Submarine slumping in the Carpathian Flysch

R O C Z N I K P O L S K I E G O T O W A R Z Y S T W A G E O L O G I C Z N E G O A N N A I. E S D E L A S O C I E T E G E O L O G I Q U E D E P O L O G N E Tom (Volume) XXVIII 1958 Zeszyt (Fascicule) 2 Kraków 1958

MARIAN KSIĄŻKIEWIĆZ

OSUWISKA PODMORSKIE WE FLISZU KARPACKIM

(Tabl. X —XVII i 25 fig.)

Submarine slumping in the Carpathian Flysch

(PI. X—XVII and 25 fig.)

STRESZCZENIE

W pracy poprzedniej (1950) w yróżniłem dwa typy ruchów maso­

w ych na dnie morza fliszow ego: pow olny sp ływ św ieżego m ateriału two^

rżący silne sfałdowania w arstw , ale bez przerywania ich ciągłości;

(w arstw ow anie spływ ow e;, i ruchy osuw iskow e prowadzące do przerwa­

nia ciągłości struktury w arstw y. W pracy niniejszej om ówiony jest drugi typ ruchów m asow ych. Obserwacje wskazują, że niem al w każdej jed­

nostce stratygraficznej fliszu zaznaczają się zjawiska osuw iskow e, cho­

ciaż w porównaniu do ław ic niezaburzonych ławic dotkniętych ruchem, osuw iskow ym jest bardzo mało.

Można wyróżnić dwa ty p y osuwisk w e fliszu. Typ pierw szy polega na tym , że kilka lub kilkanaście ław ic uległo naraz osunięciu. W typ ie drugim jedna lub co najw yżej dw ie sąsiadujące z sobą ław ice zostały dotknięte ruchem osuw iskow ym .

Typ pierw szy jest w e fliszu, o ile można sądzić na podstaw ie do-, tychcząsow ych studiów, bardzo rzadki. Jako przykłady osuwisk o du­

żych rozmiarach zostały opisane następujące struktury osuwiskowe:

1. Osuwisko w dolinie Czarnej W isełki w M alince koło W isły (Śląsk C ieszyński) w górnych w arstwach godulskich. Tutaj kilka ław ic pia-, skow cow ych osunęło się wraz ze żwiram i, w skutek czego pow stał utwór złożony ze zlepieńców zawierających bloki, otoczaki i p ły ty piaskowców., Miąższość utworu osuw iskow ego w ynosi około 40 m. (Por. fig. 1 — 5

w tekście oraz tabl. X i XI).

2. Osuwiska na Grójcu koło Żywca w górnych łupkach cieszyńskich.:

Jest tu kilka osuw iskow ych mas na sobie, złożonych z pokruszonych i pom iętych kaw ałków łupków, piaskowców i sy dery tycznych m argli tkw iących w ilastej m asie bez w yraźniejszego warstwowania. (Por.

fig. 6—8).

3. Osuwisko w Porębie W ielkiej koło M szany D olnej, w warstwach

inoceram owych, złożone z bloków i drobnych fragm entów piaskow ców tkw iących w silnie zaburzonych łupkach. Miąższość brekcji osuwisko^

wej w ynosi około 55 m. K ształty fragm entów piaskow cow ych w utw o­

rze osuw iskow ym wskazują, że m ogły być osiągnięte tylk o w tedy, jeśli były w półplastycznym stanie. (Por. tabl. XII i XIII).

4. Osuwisko w Zadzielu koło Żywca, w warstw ach istebniańskich dolnych. Wśród piaskowców w ystęp uje tu w arstw a iłu w ym ieszanego z piaskiem i drobnym żw irem , zawierająca bloki piaskow ców i sfałdo^

wane fragm enty łupków. Osuwisko w Zadzielu, o niew ielkiej miąższości (2— 3 m), jest częściowo związane z osadem utw orzonym przez sp ływ m ułow y. Osuwiska tego typu częste są w innych w arstw ach fliszu kar­

packiego (w arstw y babickie, górno-istebniańskie, inoceram owe, grodzi­

skie). (Por. fig. 9— 11).

Osuwiska pojedynczych ław ic, należące do drugiego typu osuwisk, są znacznie częstsze. Można wśród nich wyróżnić następujące typy:

a. Osunięciu ulega tylko górna część ław icy (fig. 12— 13 oraz tabl.

X IV, 1), natom iast dolna część ław icy nie jest zaburzona. Jest to typ stosunkowo częsty w e fliszu, zwłaszcza w warstwach krośnieńskich, ino­

ceram ow ych i w piaskowcu m agurskim . N adścielające łupki, czasem z konkrecjam i syderytycznym i (tabl. X IV , 1), mogą być objęte ruchem osuw iskowym , często jednak ruch się odbył, zanim łupki się osadziły.

Wśród tego typu istnieją przejścia do w arstw ow ania spływ ow ego.

b. Osunięciu ulega cała ławica, w w iększym lub m niejszym stopniu rozpadająca się na b ryły i drobniejsze fragm enty, zw ykle zaokrąglone przez zw ijanie (fig. 16, 20, 22 i tabl. X V ). Zwinięcia można rozpoznać po hieroglifach w ystępujących na obu stronach zw iniętych fragm entów (tabl. XV ) lub po ułożeniu przekątnego warstwow ania (fig. 20, rysu­

nek prawy). W niektórych przypadkach ruch osuw iskow y objawia się sfałdow aniem ław icy.

c. Osuwająca się ław ica rozpada się na p łyty, które zsuw ając się, za­

wijają w górę (częściej) lub w dół (rzadziej) sw oje przednie kraw ę­

dzie (fig. 17 i 18).

d. Dolna część ław icy ulega osunięciu, górna nie okazuje zaburzeń osuwiskowych.

e. Osuwiska zaczątkowe można rozpoznać po poprzerywaniu i roz­

sunięciu poszczególnych części ław icy (fig. 23, 24). Stw ierdzono istnienie szczelin przecinających ław ice prostopadłe do kierunku prądu (tabl. X V I oraz tabl. X IV, 2) zaznaczonego ripplemarkami, hieroglifam i prądow ym i lub lineacją prądową. Tworzenie się szczelin prostopadłych do kierunku prądu wskazuje, że rozciąganie odbywać się m ogło pod w p ływ em siły ciężkości w kierunku pochyłu dna. W niektórych przypadkach takie tensyjne szczeliny zostały w ypełnione piaskiem w ciśn iętym od dołu (fig. 25), w skutek czego pow stały piaszczyste grzbieciki na górnych (tabl. X IV , 2) lub dolnych (tabl. XVI) powierzchniach ław ic. W tym drugim przypadku grzbieciki te przypominają pozornie szczeliny z w y ­

sychania (wypełnione), z którym i oczyw iście nie mają nic w spólnego.

