Conditions of Deposition
Because of the regional distribution and regular character of the Amuri, Weka Pass, and Mungaroa Limestone and associated formations like the Whangai Shale, it is widely accepted that from Upper Mata to approximately Landon times the East Coast of New Zealand was a region of relatively quiet and stable conditions (Macpherson 1946, p. 13, and Lillie 1953, p. 75). The calcareous and muddy oozes and glauconitic sands of those times were deposited in an open sea free of coarse terrigenous detritus and were derived from a post-mature or even senile landscape. Although the source area was aged there is little doubt that some small movement occurred, and it appears probable that these were responsible for the initial slumping and consequent redeposition of the Tertiary formations and to some extent for the origin of the Whangai Shale. The siliceous clays of the latter are exceptional rock types and are only similar within the writers' knowledge to the siliceous seatearths below coal seams. Because of this it is reasonable to support that the Whangai Shale was derived by the stripping of the regolith and shallow basin deposits of an uplifted senile landscape long clothed with vegetation. The general deficiency of iron oxides and alumina in the formation indicates that the landscape was not simply lateritised, and the idea of a long-enduring vegetation explains how soils might be leached and the rocks derived from them be relatively enriched in silica.
This suggestion is complementary to Lillie's (1953, p. 78) that the landscape of origin should be “much denuded of vegetation” and is not inconsistent with his observation that the Whangai contains little organic matter. It is possible that a low country might be clothed for a long while by grasses, or scrub, and yet for these to contribute little to rivers or deltas. For comparison of the effects of rejuvenation on such a landscape, the artificial draining of heaths in the Yorkshire Pennines has resulted in the drying out, low temperature oxidation, and disappearance into the atmosphere of peats ten feet thick, and has led to the denudation of their leached underclays, all within the last century.
Following the deposition of the Whangai Shale, conditions were still quiet, but from the northward facies change of the Mungaroa Limestone into the Awhea Formation it is clear that conditions varied and that there was probably land in that direction. The coarser (but still fine) deposits of the Awhea and succeeding formations in this area, while indicating continual and slow emergence of a hinterland, contain little very coarse material such as would suggest a youthfully or maturely dissected land. At most, the deposits indicate restricted phases of uplift or localised uplifts, and appear to be at variance with the idea that the area was undergoing active folding. The characteristic twofold rhythm of sand and clay of the Awhea and other Tertiary formations is related to the terrain from which sediments were derived. Conventionally this cycle would be attributed to oscillation of relative sealevels or to periodic inundations by floods into quiet waters, but a view slightly different from the last is preferred. It is thought that particles of the two very distinct orders of size now constituting the alternating beds were carried by a series of turbid flows, that as these slowed down the coarse material was first deposited, and that when flows halted muds were precipitated.
Marshall (1928, p. 512) has shown by ball-mill experiments that long-continued grinding of rock detritus produces grains of gradually decreasing sizes until a lower limit of sand of 1.1 mm diameter is reached. Thereafter there is a sudden drop in particle size to that of clays and the common end products of milling are fine sands and muds. In nature a similar process aided by chemical weathering operates in the fluviatile and beach re-working of detritus on and around post mature landscapes, and its products are readily recognised in the sands and clays of deltas. Cyclic slumping and redeposition of the sediments of ancient coasts must result in dual rhythms of the type described above, and in general these will be distinct from the.
graded and coarser beds of orogenic areas and periods. In so far as aged landscapes are regional rather than local in extent these deposits may be expected to be regional also, and, as aged landscapes bespeak continuity of conditions the deposits should be only infrequently interrupted by coarser deposits. These characters, as well as the distinct twofold rhythm, are typical of many Lower Tertiary sequences such as the Ihungia of the southern Wairarapa and probably indicate stable conditions in the source area of the material.
This argument may be qualified in several ways. In the first place Marshall's size gradings probably apply to certain rocks only, but the principle of a sudden drop to clay sizes in general. Thus the silty beds of the dual rhythms may fit the argument as well as sands might in Marshall's context. Again the presence of occasional pebbles in the silts does not invalidate the thesis, for there are probably few beaches or deltas which carry no pebbles.
