The Geology of the Whatawhata District, South-west Auckland
M. G. Laird
By
Geology Department, University of Auckland.
[Received by the Editor, 29 March 7966.]
Abstract
The area investigated consists of a basement of Trias-Jura rocks of the Hokonui Facies, overlain unconformably in the east by the lower Tertiary Te Kuiti Group. The Mesozoic rocks constitute part of the eastern limb of the Kawhia Syncline, and comprise approximately 4,000 ft of Upper Triassic sediments representing the Otamitan, Warepan and Otapirian Stages, and nearly 6,000 ft of Lower and Middle Jurassic strata included in the Aratauran, Ururoan, Temaikan, and Heterian Stages. Lack of key fossils in the Ururoan and Temaikan stages resulted in these two stages being mapped together. Only the Warepan Stage proved to be abundantly fossiliferous, many different forms of Monotis being recognised. As a result of penecontemporaneous slumping, Warepan sequences with Monotis in the Whatawhata district are disturbed and show considerable variation from sequences recorded elsewhere. Additional evidence for slumping in both Warepan and Otapirian strata is provided by the presence of angular discordances with welded contacts.
Most of the Mesozoic rocks are indurated siltstones, although there are also rare bands of sandstone and, in the Middle Otapirian and Lower Ururoan, thin bands of fine conglomerate. Almost all the coarser Jurassic sediments contain vitroclastic material, which has been zeolitised subsequently to deposition. The secondary mineral suite is characteristic of the heulandite zone of the zeolite facies of regional metamorphism.
Tertiary rocks are restricted to the eastern half of the district, and are represented by the lower part of the lower Tertiary Te Kuiti Group. Formations present are the Waikato Coal Measures, Mangakotuku Siltstone, and Glen Massey Formation, reaching a maximum thickness of approximately 370 ft. The significant changes in thickness of these formations, both within the area and in comparison with the type section 15 miles to the north, have resulted probably from the formation of small local tectonically controlled basins of deposition.
Both the Tertiary beds and the underlying Mesozoic rocks have been disturbed by faulting during the Kaikoura Orogeny. There are two main directions of faulting. The major faults have an approximately meridional strike and apparent throws of up to 4,500 ft, while numerous subparallel smaller faults branch from the meridional faults at bearings ranging from 030° to 060°, and with throws varying up to 300 ft. A few faults have strikes at variance with the two major directions. Recognition of faulting is relatively simple where Tertiary beds are disrupted, but determination of the presence and throw of faults wholly within the Mesozoic beds depends largely on the displacement of a prominent Otapirian conglomerate band. The major fault (the Kapamahunga Fault) is regarded as mainly clockwise-transcurrent, but tilting of the country to the east of the fault through 2°-3° indicates that there is also a vertical component. Nothing is known of the sense of movement of the other meridional faults apart from the Maungakirikiri Fault, which dislocates Tertiary strata and shows scissors movement. The smaller faults striking between 030° and 060° are believed by the writer to be normal in nature.
Assuming that all the faulting was induced at approximately the same time, a PHS direction of 045° during the Kaikoura Orogeny is indicated for the district.
Introduction
The area investigated, covering approximately 26 square miles, lies south-west of Whatawhata, South Auckland (Fig. 1). The terrain consists of deeply dissected hill country rising to form part of Kapamahunga Range in the east of the district, where summit elevations commonly exceed I,oooft. Most of the area is covered with native bush or second-growth scrub and fern, the remainder being pasture. Field work was carried out as part of the requirements for the Master of Science degree during the period February, 1960, to February, 1962.
Geologically, the area consists in part of a basement of Mesozoic rocks of the Hokonui Facies lying structurally on the eastern limb of the Kawhia Syncline; this forms the western and central portions of the area. To the east this basement is overlain unconformably by lower Tertiary strata (PL 2).
Several difficulties, due in part to the stratigraphic and structural position of the basement rocks but also to the physiography of the district, were encountered in mapping the Mesozoic succession. Fossils are comparatively rare, this being characteristic of the east flank of the Kawhia Syncline (Wellman, 1956), and lithologies are generally monotonous, with few coarse sediments that can be used as marker beds. The sequence is also confused by faulting. Outcrops are discontinuous and commonly highly weathered, except those in the beds of deeply-cut streams, mainly in the central portion of the area. In the eastern and south-eastern portions of the district the Mesozoic succession is partly concealed by overlying Tertiary strata, and in the west low-lying ground with infrequent and poorly exposed outcrops and areas of dense bush make mapping difficult. The map shows the stages defined by Marwick (1951), the diagnostic fossils of most stages being found. Where these are absent lithology and structure have been used to infer the positions of stage boundaries. No fossils confined to the Ururoan or the Temaikan Stages have been found, but other evidence suggests that these stages are present in the Whatawhata district.
No difficulties were experienced in lithologic mapping of beds of Tertiary age, and the formations of Kear and Schofield (1959) have been adopted.
Previous Geological Work
No previous work has been published specifically on the Whatawhata area, although it has in the past fallen within the scope of much wider surveys. Between the years 1864 and 1877, Hochstetter, Hutton and Cox referred to portions of the district in separate reports, but no systematic mapping was done. No further work was carried out until Henderson and Grange (1926) examined the area as part of their survey of the Huntly-Kawhia Subdivision. They recognised portions of the Mesozoic succession as Upper Triassic, Lower Jurassic, Upper Jurassic, and Lower Cretaceous, but no time or lithologic boundaries within the Mesozoic beds were drawn on their accompanying maps. The lower Tertiary succession was divided into “ Whaingaroa beds ” and “ Te Kuiti beds ”.
Rear and Schofield (1959) included the Tertiary strata of the Whatawhata area in a revision and re-definition of lower Tertiary beds of South Auckland. A wider definition of the stratigraphic term “ Te Kuiti ” was adopted, and the Waikato Coal Measures and Whaingaroa beds were included with the limestone in the Te Kuiti Group.
On the 1: 250,000 geological map of the Hamilton area (Rear, 1960), the Mesozoic rocks of the district were divided on the basis of the time-stratigraphic terminology introduced by Marwick (1951; 1953).
Mesozoic Stratigraphy
Introduction
The Mesozoic rocks exposed in the Whatawhata district are between 9,000 ft and 10,000 ft thick. Approximately 4,000 ft are Upper Triassic and the rest Lower and Middle Jurassic in age.
Outcrops are commonly highly weathered, except where exposed in deep gorges, so that steep slopes are highly unstable and numerous slips and earth flows occur throughout the district. As the overlying Tertiary strata are not comparably affected, the deep weathering of the basement rocks probably resulted from prolonged exposure during the period of erosion and peneplanation following folding of the Mesozoic strata in the early Cretaceous.
Both Jurassic and Triassic sediments are almost entirely of siltstone grade, true mudstone being rare. Mud-flecks, averaging about l/10in long and lying in the plane of the bedding, are common throughout, often associated with fossil remains. These structures probably resulted from the activities of boring organisms when the sediments were being laid down (Moore and Scruton, 1957; Weller, 1960).
Minor sandstone bands occur both in the Triassic and Jurassic rocks of the area, but coarser sandstone is restricted mainly to the Jurassic. The few conglomerates are nowhere as coarse or as thick as those exposed on the western limb of the Kawhia Syncline at South Kawhia (Macdonald, 1951) and at Marakopa (Marwick, 1946).
Bands of spheroidally weathering sandstone and siltstone are more common in Jurassic than Triassic beds. Carbonaceous material is limited to the Jurassic beds, in contrast with the findings of Grant-Mackie (1959) in the Awakino-Mahoenui area, where plant fragments are much more prevalent in the Triassic than in the Jurassic. Nodules and crystals of pyrite were noted in both Triassic and Jurassic beds. Many of the Jurassic sediments, particularly the coarser ones, are highly tuffaceous, but vitroclastic material, except in one band in the Otapirian, is generally absent from Triassic rocks. The presence of secondary quartz and minerals
of the heulandite-clinoptilolite series, largely formed at the expense of intermediate volcanic glass in tuffs, combined with the presence of montmorillonite indicates that the rocks belong to the heulandite zone of the zeolite facies of regional metamorphism (Coombs, 1960). It is noteworthy that in the Triassic sediments the absence in bulk of suitable parent materials appears to have restricted the development of secondary zeolitic minerals. By contrast the younger Jurassic sediments, which contain numerous tuffs, also have a higher proportion of zeolites.
Because of lack of continuity of exposures in the district the stratigraphic column presented (Fig. 2) is necessarily composite; it is based on the study of sections in Maungakirikiri Stream and its tributaries and the headwaters of Raratawa Stream. The beds are disturbed by faulting, and recognition of the full succession and accurate calculation of thicknesses have been found to be virtually impossible.
Pre-Warepan Rocks
As only fossils common to both Oretian and Otamitan Stages were found in the Whatawhata area, it was not possible to differentiate pre-Warepan rocks. Therefore, of necessity, these two stages are treated together.
In the Whatawhata district rocks of pre-Warepan age occupy a north-south-trending strip in the eastern portion of the area (Fig. 2) representing an apparent maximum thickness of approximately 4,000 ft. The beds are highly indurated ironstained sandstones and siltstones where they are best exposed in the central portion of the strip. The poor bedding and high degree of jointing in these rocks, coupled with the presence of faulting, make determination of dips and strikes uncertain, but the rocks of this age appear to be exposed on both flanks of an anticline.
In the area where the oldest rocks are best exposed, south of Old Mountain Road in the headwaters of Maungakirikiri and Raratawa Streams, the axis of the anticline is probably concealed beneath Tertiary strata, and an accurate thickness of pre-Warepan beds cannot be determined. Approximately 2,000 ft of east-dipping strata appear to the east of the Tertiary cover in the headwaters of Raratawa Stream, whilst approximately 1,200 ft of strata, dipping to the west, are exposed in the upper reaches of Maungakirikiri Stream. Taking into account the 700-800 ft of rocks concealed beneath the Tertiary beds and assuming that the pre-Warepan beds have not thickened appreciably on the eastern flank of the anticline, the true thickness of strata of this age is of the order of 2,000 ft.
The pre-Warepan beds appear to extend beyond the eastern margin of the area, and Halohia has been recorded in a quarry adjacent to Heddons Road, immediately outside the eastern boundary of the district studied (ref. N.Z.G.S. Fossil Record form N65/503).
The only pre-Warepan fossils found were collected in the headwaters of Maungakirikiri Stream, 600 ft below the lowest Monotis beds. Here 50ft above the stream (N65/602)*, casts of ?“ Rhynchonella” sp., Hokonuia sp., and Halobia sp., sparsely distributed in red-brown siltstone, were collected.
