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Stratigraphy, sedimentology andstructure of the Ihungia decollement,Raukumara Peninsula, North Island, NewZealandJill A. Kenny aa Geology Department , University of Auckland , Private Bag,Auckland , New ZealandPublished online: 09 Feb 2012.

To cite this article: Jill A. Kenny (1984) Stratigraphy, sedimentology and structure of the Ihungiadecollement, Raukumara Peninsula, North Island, New Zealand, New Zealand Journal of Geology andGeophysics, 27:1, 1-19, DOI: 10.1080/00288306.1984.10422287

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New Zealand Journal a/Geology and Geophysics, 1984, Vol. 27: 1-19 0028-8306/84/2701-0001 $2. 50/0 © Crown copyright 1984

Stratigraphy, sedimentology and structure of the Ihungia decollement, Raukumara Peninsula, North Island, New Zealand

JILL A. KENNY Geology Department University of Auckland Private Bag Auckland, New Zealand

Abstract The 75 km2 Ihungia catchment, in the extreme northeast of the North Island of New Zealand, includes part of the northeast-trending, complex belt of Cretaceous and Tertiary rocks forming the eastern continental margin sequence of New Zealand (the East Coast Deformed Belt).

Upper Cretaceous "autochthonous" rocks, exposed in the north of the region, are mapped as extensions of the Ngaterian Hikurangi Beds (for­mation) and overlying Teratan-Haumurian calcar­eous flysch ofTapuaeroa Formation. The Hikurangi Beds contain 2 new units - the lower Mangar­akeke lithofacies (300 m thick, alternating noncal­careous sandstone and mudstone), and the Rangikohua lithofacies (200 m thick, massive, non­calcareous sandstones separated by thin siltstone lenses). These rocks were deformed by north-south­trending isoclinal folding before becoming a repo­sitory area for 3 thrust sheets, emplaced by gravity sliding from the north over the "autochthonous" material. The lowest thrust sheet, emplaced follow­ing Late Oligocene time, is composed predomi­nantly of Mokoiwi Formation mudstones, including blocks of Taitai Sandstone Member (Motuan, Albian age), and blocks of Mangatu Group lithol­ogies, including Whangai and Weber Formations (Dannevirke-Landon, Paleogene), enclosed in ben­tonite. New lithologies in the Mangatu Group are divided into sandstone, flysch, or greensand lithofacies.

The middle thrust sheet contains thick Upper Tertiary mudstone and flysch of the Ihungia For­mation, including a 10-15 m thick, normally graded, igneous conglomerate lithofacies, and a 100 m thick, laminated bryozoan, coquina limestone lithofacies. Emplacement postdates the emplace­ment of the lowest thrust and occurred later than Middle Miocene.

Received 15 March 1982, accepted 7 September 1983

Sig. I"

Mangatu Group lithologies recur in the upper­most thrust sheet, emplaced after the middle thrust.

All lithologies have been influenced by multi­phase ENE-WSW to east-west faulting and folding episodes.

Keywords Ihungia; Cretaceous; Tertiary; East Coast Deformed Belt; stratigraphy; structure; decollement; autochthon; allochthon; thrusts; Kouetumarae, Mangarakeke, Makahikatoa, Ruan­garehu Faults; new structural names

INTRODUCTION

The Ihungia valley is situated on the Raukumara Peninsula, about half way between Gisborne and East Cape, west of State Highway 35. The Ihungia River system flows north to join the lower Mata River (Fig. 1) about 40 km before it reaches the sea as the Waiapu River.

The catchment comprises approximately 75 km2 of rolling hill country rarely exceeding 600 m above sea level, dissected by a closely spaced drainage network. It spans the complex boundary between Cretaceous rocks to the northwest, which are prob­ably autochthonous (Speden 1976), and allo­chthonous Late Cretaceous and Tertiary lithologies to the southeast. The oldest rocks of the region are Torlesse terrane lithologies which cover much of the western half of the Raukumara Peninsula. These are overlain to the east by alternating sandstone and mudstone, and rare conglomerate and lime­stone, representing Clarence, Raukumara, and Mata Series. The area further to the southeast is domi­nated by sandy calcareous mudstone and thick flysch sequences of the Pareora - Lower Wanganui Series. The remaining area of the peninsula (the north, northeast, and a northeast-trending sliver separating Cretaceous and Upper Tertiary rocks), consists of jumbled lithologies, now thought to be allochthonous, of mid Cretaceous - Miocene age. These include blocks from the autochthonous material, Matakaoa Volcanics (Late Cretaceous -Early Tertiary), and Lower Tertiary lithologies characterised by pale mudstones which are usually calcareous and frequently bentonitic, and by sandy facies, often rich in glauconite.

New Zealand Journal a/Geology and Geophysics, 1984, Vol. 27: 1-19 0028-8306/84/2701-0001 $2. 50/0 © Crown copyright 1984

Stratigraphy, sedimentology and structure of the Ihungia decollement, Raukumara Peninsula, North Island, New Zealand

JILL A. KENNY Geology Department University of Auckland Private Bag Auckland, New Zealand

Abstract The 75 km2 Ihungia catchment, in the extreme northeast of the North Island of New Zealand, includes part of the northeast-trending, complex belt of Cretaceous and Tertiary rocks forming the eastern continental margin sequence of New Zealand (the East Coast Deformed Belt).

Upper Cretaceous "autochthonous" rocks, exposed in the north of the region, are mapped as extensions of the Ngaterian Hikurangi Beds (for­mation) and overlying Teratan-Haumurian calcar­eous flysch ofTapuaeroa Formation. The Hikurangi Beds contain 2 new units - the lower Mangar­akeke lithofacies (300 m thick, alternating noncal­careous sandstone and mudstone), and the Rangikohua lithofacies (200 m thick, massive, non­calcareous sandstones separated by thin siltstone lenses). These rocks were deformed by north-south­trending isoclinal folding before becoming a repo­sitory area for 3 thrust sheets, emplaced by gravity sliding from the north over the "autochthonous" material. The lowest thrust sheet, emplaced follow­ing Late Oligocene time, is composed predomi­nantly of Mokoiwi Formation mudstones, including blocks of Taitai Sandstone Member (Motuan, Albian age), and blocks of Mangatu Group lithol­ogies, including Whangai and Weber Formations (Dannevirke-Landon, Paleogene), enclosed in ben­tonite. New lithologies in the Mangatu Group are divided into sandstone, flysch, or greensand lithofacies.

The middle thrust sheet contains thick Upper Tertiary mudstone and flysch of the Ihungia For­mation, including a 10-15 m thick, normally graded, igneous conglomerate lithofacies, and a 100 m thick, laminated bryozoan, coquina limestone lithofacies. Emplacement postdates the emplace­ment of the lowest thrust and occurred later than Middle Miocene.

Received 15 March 1982, accepted 7 September 1983

Sig. I"

Mangatu Group lithologies recur in the upper­most thrust sheet, emplaced after the middle thrust.

All lithologies have been influenced by multi­phase ENE-WSW to east-west faulting and folding episodes.

Keywords Ihungia; Cretaceous; Tertiary; East Coast Deformed Belt; stratigraphy; structure; decollement; autochthon; allochthon; thrusts; Kouetumarae, Mangarakeke, Makahikatoa, Ruan­garehu Faults; new structural names

INTRODUCTION

The Ihungia valley is situated on the Raukumara Peninsula, about half way between Gisborne and East Cape, west of State Highway 35. The Ihungia River system flows north to join the lower Mata River (Fig. 1) about 40 km before it reaches the sea as the Waiapu River.

The catchment comprises approximately 75 km2 of rolling hill country rarely exceeding 600 m above sea level, dissected by a closely spaced drainage network. It spans the complex boundary between Cretaceous rocks to the northwest, which are prob­ably autochthonous (Speden 1976), and allo­chthonous Late Cretaceous and Tertiary lithologies to the southeast. The oldest rocks of the region are Torlesse terrane lithologies which cover much of the western half of the Raukumara Peninsula. These are overlain to the east by alternating sandstone and mudstone, and rare conglomerate and lime­stone, representing Clarence, Raukumara, and Mata Series. The area further to the southeast is domi­nated by sandy calcareous mudstone and thick flysch sequences of the Pareora - Lower Wanganui Series. The remaining area of the peninsula (the north, northeast, and a northeast-trending sliver separating Cretaceous and Upper Tertiary rocks), consists of jumbled lithologies, now thought to be allochthonous, of mid Cretaceous - Miocene age. These include blocks from the autochthonous material, Matakaoa Volcanics (Late Cretaceous -Early Tertiary), and Lower Tertiary lithologies characterised by pale mudstones which are usually calcareous and frequently bentonitic, and by sandy facies, often rich in glauconite.

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2 New Zealand Journal of Geology and Geophysics, 1984, Vol. 27

~ N

Severe faulting has affected the whole peninsula for as far back as the geological record can be traced (Kingma 1965).

The topographical map (NZMS 1, N80 & N81 - Tokomaru Bay, 1:63360), overprinted with the new metric grid (Y15, Y16, Z16), was used as a reference throughout this study. Field mapping was carried out on a scale of 1: 15 840. Bed thicknesses follow Ingram (1954). All fossils quoted in the text are stored by the Geology Department, University of Auckland. Fossil localities are listed in the New Zealand Fossil Record Files held by the University and the New Zealand Geological Survey, Lower Hutt.

STRATIGRAPHY

The following lithostratigraphic units are used in this paper: Mokoiwi Formation (Speden, after Bremner), Taitai Sandstone (member) (Speden, after McKay), Hikurangi Beds (formation) (Laing, after McKay), Tapuaeroa Formation (Lillie), Whangai

Fig. 1 The Ihungia catchment, showing named topographical features mentioned in the text.

