Geochemical signature of the Cenomanian to ... - Wiley Online Library

Geochemical signature of the Cenomanian to Eocene rocks in Israel - a palaeoenvironmental indicator Shimon Ilani*, Amnon Rosenfeld*, Joel Kronfeldt and Akiva Flexert “GeologicalSurzvy of Israel, 30 Malkhe Yisrael St, 95501 Jerusalem, t Department of Geophysics and Planetary Sciences, Raymond and Bezjerly Sackler Faculty of Exact Sciences, Tel Auiu University, Ranzat Aviu, 69989, Israel

ABSTRACT

Distinctive trace-metal concentrations characterize Cenomanian to Eocene marine carbonates from Israel. The Cenomanian-Turonian platform carbonates, including clayey formations, exhibit low average values ranging between 2 and 29 ppm for Zn, Cr, V, Ni, Cu, U and Co. The Santonian-Campanian and Early to Middle Eocene marine chalks show higher average concentrations of these trace-metals ranging between 3 and 56 ppm. The highest average concentrations of these metals (5-118 ppm) are found in the Maastrichtian and in the Palaeocene marine chalks and marls. The possible relationship between these metal background levels and the lithology, the biogenic productivity, the organic matter content, the iron oxide concentration, the rate of sedimentation of the studied time-rock units as well as the palaeogeographical changes are discussed. The extent of the exposed palaeo-landmasses due to tectonics, the intensity of weathering conditions and the detritus supply into the basin, control primarily the iron and trace-metal content in the studied sediments. Terra Nozia, 3, 195-202 INTRODUCTION

Many stratigraphic, tectonic and palaeontological studies have previously been carried out on the sedimentary sequence of Israel. However, very few studies have been published on the occurrence and distribution of the minor and trace elements of the non-mineralized country rocks. Detailed sampling was recently done to complement a preliminary geochemical, mineralogical and petrographical study which focussed upon the Cenomanian-Turonian sequence in Israel (Ilani, 1989). The present study deals with the trace and minor element composition of rock sequences sampled in continuous sections, complemented by occasional individual samples, representing common country rocks ranging in age from Cenomanian to Eocene. These carbonate rocks are broadly distributed throughout Israel (Fig. 1). The Cenomanian-Turonian sequence of 194

Israel is composed mainly of limestones, dolomites and marls. The sequence from the Santonian to the Eocene is composed mainly of chalks and marls. Samples were collected from fresh representative exposures, free of apparent mineralization; their stratigraphic position was determined by micropaiaeontology. The well-preserved microfossils found in the soft samples indicate that diagenetic processes were negligible. Sediments are an important store of metals released from the continents into the seas and may reflect variable geochemical environments (Chester, 1988). The primary aim of this study was to establish a geochemical baseline for the ’normal’ (non-mineralized) country rocks. The establishment of representative background metal values is a prerequisite for future geochemical prospecting and environmental quality studies in Israel and

neighbouring countries. An additional goal was to investigate the possibility of defining characteristic geochemical signatures for the various time rock units. The present study attempts to assess the possible relationships between the geochemical background of the studied rock units and their lithological variations, their environment of deposition, their organic matter content and their rate of sedimentation. METHODS

Three hundred and seventy-five representative samples of the studied sequences were subjected to geochemical analysis and palaeontological study. Each sample was dried and then ground to less than 64 p,m sieve-size. One gram of sample was leached by a 4 ml HCl: 6 ml HNOi solution and placed in an agitated water bath a t 90°C for 1 hour. The filtrate, representing the solution of the carbonates, the oxides and the clays and most of the organic matter, was analysed by atomic absorption spectroscopy. The elements analysed were iron, zinc, chromium, vanadium, nickel, copper, cobalt, silver, cadmium and lead. Another aliquot was used to determine the radiometal elements. The chemical and radiometric analyses were carried out at the Soreq Nuclear Research Center, Israel. A duplicate set of representative samples was chemically analysed at the Geological Survey of Israel for comparative purposes. The samples were analysed by inductive coupled plasma (ICP-AES) using a Jobin-Yvon JY-48 polychromator. The degree of interlaboratory reproducibility was relatively very high. The precision of the analyses, depending upon the element was between 5 and 10% of the reported values. The limit of detection for the elements examined were (in ppm): Fe-2.4;

