The Geology of the Ruakokopatuna Valley, Southern Wairarapa.

By R. J. Waghorn, M.A.

In the light of these expressed opinions it is of interest to examine the geological evidence as to the origin of the Ruakokopatuna Valley.^*

The covering strata dip towards and end abrubtly against the greywacke forming the north-western side of the valley, which has been described above as a fault-scarp. This as has already been stated, would prove tilting of a block as well as separation of blocks by faulting. The crest of the divide (beyond the westerly limit of the map) above the fault-scarp is some 1250 feet above the plane of junction of the limestone and greywacke as exposed in the Ruakokopatuna River. Since no limestone is found along this divide (all the cover having been removed since the uplift of the north-western block) the minimum amount of displacement at this point is in the region of 1300 ft. If the origin of the Ruakokopatuna valley is due to subsequent erosion along the fault-line with the removal of a great thickness of strata younger than those now present in the valley, it would be expected that these younger beds would somewhere be preserved. However, no younger rocks have so far been discovered, either in the direction in which the fault dies out, or in the direction in which the covering strata dip. On the other hand, rocks of similar age are found to the north, dipping under the gravels of the Wairarapa Plain.

Judging from the nature of the differential movements—i.e., combined faulting and tilting—in this area, it would seem that a course consequent in the fault-angle depression so formed must be established for the river. The very considerable amount of displacement strengthens this supposition. The conclusion that the valley is not due to fault-line erosion, but that it is of consequent origin would appear sufficiently well established.

The Greywacke Rocks.

These basement rocks are much folded and highly inclined, with a general strike of about N. 10° E. In the absence of fossils no certain determination of their age can be made. Their lithological character would correlate them with those rocks forming a great part of the present mountain systems of New Zealand, and classified by Marshall (1912) as of Trias-Jura age.

The Younger Rocks.

A section west to east from the Wairarapa Plains at Masterton, some 30 miles north of the area here described, reveals the Tertiary rocks dipping towards the West. The Wairarapa Limestone of Waitotaran age (Thomson 1919) forms the highest member of a series of sandstones, mudstones, and limestone. The limestone and sandstone beds form steep escarpments facing eastwards towards the coast.

In the Ruakokopatuna Valley the thick clastic strata below the limestone are absent, and the younger rocks rest upon an almost plane truncated surface of the greywacke series. Being present as a veneer on the SE. side of the tectonic valley, they strike N. 53° E. and have a general dip of about 15° towards the north-west. The greatest

thickness of the beds exposed is 700 ft. This thickness of strata occurs in the south-west portion of the area, to the west of the river. It owes its preservation to its position in the fault-angle and to the shifting of the river up the back slope of the tilted block forming the opposite side of the valley. On the dip-slope itself the limestone, where not entirely stripped away, has been much reduced in thickness through the combined agencies of erosion and solution.

The sequence in the covering strata is as follows:—

4. Upper Shelly Limestone.

3. Blue Sands 150 ft., followed by 30 ft. of brown sandy beds: both of these are unconsolidated, with numerous intercalated shell-beds.

2. Lower Shelly Limestone 250 ft.

1. Glauconitic Sand 5 ft.

Description of Strata.

1. Glauconitic Sand. The materials composing this band are extremely friable at the base, but towards the top they become progressively more compact and less glauconitic. The brachiopods Terebratulina suessi Hutton and Neothyris sp. occur throughout the band, but are most numerous towards the base, where they constitute the greater part of the rock. Of the remaining constituents the greensand in the first three feet makes up by far the greater part. In the upper two feet of the layer the greensand is replaced largely by a moderately fine calcareous sand. In addition there is a small percentage of fine argillaceous material. Under the microscope the individual grains of glauconite are seen to vary in size up to 1 mm. in diameter. They possess in the majority of cases rounded and polished surfaces, though instances are common of botryoidal and angular forms. From the varying character of the grains little can be learned as to their mode of origin.

The only fossils collected from this stratum were the brachiopods mentioned above.

2. The Lower Shelly Limestone. The glauconitic band is followed by an impure limestone rock that is composed for the major part of comminuted shells, with, in the different beds, a varying admixture of sand and sparsely distributed small pebbles. The layers vary considerably in texture and in the degree to which they have been consolidated.

The occurrence throughout these beds of graywacke pebbles, coupled with an almost complete absence of other land-derived materials, is difficult of explanation. The pebbles vary in diameter from 3 inches down to very small dimensions. The larger pebbles frequently exhibit a decidedly angular appearance and seem to have suffered little abrasion. The smaller ones, however, are more rounded though traces of angularity are still apparent. This angularity would argue a not far distant source for the greywacke pebbles. Such a conclusion seems, however, at variance with that reached after a study of the other features of the limestone.

The greater part of the limestone is made up of the fragmentary tests of Balanus. In this it shows a decided resemblance to the Te Aute limestone (McKay, 1879) which has been referred to the Plio-

stone is markedly similar to that lying below the sands, and again the broken tests of Balanus contribute the bulk of the rock. Panope is now also of frequent occurrence. Land-derived material is scarce and confined to sparsely distributed small angular pebbles of grey-wacke.