W w ielu wypadkach można na podstaw ie kierunku zaw inięcia osu­

niętych w arstw i innych cech próbować ustalić kierunek osuwania się.

W w iększości przypadków kierunek ten jest zgodny z kierunkiem p r ą ­ dów, które w danym obszarze składały osady. To m oże wskazywać,

— 125 —

że tw orzenie się osuwisk było zależne głów nie od pochyłu dna. W n ie­

których jednak przypadkach stw ierdzono kierunki skośne w zględem kierunku prądu albo naw et im przeciw ne.

Zakład Geologii UJ K ra k ó w , grudzień 1957

A b s t r a c t . Submarine slum ping in the Carpathian Flysch is described. Two types may be distinguished, in one type only one or two sandstone layers, in the other — several beds are affected by slumping. The second case is much less frequent.

INTRODUCTION

In a previous paper (1950) concerning post-depositional deform ations of the F lysch rocks in the Carpathian area tw o types of sliding m o­

vem ents w ere distinguished: in one typ e of sliding that affected sedim ents just after deposition intense contortion of sandy beds w as produced but continuity of layers was not broken; the other typ e of m ovem ent leads to disruption of beds w ith little or no folding. The first type of deform ation w as determ ined as slip-bedding and th e contortions ascribed to gravitational gliding. The other type, to w hich th e present note is devoted, is determ ined as slum ping.

Subaqueous slum ps h ave been described from m any areas; th e pa­

pers of H a d d i n g (1931), H e n d e r s o n (1936), J o n e s (1S37, 1939), C o o p e r (1943), K u e n e n (1949), B i e ł o s t o c k i (1955) m ay be quoted here only as exam ples of description of this kind of deform ation;

m any of them contain exten sive bibliographies w hat is not intended to be given in this note. Subm arine slum ps occur in th e P olish F lysch Carpathians as I pointed out in the previously quoted paper.

A subm arine slump w as also described from th e Central Carpathian Podhale F lysch by G o ł ą b (1954). R a d o m s k i (1957) m entions also th e presence of slum ped and disrupted beds in this unit. Closer re­

search show s that nearly every stratigraphic unit of the Carpathian F lysch contains deform ation caused by subm arine slum ping.

In this note it is not intended to give a com plete picture of subm a­

rine slum ping in the Carpathian F lysch as certainly the observations of the present w riter do not cover the w hole field of this problem in the area. It aims only to present a few exam ples of structures pro­

duced by slum ping that the author regards as typical and representative of slum ping phenomena in the discussed area.

G enerally speaking slum ping in the Carpathian F lysch affects either several layers of beds, or only one or tw o neighbouring beds. Accor­

dingly w e treat slum ping phenom ena w ithin th e area under tw o headings:

large scale slum ping and one-bed slum ping.

I. LARGE SCALE SLUMPS

1. S l u m p a t M a l i n k a (Upper Godula beds, Upper Cretaceous)

A very interesting case of slum ping is w ell exposed in Cieszyn Silesia south of Wisła. Near the top of the Godula beds w hich in their

upper part consist of thin-bedded and fine-grained sandstones alter­

nating With shales, a b elt of conglom erates and coarse-grained sand­

stones occur. The conglom eratic belt is presented on the map prepared by Miss J. B u r t a n (in B u r t a n , K o n i o r , K s i ą ż k i e w i c z 1937) as a fairly continuous zone w ith som e interruptions in its course.

A t the locality jV alinka the conglom eratic belt is represented by a lense cut. through by the stream Czarna W isełka (Black Vistula). The deep gorge of this stream offers com paratively good outcrops in its w alls and in the river-bed. A lso on the h ill situated east of the valley th ey are a few exposures of conglom erates although m uch poorer than in the valley.

A lready at first sight the beds in the gorge exhibit features unusual for F lysch beds. Bedding of the conglom erates is not regular, bedding planes not distinct and even. The conglom erate consists of pebbles 1 — 3 cm in diam eter; larger pebbles are rather not frequent. Quartz is the m ain component, besides black lydites and very rare gneisses and crystalline schists occur among the pebbles. Grading is not visible.

At m any places the conglom erate contains large boulders, pebbles and slabs of sandstones (Fig. 1— 4 and P late X — XI). These sandstones are fin e- to m edium -grained, w ith no conspicuous grading and only w ith some feeb ly marked lam ination, which appears only in som e fragm ents. The sandstone is glauconitic, contains little mica, and g e ­ nerally resem bles thicker sandy layers of the Godula beds. Fragm ents of sandstones occurring in the conglom erates are of various dim ensions.

Sm all fragm ents below ^{1 0} cm in diam eter are not frequent; m ore often th e diam eter is 20— 40 cm and fragm ents of this diam eter are more or less distin ctly rounded w h ilst sm aller ones are angular. Larger frag­

m ents are again not very frequent, and are generally not rounded but occur as angular blocks or slabs exhibiting som etim es som e bedding;

such slabs are on the w h ole parallel to the bedding o f th e enclosing conglom erates, although som e are in oblique position and a few even at right angles to bedding. The largest slab is 6 m long and 1,5 m thick.

If sandstone boulders or slabs are rem oved by stream erosion lar­

g e holes left a fter them can be seen in the w alls of the gorge or in th e river-bed, looking lik e potholes. A few sm aller slabs are d istin ctly folded (Fig. 1— 2) w h ilst the conglom erate in which th ey are im bedded show s no folding. O ften the contact b etw een the conglom erate and th e sandstone boulders is n ot sharp, but there are apparent gradual passa­

ges from the sandstone into the conglom erate so that no distinct boun­

dary can b e drawn betw een them . Sm all pebbles of quartz stick in the sandstone, evid en tly pressed into it w hen the sandstone was still w eakly consolidated. Therefore the surface of sandy boulders taken out from the conglom erate is covered w ith quartz pebbles sticking in the sand­

stone in a sim ilar w ay as pebbles stick in the surface of an „armoured clay-b all”. Som e contacts betw een the conglom erate and the sandstone fragm ents are sharp, som etim es one boulder exhibits a tig h tly welded contact w ith the conglom erate on one side and a sharply marked boun­

dary on the other (Fig. 2).

The sandstone boulders and slabs are not even ly distributed in the conglom erate. Som e parts of it are very rich in sandstone fragm ents

— 127 —

(Plate X), in others th ey are scarce or even absent. It does not seem that any rule governs the distribution of sandstone fragm ents but on the w h ole it appears that th ey are m ore frequent in th e conglom erates that are cropping out in th e stream -bed than in the conglom erates ap­

pearing on the slope of the hill, i. e. that th ey are m ore num erous and possibly larger toward the top of th e lens.

