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Ponui Landslide: A deep-seated wedge failure inTertiary weak-rock flysch, Southern Hawke's Bay, NewZealandJarg R. Pettinga aa Department of Geology , University of Canterbury , Private Bag, Christchurch 1 , NewZealandPublished online: 24 Jan 2012.

To cite this article: Jarg R. Pettinga (1987) Ponui Landslide: A deep-seated wedge failure in Tertiary weak-rockflysch, Southern Hawke's Bay, New Zealand, New Zealand Journal of Geology and Geophysics, 30:4, 415-430, DOI:10.1080/00288306.1987.10427545

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New Zealand Journal o/Geology and Geophysics, 1987, Vol. 30: 415-430 0028-8306/87/3004-0415$2.50/0 © Crown copyright 1987

415

Ponui Landslide: a deep-seated wedge failure in Tertiary weak-rock flysch, Southern Hawke's Bay, New Zealand

J ARG R. PETTING A

Department of Geology University of Canterbury Private Bag Christchurch 1, New Zealand

Abstract The Ponui Landslide (September 1976), in Southern Hawke's Bay, is a wedge failure in an Upper Miocene weak-rock alternating sandstone-mudstone and amalgamated sandstone succession. The planimetric area involved with failure covers 25 ha and represents the reactivation and enlargement of a previous landslide. The Ponui Landslide has a calculated volume of approximately 2.5 x 106 m3

•

Morphologically, the Ponui Landslide consists of a lateral escarpment, exhumed slide surface, translational slide block, and slide debris. Rock­mass defects have controlled the landslide block geometry. The lateral escarpment has been propagated on nearly vertical intersecting fractures Uoints and faults). The slide surface is coincident with bedding in an alternating succession of friable, porous sandstone and weak, slake-prone mudstone. Slide-plane dip is variable, ranging from 10° to 36° and this intersects the escarpment fractures, so isolating the rock wedge. The slide debris is' comprised predominantly of silty sand.

The rupture surface propagated on a thin montmorillonitic clay gouge zone parallel to bedding. The gouge represents a tectonicall y formed shear zone. Block movement occurred subparallel to strike. The partially disintegrated slide block dammed Ponui Stream, creating a lake with maximum depth of 35 m, and a total calculated storage capacity of over 2.89 x 106 m3

•

The principal failure mechanism was sliding, involving essentially one large wedge-shaped block. Rapid movement and disintegration quickly reduced much of this block to debris. Other factors contributing to failure include: (1) high rainfall over a prolonged period prior to failure; (2) presumed

Received 29 July 1983, accepted 9 September 1987

high ground-water levels with excessive pore-water pressures generated within the porous, fractured sandstones resting on impervious mudstones; and (3) reactivation and enlargement of an existing failure which occurred during the 1931 Napier Earthquake.

Keywords Hawke's Bay; landslides; Tertiary; rock-mass defects; bedding-plane sliding; montmorillonite

INTRODUCTION

The Ponui Stream, inland from Kairakau Beach (grid ref. NZMS 270, V22/438348) was dammed in September 1976 by a major landslide, named the Ponui Landslide (pettinga 1980) (Fig. O. The resulting lake at full capacity had an approximate level of 35 m above the stream bed, and a total storage capacity in excess of2.89 x 106 m3 (de Leon 1977). The planimetric area involved with the failure covers approximately 25 ha. The maximum width through the toe of the landslide is 570 m; and calculations indicate an approximate volume of landslide debris of 2.5 x 106 m3

• Study of pre-1976 aerial photographs indicates earlier failures had affected the site (Fig. 2).

The landslide terminology (description and classification) used in this paper is based on Varnes (1978).

GEOLOGICAL SETTING

Southern Hawke's Bay is located within the East Coast Deformed Belt (Sporli 1980) (Fig. 3), a region characterised by present-day tectonic activity, including major earthquakes accompanied by regional uplift, folding and faulting (pettinga 1982). Uplift rates between 0.5 and 2 mm/year have been established in Hawke's Bay (Lewis 1971; Pillans 1986) and to the south in Wairarapa (Wellman 1971a, b; Ghani 1978). This is reflected also by widespread stream incision accompanying landscape rejuvenation (pettinga 1980).

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416 New Zealand Journal of Geology and Geophysics, 1987, Vol. 30

Fig. 1 Ponui Landslide, September 1976 (V22/437347), view looking northward. The steep lateral escarpment of the wedge failure, propagated across intersecting rock-mass defects, is approximately 1000 m in length. The exhumed bedding slide plane is visible to the right hand side; note the increase in dip to the apex point of the landslide. The slide block, partially disintegrated, has blocked the Ponui Stream and valley in the foreground. The sand debris flows generated at the toe are visible in foreground.

