The Late Tertiary and Quaternary Geological History of Western Wellington

Abstract

The district discussed in the western part of Wellington Province, North Island, New Zealand. The report, which is not comprehensive, deals primarily with certain late Tertiary and Quaternary problems that have been studied in some detail during a reconnaisance survey of some 3000 square miles of country. In the concluding section an attempt has been made to give a general outline of the significant events of the later geological history of the whole district. The results of the investigation are presented in the form of thirteen articles. Article 1: Coloured fossils are common in the upper 5000 feet of a marine sequence exposed in the Rangitikei Valley. Brilliant, intense colour characterizes fossils from the upper beds of the sequence, and a progressive fading is apparent with greater depth. The amount of colour preserved can be explained as a function of time – the younger the fossil the brighter the colour. The abundance of conspicuously coloured forms and the presence of such perishable forms structures as ligaments, periostraca, and nacreous layers afford confirmatory evidence to substantiate a hypothesis that may of the rocks enclosing these fossils are Pleistocene, and not Pliocene as hitherto believed. Article 2: Radiocarbon dating of fossil wood from the Ohakea terrace in the Rangitikei Valley indicates that this terrace was formal 3050 ± 200 years ago. At Rangitawa the river has apparently cut a trench 100 feet deep and about one mile wide during the last 3000 years; at Ohingaiti, 20 miles upstream from Rangitawa, the corresponding trench is more than 200 feet deep and a quarter of a mile wide. (As the rejuvenation of the Rangitikei Valley is attributed to uplift these observations suggest that the average rate of uplift has been about one foot in thirty years at Rangitawa, and about one foot in fifteen years at Ohingaiti.) Article 3: A study of the distribution of the Paryphantidae a family of land snails, supports the suggestion made on geological grounds that, in the not-distant past, there has been a narrow isthmian land bridge trending east-west across Cook Strait. It is tentatively submitted that two forms now restricted to the South Island, Paryphanta hochstetteri obscura and P.h. bicolor, may have "hybridized" to produce the North Island form P. traversi otakia. Wainuia urnula is considered as an essentially North Island form which migrated westward along the isthmus to the D'Urville Island district from the Kapiti Island area. The isolation of Wainuia urnula on D'Urville Island has lasted long enough for the development of the sub-species W.u. nasuta. The presence of W. urnula on Kapiti Island and on the mainland opposite at paranaraumu suggests that Kapiti Island has been connected to the North Island by a land bridge which has disappeared so recently that isolation has not been sufficiently long for the formation of a new sub-species. The problem of speciation rates is briefly considered; it seems possible that Wainuia urnuls nasuta has developed as sub-species from Wainuia urnula in less than 11,000 years. Article 4: The presence of pebbles of distinctive igneous, metamorphic, and sedimentary rocks typical of D'Urville Island and the western Marlborough Sounds district, in the Halcombe conglomerates and other post-Castlecliffian conglomerates that crop out in the northern coastal part of Western Wellington, is cited as evidence for postulating a late Pleistocene land connection between the North and South Islands, across the present Cook Strait area. The Halcombe conglomerates are discussed, and it is concluded that these beds, which in places rest conformably on uppermost marine Castlecliffian rocks, are late Pleistocene deposits of beach and near-shore shallow-water origin. The pebbles of South Island rock types are considered to have been transported from a north-eastward extension of the present Marlborough Sounds district by coastal drifting along a continuous shore-line to the site of deposition of the Halcombe conglomerates. It is further suggested that during most of Pleistocene time a land bridge existed on the site of Cook Strait and that this isthmus may have lasted until 11,000 years ago. Comments are also made on several minor geological problems, such as the flattening of pebbles by beach wear, and the alignment of coasts as a response to dominant winds; later Pleistocene winds are also discussed. Article 5: Ironstone soil nodules forming buckshot gravels in western Wellington are described and discussed. It seems that the formation of a buckshot layer composed of hard mature nodules requires considerably longer than 3000 years (radiocarbon date) and probably less than about 20,000 years. It is suggested that the Mt Stewart-Halcombe anticline, with prominent cemented buckshot in soil profiles on its crest and thick uncemented buckshot layers on its flanks, is older than the Levin anticline on which buckshot appears to be completely absent. Buckshot gravel may be a useful "time and climate" indicator for deciphering Pleistocene geological history elsewhere in New Zealand. Article 6: Changes from marine through estuarine to flaviatile conditions of sedimentation are inferred from a study of the sediments in