GEOLOGIA CROATICA 45 69-86 6 Fig. 5 Pl. ZAGREB 1992 UDC 551.35:551.763 (497.13) Scientific paper The B~ttom Current. S.ed.lQl.~.qts .fC.o.n.tojJf.)tes) in the Upper Cretaceus Deep-Water Deposits in.the Northe~s·t.ern Part of Žumberak (Croatia) Ervin MRINJEK Key words: .contourit~s. contour s~quences, b9ttom currents, turbidites, hemipelagites, deep-water deposits, Upper Cretaceous, Žumberak, Dinarides. The Upper Cretaceous deep marine de~ its in NE ~art of ŽQmberak contain sedi ments deposited by bottom cilrient's ' J\1\lddy ,sandy and sandy-gravelly contourites•. The oomplete (nc;&lllive-posllive) seqdences and incomplete (negative-positive) sequences have been observoed. Apart from the sediments that have been deposited bybott 3 c=:J "'--- 15 e. 20 IJJIIII) 9 ~ 18 Cg '\ 4 10 -- 17 T•1 · •:r• Te1·e, 5 lg.•šj 11 -...r- 18 H 21 • ••a Fig. 2. Position of columns and a legend for Figs. 3-, 4 and S : l -granules and coarse arenites; 2-medium arenites and siltites; 3-micrites; 4-marly micrites; S-chert lenses and lenticular bedding cherts; 6-lenticular and irregular burrows; ?-regular lamination; S-lenses and lenticular lamination; 9-irregularly lenticular laminuion; 10- indistinct and discontinuous lamination; 11-erosion and load structures; 12-sharp transition; 13-gradual transition; 14-muddy countourites; l S-sandy countourites; 16-sandy-gravelly countorites; 17 -fine-grained turbidites; 18-hcmipelagites; 19-cornplete (negative-positive) sequences; 20-incomplete (negative) sequences and incomplete(positive) sequences; 21-polished sample, acetale peel, thin section. Slika 2. Položaj stupova i legenda za sl. 3, 4 i S: l -granule i krupni arenili; 2-srednjezmati arenili i siltiti; 3-mikriti; 4-laporovili mikriti; S-leće i lečasti proslojci rožnjaka; 6-lečasti i nepravilni tragovi bubča; 7 -pravilne l amine; 8-leće i leća s ta laminacija; 9-nepravilno leća s ta laminacija; l O-nejasna i isprekidana laminacija; ]]. erozija i tragovi utiskivanja; 12-oštarprijelaz; 13-postepeni prijelaz; 14-muljni konturiti; IS-pješčani konturiti; 16-pješčano-fljunčani konturiti; 17 -sekvencije sitnozmatih turbidita; 18-hcmipelagiti; 19-potpune (negativno-pozitivne) sekvencije; 20-nepotpune (negativne) sekvencije i nepotpune (pozitivne) sekvencije; 21-nabrusak, acetatna folija,izbrusak. by dewatering processes or bioturbation (pl. II, Fig. 2 and pl. III, Fig. 1). Isolated lenses composed of siltites and fine arenites are relatively frequent. Because of their shape and grain imbrication, some lenses could represent isolated and asymmetric ripples ("fading ripples") . Relatively long and thin climbing ripples with very gently climbing angle have been observed at a few places (pl. II, Fig. 2 and pl. III, Fig. 1). Regular and lenticular laminae as well as isolated ripples composed of fine arenites and siltites are usu­ ally randomly placed between and above discontinuous and indistinct silt and micrite laminae. However, con­ centrations and frequency of the laminae are a characteristic feature of the middle and the upper part of beds. One of the most typical features of numerous contourites is their extensive bioturbation which has modified or destroyed much of the primary sedimen­ tary structures, partially or completely altered the nature of the contacts and is probably responsible for the mixing of arenite, silt and mud together with Fe-oxide mottles.Oxides occur in baggy or pocket-like form at a millimetre and centimetre scale. This suggest that bioturbation was a continuous process, probably with several superimposed episodes (Fig. 3, 4 and 5; pl. II, Fig. 2; pl. III, Fig. 1). Certain more regular or inter­ nally structured forms are also recognized and can tentatively be identified as specific trace fossils. They include the small (up to 2 mm) elongated lenses and tubes sporadically densely spaced, which probably present a form of Chondrites, more regular oval and ellipsoid forms that are mostly similar to P lanolites or traces with chevron structure like Scolicia. 3.2. SANDY AND SANDY-GRAVELL Y CONTOURITES Sandy contourites are less common than the muddy ones. They are represented by less than 10 % in the whole succession and appear like irre gular beds from l to 17 cm of thickness, (most frequently between 4 and 6 cm). Their lower boundaries are sharp or even erosional, with load deformations. Sandy contourites as a rule are embedded in muddy contourites (Fig. 3, 4 and 5). In the most cases, they do not display any internal order except for a slightly inverse or normal grading. How­ ever, sometimes a gen tle coarsening can be seen in their middle part. Elongated grains are parallel in relation to the stratification but sometimes a weak imbrication can be also remarked. Sandy contourites are composed of very coarse and coarse areni te sized grains and, more rarely, of fine sand- 72 o E o o A1 Fig. 3. Columns A l and A2. Slika 3. Stupovi Al i A2. H Geologia Croatica 45 eJ • Cm ++ 5I• 4J•+ Ic Cm Cm Cm H ... Mrinjek : The Bottom Cu rent ... 50 E () o B Fig. 4. Column B Slika 4. Stup B. . H? Cm? Cm H? H H? Cm ~ H ---- Cm H 41-Jo 73 13bl •• Cm 13al•Jo T 12bl• e1-e2 ___ 12•1. Cm H 11ci • Cm 8di I 22lc aci 8bl•+IO Sal lO Te1-e3 1 21 o H l_ Te1-e2 201• 7 I••Io •• t ... . Te1-e3 19Jc • • fM a 74 PLATE- TABLA I Geologia Croatica 45 PLATE· TABLA I l. Alternation of thinly bedded calcilutites and marly calcilutites. Column B (Fig. 2 and 4). Izmjena tanko uslojenih kalklutita i laporovitih kalklutita. Slijed B (sl. 2 i 4). 