127

THE STRUCTURAL GEOLOGY OF THE WAITEMATA GROUP

IN THE PT. ERIN GASWORKS AREA, AUCKLAND

by R. High*

S U M M A R Y

Beds of the Waitemata Group have been subjected to an early submarine slumping phase followed by a later folding phase. Faults have been divided into four types — normal separation faults, reverse separation faults, faults (sub) parallel to the bedding and strike slip faults. Detailed stratigraphic columns provide structural control within the area.

Fig. 1. Locality map for stratigraphic columns (Roman numerals) and Figures (Arabic numerals). Inset shows locality map for Pt. Erin-Gas works area.

•Department of Geology, University of Auckland.

128

Fig. 2. Stratigraphic columns of Pt. E r i n - G a s Works area. See Fig. 1. for location of columns.

I N T R O D U C T I O N

The aim of the study was to produce a largely descriptive account of the mesoscopic structures seen in the Waitemata Group of the area, mainly a coastal section between Pt. Erin and the old Auckland Gasworks, approximately one mile west of downtown Auckland city. (F ig . l ) .

Geological data were recorded on the Auckland Harbour Board Westhaven Boat Harbour Base Plan, on scales of 1 inch to 100 feet and 1 inch to 400 feet.

P R E V I O U S W O R K

The first specific mention of this area was made by Park (1885), followed by brief reports by Fox (1901) and Mulgan (1901). They were principally concerned with the stratigraphical correlation of the Waitemata beds, the "Parnell G r i t " and the "Ponsonby T u f f horizons, and with the general structure of the area. Their findings wil l be elaborated upon later in the paper. A l l three had the

129

advantages of working with better exposures than are now present, for the development of the Auckland Harbour Bridge Motorways, growth of vegetation and weathering have reduced the quality and amount of outcrop in recent years.

S T R A T I G R A P H Y

In the Pt. Erin Gasworks area, the Waitemata Formation consists of sand­stones, mudstones and thinly bedded alternating sandstones and mudstones, and an approximately 5 metre thick bed of "Parnell G r i t " (Fig.2).

Park (1885) stated that the grit on the eastern limb of the Pt. Erin syncline reappears at Shelly Beach (now no longer present), and that just to the east of the grit there is a large fault with the higher sequence lying almost horizontally against the almost vertical grit and other beds below (Fig.3a). This fault may have been inferred by Mulgan (1901), because he believed that the volcanic grit after rising eastward into the air at Shelly Beach had been brought down to the shore platform at Pt. Acheron still showing a westward dip (Fig.3b). The persistence of the westward dip, but not the existence of the grit at Pt. Acheron, has been confirmed in the present study. Fox (1901) doubted whether this fault dipped east, (as most large normal separation faults do in this area) because the fault was concealed by landslips and vegetation at that time. He recorded a "Ponsonby T u f f ' at Pt. Acheron, and considered it to lie at least 100 feet below the "Parnell Gr i t " , thus obviating the need for the fault postulated by Mulgan.

I have been unable to find any trace of this fault or of the near vertical eastern limb of the syncline in the vicinity of the Shelly Beach cliffs. Taylor and Tonkin (1960) in a report to the Auckland Harbour Board state that the Pt. Erin Park corner disclosed "Parnell Gr i t " . However there was no other mention of Parnell Grit either in the cliffs or the shore platform near Pt. Acheron. Grit was also not recorded in the M.O.W. Bore Record sheets of the Victoria Park viaduct. During roadmaking "Parnell Gr i t " requires blasting and it is unlikely that a 5 metre bed of volcanic sandstone went unnoticed.

Stratigraphic columns for all exposures in the area are presented'in Fig. 2. Due to the non-continuity of the exposures and the presence of faults, few certain correlations can be made, but it can be stated with some certainty that the strata above the grit at Pt. Erin cannot be correlated with those at Pt. Acheron (see columns II and XII Fig. 2).

If the grit at Shelly Beach really were displaced to a position at Pt. Acheron by movement solely along the fault postulated by Mulgan, the amount of throw would have to be in the order of 750 m, but could be less i f one or more additional normal separation faults with similar attitude to the postulated fault, occurred between Shelly Beach and Pt. Acheron. Such faults actually exist (Fig. 15).