The Makara and Ponui Stream catchment headwaters adjoin the southern margin of the Maraetotara Plateau, a prominent landform in the coastal district of Southern Hawke's Bay (Fig. 3). This plateau reaches an average elevation of 500 m above sea level. The steep, deeply dissected escarpment surrounding the plateau indicates that it is an erosional remnant, attributable to the continued regional uplift during the Quaternary.

The plateau is capped by flat-lying resistant Pliocene limestone and calcareous sandstone of Te Aute Formation (Lillie 1953; Pettinga 1980; Harmsen 1985) and is underlain by a thick succession of weakly consolidated mudstones and sandstones of Makara Formation (pettinga 1980) (Fig. 3). The latter are moderately folded into a broad synclinal structure (Atua Syncline).

Numerous, very large, deep-seated landslides are recognised in the Maraetotara Plateau escarp­ment and adjoining incised catchments (peuinga 1980, 1987; Crozier et al. 1982). The Ponui Land-

slide is representative of other such failures in the district

The geology of the area in the vicinity of the landslide is relatively simple (Fig. 4). Two formations are mapped (full lithologic descriptions are given in Appendix 1):

1. The underlying Miocene Makara Formation (pettinga 1980) is comprised of an alternating suc­cession of thin-medium bedded, friable (poorly cemented) sandstone and weak mudstone (flysch). Intercalated within the succession are three thick units of composite (amalgamated) sandstone beds referred to as Ponui sandstone units A, Band C (Fig. 4 and 5). Individual sandstone beds range in thickness from 4 to 15 m, and are structureless apart from parallel and current-ripple lamination in the upper third of specific beds. Bedding plane partings are always sharp and planar (peuinga 1980; van der Lingen & Pettinga 1980). The mudstone interbeds are generally massive. One 1-5 m thick tuffaceous marker bed is recorded.

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Pettinga - Ponui Landslide 417

Fig. 2 Vertical aerial photo graph oflower Ponui Stream showing the area centred on Ponui Landslide (pre-197 6). Landslide scarplets developed by the 1931 earthquake are arrowed. (Photo: N.z. Aerial Mapping 3833/24:1964; reproduced by permission Department of Survey and Land Information.)

Fig. 3 Outline geological map of Southern Hawke's Bay with Ponui Landslide location indicated. AS = Atua Syncline; OMTZ Ocean Beach­Mangakuri Thrust Zone; A = Coastal Structural High; B = Elsthorpe Anticline; C = Inland Structural High. Inset: Location of East Coast Deformed Belt.

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sandstone units A, Band C are traced both to the north and south of Ponui Stream, and they thin markedly in both directions. They are not represented in the western limb of the synclinal complex. Makara Formation thins rapidly and is represented by a facies change to massive mudstone to the east of Ponui Landslide.

Considerable eastward-directed downfaulting associated with a major regional slump structure, extending >30 km parallel to the coastline (see Pettinga 1980, 1982, 1985) does not affect the area immediately involved in the Ponui failure. Faulting is confined to an area beyond 500 m east of the landslide.

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Makara Formation rests unconformably over a basement complex of intensively sheared, folded, and truncated Upper Cretaceous to Lower Miocene successions (see Fig. 3). The latter do not crop out near the landslide but are exposed to the northeast and south. This "basement" geology is described in detail by Pettinga (1982).

A major fault zone, the Ocean Beach­Mangakuri Thrust Zone, extends south from the Waimarama area (pettinga 1982) into the flysch succession of Makara Formation (Fig. 4). It projects into the sequence immediately beneath the lower Ponui sandstone unit A, approximately 2 km north of the landslide. The thrust is accompanied by a melange zone characterised by pervasively sheared, crushed, and chaotically (tectonically) mixed debris derived from the underlying Upper Cretaceous and lower Tertiary formations. The melange matrix is a montmorillonitic clay derived from lower Tertiary mudstones of Wanstead Formation (Lillie 1953). The thrust zone thins rapidly south of Te Apiti Stream and is further masked by large-scale deep­seated landsliding immediately north of the Ponui Landslide.

LANDSLIDE MORPHOLOGY AND BEDROCK CONTROLS

Four morphologic entities are distinguished for the Ponui Landslide (Fig. 6 and 7). (1) lateral escarpment; (2) bedding-controlled exhumed slide surface; (3) slide block; and (4) sand-flow debris.

Lateral escarpment The lateral escarpment of Ponui Landslide is > 10DO m long, is crudely linear in outline, and parallels movement direction of the main slide block. The escarpment varies in height from approximately 10 m at its extremities to more than 70 m (originally) in its central portion, although this has been substantially reduced by scarp degradation and the development of a debris apron at the foot.