Formation (Lillie), Mangatu Group (Wellman, Moore), Weber Formation (Lillie), and Ihungia Formation (Lillie, after McKay). Other informal lithofacies are used to provide mappable units within these formal names.

Stratigraphic relationships and brief lithologic descriptions are set out in Table 1. Ngaterian (late Albian) and Upper Cretaceous rocks in the Ihungia catchment are possibly autochthonous, as rocks of similar character and age further north are consid­ered to be in place (Speden 1976). Lower Creta­ceous and Tertiary lithologies are undoubtedly allochthonous, and have been mapped in 3 decolle­ment sheets (Fig. 2, 3).

MOKOIWI FORMA nON (Speden 1976, after Bremner 1934)

The Mokoiwi mudstone was described by Bremner (1934) and redescribed by Wellman (1959a), but "never adequately defined" (Speden 1976). For a detailed history of the nomenclature, see Speden (1976, p. 84).

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Kenny-Stratigraphy & structure, Ihungia decollement 3

Mokoiwi Formation, as used by Speden (1976), consists of the Mokoiwi siltstone, Taitai Sandstone (member), and volcanic rocks in Mokoiwi For­mation. The characteristic rock type of the litho­facies is siltstone, mostly medium to dark blue-grey, sometimes containing thin, fine-grained sandstone beds.

The formation occupies about 14 km 2 of the lower Ihungia catchment and the upper portion of the Mangarara Stream (Fig. 1, 2). It is separated from the overlying Tertiary beds by faults.

MOKOIWI SILTSTONE (informal name, following Speden 1976)

The dominant lithology is a crushed, dark blue­grey to black, noncalcareous siltstone, which is highly indurated. It often appears shiny and very dark where slickensided and polished surfaces are exposed, but weathers to a sombre dark grey and later a pale grey. Occasional coarser lenses within the siltstone were identified.

Hard sandston.es appear as discontinuous single beds or bedded sequences alternating with the silt­stone. They are characteristically fine grained, moderately sorted, noncalcareous, and medium grey. The beds are generally 10-30 cm thick in sequences with sandstone to siltstone ratios of from 1:4 to 1:5. Poorly sorted, coarse to very coarse grained sandstone with a fine-grained matrix, appears rarely within the siltstone. The sandstone weathers to a yellow-grey-brown colour.

Fine fractures in both the siltstone and sand­stone are filled with secondary calcite. Tiny dis­seminated grains of pyrite were observed at some exposures. Homogeneous and cone-in-cone calcitic concretions are mentioned by Vella (1959) and Speden (1976) as being common in Mokoiwi silt­stone, but were not observed within the Ihungia catchment. Sole marks, grading, and parallel lam­ination were recorded.

The rocks have been considerably folded and faulted. The siltstones are strongly sheared and the sandstone beds are broken by closely spaced frac­tures. Faulted contacts, the complexity of folding and faulting, and lack of marker horizons make accurate measurement of thickness impossible, but it is unlikely to exceed 1000 m.

TAlTAl SANDSTONE MEMBER (Speden 1976, after McKay 1887)

Taitai Beds were named by McKay after Mt Taitai (grid ref. Y15/664552 [N71/587322])*. Taitai

*Grid references are based on the national thousand-metre grid of the 1:50000 topographical map series (NZMS 260). Grid references in square brackets which follow, are equivalents for the 1 :63 360 map series (NZMS 1).

Sandstone was formalised as a member of the Mokoiwi Formation by Sped en (1976). In the type area it consists of "thick sandstone masses con­taining minor conglomerate and siltstone" enclosed by, and intertonguing with the Mokoiwi siltstone.

In the Ihungia catchment the largest mass of Tai­tai Sandstone is at Puketiti (12 ha). Other masses range in size down to only a few cubic metres. Tai­tai Sandstone blocks are all located within the Mokoiwi siltstone and occur at different levels. They probably do not represent a single bed.

The Taitai Sandstone is a dull olive green-grey to yellow-grey colour, but has a blotchy appearance in outcrop because the many joint and shear sur­faces are stained a dark chocolate brown. This massive, poorly sorted, medium to coarse grained sandstone, with fine sand to muddy matrix, is indurated and brittle, and usually noncalcareous. Siltstone and fine sandstone lenses are contained in it, and in the upper Mangarara Stream (YI6/675391 [N80/604135]) an outcrop of strongly weathered conglomerate was found. Only 2 pebbles were tentatively identifiable as granite/granodiorite and basalt or greywacke. In the same outcrop a granule conglomerate was observed. The conglom­erate is localised and laterally discontinuous and grades upwards to sandstone.

Cut sections of Taitai Sandstone reveal partings along some joint surfaces and open voids, indicat­ing a high porosity for this lithology.

Grading and fine parallel laminations were recorded at a few exposures, but no other sedi­mentary structures were seen. In the absence of bedding, no thickness can be measured. The largest mass in the area rises 170 m above the adjacent Mokoiwi siltstone.

Age and environment of deposition of the Mokoiwi Formation No fossils were discovered in Mokoiwi rocks in the Ihungia catchment. According to Speden (1976), the Mokoiwi Formation is of Motuan age and possibly late Urutawan locally.

The formation in the Ihungia catchment has been too deformed to exhibit many of the features useful in diagnosing its paleoenvironment. Speden's interpretation of the paleoenvironment is "depo­sition near the edge of a narrow shelf, possibly adjacent to deltas bordering the margin of an active landmass of high relief' (Speden 1976, p. 10 1).

Microscopic quartzite and chert fragments sug­gest that the Mokoiwi Formation may have been derived from, or had a similar source to, the Tor­lesse terrane (as described by Feary 1974; Hill 1974; Moore 1974; Hoolihan 1977; Isaac 1977) making up the main ranges to the west.

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4 New Zealand Journal of Geology and Geophysics, 1984, Vol. 27

Table 1 Stratigraphic relationships and brief lithologic descriptions.

Lithologic units

Alluvial gravels and ri ver terraces

Ihungia Formation

Weber Formation

MANGATU GROUP

Whangai Formation

Tapuaeroa Formation

Hikurangi Beds

mudstones and

flysch beds

igneous conglomerate

limestone

greensand lithofacies

grey sandstone lithofacies

flysch lithofacies

brown sandstone lithofacies

bentonite, green and red

mudstones

flysch beds

Rangikohua lithofacies

Mangarakeke lithofacies

Age

Hawera Series (Late Pleistocene - Holocene)

Upper Otaian to lower Lillburnian (Early to Middle Miocene)

Lower Altonian (late Early Miocene)

Lower Altonian (late Early Miocene)

Whaingaroan to Waitakian

Whaingaroan to Duntroonian (Oligocene)

? Dannevirke to Arnold (Paleocene-Eocene)

? Dannevirke to Arnold (Paleocene-Eocene)

? Dannevirke to Arnold (Paleocene-Eocene)

Waipawan to Bortonian (and possibly to Whaingaroan) (Eocene)

Haumurian to Teurian (Maastrichtian to Paleocene)

? Teratan to Haumurian (Late Cretaceous)

Ngaterian (latest Albian)

Ngaterian (latest Albian)

Lithologic descriptions

Aggradational deposits formed by igneous pebbles bonded by silt. Thin ash cover on hills.

Massive sandy mudstones, occasional flysch sequences, usually calcareous. Moderate microfauna, poor macrofauna. Mudstones are blue-grey in colour; thin sandstone beds are yellow-brown.

Conglomerate, of igneous origin mainly, is I bed in the flysch.

Clastic, bioherm ai, flaggy limestone enclosed in Ihungia Formation mudstone.

Cream-grey calcareous mudstone, developing flagginess.

Glauconite-rich, green-grey, massive, calcareous, fine-grained sandstones with intervening dark green-grey mudstones.

Massive or laminated, graded, friable, pale grey, fine grained to very fine grained sandstones, separated by grey mudstones

Alternating medium and very fine dark sandstones. Lenses of coarse sandstone containing ?Whangai clasts.

The above beds are underlain by thick, yellow-brown, massive fine sandstones.

Bentonitic mudstones. Swelling properties lead to ground instability. Coarse to fine grained glauconitic sandstones, and red mudstones are often associated with bentonite. Other rocks of the Mangatu Group and Mokoiwi Formation are also found.

Predominant lithology is siliceous, light grey, hard, brittle, noncalcareous shale. Limonitic and jarositic staining on joints. Calcareous, grey mudstones and noncalcareous, fine-grained micaceous sandstones, both stained, are subordinate.

Alternating, thin-bedded, medium grey, calcareous sandstones and darker grey mudstones. Coarse sand and shell fragments at the base of I bed.

Thick, massive, graded, yellow-brown, noncalcareous sandstones separated by thin siltstones.

The above beds are underlain by fossiliferous noncalcareous grey flysch.

(continued opposite page)

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Kenny-Stratigraphy & structure, Ihungia decollement 5

Table 1 (continued).

Mokoiwi Formation

siltstones and flysch

Taitai Sandstone

? Urutawan to Motuan (Albian)

? Urutawan to Motuan (Albian)

HIKURANGI BEDS (after McKay 1887)

McKay (1SS7) used Hikurangi Beds (after Mt Hik­urangi, [N71/462276]) for alternating indurated sandstone and mudstone outcropping widely to the north and west of the Ihungia catchment.

Speden (1976, p. 73) comments: "Laing (1972a, b) uses several formation names (Hikurangi Beds, Raukumara Formation and Waiorongomai Sand­stone) previously introduced informally in pub­lished and unpublished reports. Some of the names have been used in a special sense or are poorly documented, so that evaluation of their scope and content is difficult.".