In general the strata are oldest in the east of the area and become younger to the the west where they arc synclinally folded. This is seen in Fig. 3c. which

130

represents a schematic W E cross section from Pt. Erin to the Gasworks, based on present day outcrops.

S T R U C T U R E

BEDDING

The bedding throughout most of the area strikes E N E and dips to the N at an average angle of 18° (Fig. 4). There are two exceptions to this trend; near the Auckland Gasworks the beds strike approximately N-S and dip W, and at Pt. Erin the beds have been folded about a N-plunging fold axis.

FAULTING

Faults in the area can be divided into four types: - normal and reverse separ­ation faults, strike slip faults, bedding plane faults and faults (sub) parallel to the bedding.

A nomenclature based on separation (i.e. offset seen in one given plane of observation) has been used instead of terms like "s l ip" and "normal fault", to emphasise the fact that what can be seen in one cliff-section, often does not give a real indication of the actual movement along the fault. This is well known in theory, but often overlooked in practice. It is becoming increasingly evident that some of the so-called small scale normal faults within the Waitemata sediments have in fact a strike slip component of movement. Unfortunately it has not been possible to determine the net slip of any of the faults in the area studied.

Definitions.

Separation is used to denote the sense or amount of movement between the surface of the two parts of a disrupted bed, vein or any recognizable surface measured in any indicated direction.

Slip is the relative displacement of formerly adjacent points on opposite sides of the fault measured in the fault surface.

Amongst the measurements which can be made along a fault plane it is possible to distinguish the true dip separation (the distance between offset parts of one marker plane measured along the dip line of the fault) and apparent dip separation (the offset measured in any other direction on the fault plane). Given the cliff exposures in the area studied, it is the latter measurement that has been made.

Normal Separation Faults.

Those faults in which for the section given, the hanging wall has apparently moved down with respect to the footwall.

A fault zone usually consists of numerous shear surfaces which branch profusely to give an anastomosing pattern (Figs. 7 and 8). The thickness of individual shear surface varies from 0.5 mm up to several millimetres, with an average of about 1 mm. Fault zones can be over 5 cm in thickness but are

131

Shelly Beach Rd> Parnell Grit

W

Fig. 3a. Cross section between Shelly Beach and west of Pt. Erin (after Park 1855).

Pt. E r i n ^ ^ Shelly Beach Pt

^ ^ ^ ^ / / / # / ^ W E

Fig. 3b. Cross section between Shelly Beach Point and Pt. Erin (after Mulgan 1901).

Pt. Erin .concealed cliff section

St. Marys Bay Gas Works

(Pt. Acheron)

S E

Fig 3c. Cross section between Pt. Erin and Gas Works based on present outcrops.

generally thinner. Thin sections across the shear surfaces reveal a higher proportion of mud within the fault plane than in the host sandstone and often the fault plane(s) are injected with more mudstone in the vicinity of a fault mudstone intersection. The mudstone may be several centimetres thinner as a result of this depletion. The cause of the movement must be disturbance of the clay structure (e.g. shock) during fracture.

In the field the shear surfaces may be limonitized and the difference in composition is revealed in the preferential erosion of the sandstones with respect to the fault planes. In Fig. 7. the numerous shear surfaces remain mostly sub-parallel to one another, whereas in Fig. 8. there are two types of surfaces; those more or less parallel to the main fault (Set A ) and those antithetic (showing opposite sense of movement) to it (Set B), indicating the tensional nature of this type of fault.

On a larger scale the normal separation faults may diverge and unite again to

132

Fig. 4. Bedding orientation in Pt. Erin-Gas Works area.

form shear packets. If this is the case, one fault is always subordinate to the other with respect to the amount of movement. Beds within a shear packet may be contorted into a sigmoidal drag structure by movement along the fault plane. Fig. 9. not only shows typical splaying of the fault, but also provides evidence of some possible movement of sandy material, within the splays, at right angles to the dip (and possibly the movement direction) of the fault, for the distance C D is approximately 0.4 m shorter than A B . There is some drag of the hanging wall beds against the fault and this may indicate that the hanging wall block was relatively the more active during faulting. Generally however, with this type of faulting, beds show no drag. Sometimes fault splays never reunite and the move­ment is taken up along a separate, but more or less parallel fault (Fig. 10).