2. Alternation of thinly bedded calcarenites with calcilutites and mar!.y calcilut'ites. Column Cl (Fi g. 2 and 5). Izmjena tanko uslojenih kalkarenita s kalklutitima i laporovitim kalklutitima. Slijed Cl (sL 2 i 5 ). Mrinjek : The Bouom Curenl... PLATE - TABLA II 75 PLATE· TABLA II l. Thinly bedded calcarenites with chert lenses. Layers have been folded by slumping. Column Cl (Fig. 2 and 5) Tanko uslo jeni kalkareniti s lećama rožnjaka. Naslage su u formi polegle bore nastale klizanjem rotacijskog tipa. Slijed Cl {sl. 2 i 5). 2. Muddy contourites, negative-positive sequence throughout the photo. Calcilutite (wackestone) with thin and indistinct laminae. Laminae composed of siltites and fine arenites are in the middle part of sequence (ar­ row). Thinly lenticular Jamina of microquartz {black) can be scen between laminae. The sediment has been disrupted by numerous lenticular burrows filled with marlaceous micrites {light-grey). Column B. Sample 8 D. Acetate peel; X 2,5. Muljni konturit, negativno-pozitivna sekvencija prisutna kroz cijelu fotografiju. Donji i gornji dio sekvencije je kalklutit (wackestone) s tankim i nejasnim laminama silta. U sredini sekvencije su nepravilno lećast.e i utiskivanjem poremećene tamine silta i finog arenita (strelica). Između lamina nazire se tanka Jamina mikrokvarca (crno). Sediment je poremećen brojnim lećastim tragovima bušača ispunjenih laporovitim mikritom (svijetlosivo). Slijed B. Uzorak 8 D. Acetatna folija; X 2,5. - 76 sized grains. The grains are mostly well sorted, rounded, weakly to moderately spherical, grain supported with point contacts and, more rarely, planar, interpenetrat­ ing or stylolitic contacts. As in the case of muddy contourites, the grains are mostly fragments of micrites and recrystallized micrite. Some of fragments are partly clayey. Sparitic fragments and the fragments of pelagic foraminifers, bi valves and echinoderms have been rarely found. They are more or less micritized. Siliciclastic grains are rare. Quartz is much more usual than feldspar fragments. Numer­ ous quartz grains have corroded rims. On the other hand, some are fragments of composite quartz. Rare pigments and mottl es of authigenic hematite occur; the same goes for opaque mineral grains. The grains are bound together by a drusy sparite or a microsparite or/and by micrite, while syntaxial rim cement is often seen on the echinoderm fragments. Rare poikilotopic cement and dispersed silt-sized quartz and hematite grains, inside the sporadically recrystallized micrite, can be remarked. The only noticed sandy-gravelly contourite in the entire succession is placed in the middle part of col­ umn Cl in between sandy contourites, and is composed of multiple alteration of coarse arenite-sized grain in­ tervals (4-7 mm thick) and fine arenite-sized grain intervals (about lO mm thick). A few rudite-sized and weakly rounded fragments (2-7 mm) of micrite, marly micrite or skeletons are found in coarse arenite inter­ vals with parallel or slightly inclined position of their longer axis in regard to the stratification. Transitions between intervals are very gradual and sporadically very indistinct. Rare rodite-sized fragments may be also remarked inside fine arenite intervals. Unfortunately, its lower and upper boundaries are masked by exten- Geologia Croatica 45 sive chert lenses (Fig. 5; pl. V, Fig. l and 2 ). Except for poorly sorting, other grain characteristics like grain type, grain portion are nearly the same as they are in sandy and muddy contourites (pl. V, Fi g. l and 2 ). The gr ai ns are bounded together by the drusy sparite and also, but more infrequently, by poikilotopic cement. Fibrous calcite cement may be seen around some fragments. Dark-grey and dark-red nodular and lenticular chert, 0,5 to 6 cm thick, is a frequent diagenetic feature of sandy contourites. Silicification has usually taken place in the middle part of beds. The very rare chert lenses and nodules inside muddy contourites, mostly at a millimetre scale, have been originated by complete or partial silicification of si lt and fine arenite Jaminae and lenses (Fig. 3 and 5, pl. II, Fig. 2; pl. IV, Fig. l and 2 ). Chert is composite of a microquartz with the dispersed rests of brownish s pari te inside. Both chert and the s par particles are contaminated by Fe-oxides. 4. CONTOURITE SEQUENCES The contourite sequences have a regular vertical ar­ rangement although not so clearly developed as it is within fine-grained turbidites. A distinctive feature of a contourite packet is a presence of n e g a 1 i v e s e q u e n e e s in which the number and size of lam i nae and the size of grains increase upwards and of p o s i 1 i v e s e q u e n e e s in which these characteristics decrease upwards (STOW et al., 1984). Both sequences are often joined together into one n e g a 1 i v e -p o s i 1 i v e s e q u e n e e or, in other words, in one e o m p l e l e s e q u e n e e , but in this case the symmetry is not always present. Nega­ tive or positive parts can be more or less expressed or can be thinner or thicker. There is sometimes a problem in distinguishing positive