Nanoscale crystal fabric preserved in dolomite ooids at the onset of the Shuram Excursion

The Shuram excursion: A response to climate extremes at the dawn of animal life

“…In contrast, Mn enrichments reflect diagenetic addition of Mn +2 and occur in the earliest stages of cementation, presumably during or immediately after ooid deposition. We observe additional Mn variability, preserved as Mn-rich banding within the dolomite ooids associated with the onset of the Shuram excursion at the top of the Khufai Formation (41).…”

“…In contrast, the middle and upper Khufai Formation was deposited in a peritidal environment with minimal accommodation space, and restriction and evaporation. These rocks display petrographic evidence for an early, fabric-retentive, dolomite formation process (39,41).…”

“…The dolomitic ooid grainstone that records much of the onset of the Shuram excursion (δ 13 C V P DB values of 0 to 5 ) includes mudstone rip-up clasts and micritic stromatolites that nucleated on the grainstone and generated topography above the oolite. The ooids are ordered dolomite today, have a plumose texture, and are often silica-cemented or silicified, particularly in the northern Huqf outcrop area (16,39,41). These dolomite ooids preserve a radial fabric composed of fine crystals as seen in nano-scale synchrotron-based PIC mapping (41).…”

“…The ooids are ordered dolomite today, have a plumose texture, and are often silica-cemented or silicified, particularly in the northern Huqf outcrop area (16,39,41). These dolomite ooids preserve a radial fabric composed of fine crystals as seen in nano-scale synchrotron-based PIC mapping (41).…”

“…These are draped and overlain by poorly consolidated siltstones, representing a potential carbonate gap. After 10s of meters, radial to banded radial calcitic ooids intercalated with siltstones with δ 13 C V P DB values of 10 to 12 may represent a shift from geochemical conditions favoring dolomite precipitation (41) to those favoring calcite precipitation (Fig. 2).…”