The lateral escarpment has been propagated along rock-mass defects through bedrock. The bulk of the scarp face is in the composite (amalgamated) Ponui sandstone unit A, with an approximate thickness of 60--68 m. The base of the scarp coincides approximately with the base of the sandstone unit. The latter is best exposed on the opposite bank of Ponui Stream (see Fig. 14) and elsewhere to the south. Thinly bedded flysch

overlies the sandstone unit but is present only in the central and lower segments of the escarpment.

The geometric configuration of the escarpment is controlled by rock-mass defects (Fig. 8), predominantly master and major joints and faults. (Major defects are defined in terms of spacings of 3-10 m and continuity by terminating against all master defects-but may crosscut other major defects. Master defects are defined as all those which are continuous through the entire escarpment rock mass, with spacing in excess of 10 m.) Three dominant defect sets are recorded (Fig. 9). Defect sets 1 and 2 and 5 played the dominant role in controlling escarpment geometry; defect set 3 was insignificant insofar as its dip is inclined into the slope.

Since 1976, the lateral escarpment has intermittently undergone minor regression because of toppling and rockfall failures. A substantial secondary collapse of the central portion of the escarpment has obscured exposure of the fracture­controlled face with debris.

Slide surface The bedding-controlled slide (or rupture) surface has been exposed in the upper third of the landslide (Fig. 6 and 10). The bedding attitude increases from south (near the toe) to north (near the head) ranging from 10° to 36°. The exhumed mudstone surface is discontinuously coated by a highly plastic greyish­green to greenish-black montrnorillonitic clay (see XRD data, Appendix 1). Two possible origins for this clay gouge are considered:

1. Derivation may be from a series of parallel, steeply dipping faults which cut obliquely across the bedding plane (Fig. 6 and 10). These faults are accompanied by a clay-gouge zone up to 2.0 m wide. The clay gouge would thus have been smeared over the failure surface, so lubricating it during failure.

2. The plastic clay coating may have been introduced tectonically along bedding as a thin shear gouge continuation of the Ocean Beach-Mangakuri Thrust Zone.

Additional detailed field and laboratory studies have confirmed (2) as the only plausible explanation. New exposures examined in February 1983 revealed the basal contact of the Ponui sandstone unit A near the head of the landslide (Fig. 11). From this exposure it is clear that the slickensided surface with accompanying thin «50 mm) clay gouge, does not coincide with the basal

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Pettinga - Ponui Landslide 421

Fig. 6 Geomorphological map, Ponui Landslide and surrounding catchment area, lower Ponui S trearn. RB = residual slide block; A and B = areas of secon­dary collapse of lateral escarp­ments. Base plan to map is the vertical aerial photograph (Fig. 7).

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contact of the amalgamated sandstones, but rather that it is located 2-3 m (stratigraphically) below the sandstone within the thinly bedded alternating sandstone-mudstone sequence. The thin clay gouge is present along a bedding plane between an underlying mudstone and overlying sandstone. The shear-zone gouge displays some pinch and swell across the bedding surface (from <5 mm up to 50 mm). The enclosing mudstone and sandstone are closely fractured and also sheared, but the gouge seam is relatively uniform and planar and does not bifurcate into secondary shears.

Approximately 1.5 km to the south of Ponui Landslide, along strike (V22j426 329), similar, thin, clay-gouge shear zones are incorpcrated parallel to bedding within the thinly bedded flysch,

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stratigraphically c. 7-10 m below the same lower Ponui sandstone unit A (Fig. 12 and 13). The individual clay-gouge seams are of variable thickness, ranging from <2 mm to >0.5 m. The 4 m wide braided shear-zone gouge shows rapid pinch and swell along individual bedding surfaces, and in tum die out laterally in similar fashion to tension­gash structures. The adjacent mudstone is closely fractured and partially crushed. Movement directions inferred from the lozenge fabric of the mudstone and shear fabric of the gouge clays clearly indicate a northeast-eastward directed transportation of the overriding blocks, in agreement with movement determined for the Ocean Beach-Mangakuri Thrust Zone elsewhere (pettinga 1982).

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422 New Zealand Journal of Geology and Geophysics, 1987, Vol. 30

The clay pug present on the bedding slide surface of the Ponui Landslide is a sheared, montmorillonitic-rich swelling clay, whereas the clays within the shear zone of the small near-vertical faults cutting obliquely across the exhumed bedding surface are mixed-layer montmorillonite - illite -chlorite swelling clays, similar to Makara Formation mudstones (see summary, Appendix 1). The Makara

Fig. 7 Vertical aerial photograph showing the Ponui Landslide and surrounding area (post-September 1976). For geomorphological features see Fig. 6. (Photo: N.Z. Aerial Mapping 5761-FI24:1980; reproduced by permISSIOn Department of Survey and Land Information.)

Fig. 8 Upward view of landslide lateral escarpment formed in amalgamated Ponui sandstone unit A, cliff approximately 60-65 m high. Note the intersecting master amd major defects (sets 1 and 2 are identified) along which the escarpment has been propagated.