Sandstone and mudstone dikes have been injected along the fault zones and in Fig. 11 a dike with a maximum length of 1 m and thickness of 20 cm, consisting of mudstone and sandstone, both of which have lamination mainly parallel to the dike wall , has been injected upwards along a fault zone.

Bedding plane anisotropy played an important role during and after faulting. The anastomosing fault pattern through the sandstone nearly always coalesce to cut a mudstone bed as a single sharp fracture (Fig. 12). When the fault enters the next sandstone bed it again branches. The fault surface is slightly polished within the mudstone bed, but not within the sandstone. Many small scale faults termin­ate under mudstone beds and in Fig. 13 there has also been rotation of the sand-

Fig. 5. Conjugate reverse separation faults.

Fig. 6. Detail of small scale normal separation fault.

135

Fig. 10. Movement along two separate, but parallel, normal separation faults.

Fig.8. Normal separation fault zone showing shear surfaces parallel to the main fault (set A) and shear surfaces antithetic to it (set B). Drawn from photograph.

Fig. 11. Sandstone - mudstone dike along a normal separation fault.

136

Fig. 13. Rotation of sandstone blocks between nuiiii.il separation faults, which terminate beneath a . mudstone bed.

Fig. 12. Normal separation faults terminating and/or coalescing beneath a a mudstone bed.

stone blocks between the small scale faults. Some faults have been displaced along the bedding. It appears in Fig. 6 that the fault has propagated upwards and on reaching the mudstone - sandstone bedding plane, ceased, whereupon some of the movement was taken up along the bedding, and after a certain stress build-up, the fault was again initiated, a few centimetres along the bedding plane. Alternatively the bedding plane may have formed a conjugate couple with the fault. Several examples exist where the fault appears to have been displaced up to 12-15 cm by movement along the bedding.

These examples all indicate the marked anisotropy of the bedding during faulting and the relative competence of the mudstone in comparison with the sandstone at that time.

Associated with normal separation faults are two other types of minor fault­ing. Reverse faults have been observed occasionally, sometimes with associated drag of the bedding near the fault plane (Fig. 14). This figure also shows the nature of the anastomosing pattern of the fault. The distance A B is shorter than C D by nearly 50%, indicating that the movement has been taken up within the sandstone bed, probably in a direction oblique to the fault dip. The second type of faults are those antithetic to the major fault. These generally have a normal separation (Figs. 6, 8, 12 and 14) and are conjugate to the main fault, very often forming small graben-like structures within the acute conjugate angle. The resulting localized compression within a graben-structure is shown in Fig. 6.

Fig. 14. Reverse separation faults associated with normal separation faults.

137

Fig. 15. Fault orientation in Pt. Erin - Gas Works area.

The normal separation faults show a marked NW-SE trend and a subordinate WSW-ENE trend (Fig. 16). Generally the faults trend more or less parallel to the strike direction of the bed (as evident from the near horizontal NNW-WNW bedding plane fault intersections) but with a subordinate trend parallel to the dip direction of the beds.

Table 1. Conjugate Normal Separation Faults Analysis:

Conjugate Fault

Conjugate Angle Between Conjugate Fault Dihedral Bedding Stress Direction Conjugate Fault Angle And a 2 a2 0-3

076/47N 75 36 00/84 258/02 168/06 080/58N 080/58S 90 04 044/50 230/40 138/03 014/55W 082/79N 48 14 189/86 082/04 350/12 078/53S 060/64NW 83 12 205/56 040/35 306/08 006/5 IE 107/50N 44 32 143/39 306/50 047/08 154/70SW 154/70SW 74 14 116/53 320/34 222/1 1 118/83N

139

Fig.17. Stress orientation in Pt. Erin - Gas Works area. Numbers 1, 2 and 9 deduced from folds; numbers 3 to 8 from conjugate normal separation faults. Inset shows principal of stress derivation from conjugate normal separation faults.