In Southland (Campbell and McKellar, 1960), in the Awakino-Mahoenui area (Grant-Mackie, 1959), and in the Mokau area (Henderson and Ongley, 1923) thick unfossiliferous strata separate shellbeds of Monotis (Warepan) from shellbeds of Manticula (Otamitan), so that it is not unexpected that Manticula does not appear. Hence no beds older than Otamitan may be present.
Warepan Stage
Monotis (Entomonotis ) richmondiana Zittel* has long been regarded as the key fossil for the Warepan Stage, and the name has often been previously used for all specimens of the genus found in New Zealand, although Trechmann (1918: 191-6) recognised ten different forms. More recently Marwick (1953: 58) has described the new species Monotis (E .) calvata, and Avias (1953) and Brulez (1957) have described new forms from New Caledonia. Grant-Mackie (1959) recorded the presence in the Awakino-Mahoenui area of species and subspecies other than richmondiana, some of which he used to subdivide the stage. Campbell (1959: 199-200) has defined the Warepan Stage as “those beds laid down at the type locality after the appearance of Monotis richmondiana Zittel and before the appearance of an Otapirian fauna (including Spiriferina ( Rastelligera) diomedea Trechmann)”. However, in view of the numerous forms of Monotis occurring in the Warepan and the fact that the genus Monotis is usually regarded as being restricted to the Warepan, it would seem preferable to define the Warepan stage on the incoming of the genus rather than choose any one species. That the species Monotis (E .) richmondiana does not always represent the incoming of the Warepan stage is suggested by the non-appearance of that form in the western Awakino Gorge sequence described by Grant-Mackie (1959). Instead, what that author believes to be lower Warepan forms are represented by Monotis ( E .) ochotica densistriata. It is possible, however, that the non-appearance of M. (E .) richmondiana may be explained by the presence of a fault at the base of the sequence. The apparently widespread occurrence of slumping in Warepan strata {infra) can also lead to the local absence of the key fossil, a fact observed in at least one sequence in the Whatawhata area. For this reason and those outlined above, the base of the Warepan stage in this district has been mapped on the incoming of the genus Monotis.
In the Whatawhata district (and Maungakirikiri) Warepan beds form two distinct belts owing to repetition by the Kapamahunga Fault. The eastern belt, although overlain by Tertiary strata for much of its course, can be traced in a discontinuous north-trending fossiliferous band from a tributary of Kaniwhaniwha Stream, in the SE of the area, to a farm road in the NE, where outcrops containing Monotis have been reported (Dr D. Kear, pers. comm.).
The western band of Monotis could be followed with less certainty, as only two localities were found, both lying less than 200yds west of the Kapamahunga Fault in the central portion of the area. However, the position of the two localities suggests that this belt trends north-south, approximately parallel to the Kapamahunga Fault.
Rocks of Warepan age in the Whatawhata district are almost entirely of siltstone grade, the only occurrence of coarser material being south of Old Mountain Road (N65/606). Here the Monotis shell-bed is underlain by a few inches of fine, lightcoloured sandstone containing abundant fine mud-flecks.
Green mud-pellets, possibly similar to those noted by Campbell (1959) in basal beds of the type Warepan in Southland, are scattered sporadically throughout the Warepan sequences exposed in a NNW-flowing tributary of Maungakirikiri Stream, north of trig New 800. Comminuted shell fragments in Monotis beds exposed in Maungakirikiri Stream (N65/604) suggest strong wave or current action during deposition.
Fossils are common at numerous horizons, many in shell-beds varying from several inches to two feet in thickness. Many beds consist almost entirely of casts of separate valves of Monotis set in a silty matrix.
Beds containing Monotis crop out at many localities, but only one extensive sequence across the strike was seen. Although this is incomplete it has been recorded in detail for the purpose of comparison with similar sequences elsewhere.
Cooper Creek Section. Monotis- bearing siltstone crops out discontinuously for 150 yards in the headwaters of Cooper Creek. The lowest beds are partly concealed by Tertiary strata; so the full extent of the Monotis succession is not known.
Strikes vary from 085° at the base of the section to 170° at the top, and dips also vary widely, in some cases being reversed. Much of this variation can be attributed to penecontemporaneous slumping (infra) , but slickensiding and shearing in the lowest beds exposed indicate that faulting may also have played a part. The section and faunules are shown in Figure 3.
Other Monotis -bearing Localities. The only other area where the relative stratigraphic positions of beds with various species of Monotis could be determined, even roughly, was in a south-flowing tributary of Kaniwhaniwha Stream which joins the main stream 50 chains NE of the old Karamu limeworks. Three poorly exposed and widely separated outcrops of weathered siltstone were examined in this stream. The stratigraphically lowest outcrop examined (N65/526), exposed as a highly weathered bedding plane striking 355° and dipping 65° west, contained Monotis ( Entomonotis) richmondiana cf. hemispherica (Trechmann) ; M. ( E .) ochotica aff. posteroplana Westermann; and M. (Monotis ) cf. salinaria (Schlotheim). A collection could not be made from a highly weathered outcrop 10 chains upstream from the last locality, but the specimens present were observed to be strongly ribbed. From an outcrop 25 chains NNW of and stratigraphically approximately 480 ft above N65/526, with strike 280° and dip 42° SW, sparsely distributed specimens of M. ( E .) richmondiana were collected (N65/524).
In the bed of a north-flowing tributary of Maungakirikiri Stream, NNW of trig New 800, strata crowded with Monotis are well exposed for about 30 chains. However, as the strike of the beds is followed approximately by the stream and the fauna at most points along the stream proved to be similar, specimens were collected from two localities only (N65/604 and N65/605). Shellbeds were in general found to be relatively thin, thickness varying from 2in to 2ft, but all were highly crowded, the dominant fossil being Monotis ( Entomonotis ) calvata. Shell material was often retained, but the limited amount of siltstone matrix present was not noticeably calcareous.
From N65/604 were collected the following fossils: Monotis ( Entomonotis) calvata Marwick, M. ( E p achy pleura Teller, M. (E.) richmondiana hemispherica (Trechmann), Monotis (Monotis ) ?aff. salinaria (Schlotheim). Loc. 605, situated 15 chains downstream from loc. 604, appears to occupy a slightly higher stratigraphic horizon than the latter locality, and siltstone occurring here contains numerous casts of Monotis ( Entomonotis ) ochotica aff. posteroplana Westermann, and rare specimens of M. (E.) ochotica ?gigantea Avias. At the junction of the tributary with Maungakirikiri Stream a strongly-ribbed variety (not collected) occurring abundantly in a weathered clay bank confirmed the continuation of the Monotis-hearmg strata.
Ten chains SW of Old Mountain Road summit (N65/606) Monotis (Entomonotis) richmondiana Zittel and M. (E.) pachypleura Teller were collected from a highly weathered fine sandstone and siltstone exposed in a fault plane. The stratigraphic position of this locality is unknown. Kear (pers. comm.)
recorded Monotis ( E .) richmondiana from a cutting in a farm road north of the Hamilton-Raglan Main Highway, but no specimens were collected from this locality by the writer.
In the western Warepan band only two fossil localities were discovered. In a tributary of Maungakirikiri Stream, the stratigraphically lowest Monotis- bearing
outcrop seen (N65/522) contained casts of Monotis ( Entomonotis ) ochotica ? gigantea Avias, and M. ( E .) ochotica aff. posteroplana Westermann in highly sheared indurated siltstone. In highly weathered siltstone 30 yards upstream and approximately 80ft stratigraphically above N65/522 the only fossils found were M. (E.) ochotica cf. gigantea Avias, and M. sp. indet.
Half a mile north of the Hamilton-Raglan Main Highway, highly sheared siltstone with occasional deformed casts and fragments of Monotis was seen in the bed of Tunaeke Stream. Most specimens collected from this locality (N65/607) were not specifically determinable, the only species recognised being Monotis (Entomonotis) richmondiana Zittel. From the relative positions of the above two localities, it appears that the fossiliferous beds N65/607 occupy a lower stratigraphic position than those from which N65/522 was collected.
Correlation
Grant-Mackie (1959) divided the Warepan beds of the Awakino-Mahoenui district into Lower, Lower-Mid, Upper-Mid, and Upper, by using a sequence of Monotis species and subspecies. He described two similar successions from different localities, but the sequences are not borne out in the Whatawhata area. For example Grant-Mackie assigns M. ( E .) calvata to the Upper Warepan, and M. (E.) richmondiana and M. (E.) densistriata to the Lower-Mid Warepan; these three are found in one sft bed in Cooper Greek (Fig. 4). Furthermore, GrantMackie’s Upper-Mid Warepan forms are found 135 ft above the supposedly Upper Warepan M. {E.) calvata, and at N65/604 this species appears to be close to the bottom of the Monotis beds, not at the top.
M. (E .) richmondiana and M . ( E .) densistriata, which were both assigned to the Lower Warepan, though not found together by Grant-Mackie and thought by Trechmann (1918: 192) to be significantly different in age, have been found together as noted above.
In view of the convincing evidence of submarine slumping {infra) in the Warepan beds collected by the writer, it is evident why sequences examined in different districts or a different position along the strike in the same district do not bear close comparison. It appears from the foregoing that the similarity between the two sequences investigated by Grant-Mackie (1959) in the Awakino-Mahoenui district is only coincidental, and that author now believes his recorded sequences are slumped (Grant-Mackie and Lowry, 1964). Long-range detailed correlation cannot be made, and accurate subdivision of the Warepan Stage cannot be attempted until an unslumped sequence is examined.
The true thickness of the Warepan Stage in the Whatawhata district could not be determined with accuracy, as no complete sequence was seen. The greatest thickness of almost continuous Monotis-hearing strata investigated (in Cooper Greek) was approximately 380 ft, and the distribution of Monotis localities in a tributary of Kaniwhaniwha Stream suggests a thickness of about 430 ft. In the Whatawhata district the Warepan Stage is considered to have little greater extent than Monotis- bearing strata (see under Otapirian Stage) and its thickness is unlikely to exceed 500 ft.
Otapirian Stage
Campbell and McKellar (1956: 699) defined the stage as “ those rocks laid down at the type locality from the first appearance of Spiriferina ( Rastelligera ) diomedea Trechmann until the appearance of a fauna including psiloceratid ammonites”. This species was not collected from the Whatawhata district; so the position of the base of the Otapirian Stage has been determined with respect to the horizon of the highest Monotis shell beds.
