PALEOCEANOGRAPHIC DEVELOPMENTS OF THE LATE CRETACEOUS OIL SHALE SEQUENCE, SOUTHERN TEHTYS, NEGEV ISRAEL INFERRED FROM ORGANIC GEOCHEMISTRY AND STABLE ISOTOPES

The late Cretaceous organic-rich carbonate succession in Israel is part of an extensive Southern Tethyan high productivity regime, found throughout the Middle East. This unique succession has major economic potential as a hydrocarbon source and combustion material, and represents a fascinating phenomenon in the geological history of the Levant region. A thick oil shale sequence has developed under high nutrients levels with oxygen deficient bottom water conditions. A sequence of 49 m of fresh, unaltered rocks from the Oil Shale unit, its contact with the underlying top Phosphate Mbr. and the overlying Marl Mbr. were sampled from an open quarry located at Mishor Rotem, Negev where the largest oil shale reservoir in Israel is found. Reconstruction of the paleocenographic changes along the sequence is based on multi-geochemical proxies including d¹³Corg, d¹⁵Norg, %CaCO₃ and total organic carbon (TOC).  

CaCO₃ and TOC records show opposite trends at the different lithological units. In the top Phosphate Mbr. the %CaCO₃ content is very high and TOC is relatively low (2%). A sharp increase in TOC to a maximum of 17% is observed at the base of the oil shale unit gradually decreasing upwards to the top of the oil shale unit, to less than 10% while %CaCO₃ varies between 60%-72%. These vertical variations and field observations suggest that the top phosphorite sequence experienced early diagenesis induration and organic matter oxygenation. Whereas the preservation of organic matter of the oil shale unit indicates deposition under dysoxic conditions associated with increased flux of organic matter. The decrease in the organic flux towards the top of the oil shale unit seems to indicate weakening of the upwelling system.

In general, d¹³Corg values of ~-28.5 to -29‰ are highly depleted compared to typical marine organic matter values, indicating post depositional fractionation during Kerogen formation. The d¹⁵Norg records show a linear trend of enrichment (R²= 0.7) from the top phosphorite sequence (3‰) through the oil shale (3‰-5‰) and up to the marl (7‰). This trend might suggest a change in nutrient availability or perhaps influence of decomposition and preservation processes.