The Age And Distribution Of The Coal Measures In North Auckland
History of the Classification of Strata.
The table of classifications (Fig. 1) show the progressive interpretations of the earlier workers. In the left-hand column are the various beds in the sequence given by Ferrar (1925, 1934), numbered 1 to 9 from the base upwards, commencing with the Otamatea Series of Cretaceous age. The Onerahi Series he regarded as Upper Cretaceous, possibly extending into the Eocene, and the Whangarei Series as Oligocene.
In the other columns are the relative sequences as quoted by earlier writers, and shows their placing of the beds. The table is necessarily rough, since correlations were largely based on lithology, but shows clearly what may happen when this course is adopted. Those beds classed as Cretaceous have been placed opposite the Onerahi beds, and those of the Cretaceo-Tertiary group opposite the Onerahi beds. Park first put the Whangarei Series up into the Oligocene in 1903.
The coal measures were generally believed to be at the base of the sequence, but the relative position of the Whangarei and Hydraulic lime ston members was never agreed upon. McKay (1884) described Hydraulic limestone as overlying the Whangarei limestone in one of the drill-holes at Kawakawa. The Hydraulic limestone was correlated with the Cretaceous Amuri limestone, and the underlying greensand with that above the coal measures, but the fact that Tertiary fossils had been obtained from the greensand, for example, at Pahi, was ignored.
Hutton (1889) rejected the correlation of the Hydraulic with the Amuri limestone on fossil evidence, and placed the coals in the Oamaru Series. Park was not satisfied with the sequence, and revisited the northern areas, as a result of which he published a sequence (1903) similar to that of Ferrar's. He correlated the Pahi greensand with that overlying the coal measures, but gave no explanation for the superior position of the Hydraulic limestone there. He states that he interviewed the drilling superintendent at Kawakawa and was definitely informed that no Hydraulic limestone was cut in the case of the drill-hole mentioned by McKay (1884). He did not state whether the beds concerned were calcareous or not, but he describes them as clays of various sorts. He considered that McKay had correlated
them with Hydraulic limestone nearby that were thought to overlie the Whangarei limestone, and named them accordingly. McKay states in his report that the claystones were calcareous, so there is conflicting evidence on this point. The description given agrees with that of claystones that the author has seen to overlie the Whangarei limestone there and elsewhere, but which, have not been calcareous.
Ferrar does not refer to Park's paper in the text of his bulletins, and later Marshall (1916, 1924) continued to regard the Hydraulic limestone as overlying the Whangarei limestone. Marshall, Speight, and Cotton (1910) discussed the younger limestones of New Zealand and agreed that they were probably Oligocene, though Marshall later (1916) placed them as Miocene after petrological examinations. Later still (1924) he pointed out that the Hydraulic limestone at Pahi and other places lies on a greensand, which he regarded as probably Eocene and the limestone therefore not older than upper Eocene.
Ferrar considered the Hydraulic limestone at Pahi to be older than the underlying greensand, and postulated overthrusting, though this was later denied by Henderson (1936). Bartrum (1937) more recently supported Ferrar, and has described the Pahi section in detail. A recently published account of the Kamo coalfield by Hay (1947) follows Ferrar's mapping closely.
Distribution of the Coal Measures.
Fig. 2 shows the distribution of the greywackes and argillites usually regarded as Triassic and Jurassic in age, and known as the Waipapa Series in North Auckland. Their distribution is along a strip curving from north in the south to north-west in the north, and from Auckland south they strike parallel to this axis of distribution. North of Auckland few strikes
have been obtained, and in various places the Waipapa Series as mapped contains mudstones, sandstones and quartzites that do not belong in this group, but it is likely that strikes in the true Waipapa rocks will be found to be parallel to their distribution. There are additional outcrops in the far north, but as mapped in the old reports they include igneous rocks, and some of these have been omitted. The younger included rocks usually strike a little north of east, and the quartzites, notably at Ruapekapeka and Puhipuhi, form prominent ridges.