In the Marakopa district Campbell (in Grant-Mackie, 1959) collected distinctive large athyrids which he considered to be basal Otapirian. Lowry (1962) collected similar large athyrids 45ft above the topmost Monotis beds exposed on the coastal platform south of Kiritehere Beach, Marakopa, and the writer feels justified in drawing the lower boundary of the Otapirian Stage in the Whatawhata district immediately above his topmost Monotis beds.
Rocks correlated with the Otapirian Stage crop out in the Whatawhata area as two belts on either side of the Kapamahunga Fault, separated by approximately 3,000 ft of strata. They parallel the Monotis- bearing Warepan beds described earlier. The two bands of Otapirian rocks differ greatly from one another in thickness. The thickness of strata in the eastern band lying between the latest occurrence of Monotis and the earliest appearance of the Aratauran form Otapiria marshalli is approximately 2,400 ft, while the thickness of rocks of Otapirian age in the western belt totals only approximately I,oooft. This marked discrepancy may possibly be accounted for by repetition or omission of beds because of faulting, which is known to be extensive. Alternatively strong evidence for considerable clockwise transcurrent movement along the Kapamahunga Fault (infra) suggests that material from farther to the north, where perhaps sedimentation during Otapirian times was much faster than in the Whatawhata area, has been introduced into the latter district to form the western belt.
The sediments consist in general of well-bedded siltstone, with occasional pyrite nodules but lacking tuffaceous material or carbonaceous remains. Outcrops of a distinctive fine conglomerate occur at numerous localities, and apparently at several stratigraphic horizons, throughout the western belt. Petrographic examination of thin sections shows the various occurrences to be closely similar in composition. Rounded rock fragments are abundant, andesite being the most common, with a varying proportion of fragments of mudstone. Chips of quartzite and epidote-bearing rock are übiquitous but in minor quantity, and rare fragments of granophyre and some of granodiorite are also present. Plagioclase (mainly oligoclase-andesine) is the dominant allogenic mineral present, although subordinate in quantity to the lithic fragments, with minor detrital quartz and secondary calcite. Rare muscovite and several small broken crystals of clinopyroxene also occur. Crystals of authigenic pyrite (often euhedral) are always present as accessories. A sample of the conglomerate collected from a south-flowing tributary of Maungakirikiri Stream in addition to the constituents listed above also contained small quantities of a member of the heulandite-clinoptilolite series, which occurs as amorphous aggregates of crystals. The similarity in thickness and lithology of the conglomerate occurrences suggest that all outcrops of the conglomerate belong to the same band repeated by faulting. The band appears to be of wide lateral extent and is approximately 20ft thick, with sharp contacts with siltstone at top and bottom, little vertical variation in texture being observed.
This conglomerate is not represented in the eastern belt of Otapirian sediments. The only outcrop of a comparable rock type seen in this belt, exposed in the headwaters of Maungakirikiri Stream, occurs in a stratigraphic position that suggests an Upper Otapirian age. Macroscopically, the conglomerate is well bedded and light grey and its constituent pebbles range in size up to 4mm, compared with the maximum 10mm diameter of pebbles in the massive, dark blue-grey conglomerate to the west. In thin section it is seen to contain fragments of andesite and less common mudstone, but these are subordinate to large clean crystals of plagioclase (mainly andesine-labradorite), a completely different relationship from the western conglomerate. It is probably lensoid, as no further occurrences were noted. The absence of the prominent conglomerate band from the eastern belt is attributable to transcurrent movement along the Kapamahunga Fault.
The stratigraphic position of the conglomerate of the western Otapirian belt is uncertain, as no fossils characteristic of horizons within the Otapirian Stage have been found in association with it. The conglomerate crops out in a northern tributary of Maungakirikiri Stream 500 ft stratigraphically above the highest beds containing Monotis in the same tributary, and again, farther north, in Tunaeke Stream, it appears in situ only 300 ft above Monotis beds; a fault is inferred. The Maungakirikiri Stream section may also be faulted, but as there is no evidence of this the position of the conglomerate is tentatively accepted as being 500 ft above topmost Warepan beds.
The only locality where a sequence can be followed from the conglomerate up to rocks containing an undoubted Aratauran fauna is in a southern tributary of Maungakirikiri Stream, where the conglomerate is exposed one and three-quarter miles west of trig New 800. Here Otapiria marshalli first occurs, approximately 500 ft above the conglomerate.
These observations suggest that the conglomerate is of Middle Otapirian age, assuming that the rate of sedimentation throughout Otapirian times was relatively constant. However, these calculations could be considerably in error if unrecognised faulting has intervened.
Further evidence for the Otapirian age of the conglomerate is provided by the occurrence of casts of Spiriferina (Rastelligera ) n.sp. in loose boulders of the conglomerate on the banks of the large southern tributary of Unungarahu Stream (N65/615). The subgenus Rastelligera is most characteristic of beds of the Otapirian Stage. So the new form is more likely to be an Otapirian species than an Aratauran or Warepan one.
The only other fossils obtained from the conglomerate were collected from loose boulders on hillslopes bordering Unungarahu Stream (N65/613). The fauna consisted of Astarte sp. and unidentifiable brachiopods. The conglomerate comprises the oldest Otapirian bed in which fossil remains have been observed on the western side of the Kapamahunga Fault. From weathered siltstone exposed on the left bank of Unungarahu Stream, 150 ft above the conglomerate, the oldest fauna of undoubted Otapirian age in the western belt was collected (N65/614). This was a rhynchonellid, identical with that identified by Grant-Mackie (1959: 774) as characteristic of horizons in the middle and upper Otapirian. Its position in relation to the conglomerate, if undisturbed by faulting, is further evidence in support of a Mid- or Upper Otapirian age for this band.
Sparsely distributed fossils have been located at several other localities, but are not specifically identifiable or diagnostic. What are probably the youngest fossiliferous Otapirian beds in the area occur in an exposure in a tributary of Maungaokahu Stream (N65/610). A bedding-plane in siltstone, striking at 010° and dipping 75° east, one of a series evidently disturbed by faulting (infra), is crowded with casts of large Otapiria dissimilis (Cox) occupying a band 4ft thick. Ten feet stratigraphically below this bed scattered casts of Rhynchonella, ?Lima, PPalaeoneilo , and Otapiria occur, none of them specifically identifiable. In the beds of the stream, for a distance of up to 30ft stratigraphically above the Otapiria shell-bed, specimens of Otapiria dissimilis occur sporadically.
In the eastern belt of Otapirian strata only two fossil localities were discovered. At N65/608 in Cooper Creek, casts of Otapiria dissimilis were found in an exposure of highly shattered, weathered, and iron-stained siltstone approximately 1,700 ft stratigraphically above the highest Monotis beds in the same creek. Half a mile to the north a siltstone boulder containing a pocket crowded with casts of Otapiria dissimilis was found loose in the headwaters of Unungarahu Stream (N65/609). The source of the boulder has not been located, but it is unlikely to have travelled far, as 200 yards upstream the Mesozoic strata pass beneath a thick cover of Tertiary beds.
Correlation
Campbell (1956: 46-48), from a study of localities in Southland, Otago, and Kawhia recognised a basal and an upper Otapirian fauna. Grant-Mackie (1959: 773-775), as a result of study of a sequence in the Awakino-Mahoenui district, was able to recognise both basal and Upper Otapirian faunas, and also on faunal grounds to differentiate beds referable to the Middle Otapirian.
Of the forms listed by Campbell and McKellar (1956) and Grant-Mackie (1959) only the middle and Upper Otapirian " Rhynchonella 33 species and the Upper Otapirian Otapiria dissimilis (Cox) have been found in the Whatawhata district. No fossils typical of beds of lower Otapirian age were seen by the writer.
Aratauran Stage
No satisfactory definition of the Aratauran Stage has yet been given, but Campbell and McKellar (1956: 699), following Marwick (1953: 21), have placed the upper boundary of the Otapirian Stage in the type section at Otapiri Valley, below “ the appearance of a fauna including psiloceratid ammonites ”. Speden and McKellar (1958: 649) recorded the discovery by Campbell and Coombs of psiloceratid ammonites associated with Otapiria marshalli (Trechmann) 80ft above the highest bed containing Otapirian fossils on the coast SW of Roaring Bay, Otago. From the limited evidence available it appears that psiloceratid ammonites and Otapiria marshalli (Trechmann) make their appearance almost simultaneously, and that in the absence of the ammonite the lower boundary of the Aratauran Stage may be mapped on the incoming of Otapiria marshalli. The only ammonite with psiloceratid affinities collected in this district occurred approximately 400 ft above the lowest Otapiria marshalli horizon. The lower boundary of the Aratauran Stage is, therefore, here drawn below the lowest occurrence of Otapiria marshalli (Trechmann) . The upper boundary is discussed in the section on the Ururoan-Temaikan Stages.
Like the upper two Triassic stages, rocks correlated with the Aratauran Stage occur in two almost meridionally-trending bands on either side of the Kapamahunga Fault. The eastern band crops out over a width of approximately 35 chains, and relatively good exposures in the central portion of the strip allow stage boundaries to be drawn with a moderate degree of accuracy. To the south this strip disappears beneath the Tertiary cover. Rocks of the western belt are poorly exposed and the boundaries drawn, particularly the upper one, are tentative.
In the Whatawhata district beds correlated with the Aratauran Stage are represented by approximately 1,600-1,700 ft of strata, predominantly siltstone, but with some coarse sandstone bands up to 10ft in thickness. Almost all of the coarser bands are highly tuffaceous, some containing enough glassy material to be termed vitric tuffs. Coarser samples of Aratauran age typically contain a high proportion of zeolitised pumiceous material together with numerous andesite fragments, mudstone, epidote-bearing rock, quartzite, plagioclase, pyrite, granophyre, granodiorite, and biotite. The zeolite, a member of the heulandite-clinoptilolite series, commonly occurs as a replacement product in devitrified glass shards as well as forming veins and infilling interstices between detrital fragments. It occurs in amorphous aggregates, as euhedral crystals, or commonly as fibro-lamellar aggregates. Surrounding many of the zeolitised glass shards is a sheath of montmorillonite. Intrastratal solution of quartz and plagioclase crystals has occurred in some samples, with replacement by finely crystalline authigenic quartz. Some of the beds are moderately pyritised, and carbonaceous material is relatively common throughout the whole stage. No conglomerates were noted.
Fossils of Aratauran age are distributed widely throughout the district, but the fauna is meagre in species, consisting almost entirely of Otapiria marshalli (Trechmann) . Casts of Otapiria occur only sparsely in any one exposure, and at a few localities only one specimen of the fossil was found.
