Annales Societatis Geologonim Poloniae (1996), vol. 66: 245 - 267.
ORIGIN OF LITHOLOGICAL VARIATION IN THE SEQUENCE OF THE SUB-MENILITE GLOBIGERINA MARL
AT ZNAMIROWICE (EOCENE-OLIGOCENE TRANSITION, POLISH OUTER CARPATHIANS)
Stanisław LESZCZYŃSKI
In stitu te o f G e o lo g ic a l Scien ces, J a g iello n ia n U n iversity, ill. O le a n d ry 2a, 3 0 -0 6 3 K ra k ó w , P o la n d
Leszczyński, S., 1996. Origin o f lithological variation in the sequence o f the Sub-Menilite Globigerina Marl at Znamirowice (Eocene-Oligocene transition, Polish Outer Carpathians). Ann. Soc. Geol. Polon., 66: 245-267.
A bstract. Sediment features, including foraminifera and nannoplankton diversity, 8 1S0 and 5 I3C signals, total organic carbon content, and type of kerogen show that the combined influence o f periodically changing produc
tivity of calcareous material and the intermittent supply of material from land and shelf during one 414 ky eccentricity cycle are the main factors responsible for the pattern of the Sub-Menilite Globigerina Marl sequence at Znamirowice.
The vertical fluctuations from calcareous green shale and light-coloured marl to noncalcareous green shale, in the lower part o f the sequence, are shown to result primarily from temporary changes in calcareous nannoplankton and planktonie foraminifera productivity. Climate and water circulation changes forced by the 90 ky fluctuation o f the Earth’s orbit eccentricity, are presumed to be the chief cause o f the productivity variation.
Minor fluctuations in carbonate content as well as the distribution of dark-gray to black shale and marl layers are regarded as resulting from changing supply of terrigenous material and mass resedimentation events.
Intermittent and regionally changing tectonic activity, together with expansion and lateral shifting of deltaic lobes accompanied by more regular climate changes, are considered as responsible for the supply o f terrigenous material.
A bstrakt. Analiza cech osadu wraz z oceną zróżnicowania otwornic i nannoplanktonu, wartości S 180 i 5 ''’C, całkowitej zawartości węgla organicznego i rodzaju kerogenu wykazała, że badany profil ukształtowany został głównie przez zmienną produkcję materiału wapiennego oraz zróżnicowaną w czasie dostawę materiału z lądu i szelfu w czasie i pod wpływem jednego cyklu ekscentryczności orbity Ziemi o okresie 414 tys. lat.
Zmieniająca się w czasie produkcja nannoplanktonu wapiennego i otwornic planktonicznych interpretowana jest jako główna przyczyna występujących w dolnej części profilu fluktuacji przejawiających się przechodzeniem wapnistych łupków zielonych lub jasnych margli w łupki zielone niewapniste. Jako główny czynnik odpowie
dzialny za zmiany produkcji wskazywane są okresowe zmiany klimatu i cyrkulacji wody powodowane fluktua
cjami ekscentryczności orbity Ziemi o okresie 90 tys. lat.
Mniejszej skali fluktuacje zawartości CaC03 oraz rozmieszczenie w profilu łupków i margli ciemnoszarych do czarnych są interpretowane jako efekt nieregularnie zmieniającej się dostawy materiału tery gen icznego oraz występującej w nieregularnych odstępach czasowych resedymentacji masowej. Dostawa materiału terygenicz- nego kształtowana była zasadniczo przez zróżnicowną w czasie aktywność tektoniczą otoczenia basenu, rozrost i oboczne przemieszczanie się piatów deltowych, a także przez zmiany klimatu.
Key words: shales, marls, vertical sequence, cyclicity, productivity, orbital forcing, diagenesis, Eocene- Oligocene transition, Silesian Nappe, Carpathians, Poland.
Manuscript received 2 July 1996, accepted 22 November 1996
INTRODUCTION
The E ocene-O ligocene transition is recorded w orldw ide as a tim e o f m ajor changes. T hese changes are recognized as resulting from a m ajor clim ate transform ation from a w arm
“ greenhouse” type, during the E arly Eocene, to an “ ice
house” type during the O ligocene. In the C arpathian M oun tains, these E ocene-O ligocene changes are m o st prom i nently recorded in fine-grained sedim ents o f the O uter Car pathian flysch, com posed o f a few th o u san d m etres o f pre
Fig. 1 Location o f the examined section. A. Location o f the section relative to regional geology. B. Location o f the section at Znamirowice
dom inantly siliciclastic deposits o f turbidite facies associa
tion. The sedim ents o f the several m etres thick Sub-M enilite G lobigerina Marl package (G rzybow ski, 1897; B ieda, 1946;
K oszarski & W ieser, 1960) called also S heshory H orizon (V ialov, 1951 ; V ialov e t a l., 1965), L eluchów M arl M em ber (B irkenm ajer & O szczypko, 1989), and the Strw iąż G lobi
gerina M arl M em ber (R ajchel, 1990), are particularly char
acteristic o f this period in th e O uter Carpathians. The Sub- M enilite G lobigerina M arl (SM G M ) package is conspicuous by its cream -yellow m arls, rich in planktonie foram inifera.
It form s a persistent horizon at the top o f the chiefly carbo
nate-free and light-coloured Eocene sequence and at the base o f the low er O ligocene sequence dom inated by dark- coloured shales. M oreover, sedim ents o f the SM G M -type are konw n to occur w idely in the U pper Eocene o f the entire northern part o f the A lpine belt betw een the W estern Alps and the C aucasus (see R ögl & Steininger, 1983).
U nitil recently, the origin o f the SM G M has been inter
preted on the basis o f extensive foram iniferal data, o f gen
eral sedim ent features (see O lszew ska, 1983, 1984), and o f geochem ical and other m icropaleontological evidence de
rived from selected sections (e.g. G ucw a & Ślączka, 1972;
G ucw a, 1973; G ucw a & W ieser, 1980; Van C ouvering e t a l., 1981). A ccording to these interpretations, the SM GM
package is dom inated by pelagic sedim ents deposited in a cool sea during a global sea-level and CC D drop. Palaeo- geographic transform ations o f the C arpathian area and o f the global ocean as well as volcanic activity in the C arpa
thians w ere suggested as the prim ary controls o f the SM GM sedim entation (see K siążkiew icz, 1960; G ucw a & W ieser, 1980; O lszew ska, 1983, 1984; D anysh e t al., 1987).
R ecent evaluations o f the E ocene-O ligocene stratigra
phy and interpretations o f global oceanographic conditions at the E ocene-O ligocene transition (see P rothero & Berg- gren, 1992), encourage to revise the hitherto established in
terpretations o f the SM GM . M oreover, this sequence dis
plays a characteristic sedim ent v ariation reflected in rh y th m ical gradual changes o f its colour and C a C0 3 content. This variation suggests that the sequence results from fluctuating changes o f sedim entary conditions. This p eculiarity has not been exam ined previously, how ever it is o f im portance for unravelling both stratigraphy and sedim entology o f this package.
The first com prehensive analysis o f sedim ent variation o f the SM G M w as provided by K rhovskÿ e t al. (1993) from several closely spaced sections in th e C zech C arpathians.
