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Volume 78, 1950
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Two New Zealand Rivers Following Tertiary Transverse Furrows

[Read before N.Z. Science Congress, May, 1947; received by the Editor, April 11, 1949.]

Summary

The hypothesis is advanced that at Ohai (Southland) a river that was in existence until diverted very recently by capture marks the approximate course of an ancient Tertiary river, and that the Manawatu River (North Island) follows the course of a Tertiary strait through the Ruahine Range. Stress is laid on a rude correspondence between Tertiary and Recent physiography. Both rivers appear to have been tectonically initiated in furrows, at localities of maximum axial pitch, which cut transversely across the dominant trends of post-Pliocene deformation. Such furrows may mark the remnants of some ancient pattern, probably of Cretaceous or earlier date, along which a certain amount of posthumous movement took place at the end of the Tertiary Era.

Introduction

The source of the Manawatu River (Fig. 1) lies on the eastern side of the Ruahine Range, but the river, after following a south-westerly course for some distance, swings abruptly and flows in a narrow gorge through the Ruahine Range, following an approximate west-north-

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Figure 1

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westerly course. It then resumes a general south-westerly course and flows out to sea on the west coast. The course of this river, which cuts across the dominant north-east strike of the folds and faults formed in the post-Pliocene, Kaikoura, orogeny has aroused the interest of several geologists. Regional mapping suggests that the Tertiary stratigraphy sheds light on the likely origin of this river.

The Ohai coalfield lies near the extreme end of the South Island, some thirty miles north-west of Invercargill in a physiographic depression between the Longwood Mountains and the Wairaki Downs. A study of the Tertiary stratigraphy again gives a clue to the physiographic history which suggests that until very Recent times a river flowed from west to east through the depression.

A Hypothetical Wallace River
Features of Western Southland

The geology of the Ohai Coalfield has been described in part by early workers on the Geological Survey and a description has been included in Park's Geology of Western Southland.

Within sight of the Coalfield are three dominant structural and physiographical features with which the geology of the field is intimately connected.

West of the Waiau Valley (Fig. 2) and about twenty miles from Ohai, a chain of mountains stretches north–south, more, or less continuously from the Coast to beyond Lake To Anau, a distance of over one hundred miles. This chain of mountains, reaching summit heights of over five thousand feet and known by various names along its length, is referred to in this paper as the “Main Ranges.”

The geology of the chain is incompletely known, but the rocks appear from Park's mapping to be all of gneissic character, with a composition generally elose to the granitic or dioritic.

East of the Main Ranges, the Waiau Valley has also a north–south trend and marks a great syncline of Tertiary strata; steeply dipping Tertiary coal measures are known at a few localities on the east side of the syncline and marine Tertiary strata occupy the axis.

To the east of the Waiau Syncline an anticlinal chain is constituted by the Takitimo Mountains and their foothills, termed the Wairaki Downs. These hills are separated by the Ohai-Nighteaps Depression from the Longwood Mountains which, following the same generally northern trend line as the Takitimos, stretch as far as the south coast. The Takitimos, the Wairaki Downs and the Longwoods are formed of greywackes, argillites, porphyrites, various volcanic rocks, breccias, etc., and the geology of these mountains is also little known. Fossils found near Nightcaps indicate that part of the greywacke strata are of Triassic age, but probably a large part of the mountains is formed of Permo-Carboniferous strata, including volcanics.

The valley of the Aparima, east of the Wairaki Downs, is eut partly in Mesozoic strata, but broadens to the south to join the great Southland Plains. Along the edge of the Longwoods, Tertiary strata outcrop occasionally and isolated inliers of Tertiary rock interrupt the plains, hut otherwise the Southland Plains are covered by a thick blanket of

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Figure 2

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gravels enclosing occasional lenses of lignite. These gravels are largely or entirely of Recent age.