It might be argued that the fine-grained deposits simply represent the farthest travelled and finest material of a turbid flow derived from a young landscape, but the presence of occasional pebbles shows that the carrying agent was capable of transporting coarse material. It is inferred then that there was no sorting of material en route and that the present constitution of the deposits is representative of the source material.
Redeposition of Limestone
The lateral gradation of the Awhea Formation into the Mungaroa Limestone is not merely a passage by interdigitation of beds but is a transition of redeposited silts and clays into limestone. The passage cannot be traced for any individual bed, but beds can be mapped in bulk and all gradations are seen between the facies. The contemporaneous deposition of limestone and argillites was at first interpreted by the writers as meaning that the former originated as a calcareous ooze some distance offshore and that its regular shaly partings represented the farthest travelled of the fine muds of a turbidity current coming from the land. That interpretation, which may be in part correct, was unquestioned until the Manurewa Limestone was found to be graded and to show signs of redeposition. Then the Mungaroa Limestone could be examined as a possibly redeposited formation. As the Awhea Formation passes smoothly from sandy through clayey and then to calcareous sediment it is reasonable to suppose that one gradational mechanism has been at work. The separation of the sandy and muddy fractions of normal graded beds might be functions of distance travelled and of the slowing up of a turbid flow, and the separation of calcareous ooze from fine clays might be similarly governed.
The general scarcity of Foraminifera in the redeposited clays and mudstones of the area suggests that microfossils present in the original sediments may have been separated off during transport in a turbidity current and that their re-concentration as a calcareous ooze might be like that which occurs in the preparatory washing of foraminiferal samples. The large surface-to-weight ratio of microfossils would favour their floating in a turbulent stream, and allow their consequent separation and carriage far beyond the coarser detritus. One might expect, therefore that the Mungaroa Limestone could originate in this fashion and would contain many Foraminifera, but apart from the mixed faunas cited above and a relative abundance of Radiolaria (Mr. P. Vella, pers. comm.) the limestone is poor in microfossils. It may be that the fossils were broken up in transport, but it seems more probable that the concentration of naked calcareous tests and their re-mixing with water allowed a reconstitution and recrystallisation of the carbonate into the very fine-grained and apparently chemically precipitated limestone we now see.
Independent of the above observations, the presence in the limestone of occasional greensand beds is strongly suggestive of redeposition. In terms of conventional hypotheses of deposition these bands suggest rapid and complete changes in the conditions of sedimentation, but in terms of redeposition this need not be so, and a
simpler explanation is found. All that need now be supposed is that during the deposition of the Mungaroa Limestone there was a gentle warping of the sea-bed and that greensand formed on anticlinal highs. Then, as folding continued, the soft sands became unstable and slumped as a turbid flow into the adjacent troughs. The greensands of the Mungaroa Limestone possibly originated on some uplift other than the Coast Range Anticline, but it is also possible that this latter anticline was locally denuded by slumping of its soft oozes, and that its core of soft greensand was exposed. It is shown below that those soft core-rocks were quite capable of being injected into the Mungaroa Limestone some time after its deposition, and it is possible that they were also extruded as a quicksand along the crest of the Coast Range Anticline during the early stages of growth of the fold and during the deposition of the formation.
Whether the greensands were distantly or locally derived, the redeposition theory, allied with the idea of contemporaneous movement, can readily account for them. At the same time, the idea of contemporaneous folding provides us with a further mechanism of origin of redeposited limestones. If the sea floor at any time or place were covered with soft calcareous ooze and were subsequently warped, then slumping and redeposition would occur, and, with progressive folding, it would be possible for a given sediment like the limestone to have been slumped and redeposited more than once.
Summarising, we now have four possible ways in which redeposition might have affected the Mungaroa Limestone.
(a) The limestone may have been a normal ooze deposited on the sea floor, and only the thin clays may have been redeposited.
(b) The limestone may have resulted from the slumping and resorting of a mud and by redeposition of the calcareous fraction.
(c) The limestone may first have been deposited as a calcareous ooze and then have been wholly redeposited.
(d) The limestone may have been redeposited or re-worked several times.
In all this it is not implied that the Amuri Limestone has everywhere been redeposited, nor that any of the above mechanisms can be demonstrated for any particular area. It is claimed that the mechanisms have probably operated singly or in combination at many localities and that redeposition may be suspected in any area of perfectly bedded and slump-folded limestone.
