Pliocene-Pleistocene Marine Cyclothems, Wanganui Basin, New Zealand: a Lithostratigraphic Framework

New Zealand Journal of Geology and Geophysics, 1995, Vol. 38: 223-243 223 0028-8306/95/3802-0223 $2.50/0 © The Royal Society of New Zealand 1995

Pliocene-Pleistocene marine cyclothems, Wanganui Basin, New Zealand: a lithostratigraphic framework

TIM NAISH SSW between 4° and 9°. Discernible discordances of c. 1° PETER J. J. KAMP between successively younger formations are attributed to synsedimentary tilting of the shelf concomitant with migration Department of Earth Sciences of the tectonic hingeline southward into the basin. The outcrop The University of Waikato distribution of the Rangitikei Group is strongly influenced Private Bag 3105 by this regional tilt and also by three major northeast- Hamilton, New Zealand southwest oriented, high-angle reverse faults (Rauoterangi, Pakihikura, and Rangitikei Faults). A bstract The Rangitikei River valley between Mangaweka Keywords late Pliocene; Nukumaruan Stage; cyclothems; and Vinegar Hill and the surrounding Ohingaiti region in Wanganui Basin; glacio-eustacy; sea-level change; new eastern Wanganui Basin contains a late Pliocene to early stratigraphic names; Mangarere Formation; Tikapu Pleistocene (c. 2.6-1.7 Ma), c. 1100 m thick, southward- Formation; Makohine Formation; Orangipongo Formation; dipping (4-9°), marine cyclothemic succession. Twenty Mangaonoho Formation; Vinegar Hill Formation; Rangitikei sedimentary cycles occur within the succession, each of which Group contains coarse-grained (siliciclastic sandstone and coquina) and fine-grained (siliciclastic siltstone) units. Nineteen of the cycles are assigned to the Rangitikei Group (new). Six new formations are defined within the Rangitikei Group, and their INTRODUCTION distribution in the Ohingaiti region is represented in a new Wanganui Basin contains a thick (4 km) Pliocene-Pleistocene geologic map. The new formations are named: Mangarere, sedimentary fill of mainly marine strata that has accumulated Tikapu, Makohine, Orangipongo, Mangaonoho, and in various shelf and paralic environments. The northern and Vinegar Hill. Each formation comprises one or more eastern margins of the basin are now uplifted, and gently cyclothems and includes a previously described and named dipping (2-15°) marine strata are well exposed along the d stinctive basal horizon. Discrete sandstones, siltstones, and coastline northwest of Wanganui City and within deeply coquinas within formations are assigned member status and incised river valleys further to the east (Fig. 1). The Wanganui correspond to systems tracts in sequence stratigraphic Basin succession has proved to be important in studies of nomenclature. The members provide the link between the global climate change because it contains an exceptionally new formational lithostratigraphy and the sequence strati- complete on-land record of cyclothemic sedimentation in graphy of the Rangitikei Group. Base of cycle coquina response to giacio-eustatic sea-level fluctuations (Beu & members accumulated during episodes of sediment starvation Edwards 1984; Kamp &Turner 1990; Abbott & Carter 1994). associated with stratigraphic condensation on an open marine Moreover, it affords a unique opportunity to examine the shelf during sea-level transgressions. Siltstone members relationships between on-land and offshore stratigraphic accumulated in mid-shelf environments (50-100 m water records of the Pliocene-Pleistocene. Recent published studies depth) during sea-level highstands, whereas the overlying have concentrated on Pleistocene strata, especially those of sandstone members are ascribed to inner shelf and shoreface the Nukumaruan and Castlecliffian stratotype sections environments (0-50 m water depth) and accumulated during exposed along the Wanganui coastline (Beu & Edwards 1984; falling eustatic sea-level conditions. Repetitive changes in Kamp & Turner 1990; Abbott & Carter 1994). The underlying water depth of 50-100 m magnitude are consistent with a late Pliocene succession (Mangapanian-Nukumaruan) in giacio-eustatic origin for the cyclothems, which correspond Wanganui Basin, which was also deposited under conditions to an interval of Earth history when successive glaciations in of rapid basinal subsidence and high sedimentation rates the Northern Hemisphere are known to have occurred. during a period of Northern Hemisphere-driven, giacio- Moreover, the chronology of the Rangitikei River section eustatic sea-level fluctuations, should also reveal a detailed indicates that Rangitikei Group cyclothems accumulated record of sedimentary cyclicity. during short duration, 41 ka cycles in continental ice volume attributed to the dominance of the Milankovitch obliquity Between Mangaweka and Vinegar Hill, an c. 1100 m orbital parameter. thick, southward-dipping (4-9°), late Pliocene (c. 2.6-1.7 Ma) marine cyclothemic succession is spectacularly exposed in The Ohingaiti region has simple postdepositional cliffs of the Rangitikei River (Fig. 1). We have identified 20 structure. The late Pliocene formations dip generally to the sedimentary cycles or cyclothems within the succession, each of which contains coarse-grained (sandstone and coquina) and fine-grained (siltstone) units. Fine-grained lithologies contain diagnostic molluscan and foraminiferal fauna that indicate deposition in predominantly mid-shelf environments G 9403 3 (c. 50-100 m water depth), whereas coarse-grained lithologies Received 15 July 1994; accepted 9 January 1995 accumulated in a range of nearshore-inner shelf environments State Highway 1 N Other roads

Main trunk line ^ Trig station

Site no.

Old Hautawa Rd

Z ft N rt p_ ET 3 o"

3 P

O oCO O* (

a. O fD O •a

Fig. 1 Map of the Ohingaiti region showing names, physiographic features, and numbered sections referred to in the text. Inset: simplified geology of the «nuthwe«tem North Island, show ing location of alud> area. Naish & Kamp—Lithostratigraphy, Wanganui Basin 225

(c. 0-50 m water depth). Each cyclothem is characterised by a remarkably similar lithofacies architecture, corresponds in duration to the 41 ka Milankovitch obliquity frequency, and N the 20 cyclothems are correlated with oxygen isotope stages 100-58. Details of the sequence architecture, correlations with the global isotope model, and faunal characteristics are presented elsewhere (Naish & Kamp in prep. "Pliocene- Pleistocene shelf cyclothems, Wanganui Basin, New Zealand: a high-resolution facies and sequence stratigraphic analysis"). CHALLENGER

Given the remarkable exposure, the continuity of the PLATEAU record, and the dramatic lithologic and faunal changes between low and high relative sea-level deposits, the Rangi- tikei River section deserves to become a global on-land reference section recording sea-level, paleoclimatic, and AUSTRALIA environmental change. For this to be considered seriously, a PLATE detailed geologic map and formal lithostratigraphy of the PACIFIC region needs to be widely available. A formal (descrip- PLATE tive) lithostratigraphy, including a detailed description of 100 km individual cyclothems, is also essential as a framework for interpretation of the sequence stratigraphy of the sedimentary Back-arc Basin Frontal Ridge Forearc Basin Subductkjn HkuranQj Complex Trough succession, which is a related part of our study of the region. (Wanganui Basin) Some of the strata exposed in this area have been mapped pieviously by the Superior Oil Co. of N.Z. (1943) and by Te Punga (1953). Subsequent studies (e.g., Sewardetal. 1986) have applied lithostratigraphic nomenclature from western Wanganui Basin (Wanganui Subdivision; Fleming 1953) to presumed correlatives exposed in the Rangitikei River valley. Fig. 2 The regional tectonic setting of Wanganui Basin with re- A Ithough several names have been employed for late Pliocene spect to the main structural and tectonic features of the Hikurangi strata in eastern Wanganui Basin, no unified scheme is margin. The schematic section across southern central North Island currently available, and many of the existing names for units shows the eastern margin of Taranaki Basin, Wanganui Basin, the in the Rangitikei River valley are inadequately defined. axial ranges, and the eastern North Island accretionary borderland. Pioblems in lithostratigraphic nomenclature have arisen from the application of Fleming's (1953) scheme to strata in the Rangitikei River valley. Some units show dramatic changes in lithology across the basin, and the use of names derived TECTONIC SETTING from localities in western Wanganui Basin may be inappro- The Pliocene-Pleistocene tectonic setting of Wanganui Basin priate for defining type and reference localities within the with respect to the Hikurangi margin is illustrated in Fig. 2. eastern part of the basin. For those cases more appropriate Wanganui Basin is broadly elliptical, located behind the names are presented here. However, the revised nomenclature accretionary borderland of the southern portion of the retains, wherever possible, historically significant pre-existing Hikurangi margin, and is mildly deformed by northeast- names to maintain a continuity of usage. southwest-trending high-angle reverse faults. Stern & Davey The present study is concerned with establishing a formal (1989, 1990) argued that development of Wanganui Basin lithostratigraphic framework for the late Pliocene-early has been controlled by Pacific plate subduction processes. Pleistocene succession exposed in the Rangitikei River valley Pliocene-Pleistocene subsidence is attributed to an early and the surrounding Ohingaiti region (Fig. 1). We ascribe all phase of compressional downwarping driven by coupling of strata in this region, between the Mangaweka Mudstone and the overriding and subducting plates. Regional tilting of the Vinegar Hill Tephra, to the Rangitikei Group (new), and Neogene strata in Wanganui Basin is considered to be a we present here a new 1:50 000 geologic map. New detailed response to southward migration of the depocentre and coeval stratigraphic logs are also presented for the Rangitikei River uplift towards the north. Sediments deposited in eastern section, Porewa Valley section, and a 200 m deep stratigraphic portions of the basin, and described herein, have been gently drillhole sited in Simpsons Reserve (Fig. 1; New Zealand upwarped along the axial ranges at rates of 1-3 m/ka during metric Topomap NZMS 260 series, grid reference T22/ the late Quaternary (Pillans 1986), as recorded by a series of 320417) in May 1991 by the Institute of Geological and well-developed alluvial terraces in Rangitikei Valley (Milne Nuclear Sciences Ltd. We discuss the importance of applying 1973). a lithostratigraphic subdivision to high-order (6th) cyclothemic strata and comment on the problems associated with mapping cyclothems in Wanganui Basin. We explore the local STRUCTURE OF THE OHINGAITI REGION and more regional implications of our proposed litho- The Rangitikei Group strata in the study area have simple stratigraphy and make basin-wide comparisons with postdepositional structure (Fig. 3). The succession dips correlative strata in the "Wanganui Subdivision" (Fleming generally to the SSW at between 4° and 9°; steeper dips 1953; Fig. 1). We also comment on the late Pliocene (up to 15°) and more variable dip directions occur in the depositional setting of eastern Wanganui Basin and discuss vicinity of faults (Fig. 3, 4A, B). Subtle synsedimentary the origin of cyclicity preserved within the Rangitikei Group deformation of tectonic origin is evidenced by discernible strata. angular discordances of c. 1° across some unconformities. 226 New Zealand Journal of Geology and Geophysics, 1995, Vol. '8