sequences from fine-grained turbidite sequences. Negative scquences have gradual and PLATE - TABLA III l . Muddy contourite, negative-positive sequence. The lower part of sequence is marlaceous micrite. Horizontal and irregularly lenticular laminae composed of calcilutites and very fine calcarenites are in the middle part of sequence (arrows). The laminae are sporadically disturbed by burrows. Numerous burrows and laminae composed of calcilutites may be seen in the upper part. The upper part is marlaceous micrite. In the lower, weakly reverse graded part there can be a normal grading. Column B.Sample 8 B. Acetate peel; x 2. Muljni konturit, negativno-pozitivna sekvencija. Donji dio je laporoviti mikrit U središnjem dijelu vide se horizontalne i nepravilno lećaste lamine kalksiltita i vrlo finog kalkarenita (strelice), mjestimice poremećene bušačima. U gornjem dijelu, također izgrađenom od laporovitog mikrita, naziru se tamine kalksiltita i prisutni su brojni tragovi bušača. Gornji dio je jedva primjetno graduiran za razliku od donjeg dijela koji je slabo inverzno gradu iran. Slijed B. Uzorak 8 B. Acetatna folija; x 2,2. 2. Fine-grained turbidite sequence (Te1-e1-eJ. The lower part is marly micrite disturbed by multi-stage bioturbation (muddy contourite). The middle part begins with lenticular and deformed Jamina composed of fine sand and silt (interval e1) and is succeded by alternation of irregularly lenticular, regular and indistinct Jaminae also composed of fine arenite and siltite (interval eJ. The upper part is calcilutite (wackestone) with many lenticular and irregular burrows (interval eJ. Column B. Sample 6 A. Acetate peel; x 2,2. Sekvencija sitnozrnatog turbidita (Te1-e1-eJ. Donji dio je laporoviti mikrit (muljni konturit) poremećen višestrukom bioturbacijom. Središnji dio (interval e1) počinje lećastom i djelomično deformiranom laminom finog arenita i silta. Slijedi višestruka izmjena nepravilno lećastih, pravilnih i nejasnih Jamina silta i finog arenita i nejasna normalna graduiranost (interval e1). Gornji dio je kalklutit (wackestone) s brojnim lećastim i nepravilnim tragovima bušača (interval e3). Slijed B. Uzorak 6 A. Acetatna folija; x 2,2. - Mrinjek : The Bottom Curent... PLATE- TABLA III 77 78 Geologia Croatica 45 H l 1±-!:tl-8± - ~ -·-=--- a;z;a Cm o H __ ea č.:.:. .t :.:...; ~--::::~~~ t-- [l so 3 I•., I a lo ~:~ lj f-' '-l ....l ~ Cg + - --- ea E Ca ....:: u ... tj ~:....._~- 11• H?- Ca ~u '·.•.>7•i' H?- J Ca - H _l r TIT .. · · ·~ o . "' Fi~. 5. Columns C and e . Shka 5. Stupovi C l C 2 l 2. Mrinjek : The Bottom Curent ... LITHOLOGY MEAN and GRAIN SIZE STRUCTURE 50 E CJ 4 8 16 32 64,um -.Q.T -_Q···=--o:.. eO•._ ---------------- \ ( \\ ~~ EJ silt and sand ~ mud ~ shell fragments ~&b ~ bioturbation maximum velocity ~ d iscontinuous si lt lenses gradational contact -v- sharp ( irregular} contact Fig. 6. Ideal complete (negative-positive) contourite sequence (from STOW & PIPER, 1984). Slika 6. Idealna potpuna (negativno-pozitivna) konturitna sekvencija (iz STOW & PIPER, 1984). faintly visible beginning from hemipelagites whereas their to ps are sharp and clear, flat or slightly wavy. Of course, the opposite is in the case of positive contourite sequences (Fig. 3, 4, 5 and 6; pl. III, Fig. l; pl. IV, Fig. 2). Sandy contourites are commonly placed between muddy ones in the middle part of negative-positive sequences, but some of them are at the beginning of positive sequences or on the top of negative sequences. Sandy intervals can have a gen tle reverse or a normal grading, o.r even a gently co.arseni.ngjQ tp~ir middle · part (Fig. 3; 4, 5 and 6:· pl. IV; Fig. 1). n~ thiclcness of sequences varies from 5 up to 30 cm. 79 5. FINE-GRAINED TURBIDITES The fine-grained twbidites are oni y present in the "column B, and represent about 10% of all investigated successions. An ideal fine-grained turbidity sequence consists of three parts denoted by Piper's signs e 1 , e 2 and e 3 (Piper, 1978). The complete sequences (Te1-e3) and top-cut-out se­ quences (Te 1 -e 2 ) are approximately represented in similar amount The mutual bmmdaries, and the lx;>\mdaries between turbidites and the contourite sequences are usually sharp, even load-casted and erosional in some cases. The upper contact with hemipelagites is transitional instead. As in the case ofmuddy contourites, calcilutite sequences with less amount of clay and calcareous mari sequences with bigger amount of clay can be distinguished (Fig. 4; pl.III, Fig. 2). I n t er v a l "e 1 " contains the same laminae and lens types as the muddy contourites- regular, irregular, lenticular, indistinct and wispy laminae of siltites and fine arenites. The internal frame of laminae and lenses and their grain kind and frequency are the same or nearly the same as with muddy contourites. The main distinction between them is in a vertical arrangement of type, size and number of laminae and lenses. Namely, their number and thick­ ness gradually decrease towards the top of interval "e 1 ". Isolated and asymmetrical ("fading") ripples and long, thin climbing ripples are located in the lower part or even at the beginning of interval whereas indistinct, discontinuous and "wispy" laminae are placed as a rule in the upper parts of interval. Biotwbation is weaker than in other intervals (Fig. 4; pl. III, Fig. 2). I n t e rv a l "e2" is characterized by the gradually upwards decreasing amounts and size of silt-sized grains and fine arenite-sized grains. Besides the grading, there are numerous burrow traces (Fig. 4; pl. III, Fig. 2). I n t er v a l "e3" is massive. There are only sparse silt-sized and fine arenite-sized grains. This part is also disturbed by vigorous bioturbation. Transitions between intervals are gradual and therefore hardly noticeable (Fig. 4; pl. III, Fig. 2 ). 