An oligocene sink for organic carbon: Upwelling in the paratethys?

Palaeogeography, Palaeoclimatology, Palaeoecology, 60 (1987): 143-153 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

143

AN OLIGOCENE SINK FOR ORGANIC CARBON" UPWELLING IN THE PARATETHYS ? ISTVAN VETO Hungarian Geological Survey, Ndpstadion ut 14, Pf. 106, 1442 Budapest (Hungary) (Revised and accepted November 4, 1986)

Abstract VetS, I., 1987. An Oligocene sink for organic carbon: upwelling in the Paratethys? Palaeogeogr., Palaeoclimatol., Palaeoecol., 60:143 153. A synoptic review of the organic carbon content, kerogen type, biogenic siliceous sediments and planktonic remains is attempted for the often anoxic Oligocene Early Miocene sediments of the Paratethys covering a large area between the Rhine Valley and the Aral Sea. On this basis stratification of the sea and intensity of planktonic productivity are discussed for different parts of the Paratethys. The Eastern Paratethys and some parts of the Central and Western Paratethys show a Black Sea-like stable stratification, while the bulk of the anoxic sediments in the Carpathian Flysch Basin are interpreted to have an upwelling origin. The supposed Flysch Ocean upwelling regime has no good recent analogue, although it shows similarities with the Bay of California (high rate of Corg burial, laminated, diatomaceous sediments, and turbiditic intercalations) and offshore NW-Africa (abundant fresh-water discharge). Sediments of a brackish-water incursion are widespread on the bottom of the NP 23 zone. From earliest Oligocene to Early Miocene a continuous narrowing of the anoxic sedimentary belt towards the East-European Platform is observed.

Introduction The Paratethys, a large seaway flanking the south-western margins of Eurasia was separated from the Tethys at around the EoceneOligocene boundary. Its Oligocene-Early Miocene sediments, covering a large, elongated area from the Rhine Valley to the Aral Sea, often show dark colour, thin lamination and high organic contents, characteristics typical for anoxic deposits. In striking contrast to the volumetrically far less important Late Cenozoic sapropels of the East Mediterranean the geochemical-paleooceanographical aspects of the Paratethyan ones are poorly understood. In this paper the dispersed data have been collected. This was however, hampered by the 0031-0182/87/$03.50

fact that the corresponding data were published in eight languages. I hope that this synoptic review will permit a deeper insight into the relatively persistent anoxia in the Oligocene-Early Miocene Paratethys.

Geographic and stratigraphic setting Figure 1 shows the areal extension of the Oligocene Paratethys sediments. It should be noted that the extent of the Oligocene deposits is only modified by erosion while their distribution is deeply disturbed in the Central Paratethys by post-Oligocene plate motions and the resulting compression. Occurrence of Fischschiefer or its corresponding sediments are also reported from the subalpine molasse of Switzerland and France, but their cartogra-

© 1987 Elsevier Science Publishers B.V.

144

[ . _ ~ / ~-'x,~'-.-., I

I h-v.s._ ,:^

/

Parotethys

£.0.

Fig.1. Present-dayextension of Oligocene Paratethys sediments. For the Eastern Paratethys the limits of the Solenovskiy horizon, the lowermostmemberof the MaykopFormationare shown after Voronina and Popov (1984). Numbers on the map: Western Paratethys -- 1= Rhine Valley; 2= Molasse Basins; Central Paratethys -- 3= Flysch Basin; 4= Slovenian Basin; 5= North-HungarianBasin; 6= Szolnok Flysch Basin; 7= TranssylvanianBasin; Eastern Paratethys -- 8= Bulgarian Basin; 9= Crimea Lowland; 10= Western Ciscaucasus; 11 = Eastern Ciscaucasus; 12= Mangyshlak; 13= Black Sea; 14= Caspian Sea. phic presentation is not attempted. The stratigraphic setting of the sediments is presented in Table I. It has to be emphasized that in the so defined area and between the given time limits anoxic sedimentation was usual or overwhelming but not exclusive.

