The Geology of an Area North-East of Martinborough, East Wairarapa

Abstract

The area examined by the writer is about 6 miles north-east of Martinborough and lies in the Huangarua and Wainuioru Survey Districts. It includes the central portion of the Maungaraki Range bounded to the west by the Whangaehu River, the central portion of the Wainuioru Valley, and a part of the rugged "Taipos". The boundary of the area is shown in the Locality Map, fig. 1. In its western part the southern boundary of the field lies directly north of the region mapped by Vella (1949). The size of the area investigated is approximately 90 square miles. The purpose of the investigation was to map the basement rocks of the area in general, and the Tertiary rocks in detail, so that from the surface information obtained, the structure and later geological history could be interpreted. This thesis is a further contribution to the geological investigation of the Wairarapa region which is being conducted by the Honours Students of Victoria University College. As an excellent topographic map of the area is in existence this has been used as the basis for the field sheets. The 1 inch to 1 mile Wairarapa Sheet (N.l62) of the Lands & Survey Department's Provisional One Mile Series was enlarged photographically to the scale of 20 chains to the inch and the enlargements used for the field mapping. Where sufficient detail on the map was lacking air photographs were obtained and data were plotted directly on to them in the field. Unnamed streams mentioned in this report were given reference letters, and small corrections were made to the map wherever they were considered to be necessary. An unnamed range towards the eastern margin of the area has been called "the Taipos", a name by which it is known locally. The southern boundary of the Wairarapa Sheet was used to mark the southern boundary of the area investigated. The field work occupied seven weeks in the early part and four weeks in the latter part of 1949, - eleven weeks being spent in the field altogether. Occasional trips of one to two days duration were made at other times. The basement rocks in the area were not examined in detail, (their detailed structure appeared to be complicated), but their extent was mapped. Any particular information seen within them was noted, but no determined effort was made to examine them other than cursorily. The Tertiary rocks were investigated with considerable care. They were mapped in the field on a lithological basis, formations being established on the differences in lithology. Unfortunately, a large area consists of massive, fine grained mudstones containing no macrofossils so that rocks of various Tertiary stages are indistinguishable from each other in hand specimens. In general, bedding cannot be seen within these mudstones but in places dip slopes are developed on the surface so that a reasonable interpretation of their general attitude can be arrived at. Marker beds, such as the tuff bands that Vella (loc. sit.) found in his area immediately to the south, are notably absent. These mudstones comprise the greatest thickness of Tertiary strata in the area and subdivision was considered desirable. The only available method was that provided by the age determinations of foraminifera made by Dr. Finlay on samples collected at stratigraphic intervals of from 150 to 800 feet. The mapping within these mudstones is, therefore, only as accurate as the method has allowed. In other Tertiary rocks macrofossils and foraminifera have been used in an attempt to establish the age of beds. Palaeontological zoning within some formations has also been attempted. Structural conclusions are based on evidence from the field mapping, and, in the massive mudstones in particular, on evidence from foraminiferal determinations.