Formation derived clays are also a dark bluish-grey, and more silty than the bedding slide surface clay.

Faults cutting obliquely across the exhumed bedding slide plane (Fig. 10) show maximum offset of 0.40-0.50 m. The rupture surface is the same over the entire exhumed surface, which is offset by these faults. This lends further support to the propagation of the failure surface along a pre-existing

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Pettinga - Ponui Landslide

w

Contour Intervals (per I % area)

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Fig. 9 Contoured plots of poles to defects (joints and faults) measured along the lateral escarpment, Ponui Landslide. Both plots are lower hemisphere, Lambert equal-area equatorial projections. Poles contoured per 1 % area using the method of Hoek & Bray (1981). A Master defects; spacing greater than 10 m. B Major defects; spacing between 3 and 10 m. Principal defect sets 1-6 are indicated.

tectonically introduced weak clay-gouge, parallel to bedding.

No sign of the clay gouge was detected at the toe of the landslide in the bedrock exposures on the southern slope of the former Ponui Stream. Best exposures are immediately adjacent to the lake outlet (Fig. 14) where an almost continuous section was logged. The flysch underlying the Ponui sandstone unit A has changed character substantially from that beneath the exhumed slide

423

Fig. 10 Basal shear plane on bedding in thinly bedded flysch (thin sandstone beds arrowed). Note the thin discontinuous coating of montmorillonite (swelling) clay with shrinkage cracking on bedding-plane slide surface. The contact of clay coating and mudstone bedrock is indicated by a dashed line. A vertical fault trending obliquely across the exhumed bedding plane (refer Fig. 6) is visible in the foreground below hammer, with an offset of approximately 0.5 m, downthrown in foreground. Note continuation of slide surface with swelling clay coating lower right.

plane, being medium-bedded alternating sandstone­mudstone, with large, open, rock-mass defects. The contact with the Ponui sandstone is not sheared. It is probable the shear gouge has either pinched out or is ultra thin «1 mm) and so remained undetected within the underlying flysch, and coincides with a mudstone-sandstone contact.

Slide block The bulk of the rock material involved in the landslide was derived from the amalgamated Ponui sandstone unit A, which, over parts of the escarpment ridge, is capped by flysch.

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The failed wedge of sandstone rock, its geometry controlled by the intersection of the critical bedding plane and master and major fracture sets, partially disintegrated once set in motion. The remainder of the slide block (Fig. 6 and 7) shows that slide-plane geometry combined with frictional drag on the slide surface, as well as rock-block interlocking along the lateral escarpment, caused the trailing side to break up. Several large scarps developed internally in the slide block. Much of the rock material had disaggregated, giving rise to an

Fig. 11 Exposure at base of lateral escarpment, showing contact of amalgamated Ponui sandstone unit A above, and thinly bedded flysch below hammer handle. Lower right of photo shows debris resting on the slide plane of the landslide. Note also the open fractures in sandstone with limonite staining indicative of free water movement.

Fig. 12 Makara Formation flysch bedrock exposure, tributary creek of Waewae Stream (V22/ 426329). Note the thin-bedded flysch exposed in the rear scarp (fl). The topc~this scarp is inclined to the left-hand side in view (arrowed) and coincides with the contact of underlying flysch and overlying Ponui sandstone unit A. Thin-bedded flysch is also exposed in the creek bed in the lower foreground. Thin bedding­plane shears in flysch, with accompanying swelling clay­gouge seam, are exposed in the bank (X) (see Fig. 13).

unconsolidated, loosely packed, porous, silty sand with incorporated large defect-controlled blocks of sandstone. The true left side of the block slid neatly along bedding.

Sand-flow debris The lower portion of the failure block, including a substantial part of the lower slope, previously affected by deep-seated mass movement (see Fig. 2) totally disintegrated, leading to an unconsolidated, porous, silty sand debris. This debris moved into and

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Fig. 13 Thin bedding-plane shear in Makara Fonnation flysch, with accompanying swelling clay­gouge seam, exposed in tributary creek of Waewae Stream. (For precise locality refer Fig. 12, exposure in bank by X.) Note the pervasive shear fabric of the clay­gouge seam with shrinkage cracking parallel to fabric. Small shear clasts are visible in the clay. Note also the unsheared, relatively coherent overlying mudstone and moderately sheared, crushed, underlying mudstone. Inferred movement direction from the clay shear gouge is left to right (from southwest). With respect to local topography, this is upslope, and indicates the shears' origin is primarily tectonic (see text for further discussion).

across the Ponui Stream valley, blocking it. The failure appears to have been rapid. Some of the sand and silt debris (with vegetation, fences, and stock) has risen up to 6-8 m above the toe onto the southern valley wall and then downstream where it, and a substantial quantity of debris direct from the front face of the slide block, has formed a distinct debris lobe (see Fig. 6 and 7). Secondary collapse of the front face is indicated by scarps in the debris of the landslide toe.

