N. N. Gornostaev,
Associate Professor of the Institute.

Intraformational disturbances caused by underwater sliding and tectonic processes.

(An attempt at a comparative characterization).

(From the Geological Cabinet of the Siberian Technological Institute).

I.

Underwater sliding of water-saturated sediments that have rapidly accumulated on inclined areas close to the shore in modern bodies of water is a fairly widespread, or at any rate not too rare, phenomenon¹). It was therefore natural to look for traces of similar phenomena in ancient sediments accessible to us in geological sections, and the most striking example of such searches may be the work of F. Hahn on the question mentioned²).

Basing himself on the obvious manifestation of the indicated sliding in the form of disturbances in the regularity of bedding of the group of layers that slid downward, while the underlying beds, as well as the beds deposited after the sliding itself, preserve undisturbed bedding, Hahn explains by such sliding certain disturbances of this kind in the Trenton (Ordovician) limestones at Trenton Falls, New York, examined by him personally in 1911. Hahn then analyzes, chiefly on the basis of published data, a number of analogous disturbances and attributes most of them to the action of the same cause. This was already an overstatement, and, as W. Miller showed in an excellent work³), the majority of intraformational disturbances must be explained by other causes, the chief of which are differential displacements of the hanging and foot walls of the disturbed zone during tectonic bending of the entire complex of beds. Conversely, cases of indubitable underwater sliding remain only a very small number. The question has also found its way into the pages of more or less elementary textbooks⁴), but its solution has not thereby become clearer. It is therefore of some interest to attempt a comparison of the morphological features of intraformational disturbances arising from underwater sliding, on the one hand, and from tectonic causes, on the other—especially since, in the Russian literature, so far as the author knows, the question has not been touched upon at all.

¹) Arn. Heim. Ueber rezente und fossil subaquatische Rutschungen und deren lithologische Bedeutung.—Neues Jahrbuch für Mineralogie, Geologie und Palaeontologie. 1908, II Band, 136—157.
K. Andrée. Geologie des Meeresbodens. II Band. Leipzig, 1920; 272—273.

²) F. F. Hahn. Untermeerische Gleitung bei Trenton Falls (Nordamerika) und ihr Verhältnis zu ähnlichen Störungsbilder.—Neues Jahrb. f. Min. u. s. w. 37. Beilage Band, 1913; 1—41.

³) W. J. Miller. Intraformational Corrugated Rocks.—Journal of Geology, 30, 1922; 587—610.

⁴) F. H. Lahee. Field Geology. 2-nd. Ed. New-York, 1923; 184—185.
E. Kayser. Lehrbuch der Geologie Allgemeine Geologie. I Band, 7/8 Aufl. Stuttgart, 1923; 662—663.

An undoubted case of disturbances caused by submarine sliding, which yielded an altogether peculiar autoclastic rock, was observed by the author in the spring of 1924 during one of the excursions with students of the geological specialty in the vicinity of the city of Tomsk. As is known¹, on the bank of the Tom River, immediately above the city, beneath the Lagerny Garden, there are exposed sandstones and sandy-clayey shales set on end, with a generally regular bedding, containing now marine fossils, now plant remains of the Viséan stage. The thickness of individual beds ranges from several centimeters to 2–3 m, only rarely, however, exceeding 0.5 m. The bedding is almost everywhere regular, revealing here and there (for example, immediately above the “Boets”) traces of short-lived coastal erosion, repeated from time to time. These near-shore sandy-clayey deposits are intensely folded, having at the place described a very steep, almost vertical dip to the NE, with a strike NW : 30°. The relations of the erosion surfaces show that in the given case we are dealing with a normal, not overturned, dip, i.e. the younger beds lie to the NE, and the older to the SW. In parallel with this there is also a change in lithological composition—the bluish-gray, in the moist state black, clayey shales with a marine fauna, poor in sandstone interbeds, are gradually replaced toward the NE (downstream) by a sandstone facies, so that at the very “Boets” we have essentially bedded sandstones, with relatively poor interbeds of clayey rocks. In these sandstones one of the participants in the excursion found scanty plant remains in the form of small fragments of calamite stems. In addition to being crumpled into folds, the rocks are strongly broken, often into small polygonal pieces, by very numerous fractures of disjunctive displacements connected with the last phases of folding and probably continuing later as well. The latest of these displacements also affected the diabase-porphyrite dikes cutting through the Carboniferous. Portions of the Carboniferous that were subjected to the contact influence of the diabases and that consist of black, very fine-grained hornfels (narrow belts around the diabase bodies) are naturally more resistant to denudation and project along the banks of the river, for example in the form of capes, one of which is the “Boets” beneath the Lagerny Garden, well known in Tomsk and described by M. E. Yanishevsky, mentioned above. Several meters above this Boets, near the low-water line, along the towpath, among alternating rather thin interbeds of sandstones and clayey shales, there projects an interbed of a gray, clayey, indistinctly bedded, almost structureless rock, about 20 cm thick, containing a very original autoclastic formation, for which the names “conglomerate” and “breccia” are suitable—or rather unsuitable—in equal measure.

