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 L E S D E L A i g O O l f i T S G E O L O G I Q U E D E P O L O G N E
T o m ( V o lu m e ) X X X V I — 1966 Z e s z y t ( F a s c i c u l e 1) K r a k ó w 1966
STAN ISŁAW DŻUŁYlŚriSKI
O STRUKTURACH SEDYMENTACYJNYCH ZWIĄZANYCH Z NIESTATECZNYM UWARSTWIENIEM GĘSTOŚCIOWYM
( 1 2 f i g . )
Sedimentary structures resulting from convection-like pattern of motion
,(12 Figs.) STRESZCZENIE
N iestateczne rozłożenie w arstw o różnej gęstości w płynach m a m iej
sce wówczas, gdy w arstw a gęsta spoczywa n a m niej gęstej. Zakłócenie równow agi takiego uw arstw ienia prowadizi do przepływ ów w stępujących i zstępujących. Je śli różnice gęstości w yw ołane są różnicam i tem p eratu ry, przepływ y takie nazyw am y konw ekcyjnym i. P rzepływ y konw ekcyj
n e prow adzą do .rozdziału p ły n u na układy w irów kom órkow ych o p ra
w idłowej wielobocznej ibudowie, często o zarysach sześciobocznych (B e- n a r d , 1911). Wzdłuż osi ow ych wielobocznyćh kom órek zachodzi ruch w stępujący albo zstępujący w zależności od teigo, czy lepkość nadległej i gęściejszej w arstw y jest większa, czy też m niejsza od lepkości w arstw y .podścielającej ( G r a h a m , 1934).
Jeżeli na tego rodzaju ru c h zostanie nałożony ruch poziomy, to za
m iast w irów 'komórkowych pow staną w ydłużone ru rk i wirowe, przy czym dw ie sąsiadujące ru rk i będą się obracały w kierunkach przeciw nych, a cząstki płynu będą zakreślały linie spiralne (prądy lu b przepły
w y w tórne).
R uchy o om aw ianych wyżej w łasnościach w ystępują również w pły
nach, układach rozproszonych i ośrodkach półpłynnych w w arunkach izoterm icznych, jeżeli w ośrodkach ty ch pow stało niestateczne uw arst
w ienie gęstościowe ,z .powodów nie zw iązanych z różnicam i te m p eratu ry . Praw idłow y rozkład przepływ ów w tórnych, odpow iadający zupełnie ruchom konw ekcyjnym , pojaw ia się w prądach zawiesinowych o prze
pływ ie uporządkow anym i s ł a b o rozwinliętej burzliw ości (przepływ y zbliżone do statecznych).
Z jaw iska tak ie m ożna badać w prostych doświadczeniach, w których rozcieńczona zawiesina ilasta zostaje wprowadzona do szklanego naczy
nia w ypełnionego u dołu stężonym roztw orem solnym, n a którym spo
czyw a w arstw a czystej wody. Zawiesina płynie wówczas w zdłuż pow ierz
chni rozw arstw ienia i rozdziela się na podłużne sm ugi (fig. 5 A). G dy u sta je przepływ prądu zawiesinowego, owe sm ugi przeistaczają się w układy wielobocżne (fig. 5 B i C). Mogą one powstać ty lk o wówczas, gdy nad nim i znajduje się jeszcze nie rozdzielona .zawiesina. W om awia
nym przypadku spełnione są w arunki niestatecznego uw arstw ienia, po
nieważ zaw iesina ilasta je st nieco cięższa od roztw oru solnego i z wolna się weń zanurza. Owo zanurzanie połączone z ruchem poziomym wywo
łuje przepływ w tórny w podłużnych rurkach, ten zaś układa zawiesinę w smugi rozdzielone pasam i stosunkowo czystego roztw oru. Poniew aż w danym przypadku w tórne p rąd y są słabe, cząstki iłu zgromadzone raz wzdłuż liniii prądów zstępujących nie podnoszą się z powrotem , pomi
nąw szy najdrobniejszą zawiesinę, która może zataczać spiralę. Cząstki uszeregowane w sm ugi płyną razem prądem , a ruch ich zbliżony jest do ru ch u uw arstw ionego (laminarnego). Jeżeli w szystkie cząstki rozmiesz
czone są w smugach, ich układ jest trw ały przy nie zm ienionych w aru n kach przepływ u.
Bardzo podobny układ przepływ ów w tórnych m oże siię utw orzyć w prądach zawiesinowych płynących po dnie. Jeżeli zaś dno zaściela m iękki ił, to ta k i układ może zostać utrw alony pod postacią hieroglifów.
Istotnie, w śród ty c h ostatnich w ystępują odlew y grzbietów podłużnych lub ułożonych w sieci wieloboczne. Zostały one odtworzone doświadczal
nie (fig. 10) i wiem y, iż owe grzbiety pow stają w m iejscu w stępujących prądów w tórnych. C harakter tych ostatnich, jak również daleko posu
nięte podobieństw o w yw ołanych przez, n ie stru k tu r do stru k tu r konw ek
cyjnych pozwala wnioskować, iż owe w tórne p rąd y spowodowane są i w tym przypadku niestatecznym uw arstw ieniem gęstościowym. Takie uw arstw ienie niestateczne pow staje w określonych w arunkach w w arst
wie przydennej i u czoła prądu. Zachodzi to w następujący sposób:
1) W prądach o słabej ibuirzliwości ham ujące działanie tarcia o dno powo
d u je w ydatny spadek prędkości w w arstw ie przydennej i w ypadanie z niej ziarn, k tó re powyżej, w obszarze szybszego przepływ u, pozostają w zawieszeniu. W 'ten sposób w pobliżu dna pow staje cienka w arstw a o zm niejszonej gęstości, nad k tó rą przepływ a cięższa zawiesina.. Opada
n ie tej cięższej zawiesiny w dół połączone z. ruchem poziomym stw arza w arunki dla ruchu uporządkow anego w e w spom niane na wstępie rurki.
2) U w arstw ienie niestateczne tw orzy się również w następstw ie napływ u ciężkiej zawiesiny na dno zaścielone iłem o m ałej gęstości i lepkości.
3) Niestaltełczność w rozmieszczeniu gęstości pow staje w reszcie wzdłuż całej (powierzchni granicznej u czoła płynącego prądu zawiesinowego.
