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Volume 81, 1953
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Outline of Stratigraphy

The two new Southlandian species were collected from compact blue-grey silty mudstone in the south branch of Whakatahine Stream about a mile north of the Motukai Road. The associated fauna includes a Neocola closely related to N. demissus Marwick, described from the basal Tutamoe conglomerate of Gisborne district. The Wairarapa horizon is about the middle of the Southlandian sequence which comprises several thousand feet of beds, chiefly blue-grey mudstone. The lowest Foraminifera obtained from these beds, from about 500 feet above the base, appear to be Clifdenian, but Mollusca from the basal beds suggest that they are Altonian. The lower Tongaporutuan Hurupi Formation overlies the Southlandian beds, generally with disconformity but locally with unconformity. The

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fossils in the south branch of the Whakatahine River are therefore about Lilburnian or upper Clifdenian.

The name Hurupi Formation was used by MacBeath in his unpublished M.Sc. thesis (1950), and by the writer in subsequent mapping, for the Hurupi Series of L. C. King (1933, p. 334). As defined by King the Hurupi Series includes all the fossiliferous rocks at Hurupi Creek overlying the basement and underlying unfossiliferous mudstones in the Palliser Bay Cliffs north-west of Hurupi Creek. The term requires further definition to allow its use for rocks further afield. The formation consists chiefly of compact blue-grey sandy mudstones between 1,000 and 1,500 feet thick; the lower part is coarser and usually sorted into alternating layers, while the upper part is finer and typically quite massive. Generally there is a fine basal conglomerate six to twelve inches thick. The fossil Mollusca are diagnostic of the formation and were used by MacBeath and the writer to identify it in the field. King (1933, p. 334) and Dell (1952, p. 85) remarked upon the striking uniformity of the faunas from Putangirua Stream and Hurupi Creek. The Hurupi Formation is equally uniform in fossil content throughout its extent, and the field worker can rely on finding in it several of the following easily identified Mollusca: Callusaria callosa Marw., Maoricrypta radiata (Hutt.), Polinices huttoni v. Iher or closely related forms, Acominia hendersoni (Marw.), Alcithoe hurupiensis Marw., Glycymeris hurupiensis Marw., Cucullaea n.sp., Limopsis lawsi King, Dosinia cottoni Marw., or D. macroptera Fleming, Marama hurupiensis Marw., Eumarcia thomsoni Marw.

The Hurupi Formation has been identified by the above criteria overlying basement on the north-east side of the Haurangi Range, and along the valleys of the Pahaoa and Wainuioru rivers. North of the junction of the Wainuioru and the Kuamahanga Stream, the Hurupi Formation overlies Southlandian rocks for a distance of several miles along its strike.

Dell (1952, pp. 85–86) has discussed the age and facies of the Hurupi beds and classed them in the Tongaporutuan Stage. The Geological Survey paleontological staff have long regarded them as lower Tongaporutuan. Further information on the age of the formation was obtained from collections of Foraminifera made by R. A. Couper and the writer from the cliffs north-west of Hurupi Creek. Here the whole of the Hurupi Formation and a part of the underlying greywacke are repeated by a fault clearly exposed in the cliff, striking north-north-east and running out to sea a few chains west of Hurupi Creek. The results of the collection are summarised as follows:—

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Mudstone, sandstone, shell-beds more than Nukumaruan to Kapitean
3,000 feet
Fine blue-grey mudstone without macrofossils. 600 feet upper Tongaporutuan
200 feet middle Tongaporutuan
300 feet middle Tongaporutuan
Hurupi Formation. 500 feet lower Tongaporutuan
300 feet uncertain (no micro-fauna) Unconformity.
Basement (greywacke).

The lower Tongaporutuan determination is based on the presence of Bolivinita quadrilatera (Schwager) and the absence of B. pohana Finlay; middle Tongaporutuan on the presence of B. pohana without B. compressa Finlay; upper Tongaporutuan on the presence of B. compressa. The Foraminifera thus show

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that the uppermost part of the Hurupi Formation here is middle Tongaporutuan, the bulk of it, probably including the lowest beds, is lower Tongaporutuan.

Elsewhere neither B. quadrilatera nor B. pohana has been found in the formation. In most sections, however, mudstones containing B. pohana overlie the formation so that its upper beds may always represent the lower part of the middle Tongaporutuan.

MacBeath and the writer consider the Hurupi Formation to be the first beds deposited during a widespread transgression over south-east Wairarapa. Dell's remarks (1952, p. 85) are consistent with this view. He states that “deposition was rapid, in depths equivalent to comparatively shallow off shore waters to-day.” The formation is not clearly marked off from the overlying beds, grading upwards to fine-grained mudstones without Molluscan fossils. Grit, coarse sand, shellbeds, and generally well-sorted sediments are characteristic of the lower beds in most localities. Higher beds are mostly massive sandy mudstones interbedded with rare sandy phases. Probably, therefore, the lower beds were deposited in shallow water; depth increased with time at a rate probably depending on the rate of recession of the shore-line due to subsidence. The Hurupi Formation thus is not uniform in facies but represents a series of facies in a constantly changing environment of deposition.

Couper (1948) described and mapped the Bell's Creek beds, and classed them as middle Tongaporutuan, from a microfaunal determination by Dr. H. J. Finlay. The Mollusca are quite consistent with this determination. The Bell's Creek beds, in contrast to the Hurupi Formation, are fine blue-grey mudstone containing sparse, angular particles of greywacke of grit size. They overlie a conglomerate resting on the basement. The mudstones lack bedding, but apparently the sea bottom was subject to a good deal of disturbance during their deposition, for many of the molluscan fossils present have been broken during growth.

The fauna includes many small species not hitherto recorded from the Wairarapa. The largest fossils collected are Marshallena curtata (Marw.) and species of Ellicea. Marshallena curtata and Marginella aff. whitecliffensis Marw. link the Bell's Creek mudstone with the middle and upper Tongaporutuan of North Taranaki. Gemmula peraspera (Marw.) in the Mangaopari Stream is elsewhere known only from the Ormond Series of Gisborne district (Marwick, 1931, p. 32).

Fossiliferous Pliocene rocks of the district have already been discussed (Vella, 1953).

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