Otapiria marshalli and an unidentifiable species of PPseudolimea were collected from loose siltstone boulders on a slope 50ft above Maungakirikiri Stream (N65/621). Although the source of the boulders has not been traced this locality is probably near the lower boundary of the Aratauran Stage. Approximately 400 ft above this locality (N65/620) an unidentifiable psiloceratid was found in company with Otapiria marshalli and PGrammatodon sp. The fossils were distributed sporadically throughout several feet of pyritic siltstone, striking 020° and dipping 57° west, in the bed of Unungarahu Stream. From about the same stratigraphic horizon but a quarter of a mile upstream from this locality scattered casts of Otapiria marshalli were collected from siltstone (N65/619). Casts of Otapiria marshalli were found at two further localities in a large southern tributary of Unungarahu Stream, about 130 ft and 450 ft above the previous horizon. At the first (N65/616) large casts of the mollusc were found in a thin band of tuffaceous grit exposed as a dip slope (strike 350°, dip 40° west) in the stream; at the other locality (N65/617) they occurred widely scattered in weathered, iron-stained fine sandstone of a similar attitude.
Siltstones and brown sandstones striking 345° and dipping 55° west, exposed in the same tributary of Unungarahu Stream, have been taken to represent the youngest Aratauran beds in the area. The sandstones contain plentiful carbonaceous material, and from one band an unidentifiable species of Cladophlebis has been obtained (N65/618).
In the western belt of Aratauran strata the oldest fossiliferous beds exposed are carbonaceous siltstones striking 355° and dipping 60° west in a north-flowing tributary of Maungakirikiri Stream. The fossils are all casts of small Otapiria marshalli (N65/625). From approximately the same stratigraphic horizon, but one and a-half miles to the north, two separate localities in a south-flowing tributary of Maungakirikiri Stream yielded a single fossil each: Otapiria marshalli at N65/622, and Otapiria sp. at N65/623.
The highest definite fossiliferous Aratauran horizon found (N65/624) was exposed on a bend in a north-flowing tributary of Maungakirikiri Stream, approximately 800 ft above the base of the stage. Here casts of large Otapiria marshalli were collected from siltstone striking 010° and dipping 50° west.
On a track 300 ft above Orongo Stream, poorly bedded siltstone, striking 025° and dipping 27° west, contains indeterminate species of a brachiopod, a pelecypod, and a gastropod (N65/626) at approximately the same stratigraphic horizon as the previous locality.
Ururoan-Temaikan Stages
No specifically identifiable fossils typical of either stage were found, and rocks believed from their stratigraphic position to belong to the Ururoan and Temaikan Stages are treated together. In the absence of paleontological criteria the writer has used lithological evidence for correlation with the stages elsewhere in an attempt to map, at least approximately, the upper limit of the Aratauran Stage in the area.
From a Mesozoic inlier in Tertiary beds, extending along part of Fillery’s Road in the south of the area, a band of conglomerate approximately 10ft thick was recorded. The conglomerate consists of scattered pebbles up to 2in in diameter in a matrix of weathered siltstone. Rock types represented are amphibolite, andesite, greywacke, and fine sandstone. The greywacke and sandstone pebbles contain constituents similar to the coarser Aratauran sediments, but although highly altered they show no signs of devitrified glass or zeolites. The few exposures located stratigraphically above the conglomerate are deeply weathered, but consist in the main
of fine sandstones, the highest bands visible in the sequence being a medium-coarse sandstone, approximately 550 ft above the conglomerate, overlain by weathered siltstone. Below the conglomerate exposures are few and very weathered, but the lithology appears to be of the siltstone or fine sandstone grade. The conglomerate could not be traced to the north, but projection shows that it should lie stratigraphically above brown sandstones with carbonaceous fragments and leaf impressions exposed in a tributary of Unungarahu Stream (N65/618).
Near Waingaro, eight miles to the north of this district, Kear (1961) has recorded, near the base of the Pongawhakatiki siltstone, a band of conglomerate 10ft thick, consisting of greenish pebbles and containing specimens of Pseudaucella marshalli (Trechmann), a fossil generally accepted as a Ururoan marker. It is overlain by approximately 210 ft of sandstone and lies about 60ft above brown sandstone with, leaf impressions and abundant carbonaceous material. Although no fossils were found in the conglomerate or the associated sandstones of the Whatawhata district, the stratigraphic successions are so similar in the two districts that correlation of the two sequences seems justified. If this correlation is valid the lower boundary of the Ururoan Stage should be drawn between N65/618 and the conglomerate exposed in Fillery's Road.
Macdonald (1951) and Purser (1952) have recorded a conglomerate of lower Ururoan age from the South Kawhia and Port Waikato districts respectively. Macdonald (1951: 19) has noted that of all the Jurassic conglomerates examined, only the one of Ururoan age contains an abundance (25 per cent) of pebbles of metamorphic origin. The conglomerate cropping out on Fillery's Road was the only one studied in this district with undoubted metamorphic content, and this provides supporting evidence for the conclusion that the conglomerate is Lower Ururoan in age.
To the west of the Kapamahunga Fault exposures are rare and often highlyweathered. So the Aratauran-Ururoan boundary was much more difficult to map. Only three exposures of coarse sediment were located. In the south of the area a highly weathered, very coarse sandstone appears in the headwaters of a branch of Orongo Stream. Two and a-half miles to the north a poorly exposed weathered outcrop of very coarse sandstone or fine conglomerate appears, whilst farther north again, in a northern tributary of Maungakirikiri Stream, a very coarse tuffaceous sandstone with rare large siltstone inclusions is exposed. Thin sections from the outcrops, which lie approximately along the same strike line and almost certainly represent the same horizon, proved to be essentially similar in content. The main constituents are fragments of mudstone, andesite, and plagioclase, with less common quartz and pyrite, minor granophyre, epidote-bearing rock, quartzite, biotite, granitic material, and clay minerals. Glass altered to heulandite is present as a minor constituent, and authigenic replacement of plagioclase by quartz is frequent. Although there are some differences in clastic content, notably the absence of amphibolite from the western horizon of coarse sediments, this horizon may be a lateral equivalent of the conglomerate near Fillery's Road. Evidence supporting this is the comparable thickness of sediments below the coarse bands on either side of the Kapamahunga Fault and the base of the Aratauran Stage in each case. In view of the difference in thickness of Otapirian strata on either side of the fault noted earlier this evidence must be treated with caution, but the lower boundary of the Ururoan Stage is tentatively drawn beneath the coarse belt on either side of the Kapamahunga Fault.
It is also impossible to locate accurately the upper boundary of the Temaikan Stage, but its position is discussed more fully in the section on the Heterian Stage. Approximately 4,000-4,500 ft of strata lie between the upper boundary of the Aratauran Stage and an outcrop containing Belemnopsis alfurica Boehm (N65/601). This fossil occurs in beds of Upper Heterian to Lower Ohauan age at Kawhia (Stevens, 1965).
The remainder of the strata representing undifferentiated Ururoan-Temaikan, except for an isolated occurrence in Maungaokahu Stream of fine tuffaceous conglomerate showing zeolitic alteration in thin section, consists of siltstone, interspersed commonly with thin light-coloured tuff bands. Fossils are rare and nondiagnostic. A quarry in massive siltstone, situated 30 chains south of the HamiltonRaglan Highway (N65/628), has yielded ?Coenothyris sp. and a species of Pleuromya or Homomya. This latter fossil appears to have close affinities with the Temaikan forms described by Marwick (1953: 105-6) as Pleuromya milleformis and Homomya signicollina, and is therefore tentatively regarded as belonging to this stage rather than to the Ururoan Stage.
The only other identifiable fossils located were at N65/629 in Maungaokahu Stream, about 570 ft higher in the sequence than the last locality. Tests of Foraminifera, including Vaginulina and other indeterminable Lagenidae, were discovered in a fine, highly tuffaceous conglomerate, which also contained prevalent small fragments of brachiopod tests.
Heterian Stage
Fleming and Rear (1960: 43) have defined the base of the Heterian Stage m the type section at Kawhia as “ the bottom of the zone of Inoceramus galoi Boehm ”. The stage extends up to the bottom of the zone of Inoceramus haasti Hochstetter in the type section.
In the Whatawhata district only one fossil locality (N65/601) half a mile west of Mount Heretu, was discovered, the fauna consisting of; c Rhynchonella 3 sp., f Terehralula 3 sp., ?Inoceramus sp., Amherleya cf. zealandica Trechmann, Belemnopsis alfurica Boehm. Amherleya zealandica occurs in the upper part of the Temaikan Stage (Marwick, 1953: 113) whilst Belemnopsis alfurica has a time range of Upper Heterian to Lower Ohauan at the type section of the Jurassic at Kawhia (Dr G. R. Stevens, pers. comm.). This latter fossil is probably the more reliable age indicator of the two. In the Kawhia area Belemnopsis alfurica first appears in the Waikutakuta Siltstone (Fleming and Rear, 1960), which occupies only the upper 425 ft of the 1,900 ft-thick Heterian Stage. Player (1958) records 4,000 ft of Heterian strata in the eastern portion of the North Kawhia district, which lies immediately to the SW of the Whatawhata district. These beds show a north-south strike, and can be expected to continue to the north to overlie directly the rocks exposed in this district. The fossils at N65/601 are therefore almost certainly Heterian.
In view of the lack of any definite Temaikan fossils, the lower boundary of the Heterian Stage in this district cannot be delimited accurately. If the tentative lower boundary shown in Figure 2 is accepted, approximately 1,600 ft of Heterian sediments are developed in the mapped area. The upper boundary lies beyond the western edge of the district.
Rocks believed to be of Heterian age are almost entirely of siltstone grade, some of them containing mud-flecks, although several light-coloured tuffaceous bands are intercalated between siltstone bands exposed in Maungakirikiri Stream. No eastward extension of Player’s (1958: 25) Greywacke Member of the Oparau Siltstone was seen, its absence being due to either a possible intervening fault or to lateral lithologic variation. In the vicinity of N65/601 the strata consist mainly of weathered and shattered siltstone, but 50 yards downstream from the main fossil locality outcrops of poorly sorted coarser material of very coarse sandstone and fine conglomerate grade occur. Much pyritic and carbonaceous material is present.
Submarine Slumping in Upper Triassig Strata
Grant-Mackie and Lowry (1964) have drawn attention to the presence ot widespread slumping of Upper Triassic strata on portions of the western limb of the Kawhia Syncline. In the Kiritehere area, described in detail by those authors, slumped Norian (Warepan) strata are well exposed on the shore platform, and individual slump sheets can be easily recognised and mapped.