Organic geochemical, lithological and paleontological data Organic matter

The Cots content data - - range and/or average values - - are summarized in Table II. Since the samples to be analyzed were often chosen from the most organic-rich parts believed to be source rock and/or oil shale, some basins listed in Table II can not be characterized by average Cots content data. The average Co,g content, thickness and time data needed to estimate the rate of Co,s accumulation are listed in Table III. The rate of organic carbon burial shows drastic differences - - up to 1-1.5 order of magnitudes - - between the basins (e.g. Ukrainian sector of the Flysch Basin and the Eastern Molasse Basin). The determination of the mass of buried organic carbon was carried out only for the Flysch Basin and the Crimea Lowland - Ciscaucasus sectors of the Maykop

Basin, since the other land basins represent only a minor fraction of sediments and organic matter. On the other hand the Maykop Formation hidden under the Black Sea and Caspian Sea basins must contain a large quantity of organic matter. For example, the Maykop Formation under the Black Sea is reported to be 4 - 5 k m thick (Tugolesov et al., 1984). Table IV contains the data for an estimation of the mass of buried organic carbon: length, width, thickness and average Co,~ content. A value of 1.2x 1013t Co,g accumulated during Oligocene-Early Miocene time is the result of a very conservative estimation, because the Black Sea and Caspian Sea sectors were neglected and the catagenetic loss of organic carbon was not taken into account. The type of kerogen is investigated only sporadically and with different methods. The main kerogen components of the Fischschiefer are bituminous groundmass and algal liptinite, the terrestrial contribution - - spores and pollen - - being only of minor importance (Wehner et al., 1983). Gerhard (1982) studying the kerogen of the Fischschiefer, assumes a mixed, marine and terrestrial origin. The elemental composition of kerogen studied on two samples belongs to Type II (Fig.2). Shale samples representing the menilite

145 TABLE I Correlation of the often anoxic Paratethys formations Basin

Formation

Stratigraphic extent

Time span (m.y.)

Reference

Molasse Flysch

Fischschiefer Menilite

2 14a

B~ldi (1984) S~ndulescu et al. (1981)

North-Hungarian

Tard Clay

Transsylvanian

Ileanda Clay + Bisusa Creacea Maykop

NP 22 upper part of NP 21, NP 22 25, NN 2-3 a upper part of NP 21, NP 22 23 upper part of NP 21, NP 22-23 Oligocene-Early Miocene

18

Maykop

6

B~ldi (1984)

6

B~ldi (1984)

Polster et al. (1972)

aValid only for the outer units of the Flysch Basin. TABLE II Organic carbon content of the often anoxic Paratethys formations; CS=Czechoslovakian, P = P o l i s h , U = Ukrainian, R = Rumanian Basin

Formation

Rhine Valley

Fischschiefer

Molasse Western Eastern Austrian

Fischschiefer

Flysch CS and P sectors

Menilite

Average Corg content

Range of Cot~ content

(%)

(%)

2.00

CS and U sectors, outer units V sector, outer units R sector

1.86

North-Hungarian

Tard Clay

Transsylvanian

Ileanda Clay

Maykop Crimea Lowland Western Ciscaucasus Eastern Ciscaucasus Mangyshlak

Maykop

1.50

Reference

0.9 -10.0

Welte (1979) Sittler (1965)

1.0 -7.0 0.2 -7.0 1.5 -10.0

Wehner et al. (1983) Wehner et al. (1983) Wehner et al. (1983)

0.5 -10.3 a 0.2

1.9b

0.3

5.0 b

Simhnek et al. (1981), Gucwa and Wieser (1980) Korhb and Durkovic (1978), Gabinet et al. (1976) Gabinet et al. (1976)

4.2

12.0

Pauliuc et al. (1983)

0.2 -4.2

Brukner-Wein et al. (1985)

0.65 5.1

Dudich and Bombita (1983)

1.50

Shestopal (1979)

2.00

Shestopal (1979)

1.20

0.07-2.4 b 0.2

aBadak (1966) reported black shales with shale oil yield as high as 10%. bLocal average values.

2.0 b

Drozdova and Chinenov (1976) Tikhomirova (1964)

146 TABLE III Thickness, average Cot, content, time span and rate of Co,g burial for some often anoxic Paratethys formations. Sediment density used for calculation: 2.35 g/cm 3 after Polster et al. (1972) Basin

Formation

Thickness (m)

Average Cot, content (%)

Time span (m.y.)