ft »•:•:•.

|l ^ ^ SL-—-_^

t^ + _: est. +

:^-i-±r~- Manaatieka Rd"

______, Xs)

Rangitira N0.4 — A 475m —

Fig. 3 Geologic map of the Ohingaiti region showing regional structure and outcrop pattern of the newly defined late Pliocene formations of the Rangitikei Group.

Figure 5G illustrates low-angle stratal discordance across the (Fig. 4A, B). We suggest synsedimentary tilting of the shelf unconformity at the base of the Mangaonoho Formation (site occurred in response to flexural uplift in the north associated 11, Fig. 1). A decrease in dip angle of 1° occurs between with progressive southward migration of the basin depocentre successively younger formations of the Rangitikei Group during the late Pliocene. Naish & Kamp—Lithostratigraphy, Wanganui Basin 227

LITHOSTRATIGRAPHIC LEGEND Group Symbol Formation NZ Sub NZ Epoch stage Stage Undifferentiated (Ratan to Recent) alluvium State Highway Undifferentiated Waipuru MAXWELL formations A441m Road .- 4WD track Vinegar Hill •*' Main trunk line Mangaonoho s' Stream • • ^ River Orangipongo • «

Not mapped ^- Fault Makohine

RANGITIKE I + -•" Fault inferred + Tikapu + Dip + strike measured Mangarere . Trig station PAFA- Mangaweka (beaooned/unbeaconed) RANGI

Fig. 3 (continued)

Two major northeast-southwest oriented faults (Rang- area, Rauoterangi Fault is clearly exposed at site 30 (Fig. 1) it ikei and Rauoterangi) occur in the study area. Although the on Mangarere Road, and has a throw of c. 200 m, juxtaposing fault planes are exposed in outcrop at only a few places, both Mangarere Formation siltstone against Orangipongo faults display high angles (80°) of dip, have reverse Formation sandstone. Structural analysis of our geologic map d; splacement, and are upthrown to the west. Within the map indicates throws of c. 100 m across the Rangitikei Fault (Fig. 228 New Zealand Journal of Geology and Geophysics, 1995, Vol 8

4 5 Distance (km)

Fig. 4 Cross-sections illustrating the relatively simple postdepositional structure of the Rangitikei Group. Dips shallow in successively younger formations. A, Section A-A'relates the stratigraphy of the Simpsons Reserve drillcore to the regional outcrop pattern. B, Secti-m B-B' shows the structural complexity associated with the Rangitikei Fault.

4B), which is exposed on Vinegar Hill (Superior Oil Co. of the Rangitikei Group is the formation of a series of fault N.Z. 1943) and as a scarp east of Hunterville where it truncates blocks that progressively uplift strata eastward across the the Vinegar Hill Tephra. The regional strike of strata is 100- Pakihikura Fault Zone. Strike-parallel differential tilting of 110° but, adjacent to the Rauoterangi Fault, strikes between faulted blocks causes local variations in dip of strata. Strata 60° and 90° occur (Fig. 3). This difference in attitude of strata exposed in a 1 km long section in the east bank of the adjacent to the faults is attributed to drag associated with Rangitikei River opposite Rowes Road (Fig. 1) are almost limited dextral strike-slip movement along the Rauoterangi flat lying. Numerous normal and reverse faults with minor Fault. Moreover, dramatic variations in throw along both displacements (< 5 m) that deform strata on a smaller scale faults are also consistent with a dextral wrench fault pattern in the Ohingaiti region have not all been mapped. of deformation (Anderton 1981). Between Makohine and Pakihikura Streams, a 1.5 km LITHOSTRATIGRAPHIC SUBDIVISION OF wide zone of en echelon high-angle (75-90°) reverse and CYCLOTHEMIC STRATA subordinate normal faults locally complicates the outcrop distribution of the Mangaonoho and Vinegar Hill Formations. Fleming (1953) was the first to recognise that the Pliocene- We refer to this more structurally complex region as the Pleistocene succession comprised unconformity-boundrd Pakihikura Fault Zone (Fig. 3). The faults are sympathetic to sediment wedges that thicken eastwards in the direction if the Rangitikei Fault and their origin is probably associated the basin depocentre. He established and applied a compre- with minor dextral strike-slip movement along the Rangitikei hensive lithostratigraphy to strata cropping out in the Fault. Throws on the major faults range between 10 and 30 m, Wanganui Subdivision, which has been utilised widely by with greater displacement occurring towards the north where others (Seward 1974; Beu & Edwards 1984; Kamp & Turner the faults appear to bifurcate from the Rangitikei Fault. The 1990; Abbott & Carter 1994; Pillans et al. 1994). In the middle major influence the faults have had on the outcrop pattern of Pleistocene (Castlecliffian Stage) part of the record, Flemin;; s

Fig. 5 Prominent unconformities and lithologies within the Rangitikei Group. A, Channelled base of the Mangarere Formation (T22/497480). The unconformity is highlighted (dashed line), (i) Mangaweka Formation siltstone. (ii) Channel-fill conglomerate member (Mrcfm,). (iii) Aclast (c. 1 m diam.) of underlying Mangaweka Formation siltstone within channel-fill member. B, Base of the Hautan a Shellbed exposed at the type locality (GS3096; Fleming 1953), "Old Hautawa Road" at the end of West Road (T22/325484). "U" marks the unconformity. C, Base of the Tikapu Formation exposed in the east bank of the Rangitikei River (T22/436456). The prominent horizon in the cliff (S) is the Hautawa Shellbed (Tusmj; Fig 7B). D, Base of the Makohine Formation exposed in the west bank of the Rangitikei River (T22/434455). (i) Siliciclastic siltstone member Tuzmi passes gradationally upsection into (ii) Tuha Sand (Mksnij), which is sharply truncated by the upper cycle bounding unconformity (U). E, Cycle bounding unconformity (U) within the Makohine Formation (T22/434450) truncates shoreface facies within the upper portion of the (i) Tuha Sandstone (Mksmi). (ii) Coquina member Mkcmi is sharply overlain by (iii) siliciclastic siltstone member Mkzm). F, Burrowed cycle bounding unconformity (U) within the Orangipongo Formation sharply truncates (i) large-scale bidirectional trough cross-bedded sands within the top of the Ohingaiti Sand (Orsmi) and 's sharply overlain by (ii) coquina member (Orcmi), which passes upsection into (iii) siliciclastic siltstone member Orzm j. G, Unconformity (dashed line) at the base of the Mangaonoho Formation exposed in Mangamako Stream (T22/394417) truncates (i) regressive shoreline facies of the underlying Orangipongo Formation, (ii) Mangamako Shellbed (Mhcmj) above unconfromity. Naish & Kamp—Lithostratigraphy, Wanganui Basin 229 230 New Zealand Journal of Geology and Geophysics, 1995, Vol. 3S

Fig. 6 Composite stratigraphic col umn for the Rangitikei Group. Cot Chronostratigraphic relations between Rangitikei Rive Lithostratigraphy datums section, Porewa Valley, an I River Section Simpsons Reserve drillcore have been established on the basis of lithostratigraphy and geological mapping. Formation thicknesses ar.l lithologies have been derived froi • — 1.75^0.13 Ma.Vinegar Hill Tephra type and reference localities for the Rangitikei River section and are it — 1.77 Ma top of Olduvai lustrated in more detail in Fig, 7 Gephyrocapsa sinuosa Magnetostratigraphy is after Sewaid (1986), Wilson (1993), and Naish et al. (in press). Biostratigraphy is al ter Fleming (1953) and Beu A: —187*015 Ma Waipuru Ash Edwards (1984). Isothermal plateau fission track ages on rhyolitic tephra are from Naish et al. (in press). —1.95 Ma base of Olduvai