a well drilled to a depth of 443 feet at the Awahuri Dairy Factory, near Palmerston North. The base of beach deposit with abundant Chione shells now lies 338 feet below present sea-level. The age of the Chione beds may be tentatively considered as late Pleistocene, probably not older than the last interglacial and possibly younger. It is suggested that the district in the immediate vicinity of the coastal portion of the Manawatu River has been "drowned" by combination of techtonic subsidence and late Pleistocene glacio-eustatic rise of sea-level, prier to the techtonic emergence that has led to the formation of the younger terraces of the present Manawatu River. About the Chione beds some of the estuarine sediments are almost certainly related to the post-Mankato-maximum (Flandrian) rise of sea-level. Article 7: Rocks of the Hawera Series rest conformably on the youngest beds of the Wanganui Series in the "Mingaroa section", near Bulls in the Rangitikei Valley. As it has been customary in the past to consider that regional unconformity separated rocks of the Hawera Series from those of the Wanganui Series this important section is described in some detail. The record of continuous deposition of late Castlecliffian and Hawera marine sediments necessitates the revision of previous ideas concerning the dating and interpretation of late diastrophic movements. (Vigorous Kaikoura orogenic movements can no longer be regarded as having taken place during a supposed Castlecliffian-Hawera interval.) The Mingaroa fossil beds furnish interesting examples of the Tawera + Glycymeris community, the Tawera + Venericardis association, and the Notocorbula + Pleuromeris associes. Article 8: In the Rangitikei-Manawatu district rocks of the Hawera Series, of late Pleistocene age, have been folded to form several strikingly youthful asymmetrical anticlines. Extensive remnants of the original surface of the coastal plain that has been deformed are still preserved as undissected interfluve areas. The flanks of all the anticlines are drained by secondary consequent streams which in some cases feed synclinal primary consequents. It is suggested that the crest of the Mt Stewart-Halcombe anticline has been raised to a height of 560 feet above sea-level in about 20,000 years at an average of approximately 1 foot in 36 years. It seems that tectonic activity is still in progress, and the anticlines are interpreted as being live folds. The present topography closely reflects the sub surface structure as determined by geophysical surveys. The deformation of the coastal plain, which commenced in the late Pleistocene, appears to be related to faulting in the greywacke basement. Article 9: Solifluxion deposits, commonly consisting of frostriven angular fragments of greywacke set in a matrix of silty sand, are widely spread in south-west Wellington. They were formed in a periglacial environment during the Pleistocene. The youngest deposits are related to the final phase of southern Hemisphere glaciation which may be assumed as synchronous with the Mankato glaciation of North America. Solifluxion was therefore probably active in south-west Wellington until about 8000 years ago. An extensive mantle of solifluxion debris has served as the parent material of the present soil. Colonization of the area by forest vegetation cannot have begun until solifluxion censed. Article 10: Wedge structures from Kaitoke, near Upper Hutt are interpreted as fossil ice wedges, and are therefore regarded as indicating perennially frozen ground in a periglacial environment. Article 11: In southern Wellington pre-Cretaceous greywacke bedrock and ancient greywacke gravels have been deeply weathered to a bright brick-red. This distinctive weathering is “pre-periglacial”, and postdates the deposition of the gravels in the techtonic Kaitoke basin. It is tentatively submitted that the red weathering took place under warm, moist conditions, mainly in the Pliocene. Article 12: Part 1 is a description of the “Waterfall Stream Section”, near Otaki, where spicules of marine sponges have been found in beds of the Otaki Formation. Part 11 deals with an ancient marine cliff, which can be traced for 8.5 miles between Otaki and Waikanae, and is regarded as part of the thermal maximum shoreline of western Wellington. Article 13: Throughout most of its history the Palmerston-Wanganui Basin (in the northern part of western Wellington) received sediments from the older Tertiary rocks of Taranaki to the north, from the rising axial ranges to the east, and from a granitic and metamorphic terrain to the south. As the development of the basin continued periodic centripetal tilting at the margins resulted in the emergence of progressively younger sediments of the Wanganui Series. In the Rangitikei district, during the Wanganui epoch, about 7000 feet of Opoitian and Waitotaran, 4000 feet of Nukumaruan, and 3,300 feet of Castlecliffian sediments were deposited. Marine sedimentation continued from Castlecliffian into Hawera times without a break. The later history of the Rangitikei country has been dominated by intermittent regional uplift which has been progressively greater island towards the ranges. Valley-plain terraces from seven epicycles can be recognized: the youngest major terrace about 3000 years old. For