From phytoplankton to oil shale reservoirs: A 19-million-year record of the Late Cretaceous Tethyan upwelling regime in the Levant Basin

The complexity of sedimentary organic-matter formation, its source to burial, and influence on bottom-water communities are addressed in this study of the 19 million years-long Upper Cretaceous Tethyan upwelling regime in the Levant. A multi-proxy approach is amalgamated into a model through which the paleoceanographic complexity of an intense and long-lasting upwelling system is better understood. A shift in primary producer assemblages and reorganization of the bottom water bacterial consortium followed the Campanian-Maastrichtian boundary, enforcing conspicuous sedimentological and faunal changes. The siliceous and phosphatic-bearing sediments of the Campanian gave way to Maastrichtian uniform and regionally distributed organic-rich carbonates, accompanied by an increase in sedimentation rates from ∼1 to >6 cm/kyr. This has facilitated both the increase in organic-matter production and its excellent preservation, and through diagenetic pathways affected its elemental composition. These paleoceanographic modifications have also affected foraminiferal assemblages, from massive blooms of triserial (buliminid) benthic foraminifera during the Campanian which have sequestered diatom chloroplasts, to diverse trochospiral forms during the Maastrichtian using nitrate instead of oxygen for their respiratory pathways. We propose that shifts in the type of primary producers promote a change in the amount and type of preserved organic matter, in bottom water communities, on the lithological composition of the rock-mass, and ultimately on the hydrocarbon generation potential of the studied source rock. The continued rise of southern Tethyan sea level and global highstands during the Upper Maastrichtian, culminating into Arabian Plate-wide maximum flooding surfaces, cause the migration of upwelling activity away from the study area, terminating the high productivity sequence in the Levant.