1. Fig. 25 Distinctive features of Sturtian and Marinoan cap carbonates.
Distinctive features of Sturtian and Marinoan cap carbonates. (A) Sharp sedimentary contact between Sturtian glaciomarine Maikhan Ul Formation with ice-rafted dropstones (arrow) and overlying organic-rich and fossiliferous cap limestone (basal Taishir Formation), Zavkhan basin, western Mongolia (63, 143). Co-author F.A.M. points to the contact. (B) Sharp sedimentary contact between Marinoan glaciomarine Ghaub Formation (Bethanis Member) with ice-rafted dropstones and overlying cap dolostone (Keilberg Member of the Maieberg Formation) on the foreslope of the Otavi carbonate platform, northern Namibia (97). Co-author G.P.H. points to the contact. (C) Microbialaminite with roll-up structures in Sturtian cap carbonate (middle Rasthof Formation), northern Namibia. The coin is 2 cm in diameter. Roll-ups are associated with neptunian dikes and indicate that biomats were cohesive and pliable (475, 478), but the metabolic basis for their growth, inferentially below the photic zone, remains undetermined. (D) Macropeloids and low-angle cross-stratification in characteristically organic-poor Marinoan cap dolostone, Keilberg Member of the Maieberg Formation, northern Namibia (97). Sorted peloids and low-angle cross-stratification are characteristic of cap dolostones and indicate sedimentation at depths above fair-weather wave base. Coarse grain size suggests that macropeloids were less dense than pure carbonate when deposited. (E) Sheet-crack cements composed of fibrous isopachous dolomite (tinted orange by desert varnish where selectively silicified) occur regionally in the basal 2 m of the Marinoan cap dolostone in downslope settings in Namibia (425). The pen (circled) is 15 cm long. Sheet cracks and cements imply pore-fluid overpressure and an alkalinity pump, respectively, at shallow depth beneath the sediment-water interface at the onset of cap carbonate sedimentation. Overpressure could be related to rapid base-level fall due to the gravitational effect on the ocean of the loss of nearby ice sheets (419). (F) Upward-expanding mound of tubestone stromatolite (Figs. 24 and 26B) characterized internally by geoplumb (paleovertical) tubes (arrow) that were filled by carbonate mud as the mound grew. Mound margin (dotted line) is flanked by mechanically bedded dolopelarenite (lower left), Marinoan cap dolostone (Keilberg Member), northern Namibia (97). (G) Giant wave ripple (Fig. 24) in the Marinoan cap dolostone (Nuccaleena Formation) at Elatina Creek, central Flinders Ranges, South Australia. Note gradual amplification at the level of the hammer (circled, 35 cm long) and correlation of overlying layers a and b. Such unusually large and steep wave ripples occur in Marinoan cap dolostones on several paleocontinents, and the regional and global mean paleo-orientations of their crestlines are meridional (31, 443). Single wave trains aggrade as much as 1.4 m vertically, implying steady growth under the influence of long-period waves during Marinoan deglaciation (443). Intraclasts and composite grains are typically absent, suggesting that the characteristic steepness of the ripples is not related to early lithification [contra Lamb et al. (506)]. (H) Calcitized crystal fans (gray) formed as prismatic aragonite seafloor cement (Figs. 24 and 26C) in the Hayhook Formation stratigraphically above the Marinoan cap dolostone in the Mackenzie Mountains, Northwest Territories, Canada (409, 416). Growth of seafloor cement was coeval with sedimentation of lime mud (pink) from suspension. Seafloor aragonite precipitates up to 90 m thick occur in Marinoan cap limestones and mark the return of a depositional style last common before ~1.5 Ga (421, 423).
Paul F. Hoffman et al. Sci Adv 2017;3:e
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