W ywołane niestatecznym uw arstw ieniem gęstościowym ruchy typu konw ekcyjnego d a ją początek niektórym stru k tu ro m na .powierzchni la- m inacji w ew nątrz ławic piaskowcowych. Należą tu podłużne i wielo- boczne grzbiety zw iązane z niektórym i piaskow cam i skorupow ym i oraz równoległe sm ugi w ysortow anego m ateriału o grubszym lub cięższym ziairnie. Do s tru k tu r w yw ołanych niestatecznym rozmieszczeniem gęstoś
ci należą również ta k zw. hieroglify pierzaste, któ ry ch ścisły związek z om awianym i poprzedlnlio grzbietam i i w tórnym i przepływ am i w r u r kach w irow ych o typie konw ekcyjnym został w ykazany dośw iadczalnie ( D ż u ł y ń s k i , W a l t o n , 1963, 1965).
Zjaw iska analogiczne do wyżej wym ienionych zachodzić mogą wśród złożonych, lecz jeszcze n ie stw ardniałych osadów. N iestateczne uw arst
w ienie gęstośćiowe jest sam o w sobie zjaw iskiem T>owszechnym, chociaż nie zawsze prowadzi ono do ruchów o. typie konw ekcyjnym . R uchy takie w ystąpić mogą, gdy lepkość w arstw o niestatecznym rozmieszczeniu gęstości ulegnie zm niejszeniu np. pod wpływ em upłynnienia. Jednak praw idłow e ruchy o ty p ie konw ekcyjnym będą zachodżić tylko wówczas, gdy ław ice objęte ruchem nie w ykazyw ały przed upłynnieniem istotnych zm ian w miąższości.
Ruchy gęs teściowe d ają również początek talk zw anym glebom paso
wym i wielobocznyim (poligonalnym). Zjawiislka te można odtw arzać do
św iadczalnie (fig. 11 i 12), a śledząc ich przebieg stw ierdzam y jak n ajd a
lej posuniętą analogię w sam ym charak terze w stępujących czy zstępu
jących ruchów do poprzednio om awianych w tórnych przepływów. Jedy
nie z .uwagi na diUżą lepkość, ru ch y są bardzo zwolnione.
Pracow n ia Geologiczno ^Straty graficzna P A N w K r a k o w ie
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A b s t r a c t This paper ideals w ith som e sedim entary phenom ena and structures resulting from in stab ility in de'nsity stratification. It has been found that convection-like patterns o f m otion exist under isotherm ic conditions in m ovin g suspensions and plastic sedilments. T hese m ovem ents are reflected by various sedim entary structures, know n under d ifferent nam es and .recorded from d ifferen t environm ents.
INTRODUCTION
%■
D ensity gradient is an im portant factor in th e sta b ility of soft sedim ents. Consider e.ig. a sequence of tw o layers of d iffe re n t densities, one beneath th e .other. W ith denser layer a t th e bottom th a n a t th e top, th e density gradient is stable. If th e sequence is reversed, th e gradient or density stratification is unstable, since the heavier m aterial tends to tak e place of th e lighter. T he instability of th is kind is m anifested in a v ariety of stru ctu res ranging from m inor soft-sedim ent deform ations to large cru sta l displacem ents.
D ifferent as th ey m ay appear in size, environm ent and type of th e rocks involved, th e deform ations m entioned are based on th e same general principle, i.e. instability in density stratification.
The stability of a reversed (unstable) density stratificatio n depends upon, viscosity. It is e.g. possible for a layer of fluid to rem ain in equilibrium w ith higher density a t the top th a n a t th e bottom so long th e condition.:
d, — d0 652 fcv
di < 9 h 3
is fulfilled (Lord R a y l e i g h , 1916), w here: d1 — density of th e fluid a t th e top, d0 — density of th e fluid a t th e bottom , k — coefficient of m olecular diffusivity, v — kinem atic coefficient of viscosity, g — the acceleration of gravity, h — th e d ep th of the layer.
W ith declining viscosity in fluids th e in stab ility gives rise to a num ber of ascending and descending moveimemts w hich m ay follow a highly regu
la r pattern. These movemente, striving to achieve th e stable equilibrium m ay continue until * a stable density stratification, is obtained. In the m ajority of geological processes, however, m ovem ent is stepped before th e stable density stratificatio n is achieved, because of lim ited fluidity or plasticity. Thus th e interfaces betw een layers differing in 1 density m ay take th e form of m ore or less contorted surfaces w hich reflect d ifferen t stages of m ovem ents tow ards increased stability. T he resulting
sedim entary stru ctu re s a re freq u en tly indicated by d ifferen t nam es and recorded1 from d ifferen t environm ents.
The -present paper does, not p u rp o rt to give a com prehensive account of various stru ctu res w hich are believed' to resu lt from tihe above m entioned instability. Instead it deals w ith selected problem s concerning th e regular convection-like p a tte rn of m otion reflected by some sedim en
ta r y structures.
Before proceeding w ith th ese problem s it w ill be1 useful to review b riefly som e basic concepts of in stab ility as th ey a re illu strated in experim ents w ith fluids heated from below (convection currents). This question is closely related to th e subject discussed.
EXPERIM ENTS ON CONVECTION CURRENTS
Considerable am ount of experim ental and1 theoretical w ork has been dome on th e problem of tem p eratu re controlled density circulation in fluids ( B e n a r d , 1901; R a y l e i g h , 1916; L o w , 1929; J e f f r e y s ,
1 9 2S).
B e n a r d (1901) found th a t w hen a layer of standing liquid w ith a free top surface is uniform ly heated from below, th e in stab ility created by thie form ation of a lay er of less density at th e bottom , leads to a steady regim e of flow consisting of a n et of polygonal cell-vortices.
U nder uniform conditions the polygonal celi-vortices tend, to be hexagonal in shape and th e ir linear dim ensions are roughly proportional to th e thickness of th e heated layer.