"Hikurangi Beds" is used informally here as a formation name, pending formalisation by I. G. Speden and P. R. Moore (New Zealand Geological Survey). For the purposes of mapping these beds in the Ihungia catchment, they are divided infor­mally into a lower alternating sequence of subequal proportions of sandstone and mudstone beds -the Mangarakeke lithofacies; and an upper sequence dominated by thick massive sandstone beds separated by thin mudstone or siltstone layers - the Rangikohua lithofacies.

Mangarakeke lithofacies (informal)

Named for the Mangarakeke Stream in the lower Ihungia valley, where the unit is typically exposed in the stream, and on the lower northern slopes of the ridge extending eastwards from Rangikohua Trig (Trig ISS). The reference section is from Y15/61S422 [NSO/54 1 167) to Y15/631416 [NSO/555161). The lower contact with older material was not seen. The upper contact with the overlying Rangikohua lithofacies can be observed at Y15/61S421 [NSO/541166).

Mangarakeke lithofacies is composed of regu­larly alternating sandstone and m\ldstone beds, both of which range in thickness from 0.2 to 1 m. The indurated, light grey to yellow-grey, noncalcareous sandstones are very fine grained, rarely fine grained, and well sorted. The mudstones are noncalcareous and medium to light blue-grey. They fritter, leaving the sandstones protruding.

Sedimentary structures include thin-bedded tur­bidites with all divisions of the Bouma sequence

Dark blue-grey, noncaJcareous, indurated siltstone with occasional sandstone beds.

Thick lensoid bodies of hard, medium grained, noncalcareous sandstones in the siltstone.

present and in the correct order. At 2 localities -Y15/603427 [NSO/525172) and Y15/621421 [NSO/544167) - flute and groove casts beneath massive sandstone beds indicate paleocurrent flow to north (005°) and northeast (030°) respectively. Most massive parts of the sandstone units fine upwards, but some are apparently not graded. Mot­tling, presumably caused by bioturbation, is com­mon in the mudstones. Carbonaceous material occurs rarely on some bedding surfaces. Sometimes fragments of Inoceramus are concentrated at the base of sandstone units.

The beds have probably been repeated by fault­ing and the thickness is estimated to be about 300 m.

Paleontology and age of the M angarakeke lithofacies

Poorly preserved Inoceramus (Y15/f26, YI5/f27) were identified by Dr I. G. Speden (New Zealand Geological Survey, pers. comm. 19S0) as Inocera­mus hakarius Wellman, indicating the Ngaterian Stage.

Rangikohna lithofacies (informal)

Named after the Rangikohua Trig (y15/60S4IS [NSO/530162]), where the unit occurs as scattered outcrops along the ridge, and along the uppermost portion of the most western stream within the study area flowing down to Mata River. The reference section is from Y15/601420 [NSO/522163) to Y15/60342S [NSO/525173). The lower contact is apparently conformable with Mangarakeke rocks. The upper contact is faulted, or eroded away.

Rangikohua lithofacies contains thick sandstone beds with thin siltstone layers. The sand­stone:siltstone ratio is well in excess of 20: I. Each thick sandstone bed exhibits a full Bouma sequence with the A division dominating, grades up into the overlying siltstone bed, and has a sharp contact with the underlying siltstone. The sandstone units (2-3 m thick) are generally friable, brown, noncalcar­eous, fine grained, and moderately sorted, but can become coarser grained and poorly sorted. The rarely seen siltstone layers are creamy brown, non-

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8 New Zealand Journal of Geology and Geophysics, 1984, Vol. 27

calcareous, and finely laminated. They are gener­ally less than 5 cm thick and easily eroded.

Outcrops near Rangikohua Trig contain concre­tions about 5 cm in diameter, and rare, hard, non­calcareous, orange, cherty pebbles 6-8 cm long (obviously foreign to this lithology). Several basal sandstone layers are crowded with carbonised plant remains. Grooves and flute casts were often seen on the undersides off allen blocks. In situ examples were discovered under the sandstones within a huge recent rotational slump at Y15/601427 [N80/522172] and indicate local paleocurrents flowing in a westerly direction. Rotation has affected this measurement by no more than 10°.

The Rangikohua lithofacies has a thickness of approximately 200 m.

Paleontology and age of the Rangikohua lithofacies The unfossiliferous Rangikohua lithofacies is very similar to thick sandstones near the base of the Arowhanan Stage elsewhere in the region (I. G. Speden, New Zealand Geological Survey, pers. comm. 1980). However the diagnostic fossil of the Arowhanan Stage (Inoceramus rangatira Wellman) was not found.

A sample (Y15/f21) of carbonaceous fine sand­stone from a spur above the Mangarakeke Stream 3.5 km from the junction with the Ihungia River (Y15/617420 [N80/540165]) was analysed by J. I. Raine and G. J. Wilson (Palynology Section, New Zealand Geological Survey). It revealed a sparse, poorly preserved palynoflora which is of Motuan to Ngaterian age. Since the underlying Mangarak­eke rocks are Ngaterian, the Rangikohua rocks must be the same.

Environment of deposition of the Hikurangi Beds Sandstone beds in the Mangarakeke lithofacies are interpreted as turbidites. Fragments of Inoceramus up to 6 cm long are confined to these sandstones. According to Speden (1976) most species of Ino­ceramus are restricted to shelf environments, espe­cially at depths less than 150 m. If this is so with Mangarakeke rocks, turbidity currents were derived from continental shelf sands.

Increase in the thickness and proportion of sand­stone, from Mangarakeke rocks to Rangikohua rocks (distal to proximal turbidites - Walker 1967; Blatt et al. 1972; Middleton & Hampton 1973) indicates that the point source of the turbidites (perhaps a distributary channel) was advancing towards this region. The 2 lithofacies correspond to the D and C facies respectively of the submarine fan model of Ricci-Lucchi (1975). Proximity of a landmass or channel is indicated by intervening

deposits of siltstones in the upper (Rangikohua) beds, in contrast to the mudstones that are found in the lower (Mangarakeke) beds.

As with the older Mokoiwi Formation, deriva­tion of the Hikurangi Beds still seems to be from Torlesse lithologies (Kenny 1980).

TAPUAEROA FORMATION (Lillie 1953)

Tapuaeroa Formation consists of interbedded sandstone and mudstone, sometimes carbonaceous and glauconitic (Lillie 1953), widely distributed throughout the East Coast Deformed Belt. Well­man (1959a) restricted the formation in the Rau­kumara Peninsula to only those beds of Late Cretaceous age. Black (1980) has formalised the name Tikihore Formation, combining the Tapua­eroa and Raukumara Formations, because of the lack of an uncomformity between them in his study area. Nevertheless, the Tapuaeroa Formation is retained here to conform with the established stra­tigraphic nomenclature in other parts of Rauku­mara Peninsula.

In the study area, Tapuaeroa Formation is loCated along the lower Ihungia River from YI5/646435 [N80/572182] to YI5/649422 [N80/575169]. The beds are inverted. The basal contact is the Ihungia Fault (Pick 1962), and the upper contact occurs to the northeast, outside the study area.

Tapuaeroa Formation consists dominantly of alternating medium to thin bedded sandstone and mudstone in the ratio of approximately 2: 1. Beds are commonly 10-15 cm thick, although thick­nesses are variable, so that in places mudstone pre­dominates. The sandstones are light to medium grey,. hard, very fine grained, well sorted, calcar­eous, sometimes graded, usually massive, and often have fine parallel laminations towards the strati­graphic top. Some are slightly glauconitic. The mudstone units are medium grey, calcareous, in places slightly carbonaceous, and are 5-40 cm thick. Ongley & Macpherson (1928), Lillie (1953), Vella (1959), and Wellman (1959a) describe a fine shelly conglomerate packed with fragments of a small oyster and shark teeth ("Tapuaeroa Grit", Vella 1959). At Y15/646427 [N80/571173], a 1 cm thick bed of "Tapuaeroa Grit" was observed at the base of the thick sandstone bed in a fallen block. The grit contains broken fragments of Ostrea lapillicola Marwick, and greywacke and igneous clasts in a matrix of calcareous, fine to medium grained, moderately sorted sandstone, rusty in places, with rare disseminated carbonaceous fragments and mica flakes.

A summary by Vella (1959) of M. C. Pick's unpublished definitions records that calcareous

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Kenny-Stratigraphy & structure, Ihungia decollement 9

concretions up to 4 m in diameter are commonly found in sandstones near the base of the sequence. Although never discovered in situ, large concre­tions occur as float along the Ihungia River and side streams in the vicinity. Sedimentary structures in the Tapuaeroa Formation constitute the Ta. bid,.

divisions of the Bouma sequence (Blatt et al. 1980). No other internal structures or sole markings were detected.

The thickness of the formation exposed along Ihungia River is unlikely to exceed 100 m because the river tends to follow the strike.

Samples prepared for microfaunal analysis proved barren. The only macrofauna observed was Ostrea lapillicola (Y15/f28) in "Tapuaeroa Grit". Single valves are broken and crushed, mostly beyond recognition. O. lapillicola is most common in rocks of Haumurian age, although its age range of possibly Teratan-Haumurian suggests that the formation may be as old as Teratan (Speden 1973; Warren & Speden 1977; I. G. Speden, New Zealand Geological Survey, pers. comm. 1980). The Hau­murian index fossil, Inoceramus matotorus Well­man, was not detected.