Namely, almost in the middle of this interbed, somewhat closer to the hanging wall, there passes a band, about 6 cm thick, filled with variously deformed fragments of thin-bedded sandstone. The fragments (see the photograph on Plate 1) have the most irregular, bizarre external outlines; for example, from a certain central mass there extend in different directions—and not only in one plane—strange processes, sometimes narrowing, but more often even widening at the ends, so that they form, as it were, massive pear-shaped swellings attached to one another or to the central part by relatively thin stalks or plates. Sometimes such a formation has the appearance of a medusoid, conical body—still of extremely irregular form. Often such fragments lie tightly against one another—and with fairly broad surfaces—but the relations of complexity indicate the complete individuality and independence of these adjoining

masses, each of which gives its own system of offshoots, although the mechanical connection between the offshoots of one mass is very unstable. In spite of all the quaintness of the external forms of these formations, accompanied by no less quaintness in their internal structure, expressed by a very distinct fine lamination, a certain regularity can be noted in their structure. Indeed, the pear-shaped expansions of the offshoots of each fragment possess lamination turned inward, rounded, as if rolled up; where two heterogeneous fragments come into contact, their lamination, mutually adapting itself, bends around the convexities of the other; in a word, it is clearly seen that the sandstone masses described arrived at the place of their occurrence already in the form of separate fragments, but still in a plastic state; likewise the clayey substratum was also soft and plastic, and the whole picture of the formation of the rock described must have been as follows.

On the relatively steeply inclined bottom of the Visean sea, at a short distance from the shore, considerable masses of sandy sediments accumulated rapidly. From time to time these masses, owing to the rapidity of deposition, possessed a large angle of slope, which could sometimes even exceed somewhat the natural angle (under underwater conditions), and slipped downward, to great depths, into the area of deposition of more fine-grained sediments. One of such groups, which had slid downward, of thin layers of water-saturated fine-grained light sand, broke up into separate fragments, which bent in various ways even while sliding along the bottom of the basin, but were especially strongly deformed already upon stopping, when they entered at considerable speed into the soft, semi-liquid, quietly lying mass of fine dark silt. In doing so, the slid sandy parts, having the consistency of jelly or liquid gelatin, crumpled, bulged out, and partly flattened out in the silt under a somewhat rotational movement of theirs, and, moreover, collided in various ways and rolled over one another. The semi-liquid silt into which these fragments fell, offering little resistance to the slid masses, parted and crumpled, adapting itself to their form, and partly was raised into the water in the form of turbidity and, settling when everything had quieted down, slightly covered the foreign formations that had unexpectedly appeared in it, after which deposition of sediments of the usual facies for the given place again continued.

