Slump Structures in the Waitemata Beds Around Auckland
[Road before the Auckland Institute, September 13, 1949; received by the Editor, September 27, 1949.]
The paper amplifies the evidence for the frequent occurrence of submarine slumping during the deposition of the Waitemata beds.
The hypothesis that local complex folding on a larger scale which characterizes the formation is likewise due to submarine sliding is put forward as an alternative to the current tectonic explanation.
The suggestion is made that beds of andesitic grit (“Parnell Grit”) included in the sequence may be subaqueous lahar deposits.
Many years ago Turner and Bartrum (1929) pointed to the frequent occurrence of intraformational dislocations and folds in the Waitemata beds in the neighbourhood of Auckland. One example especially, south of Takapuna Beach, is figured and described as a complexly corrugated bed 2 ft. 6 in. in depth. “The probable cause is subaqueous gliding of delta-beds down the slope of the delta, when growth has caused over-loading…” (p. 882). The deformations on a larger scale shown by the Waitemata beds are attributed by them to a “moderately intense compressional force acting from a south-western direction, which has given origin to thrusts representing a special phase of the same post-Waitemata orogeny as is evidenced by the major faults near Papakura, sub-horizontal movements having temporarily superseded vertical” (p. 882). Another excellent example of “delta slumping” is well shown on one of Searle's (1944) photographs, but the deformation in this case is much less intense.
It is the purpose of the present paper to amplify the evidence for slumping and to offer an alternative to the tectonic explanation of the larger structures, namely, the working hypothesis that they may be likewise due to contemporaneous sliding. The origin and nature of the Parnell Grit will also be briefly discussed.
The writer had an opportunity of viewing some of these structures on a trip to Titirangi Bay, further at Takapuna, Muriwai, and Milford Beach during the Pacific Science Congress excursions, and at the latter and a few other spots in the company of Mr Battey on a subsequent afternoon. The writer also had the privilege of discussing the evidence with several members of the Congress, and Dr L. Bossard, who has worked in this area and who advocates a tectonic explanation to be mentioned later. It is obvious that the very short time spent in examining the Waitemata beds did not allow more than the gaining of a rough general impression. But I hope the evidence given below and the indicated lines of attack on the problem may stimulate New Zealand geologists to go over the ground in more detail in order to ascertain in what degree contemporaneous deformation may have contributed
tributed to the structure. Once a geologist has become acquainted with the features indicating slumping he soon finds to his surprise how common, and often obvious, they are, even in exposures which have been studied in detail before without their having been noted.
II. Small Scale Slump Structures
The Waitemata beds of the Auckland district consist of a series of thin mudstones intercalated between thicker light coloured sandstones, moderately indurated. Some grits and conglomeratic beds termed Parnell Grit are found locally. To the west, coarse to fine beds of volcanic origin (Waitakere, Manukau Breccia Series) represent the upper Waitemata beds. All rocks mentioned appear to have been deposited in marine environment. The age is Lower Miocene (Altonian Stage). The total thickness of the series is at least 1,200 ft. and they are found lying with small dips over wide areas. But every now and then one meets with exposures showing steep dips and sometimes intense crumpling.
In most exposures careful examination soon reveals small scale slump structures. The contorted beds may be no more than an inch thick, while the greatest thickness observed is some 2–3 feet; the bed at Takapuna already referred to and rightly interpreted by Bartrum as a slump sheet (probably the folded beds figured by Searle are somewhat thicker).
The degree of deformation varies greatly. On the one hand the bedding planes may only be slightly undulating and doubt may be felt whether this feature is not simply ripple marking. As true ripples occur with current bedding in the laminae, it is necessary to find evidence one way or the other for each individual case. Sometimes the minute crumpling of fine laminae in a thin bed, or the obvious pulling apart into separate lenses indicates the post-depositional character of the waved nature of the bedding planes.
On the other hand the beds may show violent crumpling, overturning, concertina folds alternating with steep irregular plications, repeated severing of a bed combined with overthrusting and “boudin-
age”-like forms. Characteristically no general direction of strike is then developed, and vertical sections at right angles are of almost equal complexity. Between these two extremes all degrees of deformation are represented in the slump structures of the area.