Rate of Co,, burial (106 t x km-2 x m.y.- 1)

Flysch, outer units in the U k r a i n i a n sector

Menilite

1900

1.86

max. 14

min. 5.50

N-Hungarian

Tard Clay

100

1.50

6

0.55

Maykop Crimea Lowland Western Ciscaucasus Eastern Ciscaucasus

Maykop 800 1000 1400

1.50 2.00 1.20

19 19 19

1.40 2.30 1.90

TABLE IV Dimensions, thickness, average Co,8 content and mass of buried organic carbon sediment density used for calculation: 2.35 g/cm 3 after Polster et al. (1972) Formation

Length (km)

Width (km)

Area (103 km 2)

Thickness (m)

Average Corg content

Mass of buried Corg (1012 t)

(%) Menilite Maykop (Crimea Lowland, Ciscaucasus)

1100 1500

200a 200

220 300

300 1000

2.0 1.5

3.15 10.75

"A conservative estimate of the pre-compression width.

H - index 750" Fischschiefer ";¢,

500-

"" •

•

250"

(~ • ."

i0

lbo 1go O-iodex

Fig.2. Rock-Eval pyrolysis plot of some anoxic shales. Data for the menilite formation are from SimAnek et al. (1981), for the Fischschiefer from Gerhard (1982). The Fischschiefer kerogen was analyzed for elemental composition, the C/O and H/O ratios were tentatively transformed to O and H index.

formation of the Czechoslovakian sector were studied with Rock-Eval pyrolysis by Sim~nek et al. (1981). Their kerogen proved to be between Type I and II (Fig.2). Badak (1966) reported shales from the Polish sector with a shale oil yield reaching as much as 10%. Shale oil yield of the samples studied from the Rumanian sector varies between 7 and 46 cm 3 to 100 g Corg (Grigoras et al., 1971). The H/C ratio of the kerogen from the same samples varies between 1.1 and 1.5 (the sample with the lowest oil yield was not analyzed). These data clearly indicate a very H-rich, aquatic kerogen. Based on stable C-isotope composition, bitumen IR-characteristics, n-alkane distribution and abundance of higher plant debris the organic matter of the Tard Clay is assumed to

147 have a mixed marine-terrestrial origin (Brukner-Wein et al., 1985). Rock-Eval pyrolysis of samples from a single borehole showed the presence of Type III to II kerogen (Fig.2). The organic matter of the Maykop Formation in the Crimea Lowland and Western Ciscaucasus is believed to have a sapropelic and/or mixed character. These designations from the Russian language literature are corresponding more or less to Type III to II kerogen.

Siliceous biogenic sediments The menilite formation received its name from menilites, cherts occurring in small lenses or thin bands of some centimeters to some decimeters, exceptionally 1-4 m thickness. These usually dark cherts occur throughout the ca. 1000 km long Flysch Basin, concentrated in two horizons with thicknesses of some meters to some tens of meters (Fig.3). The lower horizon is developed in all tectonic units while the upper one is restricted to units nearer to the East-European Platform. In the Maykop Formation cherts are reported only from the Mangyshlak Peninsula (Tikhomirova, 1964). Siliceous shales and diatomites are very common in the menilite formation, the former occurring in the Socka Beds in Slovenia

DUKLA

SW UNIT

SILESIAN UNIT

SKIBA

UNiT

NE

o z u~

--i

c} JO

rY

I_tJ /

...J

i

--i

Z Jupper chert

'\

\

CUl

kl_l

Fig.3. Schematic profiles for the Ukrainian sector after Gabinet et al. (1976). Dashed line = isochronous surface.

(Kuscer, 1967) and in the Maykop Formation of the Eastern Ciscaucasus (Ter-Grigoriants, 1961) as well.