In any area, however, where it is possible for a derived limestone to accumulate there must be a relative abundance of calcareous waste and freedom from other detritus, so that it would also be possible to have a primary limestone alongside that which has been redeposited. Limestones in adjacent areas and of much the same age may, therefore, have the two different origins, and even in one area any one stratum of limestone in a redeposited sequence may be composed of primary and secondary deposits.
Redeposition of Argillites.
Considering the redeposited character of the argillaceous Lower Tertiary strata, it is apparent that the very fine clays comprising large parts of formations can only have been deposited from dispersed suspensions of mud in water, and that, to allow settling, this water must have been more or less stationary The sensitivity of muddy water to gravity and to the slope of the sea floor is such that turbid flows transport sand on gradients of only 5ft per mile (Gould, 1951, p. 51). It seems, then, that redeposited silts and clays must usually be laid down almost horizontally, and that in consequence they must be deposited either on flat broad shelves or on the flat floors of closed basins. The wide extents of the lower Tertiary formations of the East Coast suggest that they were laid down on wide shelves, but there is no doubt that the area was folding during their deposition. Both Macpherson (1946, p. 17)
and Lillie (1953, p. 76) have described in some detail the intermittent growth of folds and contemporaneous erosion of anticlines in the region, and in the Opouawe area the same kind of growth and erosion has been deduced. If erosion were due to emergence of anticlines the basins of deposition between them would be separate and their deposits would tend to be distinctive. Alternatively, and with the same presupposition of emergence of anticlines, any widely distributed formations such as do occur could be explained only by imagining that the region was repeatedly and alternately raised and lowered, or that there was a fluctuation of sea level.
It is conceivable, however, that erosion may have been submarine rather than subaerial, and it will be shown that this concept fits the facts of the Coast Range Anticline particularly well. In that anticline the known stratigraphic evidence of contemporaneous folding and erosion is limited to the one unconformity below the Pukemuri Siltstone; but a study of slumping and redeposition meets the deficiency and can demonstrate both tilting of the sea-bed and the probability of contemporaneous submarine erosion.
Slumping: an Index of Earth Movement.
Slumping of beds is generally attributed to the local accumulation of deposits into irregular unstable piles such as those of deltas, or into thick and lens-like coastwise sediments, and to the triggering off of those piles by earthquake shock. In that view emphasis is on the idea that instability is inherent in the mode of deposition of beds, and few workers have conjectured any other common cause of slumping. Natland and Kuenen (1951, p. 26) give an example from California, where tilting of the sea-bed is held responsible for slumping, but it has not been recognised that tilting is a very frequent cause of submarine landsliding.
So far as the writers' experience goes, deltaic deposits are not inherently unstable, and though they may be thick, as in the coal measures of Britain, they show only trivial slumping in places away from orogenic or epeiric areas (Kuenen's 1948 examples of slumping from South Wales apply to deltaic deposits on the margin of the Hercynian Geosyncline). By contrast, it is in the young sediments of orogenic belts and in particular in redeposited sediments that slumping is common. Now, it has been deduced that very fine-grained redeposited sediments must have been deposited horizontally on a flat sea-floor, and in agreement with Kuenen and Migliorini (1950) it can further be argued that because they are deposited from broad and even flows of muddy water they must have been laid down in layers of even thickness and wide extent. It is hard to conceive of such beds being heaped up into unstable piles and equally difficult to imagine them suffering differential areal compaction. So it appears that, with the possible exception of deposits over supratenuous folds, they can have had no initial dips. In short, if fine-grained beds have been redeposited they must be adjusted to horizontality as delicately as if by a spirit level, and they must have been deposited at the lowest possible level. As they possess no potential energy of position there can be no triggering off of such energy by earthquake or other device, and for slumping of fine-grained redeposited beds to occur there must be some positional energy imparted to them by differential uplift (faulting or tilting) of the sea-bed.1 Contrarywise, we can say that contemporaneous slumping of fine grained redeposited beds is usually clear evidence of contemporaneous earth (basement) movement, and the possibility arises that phases of movement in an area could be charted with reference to dated slump structures. Further, if directions of slumping can be registered it may be possible to record the attitude of the sea-floor at particular times.
Unfortunately there are difficulties in the way of both projects. In the first case, much slumping is not readily dated, for, as will be shown, it may occur long after.