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Zn-2.0; Cr-5.0; V-10.0; Ni-3.0; Cu3.0; U-0.3; CO-3.0; Ag-1.0; Cd-1.0; Pb-10.0. The content of silver, cadmium and lead was usually below the limit of detection and therefore excluded in the geochemical assessment. Uranium was measured both by delayed neutron activation (DNA), which directly monitors uranium, and gamma-ray spectrometry using a sodium iodide crystal NaI(TI), and simultaneously monitors the thorium and potassium content in the sample. This latter technique determines the amount of equivalent-uranium based upon the *I4Bidaughter. The detection limit for potassium is 0.3% and for thorium 4 ppm. The organic matter content of the samples was determined by the potassium dichromate oxidation method. Ages and palaeoenvironment of deposition were determined from splits of each soft rock sample by micropalaeontological examination using ostracods and foraminifers. STRATIGRAPHY

Cenomanian to Eocene carbonates are widely distributed and well exposed in Israel. They were deposited in a series of basins formed upon a large platform that was periodically inundated and extended between the open Tethys Ocean and the Arabo-Nubian Massif. The stratigraphic units studied are shown in Table 1. RESULTS

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Maastrichtian

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Santonian Campanian

Cenornanian

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Fig. 1. Location map of sample sites. ( X ) = number ofsamples studied.

The average values of Fe, Zn, Cr, V, Ni, Cu, Co and U and their standard deviations are given in Table l and Figs 2 and 3. The average content value of Co and U is lower than 10 ppm. The rocks of the Ghareb, Taqiye and Menuha formations are relatively enriched in U. The contents of Pb, Cd and Ag as well as Th and K in most of the samples are below the limit of detection. The values of the metal averages of the different time-rock units follow similar lines but in different concentration levels. Nevertheless, almost constant relationships between the average values of the elements for each timerock unit can be observed. The constant relationships between Cr, Ni and Zn as well as Cr, V and Cu are demonstrated in Figs 4 and 5 . 195

N

=

number of the studied samples; SD = standard deviation; *after Shirav, 1987; -not determined.

Cenoranian

Tur oni an

Eocene

Table 1. LithostratigraF..y and geochemical data of the Cenomanian to Eocene sequence. (Kcy works: Arkin and Harnaoui, 1967; Nathan, 1969; Rosenfeld and Raab, 1974; Benjamini, 1979; Sass and Bein, 1982; Honigstein, 1984; Reiss et a/., 1985; Flexerct al., 1986, 1989a,b;Honigstein r t a l . , 1987; Reiss, 1988; Honigst ein ct a l . , 1989; Ilani et a l . , 1989; Almogi-Labin et a l . , 1990; Lipson-Benitah et al., in press).

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of the Santonian-Campanian Menuha Formation and the Eocene Avedat Group, in which the carbonate content ranges between 80 and 95%. The average trace-metal content of the elements ranges between 3.2 and 56 ppm. The most significant enrichment in the average values of the trace-elements is found in the Maastrichtian marly chalks of the Ghareb Formation and the Palaeocene marls of the Taqiye Formation, ranging between 5.1 and 118 ppm. The contents of iron from all the samples studied, range from 0.1 to 2.0%. The highest values are found in the Moza, the Ghareb and the Taqiye formations. The approximate rate of sedimentation of the whole qequence was evaluated by dividing the average thicknesses by the corresponding time intervals. In the chalky Menuha and Ghareb formations and Avedat Group, durations of biozones were taken into account as well. This was done in order to check the possibility of a relationship between the rate of sedimentation and the element content. The rate of sedimentation of the Judea Group is relatively high, 5-10 cm per 1000 years. Lower rates of sedimentation of 2-3 cm per 1000 years were found for the Eocene sequence. The lowest rates of sedimentation, 0.5-1.0 cm per 1000 years, were calculated for the Menuha, the Ghareb and the Taqiye formations. DISCUSSION

Fig. 3. Average rialues of elements ofthe studied time-rock units. The Maastrichtian bituminous average values are after Shirazl, 1987.