The distribution of the coal measures with coal along the western margin of this greywacke strip, and its extension south is striking. The coal measures are apparently found where the true Waipapa rocks constitute the basement, and usually dip west off the basement. The exception is the coal seam at Avoca, west of Whangarei, where the greywacke basement is not known to outcrop in the vicinity.
The coal measures consist of conglomerates and quarts gilts with coal seams, and pass up into greensands or calcareous sandstones, overlain by the crystalline Whangarei limestone member. Capping these are claystones that may have wider distribution than as mapped by Ferrar. These beds he has mapped as his Whangarei Series.
West of the greywacke area the peninsula consists of claystones, sandstones, and limestones intruded by dolerites in the north and serpentines in the south, and overlain unconformably by sandstones of the Waitemata Series. The former group Ferrar has mapped in his Otamatea, Onerahi and Whangarei series. He considered the Whangarei beds to be less folded than the two underlying groups, but although they are little disturbed in the eastern area where they rest on a greywacke basement, they appear to be well folded farther west, for example in the Pahi district.
Geology of the Coalfields.
At Kawakawa the Whangarei beds strike north-west and dip gently south-west. The coal measures are overlain by green sandstones grading laterally into dark grey calcareous sandstones, and these pass up into white crystalline limestone, well exposed at Waiomio. The coal thins to the dip and south, and to the north either cut out against gravels or became too wet to work. Green and i ed claystones overlie the limestone at Waiomio and again in the northern part of the field, but younger surface clays obscure the contacts. A mile and a-half south of Kawakawa these claystones outcrop in the head of a gully tributary of the Waiharakeke Stream, and a short distance down to the west in the gully are loose blocks of the Whangarei limestone resting on indurated claystones with Hydraulic limestone, mapped with the Onerahi Series by the author (1946) along with Hydraulic limestone outcropping in the bed of the Waiomio East Stream, and claystones to the west.
Whether the indurated claystones and hydraulic limestone belongs to the Whangarei or Onerahi Series is a problem of importance both as to the stratigraphy and the coal distribution. McKay considered that they overlie the Whangarei limestone, in which case the coal measures could have an extension to the west, but the author mapped them with the Onerahi Series and postulated a fault separating them from the coal-bearing area to the east. Park stated that they did not overlie the Whangarei limestone.
The coal measures are less than 50 ft. thick, the sandstones 200 ft., and the limestone also 200 ft., but its thickness beneath the claystones is irregular, due, in the author's opinion, to an unconformity. In the central area a fine conglomerate with pebbles of claystone and weathered greywacke unconformably overlies the sandstone in the road cutting on the main road north of the bridge over the Waiomio West Stream. Claystones, incorrectly mapped by the author (1946) in the Onerahi Series, occur at higher levels immediately to the east, but it is not known definitely if these overlie the conglomerate, though they appear to do so.
The distribution of the Hydraulic limestone and associated claystone is consistent with its being younger than the Whangarei limestone, but it has been mapped in the Onerahi Series, and is considered to be faulted into position. The faulting is probably older than the Whangarei claystones. The evidence for the faulting is the presence of fault breccia on the main road near the Waiomio Stream and the occurrence of the loose blocks of Whangarei limestone resting on the Hydraulic limestones and associated claystones. This is consistent with similar faulting observed by the author at Ruatangata and Whangarei, where the evidence is clearly established by foraminiferal determinations.
The most recent drill-hole sunk in the Waiomio area passed through 4 ft. of green claystone between the coal measures and the greywacke basement. It is important to determine the age of this claystone, as to whether it belongs to the Onerahi Series or an older one.
At Hikurangi the sequence is similar. The coal beds are up to 70 ft. thick, the greensands usually less than 25 ft., and the crystalline limestone up to 350 ft., but the thickness varies greatly an the drill-holes due to the unconformable relation of the overlying claystones. These were mapped in places with the Onerahi Series by Ferrar (1925), but the coal has been mined from the areas beneath them. The coal thickens to the dip to the
west as far as worked, but conditions were too wet to follow the coal to its limit. In Wilson's Colliery Ferrar reports 100 ft. of claystone beneath the coal measures, but queries the information. Where the basement is seen the coal measures rest on greywacke.