[Footnote] 1 An exception to this rule will be discussed in another paper.
beds are deposited; in the second case, there is no precise relation between directions of slumping and slope of the sea floor. It is readily visualised that in a large slumping mass, as in a landslide, there will be a fanwise spread of material and that along the arcuate margin of the slide fold axes may be oriented through 180°. In addition, it can be demonstrated (p. 544) that even with individual folds the direction of movement of the strata has sometimes been oblique to fold axes. Thus the attitudes of fold axes or other reference directions in slump folds convey little direct information on the azimuth of slumping or of inclination of the sea-bed. The study of time relations is still profitable, however, and in the following pages some attempt at interpreting slump structures in terms of time and other phenomena is given.
Piripauan Sandstone Slumping.
The Piripauan Sandstones contain slumped balls of the type shown in Pl. 33, Fig. 5, and these have clearly rolled down a sloping sea-bed, to be enclosed in softer and now eroded muds. As all of the beds in question are redeposited types, it is probable that they were not inclined on deposition, and as the slump is superficial it cannot have been caused by differential loading. The sea-bed must, therefore, have been tilted for this structure to be produced.
The sandstones also contain beds of ill-sorted and slabby conglomerate or breccia which have slid and ploughed into the underlying strata in a fashion suggesting that at the time of their emplacement they formed a submarine landslide or avalanche of mud and pebbles. Such a mass (Pl. 34, Fig. 6) consists of exotic pebbles and of slabs of Piripauan sandstone and mudstone up to a foot square and four inches thick. The commonly stratified appearance of the mass and the regular orientation of the slabs are very much like those of a locally derived scree of Piripauan sediment and are unlike typical ill-sorted conglomerates of redeposited origin. On the other hand, the presence of exotic pebbles suggests a distant origin and transportation by turbidity currents. In places the relation of the breccia to the underlying beds is markedly discordant in an overstepping fashion (Pl. 34, Fig. 3), and their angle of deposition must have been that of a talus. While this is inconsistent with deposition from a mudflow, it cannot be imagined that the scree was derived by subaerial agents or by the action of waves along the shore of an emerging island. The sandstones were probably far too soft to be broken into blocks by waves, but were probably derived as slump boudins or “pull-aparts” (Natland and Kuenen 1951, p. 91) and the exotic pebbles were probably re-derived from the Piripauan sandstones. A possible cause of slumping is shown in Fig. 13, which depicts a submarine fault in soft sediments and illustrates how the soft beds may have slid and ploughed into the adjacent strata.
Manurewa Formation Slumping.
There are few signs of slumping in the Manurewa Formation, but the disconformities that have been described above are suggestive of erosion and deposition as each member was formed. In view of the great extent and the horizontality of the sea-floor during deposition of fine-grained redeposited beds, it is difficult to see how submarine currents could be sufficiently concentrated to cause even a local scouring. One should expect rather that currents would be broad and slow, but the fact remains that in the Manurewa Formation there are repeated signs of redeposition alongside those of erosion.
To explain this we imagine an ideal case of a Pacific-type coastline where the structure of the sea-bed might comprise many growing folds with axes parallel to the coast and with their crests at greater and greater depth outwards into the ocean. Then turbidity currents originating near the shore must tend to cascade from higher to lower synclinal troughs of the sea-bed and to fill them with sediment one after the other. In an ideal early stage turbidity currents must be arrested by the first anticlinal high they meet, and their load be deposited in the first synclinal basin,
but as that basin fills the velocity of currents may only be checked and the impetus remain sufficient to carry flows of muddy water over the first submarine swell into the second basin. Just as heavy mists pouring from a high mountain valley to the next lower valley select low saddles in the intervening ridges, so turbid flows must canalise themselves at the lowest sags along the submarine anticlinal highs. As those would be composed of soft sediment, broad channels would be cut across them down to a profile where erosion and deposition were in equilibrium. It must not be imagined that the ridges would be sharp or the eroded channels deep, for the processes of submarine planation are not quite similar to subaerial erosion and aggradation. Here the processes of erosion of soft sediments and of transport and deposition are highly sensitive to slope, and are swift, so that they maintain a very subdued submarine relief and a relatively flat sea-floor.