In the Whatawhata area, where exposures are generally poor, evidence for slumping is not as clear as at Kiritehere, but the occurrence of unusual structures associated with slumping in the latter area has enabled recognition of similar features in the former. Criteria used as evidence for slumping were randomly varying dips and strikes within a small area and the occurrence of angular discordances of local extent with welded contacts. Slumping was found to be particularly characteristic of strata of Warepan age, anomalous strikes and dips and apparent overturning or rotation of beds being common.
Significant evidence for slumping is visible in the bed of a NNW-flowing tributary of Maungakirikiri Stream. An angular discordance with welded contact was observed where beds dipping at 70° overlie beds dipping at 15°, both sets having a NNW strike. Thin shell-beds containing densely packed casts of Monotis ( Entomonotis) calvata are typical of beds both above and below the contact. The crumpled nature of the shell bed and random orientation of shells within it, many shells having an attitude normal to the bedding, is in accordance with what could be expected in crumpling and drag caused by slumping, although this is not the only possible interpretation. There is no difference or gradation in lithology between beds above and below the discordance. On the slopes 100 ft above the stream beds dip gently to the SW with strike differing 30-40° to the west from the strike of the stratigraphically lower beds exposed in the stream. The strike of these beds also differs greatly from the regional strike. This series of observations suggests that at least three slump sheets (Jones, 1940) are represented in Warepan strata exposed in this valley. Variation in dips and strikes in the Warepan sequence recorded from Cooper Creek seems too great to attribute to possible interference by faulting, and it is highly probable that slumping again is the cause of the disturbed strata.
There is strong evidence that some slumping has also occurred in strata of Otapirian age. In a NE-flowing tributary of Tunaeke Stream, 30 chains north of the Hamilton-Raglan Main Highway, an angular discordance with welded contact was observed between two bands of fine conglomerate exposed in the bed and banks of the stream. The underlying strata dip at an angle of 45° to the east, whilst those above dip 25° west, both sets of beds striking north-south. Widely varying dips and strikes in apparently unfaulted strata upstream from this locality provide further evidence suggestive of slumping (PI. 2).
Elsewhere in the district rocks of Otapirian age frequently show strikes and dips at variance with the regional trend, but lack of exposures of other welded contacts prevents differentiation between the effects of slumping, faulting, and other smallscale tectonic disturbance.
Records of thinning and thickening or even absence, of Warepan beds and disturbance of strata of this age have been reported from most parts of New Zealand where they crop out, and Grant-Mackie and Lowry (1964) have construed this as evidence for widespread slumping along the western margin of the New Zealand Geosyncline. These authors also consider that slumping has been initiated by tilting of the sea floor by about 8° to the east from a normal gradient of less than |°. The writer agrees with this view, as the presence of undisturbed bedding planes in slump sheets of both the Kiritehere and Whatawhata districts indicates that slumping did not occur until the sediments had become semi-consolidated.
The fact that such a degree of consolidation was reached before slumping took place suggests that the deposits were laid down under stable conditions on a seafloor whose slope was not sufficient to cause spontaneous slumping. It seems most likely, therefore, that tilting of the sea-floor took place only after a considerable thickness of Warepan strata had been laid down, the tilting probably occurring as a result of contemporaneous tectonic movements. Thinning of strata towards the ancient shoreline as a result of slumping (Fairbridge, 1946; 88) is not evident in south-west Auckland, as Grant-Mackie (1958: 25) records a maximum of 420 ft of Monotis beds in the Awakino-Mahoenui district on the western flank of the Kawhia Syncline, which is similar to the thickness of these strata in the Whatawhata district. The lack of thinning of sequences in south-west Auckland, coupled with the general lack of deformation or thickening of slump sheets, suggests that although slumping was widespread, only limited movement of slump sheets took place.
Tertiary Stratigraphy
Introduction
Tertiary strata are represented only by beds of the Te Kuiti Group (Kear and Schofield, 1959) overlying Mesozoic rocks usually with strong angular unconformity, and separated from them by an undulating surface. The nomenclature proposed for formations and members by Kear and Schofield has been followed by the writer, and stratigraphic columns have been drawn up for two sections in the district four miles apart (Fig. 4), one 10 chains SW of Old Mountain Road summit, and the other a quarter of a mile WSW of the end of Fillery’s Road. These columns are compared with that drawn by Kear and Schofield (1959) for the type sections of the Mangakotuku Siltstone and the Glen Massey Formation and its members (Elgood Limestone, Dunphail Siltstone, and Glen Massey Sandstone) at Dunphail Bluffs, 15 miles to the north.
Waikato Coal Measures
The thickness of this formation in the district varies from about 20ft to 40ft, depending on the initial relief on the underlying erosion surface. The contact of Goal Measures with Mesozoic rocks is rarely exposed, but at many localities the basal finely laminated clays of the Coal Measures pass imperceptibly down into Mesozoic siltstones weathered to clay. At only one locality was a Tertiary basal conglomerate seen: in the headwaters of Cooper Greek a conglomerate 3ft thick, consisting of rounded pebbles containing fragments of Monotis, overlies Triassic rocks with these same fossils.
Waikato Goal Measures strata are generally poorly exposed, but the lower part was found to consist of finely laminated light-grey and brown clays, followed by weathered brown siltstones, carbonaceous layers, and coal seams varying in thickness from a fraction of an inch to 10ft (as at Whatawhata Campbell Coal Mine). No fauna has been found, but microflora from the main coal seam at Whatawhata Campbell mine (N65/519) have been described as Runangan by Couper (1960: 15).
Mangakotuku Siltstone
This formation appears to have a constant thickness of approximately 20ft throughout the district, and directly overlies the Waikato Coal Measures without any observable stratigraphic break. The basal 6-10 ft consists of blue-grey indistinctly laminated siltstone, with a prominent shell-bed, varying from lOin to 3ft thick, containing common Eumarcia crassa Marwick with less common naticids and venerids in its upper portion. Half a mile to the east of the Old Mountain Road Tertiary
section, a thin coal seam overlies the shell-beds by 6ft, while farther to the south, a quarter of a mile north of the old Karamu limeworks, the shellbed, here 3ft thick, rests directly upon a sft-thick very dirty coal seam. The wide distribution of the prominent shell-bed throughout the district will undoubtedly provide the basis for future division of the Mangakotuku Siltstone into members. This division, however, has not been attempted in this regionally restricted survey.
The blue-grey siltstone above and below the shell-bed is probably the equivalent of the “ glauconitic sandstone in the middle of the formation at Huntly which becomes basal southwards and contains thin coal seams near Whatawhata ” (Kear and Schofield, 1959). Above the basal blue-grey siltstone is 10-14 ft of siltstone, generally weathered ochre-brown, containing several bands of leached pelecypod casts.
The following microfauna collected from the prominent shell-bed near the base of the Mangakotuku Siltstone at the Old Mountain Road Tertiary section (N65/599) has been identified by Mr N. de B. Homibrook: Ammobaculites sp., Haplophragmoid.es sp., Siphonaperta sp., Vaginulina sp., Bolivina sp., Buccella sp., Cribrorotalia keari Homibrook, Elphidium sp., Cancris lateralis Finlay (early, short-chambered form), Anomalinoides subnonionoides (Finlay), Trachyleberis sp. He has recognised this as an estuarine fauna of basal Whaingaroan age, which typifies the change from coal swamps to brackish-water conditions, indicating an advancing sea.
Glen Massey Formation
Kear {in Kear and Schofield, 1959) has divided this formation into three constituent members: a basal limestone (Elgood), an intermediate siltstone (Dunphail), and an uppermost prominent sandstone (Glen Massey). The whole formation is more or less calcareous and forms prominent bluffs throughout the Whatawhata district (Plate I).
The change in lithology from the underlying Mangakotuku Siltstone is abrupt, the dominantly calcareous facies of the Glen Massey Formation emphasising the change from estuarine facies to open sea. The lithologic break is marked also by a topographic break, isolated buttes and mesas of the Glen Massey Formation resting on the surface developed on the top of the Mangakotuku Siltstone. This slightly undulating surface, formed by escarpment retreat in the Glen Massey Formation, is widely exposed throughout the district and is a valuable guide to structure (infra), as dislocations in the surface due to faulting can be plotted with a reasonable degree of accuracy.
Because the Glen Massey Formation normally crops out as escarpment sections, differentiating the three members for the purposes of mapping is difficult if not impossible. So the formation has been mapped as a single unit. The members of the Glen Massey Formation, however, can be easily differentiated in the field, and they are discussed separately in the following sections.
Elgood Limestone Member
This consists of glauconitic siltstone and fine sandstone incorporating a band of limestone of varying thickness. The basal fine sandstone is 20ft thick, the lowest 2-3 ft highly glauconitic and containing a varied fauna consisting of pelecypods, naticids, and rare corals. The upper and middle portions are only slightly glauconitic and contain rare casts of the pelecypod Panopea worthingtoni Hutton.
The bored zone noted by Kear (in Kear and Schofield, 1959: 698) between the base of the Elgood Limestone and the top of the Mangakotuku Siltstone is clearly exposed in only one locality in the Whatawhata district.
The basal fine sandstone in its upper portion grades within 2-3 ft into flaggy limestone composed largely of comminuted shell material, but in places markedly arenaceous (e.g., at the Glen Tui lime quarry, Hamilton—Raglan Main Highway). Its lensoid or wedgelike nature is demonstrated by a rapid thickening to the south. In the Old Mountain Road section the limestone is 30ft thick, whilst in the Fillery’s Road section the thickness has increased to 130 ft, a maximum for the district.
As a result of its very high calcareous content, the flaggy limestone is exceptionally prone to solution by percolating water, and, particularly towards the south of the area where the band is thickest, well-known caves are developed in it. Of these, the most important is Karamu Gave, the main entrance to which is a quarter of a mile north of the end of Fillery’s Road. Dripstone and flowstone features are well developed in the cave, the passages of which total over two miles in length. The top of the flaggy limestone grades rapidly into glauconitic calcareous sandstone which becomes finer and less glauconitic upwards until it merges with the Dunphail Siltstone member.