Fig. 1. Slump at Malinka. Sandstone blocks (p) enclosed by conglomeratic sand­

stone (z); v-denotes screes

Fig, 2. Slump at Malinka. A folded slab. For explanations, see Fig. 1

One possibility of explaining th e occurrence of blocks, pebbles and slabs of sandstones in the conglom erates is that a sandstone bed was eroded, and this erosion produced fragm ents which w ere eventually

«embedded into the conglom erate. This m ust have happened w hen the sandstone w as only w eakly indurated.

H ow ever, if boulders and slabs w ere an erosional product, it w ould be very difficu lt to understand w h y sm all fragm ents occur so seldom in the conglom erate; this presum ption does not explain either w hy som e fragm ents are distin ctly folded o f contorted. The occurrence of large unrounded slabs is also inexplicable.

It is m uch m ore sim ple to attrib ute the features observed at Ma­

linka to slum ping than to erosion. One can presum e that conglom erates togeth er w ith sem i-consolidated sandstones slid dow n a subm arine slope.

Fig. 4. Slump at Malinka. For explanations see Fig. 1.

The sandstone layer (or layers) w ere broken into pieces w hich slipped down together w ith gravels. It is possible that the sandstone layer was deposited before th e deposition of gravels, or formed an intercalation (or a few intercalations) in gravels. Overburdened by subsequent deposi­

tion, gravels and sandstone layers slid down. Fragm ents of th e sand­

stone, if not too large, w ere rolled w ithin th e slum ping mass of gravel.

Rolling produced more or less developed rounding, and, as the sand­

stone boulders w ere not y et qu ite consolidated, their surface becam e

„armoured” w ith pebbles.

As it was pointed out above, the beds exhibiting slump structures occur in a lens. The shape of th e lens is w ell given on the map of Miss B u r t a n. The lense is ca. 400 m. long (along the strike) and m ore than

— 129 —

150 m. w ide (across the 'Strike). Its northern end is truncated by a fault owing to w hich the conglom erates of the lens dipping w est (ca. 15°) are in contact w ith the typical Upper Godula beds (thin-bedded sandsto­

nes and shales) dipping east (40°). The southern end of the lens is to all appearance normal, and the conglom erates extenu ate among the sha- ly Upper Godula beds in this direction. Taking the m ean dip of the conglom erates as 15°, w e obtain the thickness o f th e slum p deposit as ca. 40 m.

The slump structure at Malinka exhibits no features w hich could indicate the direction of slum ping m ovem ent, and this can be only in­

directly determ ined.

The underlying beds, composed of thin-bedded sandstones and sha­

les, have su fficien tly developed directional features, as both current bedding and flu te marks are abundantly present. In a tributary rivu let east of th e lens flu te marks have a direction 60° 1, i. e. toward NE w h ile th e m easurem ents of current bedding g iv e directions contained b etw een 30 and 90°. From this it can be inferred that the sandstones of the Godula beds w ere deposited by currents flow ing from the sout-w est.

Covering beds are not exposed su fficien tly above the len s for direc­

tional exam ination but just close to the lens at its northern end th e U pper Godula beds are w e ll seen in river-bed outcrops. H ere the direction 70— 75° of flute marks and closely related directions o f current bedding could be measured.

Farther dow nstream , w here the strike of beds changes consider­

ably, flu te marks have direction 135— 145°, i. e. toward the south-east.

G enerally it m ay be stated that in the close vicin ity of th e slump the underlying and covering beds have features pointing out to th e north-east direction of currents. This is consistent w ith the m easure­

ments! of the direction of transport w ithin the Godula beds in other areas (Silesia, Sola valley, W adowice area), w here north-eastern and eastern direction of currents has been found by the present writer.

The conglom eratic beds of Malinka exhibit neither flu te marks nor current bedding. On the rock w all of the gorge facing a little dam on the river one can see sandstone boulders and blocks arranged in a w ay suggesting imbrication, w ith longer axes of boulders dipping w est or w est-sou th -w est. Sim ilar im bricate arrangem ent is visible, although less marked, at few other places. This is not a certain proof but seem s to point out that if boulders were transported by traction along the sea-floor, they could have been arranged w ith im brication dipping up the current. If so, this would m ean that sandstone boulders were trans­

ported from the w est. It should, how ever, be pointed out that in the Carpathians there are known several cases w here pebbles are arranged w ith im brication dipping downcurrent. This kind of im brication in grad­

ed beds was found also by K o p s t e i n (1954).

1 A ll directions in this paper are given as azimuths (from the north, clockwise).

9 R ocznik PTG.

The outcrops in question w ere visited in autum n 1957 in the com ­ pany of Professor Ph. H. K u e n e n who pointed to the author that the conglom erates containing slump boulders exhibit some orientation of longer axes of pebbles. Several m easurem ents have been executed after­

wards and it has been found that there is a lineation of longer axes in the conglom erates although not very distin ctly marked, i. e. that on a unit area no more, than 30— 40% of pebbles shows aligned longer axes. The lineation measured in the outcrops w ith slump structures is contained betw een values 70/250° and 95/275°. This means that the axis of flow approxim ates the direction east — w est. The first given value (70/250°) refers to a bed possibly not involved in slum ping m ovem ent as the con­

glom erate layer showing this lineation does not contain any boulders.

It seem s that values betw een 80/260° . and 95/275° are m ost frequent w ithin the slumped mass. Above the slump upstream the conglom erates are w ell exposed w ithout any slump structures. H ere lineation is m ost perfectly developed; at m any places m ore than 80% of pebbles is aligned w ith parallel longer axes (Plate X I, 2). Not only alignem ent of particu­

lar pebbles m ay be observed but also the same lineation is shown by strings o f pebbles running on the surface o f beds in th e sam e direc­

tion 1.

N

Fig. 5. Orientation diagram of directional features, slump at Malinka. 1-Direc­

tion of fluting in underlying beds. 2-Direction of fluting in overlying beds. 3a—3b Range of directions of current bedding in underlying beds. 4-Pebbles lineation in undisturbed conglomerates. 5-Pebbles lineation in disturbed conglomerates

1 It was thought in the beginning of sedim entary studies in the Polish Flysch Carpathians that linear arrangement of pebbles is rather exceptional and hardly so conspicuous that could be found out without the help of some statistical stu ­ dies. Moreover, although at a few places imbrication was found, the data given by it were often in conflict w ith the data furnished by measurements of current bedding and fluting. In this respect K o p s t e i n ’ s work (1954) on pebbles li­

neation in the Cambrian of Wales evoked a new interest in this method. It ap­

pears that in some series there is an apparent concordance of pebbles lineation w ith fluting resp. current bedding, in others there no such concordance, and in several cases there is no linear arrangement of pebbles. The problem requires further research.

