing package (IDRISI for Windows) was used to analyse the remotely sensed data/digital elevation model and to create different maps for investigation. A simple technique for extracting watershed boundaries and mapping hillslope units was also developed. The slope aspects facing N & NE are more susceptible to soil slip erosion than other aspects. It was also found that this erosion occurs equally on all slope classes. Four major land management practices were used in the representative area. These were pasture, exotic forest, spaced planting and reversion to bush. Nearly 95 % of erosion has occurred in pasture, 4.1 % in space planted areas, 1.3 % in exotic forest, and no erosion occurred in areas reverted to bush. The soil slip susceptibility map of the Pohangina region was created to assist in the allocation of soil conservation practices. This study has shown nearly 90% of the areas susceptible to slip erosion (2850 hectares) are presently not covered with suitable vegetation.
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    Structure and dynamics of alluvial forest in the Pohangina Valley : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Botany
    (Massey University, 1984) Lusk, Christopher Harley
    Species' population structures and replacement patterns are quantitatively described from intensive sampling of forest on alluvial surfaces of three ages. Observations and evidence from a range of other sites are incorporated to assist in tracing the development of forest on alluvial surfaces of the study area, and in examining factors influencing this development. Cockayne's postulate that: " ... the most important principle underlying succession in New Zealand forests is the relation of the different species to light." is investigated with respect to the study area. A Relatively even-aged totara-dominant dense podocarp stands have developed on floodplain surfaces made available by progressive channel down-cutting and lateral migrations. On older surfaces these first generation stands are replaced by forests dominated principally by the angiosperms tawa, titoki and mahoe. On a terrace surface c.2,000 - 3,000 years old, forest variation appears attributable to dynamic processes as well as differences in site drainage. On mesic sites tall tawa-dominant forest prevails, although recent windthrows have produced low groves of mahoe and other small angiosperms, and also apparently stimulated some podocarp regeneration. On xeric terrace sites, titoki and rewarewa dominate the canopy. Low densities of podocarps on the mesic terrace sites attest to very sparse regeneration after the demise of the dense first generation stands. The discontinuous size class distributions of podocarp species on these sites appear at least partially attributable tc speradic regeneration following major windthrows. Podocarp densities are higher on the terrace xeric sites, regeneration of matai and totara apparently being favoured by the lower vegetation density and higher understorey light levels. On these sites matai shows an all - sized stable population structure, and a cyclic discontinuous replacement of totara seems possible. Seedling growth experiments showed both totara and kahikatea to be less shade-tolerant than two large angiosperm species (titoki and pukatea, respectively) typically seen to be replacing them in old growth forests on their respective sites. Except on the terrace xeric sites, light levels measured in forest understoreys were mainly below the compensation point experimentally estimated for totara seedlings. These findings confirm that regeneration of kahikatea and especially totara is likely to be very infrequent in old growth forest on these sites. Cockayne's postulate does not completely explain species establishment patterns within these forests. However, the findings of this study lend support to his interpretation of "light relations" as the primary influence on successional trends, and suggest that regeneration of kahikatea and especially totara is likely to be largely disturbance-dependent.