Person:

Macdonald, Francis

Sheet-crack cements and coextensive intrastratal folds and breccias occur in a stratigraphically controlled, meter-thick zone, near the base of Marinoan (635 Ma) cap dolostones in slope settings. We demonstrate that sheet-crack cements on the margins of the Congo and Kalahari cratons are localized at a turbidite-to-grainstone transition, which records a transient fall in relative sea-level, preceding the larger glacioeustatic transgression. Sheet-cracks opened vertically, implying that pore-fluid pressure exceeded lithostatic pressure. When the margin of an ice-sheet retreats from a coast, a net fall in sea-level occurs in the vicinity, because of the weakened gravitational attraction between the ice-sheet and the nearby ocean. Augmented by glacioisostatic adjustment (postglacial rebound), the early regional fall in relative sea-level can mask the simultaneous rise in global mean sea-level caused by the addition of meltwater. We propose that sheet-cracks and related structures in Marinoan cap dolostones manifest pore-fluid overpressures resulting from rapid sea-level falls in the vicinity of vanishing ice-sheets.

Microbial structures in Neoproterozoic cap carbonates record the environmental processes present in the aftermath of global glaciation. The Rasthof Formation of northern Namibia is a unique carbonate depositional sequence that formed during post-glacial transgression and highstand following the Chuos glaciation. Carbon isotope profiles from four examined localities reveal that onlap was diachronous over post-glacial, syn-rift topography. The lower Rasthof Formation consists primarily of dark gray thinly (<mm) and thickly (1–4 mm) laminated microbialites that exhibit different rheological responses to the emplacement of syndepositional dikes. The thinly laminated microbialaminite facies commonly host cm-sized syndepositional folds of microbially laminated sediment called roll-up structures. In more thickly laminated facies, layers are deformed into broad decimeter-sized folds, but roll-up structures are absent. Large syndepositional carbonate clastic dikes (0.5–1 m wide) and smaller veins (0.1–0.5 m) cut across bedding in both the thinly and thickly laminated facies, but are conspicuously absent from underlying and overlying beds. These carbonate clastic dikes and veins contain convoluted microbial mats and abundant marine cements. The lack of evidence for wave action or current scouring in the form of bedforms, scour marks, or intraclasts indicates that these microbialaminites formed below storm wave base. The close spatial association of deep-water microbialaminite facies in the Rasthof cap carbonate with carbonate clastic dikes suggests that the emplacement of dikes produced both dm-sized broad folds and cm-scale laterally discontinuous roll-up structures. The emplacement of the dikes, most likely due to the release of fluids into incompletely lithified mats, deformed cement-rich thick laminites into broad folds, while thinly laminated and more slowly lithifying mats were rolled into roll-up structures. Microbialaminite facies in the Rasthof cap carbonate thus not only reflect the depositional and environmental processes that operated in the aftermath of the Sturtian glaciation, but may also provide clues for the formation of roll-up structures found in even older Precambrian carbonates.