A typical example of intermediate intensity from the Waitakere beds is shown in Fig. 1.
In this particular case the explanation of the structure might be sought in squelching upwards of the plastic, dark, fine-grained bed, while the sand above settled into pockets, due to gradual increase of overburden. Although contemporaneous, the structure might have then developed on a horizontal sea floor without any general lateral movement. The cases in which horizontal compression proves that sliding took place, show also that sufficient dip was available during sedimentation for slump movements to occur. Hence the writer is strongly inclined to invoke the same cause for the case of Fig. 1. But during the lateral slipping the mechanism of “overburden squelching” may have been induced, thus greatly adding to the complication of the structure.
In fact, the close parallel between many slump structures and structures developed in superficial deposits during periods of glacial climate (“involutions” and other results of cryoturbation or congeli-turbation, see Bryan, 1946) is probably due in both cases to combination in various degrees of compression and squelching during lateral movement.
The remarkable intensity of the deformation in many slump structures, often resembling turbulent flow, and the absence of a well-developed general direction of strike, is difficult to understand if only compression is invoked. But if the squelching process has added its influence, the area of the bedding planes within the moving mass must have been increased greatly in excess of the original amount. Before the movement started the beds were not much wider than at present, not nearly as wide and thin as would follow from smoothing out. the subsequent undulations. This is directly obvious when one considers a section at right angles to the direction of slip. As already stated, these sections also show intense deformation, while the amount of compression in this plane must have been small or zero.
Deformations are generally attributed to slumping if the contorted beds are underlain and covered by undisturbed strata of the same sedimentary cycle.
The question has been raised more than once, whether this slumping involved only the deformed beds, or whether it occurred under a moderate overburden of sediment which rode down on the deformed mass, as on a lubricant, without being itself distorted. The slumping would then not have been “open-cast” (see, for instance, Challinor, 1949). Some authors even imagine a process of squeezing out from below an overburden, without horizontal movement of the latter. It would be bold to maintain that all slumping has been of the opencast type. But in the opinion of the present author it is certainly the case with the great majority of examples so far described. Thus Jones presented evidence showing the strictly contemporaneous nature
of slumps he described from Wales because they are covered by a graded bed with a welded contact.
A slump sheet in Pembrokeshire in which the irregularities of the surface had been filled in by cross-bedded sands, before the next stratum was deposited, is shown in a paper read by the present author before the Geological Society of London (Kuenen, 1949). A clear example of the same nature is shown in Fig. 2, in which the highly irregular surface of the bed is smoothed off by material of a much darker colour than occurs elsewhere in the section, the following bed is entirely undistorted. Again, in other cases, the crests of anticlines have been cut off cleanly by erosion before the undisturbed cover was deposited.
Fig. 2—Slump sheet of Waitemata sandstone with infilling of the irregular surface by darker material before covering, Takapuna (from photograph).
Other evidence of the open-cast nature of slumping is the absence of drag in the folds directly below the cover of slump sheets. As I already pointed out in the discussion of my paper read before the Geological Society of London, the absence of even slight bending or cracking of thin, brittle, or unconsolidated beds covering slump sheets is entirely against the notion that they could have squeezed away or slid down on the contorted beds below. The development of slump-ball structures is likewise only explicable by open-cast development.
The lateral movement involved in slumping must result, at the higher end, either in thinning out, or pulling apart, of the moving strata. Very few observations of these phenomena have been published. Probably the evidence is in general inconspicuous. It must also tend to be obliterated by erosion during the sedimentary cycle on account of smaller depths or even emersion at the higher end of the slope involved. This aspect should be given special attention in future work on slumping. In the paper already referred to (Kuenen, 1949) a slump sheet in Pembrokeshire is described which shows on one side indications of stretching.
Stretching and pulling apart of minor slump sheets was mentioned above. A small amount of stretching is shown in the exposure at the month of Wairau Creek, Milford (Fig. 3).
IV. The Larger Structures
The author has little doubt in attributing the smaller structures described above to contemporaneous (open-cast) slumping. The larger structures, however, may be partly or entirely of tectonic nature and the interpretation as major slump structures to be given is merely offered as an alternative working hypothesis.
