Preliminary Statement of the Geology

Alexander McKay (1878, pp. 16-20, 1879, pp. 81-84) describing the lithological sequence in the Wairarapa district, sub-divided the Upper Tertiary as follows:—

Pliocene: (1) Gravels, fossiliferous sands, lignites, etc.

Upper Miocene: (2) Fossiliferous limestone.

(3) Fossiliferous clays.

(4) Unfossiliferous clays.

Lower Miocene: (5) Fossiliferous clays (Taipo beds).

The two higher divisions of the Upper Miocene of McKay—i.e., (2) fossiliferous limestone and (3) fossiliferous clays, are roughly equivalent to the Onoke Series of L. C. King (1933, p. 336). King considered these beds to be of Nukumaruan age, but more systematic collecting has shown that some of them are probably Upper Waitotaran. King's faunal list from the “junction of the Ruakokopatuna and Makara streams” (King, 1933, p. 338) may have included some fossils from McKay's Pliocene. This formation is particularly well developed in the Huangarua River immediately downstream from the junction of these two streams, is separated from the shell limestones by a conglomerate, and may represent the Lower Castlecliffian.

Detailed lithological sequences of the rocks concerned in the following discussion are shown in the stratigraphic columns, Figs. 1a and 1b. Column (1) is reproduced, with some alterations, from Couper's M.Sc. thesis; column (2) is my own; columns (3) and (4) were provided by McBeath. They illustrate four representative localities.

It is likely that the Nukumaruan stage is thick beneath the western Wairarapa Plains. But east of the Plains thick Nukumaruan deposits are confined to a narrow strip striking north-east near the western edge of the Maungaraki Range. They thin rapidly towards the crest of the Maungaraki Range where they consist largely of shell limestone. They thin also across the ridges between the (southern) Whangaehu Stream and the Ruamahanga River and greywackes are exposed at several places on these ridges. Greywacke conglomerates occur locally at the base of the Upper Nukumaruan in the Whangaehu Valley, and are the predominant sediments in the uppermost beds. It is argued therefore that the ridge west of the Whangaehu Valley was emergent and the greywacke already exposed even at the time of deposition of the Upper Nukumaruan.

It is clear that in this area during the Nukumaruan there was a well-marked syncline which was forming at an accelerating speed. The axis of the syncline lies approximately along the Whangaehu and Makahakaha streams and continues south-westward through the junction of the Ruakokopatuna and Makara Streams. The syncline is now in part broken by faulting along the western limb, so that it is virtually a fault angle depression at some places.

It is from within the area described above that all our specimens of Pelicaria have been collected. From the nature of the geological history it follows that the physical environment of the Waitotaran and Nukumaruan sea was constantly changing. The water was undoubtedly becoming more and more shallow; and probably it was also becoming more and more land-locked. The changes in the Pelicaria fauna may be due to this more than to any other reason. Another factor likely to produce change of faunas is the fluctuation of warm and cold conditions. C. A. Fleming (1944, pp. 207-220) has shown that the lower Nukumaruan faunas of the East Coast of the North Island are characterised by cold water elements, specially Chlamys delicatulus (Hutt) and this is followed by warmer water faunas in the Upper Nukumaruan. Actually in the Wairarapa region Chlamys delicatulus considerably transcends the Lower Nukumaruan and first appears in beds believed to be Upper Waitotaran, so that its range is somewhat greater here than in Hawke's Bay.