— 131 —

From th ese m easurem ents it follow s that th e conglom erates undi­

sturbed by slum ping exhibit a much more perfect lineation than the con­

glom erates involved in slum ping m ovem ent, and, that even taking into consideration the influence of changes of the strike of beds, on the w h ole insignificant, there is some difference of the axis of m ovem ent o f disturbed beds (approaching axis W— E) in comparison w ith the axis of flow w ithin undisturbed beds (approaching S W — NE direction, comp.

F ig - ^•

A t any rate, the supposed direction of slum ping m ovem ent does not differ m uch from the direction of currents w hich deposited both the fine-grained sandstones and the conglom erates. This m ay indicate that th e slump occurred in the direction of th e slope of the sea floor w hich controlled the direction of sand and gravel transporting currents.

There are no data perm itting the estim ation of the distance along w h ich the slump moved. Large and feeb ly distorted slabs occurring in th e slum ped m ass seem to indicate that the distance w as not great, possibly only a few hundred m etres.

It m ay only be guessed w hat w as th e cause of the slum p. P ossib ly th e direct cause was an overloading o f th e sea-floor w ith regard to its profile. The profile w as generally in equilibrium during the deposition of th e fin e- grained Godula beds as th ey contain com paratively infrequent slum ped beds. It is possible that th e deposition of thick conglom erates disturbed this equilibrium and caused local slum ping. On th e other hand the appearance of coarse-grained sedim ents after the prolonged period o f the deposition of fine-grained sedim ents certain ly points out to som e uplifting m ovem ents in the neighbouring areas, possibly as th e first singns of the Laramide orogeny ( K s i ^ z k i e w i c z 19 5 4 a). T hese m ovem ents m ight have increased the slope of th e sea-floor. It is possible that this w as th e cordillera bordering the Silesian basin from the south w hich was raising; its uplift could have steepened the d ecliv ity of th e southern slope of the trough in w hich th e Godula sands w ere deposited w h at could have caused the observed differen ce b etw een th e direction o f transport and the presum ed direction of slumping.

A striking feature of the slum p at Malinka is the absence of any c la y balls or shale fragm ents, so num erous in other slum p deposits in th e Carpathian Flysch. A ctually, the conglom erates in w hich the Ma­

linka slump structure occurs, contain very few if any sh aly interbed dings.

It is doubtful w hether such a large slump could be form ed b y slum p­

ing of a single bed. It is more lik ely that several conglom eratic beds interbedded w ith sandstones w ere involved in slum ping. It is possible that originally sandstone beds were term inations of graded conglom era­

tes, but in no case primary transition from the conglom erate into the sandstone has been observed. If such transitions existed they w ere obliterated during slumping.

This type of slum ping seem s to be very infrequent in th e Car­

pathian Flysch. Sim ilar structures can be observed in the Silesian Car­

pathians in conglom eratic beds of the Upper Godula beds w hich con­

tain som etim es fragm ents o f sandstones and shales w ith obvious signs

of contortion. These are, how ever, sm all scale features e. g. a conglo­

m eratic layer occurring in the Malinka valley dow nstream from th e great slump. The layer is only 40 cm thick and contains lum ps of lam inated sandstones and fragm ents of shales. Contortions visib le in sandstone lum ps suggest that th ey w ere slid down togeth er w ith conglom eratic material.

The right bank of the Soła river at Ż yw iec (Polish W estern Carpa­

thians) exhibits ex cellen t outcrops of in ten sely folded Lower Creta­

ceous beds (Cieszyn = Teschen beds). The outcrops that are of special interest are situated opposite the paper factory.

Here the Upper Cieszyn shales (Valanginian) are folded into an anticline, the axis o f w hich passes through a sm all tributary valley, occupied by a landslide. In th e core of th e anticline there occur th e ty p i­

cal Upper Cieszyn shales (fine-igrained and thin-bedded sandstones).

In both limbs of the anticline thick intercalations of silts containing num erous fragm ents and pebbles of lim estones occur in th e Upper Cieszyn shales. In the southern lim b th e intercalation is ca. 28 m.

thick, in the northern limb tw o layers o f „pebb le-clays” are intercala­

ted, th e low er 3 m., and the upper 15 m. thick, separated by typical Upper Cieszyn shales.

The ,,pebble-clay” contains pebbles of various lim estones, m ainly dark Tithonian lim estones, w h ite oolitic lim estones, Cieszyn lim estones (both pelitic and detrital varieties), and also fragm ents of sandstones from the Upper Cieszyn shales. Pebbles, som e of them only very sligh tly worn, are chaotically arranged in a m ass of structureless hard m arly silt w ith num erous sm all grains and fragm ents of lim estones. This deposit m ay be regarded as a m udflow sedim ent ( K s i ^ z k i e w i c z

1954).

In the northern lim b of th e anticline a thin (ca. 2 m.) intercalation of pelitic lim estones m ay be seen follow ed again by th e Upper Cieszyn

2. S l u m p s a t G r ó j e c (Cieszyn beds, Neocom ian)

Fig. 6. Slump at Grojec. Lumps of marls in shales

133

shales 8 m. thick. Above them lie m arly shales w ithout sandy in ter­

calations but containing large blocks of sideritic marls and lim estones.

Shales are strongly compressed and contorted. Their thickness is about 1 m. They are covered by a 8 m. thick layer of strongly contorted m ass of m arly clays w ithout schistose cleavage usual for th e Cieszyn shales. These clays are compact, hard and break into irregular lumps.

W eathering discloses their strong internal contortion, for th e clay is nearly black but is lam inated owing to th e presence of very thin w hitish layers w hich are better seen on w eathering. There are also very thin layers of sand. It can be seen that th e cla y consists of irregular lum ps of w ell lam inated clay cem ented by a clayey m ass w ith poorer lam ina­

tion (Fig. 6). The size of lum ps is larger nearer th e bottom . Sand strings are folded and their continu ity is broken, and if sideritic m arls occur w ithin the contorted m ass they are broken into irregular pieces (Fig. 7).