The conjugate shear angles, the three principal stresses and the angle between the bedding plane poles and a2, have been determined from the conjugate normal separation fault couples, and are presented in Table 1. It is evident that the conjugate intersection angle is unusually large for this type of fault, indicating that the angle of internal friction was in some cases very small. The angle between the bedding plane and a2, indicates that was often orientated approximately parallel to the bedding. The stress distribution is shown in Fig. 17 Nos. 3-8. There are two marked symmetry planes of stress, trending NW-SE and NE-SW, and a subordinate trend at 4 5 ° to these. Fig. 17 indicates that the principal stresses may interchange with one another, while the overall symmetry remains unchanged, i.e. a2 assumes the direction of a 3 and vice versa. In two cases CTi is vertical, while in the other four cases it is up to 5 1 ° from vertical. This, in some cases, may indicate that the normal separation faults are not purely dip slip faults but, more likely, are oblique slip faults. Two such cases are:

1. When a2 is not horizontal and lies, far from the bedding. In this case faulting may predate or postdate tilting of the bedding.

2. When a2 is (sub) parallel to the bedding, but not horizontal. Here the bedding plane anisotropy is marked, for a2 is channelled into the bedding and in this case faulting would postdate tilting of the bedding. However it is uncertain whether normal separation faults predate and/or postdate tilting.

140

Reverse Separation Faults.

Within the Pt. Erin - Gasworks area reverse separation faults are relatively uncommon and of small scale, the average apparent dip separation being 2-5 cm. It is suspected that there is a large thrust behind the Gasworks that could have a throw of many metres. The reverse separation faults differ from the normal separation faults for they arc more planar and do not form an anastomosing fault pattern. However both fault types contain a greater amount of mud within the fault plane than the host sandstone.

Bedding plane anisotropy has also been important during reverse separation fault formation. It has been observed that as the reverse separation fault (thrust) enters an interbedded mudstone or carbonaceous layer it increases in dip and assumes a lower angle of dip within the succeeding sandstone bed (Fig. 5). Folding of the beds adjacent to the thrust is probably part of the deformation prior to the thrusting, rather than due to drag of the beds caused by movement along the thrust. Within the sandstone beds the thrusts form dense networks of conjugate couples which mutually offset each other and are therefore coeval.

The reverse separation fault plane, in one case, has provided the initial plane of weakness along which a dike has been intruded (Fig. 21). The dike has a similar trend to nearby thrusts, i.e. 065°-35NW to 057°-5 1NW, and is therefore likely to have been intruded during or shortly after thrusting. Small scale conjugate thrusts adjacent to the base of the dike attest to the more or less horizontal compressive stresses at this time. Two factors suggest that the dike was introduced from below:

1. There appears to have been more disturbance at the bottom than at the top of the dike, which is also narrower than the base.

2. A carbonaceous layer has been displaced upwards from its original position.

The strike of the thrusts is E to E N E and the dip N to NNW (Fig. 14), while the concealed fault at the Gasworks is thought to strike N-S. Assuming the faults have little or no strike slip component, this trend indicates a compression from the NW quadrant.

Bedding Plane Faults and those parallel, sub-parallel and highly acute to the bedding.

Bedding plane faults are situated on top of the interbedded mudstone beds, which are usually about 10 cm thick. The faults are marked by a very clean cut fracture which is easily exposed.

At Pt. Erin there arc two certain bedding plane faults and possibly consider­ably more. Sandstone usually lies on top of the bedding plane fault, although occasionally there are packets of mudstone immediately above the fault plane. This mudstone has been highly contorted internally and probably represents packets of sheared off mudstone, formed during movement along the bedding plane fault. These faults splay at their northern extremity, where they also become slightly steeper than the bedding, although retaining the same strike. On the lower bedding plane fault there is a four metre long shear packet and the bedding relationships within the shear packet indicate that the movement along

141

I ig.18. P and T shears related to bedding plane Fig. 19. Low angle reverse separation fault movement. Drawn from photograph. f a u ) t s ( P shears) displacing carbon­

aceous ripple horizon. Drawn from photograph.

"* ,

fold ax,s 06F/W

Fig.20. Road outcrop at Pt. Erin Park. Drawn from photograph.

the fault is not reverse. The bedding plane faults were probably formed during the later phase of folding, with movement towards the trough of the syncline.

Other bedding plane faults to the E postdate the formation of the normal separation faults, which they displace by a few centimetres.