The limestone contains rare unbroken macrofossils, mainly Lentipecten hochstetteri (Zittel) and Janupecten polemicus Marwick. Mr N. de B. Hornibrook has recognised the following Whaingaroan microfauna from a sample (N65/590) collected from the basal glauconitic fine sandstone, 20ft below the flaggy limestone at the Old Mountain Road section: Globigerina cf. bulloides d’Orbigny, Semivulvulina capitata (Stache), Gyroidinoides allani (Finlay), Quadracythere sp. A sample collected from just below the flaggy limestone at the same locality (N65/591) has a richer Whaingaroan fauna than the above, and Mr Hornibrook regards it as representing a somewhat deeper environment, the Operculina probably indicating inner shelf conditions. The following microfauna has been recognised: Cribrorotalia n.sp., Semivulvulina capitata (Stache), Operculina sp., Arenodosaria robusta (Stache), Gyroidinoides allani (Finlay), Cibicides perforatus (Karrer), Nonion dorreeni Hornibrook. The following microfauna (N65/592 and N65/593), from samples collected within the flaggy limestone at this locality, has been recognised by Mr Hornibrook as consisting of typical shallow-water Whaingaroan forms, probably belonging to the inner shelf: Gyroidinoides allani (Finlay), Operculina sp., Cribrorotalia n.sp., Elphidium sp., Eponides repandus (Fichtel and Moll), Asterigerina aff. cyclops Dorreen, Textularia ototara Hornibrook. The following microfossils denoting a Whaingaroan age, from a sample (N65/594) collected from 3ft above the flaggy limestone at the Old Mountain Road section, Mr Hornibrook believes represent a deeper environment than the foregoing, probably middle to outer shelf: Globigerina labiacrassata Jenkins, Anomalinoides fasciatus (Stache), Notorotalia stachei Finlay, Globoquadrina tripartita (Koch), Cibicides thiara (Stache), Vaginulinopsis cristellata (Stache), Rectuvigerina striatissima (Stache), Uvigerina maynei Chapman, Arenodosaria robusta (Stache), Bolivina anastomosa (Finlay), B. lapsus Finlay, Bulimina pupula Stache, Gaudryina reussi Stache, Nonion dorreeni Hornibrook, Gyroidinoides allani (Finlay).
Dunphail Siltstone Member
This comprises between 30 and 110 ft of massive, finely frittering siltstone, glauconitic and sandy in the lower 20ft, with a gradational lower contact with the Elgood Limestone. The upper boundary of the member with the Glen Massey Sandstone is also gradational, but the Dunphail Siltstone can be easily distinguished from a distance by a prominent break in slope at its upper and lower boundaries. The common slope of the surface developed in the Dunphail Siltstone member is about 45°, whilst its bordering members form prominent escarpments.
Macrofossils are not common, but Chlamys williamsoni (Zittel), Cucullaea sp., and Panopea worthingtoni Hutton have been collected from near the Old Mountain Road section.
Glen Massey Sandstone Member
This is typified by slightly coarser sediments than the underlying member, and forms prominent bluffs of poorly bedded calcareous fine sandstone throughout the
area. Its maximum thickness in the district is about 200 ft. Re-collection of microfaunal samples from the Dunphail Siltstone and Glen Massey Sandstone was not undertaken by the writer, in view of previous collections made by members of the Geological Survey from a locality a quarter of a mile north of the old Karamu limeworks. A microfauna 240 ft above the flaggy limestone of the Elgood member (N65/509), i.e., from near the top of the Glen Massey Sandstone, is of Whaingaroan age.
Although the Whaingaroa Formation, which overlies the Glen Massey Sandstone to the north, is apparently not preserved in the Whatawhata district, nearly flat surfaces developed on the summits of some Tertiary blocks and accordance of tops of adjacent Tertiary buttes suggest the presence of a distinct lithologic break. This may represent the transition between the Glen Massey Formation and the overlying Whaingaroa Formation, the latter having been removed by erosion. Macrofossils consist mainly of Panopea worthingtoni Hutton, Janupecten polemicus Marwick, and Lentipecten hochstetteri (Zittel).
Discussion
Comparison of the stratigraphic columns in Figure 5 shows great variation in thickness of formations and members. Whilst the Mangakotuku Siltstone thins drastically from over 200 ft at Dunphail Bluffs to 20ft in the Whatawhata district, the reverse is true of the Glen Massey Formation. Although both lower members of this formation show great changes in thickness, the flaggy limestone of the Elgood member shows the most significant variation, increasing from 10ft in the type section to 30ft in the Old Mountain Road section and to 130 ft in the section near Fillery’s Road. These considerable changes in thickness probably result from the formation of small local basins of deposition. With the practically linear distribution of lower Tertiary outcrops in the Whatawhata district, it has not been possible, however, to obtain any accurate idea of the location of basins.
Several explanations are possible for the variations in thickness of the lower Tertiary beds. The basins of deposition may have been tectonically controlled, movements causing the downsinking of the floors of the basins keeping pace with deposition. Alternatively, the Elgood Limestone, which shows the greatest variation in thickness and is composed of comminuted shell material and a little coarse sand, could have accumulated as shell banks.
Structure
The Mesozoic strata were indurated, deformed, elevated, and denuded before the succeeding lower Tertiary beds were deposited. These latter are separated by a marked unconformity from the underlying rocks; they have been gently warped, and, together with the Mesozoic strata, were faulted by earth movements in late Cainozoic times.
Folding
The rocks of the Hokonui Facies (Wellman, 1952) exposed in south-west Auckland, Nelson, and Southland, show regional synclinal structures developed during the Rangitata Orogeny; these Wellman considered to be parts of the same major structural unit—the “ marginal syncline ”. The south-west Auckland segment —the ‘Kawhia Syncline 5 of Marwick (1946: 10) —pitches to the north at a low angle.
In the Whatawhata district the Mesozoic rocks are folded into a pre-Tertiary anticline showing gentle, slightly west of north plunge, and forming a secondary
fold on the eastern limb of the Kawhia Syncline. It is probably the southern extension of the Hakarimata Anticline, which has been recognised farther north by Rear (1960). Initial plunge of the anticline was probably greater than appears now, because post-Tertiary faulting (infra) has caused a southward tilt of the block of country that includes the crest of the fold, thus reducing its apparent northward plunge. The axis and plunge of the anticline cannot be plotted with accuracy because of great variation in strikes and dips of the flanking strata, the variation being attributed to faulting and in some cases to slumping of the beds. The trace of its axial plane, however, coincides approximately with the crest of Kapamahunga Range in the central eastern portion of the district.
The Lower Tertiary beds have dips of 5-10°, indicating that they have been gently warped. This folding has been brought about by Kaikoura earth movements, but because of interference by faults it has not been possible to recognise any fold axes.
Faulting
Where Mesozoic strata are overlain by beds of Tertiary age both sets appear to have been affected to the same extent by faulting. The episode of movement must have been subsequent to deposition of the Te Kuiti Group, and can be attributed to the New Zealand-wide Kaikoura Orogeny, which was initiated in post-Oligocene times (Macpherson, 1946).
Henderson and Grange (1926), on the geological map of the Alexandra Survey District, have shown a number of faults in this area. Several subparallel meridional faults were shown to be intersected by NE- and ENE-trending fractures, breaking up the country into large blocks.
In his geological map of the Hamilton area Rear (1960) has noted that major faulting strikes roughly parallel to the strike of the Mesozoic beds and is intense near the Waipa Fault, which is inferred to be transcurrent. In the Whatawhata district he has depicted two major N-S-trending faults and several minor ones branching from the meridional fractures and trending between 030° and 050°. A fault striking 340° from Kaniwhaniwha Stream and crossing the headwaters of Cooper Greek was mapped by him as dislocating the Mesozoic but not the Te Kuiti strata.
The writer has studied faulting in the area in some detail, and has found it necessary in some cases to omit or modify previously postulated faults and to recognise others. There are two main directions of faulting. The major faults have an approximately meridional strike and apparent throws of up to 4,500 ft. There is evidence suggesting that on at least the largest fault of this group some and probably most of the movement has been transcurrent. Numerous subparallel smaller faults branch from the meridional faults at bearings ranging from 030° to 060° and with throws varying from about 30ft to 300 ft. These faults are regarded as being mainly normal. In addition, a few faults have strikes at variance with the two major directions. There are many minor faults, but owing to the nature of the terrain their recognition is often difficult. Only those faults that are easily recognisable are described.
In the east of the area, where Tertiary strata are dislocated, it has been possible to plot faults with a relatively high degree of accuracy. Dislocation of the Elgood Limestone and the Waikato Coal Measures is especially valuable in plotting faults cutting the Tertiary. Discontinuity in the level of the slightly undulating surface developed on the top of the Mangakotuku Siltstone has been used extensively, especially in the case of smaller faults. Elsewhere, the recognition of faulting has depended largely on repetition of key beds in the Mesozoic rocks, fault contacts
of Mesozoic against Tertiary strata, and a study of aerial photographs. Shearing, brecciation, anomalous dips and strikes, topographic expression, and paleontologic evidence have also been used.
The major fracture in the area is the meridional Kapamahunga Fault, the almost rectilinear surface trace of which can be followed throughout the length of the district (a distance of six miles) and beyond its boundaries (PI. 2). For four miles, from the northern boundary of the area to Cooper Creek in the south, the trace is marked by pronounced alignment of streams. In the northern section it defines the upper reaches of Tunaeke Stream and a large northern tributary of Maungakirikiri Stream. Strata dipping vertically because of drag and high degree of shearing in siltstones cropping out in the beds of both streams, provide additional evidence for the presence of the fault. Towards its southern end, where Tertiary strata on the eastern side are faulted against Mesozoic strata to the west, the trace curves slightly and where it leaves the area it is striking at 007°. No Tertiary beds are preserved on the western side; so the extent of movement on the fault plane cannot be determined by their dislocation.
In the south hills of Mesozoic strata on the western side of the fault rise 700 ft above the general level of the base of the Tertiary to the east, implying upthrow of at least 700 ft to the west. A vertical or near-vertical fault plane is implied by the almost rectilinear fault trace. Judging from the dislocation of Mesozoic strata, the upthrow is about 4,500 ft in the south, and 3,000 ft in the north, indicating that the block of country situated to the east of the Kapamahunga Fault has been tilted 2° to 3° to the south with respect to the western block. This southerly tilt has caused the strike of strata in the eastern block to make an acute angle with the fault line (PI. 2), the strike of strata in the western block retaining a nearly parallel relationship with the fault trace.
Alignment of streams along the fault trace, mentioned above, suggests that the most recent movement was associated with the Kaikoura Orogeny. It is puzzling, therefore, that although in the north of the area a downthrow to the east of approximately 3,000 ft is inferred from dislocation of Mesozoic beds, this is not reflected in the topography. In this portion of the district in fact altitudes on the “ downthrown ” eastern side are in general slightly higher than that to the west. This problem admits of two possible solutions. One is that the main movement along the fault was vertical but occurred prior to the Kaikoura Orogeny, and the other is that movement along the fault took place in Quaternary times, but was of a mainly transcurrent nature. The author considers that the latter explanation is more likely to be correct, although the only evidence for the existence of such movement is indirect, being based on some differing lithologies in the Mesozoic strata across the fault.
