Hawke Bay Coastal Types

By C. A. Cotton

[Received by Editor February 28, 1955.]

Introduction

On the shores of Hawke Bay (Fig. 1) upheaved tracts that form actual and potential salients alternate with stretches of lowland with drowned coasts. Extensive lowland tracts surround Napier and Wairoa (Cotton, 1950). Cape Kidnappers is the seaward extremity of an actual salient formed by upheaval, being apparently developed from a seaward-pitching anticline that forms part of a coast of transverse deformation; and Mahia Peninsula is probably of generally similar origin. The strongly cliffed coast across the mouth of the Mohaka River, which is now nearly straight, though it borders a recently formed antichnal bulge between Napier and Wairoa, must be thought of, however, as a potential salient. Very recent upheaval is indicated by the features of the lower valley of the Mohaka, and unless this updomed or uparched tract has been bounded on the seaward side by a strong tectonic searp (a monoclinal or fault scarp) close to the present shoreline—an improbable hypothesis that lacks the support of collateral evidence (Cotton, 1950, p. 364)—the sea cliffs now termmating terraces that border the Mohaka River must be due wholly to coastal erosion. Cliff retreat must have followed upheaval—or the erosion that made the cliffs may possibly have gone on simultaneously with the upheaval. If the erosion was preceded by (instead of progressively accompanying) upheaval it must have destroyed a coastal salient, a segment of newly emerged coastal plain, formed on a seaward-pitching anticlinal bulge.

Fig. 1.—Locality map, Hawke Bay coast.

Order of Succession of Changes of Level of Sea and Land

Differential upheavals have been in progress quite recently, as is indicated by the geomorphic evidence (Cotton, 1950), but with only a reconnaissance knowledge of the features of the coast available dating of these movements in relation to the Pleistocene-Recent world-wide oscillation of ocean level is as yet a matter for speculation. The rejuvenated condition of the lower valley and mouth of the Mohaka River, excavated in a terrain of very soft rock, suggests that mobility has persisted within the last few thousand years, and the suggestion gains support from the fact that this district is strongly seismic; but it is reasonable to suppose that movement was already in progress as far back at least as the Last Glacial age. Such an assumption implies that potential shore-line advance due to emergence of coastal-plain salients was interfered with—interrupted and very probably reversed for a time—by the post-Glacial (or Flandrian) eustatic return of ocean water to a level much higher than that to which it had sunk in the Glacial age.

Potential Initial Forms Of Anticlinal Coastal Salients

In the diagram (Fig. 2) seaward-pitching anticlines are shown as at least potentially diversifying the coastal outline—though it must not be lost sight of that there is an alternative hypothesis that such features are progressively destroyed, or extensively modified in outline, by marine erosion as they emerge (Cotton, 1954, p. 358). No attempt has been made in the construction of this diagram to show progressive development or destruction of these features. They

Fig. 2.—A coast of transverse deformation on soft rocks: the Hawke Bay type. Seaward pitching anticlines of surface deformation are shown with potential mitially emergent forms dotted. The strip A shows an early stage of erosional development when sea-level was low; B shows the Kidnappers and C the Mohaka type of sequential development (B young and C mature erosional stages). Inset sketch: Cape Kidnappers.

are shown (fully formed) in dotted outline as smooth salients exposing upheaved sea-floor. In places these might, of course, merge with one another to make a corrugated coastal plain with a broadly crenulate outline. In another respect the diagram fails (perforce) to show alternative possibilities; for it cannot be claimed that full anticlinal development, or rather the whole of the diastrophism responsible for emergence of salients, took place prior to the Flandrian rise of ocean level to approximately its present position. It is highly improbable, indeed, that such an assumption would be correct; but it has been made to simplify construction.

Development of Present-day Forms on Anticlines

below sea-level in a way that warrants the classic theory that wave energy is generally focused on headlands and deflected from the axes of bays (embodied in the diagram, Fig. 3, left).

Such more or less, to cite an example, is the condition on the south side of Banks Peninsula. There a strong ocean current that sweeps the coast prevents accumulation of fine sediment; but on the north side of the peninsula, which is not similarly current-swept, a remarkable infilling of the sea with very fine sediment has taken place, building out the marginal shelf so that it has become a truly plane sea-floor (Cotton, 1951, p. 119). The slope is only 3 to 4 feet per mile from a depth of 8 fathoms, very close inshore, to 12 fathoms at a distance of 12 to 15 miles from the land (Cotton, 1951, Fig. 8). This very uncommon sea-floor condition allows an easterly to north-easterly swell to approach the peninsula without any refraction (as in Fig. 3, right), and the free waves continue on undeflected up Lyttelton Harbour and the adjacent bays, which are also flat-floored, so that these are a remarkable exception to the general rule that even very imperfectly landlocked bays have smooth water in them and are but little affected by ocean swell (Cotton, 1949).

Mohaka Type

If the Kidnappers type of eliffed coast is to be regarded as an example of youth in the cycle of sequential development of a coast of transverse deformation, the Hawke Bay coasts afford also an example of full maturity; it is the straight cliffed coast across the mouth of the Mohaka River. Here nothing remains of the hypothetical coastal-plain salient (dotted in Fig. 2, C) produced by seaward-pitching anticlinal upheaval, and the line of the existing coast transects the hinterland (or “old land” at the rear of a coastal plain). It is very difficult, however, to avoid assuming that a coastal feature of plunging anticlinal form has emerged here and has been entirely destroyed by marine erosion either during or subsequently to emergence (Cotton, 1950, p. 364). It is not necessary to assume that the whole, or indeed a great part, of the very considerable retro-gradation thus implied was effected since the positive movement of sea-level accompanying the post-Glacial return of the ocean; but it is in part of this late date, and development of the sea cliffs still continues.