Beds folded by the Pt. Erin syncline show a heavy concentration of faults sub-parallel to highly acute with the bedding. These faults, restricted to within a sandstone bed, are concentrated within the middle of a bed (Fig. 18), and branch profusely having mutual offsets with vertical faults. In the section viewed in Fig. 18 the faults at a very acute angle to the bedding are possibly P shears (derived from the nomenclature of Tchalenko et al — 1970) while the vertical faults may be tension faults. These would have formed in response to a left lateral shearing couple along the bedding plane faults; this sense of movement is in accord with movement towards the syncline trough. The left lateral displacement of large ripples in a carbonaceous horizon within the syncline (Fig. 19), indicate that these faults, at an acute angle to the bedding, are P shears. Development of a subordinate shear set with displacement in the opposite sense (right lateral) is also shown in Fig. 19.

Strike Slip Faults.

At the Pt. Erin Park outcrop (Figs. 20 and 22) several faults with a strike slip component trend 166°-175° and dip 2 1 ° - 4 4 ° W. The faults have an average

142

N sandstone S

20cm : v ^^Slsmall-scaie conjugate thrusts

0 10 cm Fig.22. Stike slip faults in near vertical beds. Drawn from photograph. Fig.21. Sandstone dike along reverse

separation fault. Drawn from photograph.

right lateral separation of 10 cm, and as the bedding pitches 8 5 ° - 8 0 ° on the fault plane, this separation must represent the amount of the strike slip compon­ent of movement.

Fault Relationships.

The strike slip and reverse separation faults arc thought to predate normal separation and bedding plane faults, and the normal separation faults may predate and/or post date bedding plane faults.

The normal separation faults formed in response to a (sub) vertical P load during and perhaps after compaction. The strike slip and reverse separation faults are related to the larger scale gravity controlled slumping, while the bedding plane faults are related to the later folding.

JOINTING

Although no specific joint analysis has been carried out it was noted that the joints were usually filled with limonite up to several cm thick and that most joints were restricted to within one sandstone bed, some restricted to several sandstones beds and a few extended many metres in a vertical section. There is a certain amount of parallelism of joints and faults especially where joints form a conjugate relationship to a fault (Fig. 6).

SMALL SCALE INTRAFORMATIONAL SLUMPING.

Two intraformational slump zones are exposed behind the Gasworks, the 3 m thick uppermost intraformational slump terminates rapidly towards the N E , and and lies 6.5 m above the 2.4 m thick lower slump which terminates rapidly towards the SE, so that within a few metres the slump zones become regularly bedded units. A l l structures within the slump terminate at an upper and lower decollement surfaces, which is parallel to the bedding.

Folds within the intraformational slumps vary from recumbent to steeply inclined and within the upper slump imbrication of sandstone slabs was observed. Fold axes of 171° / 00 and 3 4 6 ° / 05 were recorded from the intraformational slump zone and these parallel the large recumbent anticline (and suspected thrust) behind the Gasworks to the N E . The lower slump has been folded about this structure. As well as recumbent folds there are normal separation faults associated with the intraformational slump. A few of the latter have been recorded

143

conjugate no rma l separation faults

Fig.23. Large scale slump at Pt. Acheron. Inset shows dextrally rotated rhombo-hedral-, shaped boudins. Drawn from photograph.

within the slump, but most are located adjacent to the slump termination. These moderately low angle normal separation faults, e.g. 131° /39 N E form conjugate couples and displace the more competent mudstone beds more than the sand­stone beds, which are able to take up the fault movement within the bed. These faults indicate extension along the bedding and probably represent extensional faults at the proximal end of the intraformational slump.

L A R G E S C A L E S L U M P I N G

The large E verging, inclined to recumbent anticline behind the Gasworks may be related to the slumping. The fold axis trends 172° , parallel and coincident with the slump folds exposed just to the E of the H .M.N.Z .S . Ngapona Head­quarters, although these two outcrops probably do not represent the same slump. The slump near the H . M . N . Z . S . Ngapona Headquarters (Fig. 23) has folds verging in the opposite direction (i.e. to the W) and has an approximate thickness of 16 m. Near the base of the slump the slump beds have undergone extension to form four almost rhombohedral-shaped boudins, which have been rotated due a right lateral shear couple. Beneath the slump the regularly dipping beds have been faulted by conjugate normal faults, which probably formed due to loading of the slumped beds above. The large fault on the left of Fig. 23 displaces the slump some 3 m and other approximately N-S faults of unknown displacement are seen within the slump. The fold vergence. boudinage and bedding orientation indicate that the movement was from E to W.