In B e n a r d ’s experim ents w ith fluid's, the polygons had ascending centres, i.e. th e m ovem ent w as directed' u pw ards along th e ax is of polygons and dow nw ards along th e w alls of the cells (fig. 1). Sim ilar
Fig. 1. Przekrój komórki konw ekcyjnej Fig. 1. C ross-section through a con
vection cell (after Benard 1911)
experim ents w ith unstable gas layers, yielded cells w ith m ovem ents dow n th e axes ( G r a h a m , 1934; C h a n d r a, 1938). This dowlnward, versus upw ard m otion along th e axis of cells diepends upon viscosity.
W hen th e descent is fro m th e u p p er su rface i t m eans th a t i t is th e upper surface w hich is m ore Unstable and less viscous th a n th e low er (G r a- h a m , I.e., p. 294). The d iffe re n t behaviour of liquids and igases appears from the fact th a t th e viscosity of a gas increases w ith tem perature, whiile w ith liquids it is th e reverse.
W hen a system w ith u n stab le density stratificatio n is subjected to a horizontal shear, i.e. w hen th e vertical m ovem ents combine w ith a fo rw ard flow, th e cell-vortices change in to a p a tte rn of horizontal rolls, parallel to thie direction of flow. The neiighbourilng ro lls ro tate in opposite directions. Such a change appears to b e determ ined b y th e p rim ary arrangem ent of polygonal cells. T he polygons arranged as show n in fig. 2 are easily d raw n into longitudinal rolls, w hile those oriented as in fig. 3 tran sfo rm through th e ’’tran sitio n a l” p a tte rn to the ’’sq u a re”
p a tte rn ( G r a h a m , 1934).
C h a n d r a (1938) obtained longitudinal rolls show ing a dendritic p a tte rn , w ith d a rk hands of cliear a ir joining in th e direction from w hich thie top p la te of th e sm oke a p p a ra tu s moved (fig. 4) 1.
Fig. 2. Przekształcenia struktur szesciobocznyich w pasowe Fig. 2. Transform ations o f h ex a gonal pattern in to a pattern o f lon gitu d in al bands (modified,
after Graham 1934)
Fig. 3. Przekształcenia struktur sześciobocznych w „przejściow e”
i czworoboczne
Fig. 3. Transform ation o f h e x a gonal pattern into transitional and square patterns (modified, after Graham 1934). The arrow indicates the direction in w hich th e toip p la te m oved to ex er t
shear upon the gas
Fig. 4. Zbieżne simuigi konw ekcyjne. Czarne lin ie to sm ugi czystego ogrzanego po
w ietrza utw orzone przez prądy w stępujące. Jtasne pasy to sm ugi dym u
!Fiig. 4. ’’D en tritic” (pattern o f longitudinal convection rolls in experim ents on gases heated from b elow , and subjected to a 'horizontal shear. Dark lines inuark th e lanes
o f clear air and ascending currents (after a photograph by C h a n d r a 1938)
1 In the experim ents by G r a h a m i(1:934) and C h a n d r a (1938) th e air w ith tobacco and titanium tetrachloride sm oke w as placed and/or drawn through
a container bounded iby a hot sh eet ibelow 'ainld a cotLd plaite aibove. To produce a horizontal v elo city shear Ithe itop p late w as m oved across th e container.
As long th e horizontal shear continues, th e longitudinal rolls, once form ed, are v ery stable. However, th e transform ations discussed are reversible (as long th e re exists an instability in density stratification), i.e. the rolls bre'aik up tran sv ersally into, a .pattern of ipolygons w hen the forw ard mlotion is arrested.
CONVECTIONmLIKiE PATTERN OF MOTION IN SU SPEN SIO N S UNDER ISOTHERMIC (CONDITIONS
So far we have spoken of regular cell-vortices and sp iral m otion (in rolls) in cases w here in stab ility in density stratificatio n was brought about b y te m p eratu re differences. Now wte shall see th a t identical pattern s of m otion m ay ex ist in sus,pensions .under isotherm ic conditions (fig 5 A, B, C). It w ill be shown th a t such m ovem ents too, m ay result from instability in dlensity stratification.
Let ius 'begin w ith a steady flow in longitudinal rolls w ith opposite sense of rotation, i.e. clocik-wise and anti-clock-w ise. Such sp irals exist in sediiment-laden flows and are com m only indicated as ste'ady
’’secondary cross-currents” . The origin of thes'e secondary cross-currents
«ttWniSSMm
? 1 f---- ■ ’j
- --i«V
r
******* r \
C 1 h„
VO X** w r* * ft* tr* r& ->g?-wftg * * »*»h. ' V * V
a -< ?
Fig. 5. Układ sm ug zaw iesinow ych w prądzie cieczy w warunkach izoterm icznych.
Wzór w M ob oezn y pow staje w m iejscu ‘ustania p łynięcia. O bjaśnienia w teikście Fig. 5. F low pattern o f m oving suspensions under isotherm ic conditions. Polygonal
pattern appears in places w here th e forw ard flow of the current stops.
Explanation in text.
ils im perfectly understood. In fact, ’’the contradictions presented by various authors, indicate th e difficulty of obtaining m ore ex a ct and conclusive resu lts concerning th e steady secondary c u rre n ts” (N e m e- n y i , 1946, p. 124) 1.
The effects of Spiral cross-currents on th e distribution of suspended sedim ents are showin by th e transportation of suspended load in bands or ribbons parallel to thie flow. According to V a n o n i (1946, ip. 100),
’’th e cause of these longitudinal bands of sedim ent is not known, although it is believed th a t th e y are th e resu lt of secondary circulation and disturbances due to th e .presence of suspended load” . V a n o n i pointed out th a t even th e slightest d ep a rtu re from a uniform sedim!ent d istrib u tion wj!ll cause diensity gradients th a t can se t up secondary flows.
The flow p a tte rn of suslpensions m ay be identical w ith th a t of convection currents. This can toe dem onstrated (by experim ents using diluted and sem i-tran sp aren t suspensions of fin e clay or coal-dust. These are g ently introduced into a glass tanik w ith hypersaline solution (slightly coloured toy potassium perm anganate1) a t the bottom benteath a lay er of fresh w ater. ( D ż u ł y ń s k i , 1965). Undler such conditions th e suspen
sions Spread in bands along th e interface w hile slow ly sinking w ith increasing concentration of suspended particles a t th e base of the flow.
