U-Pb geochronology and cyclostratigraphy of the middle Ediacaran upper Jibalah Group, eastern Arabian Shield

Abstract

The Ediacaran Period (ca. 635–539 Ma) represents a major transition in Earth history, beginning with the end of the Cryogenian snowball Earth glaciation and ending with the appearance and early diversification of bilaterian animals. Whereas a steady flow of new radioisotopic ages from key stratigraphic successions has resulted in much improved age constraints for the earliest and latter Ediacaran, few ages exist for the middle Ediacaran Period. The Jibalah Group, in the eastern part of the Arabian Shield, comprises minimally deformed volcano-sedimentary successions deposited in discrete basins, previously constrained to date between ca. 620 Ma and 560 Ma. The Antaq basin is one of the largest and best exposed of the Jibalah basins and contains the Muraykhah Formation (upper Jibalah Group), which is of particular interest because it contains putative Ediacaran fossils, textured organic surfaces, multiple tuff horizons, and spectacular 1–10 m-scale siltstone-sandstone cycles. Here we report new high-precision U-Pb zircon ages from the Muraykhah Formation anchoring an astrochronological analysis of these cycles. The radioisotopic ages confirm that two strong peaks identified in the stratigraphic height domain reflect accumulation paced by the long (405 kyr) and short (131–91 kyr) eccentricity cycles, the latter of which corresponds to the m-scale cyclicity that is visually apparent in the stratigraphy. The combination of radioisotopic ages and astrochronology allows us to develop a continuous, tuned record spanning from 599.1 ± 0.4 to 590.4 ± 0.6 Ma. This record complements other astrochronological analyses from slightly younger Ediacaran strata spanning the global Shuram negative carbon isotope excursion. The integrated radioisotopic and astronomical time scale ages also provides new constraints on the Ediacaran tectonic evolution of the eastern Arabian Shield.