UNRECOGNIZED SHIFT IN MAJOR REEF BUILDERS IN MIDDLE JURASSIC OUTCROPS OF ISRAEL

The Jurassic sequence of the Levant holds some of the world’s largest petroleum reservoirs and has been rigorously researched due to its high economic potential. During the Middle Jurassic, Israel was located on the southeastern margin of the Tethys close to the paleo-equator. By the Late Jurassic, reefs formed a broad belt estimated to have been several tens of kilometers in width along the Tethyan shores. Patchy reefs rich in coral and stromatoporoids are found throughout the Middle Jurassic (Callovian) sequences of Makhtesh Gadol (southern Israel) and Mount Hermon (northern Israel). In addition to coral and stromatoporoid fossils, these outcrops are rich in benthic macrofauna, especially bivalves, gastropods, brachiopods and crinoids. Jurassic reefs and their associated macrobenthic fauna have been shown to be a reliable proxy for paleoenvironmental settings, such as sea level, and structural and climatic conditions. In our study we describe an unrecognized significant biofacies change in the major reef builders in Makhtesh Gadol (southern Israel).

Field sampling was based on randomly placed 10 m line intercept-transects that were spaced at 5 m intervals in the patchy reef subunits. The percent cover of reef builders was calculated based on the diameter of reef structures along each transect, corrected for effective sampling area (band width). Polished cross-sections were used to distinguish between coral and sponge specimens, which were otherwise undistinguishable in the field. Ten samples were analyzed using both line transects and cumulative sampling curves from bulk samples.in which.12 species of sponges and corals were found.

Analysis of the percent cover of reef components shows a shift from patchy coral reefs earlier in the Callovian to branching sponges and stromatoporoids that dominated the upper part of the Makhtesh Gadol section. This suggests an environmental change that may be associated with changes in water depths.