It is not difficult to see that the explanation given is the only possible one with respect to the process of formation of this original rock. Indeed, the form of each separate fragment, taken by itself, would be very unstable mechanically even for a very strong substance; the clayey sandstone of which our formation consists, having received such a form and being deprived of a substratum that would support it from all sides, would instantly have fallen apart. Thus, the forms of the fragments under consideration are simultaneous with the moment when the sandstone (or sand) entered the given stratigraphic position. But, on the other hand, the sandstone fragments lithologically represent a facies completely alien to the clayey mass surrounding them. Any tectonic mode of formation is completely excluded here, for, although the whole formation did experience a rather considerable dislocation, which produced, among other things, several small overthrusts near Boitsa, precisely at the given place we have a comparatively little affected block, with a well-expressed rectilinear folding (true, set on its head); the formations described also occur in the form of an interbed composed of separate bodies, isolated on all sides, lying in a homogeneous mass devoid of any slickensides. True, each fragment of sandstone is surrounded by a thin, shiny, dark-gray clayey film, resembling to some degree a slickenside and having, in essence, even a homogeneous origin, but these films wrap around each of the fragments and in no way constitute a continuous slickenside—

they do not pass from one break to another. The formation of such a film, a very thin one, was noted already in the excellent description given by P. Vollrath of undoubted submarine sliding in the Franconian Wellengebirge¹.

Further, the action of other specific factors capable of causing intraformational disturbances of the regularity of bedding—for example, the crumpling of bottom sediments by icebergs or glaciers, etc.²—is excluded if only because all these factors, by virtue of their specificity, possess comparatively easily established indications, which in the present case are entirely absent.

Already a priori one might have expected that, under the above-indicated conditions of formation of our olistoplast, its lying side, which is the surface of crumpling, should be more irregular in its outlines than the hanging surface (the surface of “self-” attachment). This feature is in fact observed, as may be seen, for example, in the appended photograph (Plate 1).

In the process of submarine sliding, the disintegration of the sliding mass into separate fragments is far from being the general rule. Rather, on the contrary, it is even an exception³, which may be encountered in any front part of a landslide (which probably occurred in the case described above), or in its lateral parts, as well as in cases of sliding of already set, hardened masses, for example, limestones. In the general case, however, the sliding mass presents a folded accumulation, and the axes of the folds must naturally be arcs convex forward, toward the lower end of the tongue, while the intensity of crumpling and the amplitude of the folds increase in the direction of the front end of the tongue⁴. The folds themselves, owing to the relatively small load from above and to the smoothness of the movement of the still soft masses controlled by water, must have smooth, rounded outlines. Sharper angles, of the type of chevron folds, may be expected only in rapidly hardening sediments (limestones) and in areas of especially strong crumpling. A feature very characteristic of the phenomenon in question will be the irregular, uneven, folded upper surface of the tongue (see Fig. 1), and there is a rather good chance that this feature will be preserved in the fossil state, because the phenomenon itself essentially occurs in places of rapid accumulation of sediments and, consequently, the landslide will soon be covered by new deposits of silt. An inevitable consequence of the indicated feature is the inconstancy of the thickness of the folded interbed observed by us in section. Only as an exception (and in undoubted cases⁵ of submarine sliding this exception is not observed) can the action of submarine erosion—derasion, in Heim’s terminology⁶—manifest itself; this will be revealed by a clear stratigraphic unconformity between the anticlinal fold crests cut off by derasion and the hanging side resting calmly upon them. With respect to the lying side, we always—since this is not a matter of the slow flowing⁷ of silty masses, which, in a semi-suspended state, are then distributed in an even layer over the bottom of the basin—observe a sharp unconformity, usually accompanied by an approximately even surface of grinding⁸, somewhat convex downward (over a great extent), which, however, will not be as perfect as in the case of true—