— Mangamako Tephra

— Ohingaiti Ash

— FAD Chlamys dellcatula LAD Crassostrea ingens LAD Pbialopecten triphooki LAD Globorotatia crassula

— 2.6 Ma Gauss/Matuyama transition

LITHOLOGY : CONTACTS :

> Conglomerate >V, Tephra

Coquina • Shellbed (thiri) Omission surface Unconformity Naish & Kamp—Lithostratigraphy, Wanganui Basin 231 formations correspond to systems tracts in terms of sequence transgressive shellbeds. This means that sequence boundaries stratigraphic nomenclature. This is particularly evident in the may lie within coarse-grained strata and may not necessarily Castlecliffian stratotype section (Abbott & Carter 1994). coincide with formation boundaries, which are placed at the However, away from well-exposed coastline and river contacts of sandstones and siltstones in the Tikapu, Makohine, sections, Fleming (1953) was not able to map individual Orangipongo, and Vinegar Hill Formations. However, in the formations. This was due to: (1) paucity of outcrop in the lithostratigraphy defined here, sequence boundaries coincide interfluves; (2) poor surface geomorphic expression of with member boundaries, which distinguish lithologic differ- sandstone and siltstone lithologies; (3) lateral changes in ences within the coarser grained units. Figure 8 illustrates lithofacies within individual cyclothems; and (4) repetition the relationships between cyclothems and members within of similar lithofacies successions from one cyclothem to the Orangipongo Formation. It is the characteristics of the another, thereby creating the potential for miscorrelation. members that define the detailed sequence architecture and Consequently, largely undifferentiated groups containing a provide the essential link between the formational litho- few prominent horizons were mapped across the Wanganui stratigraphy and the sequence stratigraphy of the Rangitikei Subdivision. In the Ohingaiti region, Superior Oil Co. of N.Z. Group strata. This contrasts with Fleming's (1953) sub- (1943) and Te Punga (1953) adopted a similar approach, division of the middle Pleistocene Castlecliff section, where mapping distinctive horizons within broad formations or the formations are essentially systems tracts. groups. Given the large number of members that can usefully be Following the recognition by recent workers that the defined within each formation of the Rangitikei Group, it Wanganui Basin contains an exceptional record of Milan- has proved impractical to ascribe members a geographical kovitch-driven cyclicity, the emphasis has changed from one name. Consequently, we have adopted an alpha-numeric of mapping lithologic units to developing integrated nomenclatural system that assigns a code to each member. siratigraphies and chronologies aimed at identifying the global The code contains three parts: (1) the abbreviated name of climatic and sea-level signatures within the sediments and the formation in which the member occurs (i.e., Vh = Vinegar their relationship to local events such as volcanic eruptions Hill Formation); (2) an abbreviated lithologic term (i.e., sm (e.g.,Pillans 1983; Turner & Kamp 1990; Pillansetal. 1994). = sand member); and (3) a numerical term denoting The corresponding change in approach has been to measure stratigraphic position of the member within the formation well-exposed sections, the correlations between sections (i.e., Vhsmi = lowest sand member of the Vinegar Hill being supported by integrated chronostratigraphies. To date, Formation). most of the published work of this kind has been undertaken within the Wanganui Subdivision part of the basin, and has been possible largely because of the lithostratigraphic framework and mapping undertaken by Fleming (1953). Our LITHOSTRATIGRAPHY work in the Ohingaiti region is directed chiefly at docu- menting the global signals of climatic and sea-level change RANGITIKEI GROUP (new) contained in the sediments, and the controls on accumulation HISTORY: A summary of the formational nomenclature used of the succession. Applying and testing sequence stratigraphic by previous workers to describe late Pliocene strata in eastern concepts (cf. Vail 1987) is one means by which we are doing Wanganui Basin is given in Fig. 9. A report detailing the this work. However, in order to do this and adequately report geology of the "Palmerston-Wanganui area" by the Superior the results, we have found it necessary to develop a compre- Oil Co. N.Z. (1943) provides the earliest significant reference hensive lithostratigraphy and geologic map as a framework to the stratigraphy of the Rangitikei Valley region. "Superior" to which we can relate our descriptions and interpretations. geologists based their mapping of Wanganui Basin on detailed In this paper we present a new lithostratigraphy and descriptions of sections sampled in Rangitikei, Turakina, geologic map for the Ohingaiti region for two reasons: (1) as Mangawhero, and Whanganui Rivers. Strata of Pliocene age a basis for sequence stratigraphic analysis (as above), and were grouped into two microfaunally defined stages, the (2) to relate the stratigraphy in the Ohingaiti region to that in Waitotaran and Nukumaruan Stages. The latter stage was the Wanganui Subdivision. We therefore subdivide Rangitikei further subdivided to include the "Lower Nukumaruan Group strata into six new formations bounded by prominent Formation" and the "Petane or Upper Nukumaruan mappable lithologic horizons. Each formation comprises a Formation". Three prominent horizons were described within package of one or more cyclothems, and includes as members the Lower Nukumaruan Formation: the Basal Nukumaruan previously described distinctive units (Superior Oil Co. of Sand, the Tuha Sand, and the Hautawa Reef. Overlying strata N.Z. 1943; Te Punga 1953; Fleming 1953): Te Rimu Sand, of the Petane Formation comprised faunas similar to those Hautawa Shellbed, Tuha Sand, Ohingaiti Sand, Mangamako of the type Nukumaruan coastal section and included three Shellbed, and Waipuru Shellbed. The formations, in ascending key beds: the Ohingaiti Sand, the Mangamako Shellbed, and order, are named: Mangarere, Tikapu, Makohine, the Waipuru Fossiliferous Sand Horizon. Orangipongo, Mangaonoho, and Vinegar Hill. Figures 6 Te Punga (1953) renamed strata of Waitotaran age below and 7A-F illustrate the detailed lithostratigraphy of each the Basal Nukumaruan Sand as Lower Rangitikei Formation. formation. An important consideration when establishing the Overlying Nukumaruan strata were assigned to the Middle formational stratigraphy was that the formations should be Rangitikei Formation (Fig. 9). mappable units. Discrete lithologic units within formations Fleming (1953) adopted in the Wanganui Subdivision, correspond to systems tracts. These units could not be mapped with minor emendation, the mapping of the Superior Oil Co. reliably across the Ohingaiti region and are assigned member of N.Z. (1943), matching the Rangitikei stratigraphy with status. Cyclothem-bounding unconformities, or sequence presumed correlatives in the Wanganui Subdivision. Some boundaries, truncate coarse-grained regressive shoreline of the names applied by Superior Oil Co. of N.Z. (1943) were fades in many places and are overlain by coarse-grained basal rejected by Fleming (1953) as they confused biostratigraphic A B to Formation Lithology Member Cycle Lithology Member Cycle Member Cycle o^ Formation TIKAPU MAKOHINE

180

LLJ

LU DC < i Silt i Sand Gravel, o z

LITHOLOGY STRUCTURES CONTACTS

CROSS-STRATIFICATION Bioturbated sifciclastic ct I siltstone E&zStaj Trough X4ffl\ Planar 1 Bioturbated Burrowed sifciclaslic | •» J Low angle ] sandstone afe unconformity l^tjjjl Bidirectional Omission MANGA |^.| Ripple lamination surface -WEKA |Ty:| Flaser cT ~ ~ Coquina |~^| Lenticular Channel scour HORC.-STRATIFICATION Silt i Sand iGravel. to | | Lamination Conglomerate ISSJ Graded bedding OTHER STRUCTURES 1=1 Bedding £•? Concretions Concretionary r-TPTq Soft sediment sandstone |~~-| Shell layer \=Ji^-\ delormation 9 O O* B

Q 8

Fig. 7 Detailed stratigraphic logs for individual formations of the Rangitikei Group. Illustrated are the thicknesses, lithologies, members, and cycle boundaries for: A, Mangarere Formation; B, Tikapu o Formation; C Makohine Formation: D, Oranginnngo Formation; E, Mangaonoho Formation: and F, Vliic^.u Hill Fornuiioii Silt i Sand iGraveli ORANGIPONGO (Or)

Thickness

MANGAONOHO IMn)

Thickness (m)

VINEGAR HILL(Vh) MAXWELL GROUP EQUIVALENT

Thickness (m) 8 8 8 3 8 8 8 I §

uiseg i 'AqdEjgqejjsoqji'7—duiB^ a B N EL g D- c" Ohakea Terrace

o ft

3 a n 6) Cycle No. 0 Cycle boundary _ Merhber boundary -a sr

Fig. 8 A, Type section of the Orangipongo Formation exposed in the east bank of the Rangitikei River opposite the Rangitira Golf Course. B, The relationship between members and cyclothems within the Orangipongo Formation exposed at the type locality.

o Naish & Kamp—Lithostratigraphy, Wanganui Basin 235

Fig. 9 Historical and new litho- Superior Oil Company TePunga(1953) Fleming (1953) This paper stratigraphic nomenclature, late of N.Z. (1943) Pliocene strata, Ohingaiti region. MAXWELL GROUP Italicised units occur as named Vinegar Hill Formation members within the Rangitikei Waipuru Sand Waipuru Fossil Beds Waipuru Shellbed Waipuru Shellbed Mb. Group. Mangamako Sand Mangamako Fossil Beds Mangamako Shellbed Mangaonoho Formation Mangamako She/bed Mb.