Although Mesozoic lithologies are in general similar across the Kapamahunga Fault, being mainly siltstone, there are two major exceptions. The first is the nonappearance east of the fault of a prominent fine conglomerate band considered to be of Middle Otapirian age ( q.v .) which appears in situ and as loose boulders in numerous widely distributed localities on the western side of the fault. The only outcrop of conglomerate of approximately comparable age on the eastern side of the fault is petrographically quite distinct from that to the west, as earlier noted, and in any case is apparently only developed locally.
It is unlikely that a conglomerate stratum of such wide lateral extent as that west of the fault could lose its identity in the short space of three-quarters of a mile when its pebbles have already travelled from a point beyond the present shoreline, 16 miles distant. Its absence to the east of the Kapamahunga Fault can be accounted for by postulating transcurrent movement of sufficient magnitude along the fault to introduce from outside this district Otapirian sediments from beyond the area of original deposition of the conglomerate.
The second exception to the generally similar lithologies on either side of the Kapamahunga Fault is provided by the presence of a relatively coarse Lower Ururoan conglomerate in the south of the area on the eastern side of the fault near the end of Fillery’s Road. The only coarse sediment on the western side of the fault at approximately the same stratigraphic horizon is represented by a band of very coarse greywacke sandstone or fine conglomerate, with quite different macroscopic and microscopic characteristics from the conglomerate to the east, as noted earlier. It is evident from a petrological study that the two coarse bands were derived from different source areas, and if they represent the same stratigraphic horizon the conglomerate east of the fault is unlikely to be the seaward continuation of the band to the west. Also, if an area of provenance lying somewhere to the west of the present coast is accepted, the much larger size of conglomerate pebbles to the east is anomalous. The anomaly disappears if the major component of movement on the Kapamahunga Fault is transcurrent. As no Tertiary beds are preserved to the west of the fault, it has not been possible to make a similar comparison of these strata across the fault.
It seems then that the Kapamahunga Fault is predominantly transcurrent. Its direction if transcurrent is compatible with the direction of similar faulting in the Waihi Mine area (Wellman, 1954), 50 miles to the NE, and also with the direction of the Waipa Fault a few miles to the east, which is regarded by Kear (1960) as transcurrent.
To the west of the Kapamahunga Fault the detection and mapping of fractures is much more difficult owing to the absence of a cover of Tertiary strata (in which dislocations caused by faults are easily recognisable). In spite of this the presence of some small faults in this area has been inferred from offsets and repetition of a distinctive band of fine conglomerate of mid-Otapirian age, and also from anomalous strikes and dips and paleontological evidence. Most of these have meridional trends, and throws of up to 750 ft are inferred from the displacement of the Otapirian conglomerate.
Additional field evidence for only two of these mapped faults was seen. A fault with prominent drag folds indicating downthrow to the west is exposed in a cutting on the Hamilton-Raglan Main Highway, 300 yards west of the trace of the Kapamahunga Fault. Although no strike can be determined, its plane is seen to be vertical or subvertical. This fault is probably responsible for the anomalous position of an outcrop of Otapirian conglomerate in a branch of Tunaeke Stream, half a mile north of the highest point on the Hamilton-Raglan Main Highway. The same fault is considered to account for the occurrence of the conglomerate outcrop in Tunaeke Stream only 300 ft stratigraphically above beds containing the Lower to Middle Warepan Monotis ( Entomonotis) richmondiana. In contrast, 500 yards south of the Hamilton-Raglan Main Highway, in a tributary of Maungakirikiri Stream, the same conglomerate appears 500 ft stratigraphically above the highest M onofh-bearing outcrop.
The Mangaokahu Fault, in the west of the district mapped, determines the N-S course of a large tributary of Mangaokahu Stream for half a mile, and accounts for highly varying dips in outcrops exposed in this stream. Farther south the trend of the fault is marked by anomalous dips and strikes in Mesozoic strata, and in the southern half of the area by offset of the Otapirian conglomerate band, downthrow to the east being approximately 750 ft.
Highly irregular dips and strikes suggest the presence of a fault zone in the area between Old Mountain Road and the Hamilton-Raglan Main Highway, half to three-quarters of a mile to the east of the Kapamahunga Fault. In at least two separate localities the Mesozoic strata are vertical.
East of the Kapamahunga Fault are numerous smaller faults, most of them trending in a NE direction and readily mapped by dislocation of Lower Tertiary strata. The Kaniwhaniwha Fault in the SE comer of the district has been mapped by Rear (1960) as broken into three faults en echelon. However, there appears to be no good evidence for this, and a single fault is sufficient to explain the field relations observed. Although evidently downthrown to the south, the amount of throw is difficult to determine by matching Tertiary formations, as grey siltstone is the only lithology cropping out in low hills on the south side of the fault. A sample from the lower part of this siltstone has been examined by Mr N. de B. Hornibrook and identified on microfaunal evidence as being Whaingaroan in age. The lower few feet of the siltstone at this locality are highly glauconitic, the glauconite content rapidly diminishing upwards until, within about 10ft of the base of the exposure, glauconite is almost entirely absent. Elsewhere in the district, glauconite characterises the upper portion of the Elgood Limestone and the lower few feet of the Dunphail Siltstone Member of the Glen Massey Formation, the glauconite content diminishing upwards, until 20ft above the flaggy limestone of the Elgood Member glauconite is insignificant. In the light of this lithologic evidence, it is likely that the lower part of the siltstone exposed to the souh of the Kaniwhaniwha Fault represents the base of the Dunphail Siltstone Member of the Glen Massey Formation. If so, comparison of height with the base of the Dunphail Siltstone Member on the upthrown northern block indicates a downthrow on the fault amounting to 300 ft to the SE.
A splinter fault leaves and then rejoins the main fault in the immediate vicinity of the old Karamu limeworks, to include a kernbut (Cotton, 1948: 393) of limestone, downfaulted 40-50 ft to the SE.
Another important fault is the Cooper Fault, which branches from the Kapamahunga Fault three-quarters of a mile NNW of the end of Fillery’s Road, striking 032°. Its trace is marked by dislocation of Tertiary beds, and by the alignment of a portion of Cooper Creek along it. In the nothern and central portions of the Cooper Fault the downthrow is consistently 100 ft to the SE, but SW of the headwaters of Unungarahu Stream the throw has increased to 150 ft.
The Maungakirikiri Fault lies half a mile to the east of the Kapamahunga Fault at its northern end and trends subparallel to it. For a distance of four miles south from its northern extremity its rectilinear trace strikes slightly west of north and is clearly marked over most of its length by Tertiary beds on the east downfaulted against Mesozoic strata to the west. Half a mile north of Old Mountain Road, although no Tertiary beds are retained on the upthrown side, the topography of Mesozoic rocks rises to 250 ft above the basal Tertiary strata immediately east of the trace, thus indicating a throw of at least this magnitude. As the succession of Mesozoic beds on both sides of the fault is not greatly disturbed, the apparent throw is not likely to be much greater than the minimum. South of Old Mountain Road, the fault crosses the headwaters of Maungakirikiri Stream and brings Waikato Coal Measures into contact with Mesozoic strata on the west, the minimum downthrow being approximately 40ft to the east. Once again the actual throw is unlikely to be significantly more than the minimum throw, as approximately a mile west of trig New 800, rocks of Tertiary age, which to the north have been removed by erosion, are found for the first time on the western side of the fault and appear to have suffered little relative displacement across it. Farther south the projected fault trace crosses the headwaters of Unungarahu Stream (and, presumably, the Cooper Fault) and dislocates Tertiary strata immediately to the south. Here, however, the direction of vertical movement is reversed, and beds on the western side of the fault are downthrown some 50ft. The Maungakirikiri Fault is therefore a scissors fault. Its southward extension crosses the headwaters of Cooper Greek and
then curves gently, with its trace convex to the west, to cut across and dislocate Tertiary strata to the south, causing a relative displacement of 100 ft with downthrow still to the west. Farther south the trace cannot be followed.
Although the Maungakirikiri Fault is a major fracture subparallel to the probably transcurrent Kapamahunga Fault, there is little evidence, apart from its almost rectilinear trace and therefore probably vertical plane, that it is also transcurrent. It is perhaps worth noting, however, that de Ridder and Lensen (1960) have regarded the scissors nature of a fault as evidence for probable transcurrent movement. In the same paper the authors (1960: 10) have stated that “a slight change in the Principal Horizontal Stress direction of the order of 2° or less can change the vertical component of a [transcurrent] fault from a normal to a reverse character or vice versa ”. In the same way, change of a few degrees in strike of a fault can result in change of character (Lensen, 1958: 1962). The Maungakirikiri Fault falls into the latter category. Although almost rectilinear for most of its length, its surface trace curves slightly but definitely to the east, south of its intersection with the Cooper Fault, where, significantly, reversal of throw first occurs.
De Ridder and Lensen’s arguments have not, however, been tested in the field, and the proposition that the Maungakirikiri Fault has a transcurrent component must be treated with caution. Certainly transcurrent movement, if any, could not have been major, as the thicknesses and lithologies of Tertiary strata on either side of the fault are similar. The thickness and lithologies of individual members and formations of the Te Kuiti Group in the area have been shown {supra) to vary considerably within the space of two or three miles. So any transcurrent movement in the fault must have been considerably less than this distance.
No horizontal offset of any fault segments can be detected where the Maungakirikiri and Cooper Faults cross. So it seems likely that the Cooper Fault is predominantly normal in nature, and vertical movement on this fault only occurred after transcurrent movement, if any, on the Maungakirikiri Fault had ceased.
Numerous faults diverging in an approximately NE direction from the Maungakirikiri Fault, together with subsidiary cross faults, create a complex structure pattern in the NE part of the field (PL 2). The majority of these faults have small throws of between 30ft and 50ft, and the surface traces are readily mappable by dislocation of Tertiary strata. Where exposed, the fault planes are subvertical, and drag folding in immediately adjacent strata indicates that the latest movement was normal.