tectonic friction. However, in the lateral parts of the tongue, and especially at its front end, there may also occur the highly characteristic crumpling of fragments and of entire folds into the underlying mass, similar to what was described above. Folds formed by a submarine landslide will all be overturned down the dip and, moreover, asymmetrical with respect to the hanging and footwall sides; the hinges of the anticlinal folds are often swollen. In a section through an ancient submarine landslide we shall observe a definite zone of crumpling, with the characteristic features listed, lying in a more calmly bedded formation; moreover, the existence of interbeds (especially thin ones) lying calmly, in conformity with the entire formation, and dividing the crumpled zone into two or more separate tiers (the terms “zone,” “tier” are used here, of course, not in their strict stratigraphic sense) is quite conceivable, except perhaps in cases of especially rapid deposition and repeated sliding. But in this case the calmly bedded interlayers will have characteristic uneven lower surfaces and regular surfaces of the hanging side, sometimes somewhat cut by the upper landslide. (See Fig. 2).

If we turn to consideration of the lithological character of the crumpled zone and of its footwall and hanging sides, it is easy to see that the crumpled zone will consist of coarser-grained, more sandy material than its hanging and footwall sides, for this material was originally deposited nearer the shore than the rocks now enclosing it. In the autoclastic rock described above this is observed quite distinctly. In general, by submarine sliding an element of a facies foreign to the autochthonous rocks is introduced, as Heim had already noted¹); moreover, a mixture of heterogeneous faunas may result, and, for example, a littoral fauna from coarse, thick shells, damaged and partly broken during the landslide, will occur in association with entirely whole, though delicate and thin, shells of deeper-water forms. Such cases are described, for example, by Schaffer²). Finally, features inherent in this phenomenon will be: the absence of certain beds in the area of sliding and, conversely, their exaggerated number, with repetition of older beds in the area of deposition of the slipped masses (see Fig. 3)—the so-called “unterzählige” and “überzählige Schichtung,” respectively³). Features of the latter kind may prove important where there are well-characterized faunistic deposits.

It has already been indicated above that in plan a submarine landslide has a tongue-shaped form. An example may be the tongue of the landslide near Zug, which occurred on July 5, 1887, more than 1000 m long, 150–250 m wide, 0.5–4 m high, and with an average slope of 4.4°⁴). It goes without saying that in the seas the phenomenon may be much more powerful, especially on the steep slopes of the greatest oceanic depths⁵), but it will always remain only local, extending in each case over a limited area.

But if in each separate case submarine landslides remain local phenomena, then, on the other hand, they have every chance of recurring frequently—since physiographic conditions favorable for sliding continue to exist. And in the case we have described, beginning with the most vivid manifestation of such a landslide that we have considered, over the course of half a hundred—

¹) A. Heim. Subaquatische Rutschungen u. s. w.; 156–157
²) F. X. Schaffer. Ueber subaquatische Rutschungen.—Centralblatt für Mineralogie, Geologie und Palaeontologie. 1916; 22–24.
³) A. Heim. L. cit.; 154–156.
⁴) A. Heim. L. c.; 139.
⁵) E. Horn. Ueber die geologische Bedeutung der Tiefseegräben.—Geologische Rundschau, 5, 1914; 422–448. (432–435).

another meter upward along the river (i.e., inland, into more ancient and at the same time deeper-water beds), we encounter numerous traces of submarine landslides of the same kind. It is curious, moreover, that still farther on, in the deeper-water beds, where in a homogeneous clayey mass with marine fossils sandstone interbeds (of the normal type) are already almost absent, the traces of submarine landslides also cease. In other descriptions of submarine landslides, too, the recurrence of the phenomenon is often mentioned, which is quite understandable.

II.