6. HEMIPELAGIC INTER V ALS The hemipelagites are thin bedded, usually between l and 13 cm, and massive. They are composed of micrites and of greater amount of clay with rare and irregularly dispersed silt and fine arenite-sized grains of sparite, quartz, muskovite and feldspar. They have more vig­ orous bioturbation than muddy contourites and fine grain ed turbidites. Hemipelagic deposits represent about 18 % of the investigated succession. They are found on the top of fine-grained turbidites or between muddy and sandy contourites. Their boundary with muddy contourites is gradual , whereas the upper boundary between them and sandy contourites or fine-grained turbidites is usually irregular and erosional (Fig. 3, 4 and 5; pl. III, Fig. 2). 7 .•. PROCESSES Bottom currents have an important effect on nature and distribution of bottom sedi ments. They result from - so circulation caused by density differences between masses; their speed being in close relation with changes of climatic and oceanographic factors. Long-term current mea­ surements show that bottom currents vary greatly in time and space, having tidal, seasonal and irregular periodicity and reversal s (STOW et al. ,1984 ). V as t areas of the oceans have been observed where currents are weak or negligible as well as areas where current ve­ locity can reach over 100 cm/s for a short period of time. Bottom currents are capable of eroding, transporting and depositing sediments up to about medium sand sized grains, of reworking coarse-grained sands and winnowing finer material away from coarser-grained sandy and gravelly sediments (STOW et al.,l984). Bottom cur­ rents transport also fine material in suspension ( average size about 0,012 mm) in thin to thick (even over 1500 m), very low concentration (0,01 to 0,3 mg/l) nepheloid layers, in some cases over thousands of kilometres (STOW et al., 1984). Suspended sediments enter the nepheloid layec from below by bottom current erosion, sediment resuspension due to burrowing organisms, internal waves, the fine tails of turbidity currents, and from vertical settling of hemipelagic and pelagic material through the wa­ ter column. So in many cases the depositional process must be considered intermediate between bottom cur­ rent, turbidity current, pelagic and hemipelagic. The vertical variation in the described contourite sequences is probably related to variation in velocity of the transporting current rather than to variation in supply. Thus complete (negative- positive) sequences represent a gradual increase, a maximum and then grafl•:al decrease in current velocity. Such fluctuations could be slow and progressive as shown by the thicker se­ quences or more rapid and sudden, as shown by thinner ones. The increase, however, in the current velocity could be gradual and decrease could be abrupt and rapid (incomplete, negative sequences), or gradual (incom­ plete, positive sequences). Horizontal and lenticular silty laminae and thin climbing ripples are related to maximum current ve- Geologia Croatica 45 locity. Since indistinct and discontinuous lam i nae and massive micrite can be seen between them, it is possible that, during maximum velocity periods, very short-time velocity fluctuations have existed. Rare sandy contourites represent intervals of very strong bottom currents capable of transporting and depositing medium and even coarse sand-sized grains, although some of them might even represent reworked turbiditic sands. The multiple and irregular alternation of erosion and/ or winnowing finer material away from turbiditic sands and gravels with deposition ofc oarse sands and fill­ ing of grain interstices with fine sands, have been probably needed for creating sandy-gravelly contourite. Depositional sorting and flocculation are probably the main origin of laminae with a gradual decrease of their thickness and number in the fine-grained turbid­ ity sequences (STOW & BOWEN, 1980). According to the authors, the alternation of silt and micrite laminae has been created by depositional sorting which was caused by shear stress changes in the lower boundary layer of flow. At the beginning, mud floc­ culation and mud floc deposition have been made impossible because of shearing so that only fine areni te­ sized and silt-sized grains could be deposited through the viscous boundary layer. This created tractive ripples and laminae. By flow weakening and mud density increasing, the efficient flocculation and very fast deposition of a "muddy blanket" have been enabled through the lower, laminar boundary layer of flow on the earlier formed ripples "}ind lam i nae. This process :has been repeated several times with fi ner and fi ner sand and silt grains, and a greater amount of mud; thinner and subsequently indistinct laminae have been