Another slump horizon occurs halfway between the Gasworks and Pt. Erin (Fig. 24) and is probably younger than the slump at the Gasworks, unless repet-

l ig .24. Large scale slump between Shelly Beach Rd. and St. Marys Rd. Drawn from photograph.

W

^ • v zones of l i que fac t i on R e d d i n g 060°/50NW

/ faul /o76*/53S / t h r u s t s 08Cf'36N

144

it ion of one horizon on a fault has occurred. There arc many differences between the two slumps:

(a) The slump plane in Fig. 24 is very irregular and cuts across the bedding, while in Fig. 23 it is regular and parallel to the bedding.

(b) The slump beds in Fig. 24 are faulted by high angle reverse separation faults, while those in Fig. 23 are boudinaged. However further west near the H .M.N.Z .S . Ngapona Headquarters an axial plane thrust and a conjugate high angle reverse fault have developed in a recumbent anticline-syncline couple. The conjugate thrusts formed during folding, when the beds were fairly mobile, for their conjugate angle is 8 8 ° . Such a large angle may in part be due to further compression and movement after fault formation.

(c) There are numerous small scale thrusts beneath the slump in Fig. 24, while in Fig. 23 conjugate normal separation faulting is present.

(d) There are two zones of sandstone liquefaction beneath the slump in Fig. 24. This is probably a response to the sudden loading of the slump. Two sandstone beds beneath the slump show boudinage, which is due to the loading of the slump and the resulting flowage of the sandstone material parallel to the bedding. None of these features are found in the other slump.

Both slumps indicate movement from E-SE to W-NW and that the cliff expos­ure shown in Fig. 23 represents a more proximal part of a submarine slump, while the slump in Fig. 24 represents a more distal part of a slump. If the beds in Fig. 24 are returned to the horizontal, the slump surface wil l dip approximately 10-11°E. Either the slump was initiated by tectonic uplift of the sediments, and thus the beds were dipping north before the slumping, or as is more likely, the exposure represents an erosion hollow formed by the distal part of the slump. This is supported by the fact that the basal decollement begins to rise again to the west in Fig. 24.

At the Pt. Erin Road outcrop (Fig. 20) a volcaniclastic grit is underlain by a sequence of folded and overturned beds. Directly beneath the grit the sandstones are riddled with fractures which form a complex network, intersecting and mutually offsetting one another. Immediately beneath the grit the sandstones show a marked variation in thickness, while the mudstones have been injected into the sandstones along fracture planes. The overturned beds further beneath the grit are more regular in thickness. Three types of faulting are present within the folded beds:

1. High and low angle reverse separation faults arc associated with the general NW to SE movement of the beds beneath the grit. The faults also strike parallel to the overturned beds. Sandstone liquefaction during slumping cause sandstone dikes to be intruded along these faults.

2. High angle reverse faults due to extension parallel to the bedding in near vertical sequences.

3. Strike slip faulting on the steeply dipping overturned beds (Fig. 22). These faults were probably due to the continuance of the S-SE movement during and

145

structural data in Pt. Erin Park area.

after the formation of the fold. Their strikes are parallel to this movement direction.

The structural relationships within the Pt. Erin Park area are represented in Fig. 25.

The cause of the folding is unknown for certain, but two possible causes are:

(a) The grit lahar may have folded and overturned the beds on emplacement. This is unlikely due to the large local scale of the folding.

(b) The grit may have been emplaced, partially solidified and then underwent gravity gliding.

Two other factors must be kept in mind. Firstly the fold axis may not represent the kinematic b axis and therefore movement was probably not at right angles to the fold axis (Fig. 17 No. 1). Secondly, all beds beneath the grit have been refolded about the N-plunging Pt. Erin syncline.

POST-SLUMP F O L D I N G

The regional dip to the NNW of the Waitemata strata is probably related to this phase of folding. A n inclined syncline is exposed in the cliffs around Pt. Erin Park, and has a trend of 0 1 2 ° / 0 8 . Previous workers (Park, Fox and Mulgan), have stated that the near vertical eastern limb of the syncline is exposed at Shelly Beach. This cliff section is now concealed by vegetation.