Initial results o f sim ilar investigations in the Polish C arpa
thians w ere m entioned by Leszczyński (1993a, b). Sedim ent
SU B-M EN ILITE GLOBIGERIN A M ARL
247
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Dark co lo u re d shale w ith thin sa ndstone & siltsto ne beds
Dark co lo u re d shale
Dark co lo u re d shale w ith thin beds of s ilicifie d sandstone A lte rna lion of thin beds of siliceous m arl and chert Thin beds of ch erts alternating with d ark co lo u re d shale
S iliceous m arl
D ark coloured m udstone, marl
& tuffite layers
A lternations of d ark and green shale, m arl, sandst. & siltstone L ighl co loured m arl w ith alternations o f dark and green shale
A lternations of d ark & green shale and marl
Green shale & thin alternations of dark coloured shale
E 3
EZZZÜD
Light co loured marl
D ark co loured marl and shale
G reen ca lca re o u s shale
G reen non -ca lcare o us shale
S an dston e in thin to thick beds & thin beds o f siltsto ne
L im onite
Fig. 2 Stratigraphy and lithofacies logs of the Zrtamirowice section
variability in the SM G M w as interpreted by K rhovskÿ e t al.
(1993) as reflecting the o rbitally forced changes o f nan
noplankton productivity. T hese authors inferred influence o f M ilankovitch cyclicity on th e developm ent o f this sequence.
A ccording to Leszczyński (1993a, b), the fluctuations o f CaCC>3 content and related changes in foram inifera assem
blages w ere forced by M ilankovitch cyclicity, w hereas the principal change in the sequence resulted from n arrow ing o f the connections betw een the N orth E uropean T ethys and the w orld ocean (see D ercourt e t al, 1985; R icou et a l., 1986).
In the next short note, Leszczyński (1993b) interpreted the entire sedim ent variability as due to a long-term change o f
sedim entary environm ent and m ass resedim entation w hich varied in tim e and space.
The presen t p aper aim s at interpreting sedim ent vari
ability o f the SM G M in one section (Fig. 1). The interpreta
tion is based on detailed m esoscopic and m icroscopic facies analysis. M oreover, analysis o f foram inifera and calcareous n annoplankton, type and am ount o f organic m atter and oxy
gen and carbon stable isotopes signal w as also applied. The collected data allow ed to interpret origin o f these sedim ents, the range and character o f sedim entation changes as w ell as the c h ie f factors responsible for th e sequence pattern.
METHODS
T he sequence o f the SM G M together w ith the im m edi
ately underlying and o verlying sedim ents, was exam ined bed-by-bed. R ock type, colour, reaction w ith HC1, bed thickness, sedim entary structures and textures, and nature o f bed contacts w ere recorded for each m esoscopically distin
guishable layer.
T hirty sam ples w ere tak en for laboratory investigations from each lithologically distinctive horizon o f the upper tw o m etres o f the SM GM sequence (Fig. 2). This part o f the se
quence displays the m ost distinctive variability o f lithology and CaCC>3 content. In the thickest m arl bed, sam ples were taken from each level th a t appeared to differ in lithology (differences in hardness, tendency to splitting, reaction with HC1). T hin sections w ere m ade only o f hard rocks. R elative abundance o f different com ponents, prim arily foram inifers, coarse lithic com ponents and m atrix, w as estim ated in thin sections. CaCO.i content w as determ ined in 30 sam ples by the standard w et titration.
F oram inifera distribution w as investigated in thirty 100- g sam ples. Sam ples w ere disaggregated through repeated boiling and subsequent crystallisation o f a sam ple m ixed with w ater and N a2SC>4 (procedure repeated up to 10 tim es).
A fter disaggregation, sam ples w ere w et sieved on 0.39 m m and 0.09 m m m esh sieves. R esidue fractions o f each sam ple w ere dried and w eighed separately. A t least 300 foram specim ens w ere exam ined in the foram inifera-rich sam ples, w hereas all specim ens w ere counted in the foram inifera- poor sam ples. Sam ples containing the counted specim ens w ere w eighed to calculate the percentage o f particular groups. In sam ples w here all foram s w ere counted th e result refers ju s t to the 100 g o f th e raw sam ple. A dditional data as to the foram inifera distribution and their size variability w ere achieved from thin sections. T axonom ic com position o f the foram inifera in the investigated section is presented after V an C ouvering e t al. (1981). V aluable inform ation concerning foram inifera assem blages o f the SM G M was found in papers by B laicher (1967, 1970) and O lszew ska (1983, 1984).
N annofossils w ere exam ined in sm ear slides o f ten sam ples. Slides w ere prepared from 5-g sam ples by settling technique. S am ple w as first crushed and treated w ith a 3 % solution o f H2O 2 for 24 hours. T he m ixture was then boiled for 30 m inutes. A fter 3 m inutes the supernatant w as poured into a b eaker that w as subsequently filled up w ith distilled
w ater so th at the colum n o f the sam ple-fluid m ixture w as 6 cm high. The m ixture w as then vigorously stirred and after 45 m inutes the supernatant w as p oured o ff w hereas the resi
due w as repeatedly treated as in the last stage. Such p ro ce
dure was repeated until the fluid becam e fully transparent after 45 m inutes o f settling. S m ear slide w as prepared from a drop o f the residue. D istribution o f n annofossils w as ex
am ined in optical m icroscope. A m ount o f nannofossils was determ ined as an average o f 20 fields o f view in the centre o f the slide, at m agnification 800x. M oreover, data concern
ing taxonom ic com position o f nannoplankton in the investi
gated section w ere taken from the pap er by V an C ouvering e t al. (1981). The paper by K rhovskÿ e t al. (1993) provided rich data on the content and diversity o f calcareous nan
noplankton in the SM G M sequence o f the C zech C arpa
thians.
Total organic carbon content (T O C ) and kerogen type w ere determ ined by the R ock-E val p y rolysis o f bulk rock sam ples at the Faculty o f G eology, G eophysics and E nvi
ronm ental Protection o f the M ining and M etallurgy A ca d em y in K raków . The determ inations w ere m ade according to the m ethod o f E sp ita lié e/fl/. 1977). B ecause o f low TOC, kerogen type was determ ined only in 9 sam ples having TO C
> 0.25% . The hydrogen and m axim um tem perature indices (HI and T max) w ere plotted in a diagram o f D elvaux e t al.
(1990).
T he oxygen and carbon stable isotope analysis was m ade on bulk rock in 30 sam ples at the Institute o f G eo
chem istry, M ineralogy and O re F orm ation o f th e U krainian A cadem y o f Sciences in K iev. Isotopes w ere m easured with m ass-spectrom eter M i 12-01 Sum y, on C O2 gas produced by phosphoric acid digestion o f carbonates contained in the bulk rock.
LOCATION AND STRATIGRAPHY
The exam ined section is located on the w estern bank o f the R ożnów reservoir on the D unajec River, som e 11 km dow nstream from N ow y Sącz, at the southern end o f the village Z nam irow ice (Fig. 1). A bout 45-m th ick sequence o f U pper E ocene-L ow er O ligocene deposits o f the Silesian N appe is there exposed (Fig. 2).