Ohingaiti Sand Ohingaiti Sandstone Ohingaiti Sand Orangipongo Formation Ohingaiti Sand Mb.

Tuha Sand Tuha Sandstone Tuha Sand Makohine Formation Tuha Sand Mb.

Hautawa Reef Hautawa Shellbed Tikapu Formation Hautawa Shellbed Mb.

Basal Nukumaruan Basal Sandstone Te Rimu Sand Mangarere Formation Sand Te Rimu Sand Mb.

with lithostratigraphic nomenclature. Group status was use of the name Rangitikei to the cyclothemic strata between assigned to late Pliocene strata in western Wanganui Basin. the top of the Mangaweka Formation and the Vinegar Hill The Upper and Lower Nukumaruan Formations were Tephra, and raise its rank to group status. Our redefinition of renamed Okiwa and Nukumaru Groups, respectively. The the name Rangitikei raises the issue of its historical Basal Nukumaruan Sand was renamed Te Rimu Sand, based preoccupation elsewhere in the Rangitikei Valley. The names on a locality in the Whanganui River valley. Lower Okiwa Upper and Lower Rangitikei Formations applied by Te Punga Group strata, below the Hautawa Shellbed, were assigned a (1953) to strata above and below the Rangitikei Group, Mangapanian age. Strata containing Chlamys delicatula were respectively, have not been adopted by subsequent workers. assigned to the Hautawan subdivision of the Nukumaruan Fleming (1953) recognised the nomenclatural difficulties and Stage, and Nukumaru Group strata were assigned to an upper therefore included Mangaweka Mudstone as a formation of biostratigraphic subdivision of the Nukumaruan Stage, the the Paparangi Group. The other units within Te Punga's Marahauan Substage. (1953) Lower Rangitikei Formation are clearly formations In applying the group names, Okiwa and Nukumaru, to and we are currently working on their redefinition. As for I ime-equivalent strata in eastern regions of the Wanganui the Upper Rangitikei Formation, subsequent workers (Seward Subdivision, Fleming (1953) recognised potential difficulties et al. 1986; Pillans et al. 1994) have applied Fleming's (1953) in lithostratigraphic nomenclature. This is particularly nomenclature by correlation with the Wanganui Subdivision, pertinent to the Nukumaru Group, where dramatic lithologic and we follow this practice here by using the name Maxwell changes occur within the group, from west to east across the Group for strata above the Vinegar Hill Tephra. In time, when basin (Fig. 10). Coarse-grained, siliciclastic sands and detailed field examination is undertaken, it may be appropriate bioclastic lenticular limestones distinguish the group in the to revise the nomenclature for strata in the Rangitikei Valley west but pass eastwards into finer grained sandy siltstones above our new Rangitikei Group. and siltstones. Within the Nukumaru Group, only the Ohingaiti Sand and Waipuru Shellbed maintain sufficiently TYPE SECTION: Strata exposed in Rangitikei River valley from consistent lithologic character to be able to be traced on field Mangarere Road (grid ref. T22/483496) to Vinegar Hill criteria across the basin. Older Okiwa Group strata are Bridge (T22/357380) constitute the type locality of the group. generally finer grained but display similar west-east fining. In the Ohingaiti region, cyclothemic strata correlated with UPPER AND LOWER BOUNDARIES: In the Ohingaiti region, the the Okiwa and Nukumaru Groups comprise broadly similar lower contact of the Rangitikei Group is unconformable with ratios of siltstone to sandstone. There is no lithologic basis massive siliciclastic siltstone of the underlying Mangaweka for recognition of two lithologically discrete groups in this Formation, and is marked by an easily recognised 1 m thick part of the basin. We propose the name Rangitikei Group layer of bluff-forming concretionary sandstone. We place the for Okiwa and Nukumaru Group correlative strata mapped upper boundary at a prominent 20 cm thick, pinky-white, in the Ohingaiti region (Fig. 9). rhyolitic tephra, which is interbedded in fossiliferous sandy siltstone previously ascribed to Maxwell Group equivalent NAME: Rangitikei Group derives from Te Punga (1953) who strata. This tephra is named the Vinegar Hill Tephra and is applied the name Middle Rangitikei Formation to strata described fully by Naish et al. (in press). In the Rangitikei between the top of the Mangaweka Formation and the River section, the Vinegar Hill Tephra occurs 130 m above Pakihikura Pumice. Te Punga's (1953) formational nomen- the base of the Vinegar Hill Formation, which includes the clature is unacceptable because his formations clearly contain Waipuru Ash (Seward 1976; Naish et al. in press). Previous numerous mappable units (formations) as we demonstrate. workers (Seward 1976; Beu & Edwards 1984) may have Moreover, Te Punga's (1953) Upper, Middle, and Lower miscorrelated the Waipuru Ash with a burrowed rhyolitic ash Rangitikei Formation nomenclature is no longer acceptable preserved in laminated silts and sands of the Tewkesbury in modern stratigraphic codes. Consequently, we restrict the Formation at Brunswick Road (R22/783502) in Kai-iwi 236 New Zealand Journal of Geology and Geophysics, 1995, Vol. 3b

EAST WEST - WANGANUI SUBDIVISION -OHINGAITI REGION

(Fleming, Coast Kai-iwi Whanganuhangs i Mangawhero Whangaehu Turakina Watershed Rd Rangitikei Nomenclature 1953) Stream Riveir River River River + drillcore River (This study

Thickness (m) O-i

100-

200-

400-

500-

600-

4. Walpuru Shellbed. T Site GS3097 (Fleming 1953) * Superior Oil Co of N.Z. (1943) 800 J Fig. 10 Schematic basinwide east-west cross-section illustrating the stratigraphic relationships between the Rangitikei Group and the stratigraphy of Fleming (1953) and Superior Oil CQ. of N.Z. (1943).

valley, western Wanganui Basin. New chemical data (Pillans AGE: A late Pliocene age (c. 2.6-1.7 Ma) has been established pers. comm.) indicate that the tephra at Brunswick Road may for the group on the basis of isothermal plateau fission track be a correlative of the Vinegar Hill Tephra, and the older (ITPFT) dating of rhyolitic tephra (Alloway et al. 1993; Naish Waipuru Ash may not be exposed at Brunswick Road. In et al. in press), paleomagnetic data (Seward et al. 1986; western Wanganui Basin, the boundary between Okiwa and Wilson 1993; Naish et al. in press), and macrofossil (Fleming Maxwell Group strata is placed at the top of the "Waipuru 1953) and microfossil content (Beu & Edwards 1984). Key Shellbed" (Fleming 1953), which is problematic as the chronostratigraphic datums are illustrated in Fig. 6 for the "Waipuru Shellbed" is easily confused with a number of strati- Rangitikei River section. The Mangapanian/Nukumaruan graphically younger and older, faunally similar shellbeds. Our Stage boundary is located in the Rangitikei Group at the base criteria for assigning the upper boundary of the Rangitikei of the Hautawa Shellbed. It is defined by the first appearance Group to the Vinegar Hill Tephra rather than to the Waipuru of Chlamys delicatula in the Rangitikei River section (new Shellbed include the following: (1) the Vinegar Hill Tephra locality), and by correlation with the occurrence of provides a regionally extensive, distinctive lithologic and Globorotalia crassula and dominantly dextrally coiled G. isochronous marker bed, which enables direct comparisons crassiformis at a site in the Mangawhero Valley (Beu & to be made between the Rangitikei Group and the established Edwards 1984). The Gauss-Matuyama paleomagnetic Wanganui Subdivision lithostratigraphy; (2) in eastern transition (2.6 Ma; Shackleton et al. 1990; Hilgen 1991) in Wanganui Basin, the "Waipuru Shellbed" is difficult to Rangitikei River was located by Wilson (1993) within the distinguish unequivocally from other shellbeds and it does Mangaweka Formation c. 70 m below the base of the not mark a distinctive lithologic change in the succession; Rangitikei Group. Paleomagnetic determinations of Seward and (3) the cyclothemic strata above the Vinegar Hill Tephra et al. (1986) and Naish et al. (in press) indicate that the upper contain more pumice and, in addition, regularly alternating Mangaonoho Formation and most of the Vinegar Hill siltstones and sandstones give way to a dominance of coarse- Formation fall within the Olduvai Normal Subchron (1.95- grained, shallow marine to nonmarine facies. 1.77 Ma). New ITPFT ages of 1.87 Ma (±0.15 Ma) and 1.75 Ma (±0.13 Ma) on the Waipuru Ash and Vinegar Hill Tephra, THICKNESS: (Fig. 6) The composite thickness of Rangitikei respectively (Naish et al. in press), support the stratigraphic Group strata in the Rangitikei River is c. 950 m. Here the location of the Olduvai Subchron determined by Seward et sedimentary succession is almost entirely exposed. Through- al. (1986). Moreover, Edwards (1976) placed the first out the Ohingaiti region, Rangitikei Group is laterally appearance of the coccolith Gephyrocapsa sinuosa in the extensive and generally maintains a consistent thickness and Rangitikei River section within the Waipuru Shellbed lithologic character. (GS3097; Fleming 1953). Gephyrocapsa sinuosa is thought Naish & Kamp—Lithostratigraphy, Wanganui Basin 237

