Eocene oil shales from Jordan - their petrography, carbon and oxygen stable isotopes

Abstract

Sediment petrographic studies combined with carbon and oxygen stable-isotope analyses are considered to be powerful tools in deciphering the depositional and diagenetic history of carbonate rocks. These studies have been found valuable for understanding the environmental controls and global changes of climate and oceanography. Two cores of Eocene bituminous marls (OS-22 and OS-23) from central Jordan with thicknesses of 222.4 m and 256.3 m, respectively, were logged. A total of 103 thin sections were analyzed for sediment petrographic investigations. Another 295 bulk rock samples from Core OS-23 were selected for stable-isotope analysis (δ13C, δ18O). The two cores show similar lithologic variations that correspond to third-order sea-level changes. The sediment petrographic study reveals the presence of six major microfacies that range from mudstones to grainstones. The distribution of these six types of microfacies in the two cores indicates an overall shallow-marine environment in an interior carbonate-platform setting. Within this environment, conditions fluctuated between open and restricted marine. The stratigraphic variability of the carbon-isotope data of Core OS-23 reflects a highly dynamic depositional system that exhibits a variable rate of organic matter accumulation in the sediments, which can be directly linked to the interaction between primary organic-matter burial and oxidative weathering of reworked organic matter. Within the overall diagenetically controlled δ18Ocarb profile an excursion is observed for a specifically enriched organic matter interval. This excursion is believed to coincide with the Middle Eocene Climatic Optimum (MECO), suggesting that next to seawater, porewaters were influenced by this event and making it the first record of this event in the region.