The mobility of the silty sand debris after failure indicates high ground-water levels prevailed at the time of movement-the debris near saturation. Observations by local farmers immediately after the event support these conclusions.

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Fig. 14 View looking to west at toe of Ponui Landslide. In the foreground, Ponui Stream is entrenched in landslide debris comprised of silty sand. The photo was

. taken within four weeks of the lake first overtopping the toe dam created by the slide block. Bedrock exposures on the true right bank of the stream in centre of view shows medium-bedded flysch overlain by amalgamated Ponui sandstone unit A. The basal contact of the sandstone unit is arrowed, adjacent to the lake outlet. Note the deep entrenchment of the stream and the unstable nature of blocky landslide debris. The contact of landslide debris and bedrock is exposed at (x).

Morphologically, the highest part of the landslide debris forming the dam was near the upstream margin. This is because the bulk of the debris was derived from the west side of the landslide, which involved an entire ridge.

The Hawke's Bay Catchment Board staff cal­culated the capacity of the lake to be some 2 890000 m3

• However, by removing the small 6 m high crest from the toe of the landslide debris, the capacity of

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the lake is reduced by nearly 35% (see Discussion). The total capacity of the lake prior to this work represented 289 mm of rainfall over the entire 200 ha catchment (de Leon 1977). (My own calculations indicate a catchment area of approximately 850 ha, however.)

MOVEMENT MECHANISM AND MODE OF FAILURE

The fundamental movement mechanism of the Ponui Landslide was one of sliding on a tectonically introduced, low-strength swelling clay seam along bedding. Subsidiary failure mechanisms include (1) flowage of the near-saturated (inferred), sandstone­derived debris in the toe of the slide block and (2) toppling and rockfall failure of fracture-bounded blocks from the lateral escarpment

The mode of failure (block geometry) is that of a simple wedge (Fig. 15). This is a simplification insofar as the bedding slide plane is warped by regional folding, having a steeper dip (up to 36°) over much of its exhumed portion above the slide block, down to 10° beneath the toe of the slide block, in the Ponui Stream valley.

Analysis from stereonet constructions (Fig. 16) indicates that the upper part of the slide block (activated by the 1931 Napier Earthquake-see Fig. 2) failed on a slope, given by the escarpment­bedding intersection line inclination (plunge), of approximately 20°. The plunge of the intersection line for the lower part of the slide block has been calculated at 6°. This emphasises the very low angles of sliding friction across potential critical rupture planes (ignoring pore pressure and cohesion effects across the failure plane). The intersection line trend is approximately parallel to movement direction of the slide block.

For the purpose of stereonet analysis above, the lateral escarpment was taken as a single planar defect inclined steeply, and inferred to intersect with the bedding-plane gouge seam. The two extreme values of dip for the latter were used in plotting (Fig. 16). This assumption, with respect to the escarpment, is justified in that examination of pre-1976 vertical aerial photos (Fig. 2) indicates the earlier development of the lateral escarpment was in a more or less straight alignment, approximately parallel to the direction of sliding movement, nearly at right angles to bedding dip. It is likely that the present escarpment geometry reflects secondary

LATERAL ESCARPMENT

PLANAR DEFECTS

BEDDING PLANE FAILURE SURFACE

Fig. 15 Schematic diagram of the failure model for the Ponui Landslide.

modification by rockfall collapse of unsupported wedge blocks. The development of a substantial debris apron is in agreement also. Defect sets 1 and 2 (Fig. 9) would have been critical to original escarpment configuration (see defects highlighted by light reflection in Fig. 1).

GROUND-WATER CONDITIONS

Exceptionally high rainfall had been recorded in the two-year period prior to failure. In 1976, four of eight months had twice or more monthly average rainfalls, and February was in excess of five times the average.

Ground-water conditions are inferred to have been such that pore-water pressures built up within the porous, uncemented, amalgamated Ponui sand­stone unit A (approx. permeability 10-4 to 10-5 m/s) resting on the impervious mudstone-dominated flysch, with the intercalated, extremely weak clay­gouge seam. Mudstone permeabilities are <l~ m/s. Water was also able to readily penetrate the amalgamated sandstone unit, along the numerous open rock-mass defects and through the permeable sandstone, to the critical interface immediately below the Ponui sandstone unit (Fig. 11).

Presumably excessive pore-water pressures negated residual shear strength along the sheared bedding contact with the tectonically introduced plastic clay. This sheared bedding surface was subsequently crucial to the gravity sliding of the entire wedge (ridge side) of bedrock into the Ponui Stream valley.