Environment of deposition of the Tapuaeroa Formation As the Tc division of the Bouma sequence is miss­ing in the Tapuaeroa Formation, and the T b or T d

divisions are poorly developed, a proximal turbid­ity current regime is inferred (Walker 1967; Blatt et al. 1972; Middleton & Hampton 1973). The presence of Ostrea lapillicola debris and carbona­ceous material suggests that 1 turbidity current, at least, transported material from a shallow upper shelf environment. Derivation continued to be from Torlesse rocks, presumably to the west.

MANGATU GROUP (Wellman 1959b, after Henderson & Ongley 1916)

Mangatu "Series" was first used by Henderson & Ongley (1916) and later (1920) and by Ongley & Macpherson (1928) for clay shales, greensands, cal­careous mudstone, and chalky limestone of the Mangatu Survey District and their equivalents elsewhere. Wellman (1959b) d.escribed the assem­blage and changed its rank to Mangatu Group. Black (1980) relegated the group to formation sta­tus in the Mangatu District, but this was ques­tioned by Moore (1981, p. 301). For the purpose of this study, the group status of Mangatu is retained. In the Ihungia catchment, 2 formations (Whangai and Weber) are recognised, separated by 4 informal lithofacies.

WHANGAI FORMATION (Lillie 1953)

Originally called the Whangai Series by Quennell & Brown (1937), after the Whangai Range west of Porangahau, Whangai Formation was described and formalised by Lillie (1953) for the siliceous shales outcropping in southern Hawke's Bay. Whangai Formation is now recognised from the East Cape region to at least Marlborough. It is predominantly creamy grey, brittle, siliceous mudstones with sul­phur-coloured stains and rusty weathering surfaces, but may also be composed of wide-ranging lithol­ogies from siliceous to calcareous, mudstone to sandstone, greensand, and dark coloured mudstone.

The largest outcrop of Whangai Form~tion in the Ihungia catchment is on the ridge between the Black Hills and Puketiti, where it is thrust over the Upper Tertiary Ihungia Formation (see below). Most exposures have been deeply weathered. The for­mation is completely fault-bounded in every expo­sure. In southern Hawke's Bay it conformably overlies the Tapuaeroa Formation in some expo­sures (Lillie 1953; Kingma 1960; Pettinga 1980), and in a few places it may prove to be a lateral equivalent of the Tapuaeroa Formation (Wellman 1959a; Pettinga 1980).

Whangai Formation is a highly variable lithol­ogy in the Ihungia area, even though not all lith­ologies reported for the Whangai appear here. The most common variant is siliceous mudstone, non­calcareous, hard, but brittle and flaky, light creamy grey to medium grey when unweathered, and often autobrecciated. Limonitic and jarositic staining on joints and weathering surfaces are ubiquitous. It weathers to a creamy white clay discoloured yellow­orange and brown by weathered jarosite and limonite. Liesegang rings are rare.

Subordinate in occurrence are calcareous shales and slightly calcareous sandstones. The calcareous shales, also auto brecciated and indurated, are medium grey on fresh surfaces with much brown staining on exposed surfaces and joints, making the .outcrop speckled brown in appearance. The sand­stones are yellow-grey to medium grey with limon­ite staining on joints. They are hard, very fine grained to fine grained, and moderately sorted, with fine parallel and slightly convoluted laminations, and locally glauconitic. Mica flakes give fresh spec­imens a glitter. A little jarositic efflorescence occurs. Pyrite is occasionally observed as tiny dissemi­nated grains in all Whangai lithologies. Crude bed­ding is rare; typically any lineations recorded are not an expression of bedding, but represent a tec­tonic fabric caused by intense shearing. Slickenside surfaces are numerous.

Whangai Formation clasts in Mangatu Group bentonitic deposits in the northernmost Ototo Fault Zone (Y15/654415 [N80/581161J) smell of oil when

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10 New Zealand Journal of Geology and Geophysics, 1984, Vol. 27

first cracked open. Concretions, thought to have eroded out of the Whangai Formation, are found as float in many streams. They are calcareous, extremely fine grained, very hard, grey, and irregu­larly shaped, with boxwork patterns of calcite veins.

Except for rare very fine parallel and convoluted laminations coloured by clays and silts, sediment­ary structures could not be found. Measurement of stratigraphic thickness was not possible. Individual exposures are never more than 100 m in vertical thickness.

The formation is poor in macrofossils (Stevens & Speden 1978) and fair in microfauna (Vella 1959). It is considered to be Haumurian-Teurian in age (Suggate et al. 1977). No fossils were discovered by the author.

Environment of deposition of the Whangai Formation The Whangai rocks within the study area are con­sidered too sheared and weathered to provide any new evidence on depositional environment (see Kingma 1962; Katz 1974; Suggate et al. 1977; Ste­vens & Speden 1978; Pettinga 1980).

Undifferentiated Mangatu Group Between the Whangai Formation and the Weber Formation, the rocks of the Mangatu Group are not mappable as formations within the Ihungia catchment, but should be regarded as "tectonos­tratigraphic" units within the decollement. To facilitate mapping (Fig. 2) the stratigraphy is recorded as 4 informal lithofacies, and bentonitic material oozing from faults and crush zones.

Brown sandstone lithofacies (informal)

The lithofacies, stratigraphically the lowest, is char­acterised by thick, massive, brown sandstones separated by thin mudstone beds, outcropping along the upper slopes of the Black Hills. It is typically exposed at the southern limit of the Black Hills, in the vicinity of Y 16/681362 [N80/611103] (refer­ence section). A thrust plane forms the lower limit of the lithofacies, while its upper contact is gra­dational into the flysch lithofacies (see below).

The brown sandstone lithofacies is composed predominatly of massive yellow-brown, fine­grained, well-sorted, noncalcareous sandstone in beds over 3 m thick. Rare carbonaceous material is scattered throughout the beds. The basal portion of some beds in the higher parts of the sequence is much coarser. Green and grey noncalcareous mud­stone clasts, up to 5 mm in length, are supported by a matrix of very poorly sorted, medium to coarse grained sandstone. The clasts appear to be in crude

alignment with bedding. The coarse material grades up to fine sandstones. Beds of weathered clay material (5-10 cm thick) are intercalated with the thick sandstone beds. No unweathered sample could be obtained. The sandstone is erosion resistant and forms high-standing hills, and is at least 200 m thick.

No fauna was obtained. However, a good recov­ery of fairly well preserved palynomorphs (Y16/f27) from a fine to medium grained sandstone midway through the sequence (at Y16/671375 [N80/607117]), by J. I. Raine and G. J. Wilson (New Zealand Geological Survey), indicated a pos­sible Early Eocene (Waipawan-Heretaungan) age. Most of the material was reworked from Teurian strata, and some from the Mata Series.

Flysch lithofacies (informal)

This is a sequence of alternating medium to fine grained, graded sandstones and very fine grained sandstone to siltstone, which occurs along the middle slopes of the western side of the Black Hills. The unit lies conformably upon the brown sand­stone lithofacies, and the upper contact is eroded away. The only good exposure i~ in a stream cut­ting, 600 m WNW of Wahingamuku Trig (YI6/673372 [N80/602115]) (reference section).

Steeply dipping, graded, fine to medium grained sandstone beds, 10-30 cm thick, alternate with fractured very fine grained sandstone 5-20 cm thick. The ratio of sandstone:very fine sandstone ranges from 1:1 to 5:1, but averages 2:1. The coarser sand­stone beds are hard, dark in appearance, noncal­careous, and contain clasts of coarse sand - granule size throughout; as a consequence, the material is poorly sorted. Some of the clasts are thought to be fragments of Whangai Formation mudstone (I. G. Speden, New Zealand Geological Survey, pers. comm. 1979). Occasionally some constituent of the matrix is slightly calcareous.

The finer grained beds (very fine sand to silt size) are softer, yellow-brown, noncalcareous, and with some parallel lenses of medium grey siltstone. Dark staining has formed on joints and fractures. Thick­ness of this lithofacies exceeds 200 m.

No fossils were found, but because this lithofa­cies conformably overlies the brown sandstone lithofacies it is possibly Eocene in age.

Grey sandstone lithofacies (informal)

The grey sandstone lithofacies is composed of nor­mally graded sandstone beds, 0.5-1 m thick, most laminated in the upper part, separated by fractured mudstone. The sandstones are friable, pale gray, very slightly calcareous, well sorted, and very fine to fine grained. Calcite is recorded in lenses parallel

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Kenny-Stratigraphy & structure, Ihungia decollement 11

to bedding and in crosscutting joints and fractures. Glauconite is a rare constituent. The lithofacies is typically exposed in a reference section along the lower Ihungia River (YI6/648420 [N80j753166j). Its stratigraphic affinities are uncertain because known outcrops appear to be fault bounded.

In the upper Te Matarau Stream, isolated expo­sures within the bentonite are in part fine-grained sandstones, and in part very fine grained sand­stones with well-developed parallel and convoluted laminae, climbing ripples, and small-scale cross bedding. At 1 horizon (YI5/627404 [N80/551149j) a weathered grey conglomerate lens with poorly sorted, noncalcareous, medium to fine grained sandy matrix is present. Its relationship with the surrounding grey sandstone is not known, as most of the landscape in this area consists of slump debris.

Total thickness is unknown. The greatest thick­ness exposed in outcrop is 7 m. No fossils were discovered in this lithofacies.

Greensand lithofacies (informal) Glauconitic sandstone and mudstone beds which make up the bulk of Sugar Loaf (YI5/627409 [N80/551153j) are grouped with the grey sandstone lithofacies in Fig. 2 for mapping convenience. This unit is also fault bounded and mostly surrounded by bentonite, but its upper limits interfinger with the base of the Weber Formation.