The section begins w ith a 5-m th ic k package o f poorly bedded green non-clacareous clayey to m uddy shales and rare, very thin interbeds o f dark-gray shales (Fig. 2). These sedim ents represent the O live-G reen Shale unit. T he bottom part o f this unit is n o t exposed, w hereas its top is transitional and needs an arbitrary determ ination. A ccording to Van C ouvering e t al. (1981), this unit m ay represent a low er part o f the P16 Z one and the N P19 Zone. F or this unit in the Czech C arpathians, K rhovskÿ e t al. (1993) suggested an up
per part o f the N P 19 and N P 20 Z one (i.e. m iddle-upper Priabonian). T hese sedim ents in th e Z nam irow ice section pass gradually upw ards into a 6.3 m -thick package com po
sed o f alternating non-calcareous green shales, calcareous green and dark-gray to black shales, and cream -yellow , beige to pinkish and dark-gray to black m arls. This pack age represents the Sub-M enilite G lobigerina M arl unit
SUB-M EN ILITE G LOBIGERINA MARL
249
(SM G M ), here under consideration. Tw o thin layers im pregnated w ith lim onite o ccur in the upper part o f the pack
age and one sandstone layer in its low er part. The “ lim onite im pregnate” occurs also in lenses w ithin m arl layers o f the upper part o f the package.
The light colour o f the SM G M causes that it distinctly stands out from the sequence below and above. The SM GM is m ade up o f generally p oorly lithified rocks falling easily apart into irregular chips. Proportion o f calcareous m aterial increases gradually up th e sequence. M arls dom inate in its upper 2 m etres. The first layer o f the calcareous fine-grained sedim ent occurs 6.3 m below the te c h n ic a lly truncated up
per part o f the SM G M package (at the SW bend o f the slope face). C om parison w ith o ther sections suggests several tens o f centim etres tectonic reduction o f the package.
A ccording to Van C ouvering e t al. (1981), in Znam i- row ice and K rosno, the SM G M package em braces the cal
careous nannoplankton zone N P 20 and m ay pass to N P 21.
Thus it is o f latest E ocene age. Sim ilar interpretations w ere proposed for the SM G M in the U krainian and the Czech C arpathians (see V ialov e t a l., 1987; K rhovskÿ e t al., 1993).
All these interpretations differ from those by B laicher (1970) and O lszew ska (1983, 1984, 1985) w ho claim ed that the S M G M o f the Polish C arpathians crosses the Eocene- O ligocene boundary. A ccording to O lszew ska (1985), the SM G M in the Polish C arpathians encom passes the entire P 17 zone and low er part o f P 18.
The overlying 25 m o f the sequence consist chiefly o f dark-brow n and dark-gray m udstones and thin- to very thick-bedded sandstones. M oreover, very thin layers o f gray siliceous shales and light-green or beige soft and hard marls occur there in subordinate am ounts. The m arls are concen
trated in the low er part o f this sequence w hereas the am ount o f silica significantly increases tow ards the top w here very thin-layered cherts appear. F urtherm ore, several tu f f layers occur there as well. This part o f the sequence is distin
guished as the S ub-C hert B eds or the Sub-M enilite Beds. It constitutes the bottom part o f the M enilite Beds. The Sub- C hert Beds represent the E arly O ligocene. N annoplankton Z one NP21 w as inferred by Van C ouvering e t al. (1981) for the sam ples taken about 2.5 m above the top o f the SM GM in the section in question. A ccording to K rhovskÿ e t al.
(1993), in the Czech C arpathians, this unit represents N P 22 Zone. C alibration o f this zone against the current Paleogene radioisotopic dates and biostratigraphy show s that it encom passes u pper part o f N P 21 and the ?entire N P 22 (see K rhovskÿ & K ućera, 1994).
RESULTS
M A C R O F A C IE S
Follow ing facies w ere distinguished on the basis o f field exam ination o f the SM G M package and the im m ediately underlying and overlying sedim ents:
- green shale, - dark shale and marl, - light m arl,
- lim onite im pregnate, - dark m udstone - sandstone.
T hese facies alternate w ith different frequency. A very distinctive vertical facies order is present in the SM GM package. D ark shale and m arl are overlain and som etim es also underlain by the green shale. B ioturbation is recorded only in the O live-G reen Shale unit and the SM G M . The u p perm ost occurrence o f bioturbation is recorded in a 9-m m thick layer o f black shale, ju s t 2 cm below the top o f the SM G M package. C h on drites in trica tu s burrow s, 0.5 m m in diam eter occur only in a 5-m m thick upper part o f the layer.
Green shale
T he green shale facies is p articularly characteristic o f the low er part o f the section and it disappears nearly com pletely in its upper part. This is a p oorly bedded sedim ent show ing a tendency to split parallel to bedding. The occur
rence o f pronounced fractures, 1 - 2 cm apart, on w eathered surfaces gives frequently the im pression o f bedding. U su
ally, there is no visible difference in texture betw een the sedim ents divided by a fissure, how ever, the texture is not hom ogeneous. Sm ooth and shiny splitting surfaces occur in som e its parts, w hereas in places the surfaces are rough and dull. The first m entioned variety w as considered as clayey shale w hereas the latter as m ore a m uddy one. T he shales displaying rough splitting surfaces tend to disintegrate into thicker and m ore irregular pieces than do the shales show ing sm ooth surfaces. M oreover, the clayey shale tends to be m ore green, w hereas the m uddy shale is rather gray-green.
Furtherm ore, the changes in colour correlate to som e extent w ith the C aC 0 3 content. The increase in C aC0 3 content is reflected in a passage to light-green or pale-green sedim ent.
C alcareous green shale occurs essentially w ithin the SM GM . The contacts o f the green shale w ith the light-col- oured m arl are vague and m any o f them are highly biotur- bated (Fig. 3), w hereas those w ith th e overlying dark-gray
Fig. 3 Passage from dark-coloured marl through green shale (lower half o f the specimen) to light-coloured marl. N ote distinc
tive bioturbation in the green shale
Fig. 4 Upward passage from light- to dark-coloured marl (top part o f specimen). A. Sedim ent change at a distance o f 10 cm in vertical section; B. detail o f A. The boundary with the overlying dark-coloured marl is indistinctive due to bioturbation. Note three bioturbation generations in the light marl. The oldest one is accen
tuated with a maze slightly darker than the enclosing rock. This feature indicates production o f the mazes in sediment o f soup- ground consistency. The youngest bioturbation is marked with the most distinctive dark-coloured burrows pointing out their pro- ducion in a softground
or black sedim ent are rath er sharp. A ccording to Ślączka and U nrug (1979), som e layers o f th e greenish-cream -col- oured fine-grained sedim ent in th e SM GM package rep re
sent v olcanic ash.
D istinctive trace fossils C h on drites intricatus, P lan o- lites isp., rarely T h alassin oides isp., H elm in th opsis isp., and A lc v o n id io p sis p h a rm a c e u s occur at som e levels in the green shale. Their m ost com m on occurrences are recorded w ithin the SM G M package. B urrow s are usually em pha
sised th ere w ith a slightly darker colouration or a slightly coarser fill w ith respect to the host sedim ent (Fig. 3). They are best visible at the contact w ith th e dark gray layers. B ur
row concentration tends to be inversely graded there. M ore
over, highly irregular changes o f sedim ent colour recorded prim arily at parting surfaces suggest a h eavy sedim ent m ot
tling.
