Paleoenvironments of the latest Cretaceous oil shale sequence, Southern Tethys, Israel, as an integral part of the prevailing upwelling system

Abstract

The Late Cretaceous succession in Israel is part of an extensive high-productivity upwelling regime that persisted over ~ 20 m.y. in the southern margins of Tethys. The deposition of a ~ 40-m thick organic-rich carbonate (“oil shale”) sequence in the Negev, Israel, indicates a major change in the evolution of this high-productivity regime and reorganization of the marine ecosystem during the latest Campanian–early Maastrichtian time. The main objective of this study was to reconstruct changes in surface water productivity and seafloor oxygenation during the deposition of the Oil Shale Member (OSM) and transitions with the underlying Phosphate and overlying Marl Members using high-resolution records of planktic and benthic foraminifera and total organic carbon (TOC) content. The updated age of the OSM, determined by integrating planktic foraminifera, calcareous nannofossils and inoceramids, suggests that this sequence spans a maximum of 1.85 m.y. from 71.6 to 69.85 Ma, with a sedimentation rate of at least 2.4 cm/kyr. Five distinct planktic (P-Types) and benthic (B-Type) foraminiferal assemblages were distinguished in the studied sequence. The change from P-Type 5 to P-Type 1 along the sequence correlates with a distinct decrease in TOC content (18.0–0.1 wt.%), and signifies an upward decline in surface productivity. Based on this relationship, it was inferred that species of Globigerinelloides (principally Globigerinelloides asper) are the best tracers for extreme eutrophic surface water, whereas heterohelicids are associated with more mesotrophic conditions. The transition in time from assemblages B-Types 5 to 1 mainly reflects a change from the predominance of praebuliminids and neobuliminids to a more diversified rotaliid-dominated assemblage, indicating an increase in bottom water aeration. The appearances of planktic foraminifera coincident with the establishment of benthic rotaliid fauna at the Phosphate-Oil Shale Member transition mark the most prominent change in the upwelling sequence. This change probably reflects moderate weakening of the eutrophic surface water productivity and a regional sea level rise resulting in seafloor deepening and improved seafloor aeration. A further step in the decrease in TOC content (to 5.5–8 wt.%) occurs in the uppermost part of the OSM (latest Campanian) and corresponds to the transitions to assemblages P-Type 1 and B-Type 1. This change suggests a shift to mesotrophic conditions, additional increase in bottom and pore water aeration, and thus the demise of the upwelling activity in the southern Tethys.