Fig. 7. Slump at Grojec. Contorted clays. Black — sideritic marls

Above the contorted clay rest a few layers of lim estones w hich are detrital and graded, as usually th e Cieszyn lim estones are, but contain in their upper part unusual am ount of fragm ents of shales and even of lam inated sandstones. Their thickness is 3 m. T hey are co­

vered by a 30 cm. thick sh eet o f clay m ixed w ith grains and pebbles of lim estones resem bling the sedim ent described above as m udflow deposit, but pebbles are neither so large nor so num erous. The m ud­

flo w deposit is surm ounted by 1,5 m. thick bed of lam inated contorted clay, above w hich lies a bed ² m. thick consisting of stron gly contort­

ed sh aly clays containing num erous lum ps o f w ell lam inated fin e- -grained sandstone. The lumps, a dozen cm. or so in length and only a few cm. thick are often internally folded (Fig. 8), th ey are also often bent and crum pled (Fig. 21). Their shapes, w edging out and contor­

tions independent from the enclosing clays indicate that th ey w ere deform ed w h en not y et quite consolidated. T h ey strongly resem ble ,,,crumpled balls” described by K u e n e n (1949) but are m ore flat than roundish. The layer w ith crumpled lum ps is covered by contorted lam i­

nated clays w ithout sandstones w hich gradually pass upward into shales w ith no internal disturbances.

The described beds are strongly tectonically engaged, and th ere are iseveral features as slickensides, sm all scale faults, calcite veins etc., due to tectonics. B ut there is little doubt that contortions, crum ­

pling, breaking of sid erites and m arls and sandstone layers into sepa­

rate lum ps and strings was caused by slum ping before the deposition of overlying shales. Probably there occurred a few slum ps in quick succession w hich deposited th ese disturbed beds, sum m arily ca. ^{2 0} m . thick.

Fig. 8. Slump at Grojec. Crumpled sandstone lumps in a structureless clay

There are few features assisting in determ ination of the direction of slum ping m ovem ent. Both clays and th e imbedded lum ps of sandstone have bends and folds closing generally toward the north-w est. T his would mean that th e sliding mass m oved from th e south-east. A s flute marks in the Cieszyn lim estones in neighbouring outcrops are directed toward east ^south-east (95— 110°), current bedding in the U pper Cieszyn shales toward south-east and fluting in these beds in a sim ilar direction (95°), it m ay be presum ed that the slum ps took place obliqu ely and nearly from an opposite direction w ith regard to th e direction of flow.

3. S l u m p s a t P o r ę b a W i e l k a (Inoc erami an beds, Upper Cretaceous)

In the southern fringe of th e tectonic w indow o f Mszana, mapped by S w i d e r s k i (1953) there occurs a narrow band of th e Inoceram ian beds. South of Poręba W ielka the main stream (Poręba) exposes a very good cross-section of these beds and their contact w ith the O ligocene Krosno beds and blaok shales of the w indow series. The contact is strongly disturbed, and the Krosno beds and black sh ales are folded together w ith Cretaceous beds. A cross-section o f an overturned fold may clearly be seen w ith the O ligocene in the core and the Cretaceous in both . lim bs (in the northern inverted lim b w ith hieroglyphs on upper sides). In the southern lim b of the fold the Cretaceous begins w ith a few sandstone beds dipping south; above them crop out strongly disturbed beds w ell observable in the streambed along a distance of 110 m. They consist o f strongly contorted and pressed calcareous shales containing num erous sm all and large blocks of sandstones (Plate X II and XIII).- The sandstones are of the type occurring norm ally in the-

— 135 —

Inoceramian beds: fine-grained, calcareous and m icaceous w ith fine la­

m ination and often current bedded. The lumps are of variable size, but th e largest hardly exceed 1 m. in diam eter and generally are m uch sm aller. Their shapes are variable too; som e are fairly w e ll rounded but m ost is more or less angular, their edges, how ever, are not sharp but sligh tly rounded or obliterated. M any of these lum ps are tw isted, and som e are fin ely folded (Fig. 20, center); both large and sm all lum ps m ay be tw isted or folded (Plate X III, 1). Shapes o f lum ps indicate that sandstones w ere still plastic during deform ation.

The dim ensions of th e slump m ass and the appearance of parti­

cular lumps indicate that the slump w as formed by sliding of several sandstone beds together w ith intervening shales. During slum ping beds w ere broken into pieces which w ere either dragged or rolled w ith som e folding and tw isting.

The thickness of the slum ped beds is about 55 m. The len gth (along th e strike) is not known, but in neighbouring stream s at a distance of a few hundred m etres no such strata are visible. A t any rate this slum p m ay be regarded as th e largest slump that so far has been discovered in the Polish Carpathians, and possibly it is only in len gth exceeded by the slum p of Malinka.

The direction of slum ping m ay be to a certain ex ten t determ ined.

Most of the lum ps, if th ey are folded, have their bends closed in th e w estern direction 3. The sam e is the direction of flow in the Inoceram ian beds w ithin the area. In the cross-section of the stream above the slump I found the follow ing directions of flu te marks: 310°, 300°; in th e Koninka v a lley (a stream east of the Poręba stream): 280°, 340°, 310°, 350°, 345° (but also 110°, 195°, 120°, these directions w ere found solely in thick-bedded sandstones w ith coarser grain). A s the direction of currents was controlled by the slope of th e sea-floor, it can b e concluded that the slum p m oved w ith th e slope.

It should be stressed that the slump mass together w ith the beds below and above is stron gly tectonized. Num erous slickensides, sm all scale faults, calcite veins cutting both sandstone lum ps as w ell as shales, com pression of shales etc., are evident features o f strong tectonic enga­

gem ent and introduction of additional distortion to already disrupted and contorted beds. The slum ped beds occur at the very base of th e Magura nappe, and it is n ot surprising that tectonic in flu en ce is so strong in this case.

1 Naturally, if bent lumps are only broken limbs of folds, no conclusion can be drawn from this fact, as bends should be closed in this case either way.

However, from the observations made on single slumped beds, as it is pointed below (p. 144), the conclusion may be borne out that folding rarely preceds disrup­

tion of beds during sliding; at first a bed is broken into lumps or sheets, probably owing to stretching, and subsequently they slid down; during sliding the frontal part of a sliding sheet bends upward or downward, and in this w ay folded lumps are formed. Thus the predominance of one direction of closures in lumps m ay be helpful in determining the direction of slumping.

4. S l u m p a t Z a d z i e l e (Lower Istebna beds, Senonian)

North of Ż yw iec, in the Soła valley, a large quarry is situated close to the road betw een Zadziele and Tresna. Thick-bedded and coarse­

grained sandstones w ith very thin intercalations of shales, and often w ithout them , are exposed here. The sandstones in this quarry are feeb ly graded, not very regularly bedded and contain fairly num erous shale fragm ents. Som e of them are typical „armoured clay-b alls”, i. e. w ith the surface covered w ith larger quartz grains and pebbles. If the clay is rem oved, the hole after it is lined w ith pebbles stuck into the enclos­

ing sandstone. This feature, fairly common in the Carpathian F lysch (Grodziszcze sandstone, Istebna beds, Ciężkowice sandstone), seem s to be particularly common in this quarry.