Titirangi Bay. On the headland coast just west of Titirangi Bay there occurs an excellent exposure of the Waitemata beds showing a curious type of structure. In the surroundings the beds are seen to dip slightly but uniformly towards the coast. Apart, from minor faults and small slump structures the general position is remarkably undisturbed. However, at the very tip of the headland the beds suddenly bend abruptly upwards at steep angles and are even locally overturned. The thickness of the mass involved is perhaps a few dozen feet. But the most striking feature is to be seen on the erosion platform fronting the sea cliff. Here the strike of the beds bands round in loops and S-shaped curves with a span measuring a few to many feet. These ares overlap and interfinger in complicated fashion, reminding one of the swash mark pattern drawn by irregular waves on a beach. Searle's photograph 2, Plate II. shows the same type of structure at Blockhouse Bay. On both sides at Titirangi. in the strike of the distorted area, the beds are seen to lie undisturbed. Right in the centre of the largest are a bed of a foot thick is found over a distance of some dozens of feet, displaying as fine and complicated slump structures as are to be met with anywhere.
Unfortunately, the light was not favourable for photography at the time of my visit and the few minutes available were too short for making sketches. The accompanying drawings (Fig. 4) are therefore only diagrammatic, drawn from memory with the aid of some snapshots. They will serve the purpose, however, of giving a rough impression of the type of structure.
The grounds for suggesting with some confidence that these structures are due to sliding are as follows. The small scale slumps of the neighbourhood demonstrate that there was sufficient surface slope available at the time of deposition to cause lateral stress. The deformation is restricted to a small area, not only in section, but, even more significantly, also in strike. While it is hard to believe that the poorly consolidated and thin series of beds could have transmitted tectonic stress over great distances so as to cause intense compression at one point of the section only, it is even more difficult to explain how sections directly alongside could have escaped deformation. Finally, the are-shaped snouts of the disturbed parts with their upthrust beds testify to differences in amount and direction of movement at very small distances apart. The surfaces along which the moving parts slid were spoon-shaped.
These features are of a type which has not yet been described, but they fit the mechanism of slumping admirably. In the case of tectonic action neighbouring sections of the crust must be shortened the same amount. The stress is equal and parallel in all parts of the sections, the beds are consolidated and by their competency a rising anticline in one section tends to pull the adjoining parts of flanking sections up with it. In the case of slumping the stress is largest at the lower end, where the greatest length of section presses down along the slope. Here maximum deformation can be localised. The sliding mass is as yet unconsolidated and hence incompetent. Therefore each section can be deformed more or less independently. The greater amount of forward movement exhibited by the centre of each are is rendered possible by the plastic nature of the sliding mass and of its substratum. The similarity between the distortions described and those occurring along the snout of some glaciers (Spitzbergen) is not merely a coincidence, but due to a close parallel in mechanics.
Milford. The structures north of Milford, along with others especially on Whangaparaoa Peninsula further north, have been described
and illustrated in some detail by Turner and Bartrum. “The base of the visible section shows that the beds below the lowest thrust ‘sole’ have been crumpled and overturned by frictional drag. Above them there are at least two thin lensoid overthrust sheets, followed by a much thicker one with a minimum thickness of at least 30 ft. The edges of its strata are indragged in most interesting fashion at the thrust-plane, and it is interrupted by two narrow zones from 15 ft. to 25 ft. in width in which sub-vertical beds are entangled” (p. 880).
After rejecting other tectonic explanations, they accept one suggested by Dr Leon Bossard. “This is that the sharply-tilted strata represent beds forced against a resistant foreland at the toe, somewhat arcuate in plan, of an overthrust mass advancing approximately from the south-west. This latter was compelled to develop temporarily sub-vertical reverse faults, and then to over-ride the obstructing mass at higher levels. Particular support is afforded this hypothesis by the fact that the over-ridden beds include especially rigid strata in a band of Parnell Grit approximately 15 ft. in thickness, ‘horses’ of which have been included in one of the zones of disturbance…elsewhere the Parnell Grit frequently provides a resistant mass upon which the weaker normal facies of the Waitemata Series are overthrust or piled up in close folds” (p. 880).