Rear (1960) has postulated a fault, pre-Tertiary in age, striking 340° from Kaniwhaniwha Stream about one mile south of trig 800 and crossing the headwaters of Cooper Creek. Farther to the north it merges with the equivalent of the writer’s Maungakirikiri Fault. The evidence for such a fault is the recorded occurrence of Athyris aff. wreyi and Hokonuia limaeformis 66 yards downstream from the highest Monotis outcrop. This places Otamitan fossils 200 ft stratigraphically above uppermost Warepan beds, although no break in the surrounding Tertiary strata is evident. The locality was visited by the writer, but no fossils were found. Subsequently (Rear, pers. comm.) the earlier-found specimens were re-examined by Dr J. B. Waterhouse, who recognised an “ arcid brachiopod ” as the only fossil present, and presumably the f Hokonuia limaeformis 3 is an inorganic structure. There being nothing to suggest that the age is Otamitan rather than Warepan or Otapirian, the only reason for inferring the pre-Tertiary fault is thus removed.
Analysis of the Fault System
Lensen (1958) has related fault bearings and the sense of displacement of these faults to a principal horizontal stress (PHS). He has recognised four types of faults (normal, reverse, and clockwise and anticlockwise transcurrent) but has suggested that in general no fault is simple in character, and that all variations in between should be expected, depending on the direction of the PHS.
In the Whatawhata district faults which are predominantly normal (if the Maungakirikiri Fault, whose nature is uncertain, is excepted) have strikes that vary between 030° and 060°, with a mean value about 045°. The transcurrent Kapamahunga Fault has a N-S strike over most of its length, and from its relation to the strike of the normal faults, it should be clockwise transcurrent.
If one period of faulting during which both normal and transcurrent movement took place is accepted, the generalised strikes of faults given above can be used to calculate the average PHS direction during the period. The direction of PHS is generally taken to be parallel to the strike of normal faults, and in the Whatawhata area would be therefore 045°. Thus transcurrent faulting makes an angle of 45° with the direction of greatest horizontal stress, which is in agreement with the theoretical value.
Wellman (1954) deduced a value of 057° for the direction of a late Tertiary PHS in the Waihi Mine area, 50 miles NE of the Whatawhata district. The difference of about 12° in PHS directions between the Waihi and Whatawhata areas can probably be attributed to slightly different times of fault formation in the two areas, to a difference in rock types in the two districts, or to a regional variation of PHS.
Geological History
Wellman (1952) suggested that rocks of the Mesozoic Hokonui Facies, which include the basement strata of the Whatawhata district, represented shelf and transitional beds deposited on the western margin of a major geosyncline, bordered to the west by the Lower Paleozoic rocks of the South Island and their northward continuation off the west coast of the North Island. This shelf slowly subsided and accumulated a great thickness of sediments. In the Whatawhata district the generally fine nature of the sediments and paucity of fossils suggest that deposition occurred some distance from land and in relatively deep water, probably near the outer margin of the shelf. Occasional rejuvenation of relief on the parent landmass to the west caused an infrequent inflow of coarser deposits containing plutonic and metamorphic material. Volcanic activity along the western margin of the shelf, although occurring in the Triassic, became more prevalent during the Jurassic, providing ash and clastic andesitic debris. Widespread submarine slumping during the late Triassic is suggestive of increased tectonic activity at this time. Uplift and erosion of geosynclinal sediments from time to time is shown by the frequent occurrence of rounded mudstone fragments in coarser Mesozoic rocks.
Emergence of the Mesozoic strata of the district, which began in late Jurassic or early Cretaceous times, was accompanied by folding into an anticline and a syncline. These strata, after being elevated by Rangitata orogenic movements, were eroded and weathered during Cretaceous and early Tertiary times, the resulting surface being one of low relief.
Submergence in the late Eocene resulted initially in the formation of coal swamps in low-lying areas, followed by slow transgression of the sea during early Oligocene times and deposition of estuarine beds on the Coal Measures. Further sinking of the land brought about open sea conditions and the accumulation of silt, detrital organic grit, and then fine calcareous sand.
Although younger Tertiary beds are absent from the district, they occur in surrounding areas, and it is probable that sedimentation continued beyond Whaingaroan times, the overlying strata being subsequently removed by erosion.
In post-Oligocene times earth movements again became widespread, culminating in the Kaikoura Orogeny of late Gainozoic age. This tectonic activity was responsible for large-scale faulting that caused considerable disruption of strata in the district.
Acknowledgments
The writer wishes to record his appreciation of the hospitality extended by the residents of the Whatawhata district and in particular by the Aitken and Fillery families. Valuable advice and discussion by members of the staff of the University of Auckland, in particular Mr J. A. Grant-Mackie, and also by Dr R. P. Suggate and Mr D. J. Young, of the New Zealand Geological Survey, has been greatly appreciated. The writer is also indebted to Mr N. de B. Hornibrook for microfaunal identifications, and to Dr G. R. Stevens for checking belemnite identifications.
References
Avias, J., 1953. Contribution a I’etude stratigraphique et paleontologique des formations antecretacees de la Nouvelle Galedonie centrale. Sci. Terre, 1 (1-2): 1-276. Brulez, G., 1957. Etude monographique des Pseudomonotinae Newell du Trias superieur de Nouvelle-Caledonie. Unpublished thesis, Faculte des Sciences, Universite de Nancy.
Campbell, J. D., 1956. The Otapirian Stage of the Triassic System of New Zealand, Pt 2. Trans, roy. Soc. N.Z. 84(1): 45-50. Geophys. 2(1): 198-207.
Campbell, J. D., and MgKellar, I. G., 1956. The Otapirian Stage of the Triassic System of New Zealand, Pt 1. Trans, roy. Soc. N.Z. 83(4): 695-704. Geophys. 3(4): 643-659. Coombs, D. S., 1960. Lower grade mineral facies in New Zealand. Rep. 21st Int. geol. Congr. Nor den 1960 , 13: 339-351.
Cotton, C. A., 1948. Landscape as Developed by the Processes of Normal Erosion, 2nd ed. Wellington: Whitcombe and Tombs, Couper, R. A., 1960. New Zealand Mesozoic and Gainozoic plant microfossils. N.Z. geol. Surv. pal. Bull. 32.
de Ridder, N. A., and Lensen, G. J., 1960. Indirect evidence for transcurrent faulting and some examples from New Zealand and the Netherlands. Meded. {tech. Bull.) Inst. Cultuurtechn. Weterh., 15: 1-13. Fairbridge, R. W., 1946. Submarine slumping and location of oil bodies. Bull. amer. Assn. Petrol. Geol. 30(1): 84-92. Fleming, C. A., and Kear, D., 1960. The Jurassic sequence at Kawhia Harbour, New Zealand. N.Z. geol. Surv. Bull, n.s., 67.
Grant-Magkie, J. A., 1958. The Stratigraphy and Paleontology of Rocks of the Hokonui System, Awakino-Mahoenui Area, South-west Auckland. Unpublished thesis, University of Auckland Library. N.Z. Jl. Geol. Geophys., 2(4): 755-787.
Grant-Mackie, J. A., and Lowry, D. G., 1964. Upper Triassic rocks of Kiritehere, Southwest Auckland, New Zealand, Pt. 1: Submarine slumping of Norian strata. Sedimentology 3(4); 296-317. Henderson, J., and Grange, L. 1., 1926. The Geology of the Huntly-Kawhia Subdivision. N.Z. geol. Surv. Bull, n.s., 28. Henderson, J., and Ongley, M., 1923. The Geology of the Mokau Subdivision. N.Z. geol. Surv. Bull. n.s., 24.
Ichikawa, K., 1958. Ziir Taxionomie und Phylogenie der triadischen ‘ Pteriidae ’ (Lamellibranch). Palaeontographica, 111, A: 132-212. Jones, O. T., 1940. The geology of the Golwyn Bay district; a study of submarine slumping during the Salopian period. Q. Jl geol. Soc. Lond. 95(4): 335-382.
Kear, D., 1960. Sheet 4, Hamilton. Ist Ed. Geological Map of New Zealand I : 250,000, Wellington: N.Z. Dep. sci. indust. Res. Geophys. 4(2): 231-238. Kear, D., and Schofield, J. C., 1959. Te Kuiti Group. N.Z. Jl. Geol. Geophys. 2(4): 685-717. Lensen, G. J., 1958. Rationalised fault interpretation. N.Z. Jl. Geol. Geophys. 1(2): 307-317. 5(1): 175-177.
Lowry, D. C., 1962. The geology of the Kiritehere district. Unpublished thesis. University of Auckland Library. Macdonald, H. A. H., 1951. A petrological study of the Jurassic conglomerate at Kawhia. Unpublished thesis, University of Auckland Library, Magpherson, E. 0., 1946. An Outline of Late Cretaceous and Tertiary diastrophism in New Zealand. N.Z. Dep. sci. indust. Res. geol. Mem. 6. Marwick, J., 1946. The Geology of the Te Kuiti Subdivision. N.Z. geol. Surv. Bull, n.s., 41. Jl. Sci. Tech., B 32(3): 8-10. ———— 1953. Divisions and faunas of the Hokonui System (Triassic and Jurassic). N.Z. geol. Surv. pal. Bull. 21.
Moore, D. G., and Scruton, P. G., 1957. Minor internal structures of some recent unconsolidated sediments. Bull. amer. Assn. Petrol. Geol. 41(12): 2723-2751. Player, R. A., 1958. The geology of North Kawhia. Unpublished thesis. University of Auckland Library. Purser, B. H., 1952. The geology of the Waikato Heads. Unpublished thesis, University of Auckland Library. Speden, I. G., and McKellar, I. G., 1958. The occurrence of Aratauran beds south of Nugget Point, South Otago, New Zealand, N.Z. Jl. Geol. Geophys. 1 (4): 647-652.
Stevens, G. R., 1965. The Jurassic and Cretaceous belemnites of New Zealand and a review of the Jurassic and Cretaceous belemnites of the Indo-Pacific region. N.Z. geol. Surv. pal. Bull. 36. Trechmann, G. T., 1918. The Trias of New Zealand. Q. Jl geol. Soc. Land. 73(3): 165-246. Weller, J. M., 1960. Stratigraphic Principles and Practice. New York: Harper and Bros.
Wellman, H. W., 1952. The Permian-Jurassic stratified rocks. Proc. 19th inter, geol. Congr. Symp. Gondwana Ser.: 13-24. notes on the stress pattern in the north-western part of the North Island of New Zealand. N.Z. Jl Sci. Tech., B 36(3): 201-206.
M. G. Laird, Department of Geology and Mineralogy, University of Oxford, Oxford, England.
•Numbers refer to Fossil Record Sheets.
for Monotis follows Ichikawa (1958)
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Transactions of the Royal Society of New Zealand : Geology, Volume 5, Issue 5, 21 July 1967, Page 131
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14,771The Geology of the Whatawhata District, South-west Auckland Transactions of the Royal Society of New Zealand : Geology, Volume 5, Issue 5, 21 July 1967, Page 131
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