In the upper part of the w all exposed in the quarry a dark band is visible, contrasting w ith the light colour of th e sandstone. This band consists of a cla y ey layer exhibitin g intense slum ping features. The layer is not of constant thickness w hich in average is 2,50— 3,00 m., and dim inishes rapidly toward th e north. The m ain body of the layer consists of clayey m ass m ixed w ith a large proportion of quartz sand (ca. 50%). The grain size of quartz sand iis ca. 0,1 mm., but there are also fairly num erous larger quartz grains and even pebbles of 5^{— 8} cm.

in diam eter. Such typ e of sedim ent m ay be regarded as a m ud-flow deposit, very common in the Carpathian Flysch.

In this m ixture of clay and sand, how ever, there are num erous lum ps o f shales and sandstones w ith apparent features of slum ping (Fig. 9). Lumps o f sh ales are folded into regular anticlines and sy n cli- nes, or rolled into balls, som e of them perfectly closed, spherical or roundish. There are also blocks of sandstones, som e of them also roun­

dish and showing internal closing of lam ination w h at points ou t that they w ere rolled into sand-balls, others are irregular in shape or round­

ed but w ith no stratification or, if an y stratification is seen, there is no internal contortion of it. Som e of th e lum ps consist of thin layers of shale and fine-grained sandstone, and rem ind much the top part of the graded sandstone beds below the slump bed. A lso som e of the sand­

stone blocks have the sam e composition and appearance as the uppermost part of the Istebna sandstones. This indicates that th ese blocks and sheets, both of sandstones and shales, w ere torn aw ay during th e slum p­

ing m ovem ent from th e underlying sandy bed. It m ay be noted, h o­

w ever, that som e sandstones, occurring alw ays as w ell rounded peb­

bles, are different from the Istebna sandstones; th ey are coarse-grain­

ed and contain mica and glauconite, absent in th e norm al Istebna beds.

A very significant feature is th e presence of clay-balls, spherical or ellipsoidal (flattened probably by compaction), armoured w ith quartz pebbles packed closely on their surface and in upper layers.

— 137 — T h e origin o f this slum p is clearly connected w ith a m udflow . P resum ably a norm al subm arine m udflow consisting o f clay, sand and gravel w hen m oving on th e sea-bot- tom either tore aw ay som e sheets o f shales covering a sandstone bed together w ith som e pieces of the sandstone, or exerting drag on them caused their slum ping. The under­

ly in g sandstone m ust h ave been partly consolidated as it did not di­

sin tegrate into sand; also' shales m ust have been w ell consolidated as their stratification is w ell preser­

ved, and evid en tly th ey did not crum ble back into clay w h en slum ­ ping. It is rather d ifficu lt to im a­

gine that w ell compacted clays w ere form ing the sea-bottom ; they m u st have had above them som e beds which caused their compac­

tion. It m ay be, therefore, presu­

m ed that th ey w ere covered by a few beds of sandstones and clays poor­

ly y et consolidated, w hich slumped and disintegrated into a m ixture of clay and sand. This m ixture d e v e ­ loped into a m udflow w hich was tearing away lum ps of underlying b etter consolidated beds.

It is characteristic that m ost of th e sandstone blocks and shales lum ps occur in the upper part of th e layer w hile th e bottom part is n early devoid of them . P ossib ly this indicates that th e slump travelled som e distance, so that th e lum ps torn aw ay from th e base, had tim e to accum ulate in the upper part, sim ilarly as shale chunks in graded sandstones tend to accum ulate in

Fig. 9. Slump at Zadziele. Fragment of the slumped pebble-clay w ith sandstone balls and blocks, folded lumps of shales and armoured clay-balls

top part (K u e n e n 1957) after travelling som e distance in a turbi­

dity current 1.

There is no evidence as to the distance covered by this m udflow , but there are indications from other areas of the Carpathians that some subm arine m udflow s could travel a distance of at least 30 km ( K s i ą ż ­ k i e w i c z, in press).

The slump layer is covered by a layer of shales 1 to 2 m thick w ith no slump structures. This shalv layer is much thicker than the shaly layers covering each sandy bed below and above the slump. It seem s that the m udflow and the resulting slum p w ere accompanied by suspended clouds of clay from w hich the covering sh ales w ere de­

posited.

There are no directional features in the described slump structure, and nothing can be said about the direction of slum ping. N eith er the covering and underlying sandstones furnish any hints as to the direction of currents w hich deposited them . L ittle is known of th e direction of fluting in the Istebna beds w ithin the area, but not far from here, in the W adowice district north-east directions m ay be noticed. In the surm ounting Upper Istebna beds sm all-scale current bedding points toward the north or north-east. Two pictures (Fig. 10) from the quarry at Zadziele represent asym m etrical flow casts w hich presum ably indi­

cate sand m ovem ent also to the north. W hether th e slump had also this direction, it is difficu lt to say.

Fig. 10. Upper Istebna beds at Zadziele. Flow casts at the base of sandstone beds.

Some current bedding visible in fine-grained layers

1 It should be noted here, however, that shale fragments occur fairly fre­

quently in many of the Carpathian sandstones near the bottom surface. Possibly this may indicate these sandstones w ere not deposited far from the source.

— 139 —

M udflow deposits are very frequent in th e Carpathian Flysch, and there is hardly any stratigraphic unit w ithout them . The G ro­

dziszcze beds (Hauterivian-Barrem ian), both Lower and Upper Istebna beds and the Babica beds abound in these deposits. The latter have been described in som e detail by B u k o w y (1956) who noticed that m udflow s could erode their bottom 1. There are fairly num erous cases of slum ping connected w ith m udflow deposits, similar to that of Za- dziele, e. g. in other occurrences of the Istebna beds, also in the Gro­

dziszcze beds etc. Fig. 11 presents an exam ple from the Inoceramian beds. In this instance it seem s to be clear that the slump m oved from

Fig. 11. Slump structure at Binczarowa, Inoceramian beds. Bed is inverted. P ebble- -clays w ith slumping in top part

left to right (beds are overturned, the picture should be reversed), i. e. toward the w est, in conform ity w ith the sense of transport in the Inoceramian beds in the area.

II. ONE-BED SLUMPS

In the instances described above several beds w'ere involved at once in slum ping. It should be underlined that such cases are rather exceptional. Much more frequent are the cases in w hich only one or at m ost two layers of sandstones underw ent slumping.