Dark shale and marl
This is a subordinate yet very distinctive facies in the exam ined sequence. D ark-gray shales to brow nish-black m arls are th e m ain constituents o f this facies. D ark non-cal- careous shale exclusively occurs in the package o f the O live G reen Shale unit. It occurs there in rare thin lam inae that usually are heavily bioturbated. L ow er boundaries o f the lam inae appear to be slightly m ore distinctive than th e upper ones. Layers o f the dark-coloured sedim ent becom e m ore distinctive up the sequence w here they are th ick er and m ore densely spaced.
W ithin the SM G M package, the dark-coloured fine
grained sedim ent occurs in five layers, 6 - 1 1 cm thick, and in several thinner lam inae. It is represented there b y calcare
ous shale and m arl. L ow er boundary o f the th ick est layers is distinctively sharp. It appears to be less d istinctive in the thin and particularly in the very thin layers (Fig. 4). M ore
over, the thickest layers are distinctively bioturbated in their upper parts (Fig. 5), w hereas th e ir low er portions display subtle horizontal lam ination accentuated by th e lam inae o f silty and fine sandy m aterial (Fig. 2). T he thin layers are rather uniform ly bioturbated and include num erous C hon
d rite s in trica tu s and P la n o lites isp. (Fig. 5). T h alassin oides isp. occurs concentrated in the top parts o f som e layers (Fig.
5). Less frequent are Ch. ta rg io n ii, A lc y o n id io p sis p h a r m aceus, ? E ch in o sp ira isp., (Fig. 6), and H o rm o siro id ea ca licifo n n is. B urrow s are accentuated w ith gray or pale- green !' .
In the S ub-C hert Beds, dark shale occurs chiefly in their upper part, w hereas dark m arl concentrates in the low er part o f this unit, w here black shale occurs in subordinate am ount and in thin and very thin layers only. It tends to overlie the beds o f the dark-brow n m udstone o r m arl and show s pas
sages to the m arl.
Light marl
This facies encom passes cream -yellow , g reenish-yel
low, beige and pinkish, usually soft, marls. T hese are typical sedim ents o f the SM G M package. T hey disappear both dow n and up the section. In the SM G M , this facies occurs in layers displaying gradual passages both dow nw ard and up
w ard to green shale (Fig. 2). In tw o layers, light m arl is sharply bounded from above by a black m arl.
Internal parts o f the light m arl beds appear to consist o f harder rock than their outer zones. M oreover, one 5-cm thick layer o f hard syderitic(?) m arl occurs in the low er part o f the SM G M package. Pyrite concretions and layers o f li- m onite im pregnate occur at som e levels w ithin th e light marl.
M ottled structures accentuated by a change in colour and texture o f the sedim ent (Figs. 3, 4, 7) are characteristic o f the light m arl w ithin the SM G M . M ottling is best re
corded on stained rock surfaces. M oreover, d istinctive bur
rows, particularly C h on drites in trica tu s and P la n o lite s isp., occur at som e levels (Figs. 4, 8). C oncentration o f these
SU B-M ENILITE G LOBIGERINA M ARL
251
Fig. 5 Inverse grading o f bioturbation in top part o f a black marl layer. Larger burrows, w ith recognizable Thalassinoides isp., Alcyonidiopsis isp., and Planolites isp. concentrate near the layer top, w hereas Chondrites intricatus occupies a deeper tier. Note two generations o f Chondrites. The older one is gray whereas the younger is whitish. A. Layer w ith distinctive concentration o f two generations o f Ch. intricatus in a deeper tier; B. Layer with chiefly gray (older generation) Chondrites
traces is recorded in thin beds and at som e levels in the thick ones. Less frequent are th e burrow s Thalassinoides suevicus (Fig. 9), T. isp., A lcyonidiopsis pharm aceus, and granulated structures resem bling E ch in o sp ira and Zoophycos (cf. Fig.
6). M oreover, single traces Z oophycos isp. and T eichichnus isp. w ere also recorded. B urrow s in thin beds are m arked with colour sim ilar to th a t o f the neighbouring, usually over- lying bed (Figs. 4, 8, 10). T his suggests bed ju n c tio n preser
vation o f th e traces. Such burrow s, and particularly Chon
drites intricatus disappear in the m iddle part o f beds thicker than 5 cm .
W ithin the S ub-C hert Beds, the light-coloured m arl oc
curs only in very thin layers and is represented there by a soft, light-green and a very hard, beige variety. The beige
Fig. 6 ? Echinospira isp. in top part o f a layer o f black marl. A.
view at bedding parallel surface; B. view in vertical section
hard m arl tends to break along flat, bedding p arallel surfaces or conchoidal surfaces oblique to bedding. A faint horizon
tal lam ination is visible in som e layers. The light-green marl
Fig. 7 M ottled structure o f light-coloured marl. Irregular and fuzzy structures indicate that m ottling was formed w hen the sedi
m ent had a soupground consistency
Fig. 9 Thalassinoides suevicus in cream -yellow marl. The bur
row is accentuated by material different from that o f the enclosing sediment
Fig. 8 Bioturbation in greenish marl overlain by thin lamina o f dark-coloured marl. Burrow s are accentuated by dark colour o f their fill. A. View on bedding-parallel fracture surface; B. View in vertical cross-section. N ote inverse grading o f burrow concentra
tion tow ards the layer top
is enclosed by black shale or is overlain by the beige marl.
The soles o f the light-green m arl are diffuse, and the m arl tends to pass upw ards into the beige m arl.
Limonite impregnate
T his is a light-brow n and yellow ish fine-grained soft rock. It occurs in tw o layers bounded by black and green shale and is rich in the C hondrites targionii burrow s. M ore
over, lenses o f lim onite im pregnate, as m uch as 10 cm thick, occur w ithin som e light m arl beds. Im pregnation w ith lim o
nite resulted presum ably from oxidation o f iron contained in the sequence, how ever, the p rim ary nature o f the tw o layers as w ell as th e type o f the original iron m inerals are unclear.
P resum ably, they form ed due to w eathering o f iron sul
phides or siderite contained in marl.
Dark mudstone
D ark-brow n to chocolate-brow n calcareous to non-cal-
careous m udstone is characteristic o f the S ub-C hert Beds.
H ighly calcareous variety occurs in low er part o f the unit. It is a m assive and irregularly b reaking rock, yellow ish on w eathered surfaces. In som e beds, the m udstone contains a significant adm ixture o f irregularly distributed coarse
grained m aterial consisting o f grains and granules o f quartz, plant fragm ents, calcareous bioclasts and shale chips up to 5 cm in size. Such m udstone occurs in beds several centim e
tres to several tens o f centim etres thick. T hey tend to overlie sandstone and pass upw ards into black shale or brow nish black marl.