From this upper limestone, where it outcrops at the Ruakokopatuna Bridge, the following were collected:—

• Panope zelandica Quoy & Gaimard.

• Venericardia purpurata Deshayes.

• Chione subsulcata Suter.

• Turritella symmetrica Hutton.

• Balanas sp.

• Ostrea spp.

Recent Species.

• The complete list of the specifically determined fossil molluscan fauna collected from the various beds is as follows, the recent species being marked with an asterisk:—

• Aethocola c.f. nodosa Martyn.

• *Ancilla australis (Sow.)

• Ancilla opima Marwick.

• *Anomia undata Hutton.

• Arca n. sp.

• *Cantharidus sanguineus (Gray).

• Chione subsulcata Suter.

• Crassatellites n. sp.

• *Glycymeris laticostata (Quoy & Gaimard).

• Glycymeris waipipiensis Marwick.

• *Lima bullata (Born).

• Macrocallista n. sp.

• Marcia plana Marwick.

• Modiolus n. sp.

• *Myodora striata (Quoy & Gaimard).

• *Musculus impactus (Herm).

• *Neothais lacunosa (Brugiere).

• *Panope zelandica Quoy & Gaimard.

• *Pecten convexus Quoy & Gaimard.

• Pecten delicatulus Hutton.

• Pecten triphooki Zittel.

• Struthiolaria acuminata Marwick.

• *Terebra tristis Deshayes.

• *Turritella symmetrica Hutton.

• *Umbonium anguliferum (Philippi).

• *Venericardia lutea (Hutton).

• *Venericardia purpurata Deshayes.

• *Zenatia acinaces (Quoy & Gaimard).

• Myllita finlayi Marwick.

It is seen that of these 29 species 16 (55%) are Recent. While it is obviously unsafe to make any definite assertion as to age on the basis of a collection as small as the one here described, it would seem that the Ruakokopatuna beds occupy a position intermediate between Marshall's Waipipi and Target Gully Series. This correlation would make it of Waitotaran (Thomson 1920) age, which Thomson himself has stated to be probably Lower Pliocene.

Geological History.

Reference has already been made to the probable Trias-Jura age of the greywackes in this area. The marked regularity of the almost plane surface of these rocks, upon which the limestones rest, suggests at first sight two possible explanations as to the mode of formation, viz.:—

(1) A wave cut platform formed during the gradual advance of the sea over the land; (2) The peneplained surface of a former land area.

That the relatively plane surface of the truncated beds of the greywacke is not a wave-cut platform is attested by the absence of typical littoral deposits. The sequence of deposits overlying the greywacke commences with a glauconitic greensand. Glauconite is at present forming on the ocean floor for the most part in depths beyond the 100 fathom line and at a considerable distance from the shore. It would seem that the chief condition favourable to the formation of glauconite is that there should be only a small amount of terrigenous material in process of deposition. Such a condition may be realized in deep water comparatively close to shores where no rivers are present to convey their load of sediment to the sea. Furthermore, it would seem possible that glauconite might form even in relatively shallow water were the supply of detrital material not excessive. Such a relation between land and sea would exist where an old peneplained land was partly submerged. Transgression over such a land would be rapid though the amount of submergence might not be very great. Owing to the peneplained state of the still emergent land, terrigenous deposition would be almost absent, and the conditions perhaps not unfavourable to the formation of glauconite. It would seem that some such explanation as this is needed to account for the transition from what is apparently a peneplained land-surface to a deposit generally regarded as of deep-sea origin. It is certainly difficult to conceive of such sudden and catastrophic movements as would be required to convert a land-area immediately into one of deep submarine deposition.

The limestone immediately succeeding the greensand represents a deep-water phase. In its upper layers it shows, by the occurrence of a bed of shells cemented in a mudstone matrix, a return to shallow water. A considerable regression of the sea is proved by the occurrence of the blue and brown sandy beds. This regression was followed by a second transgression during which was deposited the Upper Shelly Limestone. There is little doubt that other beds, now removed by erosion, were deposited above this limestone. Such beds, in their final lithological and faunistic characters would no doubt exhibit a second and complete regression of the strand. Subsequent to the emergence of these Notocene deposits, block-faulting and tilting occurred, and in the fault-angles so formed these rocks have been preserved. From the remaining portions of this area of late Notocene deposition erosion has removed the relatively weak covering-strata. The later geological history of the area has already been dealt with in the section devoted to physiography and the evidence of recent uplift noted.

New Species of Fossil Mollusca.

In the above lists of fossils collected four new species occur. These are:—

• Modiolus n. sp.

• Crassatellites n. sp.

• Macrocallista n. sp.

• Arca cottoni n. sp.

Of these, the first was collected from the Brown Sands, the other three from the Blue Sands.