to have evolved near the base of the Olduvai Subchron (c. cross-stratified, locally cemented, coarse bioclastic sand 1.9 Ma). ITPFT ages on tephra and new paleomagnetic data lenses develop within finer grained silty sandstones, and grade tor the Rangitikei River section (Naish et al. in press) indicate upsection into a larger scale, low-angle, cross-stratified that the Pliocene-Pleistocene boundary, as defined at Vrica, siliciclastic sandstone lithofacies. Soft-sediment deformation Italy (Aguirre & Pasini 1985), occurs c. 60 m below the features occur commonly. Vinegar Hill Tephra (Fig. 6) Siliciclastic siltstone member (Mrzmj). Horizontally bedded, CONSTITUENT FORMATIONS: The Rangitikei Group comprises fine sandy siltstone grades upwards into massive and i a ascending order: Mangarere Formation, Tikapu Formation, bioturbated, blue-grey concretionary siltstone, which Makohine Formation, Orangipongo Formation, Mangaonoho becomes increasingly sandy again before grading into silty Formation, and Vinegar Hill Formation. Constituent members sandstone at the base of the Hautawa Formation. are illustrated in Fig. 7A-F. Prominent members of each formation and their western Wanganui Basin correlatives DEPOSITIONAL ENVIRONMENT: Coarse-grained members of include: Mrcfmi and Mrsiri] - Te Rimu Formation; Tucm] = the Mangarere Formation represent a broad deepening- Hautawa Shellbed; Mksm] = Tuha Sand; Orsmj = Ohingaiti upwards transgressive facies assemblage. Sedimentary structures and fauna indicate deposition in shoreface to inner- Sand; Mhcmj = Mangamako Shellbed; and Vhcm4 = Waipuru Shellbed. shelf environments. Mid-shelf water depths are indicated at the base of Mrzmi by the in-situ occurrence ofthe mollusc Lucinoma galathea. Progressive shallowing to inner shelf MANGARERE FORMATION (new) depths is indicated by upward coarsening and the successive NAME: The name Mangarere Formation derives from appearance upwards within Mrzirij of Nemocardium Mangarere Stream, which enters the Rangitikei River pulchellum, Chlamys gemmulata, and Atrina pectinata immediately south of the township of Mangaweka, and is zelandica. applied to all strata located stratigraphically between the Mangaweka Mudstone and the base of the Tikapu Formation. DISTRIBUTION: Mangarere Formation is well exposed in the Fleming (1953) applied the name Te Rimu Sand to strata in north and northeast of the Ohingaiti region where it crops the Wanganui Subdivision previously mapped as Basal out in the east bank of Rangitikei River and in the south bank Nukumaruan Sand by Superior Oil Co. of N.Z. (1943). The of Mangarere Stream. Away from Rangitikei River valley, name is adopted here for the lowermost sandstone member the base of the formation is easily recognised as a low in the Mangarere Formation. vegetated bluff, and can be traced west across the region where good exposures occur along Weston Road, near TYPE SECTION: Site 1 (Fig. 1; T22/500474), Mangarere Road. Ohingaiti Reserve on Watershed Road, and in Porewa Valley. Immediately northeast of the region, Mangarere Formation REFERENCE SECTIONS: Site 2 (Fig. 1; T22/476475), east bank is exposed below Manuka No. 2 trig and in the Manga- Rangitikei River on a farm track. Site 3 (Fig. 1; T22/466473), wharariki Valley, where it is offset by the Rauoterangi Fault. east bank Rangitikei River.

THICKNESS: C. 200 m (Fig. 7A). TIKAPU FORMATION (new) NAME: Tikapu Formation derives from Tikapu Trig (Fig. 1, .AGE: Mangapanian (late Pliocene). 3) and is applied to all strata between the base ofthe Hautawa Shellbed and the base of the Tuha Sand. Superior Oil Co. of L1THOLOGY: (Fig. 7A) Basal channel-fill conglomerate N.Z. (1943, p. 17) applied the name "Hautawa" to the member (Mrcfmj). Locally, the base of the Mangarere prominent shellbed exposed above Hautawa Stream along Formation is deeply channelled (up to c. 30 m) into underlying "Old Hautawa Road" east of the Rangitikei-Turakina Mangaweka Formation east of the Rangitikei River in watershed (Site 6, Fig. 1; Fig. 5B). Fleming (1953) adopted Mangarere Stream (T22/497480; Fig. 5A) and along State the name and mapping of Superior Oil Co. of N.Z. (1943) in Highway 43 (T22/536497). Disoriented indurated clasts of the Wanganui Subdivision. The name is retained here and underlying Mangaweka Formation siltstone (up to 2 m corresponds to the basal coquina member of the Tikapu diameter), and smaller (up to 50 cm diameter) blocky, dark Formation (Tucmj). brown silty clasts of possible pedogenic origin, are randomly supported in a silty sand matrix. At Mangarere Stream, TYPE SECTION: Site 6 (Fig. 1; T22/325484; Fig. 5B), along channel-fill conglomerate is overlain by wavy laminated fine "Old Hautawa Road" 4WD track at the end of West Road. sand and silt interbeds. This site constitutes Fleming's (1953) type locality.

Te Rimu Sand—Siliciclastic sandstone member (Mrsmj). A REFERENCE SECTION: Site 4 (Fig. 1; T22/436457; Fig. 5C), conspicuous 1 m thick, well-cemented, bluff-forming Rangitikei River. concretionary sandstone marks the lower unconformable contact of the Mangarere Formation where it is not channelled THICKNESS: C. 65 m (Fig. 7B). into underlying Mangaweka Formation. Predominantly uncemented, moderately well sorted (at base), bioturbated, AGE: Hautawan (late Pliocene). Hautawa Shellbed contains brownish-grey, medium-grained, low-angle and trough cross- the age-diagnostic fauna Chlamys patagonica delicatula, stratified to parallel-laminated siliciclastic sandstone grades Phialopecten triphooki, Crassostrea ingens, and Globorotalia upsection into thick-bedded to massive silty sandstone. East crassula, and marks the boundary between the Mangapanian ofthe Rangitikei River (Site l,Fig. 1), near the lower contact, and Nukumaruan Stages in Wanganui Basin. 238 New Zealand Journal of Geology and Geophysics, 1995, Vol. 3

