Abstract
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.
Highlights
During the younger Cryogenian (Marinoan) glaciation, the continents were huddled between 30°N and 70°S latitude (Fig. 1)
The question that emerges is, do sheet-crack cements systematically accompany regressive changes from basal turbidites to lowangle crossbedded grainstones? Kennedy (1996) documented a similar relationship in the lower part of the Marinoan cap dolostone in the Amadeus Basin of central Australia, but attributed the lowangle crossbedding to differential dissolution and semi-plastic slumping in a deepwater setting
To further illustrate the coincidence of early regression and sheet-crack cements in cap dolostones, we describe a new cap dolostone from a different craton in Namibia
Summary
During the younger Cryogenian (Marinoan) glaciation, the continents were huddled between 30°N and 70°S latitude (Fig. 1). Marinoan cap dolostones are typically pale pinkish-grey (weathering to yellowish-grey) in colour and are extremely lean, containing less than 0.001 by weight of total organic carbon Their conspicuous lamination is defined by normal and reverse graded micro- and macropeloids, deposited as hydraulicallysorted silt-, sand- and granule-size (b3.0 mm diameter) spherical aggregates (Aitken, 1991; Hoffman et al, 2007; James et al, 2001; Kennedy, 1996; Xiao et al, 2004). We speculate on a causal mechanism that links early regression with the development of sheet-crack cements in cap dolostones
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