created in this way. At the end of the process, a negligible amount of si lt and fines t sand grains remained, so that al most only mud was deposited (intervals "e 2 " and "e 3 "). Fluctuations in the current velocity or interference of the low-density turbidity current with a bottom current might have caused the presence of thicker laminae above PLATE- TABLA IV l . The lowermost level is partly eroded marly micrite with indistinct lamination. The rest is very coarse sized and reverse graded arenite (sandy contourite). A great part of arenite is silicified (black). Column A2. Sample 3. Acetate peel; x 1,6. Najdonji dio je laporoviti mikrit s nejasnom laminacijom, svijetlosivo i djelomično erodiran vrlo krupnozrnastim i reversno graduiranim kalkarenitom (pješčani konturit), veliki dio kalkarenitaje zahvaćen silifikacijom (crno). Slijed A2. Uzorak 3. Acetatna folija; x 1,6. 2. Sandy contourite in the complete contour sequence. Marly micrite with indistinct and interrupted laminae composed of siltites is in irregular sharp contact with a thick lens of fine-grained calcarenite. The lens is weakly reversely graded in the lowermost part. The upper part is a marly micrite with indistinct lamination and thinly lenticular burrows. Transition from lens to upper micrite is gradual. Column B. Sample 8. Acetate peel; x2,5. Pješčani konturit unutar potpune konturne sekvencije. Donji dio koji je izgrađen od laporovitog mikrita s nejasnim i isprekidanim laminama silta je u oštrom i nepravilnom kontaktu s proslojkom finog kalkarenita. Proslojak je jedva primjetno inverzno graduiran u svom najdonjem dijelu. Gornji dio je laporoviti mikrit sa nejasnom laminacijom i tankim lećastim bušotinama. Prijelaz finoarenitnog proslojka u mikrit je postepen. Slijed B. Uzorak 8. Acetatna folija; x 2,5. - Mrinjek : The Bottom Curent ... PLATE - TABLA IV 81 82 thinner or indistinct ones. Flocculation was less effi­ cient in that case, because of greater amount of the carbonate grains with respect to the silicate grains. This is probably the reason why irregular and indistinct laminae are only present in some turbidity sequences. Depositional sorting should be involved in the contourite origin because all possible transition between contourites, fine-grained turbidites and hemipelagites could be expected. Fine-grained sediments are not so much character­ ized by the burrow varieties as they are by the intensity of bioturbation. Hemipelagites are nearly always en­ tirely and multiply disturbed by burrows but, in most cases, laminae and other primary features are at least partly preserved whereas the traces of burrows are almost absent in sandy contourites. The most intensive bioturbation is also characteristic for higher parts of the fine-grained turbidite sequences. 8. PALAEOGEOGRAPHIC SETTING Recent contourites are deposited on continental slopes, like well-laminated drifts spreading parallel with slope. They have been also noticcd in basin plains (STOW & PIPER, 1984). Fine-grained turbidites have been found in very diverse areas like slopes, subaqueous fans and on basin plains (STOW & PIPER, 1984). Columns Cl and C2 contain sandy and sandy-gravely contourites which have been originated by reworking and winnowing of turbidite sequences, and they prnb­ ably belong to a slope or upper fan, relatively near a subaqueous canyon or channel mouth. Synsedimentary slumping might point out this position (Fig. 2 and 5). On the other hand, disturbed layers in column B have been probably deposited by sliding on a slope but relatively further away from channels (Fig. 2 and 4). The layers in columns Al and A2 are likely to belong to the lower part of slope or even to a basin plain. Two petrographic types of lutites - calcilutites and marly calcilutites could be a proof of different material sources Geologia Croatica 45 or respectively may point out different paleocurrent directions or changing in a clay share. 9. CONCLUSIONS The contourites and sediments which have arisen from bottom currents, most probably contour currents, dominate in the Upper Cretaceous deep-water clastic zone in Žumberak. Muddy, sandy and sandy-gravely contourites can be differentiated according to structures and textures (primarily grain size). Muddy contourites, as a dominating member of the investigated succession, are characterized by a mutual alternation of horizontal, len tic ular, indistinct and discontinuous fine arenite and siltlaminae inside more or less marly micrite and, also, by vigorous bioturbation, mostly trough all their thickness. Sandy contourites which are considerably more infrequent in regard to muddy contourites, appear as thin and irregular beds with erosional undersurfaces. They are mostly composed of coarse areni te-sized grains that can be gently reverse or normally graded. Only one sandy-gravelly contourite composed of multiple alteration of coarse arenite-sized grain inter­ vals and fine arenite-sized grain intervals could have been seen. . Muddy contourites and a great part of sandy contourites have been transported and deposited by bottom currents. A lesser part of contourites could be re worked turbiditic sands, whereas sandy-gravely contourite might represent a manifold alternation or erosion and win­ nowing of turbiditic gravels and co~se arenites during