The toands of suspension form douible-rolls w ith descending m otion along thie central line. There is a' m otion upw ards in betw een th e toands, i.e. the hypersaline solution is welling up w hile the toands them selves are sinking down. Tlhe spiral o r incipient sp iral circulation is m ade visible toy finest clay particles w hich are caught toy th e ascending flow and describe spiral trajectories.
If the cross-currents .are weak, th e suspended particles once arranged in longitudinal toands rem ain in this position (fig 5 A). They are pushed forw ard in bands and th e ir m ovem ent is lam inar, though th e fluid itself m ay be involved in spiral motion.
U nlike tu rb u len t eddies, th e spiral or p a rtly spiral circulation associated w ith th!e tran sp o rt of suspended load, can be observed even when> th e forw ard flow (is reduced to a m inim um 2. W hen, however, th e speed is m uch increased, th e flow ceases to be steady and regular. The longitudinal rolls disappear toeing replaced by irreg u lar tu rb u len t eddies.
The steady regular, „convective” circulation discussed is neither tu rb u len t in th e lusual hydraulic sensie n o r lam inar. I t has been te n ta ti
vely 'indicated as ’’sub-turbulen't’’ ( D ż m ł y ń s k i , 1965).
The flow p a tte rn in toands, once formed, is v ery s tatole. I t resolves, however, into a netw ork of polygonal cell-vortices if th e forw ard flow stops, and w hen th e re is still an unstable density stratification. W hen all th e suspended' load is already arranged in toands, and these are in th eir
1 There are d ifferent types of „cross-currents” w hich are not related to instability in d en sity stratification. Boundary shear in straight channel's, curvatures of conduits, bottom roughness etc, can initiate th e secondary flow s. A lengthy review o f spiral m ovem ents has been recently given by K o l a f , 1956, se e also T o w n s e n d , 19151.
2 For thils reason th e spiral m ovem ent under consideration should not be identified w № the turbulent spiral m otion in cylindrical eddies along the direction of flow . Such eddiies w hich obtain energy from th e m ean flo w d issip ate through turbulent friction {see T o w i m s e n d , 19511).
stable position a t th e bottom or in a relativ ely stab le position on the interface between, th e saline and fresh waiter w ith no suspension above, the polygons do n o t form .
W ith welaik and/or n o t repeatedly revolving cell-circulation th e suspended p articles once arranged' in polygons do not reap p e ar in th e cen tres and the space betw een polygonal walls' is filled w ith a clear 'fluid (fig. 5 B, C). S uch stabilized polygons if n o t disturbed or dispersed m ay sink dow n to th e bottom of the tanlk. Given, however, a horizontal velocity, th ey m ay tran sfo rm into a p a tte rn of p arallel bands. These bands too, m ay sink dow n to thie bottom an d be preserved. If, however, th e ra te of dow n-sinking is slow er th a n th e (lateral dispersion consequent u,pon th e to ta l disappearance of circulation, th e suspended particles arranged in bands would1 spread unilformy over th e bottom of th e tank.
The bands m oving forw ard m ay b ifu rcate up-, or d o w n-current and th e resulting dendritic p a tte rn (fig. 6) is identical w ith th a t obtained in
— 10 —
Fig. 6. Rozbieżne sm ugi za'wiesiny w prądzie
Fig. 6. D iverging bands o f suspension. Flow under isotherm ic conditions
experim ents on convection in heated, smOkte cham bers (fig. 4). W ith diverging flows, lim ited am ount of suspension and: w idely separated bands th e la tte r m|ay b ifu rcate dow n-current. W ith converging flows and ab undant am ount of suspension it is the reverse (see p. 12).
W ith strongier flows, tran sv e rse rolls are form ed in fro n t of th e distal w all of a ta n k o r an y b a rrie r m)et by th e cu rren t. These tran sv erse rolls should 'be looked, upon as ’’boundary effects” in agreem ent w ith the explanation given by C h a n d r a (1938) fo r identical p a tte rn s in ex p eri
m ents on convection curren ts.
F rom th e foreigoing i t appears th a t under certain conditions the p a tte rn of flow displayed b y moving suspensions is analogous to th a t of convection currents. W ith regard1 to th e flow of suspension along the interface 'between sa lt an d fresh w ater, th e prerequisitels too, are the same, i.e. th e instability resulting from differences in density.
T he existence of sp iral m otion in tu b e-lik e bodies at th e base of th e tu rb id ity c u rre n t p ro p er (i.e. th e g rav ity flows of suspensions along the bottom) h as been inferred from th e form ation lof longitudinal ridlges on
— 11 —
m uddy bottom surfaces invaded by th e c u rre n t (D z u 1 yi n s k i and W a l t o n , 1963). These ridges show a strik in g likeness to those produced ex perim entally by C ais e y (1935) and' assigned to th e action of secondary cros's-currents.
T he question com es up w h eth er th e form ation of thiese ridges^ and th e in ferred convection-ffike m otion can bie interpreted' in term s of in stab ility engendered b y reversed: density gradients.
The im pression gained from experim ents is th a t th e prerequisites fo r this ty p e of instability a re found a t th e base and in fro n t of tu rb id ity cu rren ts. T he problem m ay be (treated as follows:
1) C onsider th e case of a rela tiv e ly slbw tu rb id ity c u rre n t w ith sm all degree of turbulence. Though, in general th e concentration' of suspended p articles increases dow nw ards, im m ediately above th e floor, th e c u rre n t velocity decreases d ue to th e viscous drag. Therefore, below a certain level correspondling to a certain critica l velocity, th e suspended particles w ill se ttle down. P artic les of th e sam e size, situated above the critical level, m ay still rem ain in suspension. This causes th e in stab ility in density stratification, i.e. th e appearance of low density la y e r close' to th e bottom i(see U n r u g , 1959). (Such in stab ility shouid' set up spiral circulation in parallel! rolls sufficient to produce th e sm all ridges on soft bottom m aterial.
2) The instability m ay also re su lt from th e onset of a heavy suspen
sion upon a fine1 bottom clay of low density. This effect is seen in expe- rimlemts w ith thle p laster - of-pa ris tu rb id ity cu rren ts spreading in a form of a th in sheet over a v e ry fine d a y . One can observe th e grow th of ridges in betw een thb ban d s of suspension, and occasionally th e grow th of point-iilntecitlons of clay.