The Longwoods and Takitimos have been described as forming a median range, but topographically they form two separate ranges broken by the Ohai-Nightcaps Depression, in which the distinction between country occupied by coal measures and that formed of the neighbouring Basement strata is striking; the summits cut in the Mesozoie strata, even in close proximity to the coal measures, reach heights of more than 1,200 feet, whilst at few places are the coal measures found at higher altitudes than 850 feet. The coal measures in this depression have all at one time been covered by alluvials, cappings of gravel overlying the measures even at the highest points. The whole depression physiographically consists of several terrace remnants which link the Southland Plains with the Waiau Valley, and from the higher points within the depression the traveller has an extensive view to the west and to the east.

Structure of the Ohai Coalfield (Fig. 3)

The general strike of the Tertiary measures in this depression, as well as the trend of the faults separating Tertiary and older rocks, is approximately at right angles to the dominant north–south trend of the main structures in Southland, and the coalfield is situated in a graben, the greywaeke and other Mesozoic rocks being usually separated from the outcropping coal measures by boundary faults. It may be suggested that certain of these boundary faults, whose course can be mapped, although their planes are rarely visible, existed before the deposition of the coal measures, but there is no clear demonstration of such early formation. Certainly, many of the faults are largely of post-Tertiary origin, judging from the steep dips shown by the coal measures in the vicinity of the faults and from the extent of folding and faulting which has affected the measures as a whole, but some of the larger faults may have been initiated during Tertiary or pre-Tertiary time.

The faults are not described in detail here; it is sufficient to notice that the southern edge of the coalfield is marked by the line of the Twinlaw fault, which juxtaposes Mesozoic and Tertiary marine strata and which is likely to have a throw of at least 2,000 to 3,000 feet, whilst the northern limit of the field has an irregular boundary composed of several faults, some of which are of considerable throw but unlikely to be of the same order as the Twinlaw fault.

Two small outliers of coal measures lie to the north of the main coalfield, separated from it by upstanding barriers of greywacke, and at one of these outliers a smooth and partly dissected surface of greywacke appears to dip under the coal measures. North of the coalfield, high smooth surfaces on the Wairaki Downs are interpreted as remnants of an exhumed pre-Tertiary erosion plane, presumably the same as the great Cretaceous peneplain described by Park and others, but the peneplain has obviously been much broken by faulting.

The Tertiary strata of the coalfield can be subdivided as follows:—

Shally limestone bands 100 feet plus
Marine mudstone with Whaingaroan microfaunas 1,100 "
Fresh-water mudstones and clay ironstones 2,300 "
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Figure 3

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Fig. 4—The Manawatu Gorge in relation to major structural elements. The Ruahine Range consists of greywacke. The Dannevirks Syncline is formed of Pliocene strata which cover greywacke cores outcropping in the Waewaepa-Oruawharo Fault-belt, East of the latter belt a composite syncline of Oligosene and Miocene beds flanks the western edge of the Whangai Range which discloses Cretaceous cores.

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Coal measures, consisting of sandstones, conglomerates, mudstones, claystones and thick lensing seams of coal 1,050 "
The basal beds rest unconformably on greywackes and porphyrites, the greywackes having yielded good Triassic fossils at the base of a bore through the measures as well as in the adjoining outcrops.

The thicknesses of these strata are approximations based on composite successions, but the order of superposition is certain. Lithologically, the important feature of the coal measures is the great abundance of boulders of granite-gneiss and porphyrite, never exceeding 1 foot in diameter, as well as the common presence of cobbles of finer volcanic rocks. The sandstones, notably felspathic, quartzose and micaceous, are largely formed from weathered material of gneissic composition, and evidently much of the material came from erosion of the Main Range; but the cobbles of porphyritic and volcanic rock probably came, not from the Main Range where such rocks are unknown, but from the Takitimo and Longwood Mountains, or perhaps from the Livingstones further north. Evidently we must envisage the whole coalfield as infilled during part of Tertiary time with coarse gravels deposited by a river coming from the west, and considerable bodies of swampy water must have extended where the stream waters were ponded from time to time, with formation of peat swamps. Although the Mesozoic and earlier rocks were probably pepeplained before the deposition of the Tertiary, it seems likely that considerable warping and faulting of the peneplained surface occurred in pre-Tertiary time, for the boulders in the conglomerates are both large and well-rounded. The nature of these boulders suggests erosion of a country of marked relief, dominated by an uplifted gneissic chain lying west of the present Waiau Valley and probably also with hills of fairly marked elevation along the line of the present Longwoods and Takitimos. Thus, the Ohai Depression is believed to mark the approximate site of an ancient Tertiary valley, an interpretation which supports partly the early view of Hutton whilst still attributing the main tectonic pattern to the Kaikoura orogeny of late Pliocene or Pleistocene age.