Alternating sandstone and mudstone change gradually from 20 cm thick, very fine grained, mas­sive, calcareous, brown sandstones with little glau­conite, and grey-brown, slightly calcareous, 10 cm thick mudstone beds at the base of the sequence, to medium to fine grained, normally graded, 20-40 cm thick, green, glauconite-rich calcareous beds with fine parallel laminations at the top of the sequence. Here, carbonate and glauconite grains are concentrated in the basal portion of each bed. Intervening mudstones are 10 cm thick, slightly calcareous, and dark green-grey.

Sugar Loaf is formed of a sequence of dipping beds approximately 200 m thick. The ridge to the south contains beds lower in the sequence, prob­ably little more than 50 m thick. Total thickness could reach 300 m.

Foraminiferal collections from the Sugar Loaf sequence, made by M. C. Pick and held by the New Zealand Geological Survey, Lower Hutt, were re­examined by G. H. Scott, who confirmed a prob­able, although not definite, Whaingaroan-Dun­troonian age for the greensand (G. H. Scott, pers. comm. 1980).

Provenance of the greensand is not known, but its characteristics are similar to the other 3 litho-

facies, which may have been derived from the Whangai Formation.

Mangatu Group bentonitic deposits In part of the study area, the rocks of the Mangatu Group occur as a veneer of impure bentonitic material accompanied by blocks of a variety of other lithologies. These rocks are concentrated in the wide crush zone on the northern Ototo Fault (Pick 1962). Elsewhere they ooze downslope from a thrust plane and from the remainder of the trace of the Ototo Fault (see beloW).

The Mangatu Group bentonitic deposits are very pale cream-grey. They swell and become very sticky when wet, and when dry display the cobweb-type deep jointing pattern typical of bentonite.

Angular blocks of noncalcareous, glauconite-rich greensand, usually of cobble to boulder size, are abundant within the benonite. Red-brown mud­stones, possibly captured from the Mokoiwi For­mation or Arowhanan or Teratan beds (I. G. Speden, pers. comm. 1979) are uncommon. Many other exotic clasts are recorded (Kenny 1980) and are most easily seen in the stream immediately south of the Blue Slip (Y15/654415 [N80/581161 j).

No macrofossils were found in the bentonitic deposits, and micropaleontological samples proved to be barren. Microfaunas (N80/fl20l) identified by Vella (1970) are poorly preserved and limited in numbers of specimens and species. They suggest an age younger than the Whangai Formation, prob­ably Waipawan-Bortonian, but possibly extending into the early Landon.

WEBER FORMATION (Lillie 1953)

The name Weber Formation was established by Lillie (1953) for creamy pale grey to white, very calcareous mudstone in southern Hawke's Bay. The name is applied to calcareous mudstone occuring as in situ remnants of a sheet overlying the green­sand lithofacies on the northern dip-slope of Sugar Loaf. Blocks are also present in Te Matarau Stream below Sugar Loaf.

The lower contact of Weber Formation is gra­dational and interbedded with the last few sand­stones of the greensand lithofacies (Mangatu Group). A thrust plane forms the upper limit of the formation.

Weber Formation is a moderately hard, very cal­careous mudstone, creamy grey in colour, some­times with a brown tinge. Typically it has dark grey blotches and occasionally fine medium-grey streaks. Dark brown staining in dendritic patterns is also seen. The mudstone weathers to a creamy white with a rusty brown colour in joints, and has an

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12 New Zealand Journal of Geology and Geophysics, 1984, Vol. 27

inCIpIent flagginess. Grey streaks, sometimes in parallel alignment giving a laminated appearance, are the only expression of sedimentary structures.

Only the basal layers (less than 10 m) are exposed in the Ihungia catchment, so no estimation of total thickness can be made. In southern Hawke's Bay, the Weber Formation is known to vary substan­tially in thickness (Lillie 1953).

Age and environment of deposition of the Weber Formation A Whaingaroan-Waitakian range is generally accepted for the Weber Formation (Lillie 1953). No new macro or microfossils were found by the author. A deep pelagic facies is indicated by Vella (1959, 1970) from the few poorly preserved spec­imens previously recovered. Dark blotches may be all that remains of disseminated pyrite grains and burrow in fillings, as described by J. R. Pettinga, University of Canterbury (pers. comm. 1977) for the Weber Formation in southern Hawke's Bay.

IHUNGIA FORMA nON (Lillie 1953, after McKay 1887)

The "Ihungia beds" were first named by McKay in 1887 (after the Ihungia River) for beds outcrop­ping in the upper Makahikatoa Stream (Y16/672359 [N80/602100] - Y16/683336 [N80/615073]). He applied the name to calcareous sandy mudstones and massive sandstones with conglomerate beds containing large crystalline boulders. The name Ihungia Formation (Lillie 1953; Kingma 1971) is used here, although Black (1980) altered the name to Te Arai Formation for the Mangatu area.

The formation covers most of the southern half of the catchment. Upper and lower stratigraphic contacts are not seen. The lowest beds are trun­cated by a major thrust.

The most common lithology in Ihungia For­mation is massive, shattered, grey to blue-grey, very fine sandy mudstone, slightly to very calcareous, occasionally containing carbonaceous fragments. It often reveals fine parallel laminations of darker colour )\Then freshly broken. The mudstone typi­cally breaks with conchoidal fracture, but cuboidal fractures are more common near faults. Iron stain­ing on joint surfaces is ubiquitous. Slickensiding is also present.

Alternating sequences of sandstone and mud­stone, rarely thicker than 4 m, and interbedded massive mudstone, are recorded in all stream sec­tions in the area. Mudstone beds in these alternat­ing units are identical to the mudstone just described. Sandstone to mudstone ratio in the units is usually about 1: 10, with the sandstone beds

generally 5-10 cm thick. Recorded extremes of the sandstone beds are I m (max.) and 3 mm (min.).

The sandstones are blue-grey to brown-grey, fine grained, very well sorted, and slightly calcareous, sometimes noncalcareous. Lower portions of each unit are massive and normally graded and have a sharp contact with the underlying mudstone. Upper parts are usually parallel laminated, rarely convo­luted, and grade into mudstone above. Fragile biv­alves and carbonaceous fragments are rare, as are convoluted laminae and microcrossbedding. Flute casts indicate current flow from 330°.

Randomly distributed yellow-white dolomite concretions are ubiquitous at all levels in Ihungia Formation (Kenny 1980) and exist in a variety of shapes and sizes (from 1 cm to over I m in diameter). The origin of these concretions requires detailed investigation, and this is not attempted here.

Foraminifera are common, but not present everywhere. Isolated, small, unidentified, thin­shelled bivalves are found in many horizons, and at some levels are concentrated in a mass of broken fragments. Terebellina-like tubes are often found.

To facilitate mapping, the Ihungia Formation was informally divided into 6 biostratigraphically determined stages or substages. A limestone lith­ofacies and an igneous conglomerate lithofacies were also identified within the lower Altonian.

Four stages were identified on the basis of for­aminifera, and in one of these a 3-fold subdivision was recognisable: Otaian, lower, middle, and upper Altonian, Clifdenian, and Lillburnian. Collections are listed under sample numbers YI6/f82-Y16jfl49.

Stratigraphic thicknesses of each stage and sub­stage in the Ihungia Formation have been tenta­tively determined using information from foraminiferal studies. They are, approximately: Otaian, 20-30 m; lower Altonian, at least 500 m (igneous conglomerate is 250 m from the top of the lower Altonian); middle Altonian, 400 m; upper Altonian, 350 m; Clifdenian, 130 m; lower Lill­burnian, 200 m. Total thickness of the Ihungia For­mation is, therefore, approximately 1600 m. The base and top were not seen because of tectonic con­tacts. This compares with estimates of 1300-1700 m (Ongley & Macpherson 1928), 1300-1500 m (Macpherson 1930), at least 550 m (Burr 1940), 3300 m (Vella 1959), and 2700 m (Kingma 1965).

Limestone lithofacies (informal)

The limestone is a pale to medium grey, laminated, crossbedded, bryozoan, bioclastic limestone, that weathers to yellow-brown on exposed surfaces. Solution has concentrated non calcareous clays into

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seam partings, producing flagginess. Shellbeds, containing fragile specimens of Chlamys (Meso­peplum) costatostriata (Marshall) broken by cur­rent action, are common (Y16/fIS6). Carbonaceous fragments are rare.

Estimated thickness of the limestone is about 100 m. At the northern end of the Ihungia airstrip, the upper and lower contacts of the limestone are seen to grade into Ihungia mudstone, each in the space of IS-20 m. A reference section is SO m below the farm track 1 km from the airstrip (Y1S/622401 [N80/S48143]).

Limestone pebbles carried down Te Matarau Stream contain large oyster fragments (indet.) and Serripecten hutchinsoni (Hutton). Age ranges of the macrofauna support the Altonian age which was determined from foraminifera. The limestone is stratigraphically below lower Altonian mudstones, therefore it is probably confined to lowermost Altonian.

Conglomerate lithofacies (informal)

A matrix-supported conglomerate, with clasts up to boulder size, of predominantly igneous origin, was deposited in a channel of unknown dimen­sions. The conglomerate has been offset by fault­ing, but although disjointed, it can be traced around the Popaingawariwari and upper Mal<:ahikatoa Stream catchments. It is not a basal conglomerate, as envisaged by numerous writers, but occurs about 2S0 m above the lowest outcrops of the lower Altonian strata in Ihungia Formation.

Rocks included in the igneous conglomerate require separate study and are not described here. Clasts identified include andesite, basalt, diorite, dolerite, gabbro, granophyre, trachyte, teschenite, gneiss, amphibolite, greywacke, and chert.