3) Anothler region of in stab ility is developed along the leading fro n t of moving tu rb id ity cu rren ts. T he sam e kind of in stab ility exists w ithin th e tu rb id flowis, in fro n t of denser clouds of suspension m oving faster th a n th e f irs t advancing and less d'enise m asses of th e suspended m aterial.
As indicated by experim ents, th e fro n tal part, of th e m oving suspension is slig h tly elevated above th e bottom (fiig. 7). T his results from th e fact th a t th e (suspension in th e im m ediate proxim ity of th e bottom is lagging behind th a t situated higher up, wWicb is n o t so m uch affected b y th e bottom friction. T hus a n overhang i® form ed w hich causes an u nstable d en sity g rad ien t and consequently a se t of ’’convective” rolls w hich m ay bring about th e form ation of ridges.
4) An im portant, and presum ably th e m ost im p o rtan t factor influencing the form ation of longitudinal ridges is th e instability in den sity distrib u tio n along th e ’’dem arcation” surface betw een th e standing cl'ear flluid and th e advancing fro n t of suspension. The frontal su rface of a m oving tu rb id ity c u rre n t can be com pared w ith an in te r fa c e betw een fluids differing in d ensity and showing an unstable density stratification. This in stab ility lead's to th e 1 formation! of characteristic lobes o r lob ate projections th a t m a rk the1 fro n tal surface o f an advancing tu rb id ity c u rre n t (Fig. 7). The lobes correspond closely to ’’ascending or descending colum ns” b ro u g h t about b y thie vertical instability in density stratificatio n (see p. 16). In both cases th e p a tte rn of cross-currents is analogous. It causes th e ’’sidew ard pushing action” ex erted Iby th e lobes whlilch d irectly contact itihie bottom i(fig. 7). This pushing action upon soft sedim ents considered as the1 m ain facto r in producing th e ridges ( D z u l y n s f e i 1 and W a l t o n , ,1963) is w ell explained in term s
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of instability in density distribution. I t is to be noted th a t th e slow ly advancing frontal lobes becom e crenulated. Thiesle secondary lobes are m ainly responsible flor th e form ation of secondary ridlges (dendritic pattern, see fig. 8) which join (the m ain ridges in the dow n-current direction ( D z u i y n s k i and W a l t o n , 1965).
From th e foregoing lit appears thalt th e re m ay be unstable density Stratification at th e bottom and in fro n t of th e flowing tu rb id ity cu r rants.
This instability m ay initiate th e convection-like1 movem ents, w hich in tu rn m ay produce a characteristic p a tte rn of sole m arkings (see p. 13).
Fig. 7. N iestateczne rozm ieszczenie gęstości u czoła prądu zaw iesinow ego. W ysokość nadwieszenia przewięktszona. a — nadw ieszone czoło prądu, w idok z góry; b — czo
ło przydennej w arstw y prądu z tw orzącym i s ię garbami p rądow ym i na ile;
c — czoło gęściejszej „ahmury” zaw iesinow ej w obrębie płynącego prądu Fig. 7. Instability in density distribution along th e leading front o f a m ovin g turbi
dity current. Overhang exaggerated, a — lobate leading front o f the current;
b — lobate front of th e current close to the bottom and dendritic ridges (plan view);
c — denser ctouds o f suspension w ithin the current
Fig. 8. O dlewy grzbietów prądowych (zbieżnych) na spągu piaskowca.
W edług zdjęcia
Fig. 8. Moulds o f converging dendritic ridges o n bottom surface o f a sandstone.
A fter a photograph
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I t should be borne in m'iind, how ever, th a t though th e presence of suspended1 load m ay be th e cause of th e convection-dilke circulation (point 1) ilt iis mot a necessary condition .for th e ir generation. If the m oving suspension consists e.g. of pure saline solutions, th e ridges too, a re being form ed. The sam e phenom enon occurs w hen th e fro n t of a pure liquid advances upon an exposed' soft clayey o r piaster-of-paris surface ( D ż u ł y ń s k i and W a l t o n , 1963). In thesie cases the ridgies are chiefly made by th e fro n ta l lobes (point 4).
SEDIMENTARY STRUCTURES REFLECTING CONVECTION-LIKE FLOW PATTERN OF TURBIDITY CURRENTS
A num ber of sedim entary stru ctu res w hich develop a t interfaces betw een th e sioft bottom and the c u rre n t can 'be in terp reted in term s of convection-like m ovem ents. These stru c tu re s are as follows:
1) Moulds of longitudinal and polygonal rildgeis on bottom surfaces of sandstones.
The ridges discussed in th e proceeding ch ap ter have th e ir replicas among th e siole m arkings. They appear as m oulds of th e stru c tu re s produced on soft bottom s (K u e n e n , 1957; D ż u ł y ń s k i arnd Ś l ą c z- k a, 1958; D ż u ł y ń s k i i and S a n d e r s , 1962; C r a i g and W a l t o n , 1982; J i p a and M i h a i l e s c u , 1964, and others). The rid-ges m ark the llnss of the asic'snd'ing secondary c u rre n ts while (the areas iin betw een the
G Q Q Q O Q O O
C O X X X X ' )
Fig. 9. Sposób po wstawiania grzbietów prądowych (a) oraz gleb pasow ych (b) w uproszczeniu
Fig. 9. S'cheimatic presentation o f the developm ent of current ridges (a) and sorted soil strip (b)
ridges correspond to th e descending flows (fig. 9). The energy im parted to th e secondary flows b y th e m ean flow land u nstable density gradient m ay lead bo scouring of th e d'eep furrow s in betw een th e ridges.
The ridges m ay b ifu rcate (fig. 8) or pass into a- polygonal p attern (fig. 10). Exam ining the polygonal stru ctu res in cross-sections one can suggest th a t m ovem ents along the axes of th e p rim a ry cell-vortices was dow nw ards. This indicates th a t the settling suspension w as m ore mobile th a n the underlying clay (see p. 6).
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2) Ridjges on surfaces of lamiination w ith in th e sandstone 'beds.