Limestones and dolomites of the ludea Group (Cenomanian to Turonian age) exhibit low average values of the studied trace metals, in the range from 2 to 18 ppm. Even the marly formations of this sequence, the Moza and the Daliyya formations, with high clay content (up to 800/0),exhibit relatively low average contents between 2.3 and 28 198

ppm. Only the bituminous marls of the Daliyya Formation (up to 2% organic matter), exhibit average trace-metal concentrations u p to 41 ppm. This is twice as much as that for the nonbituminous marls of the same formation. A higher average content of the analysed elements is found in the chalks

The distributions of the different traceelement averages in the country rocks reveal a characteristic signature for each time-rock unit (Figs 2 and 3). This may be related to several possible factors such as the lithology, the productivity of organisms during deposition, the organic matter content, the rate of sedimentation, the incorporation of scavengers such as iron-oxides and the influx of detritus from nearby weathered landmasses. The relative influence of each of these factors upon the associated metal concentrations in the rocks is evaluated as follows. Lithology

The marly formations are expected to have higher metal background values than the carbonates because of their

TBmARESEARCH

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Taqiye Fm

Ghareb Fm Ghareb (bituminous) Fm Judea Gr

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Avedat Gr Daliyya Fm (bituminous)

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Menuha Fm

Moza Frn

Cr

Y

V

U

Y

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U

V

V

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50 Fig. 4. Content ratios between Ni, Crand Z n of thestudiedrockunits. Theludea Group refers only to limestones and dolomites

greater content of clays. However, in contrast to the marls of the Moza and Daliyya formations, the marls of the Taqiye Formation have significantly higher metal concentrations, despite the fact that the clay content in the Moza Formation is greater than that of the Taqiye Formation (Table 1). Therefore, other factors must be involved in regulating these metal concentrations. Biogenic productivity

Biogenic activity is a factor which concentrates trace metals from the seawater. High diversities and population densities of the microfossils prevailed during the deposition of both the Santonian-Campanian and Eocene chalks, indicating high biogenic productivity. In general, the productivity during the sedimentation of the Maastrichtian chalks and the Palaeocene mark was similar to that of the Santonian-Campanian and Eocene chalks, but the trace

element content of the Maastrichtian and Palaeocene rocks is significantly greater. Although biogenic productivity can explain, to a certain degree, enrichment of trace-metals in the country rocks, this factor alone cannot account for the different concentrations of the trace-metals values.

trichtian Ghareb Formation (lower part) containing 10-20% organic matter (Shirav, 1987). The analysed elements there are enriched 1.5-2 times relative to the white, organic-poor chalk of the upper part of the Ghareb Formation (Fig. 3). Iron oxides

Organic matter

Organic matter can effectively absorb and concentrate trace metals (Chester, 1988). Most of the samples were obtained from non-bituminous surface exposures containing 0.1-0.5% organic matter. In organic-richer sediments, a greater metal content was preserved. For example, the bituminous marls of the Daliyya Formation, which contain up to 2% organic matter, are doubly enriched in trace elements relative to the marls deficient in organic matter. Another example of such enrichment is also found in the oil shales of the Maas-

Iron oxides can be effective scavengers of metals. The iron oxide content of the studied samples varies, with averages ranging between 0.1 and 2.0%. Certain units (Fig. 2), such as the Moza and the Daliyya formations of Cenomanian to Turonian age are enriched in iron (0.42.0%) but are poor in other metals. On the other hand, the Santonian-Campanian (Menuha Formation) as well as the Eocene sequences contain less iron (0.1-0.2%), but have a higher metal content. Hence, no direct correlation between the iron oxides and the metal content can be shown. 199