Sandstone
The sandstone facies w ithin th e SM G M pack ag e occurs as a single bed, 7.5 cm thick. It is m edium - to fine-grained, highly calcareous rock greenish-gray in fresh parts and rusty
Fig. 10 Bioturbation in light-coloured marl accentuated at a level o f slight colour change (greenish, more clayey sediment)
SUB-M EN ILITE GLOBIGERIN A MARL
253
s-
20CH cm
- / - / ■ —/ - / —/ - / —/ —/ —/ —/ — y
—S - / - / - S —/ ‘ /-A f —/ —/ —/ —S—j / —/ —/ —/ —s—
—/- S —/ S —/- / —/ —/ —/ —/ —
f —
■'S-/—/ - / —S-
150-
z—/ —/ —/ —/ — 11 12 13 1 4 15 16 1 7
18
1 9
20 21 2 2
100
- / - / • - / • / - A
N u m b e r o f fo ra m s N u m b e r o f in 100 g o f ro c k n a n n o fo s s ils
0 20 40 60 80 +2 1 0 -1 -2 -3 -4 0 10 102 103 104 10s 10s 0 200 400
CaCO,
% 6 13C
%0
TOC% 5 leO
%o N u m b e r o f fo ra m s N u m b e r o f n a n n o fo s s ils
Fig. 1 1 TOC, CaCOj, Znam irowice
C and O stable isotopes, foraminifera and nannoplankton signals in upper part o f the SM GM package at
on w eathered surfaces. T he base o f the bed is sharp, w hereas its top is indistinctive and b o u n d ed by green, non-calcareous shale. T he sandstone seem s to be a T c(d) turbidite.
The sandstone facies constitutes an im portant elem ent o f the S ub-C hert Beds. P ale-beige and brow nish-beige, m e
dium - to coarse-grained, thin- to very thick-bedded sand
stone varieties occur in this unit. The beds are poorly sorted, show a faint norm al grading near their basal and top parts.
H orizontal lam ination occurs in som e beds. Bed soles are sharp and usually flat. T op surfaces are less distinctive, how ever, they are frequently rem arkably sharp and flat. The beds are overlain by brow n to black m udstone or black shale several centim etres thick. Som e beds display a chaotic structure accentuated by irregularly distributed clasts o f dark-grey, green and black shale or marl.
SEDIMENT COMPOSITION AND M ICROFEATURES
H ydrom icas, C aC0 3 , detrital quartz and organic m atter (TO C up to 3% ) are the c h ie f m ineralogical constituents o f the exam ined rocks. Illite and m ontm orillonite, detrital quartz and m icas are recorded, besides C aC0 3 , in m arls and shales (cf. G ucw a & Ślączka, 1972). C aC0 3 content is from 0 to 70%. Its highest values are recorded in th e hardest, cream yellow and beige m arl (Fig. 11). S om e p arts o f the light-coloured m arl represent actually a m arly lim estone.
The dark-gray to black shale and m arl contain up to 44%
C aC0 3 . Quartz, in the silt fraction, am ounts to 7% o f the light-coloured m arls and up to 20% o f their dark-coloured
Fig. 12 Nannofeatures o f light marl visible in SEM micrograph.
Nannofossils constitute about 80 per cent o f the sediment volume
varieties (cf. G ucw a & Ś lączka, 1972). Pyrite is com m on in w ashed residues o f the dark-coloured shales, m arls and m udstones. G ucw a and Ś lączka (1972) recorded one layer com posed o f m ontm orillonite in the SM G M package. The sedim ent w as interpreted as pyroclastic in origin.
The carbonate m aterial in the light-coloured m arl con
sists in 50 - 80% o f nannofossils (Fig. 12; see also K rhovskÿ et a l , 1993). In beds co ntaining m ore than 50% C aC 0 3 , as m uch as about 30% o f th e carbonate m aterial consists o f tests o f planktonie foram inifers (Fig. 13). In the dark-col
oured shale and m arl as w ell as in th e green shale, C aC0 3 appears to occur predom inantly as cem ent. N annofossils and calcareous foram inifera are rare (Fig. 14). In the fine
grained sedim ent o f the Sub-C hert B eds, diatom s are addi
tional im portant m icrofossils (cf. G ucw a & Ślączka, 1972).
In thin sections, the light-coloured m arl show s usually a very irregular, clustered distribution o f the coarsest particles (i.e. chiefly tests o f planktonie foram inifera; Fig. 15). N ev ertheless, distinctive 1 - 2 m m thick lam inae rich in plank
tonie foram inifera are locally recognizable (Fig. 16). The low er boundaries o f the foram inifera-rich lam inae appear to be sharp.
The sandstone w ithin the SM G M consists p redom i
nantly o f m onocrystalline, angular quartz grains. Feldspars, m icas, rock fragm ents and bioclasts are their subordinate constituents. M onocrystalline, angular quartz grains are also the m ain constituents o f the sandstones in th e Sub-C hert Beds (see G ucw a & Ś lączka, 1972). K -feldspars, m uscovite and hydrobiotite, sm all clasts o f carbonate rocks, siliceous rocks, sporadically grains o f gneiss and plagioclase occur there subordinately. A ccording to G ucw a and Ślączka (1972), the feldspars are com m only sericitized, kaolinitized or calcitized. M oreover, these authors recorded replacem ent o f CaCC>3 by MgCC>3 tow ard the top o f the Sub-C hert Beds.
Fig. 13 Concentration o f planktonie foram inifera in the light- coloured marl (sample 7). A. Irregular distribution o f sedim ent constituents; B. D ifferent preservation o f foram inifera tests
FORAMINIFERA DISTRIBUTION
F oram inifera assem blages in th e ex am ined sam ples vary betw een 37 and 734536 specim ens p er 100 g o f the rock (Fig. 11). The distribution o f th e three distinguished groups show s a distinct correlation w ith th e sedim ent type (cf. Blaicher, 1961; Szym akow ska, 1962). The cream -yel
low m arl is the richest in foram inifera, w hereas the dark gray to black m arl and shale are the poorest. F oram inifera o f the three exam ined groups are m ost abundant in th e cream yellow m arl. Proportion o f benthonic to planktonie group is highest in the dark-gray to b lack and the greenish, less cal
careous sedim ents. O n the contrary, in the cream -yellow m arl, the proportion o f planktonie species is significantly greater. M oreover, the proportion o f agglutinating species relative to their calcareous counterparts tends to be higher in the dark-gray and black m arl and shale as w ell as in the green shale. P lanktonie foram inifera w ere absent in a sam -
SUB-M EN ILITE GLOBIGERINA M ARL
255
Fig. 14 Nannofeatures o f green and dark-coloured shales visible in SEM micrographs. Note rare occurence o f nannofossils. A. Green calcareous shale (sample 13). B. Dark-gray calcareous shale (sample 4)
Fig. 15 Irregular and clustered distribution o f the coarsest grains (chiefly tests o f planktonie foraminifera) probably due to bioturbation.