RM'geis sim ilar to those previously disculssed occur on surfaces! of in tern al lam ination and are b e st visible w hen a sandstone is p a rte d alonjg bedding surfaces. T he stru c tu re s are fre q u en tly associated w ith
’’incipient” convolutions (ten H1 a a f , 1956, 1959; K u e n e n and S a n -
Fig. 10. O dlew y podłużnych grzlbiętów prądow ych przechodzących w grzbiety w ie- loiboczne. Spąg dośw iadczalnego osadu zaw iesinow ego. W edług zdjęcia
i(lD ż u ł y ń si k i, 1005)
Fig. 10. M oulds o f longitudinal ridges passing in to a polygonal pattern. Bottom surface o f an experim ental turbidite. A fter a photograph by D ż u I y ń s> k i 1&05
d e r s , 1956; D ż u ł y ń s k i and S m i t h , 1963)1 and, th e ir orijgin is dlosely rela ted to rthie process w hich bring about th e 1 ridiges on bottom surfaces of sandstones i ( D ż u ł y ń s !k i and W a l t o n , 1965). The „in tra- s tra ta l” ridges under consideration, how ever, com prise several lam inae, and this cąlils fo r com m ents. Threte possibilities m ay be invoked in this connection; 1) th e ascending cross-currents w ere sltronig enough to w arp a set of highly plastic and d uctile lam inae into a ridge, 2) th e ridges once form ed ,u|pon a surfaoe iof lam ination grew up during th e deposition of th e subsequent lam inae ijn response1 to a m ore effective deposition in th e troughs separating th e ridgtes, 3) th e flow p a tte rn w hich produced1 th e firglt ridge® on a given bedding surface did not change d u rin g th e1 deposi- tion th e subsequent lam inae. Evidently, th e th re e .possibilities are not m u tu ally excljusiVe.
3) S orted1 bands on surfaces of lam ination.
Kelliated to th e ridges a re .sorted bands on surfaces of lam inat'ed beds.
T hey consist of heav ier o r coarser p article s and are separated1 by streak s of lighter o r finer particles. The bands tre n d in th e direction of c u rre n t flow, showing thus, how th e particles w ere distributed across the c u rre n t
1 .Similar m arkings h ave been described a s ripples (longitudinal), s e e e.g.
Q u j k l o w s - k i, 1957). For [illustration the readier is referred to D z u l y n i s fci and S m i t h , 1963, fig. 3 and 5.
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(fig. 5 A). Whlile th e d istinct ridges tend1 to develop upon cohesive and d u ctile m aterials, th e bands fo rm upon frictional sedim ents. The bands, too, are a s o rt of v e ry .Dow ridges, in m iniature. They m a rk th e lines of ascending o r descending c ro ss-c u rre n ts 1.
4) ’’F eath er” o r ’’frondescent” m arkings.
Closely allied in melchianism to th e stru ctu re s h ith erto discussed, though m orphologjcally seem ingly d iffe re n t are ’’fe a th e r” or ’’fromdes- cent m ark in g s” ( K s i q z k i 1 e w i c z , 1958; te n H a a f , 1959). T hey re su lt from sinking and branching of tu b u la r bodies of suspensions into a fluidi'zed o r p artially fluidizted1 su bstratum . These stru c tu re s w ere produced experim en tally uising a sofit tra n sp a re n t gelatine as a low d ensity la y e r .underneath th e flowing suspension. Thu© th e ir origin could be exam ined in d etail and a close relationship to th e flow of s’uslpensions in longitudinal rolls established ( D z u l y n s k i and W a l
t o n , 1963).
T he stru ctu re s hiSthbrfto discussed fo rm lunder conditions of lim ited turbulence. Increase in c u rre n t velocity accom panied b y an increase1 in turbulences leads to decay of .the reg u la r flow p attern . The effect of in stab ility in d en sity stratificatio n becomes negligible as com pared w ith o th er facto rs su|ch as (bottom roughness, cu rv atu res of channels: etc.
D ifferent velocities set up pressure differences w hich cauise cross-cur
re n ts deriving th e ir energy from th e m ean flow. There m ay be a com
p lete gradation betw een th e re g u la r convection-like and irre g u la r highly tiurbullent flow land this gradation is. reflected iin sole m arkings. E.g. th e fu rro w s in betw een th e nidges become m odified by flutes. It is som etim es im possible to discern betw een th e rounded scours produced b y strong
’’iboils” ije. th e iirrelgular ceill-vortices deriving th e ir energy from th e m ean flow, from stru c tu re s generated1 b y th e convective celil-clrc uil at ion.
I t is the g en eral character of th e m arkings assem blage (e. g. th e associa
tio n w ith flutes) w hich m ay give some inform ation concerning th e ty p e olf flow in th e im m ediate p ro x im ity of th e bottom .
O n th e o th e r hand th e m arkings discussed in this1 ch a p te r and im plicitly classed as „c u rren t stru c tu re s” m ay b e produced en tirely outside th e dom ain of ’’o rd in ary ” cu rren ts and tu rb id ity curren ts. E.g.
th e frondescent m arkings m a y appear as post-depositkmad feature® on the bottom, of th e deposited amid subsequently liquefied sand layer. T heir form ation follows a com plete disappearance of p rim a ry c u rre n t m arkings (D z u l y n s k i , 1963b). Ridges passing into a roughly polygonal p a tte rn of ’’pillow ” structures^ too, m ay form as post deposotional featu re s at th e base of load-deform ed and p artly liquefied rip p les ( D z u l y n s k i and K o t l a r c z y k , 1962). This, however, ils w h at should be expected.
The liquefaction of deposited sedim ents m eans th e form ation of a lay er of fluid. As long th e re are conditions Of instability ailong th e bounding surfaces of this layer „convective” m ovem ents m ay occur.
Betw een such ”post-depositional” stru ctu res and th e c u rre n t m ark ings proper th e re is a com plete gradation1 and in m ost cases ”it- is impos
1 The banded lam ination surfaces should not be identified w ith „parting lineation ” ( C r o w e l l , 1955) though both structures may be in tim a tely related.
The parting lineation comprise v isib le effects o f splitting and presents a picture o f a terraced surface involving several adjacent lam inae („parting-step lin ea tio n ” — M c B r i d e and Y e a k e l , 1963).
— 16 —
sible to designaite the stru c tu re s clearly in term s 'Oif th e ir tim e of form a
tion” ( D ż u i ł y ń s k i anld1 W ai 11 o n, 1963).