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N

w E

s 1 EQUATORIAL ~- ~ EQUAL-AREA ~

#3~ STEREONETS~

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Fig. 16 Lower hemisphere equal-angle stereonet analyses of wedge failure model geometry. A Geometry of upper slide block (created by 1931 Napier Earthquake enlargement of pre-existing failure), with intersection line plunge reaching a maximum value of 20° near the landslide block apex. Back analysis, ignoring other factors such as pore pressures and failure surface cohesion, suggest friction angles generated did not exceed 20°. B Geometry of the lower slide block (pre-1931 in origin), with intersection line plunge averaged to 6° near the toe. Back analysis suggests the friction angles generated did not exceed 6°.

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GEOMORPHOLOGY

In the central and lower reaches of Ponui Stream, slope development is strongly influenced by lithology and structure. The sandstone units are erosion resistant, sandstone ridges extending along strike, and exhumed dip slopes are characteristic (Fig. 6, 7, and 17). The complementary steep escarpments and bluffs forming the east-facing sides of ridges are controlled by rock-mass defects, approximately at right angles to bedding. Scree and debris aprons are developed against these scarps and bluffs, and this process is still active on the Ponui Landslide lateral escarpment.

Geomorphological mapping has shown that the sandstone scarps and bluffs are the remnants of rock­mass defect controlled lateral escarpments of wedge failures (in analogy to the Ponui Landslide). Basal slide planes are coincident with the thick amalga­mated sandstone contacts with underlying mud­stones of the flysch. These contacts are a perme­ability barrier across which shear strengths may be sufficientl y reduced by excess pore pressures with a buildup of ground water within the permeable sandstone units.

Simple wedge failures are characteristic on the slopes near Ponui Landslide (Fig. 6). Movement direction of slide blocks is inferred to have been at acute angle (normally <35°) to strike. These ancient wedge failure sites are readily recognised by their geometry, with the presence of dipping exhumed (mudstone) bedding-plane surfaces and lateral escarpments (Fig 17). Generally little colluvial debris (derived from the lateral escarpment and crown regression and degradation) remains on the mudstone bedding-plane slide surface, although minor remobilisation of lag debris and scree apron material may manifest itself as creeping earthflows (see Fig. 17). Substantial lateral escarpment modification has not occurred, the exceptions are indicated by A and B on Fig. 6. Some residual slide blocks are also recognised (RB on Fig. 6). However, generally little of the debris from these previous landslides remains. Material emplaced in the valley by Ponui Landslide is highly unstable and erodable when saturated (Fig. 14). This reflects the uncemented, unconsolidated nature of the silty sandstone debris.

SEISMOTECTONIC EFFECTS

The triggeri'lg of the upper part of the Ponui Landslide during the 1931 Napier Earthquake was noted by local farmers. This involved development

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of a series of small scarps parallel to, and nearby, the ridge crest. Total offset on anyone scarp is estimated not to have exceeded 5-10 m. It is not known if any of the scarps were apparent prior to 1931. Vertical aerial photographs during the ensuing 45 years show no signs of further movement, until sudden failure in 1976; detailed monitoring, however, was never undertaken.

It is clear from available vertical aerial photo­graphs and also historical photographs that the lower part of the landslide area prior to 1976, and also 1931, had been involved in extensive and deep­seated mass movement, presumably involving the same failure plane.

DISCUSSION

Although the occurrence of major bedding-plane faults in Tertiary weak-rock terrains in New Zealand have been recorded (see, e.g., Bradshaw & Newman 1979; Pettinga 1982), their importance with respect to major deep-seated landsliding has not. Such faults are often subtle, only recognised after detailed regional mapping, and becoming evident only from indirect data such as microfaunal age deter­minations. These bedding-plane faults, when recognised in exposure, may be accompanied by a very thin «10 mm) clay-gouge shear zone. These may be striated, and may be derived from within the formation affected by faulting (as in the numerous bedding-plane faults in the Waitemata Group flysch of the Auckland region), or may manifest them­selves by the presence of a tectonically introduced

Fig. 17 View to the southwest, from near the lateral escarpment of Ponui Landslide (V22/436348), of ancient wedge failure on southern slopes of the Ponui Stream valley. Note the Ponui Landslide lake in the foreground. The tree-covered slope comprises the lateral escarpment of the failure, formed in amalgamated Ponui sandstone unit C (exposed extreme bottom right). dipping to the right. The exhumed bedding­plane slide surface is clearly visible adjacent to and below the escarpment. formed across the sandstone-underlying mudstone (flysch) contact. The inclined planar surface has a minor. partially active earthflow located over part of its extent, along the intersection line.

lubricant (as in the shear associated with the Ponui Landslide).

Most frequently, major deep-seated landslides associated with such bedding-plane faults are present in flysch successions and reflect the tectonically active environment within which the flysch sequences have accumulated. The Makara Formation, in which the Ponui failure occurred, is but one of numerous flysch basin sequences recognised in the East Coast Deformed Belt (van der Lingen & Pettinga 1980), and it is expected that many similar deep-seated landslides have occurred, located over bedding-plane faults. Where such landslides are recognised in an area with bedding­plane faults, it is likely that the extent oflandsliding, or susceptibility for failure, is very great, in concordance with the extent of the bedding-plane fault outcrop, local topography, and geologic structure.