The essential features of the lower valley of the Mohaka River are indicated in Fig. 2, C, and Fig. 4. Rejuvenation in the lower course of the river is attributable mainly to recent upheaval localized on an axis between Napier and Wairoa (Cotton, 1950, Fig. 1). The river is a powerful one, and it covers the floor of the inner valley with a flowing stream; there is no tidal estuary. Extreme youth (since rejuvenation) is indicated by the absence of any flood plain, by the fact that the deeply incised meandering trench in which the river flows is still no wider at the bottom than the river itself, despite the extreme softness of the mudstone in which the whole valley is excavated. Wide terraces at 300 feet above the river on either side preserve almost intact the extensive flood plain that formed the floor of the valley in a penultimate cycle, and the contrast between the narrowness of the steep-sided inner trench and the width of the 300-foot terraces emphasizes the shortness of the interval since the uplift as compared with the length of stillstand preceding it, which was itself quite a

Cyclic Development of Coasts of Transverse Deformation in General

It may not be out of place to conclude this account with some remarks on coasts of transverse deformation, their place in classification, and in particular the theory of their sequential development.

Advocates of the doctrine of a continental marginal flexure intermittently or continuously active in the past (for example, Bourcart, 1950) are of the opinion that even in some stable regions longitudinal deformation of that kind is currently, or has been very recently, active. This does not, however, generally involve recognition of sharp flexure or coastline faulting such as makes tectonic-scarp coasts; and the diagnostic characteristics of coastal outlines that may be attributed to marginal flexure as generally understood are the same as those of coasts of emergence and of submergence (with the exception of a hypothetical case in which the axis of flexure reaches and crosses the sea margin obliquely, bringing features diagnostic of emergence and of submergence into juxtaposition). It may be long, therefore, before the problem of the marginal flexure and its effects in stable regions is settled satisfactorily from the point of view of geomorphology. (Compare Cotton, 1955).

In stable regions coastal evidence of transverse deformation also is, if not absent, at least obscure—that is to say, such deformation as may have taken place very long ago in regions that have since become stable. It is so overlaid with the effects of more recent changes in the relative levels of land and sea, some perhaps

epeirogenic, others certainly eustatic, that the traces of an anterior tectonic history have become obscure and no longer lend themselves to investigation by ordinary geomorphic methods.

Search for tectonic coasts, by which may be understood those produced by longitudinal and also by transverse deformation, may be confined therefore to the mobile belts of the earth, especially to the tectonically active (seismic) parts of the mobile belts, which may be expected to furnish the freshest (best preserved) examples. It may be confidently deduced, moreover, that land areas in such regions will be bounded to a considerable extent by tectonic coasts, among which those of transverse deformation will be quantitatively important. Any continental or island region that is in the throes of writhing deformation or is breaking up into tectonic units of domed, anticlinal, or fault-bounded form will thrust salients out seaward, so that its new-made boundaries will be determined at least as much by transverse deformation as by longitudinal flexure and faulting Undoubtedly many examples of transverse deformation await diagnosis. One part after another of the diversified coastline of New Zealand has been recognised as thus initiated (Cotton, 1942a; 1947, p. 372; 1950; 1955).

Though the category of transverse deformation was suggested in 1942 (Cotton. 1942a; 1942b, pp. 442, 475), and had been foreshadowed much earlier (Gulliver, 1899, p. 170), as of some importance in the classification of coasts—that is to say, of the initial forms from which actual coasts have been developed by sequential processes—no elaboration has been attempted hitherto of a cycle of sequential development applicable to such coasts. As noted on earlier pages, it is possible to recognize on the shores of Hawke Bay examples of young and mature stages of development. Probably a further elaboration or deductive analysis would be of little value, however, for the normally large scale on which the diversifying features of these coasts are initiated is obviously too large to allow of sufficiently extensive sequential development to produce significant changes that affect and modify the outlines of the coast as a whole in the short time interval that may be allowed for the current marine cycle, and probably in earlier cycles also.

Cyclic treatment is thus best reserved in most cases for individual elements or segments of such coasts, for a coast of transverse deformation will normally comprise or include a collection of coastal types that may be classified in other categories, including locally depressed and locally upheaved segments, as well as stretches of fault and longitudinal-flexure coast.

As compared with the coasts of transverse deformation diagnosed in southern Wellington and southern Westland (Cotton, 1956), the terrain on which the transverse warping of the shores of Hawke Bay has developed consists of exceptionally soft, easily eroded rocks. The tempo of erosional change of form and in particular of outline on such materials is enormously fast, and as a result exceptionally rapid cyclic development has taken place, which has here compressed the sequence of stages into so short a span of time that even development to maturity has been possible.

References

Bourcart, J., 1950. La théorie de la flexure continentale, C. R. Cong Internat Géogr (Lisbon), 2 pp. 167-190.

Cotton, C. A., 1922. Geomorphology of New Zealand: Part I. Systematic. Wellington Dominion Museum