As an exam ple of this ty p e of slum ping a distorted sin gle bed is described first. A t Tresna in the Sola v a lley (W estern Carpathians) there is a good outcrop in the Godula beds near the bridge. The Godula beds consist here of even ly bedded sandstones which are not conspicu­

ou sly graded, but th e top part of sandy layers is alw ays m ore fin e - -grained, lam inated and richer in mica and plant detritus.

One sandstone bed exhibits different features. A t th e base it is even ly bedded but a few cm. above the sole shows contortions and inter­

1 A fter giving this paper to the Editor I came across the paper of J. C. C r o- w e 11 (Origin of pebbly mudstones, Bull. Geol. Soc. Aroer., vol. 68) in which he described plow ing action of submarine m udflows (p. 999).

nal folding, w hich becom es more intense toward th e top (Fig. 12). The coherence of the sandstone changes toward th e top too. A t th e base the sandstone is firm and compact w h ile th e distorted part is m ore friable

although still com paratively hard, m ore muddy, and contains much more mica and plant detritus than th e lower part. Bands of shales, apparently deriving from the covering shales are folded together w ith the sandstone. There are several closures of sm all overturned and even plunging folds, all closures bent toward th e east. This would m ean that this was the direction of slumping. The general autlook of th e bed seem s to oonfirm such an inference, as the folded mass at A in Fig. 12 seem s to be a rootless ,,nappe” w hich slum ped to the left from beyond th e shales at B. Its composition, plen tifu l mica and plant detritus indicate actually that the slum ped mass belonged to th e upper part of the originally graded bed. The direction of slum ping coincides w ith the direction of flu te marks (30— 80°) in the outcrop. This m eans that the slump follow ed the general slope of th e sea-floor.

Fig. 13. One-bed slump, Magura sandstone, Skawica. Note the undisturbed base of the bed

In the described case only th e top part of the bed underw ent slum ping, w h ilst the bottom part does not show any trace of distor­

tion. There are num erous cases of this sort, and tw o kinds m ay be di­

stinguished depending on w hether the covering shales are involved or not into slum ping m ovem ent. P late XVII, 3, 4 and Fig. 13 represent a fairly frequent case in which slum ping produced large balls pushed one on another. Probably to this typ e of internal slum ping belong the

featu res presented on Fig. 14 w here no other signs of slum ping are visib le except the trace of contact b etw een the low er unaffected part and the up­

per slid portion of the bed. Such features m ay be observed in thick- and

— 141 —

Fig. 14. Trace of internal slumping in sandstone bed. Inoceramian beds, Zaryte

m edium -bedded sandstones; I observed them in th e Lower Gouula basal sandstone (e. g. quarry at Porąbka), in the Inoceramian beds, in the H ieroglyphic beds and in the Magura sandstone in w hich th ey seem to be particularly frequent. Som etim es one can see on the contact som e m inute corrugation (PI. XV III, ⁶) or som e sort of contortion but usu ally the surface is smooth and both portions are w e ll w elded.

Fig. 15. One-bed slump. Krosno beds, Besko. Slumped bed contains siderites (s) and shale fragments (b)

A remarkable case belonging to the type of slum ping w here only th e upper part of a sin gle bed underw ent slum ping together w ith covering shales m ay be given here from th e Krosno beds, so w ell expos­

ed at Besko. In m y previous paper (1950, p. 494) an instance w as reported in which sideritic concretions occurring as a rule in shales, occur in sandstones owing to slum ping of a sandstone bed together w ith covering shales. P la te XIV, 1 and Fig. 15 present this case w h ere concretions and shaly lum ps are m ixed w ith a strongly corrugated sandstone. Incidentally, the presence of concretions in slum ped beds points out that these concretions m ust have been formed very early, at any rate before the deposition of the covering sandstone, and are th en probably of a syngenetic, and not a diagenetic origin.

Slum ps involving only the upper part of the sandstone bed, w ith or w ithout covering shales, are num erous in the Carpathian Flysch, and probably occur in all stratigraphic members. I know exam ples of this kind of slum ping from the Lgota beds, Godula sandstone, Ino- ceramian beds, H ieroglyphic beds, Krosno beds and the Magura sand­

stone. This type seem s to be closely related to the structures of contorted beds, described by the present w riter (1950) as ,,slip-bedding”. On the other side, slip bedding is related and to a certain extent, identical, w ith „convolute bedding” of K u e n e n (1953, cf. also T e n H a a f 1958).

There are certainly cases w here slip bedding is d ifficu lt to separate from convolution, particularly it chunks and lumps of shales are absent in the deformed bed, and w hen contortions are expressed in regular folding. At any rate, if a corrugated bed contains fragm ents of shales, what means that some slipping occurred after deposition of covering shales, such a case m ay be regarded as an interm ediate link betw een convolute bedding and slum p structures. Thus the follow ing distinction can be made:

C onvolute bedding — no horizontal slipping.

Slip bedding — convolution distorted by slipping, but continuity of bedding (lamination) is not broken. N um erous recum bent folds, in extrem e cases chunks of shales folded together with sandstone.

Slum p bedding — top part of beds slipped horizontally (PI. XV III, 5).

One-bed slum p — w h ole single beds slum ped and disrupted.

W hen the w hole bed is slum ped, it is usually broken into sm all­

er or larger pieces w ith all features of slum ped sandy sedim ents. The m ain feature of such beds is rolling into sand-balls, or m ore often, into sand-cylinders. D im ensions of such forms are variable: from sm all lum ps a few cm thick to large blocks ca. 1 m. in thickness. Som etim es the balls and cylinders are packed closely and m ore or less w elded, w ith no shales betw een (PI. XVII, 1, 2) but much more frequent are cases w hen they are m ixed w ith shales (Fig. 16). Som etim es beside roundish blocks there occur also angular sandstone blocks, that did not undergo rolling w hen sliding. In a few cases th e slum ped bed consists only of angular blocks scattered in shales.

Fig. 16. One-bed slump. Godula beds, Malinka

— 143 —

In several instances the slum ped bed consists of flat sheets of sand­

stone separated from each other, w ith their edges bent up and rolled back on one side. Fig. 17 dem onstrates such a case. Here a shaly layer

Fig. 17. Sheet-like slump, Inoceramian beds, Poręba Wielka. Note the upturned noses of sandstone sheets

Fig. 18. Various types of terminal bends in slumped sheets

ca. 0,5 m . thick, contains tw o sandy layers w ith bent up term inations.