Bedding plane faults and faults subparallel to the bedding are found on the steeply dipping western limb, (Figs. 18 and 19), most movement being towards

\

146

the syncline trough. The cause of the folding is probably of a compressional nature rather than

gravity controlled, and therefore 6{ must have been approximately W-E. (Fig. 17 No . 2). The nature of the folding seems similar to the N-trending anticline exposed on the eastern side of the Mustek Point Peninsula (Chappell 1963).

The Pt. Erin syncline is seen to refold the folded beds beneath the grit and reconstruction of the original fold axis beneath the grit is shown in Fig. 26. Two possibilities exist. One can assume that the fold is a flexural slip fold, (a reason­able assumption given the presence of bedding plane faults) and therefore the angle between the two fold axes remains constant. The earlier fold axis, 0 6 3 ° / 1 0 , is rotated along a small circle about the later fold axis, 0 1 2 ° / 0 8 , until it reaches the horizontal. This is at 0 6 2 ° ("possibility I" Fig. 26). It is more likely that the bed immediately above the grit was more or less horizontal initially. Unfold­ing of the earlier fold axis on this basis gives a new trend of 229/26 ("possibility II" — Fig. 26 ), which has an unusually steep plunge, but the NE-SW trend is still maintained. It is unlikely that the movement of the grit was at right angles to the fold axis.

SYNOPSIS O F T H E D E F O R M A T I O N A L H I S T O R Y

The intraformational slumping probably immediately precedes the large scale slumping which is thought to be penecontemporaneous, for the intraformational slump, in one case, is seen to be folded within a large fold which is probably

147

related to the large scale slumping. This indicates that the bedding plane anisotropy is well developed shortly after the deposition, as otherwise the upper and lower decollement planes of the intraformational slumping would not follow the bedding as rigidly.

Reverse separation faults are known to have developed before and after the larger penecontemporaneous slumping. These faults predate the normal separ­ation faults, except when the normal separation faults are associated with localised extension within, or at the proximal end of the slump mass. In this case reverse and normal separation faults are likely to be coeval. Again, in both types of faulting bedding plane anisotropy is marked. The large dihedral angles of the conjugate faults opposite ai indicate that there may have been a very low angle of internal friction and that the normal separation faults formed at an early stage, when the sediments were fairly mobile. However rotation of the shear planes during compaction wil l increase the dihedral angle between the conjugate faults and therefore the angle of internal friction may have been larger during faulting. Bedding plane anisotropy may also affect the dihedral angle between the conjugate faults. This is supported by the morphology of these faults within the sandstones. Nevertheless normal separation faults are seen to displace large slump horizons.

The strike slip faults within the overturned beds at Pt. Erin, probably formed during folding associated with movement of the grit which lies immediately above.

A later phase of tectonic folding with associated bedding plane faulting, refolded the earlier folds occurring beneath the grit at Pt. Erin, although some of the normal separation faults probably post-date this folding.

1 wish to thank Dr K . B . Sporli for reading the manuscript and together with Dr P .F . Ballance, for many helpful comments. Thanks are also due to the Auckland Harbour Board for providing the field maps used in this study, and to Miss A . Donaldson for typing the manuscript.

A C K N O W L E D G E M E N T S

R E F E R E N C E S

Chapelt, J. 1963 The Geology of the Musick Point - Bucklands Beach Area. Tane 9 : 33-9.

Fox, C .E . 1901 The Volcanic Beds of the Waitemata Series. Trans. R. Soc. N.Z., 34: 452-93

Mulgan, E . K . 1901 On the Volcanic Grits and Ash Beds in the Waitemata Series. Trans. R. Soc. N.Z., i4 .414-35.

Park, J. 1885 Waitemata, Eden and Manukau Counties. N.Z. Geol. Surv. Rep. Geol. Exp. 17; 1885-1866 : 147-64.

Taylor, U . K . , Tomkin, R . M

1960 The Construction of the Approaches to the Auckland Harbour Bridge, (unpubl.)

148

Tchalenko, J.S., 1970 Structural Analysis of the Sasht-e Bayaz (Iran) Abraseys, N . N . Earthquake E ractures.

Bull. Am. Geol. Sot: SI : 41-60.