Note on the Geology of Kauroo Hill.
Kauroo Hill (Loc. 100 on Plate 36 of Part I of this series) lies twelve miles due west of Oamaru, from the higher portions of which it may be seen standing out prominently in front of the Kakanui Range. The only reference to it in the geological literature appears to be McKay's (1894, p. 31) comment that “its southern base is formed of Palaeozoic rocks, and the higher portions of volcanic rocks covering the edges of the denuded quartz-grits, which make rapidly to the north-west, north and north-east, and cover a considerable extent of country between the Kauroo Creek and the Kakanui River.” Actually the structure of this hill is much more complex and interesting. (See Fig. 3.)
As may be inferred from other comments by McKay (loc. cit.), and as Cotton (1917, Fig. 2, Plate xxxi; 1922, Figs. 147, 152) has illustrated, the north-eastern slope of the Kakanui Range is an almost completely stripped portion of the Cretaceous peneplain passing downwards beneath its covering sediments in the broad valley of the Kakanui River. On the higher portions of the range are residual masses of Late Tertiary basic lavas either resting directly on the greywackes and argillites which are exposed on the Cretaceous peneplain or are separated therefrom by a small thickness of quartz-grits, the upper surface of which marks the local level of the “Miocene”* peneplain. (See Part I, Fig. 2, Section X–X, and notes on Locs. 89–95 given above). On the lower portion of the slopes of the range a residual mass of the covering strata capped by a doleritic basalt forms a mesa (Cotton, 1922, Fig. 152) termed Government Hill (Loc. 87), which differs from the above-mentioned lava-capped residuals in that its igneous cover has all the petrographic features which distinguish the usually sheet-forming pre-Miocene basic igneous rocks of Oamaru, Mount Charles and Moeraki from the more alkaline late Tertiary lavas herein described. The features of the older rocks have been briefly noted by Hutton (1889) and Marshall (1925), and will be discussed in detail elsewhere. Kauroo Hill, so far as is known at present, is unique in Eastern Otago in that basic igneous rocks of both Older and Later Tertiary age occur in close association separated by the trace of the “Miocene” peneplain.
The Cretaceous peneplain, sloping down from the summit of the Kakanui Range, is broken by a fault which forms the tectonic boundary of the Kakanui Valley adjacent to Kauroo Hill, the region beneath that valley and hill having been thrown down some 200–300 feet. The downthrown surface of the peneplain is for the most part concealed beneath Kauroo Hill, but on either side thereof the removal of the sediments covering the peneplain has brought into striking relief a fault-line scarp through which emerge the gorges of the superposed valleys of Kauroo River and Fuchsia Creek.
Resting on the peneplain beneath Kauroo Hill is a layer of quartz-grit about 50 feet thick and well cemented on the eastern side of the hill, but over twice as thick, feebly cemented or even friable on the north-western. This is probably the equivalent of the Limonitic Sandstone of von Haast (1877) and McKay (1887) and the Herbert Series of Brown (1938). It is followed by glauconitic mudstone. Where the contact of these two formations is exposed (at the point where the line of section of Fig. 3 crosses the little valley at the eastern foot of Kauroo Hill), the glauconitic mudstone appears to rest disconformably on the sandstone and its basal layer contains rolled pebbles of quartz up to two centimetres in diameter. Higher up, a layer of white weathered silt-stone and mudstone (exposed on the eastern side of Slaughter Creek) seems to be without trace of glauconite, though interbedded in the glauconitic mudstone, which, as a whole, may correspond with the Lower Greensand of McKay and the Otepopo Series of Brown. It is almost indistinguishable
[Footnote] * See footnote on p. 85.
from the Abbotsford Mudstone of the Dunedin District, and like it, exceedingly prone to land-slip movements. About 600 feet thick of this mudstone occurs below a sheet of dolerite-basalt, approximately 50 feet thick, forming the flat-topped northern and southern spurs of Kauroo Hill. This rock (5719) has the petrographic characteristics of the Older Tertiary dolerite-basalts as distinct from those displayed in the Late Tertiary basic lavas of comparable grain-size. The concealment of the lower margin of this sheet by creeping soil prevents decision as to its intrusive or effusive character, though the probability of the former is suggested by comparison with similar rock-masses near Moeraki.