Physiography of Ohai Coalfield

A view from the Southland plains in the direction of Ohai and Nightcaps is striking, as the eye follows continuously the smooth plains to a broad, flat, surface, with incised streams separating the steep northern face of Twinlaw from the hills to the north of the coalfield; to all appearances a major river flowing eastwards from the Waiau Valley might be expected; yet there is no stream flowing through this apparent gap, and the largest streams meandering within the coalfield are of little size. The Ohai field is a region of very extensive Recent alluvial flats contrasting markedly with the meagre flow of water in the present streams whose catchment areas are remarkably small. The explanation of this phenomenon lies in the physiographical history after the Pliocene orogeny, a history which links the geography of the Tertiary period with that of the present day.

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A closer examination of the topography of the coalfield reveals three comparatively flat surfaces at different levels separated by steep slopes. In the Nightcaps region these surfaces in ascending order are:

1. The river flats adjoining the Wairio stream (elevation approximately 500 to 550 feet between Nightcaps and Wairio), flats which slope to the south and widen very considerably at Wairio. The present stream flows only some five to ten feet lower than this surface.

2. The flat surface, a terrace remnant, on which the town of Nightcaps (elevation approximately 600 feet) and the group of houses on the Nightcaps–Tinker Town Road (elevation approximately 600 to 630 feet) are situated. These terrace remnants have a general southerly slope.

3. The top of White Range (maximum elevation approximately 825 feet), actually a flat gravel-covered plateau, partly dissected by streams flowing south-west.

In the Ohai region, similar surfaces can be easily detected, namely:

1 The Morley Valley sloping to the west with an elevation of 500 feet near the Morley bores (north of Trig T), increasing to 600 feet near Black Lion Mine (south-east of Trig U). The flats of the Orauea Stream, etc., accord with, the Morley Flats.

2. The plain on which Ohai is built (elevation approximately 690 feet at Ohai). Near Ohai itself it is not possible to specify a general direction of slope. This Ohai surface is covered by a layer of gravels, reaching a maximum thickness of 50 feet to the west of Ohai.

3. The White Range surface already noted.

All these flat surfaces are covered by gravels and mark ancient periods of still-stand.

If one looks from Ohai or Little Wairio Hill towards the eastern flanks of the Takitimos and slightly west of the upper course of the Morley Stream, one perceives a remarkable series of flat shoulders lying in one plane with a general southerly slope; these shoulders lead the eye through a marked high gap to the valley lying between the eastern flanks of the Takitimos and Letham Bush, where the head waters of the Wairaki River flow south previous to swinging abruptly on a south-westerly course (Fig. 2). The prominent shoulders suggest that the principal drainage of this part of the Takitimo Mountains formerly flowed through the Ohai field. These shoulders can be traced as probably accordant with the White Range surface. Moreover, there are traces of a very high and remarkable flat (between 1,050 and 1,100 feet in elevation) preserved on the south-east and south-west slopes of Mt. Franklin, whose summit projects as a steep monadnock above this surface. The same surface can also be followed around the slopes of Mt. Linton and at some height above the bed of the Linton Creek for a considerable distance norths wards into the Wairaki Downs, where a high surface bevels the hill tops as far as the south edge of the Wairaki River.

