monoclines and thence into undeformed rock. There is no evidence in the Coal Measure Series and in subsequent series of external forces such as would cause this jointing, and it is evident that shrinkage is the main cause of their existence.

A similar origin for this type of jointing was postulated by Leith (1923, p. 33) who, referring to joints related to the contraction of a crystallising and cooling mass of lava, said that, “Similarly, joints may be very abundant in flat lying, consolidated beds of sediments, which plainly have not been disturbed by great exterior forces. One of the causes in this case is doubtless the change in volume incidental to the drying and settling of the beds. Mud cracks are one manifestation of this process. Joints may die out above or below; there may be evidence that jointing in a given bed was complete before the next layer of sediment was deposited. They are likely to be especially abundant near the contacts of different beds or formations (a fact often noted by well-drillers in search of water).” He said further (op. cit., p. 50) that this “type of local tension jointing is developed by the drying out of a sediment, resulting in the formation of mud cracks and of shrinkage cracks on a large scale. The joints so formed lack regularity and persistence, vertically and horizontally.” He quoted the flat lying sedimentary Palaeozoic beds of the Mississippi Valley, the many joints in which are probably due to the drying and settling of the formation. “The topography of the basement controls the settling to some extent, and therefore the distribution of the resulting joints. Accompanying the vertical joints caused by tension, there are horizontal joints caused by shearing, due to the differential expansion and contraction of the different layers. Mud cracks are often associated with cross breaks causing the upper layer to separate and curve up from the lower layers.” No such horizontal joints have been seen in the shrinkage-jointed Coal Measure Series of the Waikato district unless the thin beds showing fracture cleavage and the contorted layers of carbonaceous shale may be a manifestation of this. It seems more likely that the cross breaks associated with mud cracks are due to rapid drying, which with buried sediments would be slow and gradual.

Shrinkage is not confined to the claystones. The peaty matter which now forms the coal underwent considerable reduction in volume, but it is difficult to estimate with any degree of accuracy how much shrinkage took place in the peaty mass after burial because the degree of compacting of the lower layers before burial would affect subsequent volume reduction of the whole mass. According to Ashley (1907), one foot of coal is formed from about 3½ feet of well compressed peat and so the amount of shrinkage would be at least 60 or 70 per cent. Kendall (1922, pp. 64–65), after quoting Lomax, and Stopes and Watson, believes “that the reduction in passing from the state of wet undisturbed peat will not be much less than 15 or 20 to 1.” Shrinkage of the coal-forming material would have no effect on the underlying beds. The cleat in the coal is the internal effect of this shrinkage, and externally the overlying claystone would be affected with the formation of the shrinkage joints. The

by the shrinkage on consolidation and drying of a moderate thickness of beds.” (Firth, 1930, pp. 96–97).

Slipping due to shrinkage of the clays, to slumping, and to the weight of overlying sediments may conceivably give rise to very large joints or small normal faults under favourable conditions imposed by situation on a sloping or uneven surface. Such faults would affect both claystones and coal, would be irregular, would die out vertically and horizontally, and would be confined to the Coal Measure Series. The overlying series of rocks would be undisturbed and would therefore show no evidence of the faulting below. The existence of such faults has already been noted. “Faults, overthrusts, and unconformities may as a rule be classed among what I have called the posthumous type of interference, though in many cases true faults appear to have achieved a portion of their total movement contemporaneously with the deposition of the seams, or during the interval between seam and seam. An illustration of a contemporaneous fault is found at the Barrow Colliery, near Barnsley, where, on the downthrow side of the fault and parallel with it, the Thorncliffe Thin Coal swells up from 3 feet to 5 feet 6 inches, and carries a strip of cannel absent elsewhere in the mine. Of a fault moving between seam and seam an example is furnished at Whitwood, where a lower seam is thrown to the extent of 60 feet while an overlying one is unbroken. The case of a fault affecting an upper and not a lower seam is noticed at Aldwarke Colliery.” (Kendall, 1922, p. 61). A more general observation is that of Leith (1923 p. 75) who said: “Normal faults may be locally developed in nearly flat-lying sediments. Here the cause of tension may be shrinkage and settling due to drying and recrystallisation. A displaced layer is sometimes covered by a continuous layer of sediment, suggesting that the faulting was contemporaneous with deposition. Often no other causes are discernible, but it is not possible to exclude hypotheses of regional or deep-seated tension related to major earth movements.” He gave no examples of these faults; and, as remarked before, there is no evidence to show that these faults could have been produced by any external forces, because they are confined to a particular series and all their properties can readily be explained from their origin as shrinkage and slip lines in this series.