Planktonic remains The anoxic sediments of the Maykop Formation, the Ileanda Clay, the Tard Clay and the Fischschiefer from the Rhine Valley are characterized by the absence of planktonic foraminifers and the frequent occurrence of a rich brackish nannoflora (B~ldi, 1984; Doebl et al., 1976). On the other hand, the Fischschiefer from Eastern Bavaria (Gerhard, 1982) and the Schistes ~ Meletta, Early Oligocene organicrich sediments from the French Alps (Charollais et al., 1980) are characterized by the presence of planktonic foraminifers. The overwhelming majority of planktonic remains reported from the menilite formation shows normal marine characteristics (Fig.4). The outcrop area of tectonic units is presented in Fig.5. According to Krestel (1961) and Gurzhiy and Ripun (1970) the freshwater diatom floras studied from the Skiba Unit were transported into the Flysch Ocean by rivers. Although the mass occurrence of freshwater diatoms in the Pouzdrany Unit (Reh~kov~ cited by Krhovsk~, 1981a) and the laminae composed of the freshwater diatom Melosira (Krhovsk~, 1985) in the lower chert horizon of the ~d~nice Unit are difficult to explain by this way, it should be noted that, according to Fourtanier (1984), mass occurence of freshwater diatoms has also been found in normal marine Miocene sediments off the Angola coast. Highly interesting are the brackish-water nannoplankton assemblages reported by Lebenzon (1973) and Krhovsk~ (1981a) from the Skiba Unit and 2d~nice Unit, respectively. In both cases they occur in a nanno-chalk/marl layer overlying the lower chert horizon, at the bottom of NP 23 zone. These sediments are characterized by a bloom of Reticulofenestra ornata, which is generally considered as a proof for brackish-water environment. Sediments underlying and overlying this nanno

148 Tectonit Uni[

CS

P

R

U

Pouzdron~j

CS

s u m~ ~~ . ~ . ~~ ~

cQnyons~.~

~.~

Zd6nice 02 04

Silesian

114

013 114 014

Maguro Skibo Duklo

~ 03

013 dr~8

05 015 1 7 ~7 I l l 1~5 012,,Q~1e 1 1 k-~9

013

05 O6 16

Marginal

F-rl ~-~.,

~

~

Fig.4. Planktonic remains reported from the menilite formation. marine planktonic foraminifers nannoplankton diatoms

brackish

fresh-water

• •

•

[]

Fig.5. Present-day extension of the Paleogene deposits in the Central Paratethys simplified after Balla (1984) and B~ldi (1984). Po = Pouzd~any Unit; Z= ~d~nice Unit. Contours of submarine canyons in Southern Moravia after Picha et al. (1978, fig.1.). CS, P, U, R: see Fig.4.

&

& - - & :marine and fresh-water diatoms together; hachured area = absence of tectonic unit. CS, P, U, R = Czechoslowakian, Polish, Ukrainiar/, Rumanian sectors. References: l=Krhovsk#, 1981a, 2=Hanzlikov~, 1981; 3=Str~mik and Hanzlikov~, 1963; 4=Jur~§ov~, 1974; 5= Gabinet et al., 1976; 6= Dicea and Dicea, 1980; 7= Lebenzon, 1973; 8=Kaczmarska, 1982; 9=Krestel, 1961; 10 =Stefanescu et al., 1979; 11=Gruzman, 1972; 12= Gruzman, 1981; 13 = Olszewska, 1982; 14= Van Couvering et al., 1981; 15=Andreeva.Grigorevich and Gruzman, 1978; 16= Gurzhiy and Ripun, 1970; Krhovsk#, 1985.

chalk/marl contain a typical marine microplankton assemblage (Stef~nescu et al., 1979; Hanzlikov~, 1981).

Discussion Summary of differences between the basins The substantial differences in the rate of organic carbon burial, lithology, and microplankton content mentioned above and listed in Table V call for regional differences of the anoxic regime as I try to demonstrate in the following. The Maykop Sea was characterized by abundant freshwater supply. Rivers draining the

huge surrounding land areas with humid climates - - Russian, Siberian and Turkestan platforms - - delivered more water than the sea lost by evaporation. The resulting positive water balance caused a stable vertical stratification similar to that of the present-day Black Sea (B~ldi, 1984). Rivers transported also a substantial amount of nutrients into the Maykop Sea which caused a higher than average planktonic productivity. The high rate of Corg burial and the mixed character of kerogen are explained by increased planktonic productivity and abundant supply of terrestrial plant debris. Thus, the anoxia of the Maykop Sea was an euxinic-type one. Cherts reported from the Mangyshlak Peninsula indicate a local, very high planktonic productivity. The mechanism responsible for this is unknown because of the lack of comprehensive data. The abundance of marine planktonic remains and the scarcity of brackish forms make it very likely that rivers entering the Flysch Basin were relatively small" and/or the isolation was not complete enough to produce a Black Sea type stable stratification. One noticeable exception is the nanno-chalk/marl overlying the lower chert horizon. A valid

