Nanoscale Crystal Fabric Preserved in Dolomite Ooids at the Onset of the Ediacaran Shuram Excursion

A remarkably preserved dolomite oolite from the onset of the Ediacaran Shuram excursion—a critical, yet enigmatic, interval in Earth’s history—offers new insights into pathways of dolomite formation during the Neoproterozoic era. We examine ooids from the Khufai Formation using electron backscatter diffraction (EBSD) and polarization-dependent imaging contrast (PIC) mapping to characterize crystal preservation and orientation at the nanoscale. For comparison, we analyze two other well-preserved Ediacaran dolomite fabrics from the same stratigraphic succession: spherulitic dolomite from conical stromatolites in the Buah Formation, and fibrous dolomite cements from a supratidal pisolite in the Birba Formation. The Khufai ooids exhibit distinctive characteristics: fibrous radial crystals organized into plumose (feather-like) bundles, with the c-axis consistently oriented perpendicular to the elongation direction (length-slow), concentric banding with evidence of abrasion, and microfabrics indicating competitive growth dynamics. This combination of features is less consistent with typical replacement processes, including mimetic or synsedimentary dolomitization, than with primary precipitation of crystals with a dolomite lattice, likely imperfectly ordered at nucleation. Primary dolomite precipitation in ooid shoals may reflect a rare and still unknown set of environmental conditions during the onset of the Shuram excursion.