A. Irregular distribution due to mottling and filling of burrow tunnels. Note tiny burrow on the left, filled with crushed foraminfera tests, (sample 30); B. Sediment mottling and tube of a burrow filled with intact foraminifera tests probably due to passive filling o f the tube (sample 25)
pie from the low erm ost exam ined layer o f green shale. A g
glutinated species w ere n o t recorded in tw o sam ples o f black m arl and in one sam ple in th e thickest layer o f th e cream - yellow m arl. O ne sam ple taken from a 2.5 cm thick layer o f hard m arl in the S ub-C hert B eds, 4 m above the SM G M top, contained exclusively rare and tiny planktonie species, 0.03 - 0.05 m m in size (Fig. 17) . These foram inifera w ere re
corded in thin section only. For com parison, in the SM GM sequence, the size o f p lanktonie individuals ranges 0.1 -0.3
Fig. 16 Relics o f lamination in the light-coloured marl. The lamination is accentuated by distribution o f coarse grains, chiefly tests o f planktonie foraminifera
F ig . 17 M ic ro f e a tu r e s o f th e lig h t-c o lo u re d m a rl o f th e S u b - C h e rt B e d s . A t sa m e m a g n if ic a tio n as in F ig s. 13A , 15, 16, it a p p e a r s th a t fo ra m in if e ra a re a b s e n t in th is s e d im e n t. A . O n ly s in g le , la rg e r f o ra m in if e ra a p p e a r a t th is m a g n ific a tio n ; B. S ig n ifi
c a n tly h ig h e r a m o u n t o f fo r a m in if e r a is v is ib le a t a h ig h e r m a g n i
fic a tio n . N o te th e c o m m o n o c c u r r e n c e o f p y r ite (b la c k ) in c h a m b e rs o f f o ra m in if e ra te s ts a n d th e p a r ra lle l a r ra n g e m e n t o f e lo n g a te d g r a in s ( m ic a fla k e s a n d c o a lif ie d p la n t f ra g m e n ts )
F ig . 18 N a n n o f o s s ils in sm e a r slid e s . A . H ig h ly c a lc a r e o u s m arl (s a m p le 8); B. D a r k -b r o w n sh a le . N o te s in g le , h ig h ly c o r ro d e d an d r e c r y s ta liz e d s p e c im e n s (s a m p le 4)
the less calcareous sedim ents (Fig. 18B), and th e hardest m arl (Fig. 19).
m m . C orroded tests o f calcareous species are frequent in the less calcareous sedim ent.
NANNOFOSSIL DISTRIBUTION
The content o f calcareous nannofossils in sm ear slides varies betw een 0 and 34 specim ens per one observation field, depending upon the sedim ent ty p e (Fig. 11). The m ost num erous occurrences are recorded in the highly calcareous w eakly cem ented light-coloured m arl (Figs. 12, 18A). In contrast, nannofossils disappear in the non-calcareous shales. In the dark-gray to black m arls and the slightly cal
careous green shales, th e assem blages are significantly im poverished (Figs. 14, 18B). M oreover, poorly preserved, corroded and recrystallized specim ens occur frequently in
TOC AND KEROGEN TYPE
TO C content in the exam ined sam ples ranges betw een 0 and 3.12% (Fig. 11). The highest values, 1.29 - 3.12 %, w ere recorded in the dark gray and black m arls and shales. Except for one sam ple, values below 0.1 %, w ere recorded in the green shales and m arls. In the low erm ost sam ple o f the green shale (sam ple 29), TO C reaches 0.25 %. In eleven m arl sam ples, TO C values close to zero w ere recorded.
C haracteristically, slightly elevated T O C values occur in layers contacting w ith dark gray and b lack m arls and shales.
The results o f the R ock-E val P yrolysis p lotted in the diagram o f HI (hydrogen index, i.e. m illigram s o f h y drocar
bons evolved during kerogen breakdow n, d ivided b y w t % TO C content, x 100) versus m a x T (tem perature o f m axim um hydrocarbon evolution from kerogen, °C), indicate th at the
SUB-M EN ILITE G LOBIGERINA M ARL
257
Fig. 19 Nannofossils in hard marl. Note the poor preservation, overgrowth and recrystalization of specimens (sample 8); A.
Specimen to the right of the centre disappears in newly formed mineral matter; B. Concentration o f highly altered nannofossils
organic m atter o f the analyzed sam ples is dom inated by a hydrogen-poor T ype III com ponent (Fig. 20).
Fig. 20 Type of kerogen determined with Rock-Eval pyrolysis.
Type III suggests predominance of terrigenous organic matter in examined samples. A. Type of kerogen indicated by relation be
tween hydrogen index versus temperature o f maximum hydrocar
bon evolution ( T m a x ) ; B. Type of kerogen indicated by relation between hydrogen index and oxygen index; C. Type o f kerogen indicated by relation between total organic carbon content (TOC, in %) and amount of hydrocarbons evolved from thermal alteration of the kerogen (S. in milligrams, normalized to sample weight)
IMMATURE I OIL ZONE I G A S Z O N E
OXYGEN INDEX (m g CO*/g TOC)
TOC
ISOTOPES CaCO,% S” C%0 I o
V alues o f 8 O range betw een -1.6 and -4.3, w hereas 8 1JC range from +1.2 to -3.6 (Fig. 11). The highest values o f indices are recorded in th e least calcareous sedim ents, p a r
ticularly in the green shales. The low est values are charac
teristic o f the highly calcareous cream -yellow and beige m arls. N evertheless, th e highest ö 13C w as recorded in the dark-gray and black m arls and shales. In contrast, § 180 was there low , and sim ilar to th a t in the highly calcareous beds (beige- and cream -yellow m arl).
T he fluctuations o f both indices show a strong positive correlation (correlation coefficient, r = 0.765) throughout the section in the green shales and the cream -yellow to beige m arls (Fig. 21). The increased carbonate content in these deposits correlates quite w ell w ith the negative shifts o f the 5 * 'C values H (r = -0.589) and less distinctively w ith the negative shifts o f 5 I80 (r = -0.435). C orrelation is disturbed in the dark-gray to black m arls and shales. ô I80 tends to be significantly low er in these sedim ents than in th e adjacent green shales and light-coloured m arls (negative shift), w hereas 5 I3C tends to show slightly higher values (positive shift).
-13,
DISCUSSION
SEDIM ENT M ACROFEATURES
The features o f the light-coloured m arl and the associ
ated green shale suggest th e ir sedim entation thorough parti- cle-by-particle fallout from the w ater colum n. Thus, these deposits can be considered as pelagites. The features o f the dark-coloured shale and m arl layers (lam inae) as well as o f the sandstone beds, such as norm al grading, sharp soles, in
verse concentration o f burrow s in the vertical layer section, point to deposition by turbidity currents. Som e layers o f the dark-coloured shale and m arl, show ing indistinctive bases, could have been deposited by slow particle-by-particle sedi
m entation. In contrast, th e green lam inae enriched in ter- rigeneous m aterial and displaying sharp bottom surfaces are hem ipelagites. The dark-coloured m udstones occurring in the S ub-C hert Beds appear to be deposited by high-density turbidity currents and m ud flows.
T he occurrence o f flat lam ination w ith foram inifera concentrations in the light m arl suggests local rew orking by bottom currents. The extent o f such rew orking cannot, how ever, be determ ined as the sedim ent is heavily bioturbated.