the last few thousand years, along the Rangitikei coast, the land has gained at the expense of the sea due to: (1) continued emergence of the coastal plain caused by uplift, together with some emergence brought about by the fall of sea-level following the thermal maximum: and (11) building out of the coast by progradation. There are three generation of coastal sand dunes. These may be closely related to volcanic ash showers that have dominated late Pleistocene history in the central North Island. It is suggested that in the northern area of western Wellington Vigorous late movements of the “Kaikoura Revolution” Commenced at the time of deposition of the coarse conglomerates of upper Castlcliffian age, and have continued intermittently to the present day. The folding of beds of the Hawera Series into youthful anticlines, following a slight pause in techtonic activity, marks a very late phase of earth-movement. Study of the complex central area of western Wellington, south of the Manawatu River to Paekakariki, is incomplete. Fossils are very rare in the surface rocks (Otaki Formation) but richly fossiliferous samples have been obtained recently from bores at many localities. So far no extinct species has been recognized among the macrofossils, suggesting that the oldest beds penetrated are unlikely to be older than upper that Castlecliffian: most of them are probably much younger. Preliminary study of bore-logs shows that the greywacke basement is highly irregular in some districts, as for example near Levin, where the Poroutawhao greywacke ridge is actually above sea-level, although buried beneath dune sand. It seems likely that the northern coastline of a late Pleistocene land bridge across Cook Strait lay a little to the west of Poroutawhao. The western margin of the greywacke of the Tararua Range consists mainly of ancient fault scarps modified by marine erosion. There have probably been several episodes of faulting. Near Te Hero the greywacke has been trimmed four times by the sea. The penultimate line of cliffs forms the landward boundary of the Otaki Formation: and it seems probable that the youngest cliffs mark the position of the thermal maximum shoreline. In the southern area, south of Paekakariki, the greywacke terrain is dominated by tectonic relief resulting from dislocation and deformation accompanying the upheaval of a peneplain, probably of upper Miocene age, Remnants of the ancient surface are deeply weathered. Majoy earth-movements which led to the disruption of the peneplain, produced great tectonic scarps and basins in which coarse alluvial gravels accumulated. During later movements greywacke was thrust over these ancient gravels at some places. In the Kaitoke and Rikiorangi basins the gravels are red-weathered indicating that the prominent red weathering in southern Wellington post-dates the break-up of the peneplain. The widespread solifluxion deposits of the periglacial landscape of south-west Wellington are confidently regarded as Pleistocene in age. Untilted marine benches around the shores of Porirua Harbour may record later Pleistocene glacio-eustatic changes of sea-level The pre-Cretaceous greywackes are continuous throughout western Wellington. It is inferred that the Ruahine, Tararua, and Rimutaka Ranges have resulted from the upheaval and deformation of an upper Miocene peneplain. The upheaval of the Ruahine Range apparently began in the late Miocene and/or early Pliocene, and the Tatarua and Rimutaka Ranges may have been born at the same time. If the Kaitoke and Rikiorangi basins were blocked out in early Pliocene time it would seem that the red weathering of the gravels in them is likely to date from the middle and upper Pliocene. This supports the suggestion that the red weathering is Pliocene as indicated by direct comparison with Pliocene red weathering Pliocene as indicated by direct comparison with Pliocene red weathering known overseas. Pleistocene solifluxion deposits which commonly mantle the greywacke throughout western Wellington, overlie red-weathered rocks at many localities. The red-weathering is thus clearly “pre-solifluxion” A Striking change from a warm, moist climate (which produced red weathering) to a cold periglacial climate (which favoured solifluxion) is indicated. Materials of volcanic origin are of special importance as stratigraphical and chronological markers from the beginning of the Castlecliffian to the present day. It seems that further work on ash showers will greatly increase the precision of our knowledge of late Pleistocene history. The Palmerston-Wanganui basin, in the northern part of western Wellington contains some 14,000 feet of strata which represent a virtually continuous, and perhaps unique, record of Pliocene and Pleistocene marine sedimentation. In marked contrast the southern part of western Wellington, less than 50 miles from the margin of this basin, consists of a greywacke mass that appears to have been emergent throughout the Pliocene and Pleistocene. It seems possible that significant events in the complicated history of the landmass in the southern area may eventually be dated against a time-scale founded on the marine sequence of the northern basin. The area is particularly suitable for the establishment of an absolute radiocarbon chronology.