Some of the conglomerate constituents have obviously undergone a number of deposition/ero­sion. cycles. The multicyclic specimens are very hard, very dense, well rounded, and polished. Other constituents are softer and angular, and appear to have experienced only 1 cycle.

Calcite often coats pebbles or fills voids between them. Carbonaceous fragments and broken shells of molluscs (Y16jfIS2, YI6/fIS3) are also recorded.

The total thickness of the unit was never seen, but it is probably about 1O-IS m. The lower con­tact is sharp and irregular with the Ihungia mud­stone. The upper contact is gradational into the mudstone. Provenance of the igneous conglomer­ate has. not been investigated.

Environment of deposition of the Ihungia Formation A depositional environment for the Ihungia mud­stone has been deduced by the author from fora-

miniferal studies. The formation was deposited in the outer shelf - upper slope region, in which a sea­floor temperature of 10°C and salinity of 30-40%0 prevailed (Murray 1973; Boltovskoy & Wright 1976). Occasional influxes of turbidites (probably from the northwest) caused flysch-type deposits to build up in lenses.

The bioclastic limestone lithofacies exhibits fea­tures characteristic of an offshore barrier, perhaps biohermal "in form. Evidence from both micro and macrofauna indicated deposition of calcareous det­ritus in shallow conditions. High water tempera­tures and moderately high salinities are indicated by foraminiferal studies (Murray 1973; Boltovskoy & Wright 1976). The worn appearance of most macrofossils hints at wave or current action. Moderate to strong currents are also indicated by the zoarial forms of bryozoa present (Crabb 1971; Whitten 1979). With a subsequent rise in sea level, muds covered the barrier.

STRUCTURE

The structure of the Ihungia catchment is sum­marised in Fig. 2. A very distinct NNW -SSE trend is seen for both megascopic faults and folds over most of the area, although to the south the trend swings NNE-SSW. The catchment can be divided into 2 megascopic structural regions - a northern region which is part of an apparently autochtho­nous Upper Cretaceous sequence traced further to the north and west out of the Ihungia catchment (I. G. Speden, pers. comm. 1980), and an allo­chthonous region to the south which consists of 3 decollement sheets* emplaced over the autochtho­nous domain. Both structural regions have been affected by phases of large-scale crosscutting fault­ing over a long interval before, during, and after emplacement of the allochthon.

The lowest thrust sheet was emplaced after the Oligocene (the youngest lithology incorporated in this sheet is Weber Formation of Waitakian age, which was already lithified before movement). The middle and upper thrust sheets were emplaced after the Middle Miocene, following deposition of Ihun­gia Formation. No other dating of fault or fold movements has been possible. However, a relative

*Decollement or thrust sheet is defined, for this paper, as detached and deformed strata, here probably resulting from gravity sliding, with a low-angle basal fault sepa­rating them from stratigraphically or structurally lower lithologies.

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I'IIuII CGrnptssian IlIs~ • EJIInIion Ills N

'NItW-ESf .....

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New Zealand Journal of Geology and Geophysics, 1984, Vol. 27

c-

NomoI fa .... orienlld IIIP"IimIIII¥ 020"185° /tW. Pt. ...... Oigane.

Rewtw fa. orienIId 060"180" SE. Pt. ...... Oigane.

NomoIIIIAIs ariInIId 155°185° E. ,...._011....".."O"h ... Pt. ...... Oigane.

~ III IMut 1hNst ... m nonh • _ I_ II .. 0IigacenI. ConIIins c-Iftd ~ TII1iary IiIIdagia

M_ 011 IICII1Iwn OIIIID h ...

MMmn 011 MlngaqUb II1II ........ 0. hub.

8eginning III NE-SW .. E- W CIIIIIIrIIIian w4IidI ~ ...

ill o..m.ry .......... Ih .... l1li ...... DIDID h'*'l.

~ III IIIddIl MIll .... ConIIina u,.... Tlllilry ~ IIIno /tW • ?NNE. TIIIIiInI ..... caulld by dq" ....,.. IhNst ... fCaribri Iu .. Ind T UIbu h'*'l.

~ III NE-SW .. E-W ~ MMmn 011 IhungiIlnd 0.... hub. Megaapt IaIding in 1OUIh.

E ...... _III~IhNst .... III~c-IIII~Tlllilry ...................... 1ian in Ina......-u,.... T"'-ry __ billie_ ... ~ RIIIngIIIhu fNII.

CIIIIinuoIion rI NE-SW .. E-W ~

........, __ .0. fI,* _ ... ,.,... .. it

Wlihlll. WliptPl. MabmIb l1li MIbhiII:IIDI flub.

III ..... ~ Watti ga_obI. ....... ...n-i Iftd ..... flub.

Fig. 4 Summary of faulting and folding phases in response to compression and extension forces.

sequence of events has been demonstrated (Fig. 4). The overall east-west horizontal compression evi­dent in Fig. 4 is not quite so obvious in Fig. 2, apart from the most recent phases. Effects of thrusting have hidden many of the structural pat­terns produced during the compression phases.

has also taken place. The Rangikohua lithofacies is structurally more competent than the underlying Mangarakeke lithofacies. No folding of any kind was observed owing to poor exposure.

Three phases of faulting have been identified, which affected the autochthonous lithologies before the emplacement of the lowest allochthonous sheet. In the Mangarakeke Stream and Mata River areas, dip-slip normal faults oriented 0200/85°NW (ten­sional regime) have been cut by reverse faults ori­ented 0600/800SE (compressional regime). These phases were followed by a further regime of ten­sion, indicated by normal faults oriented about \55° /85°E. Most major faults have offsets of 200-500 m.

Autochthon The Mangarakeke lithofacies is repeated along the Mata River by upright isoclinal folds with axes trending about NNW. Elsewhere the structure seems simple, probably because most areas are obscured by recent slumping, but a continuation of isoclinal folding is suspected. Some flexing of beds

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Kenny-Stratigraphy & structure, Ihungia decollement 15

Lowest thrust sheet The lowest thrust sheet incorporates packets of Mangatu Group lithologies included in Mokoiwi siltstone, and is now exposed across the middle Ihungia valley. The basal plane of this thrust sheet has been concealed by later downfaulting (Fig. 3). Field relationships suggest that bentonite of Dan­nevirke (Eocene) age is being squeezed up faults from beneath the Mokoiwi lithologies of Motuan age. Evidence from Alliance Petroleum's Te Puia­I well (Laing 1 972a) drilled 6.4 km NNE ofPuketiti Trig, shows the Mokoiwi to be a rootless mass "floating" on Mangatu bentonite. Micropaleonto­logical samples indicate that the Mangatu Group lithologies in the well are not overturned (Katz 1974). Therefore, it seems likely that the Mokoiwi in the Ihungia catchment was thrust over Mangatu lithologies, and incorporated jumbled packets of them prior to or during emplacement of the thrust sheet.

There is no evidence for a source of the thrust sheet within the Ihungia catchment, and lack of similar lithologies to the east or south could rule out an eastern or southern source. If we assume sliding from the west or north, the decollement must have overridden the Upper Cretaceous auto­chthon, but it has left no trace of its passage. No evidence was found of horizontal shortening in the autochthon. Therefore, the decollement is unlikely to be the result of regional compression in this area, and can be regarded as probably due to gravity sliding from the north or west.

Within the thrust sheet, attitudes and throws of faults are difficult to measure because of the abun­dance of bentonite in these regions. The Ototo Fault may predate the thrust; it is suggested that the fault was still moving or was reactivated, as it now off­sets the thrust sheet.

The incompetent nature of the tectonised silt­stone in the Mokoiwi Formation contributes to the structural complexity observed within this area. The formation is always crushed and contorted, and shear planes are common. Slope wash material and deep gully erosion disguise the mesoscopic folds. Bands of sandstone 10-20 cm thick in the siltstone may be traced for short distances; some are iso­clinally folded, usually with apparently subhori­zontal axial planes.

Structures within each block. of Mangatu Group material in the sheet are also difficult to interpret because of the isolation and small size of the blocks. Isoclinal folding is common in the Whangai For­mation. The rocks are also often finely fractured, sheared, and highly contorted. Bedding in tecton­ically separated blocks of the greensand lithofacies and Weber Formation has remained intact. The whole of Sugar Loaf (Y 1 5/623410 [N80/55151535]),

Sig.2

composed of the greensand and Weber Formation mudstone, comprises an open syncline plunging 32° to ENE, axial plane 03Y/45°SE.

The Mangarakeke Fault (new name, Table 2: a) now separates the autochthonous domain from the allochthonous domain and has down thrown the lowest thrust sheet at least 600 m to the south, so that the basal plane is now buried. Faults parallel to the trend of the Mangarakeke Fault occur in the Sugar Loaf region and north of Puketiti. A north­west-southeast subhorizontal extension direction is suggested by the whole pattern of faulting in this phase (Fig. 4).

Prior to emplacement ofthe middle thrust sheet, movement began on the 2 splinters of the northern Ihungia Fault (Table 2: b, c).

Middle thrust sheet The middle thrust sheet, which consists entirely of the Ihungia Formation, is exposed in the southern half of the catchment. The undulating northern edge of the sheet can be traced in outcrop below the road east of Ihungia and on the southern slope of Te Matarau Stream (Fig. I, 2). The basal thrust plane dips about 100S and has large amounts of bentonite oozing from it.

A NNW-SSE tensional regime at the time of emplacement of the middle thrust sheet is indi­cated by the conjugate Karikarihuata and Tuakau Faults (Table 2: d, e). Conjugate faults cutting across the northern end of the Black Hills also demon­strate northwest-southeast tension. Tension may have been caused by dragging within the thrust sheet on the underlying material.