.cu

Cr

Fig. 5. Cmztent mtim betwen V , Crniid Cuof thestudied rock units. Thr [iideflGroup refers on1.y to limestones and dolomites Rate of sedimentation

Palaeogeography

The sequence ot rocks under study can be defined by three different rates of sedimentation (Table 1): 5-10 cm per 1000 yr, 2-3 cm per 1000 yr and 0.5-1.0 cm per 1000 yr. Probing the three major lithostratigraphic units, the Cenomanian - Turonian Judea Group, the Senonian-Palaeocene Mount Scopus Group and the Eocene Avedat Group, it appears that there is an inverse correlation between the metal content of the rocks and the rate at which they were deposited. It seems that a longer contact of the sediments with the seawater, reveals a higher content of the trace metals. However, the rate of sedimentation of the Menuha Formation is similar to that of the Ghareb and the Taqive formations (0.5I cm/1000 yr), but the average metal background level of the Menuha Formation is significantly lower. Therefore, the rate of sedimentation cannot be the sole factor explaining the differences of metal concentrationsin these formations.

During the deposition of the Cenomanian to Turonian sediments the sea was extensive and shallow (Flexer et al., 1989a). In Late Turonian to Santonian times, landmasses started to be exposed due to tectonic activity (Braun et a / . , 1987; Flexer et a l . , 1989a). This uplift is denoted by the palaeokarst phenomena that are found in the upper part of the Turonian sequence (Weiler, 1966). Exposed landmasses during the Cenomanian-Turonian times could not efficiently contribute sigruficant detritus throughout the quiet, shallow epeiric sea that covered a broad area of the platform. Deepening of the sea, and the tilting of the platform during Santonian and Early Carnpanian times (Flexer et a l . , 1986), resulted in an increase of detrital material contribution from the continent into the sea. This is reflected in the moderate iron and trace-metal concentration in the chalk of the Menuha Formation. In Maastrichtian to Palaeocene times,

200

relatively large anticlinal areas were exposed due to intensive folding and uplift (Arkin et al., 1972; Flexer et al., 1989b).On the exposed anticlines, ironrich soils and laterites were formed indicative of intense weathering (Starinsky, 1964; Nathan and Shahar, 1967; Sass and Freund, 1967; Shiloni et a / . , 1988; Ilani et a / . , 1989). The soilforming processes released appreciable iron and associated metals into the sea (Shiloni et al., 1988; Ilani et a / . , 1989). Mainly detritic sediments (enriched in palygorskite), from nearby landmasses were deposited in the basins (Nathan, 1969). Moshkovitz and Eshet (1989) report plentiful terrestrial palynomorphs in the Late Maastrichtian to Early Palaeocene marine sediments of Israel. This indicates a well-developed transportation system from the landmasses to the basins. In other parts of the world during the Palaeocene, increased continental weathering was accompanied by river borne sediment influxes into the sea

(Delaney and Boyle, 1988). This appears to be reflected in Israel by the higher metal concentrations found in the Ghareb and the Taqiye formations. During the Eocene times one of the largest transgressions of the Tethys Ocean occurred, which covered the Syrian-Arc anticlines and large parts of the Arabo-Nubian Massif. As a result of this large transgression, the landmasses supplying detritus and metals were probably inundated, explaining why only moderate trace-metal concentrations were encountered. Palaeogeographical configuration and the intensity of tectonic activity may thus have played the dominant role in the supply of metals to each depositional basin. ACKNOWLEDGEMENTS

We would like to thank Dr Y. Nathan for helpful remarks; A. Strull, Y. Assael and G. Liphshitz from the Soreq Nuclear Research Center at Yavne, and Ms Sara Ehrlich from the Geological Survey of Israel for the Chemical analyses; Ms Bevie Katz for editing the manuscript; Camille Alafi and S. Levy from the Geological Survey of Israel for technical assistance.

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