very strong currents. In most contourites, one can see the vertical ar­ rangement of laminae and lens type and size and also average grain sizes, or complete sequences (negative­ positive) and incomplete sequences (negative or positive) ·respectively and whose origin is in the close relation to the current velocity. PLATE- TABLA V 1. Sandy-gravelly contourite. Alteration of coarse calcarenite intervals with fine calcarenite ones. Transitions are gradual and sporadically indistinct. Rare, poorly rounded and imbricated rudite grain can be seen inside the fine areni te intervals. Sample from the lower part of the sandy-gravelly contourite with initial silicification (black). Column Cl. Sample 2. Acetate peel; x 2,7. Pješčano-šljunčani konturit..Izmjena inte~al~ krupnog kalkareriita s intervalima finog kalkarenita. Prijelazi između intervala su postepeni ili Čak mjestimice nejasni. Unutar intervala finog arenita mogu se zapaziti slabo zaobljeni ruditni fragmenti blago kosog (imbriciranog) položaja u odnosu na slojevitost. Donji dio pješčanoŠšljunčanog kon turi ta čiji je početak zahvaćen silicifikacijom (crno). Slijed Cl. Uzorak 2. Acetatna folija; x 2,7. 2. Sandy-gravelly contourite. Alternation of coarse calcarenite intervals with fine calcarenite intervals. Tran­ sitions are gradual and sporadically indistinct. Rare, poorly rounded and imbricated rudite grains can be seen inside fine arenite intervals. The top of the sandy-gravelly contourite. Column Cl. Sample 3. Acetate peel;x 2,7. Pješčano-šljunčani kon turi t. Izmjena intervala krupnog kalkarenitaa s intervalima finog kalkarenita. Prijelazi između intervala su postepeni ili čak mjestimice nejasni. Unutar intervala finog arenita mogu se zapaziti rijetki, slabo zaobljeni ruditni fragmenti blago koso g (imbriciranog) položaja u odnosu na slojevitost. Završni dio pješčano­ šljunčanog konturita. Slijed Cl. Uzorak 3. Acetatna folija; x 2,7. Mrinjek : The Bottom Curent ... PLATE- TABLA V 83 84 Sandy and sandy-gravelly contourites probably belong to a slope or upper fan environment, relatively near a subaqueous canyon or channels, while muddy contourites have been deposited on a lower part of slope, relatively farther away from channels or even on a basin plain. 10. REFERENCES AUBOUIN, J., BLANCHET, R., CADET, J .P.,CELEf, P., CHARVET, J., COHOROWICZ, J., COUSIN, M. & RAMPNOUX, J.P. (1972): Essai sur la geologie des Dinarides. - Bull. Soc. geol. France, (7), 12/6, 1060-1095, Paris. BABIĆ, Lj. (1973): Bazenski sedimenti gornjeg titona, beriasa i valendisa, zapadno od Bregane.-Geol. vjesnik, 26, 11-27, Zagreb. BABIĆ, Lj. (1974 ): Razdoblje otriv-cenoman u Žumberku: Stratigrafija, postanak sedimenata i razvoj prostora. Geol.vjesnik 27, 11-33, Zagreb. BABIĆ, Lj. & Zupanič, J. (1976): sedi menti i paleo geografija zone Globotruncana calcarata (gornja kreda) u Baniji i Kordunu (središnja Hrvatska). - Geol. vjesnik, 29, 49-73, Zagreb. Bl.ANCHET,R,CADET,J. P .. CHARVEf,J.&RAMFNOUX, JP.(l970): Sur l' existence d'un important domaine de flysch tithonique cretace inferieur en Yougoslavie: l'unitee du flysch bosniaque. -Bull. Soc.geol. France, (7), 11/6, 871-880, Paris. CARON, M. & COUSIN, M.(l973): Le sillon slavene: les formations terrigenes cretacees des unite extertnes au Nord-est de Tolmin (Slovenie occidentale). - Bull. Soc. geol. France, (7),14/1-5, 34-35, Paris. CHAR VET, J .(1972): A per u geologique des Dinarides aux environs du meridien de Sarajevo. - Bull. Soc. geol. France, (7), 12/6, 986-1002, Paris. Geologia Croatica 45 GONTIDER, E.G., FAUGERES, J.C.& STOW, D.A.V. (1985): Contourite facies of the Fayro Drift, Gulf ofCadiz. -In: STOW, D.A.V. &PIPER, D.J.W. (Eds.): Fine-grained sediment: Deep-Water Processes and Facies.- Blackwell, 275-292, Oxford. MRINJEK, E.(l988a): Sitnozrnati turbiditi i konturiti u dubokovodnim sedim entima gornje krede kod Slunja i u Žumberku.- Geol. glasnik, pos. izd. VI, Zbornik radova VI Skup. Sed. Jug., 135-149, Titograd. · MRINJEK, E.(l988b): Sitnozrnati turbiditi cenomana u prelaznom pojasu unutrašnjih i vanjskih Dinarida sjeveroistočno od Slunja. - Geol. vjesnik, 41, 181- 196, Zagreb. PIPER, DJ .W. (1978): Turbidite Muds and Silts on Deep Sea Fans and Abyssal Plains. -In: STANLEY, D. J. & KELLIN, G.(Eds.):Sedimentation in Sub­ marine Canyons, Fans and Trenches.- Dowdcn, Hutchinson and Ross, 163-176,Stroudsburg. STOW, D.A.V. & BOWEN, A.J.(I980): A physical model for transport and sorting of finegrained sediment by turbidity currents. - Sedimentology, 2 7 /l, 31- 46, Oxford. STOW,D.A.V. & HOLBROOK, J.A. (1984): North Atlantic contourites: an overview.