Third-Order Microbialitic Cycles of Kuwait Formation: A Gateway to Early Oligocene Orbital Obliquity

The outcrop study of microbialites is crucial for developing depositional models that can explain their occurrences and distribution. This article examines the presence of five distinct microbialitic beds exposed at the Jal Az-Zor escarpment over the Kuwait Formation in northern Kuwait, which has been misinterpreted as calcretes. This work will also attempt to highlight their global importance, by linking them to astronomical orbital obliquity modulation.To characterize these microbialitic beds, detailed field observations combined with several outcrop samples subjected to elemental analysis, thin section, scanning electrical microscope, and strontium radiometric dating analyses.Two main microbial types are recognized over the Jal Az-Zor escarpment; these are thrombolites and stromatolites. Thrombolites can be further divided into clotted (pustular) and domal (colloform) structures. The stromatolitic beds described herein are identified for the first time over the Jal Az-Zor escarpment. Microbialites of the Kuwait Formation are characterized by fossilized filaments, flagellates, bacteria, and extracellular polymeric substances biofilms that were deposited in subtidal to supratidal settings. Stable strontium isotope ( 87 Sr/ 86 Sr) dating of the five microbial beds demonstrate that they were deposited during the early Oligocene (Rupelian), where the exposed beds have been dated at 33.99 Ma, 32.6 Ma, 31.5 Ma, 30 Ma, and 29 Ma respectively, suggesting a 3 rd order sequence stratigraphic hierarchy. Such dates correspond to the La2004 obliquity node of New Jersey, USA highlighting the global importance of these beds.Despite the eustatic sea-level rise during the Rupelian, the exposed depositional sequences are characterized by an average 2.5 m cyclic nature. This observation combined with the identified ages of each microbial bed, suggests that the Jal Az-Zor area was subjected to a continues and gentle uplifting rates that resulted in maintaining a consistent shallow setting. This work concludes that within subtidal and intertidal settings global sea-level fluctuations can be captured regardless of local tectonic activity.