Disturbances in the regularity of bedding of only certain layers of a formation are incomparably more widespread than landslides; in comparison with these layers they are more tranquil, being caused by the differential displacement of individual beds one over another during the bending of the entire formation. When a more or less thick series of beds is bent—whether because it constitutes a somewhat overturned limb of a normal fault or reverse fault, or because it is caught up in regional folding—the individual beds of this series must in any case slide one over another, producing between them inclined surfaces of abrasion (see Fig. 4). If, as often happens, the series is composed of beds of unequal strength and hardness, then the softer and more plastic beds will be more easily crumpled and abraded, and in them there will often appear very intense small disturbances, having, moreover, a somewhat different manifestation depending on the cause by which the bending of the beds of the formation is conditioned—or, more precisely, on the depth at which this bending takes place. Let us first examine the phenomena that occur during regional folding, when bending proceeds under the great load of a thick geosynclinal formation. If we are dealing with relative sliding of still small amplitude, then the connection of the footwall and hanging-wall sides with the weaker bed intercalated between them is disrupted only in a limited number of places; but this bed itself undergoes, within itself, more or less sharp reverse faults with small, acute turns of the limbs. The reverse-fault fissures do not extend beyond the weak bed and have a more or less steep dip with respect to the stratification. Such a primary stage of the disturbances under consideration is very clearly visible in Figure 5, borrowed from the above-cited article by W. J. Miller¹). It shows, in plan, displaced layers of quartzite and phyllite, standing almost on end, exposed at Baldhead Cliff near Ogunquit, Maine. The quartzite layers in nature stand out less sharply than in the figure, and in general the entire crumpled zone, about 9″ thick, is not sharply separated from the straighter, predominantly quartzitic beds on either side. Attention is drawn to the almost complete symmetry of the picture with respect both to the footwall and to the hanging-wall sides, so that the side of the latter can be distinguished only by the more developed bending of the limbs in the upthrown parts. The resulting picture is easy to explain mechanically. Indeed, from a mechanical point of view, the deformation experienced by the zone under consideration is shear. Let us imagine (Fig. 6) a cube \(ABCD\) with a sphere inscribed in it and subject them to a shear deformation. The cube will become a parallelepiped \(A'B'C'D'\), and the sphere an ellipsoid. The directions parallel to \(A'C'\) and \(B'D'\), i.e., the axes of the ellipse, will respectively be the directions of greatest extension and greatest compression, while the directions \(E'F'\) and \(G'H'\) will be the planes of greatest shear—moreover, along them no change of linear dimensions occurs. Since in Figure 5 unquestionable sliding occurred along the reverse-fault cracks, indicated by the turning of the limbs, it follows to conclu—

¹) W. J. Miller, l. c., 594–595.

... to be read in accordance with Leith¹ and Schreiner² and contrary to the opinion of Van Hise,³ that fractures under such deformations are produced not along the directions of greatest extension, but along the planes of maximum shear, i.e. \(G'H'\). This makes it possible even approximately to determine the magnitude of the deformation.

With the further development of the deformation, in view of the ever greater inclination of the direction \(G'H'\) of our parallelepiped, the reverse faults will become more and more inclined and will asymptotically approach parallelism with the bedding surfaces. At the same time the number of fractures increases greatly, to the point that an already stressed cleavage is obtained, while still inclined to the bedding surfaces. The turns of the limbs of our small reverse faults are stretched and disappear. A separateness appears also along the direction \(E'F'\)—parallel to the bedding surfaces. The shear deformation is most concentrated along the contact surfaces of the crumpled zone with the hanging and footwall sides, and here the fractures of the \(F'H'\) system, being strongly inclined, practically merge with the \(E'F'\) system. Therefore the separateness along \(G'H'\) acquires an S-shaped bend, standing more steeply in the middle part of the zone. Owing to stretching, in this middle part there appear short rupture cracks, dying out toward the edges, parallel to \(B'D'\), transverse (but not perpendicular) with respect to the \(G'H'\) system. Such a moment may be seen in figures 131 and 132 in Van Hise⁴ and in the excellent photographs, figs. 9 and 10, in Leith.⁵ The schematic drawing 7, borrowed from the latter, conveys the essence of photograph fig. 10 so vividly that it scarcely requires explanation. With further development of the phenomenon, the thickness of the middle zone becomes ever smaller, whereas the thickness of the marginal parts, with cleavage parallel to the bedding, relatively increases; and, finally, we obtain complete leafing of the crumpled zone, as often happens, for example, in the folding of alternating more or less thick beds of sandstone and thin interbeds of clay shale, the latter breaking up directly along its fissility so much that no sample can be taken from it. Outcrops of this kind are so common in the practice of every geologist that there is scarcely any need to cite specific examples; however, the author may point to an outcrop on the left bank of the Irtysh, 3 kilometers below the settlement of Izvestkovyi (Semipalatinsk region) and 1 km above the Kendyк hill, where the layers of strong graywacke sandstone, 8–10 to 30 cm thick, crumpled into steep folds, have transformed the alternating layers of clay shale, now up to 10–15 cm thick, directly into continuous “slickensides,” crumbling between the fingers into the finest shiny flakes.⁶ The intensity of such bedded cleavage is proportional to the difference in thicknesses of the strong and soft beds and to the difference in their softness and plasticity. It should also be noted that the development of such bedded cleavage, being confined to fold formation, has nothing in common with shaliness—cleavage independent of bedding and developing in rocks already crumpled into folds or, in general, in compacted hard formations.