On the Mt. Franklin surface the loose boulders in the gravel reach a diameter of two feet, and boulders of similar dimensions, have been found on one of the shoulders west of the upper branch of the

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Morley Stream and a short distance south-west of Trig Z. The coal measures at White Range are covered by boulders, often difficult to separate from the Tertiary conglomerates and probably partly derived from them; thick boulder beds are also piled against the shoulder of Trig H. and underlie a smooth surface which slopes south-east from the shoulder. All these gravel-covered surfaces can be correlated by eye, giving a conception of an ancient terrace system with a general slope to the south-south-east, representing the highest surface of alluvials in the field. The cycle during which this surface was formed is termed in later discussion the White Range cycle.

Accordant with the Ohai surface are comparatively flat stretches of ground above Black Lion Mine (elevation 700 feet) and north of it; the flats are covered by boulders of 2ft. maximum diameter; there is also a shelf cut in coal measures south of Franklin Fault and west of Meadow Creek. Farther north the surface only occurs as very dissected remnants and is not easily distinguished, but south of the Orauewa Stream the Ohai surface is probably represented by a very distinct and long smooth surface (elevation between 650 feet and 700 feet) that projects north-east from the Twinlaw scarp. A shoulder (elevation 640 feet) on the edge of the Twinlaw scarp west of the Blue Bottle Creek can be assigned to the Ohai surface and also the wide flat-topped hilly ground (elevation 600 to 630 feet) east of the Blue Bottle Creek. On the western margin of the coalfield the Ohai surface cannot be followed clearly as the western edge of the Ohai plain has been considerably dissected very recently. West of Birchwood the only flat surfaces that may be accorded with the Ohai surface are at a higher elevation and a high surface on the road between Orawia and Clifden may be only very tentatively regarded as the projection of the Ohai surface.

Ancient River Hypothesis

Within the Ohai field the slope of the Ohai surface, accepting the remnants described above as belonging to it, is approximately southcast towards Little Wairio Hill. It is probable that this Ohai surface and the White Range surface are remnants of the alluvial flats of a river system which flowed east, from the present Waiau Valley, draining the catchment area of part of that valley.

The conception of such an ancient east-flowing river, here termed the Wallace, explains in a satisfactory manner the changes in drainage between the time of deposition of the coal measures and the present day, and these changes, as they are believed by the writer to have occurred, are now presented chronologically with additional supporting evidence.

A study of the stratigraphy of the Coal Measures suggests that during the early part of the Tertiary period a large river flowed eastwards bringing great quantities of material from the Main Ranges. After the deposition of the measures a lacustrine phase followed consequent on gradual submergence which later caused the Tertiary sea, formerly lying east of the coalfield, to encroach over a large part of Southland and over most of the Waiau Valley, though it possibly left certain parts of higher masses of rock uncovered.

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During Pliocene and post-Pliocene time there were the considerable earth movements of the Kaikoura orogeny, leading to the formation of the main structural clements as we know them now and to the re-emergence of the larger part of Southland. The Main Ranges and the median ranges comprising the Livingstones, the Takitimos and the Longwoods may have first emerged as island chains of pronounced relief, and by the time that most of Southland had become land streams would have been well-established. One of the principal streams, the Wallace, continued to follow the ancient tectonic and physiographic gap at Ohai, which probably had not been completely obliterated by the Kaikoura (Pliocene) orogeny, and, indeed, was more likely accentuated by it. From the high land of the Takitimo Mountains, then possibly uplifted in the form of a dome, streams flowed south to join the Wallace in the Ohai Depression. During the White Range cycle, the first which we can clearly perceive consequent on the Pliocene crogeny, erosion quickly removed any cover of Tertiary strata that may have existed in the Takitimos and Wairaki Downs and formed the high shoulders carved on greywacke which now lead the eye so foreibly towards the north.

In the Ohai region, both the Wallace and the tributary streams from the Takitimos flowed through Tertiary strata and the main river flowed out to the Southland Plains at some point between the lower slopes of Trig H and Wairlo Hill. Once the streams had levelled their courses an aggradational phase followed, and gravels containing large boulders were deposited on the surface previously planed. These boulders were piled against the shoulders of Trig H, where the erosive power of the streams had previously cut away much of the coal measures stepped against the Quested fault, and similar boulders were laid on the slopes of the present Mount Linton and Mount Franklin whose summits then stood as low hills, and on White Range and near Little Wairio Hill.