CONVECTION -LIKE MOVEMENTS IN W ATER-SATURATED PLA STIC SEDHMCEINTS
The concepit of convective m ovem ents consequent upon unstable density gradient offers a possible explanation for a numlber of sedim en
ta ry stru ctu re s w hich tend to develop in soft layered sedim ents no m at
te r w hat th e ir sedim entary environm ent. It ils beyond th e scope of this publication to discuss th e full v ariety of ’’post-depositional’’ stru ctu res resulting from instability in density stratificatio n 1. W e concentrate on those only Which show a reg u lar convection-like p a tte rn and th is brings us logically to the controversial problem of ’’patterned groundis” (W a s h - b u r n , 1956).
The folloiwing dliscussion ils based on th e experim ents decriibed alread y in ea rlie r publications (ID ż u ł y ń s k i, 1963 a ; B u t r y m e t al., 1964). The .procedure of preparing unstable density stratificatio n in mo
del m aterials was as follows; Fine sand, coal-dust or sim ilar clas'tics were gently sieved through w ate r onto a settled clay, or diluted p laster of piaris susipensions were g en tly introduced in to a w a te r ta n k w ith settled clay to produce a uniform lay er of 'heavier m aterial upon the clay. Then, th e tanks were j air red o r slightly tilted. In a new series of experim ents plaster of paris was su b stitu ted for the clay settled from w ater suspen
sions, and th e sand w as sieved th ro u g h th e w a te r upon th e still soft and plastic pllas'ter-of partis deposit. This technique p erm itted 1 perm anent specim ens of th e stru ctu re s produced to be O b ta in ed .
a) D eform ations of layers u n d er conditions of instability in the absence of horizontal displacem ents
Consider th e in stab ility of a sequence w ith a loose sand at th e top, and clay at the bottom. Suppose th e1 top layer is u n ifo rm in thickness and composition, the stre n g th of th e clay sufficient to su p p o rt th e load, and th a t th e whole sequence is waiter-'saturated'. Such a sequence m ay brealk down w hen th e stren g th of th e low er layer is descreased by lique
faction or increasing plasticity. This is frequently observed in th ix o tro - pic clays and' looseily 'packed1 sands. G iven 'a slight m echanical im pulse, e. g. a shock or local overloading, the sedim ent w hich is prone to lique
faction m ay pasis from a solid to a liquid or semil-liquid p h ase;1
In absence of horizontal displacem ent, th e disturbance of equilibrium w hich follows th e liquefaction gives rise to ascending and descending m ovem ents sim ilar to those1 produced by heating of a fluid1 from bellow.
W ith th e low er lay er 'less viscous th a n th e upper (fiig. 11 a), cine obser
ves th e grow th of colum nar clay intrusions '(involutions) w hich m ay break through th e sand; layer, and form ro u n d ed humlmodks m ore or less uni
form y distributed1 over th e to p su rface (fig. 11 b). W hen th e flow of clay continues, th e hum m ocks grow larg er pushing th e top m aterial aside so
1 Load deform ation and the resulting structures, clastic intrusions (sand or clay dykes and sand or m ud volcanoes etc) belong ihere. T hese structures have been described in num erous publications and there is a considerable literature
devoted to th e experim ental production o f such structures.
— 17 —
to form th e ring-liike stru ctu re s (fig l i b ) . F inally the sta te is reached w hen th e m argins of th e rings com e to m u tu al contact (fig. 11 c). This unevitably leads to ipalyganal, and in case of uniform ity, to hexagonal p attern s. Whien th is stage is reached, the area occupied b y th e heavier m aterial on the top surface is a minimum.
Fig. 11. P ow staw an ie ‘gleb wielobocznych w w yniku zaburzenia rów now agi w n ie statecznym u w arstw ieniu gę^to.ścmwym w imiękkich, p lastycznych i upłyinnia!jących
się osadach. O bjaśnienie w tekście
Fig. 11. Formation of 'polygonal structures from unstable d en sity stratification in cross-section and in ,pla'n v ie w (righ't). Upper layer — sand, low er — clay. After
experim ents. E xplanation in text
Judging from, experim ents th e polygonal p a tte rn s are relativ ely stable, p articu la rly w hen [the denser m aterial descending along the waills of polygons reaches th e bottom (or a layer of hligh viscosity) so to form
„rooted polygons” (fig. 11 d). Even suspended polygons m ay be stable, though th is is m ainly due to th e fact th a t a t th is stage, th e clay rap id ly looses the excess of w ate r 1 and its fluidity decreases.
If, however, th e re ils a continuous flow of clay from below, and th e viscosity of Ithe clay sufficiently low, the top m aterial m ay g ath er at the bottom in form of polygonal ridges (fig. 11 e).
The size of polygons largely depends Upon th e thickness of the mobile
1 The expulsion of water leadis to the form ation of concave polygonal fields and -cracks (fig. 11 c2). Larger cracks preferably d evelop along the m ’anginls of.
polygons (fig. 11 c2), and th e form at ion of ioracks takes ipllace under w ater as w e ll, as under su b-aerial conditions.
2 R o c z n ik
— 18 —
low er layer of low viscosity. The thicker is th a t layer, th e larger are the polygons.
We n ex t (proceed to exam ine th e case of ia hi|ghly m obile sand1 deposi
te d upon a sodft b u t relativ ely viscous d a y . To obtain, such a sequence in experim ents it i® useful to sieve sand rapidly thorough w a te r on to the surface of settled clay. U nder siuch conditions, w hich m ay ibe com pared w ith liquefaction of a sand lay er «Deposited uipon th e viscous clay, the in stab ility (takes a form of descending sand columns. These, if closely sett, have polygonal outlines dm horizontal sections '(fig. 12). T he situation is th u s reverse to rt'ha't previously (described since the instability in th is case begins w ithin the u p p er layer (see p. 6).
i • *
v *
Fig. 12. Struktury w yw ołane grzęźnięciem upłynnionego piasku w ił o lepkości w iększej niż lepkość upłynnionego piasku. W przekroju pionow ym i poziom ym Fig. 12. P en d en t structures produced by descending liquefied sandy suspensions into a clay of relatively high viscosity in vertical (left) and horizontal sections
(right)
W here density differences ane less ex trem e and th e viscosities of both layers are high th e n th e interface betw een th e layers sim ply becomes undulating. W ith low viscosities of both th e u'pper and low er lay ers th e sand is observed to m ove dow nw ards in vertical stringers, th e m ud1 being pushed up in betw een th e sandy threads. The resulting sediiment appears as a layer of gradded m uddy sandstone w ith characteristic vertical striping { D ż U ł y ń s k i anld W a l t o n , 1963).