In bends of strata connected with faults or reverse faults, and also with shallow folding of epicontinental formations of small thickness, generally under conditions close to the ground surface, with not too considerable

under the load of the overlying sediments, the phenomenon of sliding remains essentially the same, but acquires somewhat different morphological features. Namely, the displacements of parts of the crumpled zone gain greater freedom and amplitude in the vertical direction—we have before us not so much shearing along subparallel low-angle thrusts as true folding, i.e. small-scale folding (provided the crumpled zone is sufficiently plastic). In exactly the same way as in the case considered above, with small degrees of differential displacement of the top and bottom we have a disturbed zone that gradually passes into them without rupture; moreover, in contrast to displacements of the deep-seated type, there are as yet no thrusts at all (see Fig. 8). They appear later—when both the horizontal and the vertical components of displacement are sufficiently large; in this case the overturning of the thrust wings is distinguished by more rounded and smoother outlines (Fig. 9). Figures 8 and 9 were made by the author from specimens collected in 1924 from an almost horizontal, weakly folded (dip angle up to 10°—12°), but rather well broken by small thrusts, formation of regularly bedded thin layers of gray- and black-green fine- and fine-grained graywacke sandstones and sandy shales, which in age belong to the productive deposits of the Kirghiz Steppe¹). This formation—apparently the hanging wall of the “erosional thrust” of Ulymtas²)—is exposed along a gully entering the Chagan River from the left, 28 kilometers south of Dolonskaya station (Semipalatinsk Province). The rocks of the individual beds differ little lithologically from one another, are rather hard, and therefore tangential compression was relieved mainly in thrusts; the individual dislocated beds also show a tendency toward fractures rather than bends. For this reason the specimens in question are not entirely typical. In both cases the crumpled layer has not yet lost its connection with the top and bottom and passes gradually into them, through 2–4 less dislocated interbeds, each 1.5–2 mm thick.

A more typical case is represented by Figures 10 and 11, borrowed from Grabau³), depicting disturbances of a similar type in limestone beds at Cape Gaspé, Quebec, Canada. In them one clearly sees sharp and regular tectonic surfaces of separation between the top and bottom and the disturbed zone itself—as a consequence of strong sliding with a significant difference in plasticity between these different beds, as well as the overturning of saddles upward along the rise of the beds. Owing to the strong development of folding, somewhat displaced in the same direction, the symmetry of the folds with respect to the hanging and the lying side is already lost. With further development of differential displacements, ruptures of the folds of the disturbed zone are already obtained (Fig. 12, taken from the same source).