Both layers are only a few cm. thick. The term inations are bent backward so that they look like recum bent folds w ith their noses upturned. Backward bends of these noses may be m ore or less closed (Fig. 18) but usually there are shales infolded. It seem s that the des­

cribed type of slum ping m ay be regarded as an uncom pleted slump:

a sandstone layer w hen slipping down was disrupted into a few flat sheets, the edges of w hich rolled up a little during slum ping, but otherw ise sliding sheets becam e neither distorted nor folded. If slum ping m ovem ent continued, th ey would probably have been rolled up com ple­

te ly into balls or cylinders. The upper slumped layer in Fig. 17 repre­

sents a more advanced stage towards com plete rolling up. T hese cases remind m uch beds folded back on them selves described by C o o p e r (1943, Fig. 3—4).

Exam ples of this typ e of slum ping are fairly num erous in th e Car­

pathian Flysch, particularly in th e Krosno-and Inoceramian beds. It should be added that noses of sliding sheets are not necessarily ben t upward, in som e cases, although seem ingly less num erous, they are bent downward (Fig. 18, below). In either cases the presence of bends helps in determ ining the direction of slum ping. In all exam ined cases the determ ined direction of slum ping is approxim ately conform able w ith the direction of currents, what allow s to infer that slum ping fo l­

lowed the slope of th e sea-floor.

More frequent than sh eet-lik e slumps are cases w hen slum ped beds w ere disrupted into num erous pieces which during slum ping became bent, tw isted, folded, rolled up etc. In these cases w e have a discon­

tinuous layer composed of lum ps sticking in shales. There are instances when one can surm ise that more than one bed was disrupted into lum ps but m ostly there are indications that only one bed produced slum ped lumps.

Fig. 19. Sliding and rolling up o f sandstone layers

Slum ping lum ps m ay be rolled up in more than one way: their downslope end m ay be bent upward or downward (Fig. 21); in the first case hieroglyphs and other bottom features occur on both sides of th e rolled lum p (Fig. 19 and P late XV). W hen grading or current bedding are visible, it is easy to find out w hether the original bottom is inside of th e lump or, as the case is in Fig. 20, right, it is now the outer su r­

face of the lump. So far no case has been found w ith hieroglyphs inside, w h ile lum ps w ith hieroglyphs on the outside surface are fairly numerous.

Rolled lumps m ay also be formed by folding of the slum ping bed and subsequent disruption (Fig. 19, bottom); in this case one can exp ect to find lum ps rolled either ways, their bottom surfaces on external

— 145 —

surface of som e lumps, and inside in others, but curiously, I do n o t know such instances w hat probably indicates that disruption into lumps;

is generally not preceded b y folding. Fig 22 presents th e case in w hich

Fig. 20. Rolled up slumped sandstone lumps. First and third specim en from th e Magura sandstone, Sidzina, second from the Inoceramian beds, Por^ba W ielka. N ote

bent current bedding in the third specimen. X 0,5

Fig. 21. Folded sandstone lump in shales, slump of Grojec, Cieszyn beds. X 0,5

Fig. 22. Folded and disrupted bed by slumping. Magura sandstone, Skawica

disruption was probably preceded by folding. V ery fe w exam ples are known to m e, in w hich th e w h ole bed is folded b y slum ping, but w ith no disruption.

In all these cases, if hieroglyphs are present at the bottom surface, th ey are clearly deform ed, bent or tw isted, but curiously, in spite o f

It R ocznik PTG.

evident sliding and rolling their shapes are fairly w ell preserved {Plate XV), although in some instances th ey m ay be strongly flattened.

Single beds disrupted by slum ping into sm all pieces are fairly nu­

m erous in the Carpathian Flysch, particularly in the Inoceramian beds, Krosno beds and T1/a g u ra sandstone, they also occur, but less frequently, in the Godula and Lgota beds, and in the Upper Cieszyn shales.

W hile one-bed slumps in w hich the w hole bed or the top part is slum ped are fairly frequent, very rare is the case w h en th e bottom part exhibits slump structures, w h ilst the top part is not disturbed.

The origin of such beds is not quite clear. Two explanations are possible; either the bed slum ped before the top part was deposited, i. e. that over the slumped bed a turbidity current passed and deposited its normal sedim ent in such a w ay that it sm oothed the slump and covered it w ith its m aterial which w elded to th e slum ped material; or, that the just deposited bed before the end of deposition, began to slum p and roll. The second explanation is less satisfactory, as it would m ean that the fresh ly deposited m aterial had already such a co­

herence that it could slide and' roll w ithout disintegration into sand.

III. INITIAL SLUMPING

Comparing various types of slum ping one can trace stages through which a slumped bed goes through, until it becom es only a m ass of loose lumps. N ot a ll slum ped beds arrived at this stage, having stopped a t earlier stadia.

From all available evidence one can infer that rather disruption and not folding is the m echanism of slum ping, therefore one can im agine that in the first stage of slum ping stretching fractures should appear along which the bed parcels out in sheets or lumps. In fact, there are .several instances w hen such initial stages of slum ping m ay be observed

in F lysch rocks.

Fig. 23. Initial slum ping expressed in disruption. Magura sandstone, Skawica

One type o f stretching is the occurrence of thin len se-like sh eets of sandstone (Fig. 23) separated b y distances of a few cm.; som e of them show dow nbuckling and extenuation (Fig. 24); if bottom hieroglyphs are present, they are deform ed, flattened or tw isted. T hese features point out to sliding that has not gone too far, w ith som e initial bending and deform ation. The ty p e of deform ation represented in Fig. 23 m ay be determ ined after K u e n e n (1953, 1953 a), as „pull-apart” structure.

— 147 —

It should, how ever, be emphasized that structures like those in Fig 24 m ay be caused also in purely sedim entary way, if fresh ly deposited sand sinks into underlying yielding clay already during deposition by some sort of load casting, w h ile the depositing current has sufficien t

Fig. 24. Initial slumping, Lgota beds, Rzyki

v elo city for transporting finer grades farther away. Truncations trans­

verse to bedding in the low er part of Fig. 24 points clearly to disruption and lateral m ovem ent. Combined action of dow nsinking and sliding w as described by P. M a c a r (1948). It m ay be noted here that in the Krosno beds there occur ellipsoidal bodies of lam inated sandstones im ­ bedded in clays and rolled up from all sides upward w hich seem to be produced only by dow nsinking of sand into clay w ithout lateral m ovem ent.

Fig. 25. Cracks in sandstone bed, filled by sand from below. Krosno beds, K am e­

sznica. Drawing refers to cross-section of the bed figured in Plate VII, 1

Another sym ptom o f in itial stretching is th e presence of cracks filled from below , or possibly also from above, by sand. Cracks m ay fissure th e sandstone bed to the very top, and in this case th e in filling sand form s sm all dikes or vein s sticking out as sand ridges above th e upper surface of the fissured bed. Cracks m ay also be closed from above, and in these instances sand intrusions form m iniature neck-like veins, dikes

so*