Structure of the Ohai Coalfield (Fig. 3)
The general strike of the Tertiary measures in this depression, as well as the trend of the faults separating Tertiary and older rocks, is approximately at right angles to the dominant north–south trend of the main structures in Southland, and the coalfield is situated in a graben, the greywaeke and other Mesozoic rocks being usually separated from the outcropping coal measures by boundary faults. It may be suggested that certain of these boundary faults, whose course can be mapped, although their planes are rarely visible, existed before the deposition of the coal measures, but there is no clear demonstration of such early formation. Certainly, many of the faults are largely of post-Tertiary origin, judging from the steep dips shown by the coal measures in the vicinity of the faults and from the extent of folding and faulting which has affected the measures as a whole, but some of the larger faults may have been initiated during Tertiary or pre-Tertiary time.
The faults are not described in detail here; it is sufficient to notice that the southern edge of the coalfield is marked by the line of the Twinlaw fault, which juxtaposes Mesozoic and Tertiary marine strata and which is likely to have a throw of at least 2,000 to 3,000 feet, whilst the northern limit of the field has an irregular boundary composed of several faults, some of which are of considerable throw but unlikely to be of the same order as the Twinlaw fault.
Two small outliers of coal measures lie to the north of the main coalfield, separated from it by upstanding barriers of greywacke, and at one of these outliers a smooth and partly dissected surface of greywacke appears to dip under the coal measures. North of the coalfield, high smooth surfaces on the Wairaki Downs are interpreted as remnants of an exhumed pre-Tertiary erosion plane, presumably the same as the great Cretaceous peneplain described by Park and others, but the peneplain has obviously been much broken by faulting.
The Tertiary strata of the coalfield can be subdivided as follows:—
|Shally limestone bands||100 feet plus|
|Marine mudstone with Whaingaroan microfaunas||1,100 "|
|Fresh-water mudstones and clay ironstones||2,300 "|
Fig. 4—The Manawatu Gorge in relation to major structural elements. The Ruahine Range consists of greywacke. The Dannevirks Syncline is formed of Pliocene strata which cover greywacke cores outcropping in the Waewaepa-Oruawharo Fault-belt, East of the latter belt a composite syncline of Oligosene and Miocene beds flanks the western edge of the Whangai Range which discloses Cretaceous cores.
|Coal measures, consisting of sandstones, conglomerates, mudstones, claystones and thick lensing seams of coal||1,050 "|
|The basal beds rest unconformably on greywackes and porphyrites, the greywackes having yielded good Triassic fossils at the base of a bore through the measures as well as in the adjoining outcrops.|
The thicknesses of these strata are approximations based on composite successions, but the order of superposition is certain. Lithologically, the important feature of the coal measures is the great abundance of boulders of granite-gneiss and porphyrite, never exceeding 1 foot in diameter, as well as the common presence of cobbles of finer volcanic rocks. The sandstones, notably felspathic, quartzose and micaceous, are largely formed from weathered material of gneissic composition, and evidently much of the material came from erosion of the Main Range; but the cobbles of porphyritic and volcanic rock probably came, not from the Main Range where such rocks are unknown, but from the Takitimo and Longwood Mountains, or perhaps from the Livingstones further north. Evidently we must envisage the whole coalfield as infilled during part of Tertiary time with coarse gravels deposited by a river coming from the west, and considerable bodies of swampy water must have extended where the stream waters were ponded from time to time, with formation of peat swamps. Although the Mesozoic and earlier rocks were probably pepeplained before the deposition of the Tertiary, it seems likely that considerable warping and faulting of the peneplained surface occurred in pre-Tertiary time, for the boulders in the conglomerates are both large and well-rounded. The nature of these boulders suggests erosion of a country of marked relief, dominated by an uplifted gneissic chain lying west of the present Waiau Valley and probably also with hills of fairly marked elevation along the line of the present Longwoods and Takitimos. Thus, the Ohai Depression is believed to mark the approximate site of an ancient Tertiary valley, an interpretation which supports partly the early view of Hutton whilst still attributing the main tectonic pattern to the Kaikoura orogeny of late Pliocene or Pleistocene age.