At Kamo the coal measures are at least 50 ft thick and rest on greywacke where the basement outcrops, but none of the drill-holes penetrated to the basement. The greensand is just under 200 ft. thick, and the limestone up to 160 ft., but again extremely irregular in thickness, owing to the unconformity separating it from the overlying claystone and sandstone, which have a maximum recorded thickness of 270 ft. in the drill-holes. Two drill-holes (Nos. 4 and 5 of Hay, 1947) cut sandy conglomerate between the coal measures and the greensand above.
The claystones at the north-east of the field have been mapped by Ferrar and Hay with the Onerahi Series, but the author considers these probably overlie the Whangarei limestone. Hay's section (through drill-holes 2, 5, and 13, opp. p. 173, loc. cit.) clearly shows that drill-hole 13 was inconclusive. It is unlikely that a claystone slope could have persisted during the deposition of all the Whangarei beds, and should the claystones at 13 belong to the Onerahi Series, the fault postulated by Ferrar would be the more likely interpretation. Some miles to the west, at the Ruatangata lime-works, the claystones unconformably overlie the Whangarei limestone. The loose blocks of limestone occurring at Kawakawa, Ruatangata and elsewhere may have been residual before the deposition of the claystones.
Recent Age Determinations in North Auckland.
The following age determination was made by Dr. Finlay from formaminiferal rocks forwarded to him during the author's stay in North Auckland:—
Black Shale, Mangonui area; Tapuwaeroan.
Calcareous mudstone, half a mile south of Whangarei Hospital; Te Urian.
Specimens collected by Mr. C. A. Fleming from drill-holes at White Hills, Silverdale; mixture of faunas from Whangai to Bortonian.
Hydraulic limestone, Weiti Stream, one mile west of Silverdale. Soft limestones beneath Kaihu sands, from drill-holes at Mangawhare, Dargaville; three specimens.
Hydraulic limestones from 1,400 ft. drill-hole, Ruawai; specimens from various depths.
Calcareous mudstone, main road between Warkworth and Waiwera. three-quarters of a mile south-cast of Trig G. Calcareous shale, old lime-pit, Redvale.
Greensand, Pahi, beneath Hydraulic limestone, north contact. Marly mudstone from dyke in Whaingaroan sandstone, Pahi.
Greensand, 100 ft. below Whangarei limestone, Whangarei Hospital. Greensand below Whangarei limestone, Ruatangata, three specimens, one immediately below the limestone.
Greensand beneath crystalline sandy limestone and banded calcareous sandstone, Skelton's Hill, Paparoa.
Greensand beneath white crystalline limestone, Okokaho, north-west of Kawakawa (could possibly be Bortonian).
Dark-coloured mudstone, main road, Waiare.
Banded sandstones, Pahi, three-quarters of a mile north-west of township.
Greensand immediately beneath Whangarei limestone, Whangarei Hospital.
Light-coloured sandy limestone from old quarry, Dairy Flat, resting on dark grey shales.
Harrington (1947) has also mentioned limestones considered to be Upper Cretaceous, Bortonian and Whaingaroan, from the Hokianga district.
Thus we have a number of previously undetermined Cretaceous rocks, some of which have been mapped in the Onerahi group, and their relationship to the overlying Tertiary beds is unknown. The Bortonian faunas have been obtained largely from argillaceous limestones of the Hydraulic type, and in one case from a greensand beneath. There appears to be widespread limestone or limestones of this stage, and these will constitute the Onerahi limestone proper.
No Kaiatan rocks have turned up so far, and it appears likely that this break marks the first folding of the Onerahi beds and their erosion off the greywacke blocks to the east, since although they extend to the western margin of the greywacke the younger Whangarei and Waitemata beds usually overlap directly on to the older rocks over their stripped surface. However, assuming the Onerahi limestone to be Bortonian, the overlying claystones and shales may extend up into the Kaiatan, and the orogenic movements would have then to be limited to a small portion of Kaiatan time and/or Runangan time.