- In: STOW, D.A.V. & PIPER, D.J.W. (Eds.): Fine-grained sediment: Deep-Water Processes and Facies. - BLackwell, 145-256, Oxford. STOW, D.A.V. & PIPER, D.J.W. (1984): Deep-water fine-grained sediments: facies models. - In: STOW, D.A.V. & PIPER, D.J.W. (Eds.): Fine­ grained sediments: Deep-Water Processes and Facies. -BLackwell, 611-646, Oxford . Sedimenti pridnenih struja (konturiti) u dubokovodnim naslagama gornje krede sjeveroistočnog Žumberka E. Mrinjek Istraži vane naslage se nalaze u sjeveroistočnom dijelu Žumberka i pripadaju roni gomjokrednih dubokomorskih taloga, smještenoj na sjeveroistočnoj strani prostranog karbonatnog šelfa (Vanjski Dinaridi, BABIĆ & ZUPANIĆ, 1976). Naslage karakterizira izmjena konturita te u mar joj mjeri sitnozrnatih turbidita i hemipelagita. Mogu se razlikovati tri tipa konturita - m u l j n i , p j e š č a n i i p j e š č a n o - š l j u n č a n i konturiti (sl. 3, 4 i 5). Muljni konturiti, čija debljina varira od 3 do 22 cm, su najčešći tip. Međusobr~. granica između samih m uljnih kon turi ta i kon turi ta i hemipelagita je postepena, dok je kontakt s pješčanim konturitima i finozrnatim turbiditima oštar ili čak oštro nepravilan (sl. 3, 4 i 5). Muljni kon turiti su izgrađeni od mikrita s promjenjivom količinom gline kao i od karbonatnih i siliciklastičnih čestica veličine finog arenita i silta. Silikatne čestice su podređene u odnosu na karbonatne čestice. Njihova glavna teksturna karakteristika je prisutnost pravilnih, nepravilnih, lećastih i nejasnih Jamina silta i finog arenita. Prosječna debljina laminaje od 0,5 do 5 mm iako lamines mogu biti između 0,1 do 0,2 mm tanke. Osim normalno graduirane, tamine mogu biti inverzno ili normalno­ inverzno graduirane. Prisutni su brojni tragovi bušača koji su poremetili ili prekinuli tamine {tab. II, sl. 2; tab. III, sl.l). Pješčani konturiti su rjeđi od m uljnih konturita. Dolaze kao nepravilni slojevi, debljine l do 17 cm, izgrađeni su od karbonatnih i siliciklastičnih čestica veličine krupnog arenita. Njihova glavna karakteristika je blago inverzna ili normalna graduiranost, ili čak inverzno­ normalna graduiranost (sl. 2, 4 i 5;tab. l, 2 i 3). Jedan jedini pješčano- šljunčani konturit se nalazi u srednjem dijelu stupa Cl i izgrađen je od višestruke Mrinjek : The Bottom Curent ... izmjene intervala krupnog arenita debljine 4-7 mm i intervala finog arenita debljine oko lO mm. Nekoliko rudi tnih fragmenata (veličine 2-7 mm) mikrita, glinovitog mikrita ili skeleta nalaze se unutar krupnog arenitnog intervala. Prijelaz između intervala je vrlo postupan i mjestimično nejasan (sl. 5; tab.V, sl. l i 2 ). Postoji u određenoj mjeri pravilna vertikalna uređenost unutar konturita. iako ne toliko jasna kao unutar finozrntih turbidita, pa se može govorti o n e g a t i v n i m s e­ k v e n e i j a m a u kojima broj i veličina Jamina i veličina zrna rastu prema gore i o p o z i t i v n i m s e k v e n e i j a m a u kojim te karakteristike o padaju prema gore (STOW et al., 1984). Cesto puta su obje sekvencije pridružene zajedno u jednu n e g a t i v n o- p o z i t i v n u s e k v e n e i j u pri čemu simetrija ne mora biti uvijek prisutna, tj. negativni ili pozitivni dijelovi mogu biti jače ili slabije izraženi, odnosno deblji ili tanji (sl. 3, 4 i 5). Pješčani konturiti su obično smješteni između muljnih konturita u središnjem dijelu negativno-pozitivne sekvencije. Pješčani konturit (interval) je obično blago inverzno ili normalno graduiran ili samo blago pokrupnjuje u svom središnjem dijelu (sl. 3, 4 i 5; tab. IV, sl. l, 2 i 3). Sitnozrnati turbiditi su prisutni samo u stupu B. Istog su petrografskog sastava kao i konturiti ali za razliku od njih, njihove strukturne i tekstume karakteristike imaju prav ilni ji vertikalni raspored. Njihova idealna sekvencija se sastoji od tri dijela označenih Piperovim oznakama "e 1 ", "e 2 " i "e 3 " (PIPER, 1978). Kompletne sekvencije (Te 1 -e:J i odrezane sekvencije (Te 1 -e 2 ) su približno podjednako zastupljene u istraživanom slijedu (sl. 4). Njihova debljina je između 4 i 9 cm. Njihove međusobne granice i granice između njih i konturita su obično oštre i erozivne ili čak poremećene utiskivanjem, dok je krovinski kontakt s hemipelagitom prijelazan. Glavna karakteristika donjeg dijela sekvencije (interval "e 1 ")su pravilne, nepravilne, lećaste, nejasne i "wispy" tamine siltita i finog arenita. Osim lam ina, interval "e 1 " sadrži i izolirane rip love i tanke "penjajuće" riplove. Bioturbacijaje slabije izražena nego u ostalim intervalima (tab. III, sl. 2). Postepeno smanjivanje količine i veličine siltnih i finoarenitnih čestica je glavna karakteristika intervala "e 2 " . Osim graduiranosti prisutni su brojni tragovi bušača (tab. III, sl . 2). Interval "e," je masivan sa samo rijetko raspršenim siltnim i finoarenitnim česticama. U pravilu je gotovo uvijek jako bioturbiran. Prijelazi između intervala su postepeni i stoga teško primjetni (sl. 4; tab. III, sl. 2). Hemipelagiti su tanko uslojeni, obično između l i 13 cm tanki, masivni·, s više gline i jače izraženom bioturbacijom nego muljni konturiti i sitnozrnati turbiditi. Nalaze se između m uljnih konturita, muljnih i pješčanih konturita ili su kontinuirani. Pridnene struje imaju snažan utjecaj na distribuciju i sastav taloga dna d~jih oceana i mora. Za razliku od površinskih i pri površinskih struja koje nastaju puhanjem vjetra uzrokovanog razlikama u gustoći, odnosno razlikama u pritisku zraka, pridnene struje nastaju termohalinom cirkulacijom uzrokovanom 85 razlikom u gustoći pojedinih dijelova mora, te je stoga njihova brzina u uskoj vezi s promjenama klimatskih i oceanografskih faktora. Dugotrajna praćenja i mjerenja pridnenih struja otkrila su značajna dnevna, sezonska ili vremenski nepravilna variranja njihovih brzina kao i velike promjene njihovog smjera kretanja (STOW et al., 1984). Također su otkriveni veliki dijelovi morskih dna sa slabim ili beznačajnim djelovanjem struja, kao i dijelovi dna sa snažnim djelovanjem struja gdje u kratkim vremenskim