LITHOLOGY: (Fig. 7B) Siliciclastic sandstone member Rangitikei River to the Ruahine Range. The name is derive I (Tusmj). East of Rangitikei Valley, a conspicuous 2-3 m thick from Tuha Stream, which enters the Rangitikei River from bluff-forming concretionary sandstone, which marks the the east 3 km north of Ohingaiti township. The name and lower gradational boundary of the Tikapu Formation, passes definition are retained here, and the name applies to the upsection into a 2 m thick interval of crudely bedded to lowermost siliciclastic sandstone member of the Makohine massive and bioturbated, sparsely fossiliferous, fine silici- Formation. clastic sandstone. TYPE SECTION: Makohine Formation is continuously exposed Hautawa Shellbed—coquina member (Tucni/). A mollusc- in Rangitikei River from Site 21 (Fig. 1; T22/434455) near rich and brachiopod-rich shellbed supported in a matrix of the Otara Road bridge to Site 19 (Fig. 1; T22/387438*, siliciclastic fine sandstone to fine sandy siltstone uncon- Makohine Stream. formably overlies Tusmi in the east and rests directly upon Mrzni| in the west of the Ohingaiti region. The unconformity REFERENCE SECTIONS: Mksrri] to Mkznii (Fig. 7C) ate is marked by small-scale erosional relief and is sporadically exposed at Site 7 (Fig. 1; T22/434450), Otara Road. At Site penetrated by ophiomorpha burrows, which are preserved in 20 (Fig. 1; T22/438444), Mksm2 to Mkzm3 (Fig. 7C) crop the underlying sandstone. The upper portion of the shellbed out in cuttings along Otara Road. becomes less fossiliferous and is sharply overlain by massive siliciclastic siltstone. The shellbed thickens westward from THICKNESS: C. 165 m (Fig. 7C). 0.5 m in Rangitikei River (Site 4, new locality; Fig. 1) to 4 m AGE: Hautawan (late Pliocene). at the type section. LITHOLOGY: (Fig. 7C) Makohine Formation cyclothernu Siliciclastic siltstone member (Tuzm/). Massive, blue-grey, strata comprise four lithologically similar siliciclastic bioturbated, concretionary, siliciclastic siltstone coarsens sandstone, siliciclastic siltstone, and coquina members. upsection into a fine sandy siltstone. The gradational upper contact is marked by interbeds of sandstone that progressively Siliciclastic sandstone members (Mksm/_4). Mainly thicken upwards into overlying Makohine Formation. uncemented, sparsely fossiliferous, intensely bioturbated, brown to grey, massive to wavy laminated, siliciclastic silly DEPOSITIONAL ENVIRONMENT: The faunal content and sandstone grades upsection into moderately well sorted, sedimentary structures are consistent with an inner shelf bioturbated, low-angle and trough cross-stratified siliciclastic environment of deposition for the siliciclastic sandstone sandstone. Siliciclastic sandstone members Mksmj and member (Tusmi). Hautawa Shellbed (Tucmj) was deposited Mksiri2 comprise flaser-bedded sandstone in their uppi r under conditions of terrigenous sediment starvation and portions. Members Mksm2 and Mksrr^ comprise probable represents stratigraphic condensation on an open shelf during storm and channel-fill molluscan shellbeds, respectively. marine transgression. Fleming (1953) described an open Several distinctive layers of concretionary sandstone that marine shelfal fauna dominated by Barbatia, Chlamys, occur at the base of Mksmi mark the base of the formation, Purpurocardia, Tiostrea, Turritellidae, Bryozoa, and affording it easy recognition within the Ohingaiti region Brachiopoda for the shellbed. Faunal changes upsection (Fig. 5D). within the shellbed indicate a progressive increase in water depth from inner to mid-shelf environments. The successive Coquina members (Mkcm/^). Siliciclastic fine sandy siltstone upsection occurrence, in siltstone member Tuzirt], of Chlamys mollusc-rich and brachiopod-rich shellbeds sharply overlie gemmulata, Amygdalum striatum, Nemocardium pulchellum, all four siliciclastic sandstone members. The lower boundary and Panopea zelandica are interpreted as a shallowing from of the shellbeds is marked by erosional relief (up to 1 m at mid-shelf to inner shelf environments. base of Mksmi; Fig. 5E) and ophiomorpha burrows that penetrate into the underlying sandstones. Shellbeds are DISTRIBUTION: Tikapu Formation is well exposed in the abruptly overlain by blue-grey siliciclastic siltstone. Rangitikei River (Fig. 6B), Porewa Valley, and above Hautawa Stream. The lower coarse-grained members form a Siliciclastic siltstone members (Mkzmi-4). Uncemented, distinctive low bluff, which is easily identified in the field massive and bioturbated, blue-grey sandy siltstone and and can be traced from the Rauoterangi Fault west into the siltstone grade upsection into massive and crudely bedded, Makohine Stream gorge. West of Makohine Stream, the base bioturbated, siliciclastic silty sandstone near the base of the of the formation is only sporadically exposed, but can be overlying siliciclastic sandstone members. Mkzm3 includes followed to the type section where Hautawa Shellbed is a 20 m thick section of normally graded, decimetre scale, continuously exposed for several kilometres westwards fine sandy siltstone beds (Site 20, Fig. 1). towards the Turakina River. The shellbed thins dramatically towards the east. In contrast, the underlying sandstone DEPOSITIONAL ENVIRONMENT: The dominant molluscan member thickens towards the east. fauna, Gari lineolata, Scalpomactra scalpellum, Panopea zelandica, Fellaster zelandiae, together with the occurrence MAKOHINE FORMATION (new) of a variety of traction-emplaced sedimentary structure-., indicate siliciclastic sandstone members were deposited in NAME: The name derives from Makohine Stream (Fig. 1) and transitional and shoreface depositional environments. applies to all cyclothemic strata between the base of the Tuha Coquina members accumulated due to stratigraphic Sand (Mksmi) and the base of the Ohingaiti Sand (OrsmO. condensation on the inner to mid shelf during marine Superior Oil Co. of N.Z. (1943, p. 18) introduced and applied transgression and are dominated by the molluscs Chlamys, the name "Tuha Sand" in surveys of the region east of Tawera, Pleuromeris, Tiostrea, Gari, Nucula, Scalpomactra, Naish & Kamp—Lithostratigraphy, Wanganui Basin 239 and Brachiopoda. Siltstone members are dominated by the and water ejection structures are also common within mid-shelf molluscan taxa N. pulchellum, A. striatum, and members. High-angle planar cross-stratified sandstone occurs Neilo australis. at the top of Orsm, (Site 19, Fig. 1).

DISTRIBUTION: Except for sections in Rangitikei River, along Coquina members (Orcm/_5). Siliciclastic fine sandy siltstone Watershed Road, and in Porewa Stream valley, outcrops of supports a variety of mollusc and brachiopod shellbed Makohine Formation are sporadic within the Ohingaiti map members, which sharply overlie siliciclastic sandstone region. Good but discontinuous exposures occur east of members (Fig. 5F). The lower boundaries of shellbeds may Rangitikei River near Tuha Trig (Fig. 1) and in cuttings along exhibit erosional relief and are often penetrated by the F'eka Road. In the west, exposure is typically poor, with ichnofossil ophiomorpha. Blue-grey siliciclastic silts abruptly isolated outcrops in the hill country either side of the Porewa overlie the lower three coquina members. Orciri4 and Orcm5 Valley. Tuha Sand was not mapped by Fleming (1953) in the pass gradationally upwards into sandstone. Wanganui Subdivision, but was recognised by Superior Oil Co. of N.Z. (1943) as a "15 ft thick fine brown muddy Siliciclastic siltstone members (Orcm/_s). Uncemented, sandstone with a fossiliferous zone at the top" in the Turakina massive and bioturbated, blue-grey, fine sandy siltstone and Valley, and as a "poorly-developed fossiliferous silty sand" siltstone, sharply overlie coquina members Orcm(_3 (Fig. 5F) in the Mangawhero Valley c. 65 m above the Hautawa and upper sandstone members. Siltstone grades upsection into Shellbed. massive and often crudely bedded or laminated, bioturbated, siliciclastic silty sandstone. ORANGIPONGO FORMATION (new) Ohingaiti Ash member (Ortmj). A 1.5 m thick grey, rhyolitic N AME: Derives from Orangipongo Road and Orangipongo tephra, named "Ohingaiti Ash" by Seward (1976), crops out Trig and is applied to all cyclothemic strata between the base in the east bank of the Rangitikei River in the type section of the Ohingaiti Sand (Orsm;) and the base of the opposite the Rangitira Golf Course (Fig. 1, 8). Moderately Managamako Shellbed (Mhcmi). Superior Oil Co. of N.Z. lithified, bioturbated, horizontally bedded, glassy, silty fine (1943, p. 19) applied the name "Ohingaiti" to a prominent sands of the tephra member are interbedded within massive sandstone clearly exposed for several kilometres along-strike to decimetre-scale, crudely bedded, siliciclastic fine sandstone in bluffs on the east bank of the Rangitikei River opposite belonging to Orsm4. The lower contact is abrupt, whereas the township of Ohingaiti (Fig. 5F). Fleming (1953) adopted the upper contact grades upwards into overlying fine the name for a sandstone in the Wanganui Subdivision. The sandstones. name is retained here with member status and corresponds tc the lowermost sandstone member of the Orangipongo DEPOSITIONAL ENVIRONMENT: Sedimentary facies and faunal Formation (Orsm]). characteristics of the Orangipongo Formation cyclothemic strata are similar to those of the Makohine Formation. TYPE SECTION: The entire Orangipongo Formation is exposed Siliciclastic sandstone members were deposited in transitional in the east bank of the Rangitikei River between Site 9 (Fig. inner shelf to shoreface environments. Coquina members that 1: T22/393436) and Site 22 (Fig. 1; T22/394426), directly accumulated during transgressions are ascribed a shoreface opposite the Rangitira Golf Course (Fig. 1). The type section, to shelfal depositional environment, whereas siltstone which is aligned subparallel to the regional dip, is illustrated members accumulated during episodes of sea-level highstand in Fig. 8. The type locality for the Ohingaiti Tephra (Orim,) aggradation, and show an upsection-shoaling from mid to occurs within the section at Site 10 (Fig. 1). inner shelf depositional environments.