E ffects of horizontal m ovem ent on 'unstable d ensity stratificatio n of soft sedim ents
Consider now th e sequence w ith sand at th e top, and' clay a t th e bot
tom subjected1 to a horizontal displacem ent. S lig h t tillin g of a tra y w ith th e sequence prepared is u su ally enough to break dow n th e unstable equilibrium . The clay colum ns w hich b u rst upw ards becom e im m edia
te ly elongated and' tran sfo rm in parallel strip s trending in th e direction of sedim ent creep. In each of these strip s th e re is an upward: m ovem ent iof clay along th e cen tral line. The to p m aterial is pushed aside anld pres
sed into a p a tte rn of p arallel ridges m arking the lines of dow nw ard m ovem ent (fig. 9 b). Because of high viscosilty and in absence of continu
ing inflow of energy necessary to m aintain the spiral circulation, th e particles involved in th e m ovem ents seMom if ever describe th e full
— 19 —
sp iral trajectories. O nce arranged iin strips, th e y move dow n-slope along th e already established p attern .
Occasionally th e stru ctu re s showing th e ’’transitional” p a tte rn indi
cated by fig. 3 b are form ed. T hey are d u e to the prim ary orientation o f polygons unfavourable to the change into strip s (see p. 7) !.
A pplication of th e results to ’’patterned grounds”
The foregoing considerations offer an explanation to a num ber o f stru c tu re s comiprised und'er a general te rm of ’’p a tte rn e d grounds^
( W a s h b u r n , 1956). This problem has already been d ealt witth else
w h ere ( B u t r y m e t al. 1964) and th e re is no need to dw ell oin th is subject. lObviously not all of th e stru c tu re s indicated iais p a tte rn e d groundis would fa ll in to a cathegory of stru ctu re s resu ltin g from in stab ility in d ensity stratificatio n {e.g. th e ice-w edge polygons)2. Thtis instability, how ever, explains th e stru c tu re s such as so rted polygons, circles anid strips. I t accounts also fo r a p a rt of nonsorted 'polygons.
The concept of ’’convective” m ovem ents a s a cause of polygonal soils and sorted strip s has already foeen invoked !by a num ber of au th o rs (L o w, 1925; G r i p p and i S i m o n , 1933; G r i p p 1951; S o r e n s e n , 1935;
R o m a n o v s k y , 1939; and others). Too m uch em phasis, how ever, has been laid dow n upon tem p eratu re and m oisture controlled d en sity c u r
rents. These tu rn e d o u t to ibe incapable of m oving clastics 'against gravi
ty, as req u ired iby mo^t of th e convection hypotheses. However, as indi
cated b y S o r e n s e n '(1935), th e superposition of th e’ coarse d eb ris upon th e fin er m aterials (in polar regions) is n o t dependent lupon th e processes w hich lead to th e form ation of polygons b u t vice versa. U nstable s tr a ti
fication ex ists in jpoilar regions, a n d is due to th e segregation caused by fro st-h eav e o r deposition of heavier clastics upon th e frozen silts. The refreezing of such unstable sequences m ay in itia te th e convection-like m ovem ents.
i
CONCLUDING REMARKS
Sedim ents e x h ib it a v arie ty of stru ctu re s w hich in th e p resen t clas
sification are separated from each other, given d iffe re n t nam es and in ter
pretations. Tracing th e form ation of sedim entary stru ctu re s in experi
m en ts one sees that, a n u m b er of seem ingly ’'in d ep en d en t” stru c tu re s are stages o r transitional form s in one chain of physical phenom ena, belong
ing to a silngle physical process. O n th e o th e r hand, th e sam e physical processes operating und'er d ifferen t conditions and in d ifferen t e n v ir o n m ents give rise to th e stru c tu re s which m ay ibe m orphologically d iffe re n t b u t genetically closely allied to each 'other.
I t is realized th a t th e present classification of sedim en tary stru c tu re s w ith its overgrow n term inology is inadequate. A new approach to this question is needed. This new approach should be based upon experim ental studies and a close cooperation betw een geologists and hydirodynamicist.
1 Such structures may b e compared w ith ’’sorted step s” in th e m eaning o f W a s h b u r n (1996).
2 T h ese 'may be compared w ith dessi'cation cracks.
2*
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Acknow ledgem ents: The w riter is g ratefu lly indebted to D r J. K o t - l a r c z y k , Dr A. R a d o r n s k i and Dr R. U n r u g for discussion and critical reading of thiis paper.
Geological L a boratory
of the Polish A c a d e m y of Sciences K rakow
A d d e n d u m . A fter .this ipalper wals subm itted to th e Editors., an i<m!portant (publi
cation b y E. V. A r't y u s ' h k o v appeared in the Decelmlber issu e o f Izvestia A kadem ii Naulk UiSISIR, ser. igeoil., 1965 under the title: C onvective deform ations develoiped in feeibly llithiilfied sedim entary rocks. The R ussian author d eals w ith sediimemltary structures arising from instability in denisity stratification in a treatm ent sim ilar to that o f the p resent a'ooount.
WYKAZ LITERATURY REFERENCES
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ser. 7, p. 62—144.
B u t r y m J., C e g ł a J., D ż u ł y ń s k i S,, N a k o n i e c z n y S. (1964), Nowia in terpretacja „struktur peryglaicjalnyoh” N ew Interpretation o f ’’periglacial stru ctures”. Folia Q uatem a ria , 17, p. 1—34, Kraków.
C h a n d r a K. 1938, Instability o f fluids heated from below. Proc. Roy. Soc., ser. A, 164, p. 231—242, London.
C r a i g G. Y., W a l t o n E. K. (1902), Sedim entary structures and Palaeocurrent directions from th e S ilurian rocks o f Kircudbrightshire. Trans. Edinb. Geol. Soc.,
19, 100— 119.
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