At Little Wairio Hill there is a curious capping of gravel containing boulders of granite-gneiss, porphyrite, greywacke and schist. All the boulders are well-rounded and, reaching diameters of over 2 feet, are of a size unknown in any of the conglomerates in the coal measures and only to be likened to the boulders found on the high flat surface near Mount Franklin, and on the shoulders near Trig Z. They do not necessarily belong to the White Range cycle, and, indeed, from their present low position, are best regarded as rewashed material of that cycle, but they are important evidence of drainage from the direction of the Main Range in Recent time.

Following on the White Range cycle there was renewed uplift and a period of great down-cutting followed. Along the course of the Wallace River the ground was levelled with the formation of great flats covered, during the following aggradational phase, by the deposits of gravel, at least 40 feet thick at Ohai, that underlie the great Ohai surface. The Outlet for the Wallace River at the end of the Ohai aggradational cycle can be localised as either immediately north of Little Wairio Hill, where the minimum elevation is approximately 580 feet, or through the Wairio Graben, south of Little Wairio Hill, where the minimum elevation is approximately 540 feet. These minimum

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elevations cited are the lowest breaches in the ground stretching from White Range to Wairio Hill through which the Ohai surface might be traced accordantly—to accept any other points is to make dislocation of thesurface by very recent faulting necessary. Gravels are known at both points.

The valley of the Wairio Graben is topographically remarkable because in its upper part it is very flat and in accordance with its surface and west of it there are flat surfaces adjoining the Bluebottle Creek, which may possibly be accordant with the Ohai surface; it seems likely that the Wallace River, probably in some senile stage later than the Ohai aggradational cycle, passed through this valley, which is a striking physiographical feature unlikely to have been eroded by the small and very short Gorge Creek. The Bluebottle Creek has a wide upper valley and the water from the upper part of the Bluebottle, a former tributary of the Wallace, is likely to have been formerly discharged to the east. At the close of the Ohai cycle we can visualise a large river meandering sluggishly eastwards across a wide valley which stretched from the Twinlaw scarp to Meadow Creek and perhaps some distance north of Malakoff Hill. Possibly some, if not all, of the drainage which formerly flowed from the Takitimos had already been diverted to follow its present course through capture by the present Wairaki River.

Between the Ohai cycle and the erosive cycle which scoured out the present Morley and Wairio Valleys, the Wallace River was diverted to become the Waiau as we know it now. The cause of this diversion is obscure. Tilting may be invoked which would result in increasing the erosive power of a capturing stream to the west of the coalfield; perhaps a limestone barrier which formerly prevented the capture was worn clown to the critical point at which the drainage was diverted, or probably—most likely of explanations—a consequent stream which had become installed in the lower reaches of the Waiau syncline was able, simply by its greater erosive power resulting from its much steeper grade, to capture the upper part of the ancient Wallace. This period of capture probably coincided with a period of emergence, with or without tilting, and possibly accompanied by minor faulting. The capture of the Wallace, which reached the sea coast by a circuitous route, probably accelerated the erosive cycle initiated by uplift. The lower part of the senile Wallace perhaps continued as a remnant flowing east for some time with a very limited catchment area, but eventually most of the catchment basin within the coalfield went to feed the more active streams flowing to the west. Thus, great quantities of material must have been scoured out by the early Morley and the Orouewa and the Bluebottle, now diverted to become a tributary of the Orouewa.

It is very difficult to visualise the drainage which scoured out the valleys of the Morley and the Orouewa without imagining a very much greater rainfall than the present: even making allowances for the higher land masses and consequently greater area that must have been drained during this scouring, the catchment area cannot have been very much larger than at present; some profound elimatic change is likely to have occurred possibly simultancously with the retreat of the glaciers in the country to the west and north-west.