There remains little to add to the above-mentioned characteristic features of the manifestation of differential displacements. First of all, since we have favorable conditions for the manifestation of this type of disturbance, there is every reason to suppose that it will not be confined to a single horizon, but will be repeated more or less frequently on several, so that sometimes two very strongly disturbed beds will be separated from one another by a very thin, but almost perfectly regular interbed (which, incidentally, is found precisely at Cape Gaspé⁴)); and in general, in almost all observed cases, the disturbances were not confined to one horizon, but embraced at least two. The character of the disturbances, determined by more or less large tectonic phenomena, is preserved over large areas, since these tectonic phenomena—

¹) N. N. Gornostayev. Ibid., pp. 7–8 and 12–13.
²) B. Willis. The Mechanics of Appalachian Structure—13th Annual Report of the W. S. Geological Survey, 1893; 211—289 (223).
³) A. W. Grabau. Principles of Stratigraphy. New York, 1913,—783.
⁴) W. J. Miller. Ibid., p. 591.

spread far their sphere of influence. At the same time, the disturbances reveal a close connection with general tectonics—becoming stronger in the places of greatest development of the latter and possessing an extension concordant with it. These disturbances are naturally confined to the softest, most plastic beds; it is curious that almost everywhere platy intraformational dislocations are confined to thinly bedded calcareous rocks—sediments sufficiently plastic and at the same time elastic, allowing wide variations in the flexibility of individual layers.

We see that intraformational disturbances caused by subaqueous sliding, on the one hand, and by tectonic causes, on the other, differ in very many essential features, permitting them to be distinguished from one another fairly well—something which until now has been done very imperfectly.

Mixed features characterize intraformational disturbances produced in some more plastic and soft bed of an almost horizontal formation by the slight slumping of a hanging block relative to the given bed, when this bed has not yet lost its coherence and is only crumpled into a series of small folds. Such differential movement, taking place under the influence of gravity, is, from the mechanical point of view, a displacement of the soft bed and therefore produces in it changes of the same kind as those produced by differential displacements from the bending of the entire complex of beds. In genesis, however, the phenomenon stands closer to the first type. Consequently, the morphological characterization of such disturbances will also be distinguished by a certain peculiar mixture of features: the folds in the bed will correspond to the second type, as will their confinement to a softer and more tender bed; the absence of connection with tectonic causes, and the overturning of folds and overthrusting in the direction downward along the dip of the beds, distinguish this type, which has been observed in a number of lake deposits of North America ^1).

For clarity, the characteristic features established above for intraformational disturbances caused by the two principal causes described above are summarized in the accompanying table.

As for the remaining causes capable of producing intraformational disturbances, since they are due to specific factors (e.g., the action of ice, the intrusion of magma), they are easily recognized by this very specificity and, occurring relatively rarely (perhaps excepting the action of magma), are of lesser importance. Therefore they have been excluded here from consideration, in which the author has aimed not so much at an exhaustively complete description of all possible observed forms of the phenomenon as at elucidating its principal genetic features.

In the work of W. Miller ^2), cited many times above, there is a rather detailed (and very successful) description of all possible types of intraformational disturbances, while the action of ice in particular is characterized in considerable detail by T. H. Lahee ^3).

I take this opportunity to express my deep gratitude to the geologist of the Siberian Branch of the Geological Committee, K. G. Tyumentsev, to whose kindness and skill I owe the opportunity to append the excellent photograph of the original autoclastic structure described above.

Tomsk, 1925.

^1) W. J. Miller. L. c., 595—610.
^2) Idem. L. c., 604—610.
^3) T. H. Lahee. Field Geology, 180—184.

Characteristics of intraformational disturbances caused by:

Fig. 1

Fig. 2

Normal sequence of layers

Deposit with sliding

Upper-layer bedding

Lower-layer bedding

Fig. 3

Fig. 4

Fig. 5

Fig. 6

Visible labels in the left diagram: \(B\), \(H\), \(C\), \(E\), \(F\), \(A\), \(G\), \(D\).

Visible labels in the right diagram: \(B'\), \(H'\), \(C'\), \(E'\), \(F'\), \(A\), \(G'\), \(D'\).