An important contrast also documented from the Ponui catchment is that similar deep-seated large failures may be propagated in a bedded sequence where the permeability contrast between strata is extreme, with no bedding-plane faults accompanied by a thin, weak, swelling clay gouge controlling the failure surface.

The tectonic origin of the stratigraphically contained thin clay seam forming the failure surface of the Ponui Landslide contrasts with that of the large deep-seated failures recognised in the central North Island, where similar areally extensive clay seams are recognised (Stout 1977). The origins of the latter have not been discussed by S tout, but

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Thompson (1981) infers these to be primarily sedimentary in their development, related to the submarine alteration of thin volcanic ash layers. In both regions, however, the importance of rock-mass defects and material properties cannot be overemphasised in the controls they exert on large deep-seated landsliding, often on failure planes inclined at less that 10°, and the role these factors play in geomorphic evolution of the regions. Initiation of such failures by major earthquakes may be crucial, but can not as yet be further evaluated.

ACKNOWLEDGMENTS

The Ph.D. project on which this paper is based was carried out under the supervision of Mr W. M. Prebble, Dr P. F. Ballance and Associate Professor K. B. Sporli, Department of Geology, University of Auckland. The author is indebted to Dr M. 1. Crozier (Victoria University of Wellington), Mr W. M. Prebble (University of Auckland) and Mr D. H. Bell (University of Canterbury) for reviewing the manuscript.

Financial support for the Ph.D. study was received from the National Water and Soil Conservation Organisation (MWD), and a Postgraduate Scholarship from the University Grants Committee. Research grant funding from University of Canterbury allowed for additional fieldwork in 1983-84.

Special thanks also to Mr Peter Sherning (Waimoana) for assistance with accommodation; and Mrs Jan Graham and Professor W. B. Bull for assistance in the field. The technical services of Mr A. Downing (photography), Mrs S. Tye (typing), and Ms L. Leonard (draughting), University of Canterbury, are also acknowledged.

REFERENCES

Bell, D. H.; Petting a, J. R. 1984: Presentation of geological data. In: Brown, I. R. ed. Proceedings of Symposium on Engineering for Dams and Canals. IPENZ proceedings of the Technical Group 9(4G): 14.1-14.35.

Bradshaw, 1. D.; Newman, J. 1979: Low angle thrusts in Cenozoic rocks in Canterbury, New Zealand. New Zealand journal of geology and geophysics 22: 435-442.

Crozier, M. J.; Gage, M.; Petting a, J. R.; Selby, M. J.; Wasson, R. J.1982: The stability of hills lopes. In: Soons, 1. M.; Selby, M. J. ed. Landforms of New Zealand. Auckland, Longman Paul. 392 p.

de Leon, P.1977: Ponuislip, Hawke's Bay. Soil and water 13-15: 25 p.

Ghani, M. A. 1978: Late Cenozoic vertical crustal movements in the southern North Island, New Zealand. New Zealand journal of geology and geophysics 21: 117-125.

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Hannsen F. J. 1985: Lithostratigraphy of Pliocene strata, Central and Southern Hawke's Bay, New Zealand. New Zealand journal of geology and geophysics 28: 413-434.

Hoek, E.; Bray, J. 1981: Rock slope engineering. 3rd cd. London, The Institute of Mining and Metallurgy. 358 p.

Lewis, K. B. 1971: Growthrateoffolds using tilted wave­planed surfaces; coast and continental shelf, Hawke's Bay, New Zealand. In: Collins, B. W.; Fraser, R. ed. Recent crustal movements. Bulletin of the Royal Society of New Zealand 9: 225-231.

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

Petting a, 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. 602 p.

--1982: Upper Cenozoic structural history, coastal Southern Hawke's Bay, New Zealand. New Zealand journal of geology and geophysics 25: 149-191.

--1985: Seismic evidence of the offshore extension of the Kairakau-Waimarama Regional Slump, Hikurangi Margin Cruise 1121. In: Lewis, K. B. ed. New seismic profiles, cores and dated rocks from the Hikurangi Margin. New Zealand Oceanographic Institute field report.

--1987: The Waipoapoa Landslide: a deep-seated complex block slide in Tertiary weak-rock flysch, Southern Hawke's Bay, New Zealand. New Zealand journal of geology and geophysics: (this issue).

Pillans, B. 1986: A late Quaternary uplift map for North Island, New Zealand. In: Reilly, W. I.; Hartford, B. E. ed. Recent crustal movements. Royal Society of N ew Zealand bulletin 24: 409-417.