Unfortunately, from the data available or ever likely to be available, there is no means of calculating the total shrinkage that the Coal Measure Series in the Waikato district has undergone. The total thickness varies from about 80 to 300 feet, and because many of the bores in this district did not reach the basement rock the different thicknesses of the undermeasures, as well as the contours of the basement rock, are not everywhere known. The old land surface was, however, a gently undulating one. From one to three thick seams of coal are present and the large shrinkage of this vegetal matter introduces another factor. Furthermore, the coal measure claystones are not uniform lithologically and show marked variations, the result of the characteristic lenticularity of these estuarine beds. From a study of the coal measures as a whole, sedimentation was in general continual, though diastems of greater or less magnitude

occurred as well as “hesitations” in the rate of subsidence, and during these periods some consolidation of the clays took place before they were affected by the loading of sediments subsequently deposited. The shrinkage of muds on solidification may amount to 20 or 50 per cent. or more (Twenhofel, 1926, p. 526), and during Coal Measure times in the Waikato district the deposited clays and muds were forced to adapt themselves to shrinkage by slumping, and probably flow, under various conditions imposed by the weight of overlying material and by an uneven floor. By uneven floor, the writer refers not only to the original basement Mesozoic rocks but also to subsequent surfaces of partly consolidated clays, which were not necessarily parallel to the original or to any later temporary floor.

Summarising then the mode of origin of these contemporaneous faults, they are the result of the slipping or slumping under load of dominantly argillaceous sediments. The movement is caused in the first place by the shrinkage of the clays on loss of water, but is assisted by the weight of overlying material. A necessary contributing cause is an underlying sloping surface of deposition, and so gravity plays an important part in causing the slumping. The strike of the faults would therefore follow the contours of such surface. Assistance to the movement would be given by different amounts of shrinkage caused by different thicknesses of sediment beneath the seam of “coal.” Because the peat-like material would probably be coalified before much consolidation of the enclosing muds had taken place (see page 105), it is evident that these faults would occur principally as displacements or fractures of the coal seam or seams. The slumping or slipping movement might, in the muds, be absorbed as adjustment by flow, although it is possible that large joints or small faults, similar to the shrinkage joints described on page 103, might occur confined to the claystones only. Their formation would depend on the degree of consolidation of the muds. For obvious reasons all the contemporaneous faults seen are those which cause displacements of the coal seam or seams, but it is reasonable to regard these faults as being developed principally as fractures of the coal. Characteristically, these faults would have a variable amount of throw and would not be continuous over long distances. Contemporary faulting which took place during growth of the peat swamp and was caused by settlement of the underlying clays, would be characterised by a fracture or displacement of the undermeasures passing into the base of the seam without effect on the top of the coal. The seam on each side of the fault would be different in thickness and, although a step would be present on the floor of the seams, the roof would be continuous. Again, because every surface of deposition in the estuary did not necessarily remain parallel to the original surface, faulting at different periods would not necessarily follow the same lines or affect the same parts of the series. Thus, where two seams of coal occur, faults may be found in the upper seam and not in the lower, and vice versa, and their strike need not be the same in each case. By the time that the original basin-shaped depression had been more or less filled the irregularities would be smoothed over and the dips everywhere slight, so that there would now

not necessarily be the same conditions or reasons for contemporaneous faulting except perhaps where younger beds overlap on to an irregular surface. Therefore an overlying deposit might be unfaulted.

The main features of these contemporaneous faults are illustrated by the block diagram in Fig. 1. Probably any smaller faults lying at an angle to a larger fault have been developed as secondary slips, as shown in the lower right-hand section of the block.

Fig. 1.—Block diagram, showing main features of a “contemporaneous” fault with possible development of a cross fault of similar origin.

The following descriptions are of contemporaneous faults which occur in the several mines of the Waikato district, and with which the writer has had personal experience. These coal mines are worked on the bord and pillar system, and an examination of the colliery plans does not always reveal the extent to which the small faults occur in the workings. As a rule, only the large block faults (formed during the Kaikoura orogeny at or about the close of the Tertiary period, and displacing all the rocks including the basement rocks) and the larger or more continuous of the contemporaneous faults are marked on the mine plans, and the recording of the many small faults shown in Figs. 2 and 3 is due to the detailed survey of these particular districts of the Pukemiro Colliery by Mr H. N. Davies.