intervalima brzina struje može biti veća od 100 cm/sek., tako da su sposobne erodirati, transportirati i deponirati taloge sa zrnima do veličine srednjeg pijeska, prerađivati krupni pijesak ili ispirati sitnozrnati materijal iz šljunka i vrlo krupnog pijeska (STOW et al.,l984). Utvrđeno je da pridnene struje prenose finozrnatu suspenziju prosječne veličine zrna od oko 0 ,012 mm u tankim do debelim (čak i preko 1500 m) nefeloidnim slojevima vrlo niske koncentracije (0,01 do 0,3 mg/l) tisućama kilometara daleko. Suspendirani materijal potječe od taloga dna pokrenutog erozivnim djelovanjem pridnenih struja, a dio može potjecati i iz repa razrijeđene turbiditne struje kao i od pelagičkih i hemipelagičkih čestica zahvačenih pridnenim strujama. Stoga takvo složeno porijeklo materijala nošenog pridnenim strujama može objasniti sve moguće prijelaze između konturita, sitnozrnatih turbidita i hemipelagita i/ili pelagita prepoznate i u istraživanim slijedovima. Vertikalne varijacije konturnih facijesa uočene u istraživanim slijedovima prvenstveno su u vezi s promjenjivošću brzina pridnenih struja, dok je izvorište materijala moglo imati mali ili nikakav utjecaj. Opisane negativno- pozitivne sekvencije bi stoga predstavljale postepeni rast, maksimum i postepeni pad brzine struje. Takve fluktuacije su mogle biti relativno spore, na što ukazuju deblje sekvencije, ili pak brze i nagle, na što ukazuju tanje sekvencije~orast brzine struje je mogao biti postepen, a slabljenje naglo (nepotpune negativne sekvencije) ili obrnuto,~tj. nagli porast brzine, a zatim postepeno slabljenje (nepotpune pozitivne sekvencije). Horizontalne i lećaste lam ine finog arenita i silta kao i tanki penjući riplovi vezani su za period maksimalnih brzina struja. S obzirom da između njih postoji i prostor ispunjen nejasnim i isprekidanim laminama kao i mikritom, očito je da su unutar perioda maksimalnih struja postojale kratkotrajne sekundarne fluktuacije brzina na što ukazuju i neke lam ine s blago normalno ili blago reversno graduiranom građom. Rijetki pješčani konturiti mogu predstavljati interval~ izrazito.snažnih pridnenih struja koje prenose i talože srednji ili čak krupni pjesak, ali isto tako neki od njih mogu predstavljati manje ili više in situ prerađivane krupne turbidne pijeske. Snažna ali i vrlo promjenjiva pridnena struja je bila potrebna za stvaranje pješčana - šljunčanog konturita kod čijeg je postanka vjerojatno postojalo višestruko i nepravilno izmjenjivanje procesa erozije i ispiranja turbidnog pijeska i šljunka s procesima taloženja krupnog pijeska i ispunjavanja međupornog prostora sitnim 86 pijeskom. Taloženju siblozrnatih turbidita pripisivani su različiti načini (STOW & PIPER, 1984) ali se čini da najprihva­ tljivije obj~njenje daju STOW & BOWEN (1984). Po njima je izmjena Jamina silta nastala depozicijskim sortiranjem zbog promjene sila naprezanja u donjem graničnom sloju. U početku naprezanje ne dopušta flokulaciju i taloženje mulja nego se kroz viskozni dio graničnog sloja talože čestice finog arenita i krupnijeg silta koje vučenjem fonniraju riplove i lamine. Slabljenjem toka i povećanjem koncentracije mulja omogućena je uspješna flokulacija i vrlo brzo taloženje "muljnog pokrivača" kroz najdonji laminirani dio toka na već prije formirane riplove i lam ine. Taj proces se ponavlja više puta ali sa sve sitnijim česticama silta i sve većom količinom mulja, tako da depozicijskim sortiran jem dolazi do sve tanjih, a zatim i nejasnih lamina silta. Nakon toga preostaje u toku neznabla količina silta i najfinijeg arenita pa se taloži mulj što odgovara srednjem i gornjem dijelu sekvencije (interval "e2" i "en. Kod promjena kod kojih nisu uvijek vidljive, a niti . pravilno raspoređene karakteristike siblozmatih turbidita, moguće je da je došlo do miješanja djelovanja turbidne struje s pridnenom strujom ili pak do prevlasti djelovanja ove potonje. Također treba naglasiti da u sastavu Geologia Croatica 45 istraživanih turbidnih sekvencija manje ili više prevla­ davaju karbonatne čestice što sigurno onemogućuje uspješnu flokulaciju i time depozicijsko sortiranje kako je bilo interpretirano analizom siliciklastičnih turbidita (STOW & BOWEN, 1980) paje to moglo dovesti do razvoja nejasnih lamina i jedva primjetno g gradu iranja. S druge strane mehanizam depozicijskog sortiranja mogao je barem djelomično postojati i kod konturita na što upućuju i svi mogući prelazi između kon turi ta, sitnozmatih turbidita i hemipelagita. Tipove siblozmatih naslaga ne karakteriziraju toliko posebni tipovi bušača koliko intenzitet bioturbacija. Tako su hemipelagiti gotovo u pravilu potpuno višestruko bioturbirani različitim tipovima bušača. Muljni konturiti su također bioturbirani kroz cijelu svoju debljinu ali su u većini slučajeva barem djelomično sačuvane lam ine i ostale karakteristike, dok kod pješčanih kon turi ta gotovo da tragova bušača nema. Kod sitnozrnatih turbidita intenzivnija bioturbacija je vezana za gornje dijelove sekvencija. • Pješčani i pješčano-šljunčani konturiti vjerojatno su taloženi na padini ili višim dijelovima lepeze, relativno blizu podmorskog kanjona ili kanala, dok su muljni konturiti taloženi na donjem dijelu padine ili lepeze, ili čak na đnu bazena. Manuscript received March, 31. 1992. Revised manuscript accepted July, 20. 1992.