M oreover, the occurrence o f the levels rich in discrete bur
row s (C hondrites, Planolites, Thalassinoides) that are filled w ith sedim ent different from the host rock, suggests a m ore com plex prim ary differentiation o f the sequence. The very thin lam inae m ight have even been blurred by burrow ing anim als. M oreover, bioturbation could have significantly changed the original foram inifera and nannoplankton distri
bution as w ell as the original sedim ent com position. The bioturbation w as accom plished m ainly by the anim als w hich penetrated that shallow est part o f the sedim ent w hich had consistency o f a dense fluid, i.e. soupy sedim ent. Such bioturbation is displayed by th e fuzzy, poorly individualized
■ / s —/ —/ —/ —
—s—s —/ —/■—/■
200 cm
150
■/—/—/ —/—/ - k/«*/-—/- s —/•
■/ —/ —/■—/ —/ — / - S —/ - / —/ Ą
/ - / - / - / - / -
• / • / • / - / - Z -
0 20 40 60 80 +2 1 0 >1 -2 -3 -4
10CH
5 0 - ;
/ —/ —/ —/ —y—.
•/—/—/ —/—s—
' —/ —/ —/ —/ —Z / - / - / —s—
-2 -3 -4 -5
8 1<lO
%o
18 11
Relationships between CaCC>3 content, 8 O, and 5 " C Fig. 21
in light marls and green shales
and very irregular structures accentuated by slight changes in the sedim ent colour (Figs. 3, 4, 7, 8B, 10). In contrast, the distinctive burrow s w ere produced at a greater depth below the bottom surface, w here the sedim ent consistency w as o f a soft-ground type. The features o f the surrounding sedim ent together w ith the confined occurrence o f the burrow s (i.e. at discrete sedim ent levels) indicate, how ever, th at only a m i
nor blurring o f the sedim ent structure w as due to their pro
duction.
The upw ard passages from the dark-coloured sedim ent to the green shale im ply redeposition o f som e part o f the green shales. The occurrence o f planktonie foram inifera in all exam ined sam ples indicates th at sedim ent w as redepo
sited from a zone dom inated by o pen-sea sedim entation. It m ight have been an outer sh e lf o r u p p er slope tract.
The presence o f burrow s filled w ith dark sedim ent in the top parts o f som e light-coloured m arl layers (Fig. 4) as w ell as the lack o f distinctive norm al particle grading in the
SU B-M EN ILITE G LOBIGERINA M ARL
259
overlying dark-coloured m arl, both suggest that som e layers o f the dark m arls m ight have also been deposited by a slow particle-by-particle fallout from the w ater colum n. Such kind o f deposition m ay be ascribed essentially to the very thin lam inae o f the dark-coloured fine-grained sedim ent.
FORAM INIFERA
T he distinctive correlation betw een the abundance o f the three distinguished foram inifera groups and the sedi
m ent type can result from different sedim entation conditions o f the foram inifera-rich and the foram inifera-poor sedi
m ents. H ow ever, the recorded variability o f the foram inifera distribution can also be explained by differences in preser
vation potential (susceptibility to dissolution) o f individual species w ithin the distinguished groups.
V an C ouvering et al. (1981) interpreted the foram ini
fera assem blages recorded in the SM G M section at Z nam i
row ice as characteristic o f cool-tem perate, high latitudes.
T he lack o f species indicative o f w arm , subtropical condi
tions w as em phasized by these authors. The assem blage o f benthonic foram inifera w as interpreted there after K siążkie- w icz (1975), w ho suggested sedim entation o f these deposits at bathyal depths. O lszew ska (1984) argued that the p re
dom inance o f plankton in the SM GM foram inifera assem blages indicates sedim entation beyond the sh e lf edge in an open m arine realm . In h er view , the assem blage o f ben
thonic foram inifers is indicative o f 600-2000 m w ater depths and show s evidence o f gradual shallow ing. It is the shallow ing that she considered responsible for the transfor
m ation o f the assem blage recorded up the sequence.
The results o f the present analysis differ from those published by Van C ouvering and others (1981) in that all three foram inifera groups w ere recorded in the entire exam ined section. V an C ouvering et al. did not m ention neither agglutinated species in the upper part o f the SM G M nor cal
careous species in the low er part o f the section.
CALCAREOUS NANNOFOSSILS
T he recorded calcareous nannofossil distribution is thought to result prim arily from different sedim entary con
ditions o f th e nannofossil-rich and the nannofossil-poor sedim ents (see K rhovskÿ et a l., 1993). D iagenetic m odifica
tions are considered as o f a m inor significance.
Intensified nannoplankton sedim entation occurs in tim es o f its enhanced production. This happens generally in periods o f increased nutrient supply that often follow s inten
sification o f w ater circulation in the sedim entary basin. The last m entioned process is strongly dependent on geographic location o f the area as w ell as on the global clim ate pattem and the sea-level stand. P roductivity changes are recorded at different scales. Som e changes are proven to follow the M i- lankovitch orbital cyclicity.
The distribution o f calcareous nannofossils in sedim en
tary sequence m ay, how ever, be m odified due to dissolution o f the less resistant form s during degradation o f the organic m atter contained in the sedim ent (see e.g. Em erson &
Bender, 1981). The extent o f such m odification in the exam ined section is suggested by the occurrence o f poorly p re
served, partly dissolved and recrystallized nannofossils (Figs. 18B, 19; cf. V an C ouvering e t al., 1981; K rhovskÿ et al., 1993). T he less resistant form s m ight have been com pletely dissolved, particularly w here higher am ounts o f m ore easily degradable organic m atter w ere present.
A ccording to V an C ouvering e t al. (1981), the p redom i
nance and abundant occurrence o f Isthm olithus recurvus to gether w ith other reticulofenestrids, p articularly Cocco- lithus pelagicus, as w ell as th e occurrence o f C hiasm olithus oam aruensis and the rarity o f discoasterids and Sphenoli- thus p seudoradians in the SM G M o f th e section in question (7 sam ples), sim ilarly to its foram inifera assem blage, are all indicative o f a cool-w ater environm ent (cf. A ubry, 1992).
T he variability o f the nannofossil assem blages o f the SM G M in the Czech C arpathians w as interpreted by K rhov
skÿ et al. (1993) as resulting essentially from variations o f nannoplankton productivity related to the M ilankovitch cy
clicity. T heir data show , how ever, th at the difference be
tw een the assem blages o f the less and the m ore calcareous sedim ent is not very distinctive. It is displayed chiefly by increased am ounts o f N o elaerhabdaceae in the m ore cal
careous sam ples. This enrichm ent w as suggested to result from higher susceptibility to dissolution o f these form s. The interpretation by K rhovskÿ et al. (1993) suggests, how ever, that such changes w ere o f m inor significance in their sec
tion. A ctually, the less calcareous greenish clay in th a t sec
tion resem bles the green shale and m arl in Znam irow ice.
T he TO C contents o f such sedim ent w ere show n in the Z nam irow ice section to be higher than in the light-coloured marl. Therefore, the probability th at th e recorded differ
ences in nannofossil assem blages recorded in the section studied by K rhovskÿ et al. (1993) m irror also the prim ary differences in TO C content o f these sedim ents seem s to be quite high. H ence, consideration o f the influence o f dissolu
tion on m odelling o f this assem blage, at least in som e parts o f that sequence, appears to be necessary also there.