Fig. 7

Shear deformation

Fig. 8

Fig. 9

Fig. 10

Fig. 11

Fig. 12.

Fig. 13. Photographic image.

Explanation of the Figures.

Fig. 1. Folding produced by an underwater landslide. A bed of Lower Paleozoic limestone conglomerate (Beekmantown limestone, outcrop in Logan Branch of Spxing Creek, Center Co., Pa., U. S. A.). Characteristic is the flatly convex downward boundary with the underlying bed and its uneven, irregular upper surface. After Th. C. Brown, Notes on the Origin of Certain Paleozoic Sediments, etc. Journ. of Geol., 21, 1913; 241–244, and W. f. Killer: Op. cit.; 602.

Fig. 2. Diagram of the structure of a horizontal interbed between two zones successively deformed by underwater sliding.

Fig. 3. Diagram of the sequence of deposits in the area of development of underwater slides. In the area from which the masses slide, a deficiency of beds is obtained (unterzählige Schichtung); in the area where the landslides come to rest, a repetition of layering (überzählige Schichtung) occurs. After Arn. Heim. Op. cit.; 157.

Fig. 4. Diagram of differential displacements of beds during folding. The arrows indicate relative displacements of the beds. After Lahée.

Fig. 5. Plan of overturned beds of phyllite and quartzite at Baldhead Cliff near Ogunquit, Me., U. S. A., with the effect of differential displacement. The black lines are quartz. After W. J. Miller: Op. cit.; 595. (See text.)

Fig. 6. Shear deformation. Transformation of a sphere into an ellipsoid: A′ C′—direction of greatest extension; B′ D′—of greatest contraction. E′ F′ and G′ H′—planes of (maximum) shear. After Leith. Op. cit.; 20.

Fig. 7. Cleavage formed in a bed of schistose quartzite enclosed between two beds of massive quartzite. Paraboo quartzite, Wisc., U. S. A. The fine, curved cracks are cleavage caused by differential displacement of the massive beds. The sharper cracks are parallel to the schistosity (E′ F′ in Fig. 6) and perpendicular to the greatest extension (⊥ B′ D′ in Fig. 6). After Lahée. Op. cit.; 26.

Fig. 8. Specimen of graywacke sandstone shale from an interbed deformed by differential sliding higher and lower lying beds. The bed dipping into the river Chagaya (tributary) of the ravine toward C from Lake Balykty-kul (Semipalatinsk Province). On the surface of the interbeds there are also many small wrinkles, parallel to one another and to the principal deformation.

Fig. 9. A fold passing into a micro-overthrust. From the same place.

Figs. 10–12. Intraformational disturbances in Devonian beds at Cape Gaspé, Quebec, Can. After A. W. Grabau. Op. cit., 783. (See text.)

Fig. 13. Photograph from an autoplastic cast formed during an underwater landslide. Cape Boets on the Tom River near the city of Tomsk.

1. C. K. Leith. Structural Geology. London. 1914. 16–25. ↩↩↩

2. R. Schreiner. Scherung, der Zentralbegriff der Tektonik—Centralblatt für Min. u. s. w., 1924, 472. ↩↩

3. C. K. Van Hise. Principles of North American Pre-Cambrian Geology. 16-th Annual Report of the United States Geological Survey. Part I, 1896; 636–643. ↩↩

4. Op. cit., 652–653. ↩↩

5. Op. cit., 24–25. ↩↩

6. N. N. Gornostayev. Gradus \(\frac{50^\circ-51^\circ\ \text{N. lat.}}{48^\circ-49^\circ\ \text{E. long. (from Pulkovo)}}\) (Preliminary report). Izvestiya Sib. Otd. Geologich. Komiteta, 5–2; Tomsk, 1925; 12. ↩↩

7. A. Heim, L. cit., 153 and F. F. Hahn. 2. cit., 24. ↩

8. Cf. the cited work of P. Vollrath. ↩