149 TABLE V Organic geochemical, lithological and paleontological differences between some often anoxic Paratethys formations Formation

Kerogen

Rate of Cor~ burial

Biogenic siliceous sediments

Planktonic remains a brackish

Fischschiefer

mixed

low-moderate ?

non-reported

Menilite Tard Clay

marine mixed

high very high moderate

common-abundant non-reported

Maykop

mixed

high

moderately developed

marine

absent common French Alps + eastern Bavaria common Rhine Valley scarce common common in the nonlaminated parts common

aData from B~ldi (1984), Charollais et al. (1980), Doebl et al. (1976), Gerhard (1982), Hanzlikov~ (1983) and Popov et al. (1985).

model for the anoxia should be able to explain both the absence of a brackish-water surface layer during deposition of the menilite formation and the relatively short-lived brackishwater incursion. The anoxia in the Tard Sea and in Transsylvania was characterized by stable stratification of the water body (B~ldi, 1984). Profiles from the Tard Clay reveal the abundance of planktonic organic matter and/or an important contribution of terrestrial plant debris (Brukner-Wein et al., 1985). In the case of the Fischschiefer Sea stable stratification due to a brackish surface layer is reported from the Rhine Valley, while for Eastern Bavaria and the French Alps this model does not work. Thus, for a large part of the Fischschiefer Sea the reason for the anoxia remains unknown. The anoxia in the Flysch Basin

Working far from the field the author as a geochemist was unable to analyze the tectonic, palaeogeographic, and climatic conditions governing the beginning, evolution, and destruction of anoxia. Therefore, it is only intended here to elucidate which model of anoxia has to be refuted and which is the most likely one. The predominance of marine forms among

planktonic remains (see Fig.4) makes a stable stratification produced by a brackish surface layer very unlikely. Other variations of stable stratification, caused either by hypersaline brines (e.g. Orca Basin) or by the sharp density contrast between cold bottom water and warm surface water (e.g. Lake Tanganyika) can not be seriously considered. High algal productivity is the most likely mechanism for anoxic bottom conditions, high rate of organic carbon burial, H-rich kerogen, and abundance of biogenic siliceous sediments. The nutrient supply needed for it can be produced either by upwelling deep waters or by rivers. The river influence is documented by the common although minor occurrence of freshwater diatoms in the Skiba and Zd~nice Units. Since the rate of organic carbon burial in the Tard Sea, strongly influenced by freshwater input, was much lower than that in the normal marine Flysch Basin (see Table III), higher productivity in the latter can not be explained by river delivered nutrients. Thus, the high algal productivity and resulting anoxia were very likely caused by upwelling deep water, as already suggested by Afanas'eva and Tkachuk (1980) and Pokorny (1980). Despite this conclusion, a widespread, relatively short-lived brackish-water incursion into the Flysch Basin has to be admitted, since