So far as it concerns the Manurewa Formation this argument is not exclusive, for wave action or tidal flow may also cause erosion, but the process outlined is probably general and of world-wide application, and it explains the disconformities of the Manurewa Formation within the synthesis attempted here.
Mungaroa Limestone Slump Folds.
The slump folds of the Mungaroa Limestone are discussed at length in the third part of this paper. Intense slumping after the deposition of the formation probably coincided with a major tilting and with the formation of the unconformity of the base of the Pukemuri Siltstone. Less intense slumping probably occurred on several later occasions.
Pukemuri Siltstone Slumping.
The angular unconformity at the base of this siltstone is evidence that the formation was deposited after a phase of intensified movement, and the nature of the deposit shows that movement continued for some time. The formation is characterised by involved pseudo-tillites in its lower half and by highly attenuated folds in its upper portion (Pl. 33, Fig. 6). These folds, which are drawn out in the ratio of twenty to one, have such “plastic” forms as to indicate that the clays which comprise them must have been exceedingly soft at the time of folding. There are no signs that the formation was loaded by another formation at the time of slumping, and it is inconceivable that the laminated silts of which it is composed were laid down on a slope. It follows that the beds have been tilted and that the clays have simply “flowed” down an incline. The intense contortion of beds does not mean necessarily that they were involved in any violent sliding, and the fact that the pseudo-tillite remains unmixed with the bulk of the formation suggests that movement was slow and perhaps comparable in its velocity with that of soliflual creep. It is difficult to know whether the anticline had emerged at this stage, but that is not unlikely, and it is certain that the crest had risen at least to wave base and was subject to shallow-water erosion.
Kandahar Formation Slumping.
As this formation is even now very prone to slumping or landsliding, following recent rejuvenation of the landscape, and as it has been folded into an anticline, it is to be expected that it must have slumped, perhaps several times, during its uplift. The formation is so soft that any face cut by rivers or any large cutting which might make a considerable exposure rapidly becomes a slide, and it is impossible to distinguish any single slump structure in the formation as belonging to a particular period. Both Macpherson (1951) and Lillie (1953) attribute the slumping in their closely comparable Benmore and Wanstead formations to the presence of bentonite in the clays, and the slumping in the Kandahar formation may be similarly explained.
However little one might learn about the early slumping of the Kandahar Formation, one fact of the greatest importance emerges, namely, that this and other very
thick formations that are still thoroughly incompetent can never have transmitted much “drag” across bedding or thrust along bedding planes, and that consequently most thrust and fold structures in them must have been caused by gravitational collapse. Even the major folds which run through these formations can never have been the direct result of compression and arching, but must be passive or draped folds and have been caused by elevation or subsidence of the basement rocks below.
This conclusion is obvious, but the important application of it is not immediately apparent. Many rocks, like the redeposited and now lithified Cretaceous strata of the region, are not very different in composition or origin from the Kandahar Formation. While some structures of these strata were clearly developed after lithification, there are many others which were probably due to slumping of one kind or another but which cannot now be easily distinguished from tectonic structures. The writers have observed in the Kandahar Formation pseudo-tectonic structures such as boudins, attenuated beds and “pebbles”, drawn-out crush-breccias, sheared formations, and so on, and consider that the majority of similar forms in the Cretaceous strata may be better attributed to slumping than to “compression”.
If it is accepted that “slump erosion” may be due to tilting of the sea-bed it is reasonable to ask what might be the angle of slope on which sliding would occur. From the standpoint of mechanics, thick redeposited and perfectly stratified formations sliding on laminae of clay must be only slightly more stable than equally thick masses of clay. Because some Tertiary clays flow under subaerial conditions on angles of less than 5° it seems probable that submarine slumping may occur on even gentler slopes, perhaps as low as one or two degrees. These assessments are compatible with those of Milne, Heim, and Arkangueslsky, who, according to Fairbridge (1946, p. 84), have demonstrated that submarine slumping occurs on slopes of 2° to 3°, and who claim that slumping of normal sediments is inevitable on all slopes of more than 5°.