A northwestern source for this thrust sheet is apparent from the 337%6° (bearing and plunge) extension direction from conjugate faults, but no Miocene lithologies of this age have yet been dis­covered in the Raukumara Range to the northwest. It is more likely from the present-day distribution of formations that the source of the thrust sheet was to the NNE. Whatever the source, the thrust overrode the lowest thrust sheet and probably the autochthonous domain as well.

The subhorizontal east-west-directed compres­sion which operated prior to thrusting of Upper Tertiary material (Fig. 4) was certainly present after thrusting had ceased. Reverse movements on the Ihungia and Ototo Faults continued in response to the east-west compression, and a new reverse frac­ture was initiated further south.

Good exposure within the Ihungia Formation itself is restricted almost entirely to the Popain­gawariwari and upper Makahikatoa Streams. Here, complex mesoscopic faulting and folding, which cannot be seen elsewhere, is displayed. Slicken­sided surfaces are common. Faults are often filled

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16 New Zealand Journal of Geology and Geophysics, 1984, Vol. 27

with sheared material or calcite, and in general the mudstones have undergone greater deformation than the more competent intervening sandstones. Rare conjugate faults show a general pattern of northwest-southeast extension, except in the middle reaches of Makahikatoa Stream where compres­sion is oriented northwest-southeast.

On a megascopic scale there is 1 north-south­trending syncline in the Upper Tertiary material, broken by later faulting (see below) and flanked by anticlines on both sides. A steep easterly dip of axial planes, and a gentle plunge of the fold system towards the south, is inferred from the outcrop pat­tern. A swing in bedding attitudes from the general northwest-southeast strike to a northeast-south­west strike occurs in the southernmost streams, and suggests refolding around an approximate 2800j300(bearing and plunge) axis.

Uppermost thrust sheet The uppermost thrust sheet is located on the south­eastern boundary of the Ihungia catchment, over­lying the middle thrust. The uppermost sheet includes similar Mangatu Group lithologies to the lowest thrust, and it is conceivable that they may have the same source area. The gently undulating

basal plane of the uppermost sheet dips approxi­mately 1400jlOOW, and is traceable around the midslope region of the Black Hills (Fig. 2).

The sheet contains dominantly Whangai For­mation and the brown sandstone and flysch lith­ofacies of the Mangatu Group. As in the lowest thrust, the Whangai Formation is sheared and con­torted, but very weathered. Bedding in the brown sandstone and flysch lithofacies of the Black Hills region strikes regularly NNW-SSE at a constant dip of 600WSW. Minor meso scopic faults, probably initiated during thrust emplacement, cut the beds.

Evidence of east-west compression associated with emplacement of this sheet has been detected within the sheet and in the Upper Tertiary mud­stones beneath the thrust plane. Much of the defor­mation has been transferred from the relatively competent Mangatu lithologies to the less-compe­tent Ihungia Formation mudstone below the thrust plane. The Ruangarehu Fault (new name) and other faults in the Waipapa Stream are thought to have been initiated in this way.

Continuation of northeast-southwest to east-west compression after the thrust emplacement caused rejuvenation of faults in the northern Black Hills

Table 2 Orientation and slip direction of some faults in the lhungia valley. Faults listed in chronological order. Slip direction obtained using displacement of 2 planes or by measuring conjugate faults.

Fault Orientation Sense Slip

a Mangarakeke 0700/85°S normal 600 m down to S b Northern Ihungia

(western splinter) l600/800W reverse (dextral) ? c (eastern splinter) 165°/800W reverse (dextral) ? d Karikarihuata 065°/8YN normal 300 m along 280°/80° e Tuakau 065°j700S normal ? f Fault cutting 0000/8YE dextral (normal) 30 m along 172°/32°

through limestone g Waihua 02Yj700NW reverse (dextral) 800 m along 250°/63° h Waipapa 0200/85°NW normal (sinistral) 650 m along 216°/72°

Makomako 00r/800E reverse (dextral) 700 m along 019°/70° j Makahikatoa 00Y/85°E normal (sinistral) 300 m along 032°/68° k Fault crossing track 058°j700NW sinistral (reverse) 75 m along 054°/11°

to airstrip Kouetumarae No 3 045°/700NW dextral (reverse) ? m along 237°/30°

m Wahingamuku No 1 025°/85°SE sinistral (normal) 85 m along 026%5° n Wahingamuku No 2 025°/85°SE sinistral (normal) ? m along 027%5° 0 Wahingamuku No 3 0100/8YE sinistral 75 m along 010%0° p Wahingamuku No 4 047°/85°SE sinistral (reverse) 50 m along 050% 1 ° q Popaingawariwari No 1 032°/8YSE some combination of ?

sinistral and normal sense

r Popaingawariwari No 2 0300/85°SE dextral (normal) 350 m along 2{)8% 2° s Popaingawariwari No 3 0200/8YSE dextral (normal) 650 m along 19r/30° t Popaingawariwari No 4 175°/8YSE dextral (normal) 400 m along 174% 1° u Puketiti 1 55°/85°NE sinistral (reverse) 300 m along 156% 1°

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Kenny-Stratigraphy & structure, Ihungia decollement 17

area and minor offset of the last thrust plane fur­ther south. Compression then moved slightly to WSW-ENE and initiated more faulting in the Ototo Fault region. Faults parallel to the trace of the Ototo Fault developed to the east. However, movement on the Ototo Fault itself was negligible: the upper­most thrust plane was down thrown to the west by less than 15 m. Associated minor faulting occurred in the middle Makahikatoa Stream. The dextral strike-slip fault oriented north-south, which cuts the limestone lithofacies (Table 2: f), is the only fault which bears no obvious relationship to the WSW-ENE compression.

The most recent faulting in the Ihungia catch­ment is the most obvious, and comprises 2 fault sets: one trending north-south with east-west sub­horizontal compression, and the other trending southwest-northeast with both east-west compres­sion and north-south extension axes subhorizon­tal. These faults are also characterised by splintering and fault wedging as a result of rotation of fault­separated blocks, but lack of marker horizons pre­vents detailed measurements of rotation.

The Waihua, Waipapa, Makomako Faults (Pick 1962) and Makahikatoa Fault (new name) belong to the fault set which has developed from east-west subhorizontal compression and steep extensional forces. They trend north-south and exhibit a dip­slip mode. The Waihua and Waipapa Faults and Makomako and Makahikatoa Faults constitute conjugate couples (Table 2: g, h, i). They all merge near the Takapau-Waitahaia Road and continue northwards as 1 steeply dipping fault, which offsets the edge of the middle thrust sheet sinistrally by approximately 100 m and is then lost in Mokoiwi Formation.

The other fault set in this phase has also developed horizontal east-west compression, but in addition has a north-south extension direction. Fracturing, therefore, has evolved in a diagonal trend from these forces with a dominant strike-slip mode. Offsets on some of these faults were able to be measured (Table 2: k-u).

The last movement on the Ihungia Fault occurred along its southern extension during this phase. The fault is oriented 0100/800W and has slipped 200 m along 311 °/78°. Where the Kouetumarae Fault (new name) and southern Ihungia Fault join just north of the mouth of Te Matarau Stream, the amount of throw is reduced significantly and seems to die out northwards. Movement on the northern sec­tion of the Ihungia Fault is not thought to have occurred in this phase. The southern Ihungia Fault and Kouetumarae No. 3 Fault constitute a conju­gate couple which complies with the east-west sub­horizontal compression forces.

Sig. 2·

No movement on the Ototo Fault took place during this final stage of faulting, but reactivation in the future is not ruled out.

DISCUSSION

Lack of marker horizons makes interpretation of many of the minor fault movements difficult, and bentonite conceals much valuable information. However, major faults have been recognised and described.

Similar thrusting was identified by Stoneley (1968) in the Maungahaumia area about 40 km to the southwest of Ihungia. He believed some of the thrust complexes advanced individually and over­rode earlier ones. An initial phase of thrusting late in the Waitakian also correlates with the post-Oli­gocene date for the Ihungia area. However, Sto­neley believed the gravity slides were emplaced towards the southwest from the Te Puia - Hiku­rangi area, which was uplifted in mid-Waitakian times and again in the post-Otaian. My investi­gations in the Ihungia region have established that Stoneley's "high" is itself a gravity slide and/or decollement, probably emplaced about the same time from a source to the north or west, and there­fore is unlikely to be the source for his gravity slides. Obviously more information is needed on mechanisms of thrust emplacements and source regions before a reasonable history, which is undoubtedly more complex than outlined in this paper, can be described. A greater understanding of the stratigraphic and structural relationships in the Mangatu Group will be of assistance.

Speden (1976) has interpreted the outliers of Mokoiwi Formation, from the Tapuaeroa valley to Te Puia, as klippen. Most other geologists who have worked in the area have favoured the interpreta­tion of the outliers as structural highs. Speden's interpretation is preferred for the Mokoiwi For­mation in the Ihungia catchment. He also states (1976, p. 114) "there is general agreement that the latest Landon (late Oligocene) and earliest Pareora (early Miocene) was a time of great tectonic activ­ity in the East Cape region". His investigations to the north made it possible to "restrict the possible source of the main Mokoiwi klippe to the west and northwest, to the region of the main Raukumara Range" (Speden 1976, p. 166).

In paleogeographic reconstructions of the Upper Tertiary of the North Island (Sp6rli 1980; Ballance et al. 1982) the Ihungia catchment falls within the arc-trench-gap adjacent to the Hikurangi trough. In this environment, east-west compression (at right angles to the strike of the system) would be expected. Apart from emplacement of 3 thrust

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sheets from the north or west, most faulting or folding has occurred in response to approximately east-west subhorizontal compression forces oper­ating since the time of emplacement of the first thrust sheet (Fig. 4).

