--> --> A Regional Investigation of an Under-Considered Source Rock in the Eastern Mediterranean: Organic Rich Carbonates From the Eocene of the Levant Margins

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A Regional Investigation of an Under-Considered Source Rock in the Eastern Mediterranean: Organic Rich Carbonates From the Eocene of the Levant Margins

Abstract

Lower to Middle Eocene oil shale deposits are known to occur in Jordan, where they are economically mined together with similar Late Cretaceous source rocks that are better known regionally. To the west, no substantial Eocene organic-rich source rocks have been described from within the Levant Basin, where the high productivity upwelling system is believed to have terminated in the upper Maastrichtian. Here we report on a thick organic-rich interval in the Eocene Levant basin and compare it to bituminous Eocene sediments from the Eratosthenes Seamount (ES) offshore Cyprus. Recent onshore coring from northern Israel has revealed a 150 m Eocene sequence composed of organic-rich chalks interleaved with displaced neritic limestones. TOC values range between 1.5 and 14%, averaging 4.5%. Displaced limestones are composed of a variety of poorly cemented mud- and wackestones with low-diversity assemblages of large benthic foraminifera associated with planktonic foraminifera, suggesting original deposition under low-energy conditions within the oligophotic zone on the outer ramp. Transport overprints include soft-sediment deformation (SSD), partially lithified rip-ups, folds, small diapirs, bed-scale imbrication, brecciation and synsedimentary shear. These features indicate detachment, movement and emplacement following initial sedimentation, in some cases more than once. Emplacement occurs into a chalk facies that can vary in appearance from darker (higher TOC) and lighter (lower TOC) chalk lithofacies. Elemental XRF data combined with TOC content, petrophysical measurements, and sedimentological features of the chalks were used for clustering via principal component analysis. High Zn, P, S and Cr values cluster with organic-rich samples, which are interpreted to represent the affinity of micronutrients to marine OM, and to natural in-sediment sulfurization processes connected with its preservation, as also observed by the occurrence of sulfur-bound biomarkers. Ca clustered broadly with organic-lean samples. The OM-rich chalks are deep marine and are similar to the Upper Cretaceous oil shales that are widespread in the Levant. However, elements of terrestrial affiliation (i.e., Al, Si, Fe and K) and high magnetic susceptibility values clustered with the highest OM-rich samples, and together with the transport of benthic fauna in the interleaved limestone facies, demonstrate a link between sedimentary transport and accumulation and/or preservation of OM. When compared with data from ODP Leg 160 Hole 966F drilled in the ES, chalk facies of both studied sections indicate a similar distribution of biomarker indices (e.g., chain length distribution of long and short n-alkanes carbon preference index, and the pristane/phytane ratio). However, the Israel section has a higher contribution of terrestrial organic matter relative to the ES section. A major hiatus separates organic-rich Eocene chalks from the overlying Miocene coarse-grained bioclastic limestones in the ES section. This surface is enriched in glauconite, suggesting a prolonged period of reduced sedimentation or of non-deposition. The glauconite grains probably developed in a deep-water environment, as suggested by planktonic foraminifera preserved within the grains. Our data indicates that the two sections were deposited in similar open marine depositional environments, likely as part of a regional high productivity system with local variation related to transport mechanisms in the more proximal successions in the eastern part of the Levant. High primary productivity in surface waters of deep-water environments and the consequent deposition of organic-rich chalks was therefore a widespread eastern Mediterranean phenomenon extending from Jordan, across Israel, and as far as offshore Cyprus. Consequently, we propose to re-evaluate Eocene deposits as potential source rocks for oil and gas formation in the deep Levant Basin, and elsewhere in the eastern Mediterranean.