150

the brackish-water chalks/marls reported from ~d~nice and Skiba Units are not isolated phenomena; lithologically a n d stratigraphically corresponding sediments are widely known in the Flysch Basin (see Krhovsk~, 1981b). Good recent analogues of this situation are not known, but the NW-African coastal upwelling shows important similarities. According to Diester-Haas and Chamley (1982) the NW-African coastal Quaternary records show periodical changes of upwelling intensity; sediments deposited during periods of relatively weak upwelling witness an increased river influence (Senegal, Gambia, etc). Obviously the large water mass of the Atlantic Ocean did (and does) not permit development of a brackish surface layer, while the much narrower Flysch Basin allowed the NP 23 zone brackish-water incursion. Consideration about the cause(s) of this incursion, such as uplift of the area causing the connection with the World Ocean, sea-level fall and/or colder, more humid climate, are beyond the scope of the paper. As to the geographic setting and dimensions a recent analogy is furnished only by the upwelling regime of the Bay of California (Fig.6). The striking difference between the straight Bay of California and the very curved Flysch Basin is deceiving, since according to the paleomagnetic reconstruction of Ba~.enov et al. (1980) the Flysch Ocean had a less curved shape and the loop of the Carpathians was formed very likely in Late Tertiary time. The Quaternary of the Bay of California and the menilite formation show striking similarities, despite the quite different tectonic settings of their deposition - - extension versus collision. These include high rate of Cor~ burial, laminated, diatomaceous sediments, and turbiditic intercalations. These common features can be explained by an interplay of upwelling, steep bottom morphology, and intense seismicity. Thus, the Oligocene Flysch Basin shows substantial similarities both with the Bay of California (geographic setting, steep bottom morphology, intense seismicity) and with off-

Q.

b°

.

,o

Fig.6. a. Present-day extension of the menilite formation, b. Recent sediments of the Bay of California with Cots content over 2% (after Summerhayes, 1983).

~vers

C.

b,

~ivers upv/slling,~ ~

Fig.7. Ideograms for the three upwelling regimes, a. Bay of California. b. Offshore NW-Africa. c. Early Oligocene Flysch Ocean.

shore NW-Africa (importance of freshwater discharge), both. A highly simplified comparison between the three upwelling regimes is proposed in Fig.7.

Paleogeographic speculations The fact that in sharp contrast to the Western and Eastern Paratethys basins the

151

Central Paratethys basins are not in autochthonous position, must be first of all taken into consideration. Following the seismical findings of Tomek (1984) not only the Flysch Basin, but even a large part of the Central Carpathians is underlain by the East-European Platform. The sediments deposited at the outer margin of the Flysch Basin are covered by the nappe structures of the Flysch Basin. The only available data about them are from Southern Moravia (Picha et al., 1978). Here fossil submarine canyons cutting the Paleozoic basement and Mesozoic sedimentary cover of the Bohemian Massif (Fig.5) are filled with dark, pelitic Early Oligocene sediments with thicknesses of several hundreds of meters to 1 kilometer. Their Cot~ content can reach up to 8%. These organic-rich sediments suggest at least dysaerobic conditions on the bottom of the outer, marginal part of the Flysch Ocean. Gabinet et al. (1976) studied in detail the stratigraphic relation of the menilite formation with the overlying normal marine Krosno Formation in the Ukrainian sector. They have found their contact to become older and older as one goes away from outer to inner tectonic units (Fig.3), while the beginning of the anoxic sedimentation is believed to be synchronous. Essentially the same picture can be found in other sectors of the basin, too (Jerzmhnska and Kotlarczyk, 1976; Shndulescu et al., 1981; Roth and Hanzlikov~, 1982). Going from the outer to the inner units not only the oxygenated sedimentation but also the thrusting started more and more earlier resulting in an earlier shallowing of the inner parts of the Flysch Basin (Beer and Shcherba, 1984). These facts suggest that the restricted development of anoxia in the inner units was the result of an interaction of the rising of sea bottom above the oxic-anoxic interface and the outward narrowing of the high planktonic productivity area. According to ichtyological studies of Jerzma~ska (1968) the thickness of the upper, oxygenated layer reached at least half a kilometer.

Conclusions (1) The anoxia in the Eastern Paratethys, the smaller Central Paratethys seas and the Rhine Valley was caused by an upper brackish-water layer, as already clearly stated by Bfildi (1984). (2) The anoxia in the Flysch Ocean was caused by a very high planktonic productivity. Nutrients necessary for it were most likely supplied by upwelling. (3) Rivers deeply influenced the narrow Flysch Ocean; for a short period at the lower part of the NP 23 zone upwelling ceased and an upper brackish water layer was generated.

Acknowledgements Z. Balla reviewed earlier drafts of the manuscript. M. B~ldi-Beke, M. Haj6s and A. Nagymarosi helped the author in micropaleontological problems. The author is very indebted to J. Urb~nek (Geological Survey of Prague) for the Rock-Eval pyrolysis on Tard Clay samples.

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