TOC CONTENT AND KEROGENE TYPE
T O C content in the exam ined sam ples show s a n otice
able variability. As pointed out earlier, the sedim ents dis
playing the highest T O C contents bear features im plying sedim entation essentially from m ass-gravity flow s. This suggests in turn, that a zone o f increased organic m atter ac
cum ulation existed in shallow er environs o f this area. The present position o f the section suggests that it could have been located in a zone som ew here w ithin the southern m ar
gin o f the S ilesian Basin. The presence o f planktonie fora
m inifers in the resedim ents o f the section in question indi
cates that it was located in an open-sea influenced zone (cf.
O lszew ska, 1983). The im poverished benthic foram inifera assem blages im ply dysaerobic or p o o rly areobic conditions at that area o f the basin bottom .
The slightly elevated TO C contents in the layers o f the light-coloured m arl adjacent to the dark-gray to black m arls and shales m ay result from bioturbation. This is suggested by the burrow s filled w ith the dark-coloured m aterial. E le
vated TO C contents in the green shale or the light-coloured m arl overlying the dark-coloured sedim ent are also ex
plained in th e sam e w ay. T hese sedim ents could becom e en riched in organic m atter w hen bioturbation affected syn
chronously the layer o f the dark sedim ent and the overlying green shale or marl.
T he predom inance o f th e T ype III kerogen in the exam ined sam ples suggests th a t it m ay consist m ainly o f polycy- clic arom atic hydrocarbons and oxygenated functional groups (chiefly phenols; see Tyson, 1995). A ccording to T issot and W eite (1984, fid e Tyson, 1995) these constitu
ents are derived essentially from continental plants. The subordinate m arine organic m atter in the SM G M o f the sec
tion in question is represented by diverse dinoflagellate cysts (see V an C ouvering e t al., 1981). H ow ever, one has to note the statem ent by T yson (1995) that the signal o f Type III kerogen can be present also in sedim ents w hose kerogen is dom inated by aerobically degraded am orphous organic matter. M oreover, T yson (1995) stressed that selective p res
ervation o f the m ore refractory terrestrial m aterial (oxida
tion o f th e T ype II am orphous organic m atter to the T ype III kerogen), can reinforce form ation o f the T ype III kerogen. It cannot also be excluded th at the type o f organic m atter con
tained in the analyzed sam ples w as to som e extent deter
m ined by its oxidation in th e exposure. The organic m atter o f predom inantly m arine origin was recorded in the SM GM by Gucvva and W ieser (1980). H ow ever, the source o f their data is not very precise. I f th e organic m atter o f the TOC- rich sedim ents is chiefly o f terrestrial origin then it points to resedim entation from a zone influenced by a river mouth.
O therw ise, it w ould have originate in a zone o f very high organic productivity such as th at observed in the upw elling affected areas.
A distinctively increased accum ulation o f m arine or
ganic m atter appears to be recorded in a sam ple taken from a 2.5 cm thick m arl layer o f the Sub-C hert Beds, 4 m above the top o f the SM GM . T he lack o f bioturbation and benthic fauna in this sequence indicates deposition on an anoxic bot
tom . Such setting also favours preservation o f the easily de- gradable organic m atter th at form s kerogen Type II and I.
ISOTOPES
18 13
The 5 O and 8 C values recorded in the exam ined sam ples show som e correlation w ith C aC0 3 content. H ow ever, the values o f both isotope indices show a negative shift, up to 2 prom ille, relative to the data published by K rhovskÿ et al. (1993) from the SM G M o f one section in the C zech C arpathians. S edim ents in that section appear to be slightly less calcareous than in Znam irow ice. M oreover, dark-coloured m arls and shales are absent there except for the highest exam ined part o f the sequence. H ow ever, sedi
m ents o f this part o f the sequence are considered in the P ol
ish C arpathians as belonging to th e Sub-C hert Beds. U nfor
tunately, th e data p resented by K rhovskÿ et al. ( 1993) do not show how 5 180 and S *'C in brow n m arls do relate to those in^green shale and m arl. A strong positive correlation o f 5 1 0 and 8 I3C is recorded in their section. They suggested that an environm ental signal is reflected by the isotopic p a t
tern they have recorded. H ow ever, the tendency o f the changes o f th e isotope signal recorded by these authors, an
tithetical to the global trend, they declared as com plicating their interpretation.
T here are tw o basic possibilities for th e origin o f the 5 180 and 5 I3C signal recorded in the exam ined section: (1) specific history o f the sedim ent, (2) inaccurate determ ina
tion o f the isotope content. V alues o f S 180 and § 13C low er that the globally reported ones w ere also recorded in bulk sam ples o f late E ocene sedim ents cored in the southern L ab
rador S ea (A rthur et al., 1989). E arly diagenesis related to shallow -burial organic m atter decay w as there inferred as responsible for the recorded 5 1S0 and 5 13C values. Such interpretation appears also to fit to th e here exam ined rocks.
M oreover, the discrepancy in the style o f th e change o f the isotope values betw een the green shale and light m arl on one side and the dark m arl on the o ther side im plies th at isotope signal in the SM G M sequence w as m odelled by diagenesis in at least tw o stages.
In the first stage, C aC0 3 enriched in "C w as form ed by degradation o f organic m atter in shallow burial conditions and release o f isotopically light C O2 (see W right & T ucker,
1990). Such process was presum ably the m ain control re
sponsible for the consequently low est ô I80 and 8 I3C values in the hardest light-coloured m arl (m ost strongly affected by cem entation) as w ell as for the rem arkably low er 8 180 val
ues in the dark-coloured m arl in the u pper part o f the sam pled section. These rocks probably underw ent the m ost in
tensive recrystalization o f the above indicated type (cf.
B roecker & Peng, 1982; S hackleton et al., 1983). Its pro gression at shallow burial is indicated also by the w ell-pre- served, not com pacted burrow s as recorded in the strongest cem ented marl (carbonate concretions; see also B ohrm ann
& Thiede, 1989). Siderite, rodochrosite and ankerite co n cre
tions could have been form ed due to such diagenetic process as well. This kind o f diagenesis w as also p ointed out by K rhovskÿ et al. (1993) as the only one responsible for the observed features o f the sedim ents.
The very low TOC values recorded in th e light-col
oured m arl can result from a n early com plete degradation o f its prim ary organic m atter. M oreover, the above m entioned features, such as the isotopic signal, high degree o f lithifica- tion and carbonate concretions, suggest that these sedim ents m ust have originally contained considerable am ount o f eas
ily m etabolizable organic substances. C onsequently, one can suppose that the sedim ents displaying higher S 1 O and 5 ,3C (e.g. sam ples 5, 6, 14, and 15), m ight have contained low er am ounts o f such substances. M oreover, the ô 180 v al
ues recorded in the latter sedim ents m ust therefore have been altered least at this stage o f diagenesis.
The second stage o f diagenesis occurred during deep burial, after form ation o f m ethane from the organic m atter that escaped the earlier oxic degradation. This process af
fected prim arily the significantly T O C -enriched dark gray and black m arls and shales (e.g. sam ples 4, 16, 20, 23, 25, 27, and 30; see Fig. 11). T heir carbonates show higher v al
ues o f S 13C and, therefore, suggest recrystalization influ
enced by C O2 enriched in 13C. Such C O 2 is know n to form due to therm al alteration o f organic m atter (see e.g. T ucker
& W right, 1990). The enrichm ent in 13C labels the carbon
ates form ed from such C O2. C arbonates produced during