ACKNOWLEDGMENTS This paper is based on part of my M.Sc. thesis which was undertaken at the Geology Department, University of Auckland. I would like to thank all the people at the U ni­versity and at the Geological Survey in Lower Hutt who helped with that project. I would especially like to thank Dr Peter Ballance, Associate Professor Berhard Sporli, Dr Ian Speden and Mr Ian Mackenzie for helpful discussions on this paper and for reviewing the manuscript.

I am obliged to the farmers in the region - Messers R. G. Eivers (Ihungia and Mangara Stations), P. Rouse (Kiteroa Station), A. H. Williams (Ruangarehu Station), and D. O. E. Williams (Puketiti Station) - for granting me access to their land. Special thanks to the Williams family of Ruangarehu Station with whom I stayed during my weeks of fieldwork.

REFERENCES Ballance, P. F.; Pettinga, J. R·.; Webb, C. 1982: A model

of the Cenozoic evolution of northern New Zealand and adjacent areas of the southwest Pacific. Tec­tonophysics 87: 37-48.

Black, R. D. 1980: Upper Cretaceous and Tertiary geol­ogy of Mangatu State Forest, Raukumara Penin­sula, New Zealand. New Zealand journal of geology and geophysics 23: 293-312.

Blatt, H.; Middleton, G.; Murray, R. 1972: Origin of sed i­mentary rocks. New Jersey, Prentice Hall, 634 p.

1980: Origin of sedimentary rocks. 2nd ed. New Jersey, Prentice Hall, 782 p.

Boltovskoy, E.; Wright, R. 1976: Recent foraminifera. The Hague, Dr W. Junk, 515 p.

Bremner, C. St J. 1934: Report on the geology, oil pos­sibilities and source rocks of the northern Ruatoria and East Cape District, New Zealand. Vacuum Oil Co. New Zealand Geological Survey, Lower Hutt, unpublished open-file petroleum report no. 123.

Burr, 1. L. 1940: Report on a reconnaissance survey of East Cape District. New Zealand Oil Exploration Co. Ltd. New Zealand Geological Survey, Lower Hutt, unpublished open-file petroleum report no. 180.

Crabb, P. L. 1971: Bryozoan zoarial paleoecology of the Waitemata Group. Unpublished M.Sc. thesis, lodged in the Library, University of Auckland.

Feary, D. A. 1974: Geology of the Mesozoic "Basement" in the Waioeka Gorge, Raukumara Peninsula. Unpublished M.Sc. thesis, lodged in the Library, University of Auckland.

Henderson, J.; Ongley, M. 1916: Gisborne and. Whata­tutu Subdivisions. New Zealand Geological Survey 10th annual report: 7-9.

1920: The geology of the Gisborne and Wha­tatutu Subdivision. New Zealand Geological Sur­vey bulletin 21: 88 p.

Hill, P. H. 1974: Taitai Series rocks at Te Kaha. Unpub­lished M.Sc. thesis, lodged in the Library, Uni­versity of Auckland.

Hoolihan, K. 1977: Basement geology of the Motu River area, Raukumara Peninsula. Unpublished M.Sc. thesis, lodged in the Library, University of Auckland.

Ingram, R. L. 1954: Terminology for the thickness of stratification and parting units in sedimentary rocks. Geological Society of America bulletin 65 : 937-938.

Isaac, M. J. 1977: Mesozoic geology of the Matawai Dis­trict, Raukumara Peninsula. Unpilblished Ph.D. thesis, lodged in the Library, University of Auckland.

Katz, H R. 1974: Recent exploration of oil and gas. In: Williams, G. J. ed. Economic geology of New Zealand. Australian Institute of Mining and Metal­lurgy monograph 4 : 463-480.

Kenny, J. A. 1980: Geology of the Ihungia catchment, Raukumara Peninsula. Unpublished M.Sc. thesis, lodged in the Library, University of Auckland.

Kingma, J. T. 1960: Outline of the Cretaceo-Tertiary sedimentation in the eastern basin of New Zealand. New Zealand journal of geology and geophysics 3 : 222-234.

1962: Sheet II-Dannevirke. Geological map of New Zealand 1:250000. Wellington, New Zealand. Department of Scientific and Industrial Research.

1965: Sheet 6-East Cape. Geological map of New Zealand 1:250000. Wellington, New Zealand. Department of Scientific and Industrial Research.

1971: Geology of Te Aute Subdivision. New Zealand Geological Survey bulletin 70: 173 p.

Laing, A. C. I 972a: Te Puia-I (N80/02). Alliance Pet­roleum New Zealand Ltd. New Zealand Geologi­cal survey, Lower Hutt, unpublished open-file petroleum report no. 257.

1972b: Geology and petroleum prospects of Ruatoria area, East Coast, North Island, New Zealand. The APEA journal: 45-52.

Lillie, A. R. 1953: The geology of the Dannevirke Sub­division. New Zealand Geological Survey bulletin 46: 156 p.

McKay, A. 1887: On the geology of east Auckland and the northern district of Hawke's Bay. New Zealand Geological Survey, reports of geological explora­tions, 1886-1887, 18: 182-219.

Macpherson, E. O. 1930: Geological reports on Area 33 (Waiapu). Taranaki Oilfields Ltd. New Zealand Geological Survey, Lower Hutt, unpublished open­file petroleum report no. 48.

Middleton, G. V.; Hampton, M. A. 1973: Sediment grav­ity flows; mechanisms of flow and deposition. In: Bouma, A. H. et al. ed. Turbidites and deep-water sedimentation. Society of Economic Paleontolo­gists and Mineralogists. Pacific section. Part 4.

Dow

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Moore, P. R. 1974: Cretaceous stratigraphy and structure of western Koranga Valley, Raukumara Peninsula. Unpublished M.Sc. thesis, lodged in the Library, University of Auckland.

1981: Upper Cretaceous and Tertiary geology of Mangatu State Forest, Raukumara Pen'insula. New Zealand journal of geology and geophysics 24 : 301-303.

Murray, J. W. 1973: Distribution and ecology of living benthic foraminiferids. New York, Crane, Russak and Co. Inc, 274 p.

Ongley, M.; Macpherson, E. O. 1928: Geology of the Waiapu Subdivision, Raukumara Division. New Zealand Geological Survey bulletin 30. 79 p.

Pettinga, J. R. 1980: Geology and landslides of the east­ern Te Aute District, southern Hawke's Bay. Unpublished Ph.D. thesis, lodged in the Library, University of Auckland.

Pick, M. C. 1962: The stratigraphy, structure and economic geology of the Cretaceo-Tertiary rocks of the Waiapu District, New Zealand. Todd Oil Explo­ration Co. Ltd. New Zealand Geological Survey, Lower Hutt, unpublished open-file petroleum report no. 608.

Quennell, A. M:; Brown, D. A. 1937: Dannevirke Sub­division. 31st annual report of the New Zealand Geological Survey.

Ricci-Lucchi, E 1975: Depositional cycles in two turbi­dite formations of northern Apennines (Italy). Journal of sedimentary petrology 45(1): 3-43.

Speden, I. G. 1973: Distribution, stratigraphy and stra­tigraphic relations of Cretaceous sediments, western Raukumara Peninsula, New Zealand. New Zealand journal of geology and geophysics i6: 243-268.

1976: Geology of Mt Taitai, Tapuaeroa Val­ley, Raukumara Peninsula. New Zealand journal of geology and geophysics 19: 71-114.

Sporli, K. B. 1980: New Zealand and oblique-slip mar­gins: tectonic development up to and during the Cainozoic. in: Ballance, P. E; Reading, H. G. ed. Sedimentation in oblique slip mobile zones. inter­national Association of Sedimentologists special publication 4: 147-170.

Stevens, G. R.; Speden, I. G. 1978: New Zealand. Pp. 251-328 in: Moullade, M.; Nairn, A. E. M. ed. The Mesozoic, A. The Phanerozoic geology of the World. II. Amsterdam, Elsevier Scientific Publishing.

Stoneley, R. 1968: A lower Tertiary decollement on the East Coast, North Island, New Zealand. New Zealand journal of geology and geophysics 11 : 128-156.

Suggate, R. P.; Stevens, G. R.; Te Punga, M. T. ed. 1977: The geology of New Zealand. Wellington, New Zealand Goverment Printer, 2 vol.

Vella, P. 1959: General geological and paleontological report on Ruatoria Lease area. Todd Oil Explo­ration Co. Ltd. New Zealand Geological Survey, Lower Hutt, unpublished open-file petroleum report no. 585.

1970: Paleontological reports, East Coast area. Alliance Petroleum New Zealand Ltd. New Zealand Geological Survey, Lower Hutt, unpublished open­file petroleum report no. 258.

Walker, R. G. 1967: Turbidite sedimentary structures and their relationship to proximal and distal deposi­tion environments. Journal of sedimentary petrol­ogy 37: 25-43.

Warren, G.; Speden, I. G. 1977: The Piripauan and Hau­murian stratotypes (Mata Series, Upper Creta­ceous) and correlative sequences in the Haumuri Bluff District, south Marlborough. New Zealand Geological Survey bulletin 92. 60 p.

Wellman, H. W. 1959a: Divisions of the New Zealand Cretaceous. Transactions of the Royal Society of New Zealand 87 (J and 2): 99-163.

1959b: Mangatu Group. in: Fleming, C. A. ed. international stratigraphic lexicon 6(4): 217.

Whitten, R. E 1979: Systematics and ecology of northern Hauraki Gulf bryozoa. Unpublished Ph.D. thesis, lodged in the Library, University of Auckland.

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