REFERENCE SECTIONS: Site 19 (Fig. 1; T22/387437), State DISTRIBUTION: The formation is well exposed in banks of Highway 1. Site 17 (Fig. 1; T22454433), Mangamako Road. the Rangitikei River and within the deeply incised Mangamako Stream gorge. However, over much of the THICKNESS: c. 170 m (Fig. 7D). Ohingaiti region, exposure is generally poor and confined mainly to road cuttings. Of these, the best outcrops occur AGE: Nukumaruan (late Pliocene). along Watershed Road, Mangamako Road, and State Highway 1 (Fig. 1). Immediately west of the mapped area, LITHOLOGY: (Fig. 7D) Orangipongo Formation cyclothemic Orangipongo Formation is well exposed in cuttings towards strata comprise seven lithologically similar siliciclastic the end of Agnews Road. Between road cuttings and river sandstone members, five lithologically similar siliciclastic sections, the base of the Orangipongo Formation is poorly siltstone members, five coquina shellbed members, and a expressed geomorphologically and is seldom exposed. tephra member. Figure 8 illustrates Orangipongo Formation cyclothemic strata and emphasises the relationships between the formational stratigraphy and the sedimentary cycles that MANGAONOHO FORMATION (new) occur within the formation. NAME: The formation is named after the settlement of Mangaonoho, located 4 km southwest of Ohingaiti township Siliciclastic sandstone members (Orsm/_j). Uncemented, (Fig. 1), and is applied to all strata between the base of the sparsely fossiliferous, intensely bioturbated, massive to wavy Mangamako Shellbed and a prominent fossiliferous sandstone laminated, brown, siliciclastic, silty fine sandstone grade member (Vhsmj) that marks the base of the Vinegar Hill upsection into well-sorted, bioturbated, bidirectional low- Formation. The Superior Oil Co. of N.Z. (1943, p. 20) angle and trough cross-stratified siliciclastic sandstone. ascribed the name "Mangamako" to a "50 ft thick, highly Intensely bioturbated flaser-bedded facies occur in the upper fossiliferous, muddy shell-sand zone" in Mangamako Stream, portions of members Orsm4 5 7. Soft-sediment deformation a tributary of the Rangitikei River (Fig. 1). Fleming (1953) ! 240 New Zealand Journal of Geology and Geophysics, 1995, Vol. : 8 adopted "Superior's" mapping with minor emendation in the DEPOSITIONAL ENVIRONMENT: All four coquina membeis east of the Wanganui Subdivision. We retain the definition represent stratigraphic condensation during a period of marine and name in this study. Mangamako Shellbed corresponds to transgression. Mangamako Shellbed (Mhcmi and Mhsiri ) members Mhcmi and Mhsiri] (Fig. 7E). was deposited within the shore-connected sand prism in a position further shoreward than older coquina members. The TYPE SECTION: Site 11 (Fig. 1; T22/394417), Mangamako faunal content and sedimentary structures show dramatic Stream. Between Sites 28 and 14 in the banks of the Rangitikei upsection deepening from nearshore (e.g., Amalda austral is River (Fig. 1). and Ruditapes largillierti) to inner (e.g., Tawera) and mid- shelf environments (e.g., Chlamys, Tiostrea, and Bryozoa). REFERENCE SECTION: Site 12 (Fig. 1; T22/328423), A shallow-water foreshore-shoreface fauna is dominated by immediately north of the cemetery on Murimotu Road Dosinia spp., Bassina yatei, Fellaster zelandiae, and Lutraria (Mangamako Shellbed; Mhcmi and Mhsir^). solida within the lower part of coquina member Mhcm4 (She 14, Fig. 1). Fauna preserved in coquina member Mhcni2 and THICKNESS: c. 135 m (Fig. 7E). Mhciri3 are similar to many older coquina members that accumulated in an open marine shelf environment. Siltstone AGE: Marahauan (late Pliocene). members contain molluscan fauna indicative of sheltal deposition, whereas sandstone members are interpreted as LITHOLOGY: (Fig. 7E) Mangaonoho Formation strata show a having accumulated in transitional inner shelf to shoreface cyclic signature similar to that of the Makohine and depositional environments. Orangipongo Formations. The formation is subdivided into members. DISTRIBUTION: Mangamako Shellbed is well exposed n Mangamako Stream. Although only sporadically exposed Mangamako Shellbed—coquina member Mhcmi and west of the Rangitikei River, the formation can be traced sandstone member Mhsmj. This prominent basal 15 m thick across the Ohingaiti map region due to the distinctive sleep member (Mhcmi) comprises a lower 1 m thick siliciclastic slope geomorphology produced by the basal Mangamako fine sandy silt supported shellbed, which unconformably Shellbed member. East of the Rangitikei River the formati. >n overlies Orangipongo Formation strata. The lower contact is thins and appears to lose its distinctive lithologic character. marked by very low angle truncation of the underlying beds West of Hunterville, where exposures of Mangaonolio (Fig. 5G). Upsection, a densely packed, reworked (at base), Formation equivalent strata occur along Ongo Road, (lie trough cross-stratified, pebbly shell conglomerate is scoured formation coarsens dramatically towards Turakina River. into underlying mollusc-rich silty sands. Mollusc valves Siltstone members grade laterally into fossiliferous silly exhibit imbrication aligned parallel to bedding. In the upper sandstone and sandstone. portion of the shellbed, a siliciclastic fine sandstone (Mhsmj) supports abundant in-situ or near-situm molluscan fauna, and passes upwards into less fossiliferous, horizontally bedded VINEGAR HILL FORMATION (new) sandy siltstone. Prominent sharp discontinuities, which are NAME: Derived from Vinegar Hill, 5 km northeast of the penetrated by ophiomorpha burrows, occur in the upper half township of Hunterville (Fig. 1), the name Vinegar Hill of the coquina member. A 20 cm thick reworked megascopic Formation is applied to strata between the base of a prominent tephra was observed within Mangamako Shellbed Member fossiliferous sandstone (Vhsnij; Fig. 7F), located in the (Mhcmi) in Simpsons Reserve drillcore, and comprises a Rangitikei River 1.5 km upstream from the Vinegar Hill concentrated layer of hornblende, hypersthene, minor augite, Bridge (Site 15, Fig. 1), and the top of the Vinegar Hill Tephra. and rare andesitic volcanic glass within siliciclastic sandy The formation includes New Zealand Geological Survey siltstone. The tephra does not appear to be present at the type fossil locality GS3097 (Fleming 1953) within the Waipuru section. Shellbed (Site 24, Fig. 1). The name "Waipuru" was applied by Fleming (1953) to the "Waipuru Fossiliferous Sand", a Coquina members (Mhcm2 and Mhcm^. Siliciclastic fine unit previously mapped westward from Rangitikei River to sandy siltstone supports mollusc and brachiopod shellbed Kai-iwi Stream by Superior Oil Co. of N.Z. (1943, p. 20). members, which sharply overlie siliciclastic sandstone The stratigraphic locality GS3O97 (Fleming 1953) is shown members. The lower boundaries of shellbeds exhibit erosional in Fig. 7F and corresponds to an interval of fossiliferous relief and are penetrated by the ichnofossil ophiomorpha at siltstone (Vhzm3) and a shellbed (Vhcm4) in a road cult tug many sites. Mhcm3 occurs conformably within sandy siltstone on SHI immediately north of the Hunterville-Fielding road lithologies. intersection. The name derives from Waipuru Stream (Fig. 1) and is retained here with member status to maintain Siliciclastic sandstone members (Mhsmj and historical continuity of usage. Brownish-grey, bioturbated, massive to laminated siliciclastic silty fine sandstone grades upsection into bidirectional low- TYPE SECTION: Vinegar Hill Formation is exposed in banks angle cross-stratified, wavy laminated and flaser-bedded of the Rangitikei River south of Site 15 (Fig. 1; T22/3633S8) siliciclastic sandstone. and in cliffs opposite the Vinegar Hill Domain. Faulting complicates exposure in sections downstream of the bridge, Siliciclastic siltstone members (Mhzmi^). Massive, blue- but middle portions of the formation are well exposed along grey, bioturbated, siliciclastic fine sandy siltstone coarsens a farm track at Site 23 (Fig. 1) and along Vinegar Hill Road upsection into decimetre-scale, crude horizontally bedded and (Sites 16,24; Fig. 1). The Rangitikei River section, farm track, wavy laminated, fine silty sandstone. Upper contacts are and road exposures in the vicinity of Vinegar Hill lire gradational into siliciclastic sandstone facies. designated as the type locality of the formation. Upper Naish & Kamp—Lithostratigraphy, Wanganui Basin 241 portions of the formation, including the Vinegar Hill Tephra, DISTRIBUTION: Vinegar Hill Formation is exposed in the south are accessible in gullies between Vinegar Hill and Cooks of the Ohingaiti region, although the most continuous Roads. exposures are confined to the Rangitikei River valley.

