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Art. 5.—On the Character of the Contact between the Ngaparan Beds and the Underlying Bed-rock.
[Read before the Otago Institute, 8th November, 1921; received by Editor, 12th November, 1921; issued separately, 1st February, 1923.]
An examination of the surface of the bed-rock on which the Ngaparan lignitic beds rest should afford much useful information as to the configuration of the ancient land—surface immediately prior to the deposition of the quartz drifts which constitute the lowermost sediments of that series. Unfortunately, in consequence of faulting and the presence of superficial deposits of Recent age along the line of contact, it has been difficult to gather trustworthy data on this interesting subject, and I can only give a few examples that have come under my own observation.
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At Cape Rodney, on the east coast of North Auckland, the fossiliferous breccias at the base of the Waitemata series of Oamaruian age rest on the uneven surface of a jagged ridge of Trio—Jurassic argilhtes and grey-waeke.
Farther south, at the west end of Motutapu Island* in the Waitemata Harbour, marine erosion has exposed a good section of the basement beds of the same Cainozoic formation abutting against and overlapping the Trio-Jurassic rocks which rise up somewhat abruptly above sea-level, forming a ridge that runs to the eastward. As the lowermost beds of the younger series are not uncovered, it is impossible to say how far they overlap the uneven bed-rock on which they rest; but the character of the material tends to show that no great thickness of strata exists below the breccia exposed on the beach.
At Puketawai, lying six miles east of Te Kuiti. a ridge of Middle Mesozoic rocks rises up through the Oamaruian coal-measures, thereby interrupting the continuity of the coal. Though the actual contact is not seen, boreholes have shown that the sides of the ridge are steep. Other outcrops of the bed-rock occur to the north and west, a circumstance which seems to show that the coal-measures in this area were deposited on a terrain dissected in the early Cainozoic into hollows and ridges. The floor of the same coal-measures at the old Taupiri workings as exposed along the outcrop is not rugged but gently undulating.
On the Mount Arthur tableland, Nelson, the Oamaruian formation is represented by a thin bed of gravelly conglomerate overlain directly by a horizontal sheet of hard limestone. Here the conglomerate rests on a fairly even peneplained surface of Palaeozoic rock.
Near Stony Creek, in the Takaka Valley, the same Cainozoic limestone lies directly on a remarkably even platform of Ordovician crystalline limestone.† To the north of this, in the Aorere Valley, isolated blocks of the Oamaruian limestone are underlain by quartz sands which rest on a peneplained surface of Palaeozoic slaty argillite.
In the upper part of Big Hill Creek, to the west of Big Hill, in the Awamoko Survey District, Oamaru, a sharp pinnacle of phyllite projects into the quartz sands of the Ngaparan lignitic series.‡J In the same area I recently discovered an isolated ridge of russet-brown semi-metamorphic Kakanuian schist projecting into the quartz sands of the lignitic series at a place a mile from Ngapara, close to the sand-pit facing the main road and railway to Oamaru. The outcrop begins at the south-east corner of Section 14.A, and passes into Section 15a of Block VII for a distance of 50 yards or more. It rises into the sands to a height of 40 ft. It was noted that the strike of the Palaeozoic rock is N.W.-S.E., and the dip, though this is somewhat obscure, to the south-east at high angles. The slope of the schist-ridge on the side presented towards Ngapara is steep. To the eastward the slope is gentle. The sands wrap around and ride over the ancient ridge, rising above it to a height of 180 ft.
At Port Craig, south of Mussel Beach, on the south-west side of Te Waewae Bay, Southland, the breccias, conglomerates, and limestones at the base of the Oamaruian are seen resting on a deeply eroded and uneven floor of diorite.§
[Footnote] * J. Park, Rep. Geol. Explor. during 1886–87, No. 18, p. 225, 1887.
[Footnote] †J. Park, Rep. Geol. Explor. during 1888–89, No. 20, p. 226, 1890.
[Footnote] ‡J. Park, N.Z. Geol. Surv. Bull. No. 20 (n.s.), p. 21, 1918.
[Footnote] §J. Park, The Geology and Mineral Resources of South-west Southland, N.Z Geol.. Surv. Bull. No. 23 (n.s.), p. 51, 1921.
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In 1906 Dr. J. M. Bell called certain uplands in Westland a “ fossil peneplain,” and’ in 1916 Professor C. A. Cotton described the even, gentle slopes descending from the uplands to the west towards the middle Kakanui Valley, and forming a conspicuous land-feature from almost all the higher ground around Oamaru, as a “fossil peneplain” or “fossil plain.” Dr. J. A. Thomson, F.G.S., has done the same, but I am at the moment unable to put my hand on the reference. Certain mountain areas in Central Otago have also been described by Professor Cotton as “ fossil peneplains.” The latter has on more than one occasion followed the Davisian nomenclature, which describes an ancient peneplain laid bare by the ordinary processes of denudation as a “ stripped peneplain.” This, I think, is a misuse of a common English -word in a way that probably conceals the real meaning of the users, and, moreover, seems to convey the subtle and erroneous inference that the surface of the peneplain as it now exists, is the same surface as that on which the covering sediments were laid down. Of all the processes of denudation there is none capable of uncovering a peneplain at such a rapid rate as to justify the use of the term “ stripped peneplain.” The uncovering of a peneplain must, from, the nature of the agencies involved in the work, be a slow process, conceivably spread over scores of thousands of years. To my mind the term “ uncovered peneplain ” seems to fit the conditions and circumstances of the uncovering with more accuracy than the term “ stripped peneplain.”
The use of the term “ stripped peneplain,” besides implying an action that we know to be untrue, like the term “ fossil peneplain,” conveys the impression that the uncovered peneplain as we now view it is the actual surface on which the cover of younger strata actually rested.
If the wastage or denudation of the covering strata proceeded at such a rate as to expose the orginal surface of the peneplain everywhere at approximately the same date, and if, after this event had come about, denudation were held in abeyance, then we should behold what was in deed and in fact a true “ fossil peneplain.”
The first condition is not impossible. If the covering beds consisted of weak unconsolidated sediments of fairly uniform character the rate of degradation might be fairly uniform over a wide area. But there is only one agent that could stay the progress of denudation, and that is a protecting covering of ice. Such an ice-sheet may have existed on the uplands during the Pleistocene, but clearly’ on the disappearance of the ice denudation would once more begin its activities.
The frequent tors of rock on the Barewood peneplain and high stacks on the table-topped summit of the Rock and Pillar and, Old Man ranges are an evidence of the progress of denudation since the Otago peneplain was uncovered. Moreover, they prove that the present surface of the relics of the ancient peneplain in Central Otago is not the original peneplained surface. For that reason I think that the term “ fossil peneplain ” or “ fossil plain,” though perhaps not so inaccurate as the term “ stripped peneplain,” nevertheless connotes a condition that from the nature of things must be almost impossible of attainment.
The character of the rock-floor on which the Oamaruian rests possesses a far-reaching significance. When many observations are placed on record as to the contour of the floor on which the basement beds of the Oamaruian rest we shall be able to speculate on the configuration of the maritime lands of older Cainozoic times with a confidence we do not now possess.