The cap of Kauroo Hill is a steep cone rising about 280 feet above the surface of the doleritic basalt. It consists chiefly of finegrained occasionally vesicular iddingsitic nephelinite (5718, 5739) resting on a thin flow of slightly zeolitic doleritic feldspar-olivine basalt (5788). Both of these are described above, and are clearly comparable with Late Tertiary lavas. The lower flow is separated from the underlying dolerite by not more than a few feet thick of coarse quartzite gravel and brown chloritic sandstone, which is exposed on the north, east and south sides of the peak. The boulders seem to have been derived from locally silicified portions of the quartz-grits, such as rest on the Cretaceous peneplain on the upper parts of the Kakanui Range, and may be supposed to have been deposited as flood-plain gravels when the older dolerite was laid bare during the formation of the so-called “Late Miocene” peneplain, and may mark the approximate level of that peneplain in the neighbourhood of Kauroo Hill, namely about 650 feet above the Cretaceous peneplain. If that be so, the angle of inclination between the two peneplains is here rather less than four degrees.
The Late Tertiary lavas in the vicinity of the peak comprise not only the two masses mentioned, but also an extension of the zeolitic dolerite-basalt capping the hills to the east-north-east of Kauroo Hill (5738), and a mass of iddingsitic nephelinite forming the eastern buttress of the peak (5717, 5736, 5761, 5788), and thinner but still massive outliers extending nearly a mile further to the east and south-east (5714, 5740, 5770). The localities whence the several described specimens were collected are denoted by the two last digits of the corresponding specimen-numbers on the map (Fig. 3). The steeper slopes beyond these massive outliers are littered by drifted residual blocks of nephelinite.
The fact that the outlying masses of Late Tertiary lava rest in situ on the glauconitic mudstone two or three hundred feet below the assumed level of the Tertiary peneplain may have one of two possible explanations. The more probable hypothesis assumes that following a small uplift of the Tertiary peneplain, occurring before the Late Tertiary eruption had broken out, the older dolerite became the cap of a mesa rising above the surface which was quickly eroded out of the very weak surrounding mudstone. The Late Tertiary lava either broke through the older dolerite and flowed over its edge on to the surface of the surrounding mudstone (as indicated in Fig. 3), or, having been erupted from a vent at the eastern side of the
dolerite-capped mesa, accumulated against its flank until it flooded over its upper surface. The overlapping of an eroded scarp of Caversham sandstone by the early basaltic lavas at St. Clair may be recalled in this connection. (See Part I of this series, fig. 1.) The second variant of the hypothesis seems, however, on topographic grounds, to be less acceptable than the first, which is not without its difficulties. An alternative hypothesis holds that the Late Tertiary lavas on Kauroo Hill was derived from the vent adjacent to the fault-line on the western side of Fuchsia Creek, and flowed eastward on the Miocene peneplain covering and extending beyond the mass of older dolerite. Later, when uplift and erosion had allowed the excavation of the valleys of the Kauroo River and Fuchsia Creek, extensive mass-movements in the glauconitic mudstone resulted in the downward slumping of mudstones and their lava cover on the eastern side of Kauroo Hill. The margin of the older dolerite on this side is, on this view, a slump-scarp comparable with that occurring on the north-eastern face of Swampy Hill, near Dunedin (see Benson, 1940, fig. 5), except that a remnant of the subsided portion of the nephelinite remains tilted against this scarp to form the eastern buttress. This explanation, however, is difficult to reconcile with the formation of the valley of Fuchsia Creek in its present position, and cannot yet be supported on petrographic grounds, since the only specimen obtained from the Fuchsia Creek volcanic vent differs from any microscopically examined lava on or east of Kauroo Hill, in that it is apparently atlantitic in composition (5737). So far, however, only a very hasty examination of the Fuchsia Creek vent has been made, and further study would be desirable.
Whichever (if either) of these hypotheses be true, it seems clear that uplift following the Late Cainozoic eruptions caused the rejuvenation of the valleys draining the area, and the formation of the steeper lower slopes of Kauroo Hill down which have drifted masses of residual boulders, chiefly of nephelinite, accumulations of which cap and protect the lower spurs of the main hill. It also made possible the removal of the covering strata from the Cretaceous peneplain west of Kauroo Hill, and the superposition on to and incision into that surface of the streams which originated as consequent streams on the tilted surface of the Late Tertiary peneplain when the Kakanui Range was being lifted up to its present elevation.