A second consideration concerns the broad characters of uplift and erosion of anticlines and of the infilling of synclines. It is in the nature of turbidity currents that they, like rivers, must seek the lowest level and that they should deposit their load as their velocity decreases. Thus synclines will be sites of deposition and, as described above, anticlines may be the scenes of contemporaneous submarine erosion. With relatively rapid folding, erosion and deposition might just keep pace with the tilting of the sea-bed and so maintain stable slopes of fold limbs at about 2°. With any faster folding, slumping would necessarily ensue until a stable slope was reestablished and the sea-floor as a whole would remain relatively flat. Any halt in folding would, of course, allow deposition to restore horizontality in a very short time.
Special interest attaches to the submarine erosion of growing anticlines as they reach wave base, for the soft young sediments at their crests must be eroded by every severe storm and the debris must be carried away by turbidity currents. It is unlikely, therefore, that any fold of very young sediments can ever emerge from the ocean. The net effect of wave-base erosion, of redeposition, and of slumping must be to maintain a nearly horizontal sea-bed, and thus any emergence in an area of young sediments must tend to be regional and of a plain rather than of island chains.
There are marked exceptions to the above generalisations in the East Coast area, for it has sometimes happened that rising folds were stripped of all the soft cover at their crests and that their hard basement cores were exposed. These cores were not subject to slumping and rapid erosion, and as uplift continued they emerged at the surface of the sea as long and rugged island chains of old rocks. Such a chain probably gave rise to the conglomerate at the base of the Kandahar Formation, and throughout the Tertiary such ridges of Trias-Jura greywacke or of somewhat softer Cretaceous strata probably gave rise to the greywacke pebble beds which occur in the Tertiarv sediments.
Fig. 1.—Paralloid Fold, diameter 6ft, Mungaroa Limestone, Te Kau Kau Point. Fig. 2.—Same as Fig. 1 with sandstone dyke. Fig. 3.—Same as Figs. 1 and 2 with 18in thick sandstone dykes and sills. Fig. 4.—Skew Fold, Te Kau Kau Point. Note oblique fissures. Fig. 5.—Mud Ball. Piripauan Sandstone Fig 6—Attenuated distortional slump folds, Pukemuri Siltstone, Opouawe River.
Figs. 1 and 2.—Greensand dykes in Mungaroa Limestone, Te Kau Kau Point. The 20ft. dyke of Fig. 2 has been etched out by wave action. Fig. 3.—Scree (submarine ?) of Piripauan Sandstone in Piripauan Sandstone. Fig. 4.—Calcite filled joints in Mungaroa Limestone. (Photo: M. King.) Fig. 5.—Greensand dykes in Whangai argillite. Fig. 6.—Slumped conglomerate in Piripauan Sandstone. (Adjacent to Fig. 3.)
Fig. 1.—Lower extremity of a 60ft long drag fold. Mungaroa Limestone, Te Kau Kau Point. Note how passages of thrust fault into bedding plane slide allows attenuation with least distortion. (Photo: D. Kelly.) Fig. 2.—General view of slumped limestones, same locality as 1. Fig. 3.—Slump Drag-Folds in Amuri Limestone, Kaikoura Peninsula, Marlborough. (Photo: G. Shaw.)
In late Pliocene times narrow reefs of greywacke rising from deep water were particularly prominent in the Wairarapa district, and when subsequent uplift raised all of that district above sea level rugged chains of hills were exposed. These unexpectedly jagged ranges, 500ft above the neighbouring flat but synclinally warped country, are very characteristic of the district and are called “taipo” ranges. The Taipos, occupying the crests of asymmetric anticlinal highs, are usually strongly faulted on one side and may be lightly faulted on the other. Many are still undergoing uplift, and, while they may be regarded as young horsts, their topographic expression is that of ancient residuals.
It is now possible to explain how formations like the Whangai or Wanstead might be deposited across a wide terrain which consisted at the time of deposition of a succession of actively warping and eroding folds. As the sea-floor was substantially flat and horizontal, and as sediments were spread out very smoothly over vast areas by turbidity flows, it follows that a continuous formation might be deposited over what are now separate basins. Subsequent erosion on the anticlinal highs would not affect deposition already accomplished or lithological correlation. Indeed, it might be expected that facies variations would be independent of structure and that there could be a steady facies variation across several troughs. The hundreds of miles of distances over which broad lithological correlations can be made on the East Coast are thus compatible with continuous folding, with the present topography, with unconformities, numerous disconformities and discontinuities, and with the pronounced slumping of strata.