All the contemporaneous faults seen are normal faults with a dip (variable) of about 50°. The majority have a throw of a few feet only (see Fig. 2), but faults with greater throw, up to 30 feet and perhaps more (see Figs. 3 and 4), are not uncommon. Faults are numerous where the seam dips comparatively steeply, and generally the displacement is “downhill,” although it appears that, under some conditions, adjustment, or possibly re-adjustment, results in a small fault dipping in the opposite direction. These variations can be seen in Fig. 2, where can be seen also the lack of persistence laterally of some of the faults. In this part of the mine the general downhill dip of the faults should be noticed, and although no bores have been drilled to the basement rock it can be inferred from neighbouring bores that an undulation or projection of the basement rock occurs. During deposition and settlement the sediments slumped down the surface and the strike of the faults will therefore follow the contours of this old surface. Similar faults are found in almost flat-lying parts, though there they are not so numerous and commonly follow irregular or sinuous courses. A fault may be cut off by another meeting it at an angle (see Fig. 1), and it is not unusual to find that a fault occurring in one bord is absent in the neighbouring bords. The variable throw of these faults is therefore explained.

Fig. 3.—Plan of portion of Machine Bords Section, North Mine, Pukemiro Collieries, showing the variability of a large contemporaneous fault. Other numbers indicate height in feet above sea level.

Fig. 3 shows a larger contemporaneous fault which starts as a roll and increases to 30 feet in throw. The further behaviour of this fault is unknown. A stone drive or rise was put up as shown on the right of the plan and it was seen here that the fault extended into the claystones with the same rate of dip as it had in the coal. From its origin as a roll in the coal seam it may be concluded that this fault does not extend into the underlying basement rock.

That the coal-forming vegetal mass had been consolidated practically to its present form is shown by the fact that there is no change in the cleat right up to the fault plane. The coal also maintains its usual appearance with horizontal bedding of the lenticles of bright coal (see Penseler 1930A) and at the fault plane itself evidence of fracture in some comminution or crushing of the coal is commonly seen. The fault plane or zone is usually narrow (not more than an inch or two wide), filled with powdery matter, and called a “sooty-back” by the miners. A “sooty-back” usually indicates the presence of a step in the floor or roof, or both. Some of these faults seen at Rotowaro do not pass right through the seam and affect the claystone floor and lower part of the seam only (see page 107). In some places, the coal is crushed within a foot or two of the fault, but at others it is normal and clean right up to the fault and no crushed coal is present. When crushed, the coaly material may be cemented by the deposition of calcium carbonate from circulating solutions, and the writer has noticed, particularly with the large fault shown in Fig. 3, that the upthrow part of the seam was crushed and cemented with calcium carbonate, whereas on the downthrow side the coal was clean and not crushed. The claystone along the fault planes, where exposed, is polished or distorted by “drag” into the fault zone. Commonly some gouge is present. In the majority of the smaller faults a foot or so at the most of the claystone is exposed (usually on the floor because, where possible, coal is left to support the roof) and the extent or persistence of the faults into the claystone could not be determined. If, as is probable, the claystones were only partly consolidated when the faulting occurred, the faults would soon die out or the movement become distributed into a number of small slips, or even into a flow of the semi-plastic material, but with the large fault of Fig. 3 the fault, as already mentioned, was persistent for at least 30 feet below the coal on the upthrow side. From studies at Rotowaro, however, these faults must die out sooner or later in the claystones without affecting the basement rock, and upwards they would not pass out of the Coal Measure Series unless as an exception to the rule and under special circumstances.

Fig. 4.—Part of Nos. 1 and 3 Mines at Rotowaro. Workings in upper seam (No. 1 Mine) shown white, and workings in lower seam (No. 3 Mine) shown black.

Part of the workings at the Rotowaro Collieries are shown in Fig. 4. Two seams of coal are present here and both are mined. Unfortunately, most of the workings shown (those in one seam underlying those in the other) are now inaccessible, particularly those in the upper seam, and only the larger faults were marked on the mine

plans. Many small faults do, however, occur, and on consideration of the non-persistence of the larger faults from one seam to the other it is evident that these smaller faults of small extent die out quickly above and below the coal seam.

Of the larger faults shown in Fig. 4, it might possibly be incorrect to say that they do not displace both seams, even if to only a slight extent in one seam, because one or more of the different smaller faults in a particular seam may represent either a continuation of the large fault or a distribution of the slip into several small, more or less parallel faults. On the other hand, if the large fault were not continuous, later faulting might have occurred in approximately the equivalent overlying beds because of the persistence of a sub-parallel sloping surface of deposition. In other words, although the cause of the faulting in each seam was the same, the two faults or sets of faults were not strictly contemporaneous with each other and are not continuous. It is impossible, for reasons already given and also because of the difficulty of differentiating the smaller faults, to correlate a large fault in one seam with smaller faults in the other. What evidence there is points rather to the discontinuity or rapid dying out of even the larger of these contemporaneous faults, and in any case it is quite clear that a 26 foot fault, for example, in one seam is not represented by a 26 foot fault in the other seam.

Fig. 5

Fig. 6