Sparli, K. B. 1980: New Zealand and oblique-slip margins: Tectonic development up to and during the Cenozoic. In: Ballance, P. F.; Reading, H. G. ed. Sedimentation in oblique slip mobile zones. International Association of Sedimentologists special publication 4: 147-170.

Stout, M. L. 1977: Utiku Landslide, North Island, New Zealand. Geological Society of America reviews in engineering geology 3: 171-184.

Thompson, R. C. 1981: Landsliding in Cenozoic soft rocks of the Taihape-Mangaweka area, North Island, New Zealand. Special Bulletin de Liaison des Laboratoires des Ponts et Cheussees 10: 93-100.

van der Lingen, G. J.; Petting a, J. R. 1980: The Makara Basin: a Miocene slope-basin along the New

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Zealand sectorofthe Australian-Pacific obliquely convergent plate boundary. In: Ballance, P. F.; Reading, H. G. ed. Sedimentation in oblique slip mobile zones. International Association of S edimentologists special publication 4: 191-215.

Vames, D. J. 1978: Slope movement types and processes. Chapter 2 in: Schuster, R. L.; Krizek, R. J. ed. Landslides, analysis and control. National Academy of Sciences, Transport Research Board special report 176: 11-33.

Wellman, H. W. 1971a: Holocene tilting and uplift on the White Rocks coast, Wairarapa, New Zealand. In: Collins, B. W.; Fraser, R. ed. Recent crustal movements. Bulletin afthe Royal Society of New Zealand 9: 211-215.

--1971 b: Holocene tilting and uplift on the Glenbum coast, Wairarapa, New Zealand. In: Collins, B. W.; Fraser, R. ed. Recent crustal movements. Bulletin of the Royal Society of New Zealand 9: 221-223.

APPENDIX 1

Field lithologic descriptions

The following descriptions are based on the engineering geological field descriptions for rock material as outlined by Bell & Petting a (1984).

Makara Formation (predominantly flysch):

1. Mudstone: Unweathered, moderately weak to weak, dark greyish-green (saturated) to light bluish-grey (dry), massive calcareous montmorillonitic MUDSTONE. KB.: (1) Prone to slaking (with shrink-swell behaviour). (2) Grain-size analysis: clay 32-50%; silt 50-63%; sand 2-6%.

2. Sandstone: Unweathered to moderately weathered (includes Ponui sandstone units A, B, and C), moderately weak to weak, light greyish-green to light yellowish-grey, finely to coarsely layered, quartzofeldspathic fine SANDSTONE: slightly calcareous, slightly silty.N.B.: (1) Occasional concretionary beds. (2) Thicker beds lower in sequences are graded.

3. Tuffaceous units: Unweathered to moderately weathered, weak to very weak, light-bluish grey to white SANDSTONE, SILTSTONE, and MUSTONE. N.B.: Very thin to thickly bedded (10 rom to (+)1 m), quartzofeldspathic and vitric, noncalcareous.

Te Aute Formation

1. Coquina limestone: Slight to highly weathered, very strong to moderately weak, light yellowish-orange to light yellowish white COQUINA LIMESTONE.

2. Calcareous sandstone: Moderate to completely weathered, moderately strong to weak, light ye!lowish­orange to light bluish-grey fossiliferous calcareous SANDSTONE.

Clay mineral XRD analyses

Mako.ra Formation

Two clay mineral structures have been identified from representative Makara Formation mudstone samples: chlorite (peaks 7.31 A, 3.59 A and interlayered illite-montmorillonite (and chlorite) (peaks 15.24 A, 10.40 A, 5.09 A and 3.38 A). Several samples were glycolated and subsequently heat treated. The montmorillonite clay peak shifted to 17.3 A on glycolation and disappeared on heating. The illite peak was enhanced, indicating the collapse of the montmorillonite structure to that of illite (mica).

Several samples were analysed qualitatively from X­ray charts to determine an approximate percentage of montmorillonite clay in the bulk rock. Percentages ranged from 15 to 40% (mixed-layer swelling clay).

There was no variation in mineralogy recorded from the shear gouge accompanying small faults which trend obliquely across the exhumed slide plane.

Shear gouge clays from Ponui Laruislide rupture surface

Clay mineralogy determinations show a clear distinction from the clays present in the minor faults trending obliquely across the exhumed slide plane (see above descriptions). Samples X-rayed showed approximately 60-70% montmorillonite, 10-15% quartz, 10-15% calcite, with minor mica and feldspar. One sample was glycolated and subsequently heat treated. The montmorillonite peak shifted from 15.23 A to 17.33 A basal spacing on glycolation, and disappeared on heat treatment, with enhanced 9.93 A mica peak.

The results clearly indicate a pure montmorillonite clay is present, with no evidence of interlayered illite­montmorillonite structures. Chlorite was absent.

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