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The Manawatu River and Gorge

Just as, from a study of the palaeogeography combined with the evolution of the Recent physiography, it seems likely that the present topography at Ohai is based on a pattern sketched before or during early Tertiary time; so with the same working hypothesis one may approach the Manawatu Gorge.

Of the several theories advanced to explain the origin of the gorge by which the Manawatu River cuts through the Ruahine Range (Fig. 4), only those of Adkin, Cotton and Ongley stress the marked axial pitch of the Ruahine anticline in the vicinity of the Gorge. Ongley's paper contains a good summary of all other authors' views and covers most of the available evidence. The Ruahine chain is essentially anticlinal in terms of its Tertiary cover, and he has described an axial depression on this anticline.

Reconnaissance field mapping in the Dannevirke Subdivision has confirmed Ongley's structural interpretation, and added some significant details bearing on the early history of the gorge. Whilst the main trough of Cretaceous and early Tertiary sedimentation appears to have been situated approximately along the Whangai Anticline, during early Pliocene time this axis appears to have shifted westwards following some differential uplift along the Whangai axis, and at intervals considerable volumes of greywacke conglomerate were deposited along with the Pliocene limestones in a region lying between the present Ruahine and Whangai Ranges; the only likely source for these conglomerates was the Ruahine Range, which must be postulated as being then partly emergent. At the Gorge itself, the Tertiary succession, resting directly on the erosion plane, and truncating the greywacke, represents most of the Pliocene stages and shows numerous bands of sandstone, and conglomerates formed of large greywacke boulders (maximum diameter 2ft.); these sediments are of shallowerwater character than the Pliocene strata lying immediately west of the Waewaepa Range. Fleming and the writer have concluded that the vicinity of the Gorge was a strait during the Pliocene, and Fleming has already referred to this strait in his paper on Pliocene climatic changes.

The Castlecliffian stage in the Dannevirke subdivision is represented by interstratified conglomerates, lignites, pumice sands, and mudstones —these last, bearing a fauna which Fleming has interpreted as of typical mud-flat facies. The stage represents the final infilling and emergence of the Tertiary basin and at the Manawatu Gorge the Castleclifflian sediments rest on a worm-bored surface of Nukumaruan beds.

The coincidence of the present Manawatu River with the site of the ancient strait is very striking and recalls Cotton's definition of an anteconsequent River as “guided by the first wrinkles of the surface as it emerged from the sea and maintained the consequent courses thus assumed during a continuation of the movements…” It seems, then, reasonable to assume that on the early emergence of land the strait, became the course of an incipient river draining the declining Castleclifflian mud flats—a river whose course may have been roughly coincident with that of the present Manawatu. Never-

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theless the evidence on the history of the river remains obscure for the time of the final Kaikoura orogeny, already heralded by preliminary movements during the Pliocene ages. The obstacle presented by the greywacke bar at the gorge has been magnified because there has been a tendency to ascribe all movements affecting the late Tertiary beds to a rapid and final Kaikoura orogeny; if, however, we conceive of many of the movements as commencing before and during the Castleclifflan age, we can visualise a considerable infilling of the basin to the east of the gorge and the river as being initiated on a surface in which the bar was covered. Later and considerable movements of the Kaikoura orogeny certainly occurred in the form of reversed faults cutting across the course of the river, but it is assumed that the corrasive power of the river was sufficient to keep pace with these differential uplifts. As the Pliocene succession thickens markedly east of the gorge, it is likely that the Kaikoura movements imparted to this wedging succession gave the river a comparatively steep gradient.

The only direct physiographic evidence bearing on the early history of the gorge consists of a small area of mature topography with marshy streams perched above deeply ravined slopes; it may represent a block of ground differentially rejuvenated in relation to now-eroded country to the east or a surface which was once continuous for a considerable distance to the east and now completely removed with the exception of the portion near the gorge.

The writer's views correspond in many ways with the ideas earlier expressed by Adkin. who insisted on close tectonic control of the Manawatu's course, although Adkin had not the supporting stratigraphic evidence presented here.