THICKNESS: c. 135 m (Fig. 7F). DISCUSSION AGE: Marahauan (late Pliocene). Regional correlations and depositional setting LirHOLOGY: (Fig. 7F) Vinegar Hill Formation has much in Figure 10 illustrates east-west correlations of late Pliocene common with the Makohine, Orangipongo, and Mangaonoho sediments across Wanganui Basin, based on maps and Formations that underlie it, as it comprises cyclothemic strata stratigraphic columns published in Superior Oil Co. of N.Z. that can be subdivided into groups of lithologically distinctive (1943), Fleming (1953), and in this paper. The cross-section members. is necessarily partly schematic, because individual cyclothems cannot be mapped across major interfluves, and therefore some correlations are inferred. It is also a composite cross- Coquina members (Vhcnii_^). Within the Vinegar Hill section, as each of the named sections are built up of strata Formation, four coquina members comprise a dense that dip to the south, and therefore older units occur many molluscan fauna supported in siliciclastic silty sandstone and kilometres to the north of the younger units. Furthermore, vary in thickness from 1 to 5 m. It is likely that Superior Oil we have not presented new stratigraphic descriptions for strata Co. of N.Z. (1943) included Vhcrri| 2,3 in their mapping of exposed in river sections west of the Ohingaiti region, which the "Waipuru Fossiliferous Sand". Vhcirij 345 occupy the almost certainly contain a more detailed lithostratigraphy than basal position of cycles, whereas Vhcm2 occurs in a mid- described by earlier workers. Nevertheless, several fundamental cycle position. features of the late Pliocene stratigraphy emerge from Fig. 10. Notwithstanding the lack of a significant lithologic Siliciclastic sandstone members (Vhsm]_^). Uncemented, distinction in the Rangitikei River section between units bioturbated, massive, and moderately fossiliferous silty fine above and below the Ohingaiti Sand, there is clearly a major sandstone passes upsection into wavy laminated and flaser- lithologic distinction across the Wanganui Subdivision bedded fossiliferous sandstone. Sedimentary structures in between the muddy Okiwa Group and the sandy and members Vhsm2 and Vhsm4 have been largely obscured due calcareous Nukumaru Group. During deposition of the Okiwa to intense bioturbation; however, wavy lamination and crude Group and the lower Rangitikei Group, the basin depocentre horizontal bedding does occur. Vhsmj comprises flaser- was located between Mangawhero and Turakina Rivers, bedded and wavy-laminated sandstone. where the thickest and finest grained sediments now occur. The lesser accommodation generated toward the basin Siiiciclastic siltstone members (Vhzmi_4). Uncemented, margins during this time (early Nukumaruan) is reflected in massive and bioturbated, moderately fossiliferous sandy the occurrence of sandy regressive shoreline deposits that siltstone grades upwards into sandstone. Lower portions of prograded towards the depocentre during sea-level highstands Vhzm2 and Vhzrn^ contain an abundant in-situ molluscan and initial stages of sea-level fall, and which now help to shelfal fauna. define the lithologic character of the cyclothems. Some sandstone units appear to have accumulated across the entire Waipuru Ash member (Vhtmi). A 2 cm thick, grey, basin, and these are the horizons that Superior geologists bioturbated, silicic volcanic ash, named the "Waipuru Ash" mapped during their evaluation of its hydrocarbon pros- by Seward (1976), is interbedded in fossiliferous sandstone pectivity. The bidirectional interfingering nature of sandstone member Vhsm4 (Site 15, Fig. 1, T22/361387; Seward 1976; units implies that the sandy sediments, at least, were sourced Naish et al. in press). The underlying contact is sharp, from two directions at the time, one from the northeast and irregular, and intensely burrowed. the other from the northwest. During deposition of the Nukumaru Group and the upper Vinegar Hill Tephra member (Vhtm2). A 20 cm thick pinky part of the Rangitikei Group (late Nukumaruan), the basin grey silicic volcanic ash and pumice horizon preserved within depocentre was located east of its earlier position in the fossiliferous sandy siltstone crops out in a cutting along vicinity of the Rangitikei River valley, as judged by the Vinegar Hill Road (Site 16, Fig. 1, T22/350387). This unit is thickness distribution of strata and the occurrence of finer named the "Vinegar Hill Tephra" and described fully in Naish grained deposits (Fig. 10). It is the combination of the late et al. (in press). Nukumaruan persistence of siltstone deposition and the early Nukumaruan occurrence of sandstone in the Rangitikei River DEPOSITIONAL ENVIRONMENT: We report an open marine valley, a pattern resulting from depocentre migration and shoreface to inner shelf faunal assemblage for coquina sediment supply, which caused the lithologic distinction members, the dominant molluscs being Purpurocardia, between the Okiwa and Nukumaru Groups to diminish Tiostrea chilensis lutaria, Notocorbula zelandica, Tawera eastwards toward the Ohingaiti region, thereby warranting subsulcata, Gari lineolata, Maorimactra ordinaria, and the erection of the Rangitikei Group. The ratio of sandstone Bassinet yatei. Intense bioturbation and diagnostic open to siltstone in late Pliocene cyclothemic strata of the marine fauna are indicative of an inner to mid-shelf Rangitikei Group exposed in the Ohingaiti region is depositional environment for the siliciclastic siltstones. The characteristic of eastern Wanganui Basin, and our proposed occurrence of ripple-laminated and flaser-bedded structures lithostratigraphic nomenclature may be more applicable to in upper portions of sandstone members suggests tidal eastern portions of the Wanganui Subdivision than previously conditions during deposition. established schemes. ! 242 New Zealand Journal of Geology and Geophysics, 1995, Vol. 8

The dominance of siltstone in the Okiwa Group and in REFERENCES the lower part of the Rangitikei Group, together with the Abbott, S. T; Carter, R. M. 1994: The sequence architecture of IT .J- occurrence of thick cyclothems, suggests that basin infilling Pleistocene (c. 1.1 -0.4 Ma) cyclothems from New Zealat d: was strongly aggradational during the early Nukumaruan. facies development during a period of orbital control on iea At this time, sedimentation rates probably matched level cyclicity. In: de Boer, P. L.; Smith, D. G. ed. Orbi al subsidence rates, and similar depositional environments were forcing and cyclic sequences. International Association of maintained during accumulation of successive cycles. In Sedimentology special publication 19: 367-394. contrast, during the late Nukumaruan (Nukumaru Group and Aguirre, E.; Pasini, G. 1985: The Pliocene-Pleistocene bounds y. upper Rangitikei Group deposition), basin infilling was Episodes 8: 116-120. strongly progradational. Cyclothems are thinner and coarser Alloway, B. V.; Pillans, B. J.; Sandhu, A. G.; Westgate, J. A. I9"3: grained than the underlying cyclothems and show a significant Revision of the marine chronology in Wanganui Basin, tv-w upsection basinward shift to shallower water facies. The Zealand, based on isothermal plateau fission-track dating change from aggradation to progradation during the mid of tephra horizons. Sedimentary geology 82: 299-310. Nukumaruan is probably of tectonic origin, and results from Anderton, P. W. 1981: Structure and evolution of the South Wanganui a decrease in the rate of subsidence in the Ohingaiti region Basin, New Zealand. New Zealand journal of geology and concomitant with migration of the tectonic hinge line geophysics 24: 39-63. southward into the basin, thereby resulting in coeval Beu, A. G.; Edwards, A. R. 1984: New Zealand Pleistocene ;ind progressive uplift of its northern margin. Late Pliocene glacio-eustatic cycles. Palaeogeography palaeoclimatology palaeoecology 46: 119-142. Origin of cyclicity in the Rangitikei Group Edwards, A. R. 1976: Gephyrocapsa and the Pliocene-Pleistoct ne boundary in the New Zealand region. Geological Society The lithostratigraphy documented in this paper for the of New Zealand, Hamilton conference abstracts. 2 p. Rangitikei River valley, between Mangaweka and Vinegar Fleming, C. A. 1953: The geology of Wanganui Subdivision. t,-:w Hill, illustrates a remarkable on-land succession of nearly continuously exposed late Pliocene-early Pleistocene marine Zealand Geological Survey bulletin 52. cyclothems. The 20 sedimentary cycles revealed in this part Hilgen, F. J. 1991: Astronomical calibration of Gauss to Matuyama of the basin are considered to have formed through the sapropels in the Mediterranean and implications for he interplay of subsidence, sediment supply, and glacio-eustatic geomagnetic polarity timescale. Earth and planetary sc:e,

Scward, D. 1976: Tephrostratigraphy of marine sediments in the Stern, T. A.; Davey, F. J. 1990: Deep seismic expression of a foreland Wanganui Basin, New Zealand. New Zealand journal of basin: Taranaki Basin, New Zealand. Geology 18: 979-982. geology and geophysics 19: 9-20. Superior Oil Company of New Zealand 1943: In: Feldmeyer, A. E.; Seward, D.; Christoffel, D. A.; Lienert, B. 1986: Magnetic polarity Jones, B. C; Firth, C. W.; Knight, J.: Geology of the stratigraphy of a Plio-Pleistocene marine sequence of North Palmerston-Wanganui Basin, "West Side", North Island, Island, New Zealand. Earth and planetary science letters New Zealand. (Typescript and maps filed with Institute of 80: 353-360. Geological and Nuclear Sciences Ltd, Lower Hutt; see Shackleton, N. J.; Backman, J.; Zimmerman, H.; Kent, D. V.; Hall, Feldmeyer, A. E.) M. A.; Roberts, D. G.; Schnitker, D.; Baldauf, J. G.; Te Punga, M. T. 1953: The geology of Rangitikei Valley. New Despraires, A.; Homrighausen, R.; Huddlestun, P.; Keene, Zealand Geological Survey memoir 8: 46 p. J. B.; Kaltenback, A. J.; Krumsiek, K. A. O.; Morton, A. C; Murray, J. W.; Westberg-Smith, J. 1984: Oxygen isotope Turner, G. M.; Kamp, P. J. J. 1990: Palaeomagnetic location of the calibration of the onset of ice-rafting and history of glaciation Jaramillo subchron and the Matuyama-Brunhes transition in North Atlantic region. Nature 307: 620-623. in the Castlecliffian stratotype section, Wanganui Basin, New Zealand. Earth and planetary science letters 100: Shackleton, N. J.; Berger, A.; Peltier, W. R. 1990: An alternative 47-50. astronomical calibration of the lower Pleistocene timescale based on ODP Site 677. Transactions of the Royal Society Vail, P. 1987: Seismic stratigraphy interpretation using sequence of Edinburgh 81: 251-261. stratigraphy, part 1: seismic stratigraphic interpretation procedure. In: Bally, A. N. ed. Atlas of seismic stratigraphy. Slern,T. A.; Davey, F. J. 1989: Crustal structure and origin of basins American Association of Petroleum Geologists studies in formed behind the Hikurangi subduction zone, New geology 27: 1-10. Zealand. In: Price, R. A. ed. Origin and evolution of sedimentary basins and their energy and mineral resources. Wilson, G. S. 1993: Ice induced sea-level changes in the late International Union of Geodesy Geophysics and American Neogene. Unpublished Ph.D. thesis, lodged in the Library, Geophysical Union. Geophysical monograph 48: 73-85. Victoria University of Wellington.