Nearly all of the above citation can be accepted when attributing the structures to slumping. We only need to replace “overthrust” by “slumped mass.” The process suggested is similar to that described by Jones (1940) from Wales: “It is clear in several exposures, that at certain places the mass failed to move the underlying sediments to the depth that prevailed farther up the slope; at those places it mounted the obstacle in a ‘leap-frog’ “(p. 370).
However, there are several features which render support to the explanation by gravity sliding.
The structures at Milford are much like those at Titirangi Bay, but on a larger scale and more intensely compressed; the “swash mark” -structure is widely represented. The evidence is strongly in favour of attributing the Titirangi structures to slumping and much of it can be applied with equal force to the larger structures.
In the writer's (admittedly limited) experience of tectonic thrust sheets there may occur a mylonite at the sole of a nappe, but the amount of deformation and drag folding in the over-ridden rocks is very slight as compared to that of the thrust mass. Here, at Milford, however, the deformation quickly rises in intensity from zero as one approaches the thrusts. These are poorly marked because below them there is almost equal complexity of structure as above, while the extent is hardly smaller. Anything comparable to mylonite is absent.
The writer found no true slickensides, although one would expect them in abundance in such intensely distorted sandstones and shales, had these been crumpled under a heavy overburden after consolidation. Before consolidation tectonic thrust could not have been transmitted. The planes on which sliding is thought to have occurred may show smoothing, resembling slickensiding, but the
writer is unable to say whether this feature occurs at Milford. True slickensides, however, should be found on bedding planes and cracks throughout the intensely warped beds if the deformation were tectonic.
The remarkable intensity of the crumpling, coupled with the no less astonishing variability in strike, testifies to the great plasticity of the beds during the deformation.
Turner and Bartrum suppose, as quoted above, that sub-horizontal movements temporarily superseded the normally vertical displacements of a post-Waitemata orogeny. This is not a very attractive hypothesis. But the improbability is even greater, that a thin sheet or poorly consolidated sandstone was pushed over beds of the same composition without any internal deformation, the sheet measuring many miles in the direction of movement. No “traineau écraseur” was available, no nappe pushing from the rear. How, then, was the tectonic thrust localised in the upper sheet of Waitemata beds? This difficulty vanishes when it is assumed that the sheet slid down on a slight slope merely under the action of gravity. The angle need not have been more than a couple of degrees, but may have been more.
The Parnell Grits comprise fine-grained volcanic grits and tuffs, and in some places coarse-grained breccias and conglomerates. These crop out here and there and appear to have a random distribution. At Milford the writer observed that there is more or less pronounced grading from coarser to finer upwards in some bands. The lower contact with normal Waitemata beds is irregular (Fig. 5) and in the grits bedding planes also tend to be irregular, both features indicating gullying. Moreover, large and small masses of normal sandstones are found incorporated in the grits. Beds of the latter may end abruptly, continuing as suddenly again many yards away, while isolated portions are highly warped. Chunks of shale are scattered through some parts of the grit.
Each individual detail is hard to account for, but at any rate the entire composition is chaotic. The great irregularity of the lower and internal contacts, and the poorly consolidated nature of the large fragments exclude the possibility that the grit is a beach formation. Neither can direct deposition on the sea floor by eruptions be assumed. because in that case the beds should have come to lie on a smooth
bedding plane while the inclusions would be difficult to explain. Hence the writer ventures to suggest that the grits were brought along by mud flows from a distance. These flows developed sufficient erosive power to cut a gully in the freshly deposited substratum of Waitemata beds. The mud flows are thought to have originated subaerially as “lahars” on the slopes of the volcano (es?) producing the andesite material and to have travelled onwards along the sea floor after reaching the coast. If this mode of origin is accepted, the observed features can be explained more readily than by production from local vents, as suggested by Turner and Bartrum. In the case of mud flows, it should be found that the Parnell Grit tends to occur in elongated strips pointing towards the volcanic area from which the material is supposed to have been carried by the lahars.
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— 1947. The Geology of the Silurian Rocks West and South of the Carneddau Range. Radnorshire. Quart. J. Geol. Soc. London, vol. 103, pp. 1–36.
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Turner, F. J., and Bartrum, J. A., 1929. The Geology of the Takapuna-Silvered District, Waitemata County, Auckland. Trans. N.Z. Inst., vol. 59, pp. 864–902.