Considered from a purely physiographical aspect, the land forms represented by the Manawatu Gorge are remarkable and difficult of explanation, but when Tertiary palaeogeography and tectonic evolution are taken into account. their origin can be more reasonably explained. In general, the evidence at the Manawatu Gorge is less clear than in the Ohai region but suggests the same Tertiary initiation of physiographic elements foreshadowing present land-forms, and these physiographic elements seem to be directly related to tectonic accidents.

Axial Depressions and Tertiary Palaeogeography

In Otago Benson insists that “the present Taieri-Waihola-Tokomairiro depression, which is a faulted syncline resulting from post-Pliocene crust-movements is parallel to and at most a few miles west of the Mid-Cretaceous fault-bounded depression. Once again, the tectonic character of Eastern Otago seems remarkably persistent.”

The tectonic development of the Dannevirke region is in accordance with Benson's view in suggesting that early in the Cretaceous-Tertiary a north-east trend similar to that of the Post-Pliocene movement becomes dominant; but the writer is here concerned to focus attention again on the survival (both in the Damievirke and Ohai regions) of some transverse furrowing very roughly at right angles

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to these principal Kaikoura trends. Benson and Paterson have already drawn attention to such features, notably at the Shag Valley fault in Otago. Adkin also appears to have early recognised the existence of a transversal element in the Tertiary tectonic pattern, although his evidence at the time was extremely slender.

The palaeogeography of the district near the present Manawatu Gorge is unknown for pre-Pliocene time, but at Ohai fluviatile conglomerates of the coal measures belong to either the early Tertiary or even the late Cretaceous stages. The ancient Wallace, whose existence during Recent times is postulated (on strong grounds), evidently followed a structural “low” for it flows through a marked fault depression cutting transversely the main Kaikoura trends. But it seems likely that its course was more than structurally controlled, that it did not follow that course through “working down” to find a structural weakness, but that from the first it flowed through a gap which had existed earlier during Tertiary time, that the river was tectonically determined in so far as its Recent history is concerned.

It is interesting to consider what decided the course of the more ancient Tertiary Wallace, the river which deposited the conglomerates of the coal measures. Admittedly, we cannot be sure whether this more ancient stream was a single and prominent feature or one of many parallel streams pouring their burden eastwards, but the coincidence of the thick conglomerates with a graben cutting transversely the Kaikoura trends leads to the not entirely idle speculation that this graben, with some evidence of axial pitch north of it, may mark the accentuation of an old pre-Tertiary tectonic element coincident with the ancient Tertiary Wallace, a tectonic element which has been revived posthumously during the Kaikoura orogeny.

A similar cause can also be tentatively advanced for the position on an axial depression occupied by the Tertiary Manawatu Strait, and many other similar depressions are likely to have special significance in reconstructing the geography of the Tertiary period.

The writer is grateful to the Director of the Geological Survey for permission to publish this paper.

References

Adkin, G. L., 1930. The Origin of the Manawatu Gorge, N.Z. Journ Sc. and Tec., vol. 11, pp. 353–356.

Benson, W. N., 1941. The Basic Igneous Rocks of Eastern Otago and their Tectonic Environment. Trans. R.S.N.Z., vol. 71, pp. 208–222.

Cotton, C. A., 1922. Geomorphology of New Zealand, Dominion Museum, Wellington.

Fleming, C. A., 1941. Molluscan Evidence of Pliocene Climatic Change in New Zealand. Trans. R.S.N.Z., vol. 74, pp. 207–220.

Lillie, A. R. and Fleming, C. A. 1941. Dannevirke Subdivision. 35th An. Report Geological Survey Branch, S.I.R. Dept.

Ongley, M., 1935. Manawatu Gorge. N.Z. Journ. Sc. and Tec., vol. 16, pp. 249–260.

Park, J., 1921. The Geology and Mineral Resources of Western Southland. N.Z.G.S. Bull., 23.

Paterson, O. D., 1941. The Geology of the Lower Shag Valley, N.E. Otago. Trans. R.S.N.Z., vol. 71, pp. 32–58.