The Runangan greensand horizon has been found in widely spaced areas, and usually is associated with overlying crystalline limestone. These are the oldest rocks found above the coal measures. In one place the greensand extended up into the Whaingaroan, and in another was still Runangan immediately beneath the limestone, so that the limestone will be likely to differ in age slightly from place to place, at the base, at any rate. The limestone was probably not continuous, and the lithology varies.
The claystone above the Whangarei limestone appears to have been laid down after faulting along the margins of the greywacke area, since in two places Cretaceous beds have been found faulted against rocks of the Whangarei Series, and the claystones seem to transgress the two. They are possibly the equivalent of Fellar's top Whangarei sandstones and the Waitemata beds.
There is thus a well-developed Bortonian limestone of the Hydraulic type, and a crystalline limestone, known as the Whangarei limestone, which overlies greensands of Runangan, and in one case Whaingaroan age. This clearly establishes the relative ages of the Onerahi and Whangarei limestones, but there are limestones of the Hydraulic type comparable in age with the Whangarei limestone in the western area.
Conclusions on the Age of the Coal Measures.
The coals usually overlie the Waipapa Mesozoic rocks, but drill-hole information shows that they also rest on claystones in part, and it is important that the age of these claystones be ascertained. The coals are overlain by greensands and sandstones, from which no age older than Runangan has so far been determined. Information is required as to the age of the greensand immediately above the coal.
The coals have been regarded usually as the basal part of the Whangarei group, and would in that case be basal Runangan or even Kaiatan. Coals in the South Island associated with quartz gilts are now known to be Cretaceous in age, even in cases where overlain by Tertiary rocks, and it is necessary to consider whether this may be the case with the North Auckland coals, as this would have an important bearing on their distribution.
Although the coal measures have been found in widely spaced areas, there have not so far been found any Cretaceous or Onerahi rocks between them and the Whangarei greensands. On the other hand, the basal Whangarei beds have been brought to the surface by folding and erosion in the western area, but no coal measures have been found associated with them, and this also holds in the case of the Onerahi Series. The greywacke does not come to the surface in the western area, suggesting either that the folding of the Tertiary beds was not great, or that the Cretaceous beds are thick. The latter appears more likely.
If the theory of load metamorphism of coals as postulated by Wellman (1947) be applied to the North Auckland area, a thickness of overlying sediments is implied amounting at Kawakawa to 7,000 ft., at Whangarei a little less, and probably more to the east along the axis of the greywacke block, which would then have been a geosynclinal area. Little is known of thickness in the area. The Whangarei greensands and limestone seldom exceed 400 ft.,
and the overlying claystones are of unknown thickness, though a late Tertiary peneplanation that completely truncated the younger rocks of the North Auckland Peninsula may have removed a considerable thickness of these claystones and sandstones. Ferrar gives a total thickness of 1,500 ft. for the Onerahi group of beds, yet at Motatau the author has described a limestone member thicker than 1,500 ft. The thickness of Cretaceous beds beneath the Onerahi Series is unknown. A group of claystones south-east of Kaikohe are sufficiently indurated to be used as road material on secondary roads, but the age of these is not certain. However, consideration of the rank of the coals suggests that there were 7,000 ft. of sediments overlying them at one time, either before or after the Whangarei beds were laid down, if the coals are Cretaceous, or after if the coals are younger than the Onerahi beds.
There are thus two important problems concerning the coal measures of North Auckland. The first is to determine their age. Those in the eastern portion appear to be the same age, and there is probably no good reason to believe that the age of the coal at Avoca is different. It may well be of the same age, but have had less sedimentary cover. Foraminiferal evidence should be sought from the beds above and below the coal measures, especially from the limestone and greensand members. Lithologic correlation of claystones may waste considerable time. Drill-holes shoud be sunk in critical areas, notably in the Kawakawa area west of the fault postulated by the author.
The second problem is the possible distribution of the coal, largely dependent on the solution of the first. Since coal has been found beneath claystones formerly mapped with the Onerahi beds, close remapping of the areas west of the greywacke area is required. If the coals are younger than the Onerahi Series, the fact that they do not outcrop to the west, except at Avoca, suggests that they do not extend far from the greywacke area, but if they are Cretaceous, their possible distribution is at present completely unknown.
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