Geomorphic evolution of a storm-dominated carbonate ramp (c. 549 Ma), Nama Group, Namibia

Abstract

-The well-exposed Hoogland Member (c. 549 Ma) of the northern Nama Group (Kuibis Subgroup), Namibia, represents a storm-dominated carbonate ramp developed in a foreland basin of terminal Proterozoic age. The ramp displays facies gradients involving updip grainstones which pass downdip into broad, spatially extensive tracts of microbial laminites and finely laminated mudstones deposited above and below storm wave base. Trough cross-bedded, coarse grainstones are shown to transit downdip into finer-grained calcarenites, irregular microbial laminites and mottled laminites. Siliciclastic siltstones and shales were deposited further downdip. Platform growth was terminated through smothering by orogen-derived siliciclastic deposits. Ramp morphology was controlled by several different processes which acted across many orders of magnitude (millimetres to kilometres), including in situ growth of mats and reefs, scouring by wave-produced currents, and transport and infilling of coarse-grained carbonates and fine-grained carbonates and clastics. At the smallest scale, 'roughening' of the sea-floor through heterogeneous trapping and binding by microbial mats was balanced by smoothing of the sea-floor through accumulation of loose sediment to fill the topographic lows within the upward-propagating mat. At the next scale up, parasequence development involved roughening of the sea-floor through shoal growth and grainstone progradation, balanced by sea-floor smoothing through shale infilling of resulting downdip accommodation, as well as the metre-scale topographic depressions within the mosaic of shoal-water facies. At even larger (sequence/platform) scales, roughening of the sea-floor occurred through aggradation and progradation of thick carbonates, balanced by infilling of the foreland basin with orogen-derived siliciclastic sediments. At all scales a net balance was achieved between sea-floor roughening and sea-floor smoothing to maintain a more or less constant ramp profile.

Superheavy pyrite ( 34Spyr > 34SCAS) in the terminal Proterozoic Nama Group, southern Namibia: A consequence of low seawater sulfate at the dawn of animal life

“…The ubiquitous distribution of storm deposits in the studied Nama Group carbonates (DiBenedetto and Grotzinger, 2005) indicates that the Nama basin sediments were regularly reworked and ventilated by storm activity, which likely resulted in intense aerobic reoxidation of dissolved sulfi de within these sediments. In modern coastal marine sediments that are regularly ventilated by storm activity, up to 90% of reduced sulfi de is believed to be reoxidized before being buried as pyrite (Jorgensen, 1982;Berner and Westrich, 1985;Canfi eld and Teske, 1996).…”

“…These units were deposited in shore-associated settings ranging from upper shoreline/tidal fl ats to below-wave-base lower shoreface, and comprise calcisiltites, calcarenites, heterolithic interbeds, grainstones, and microbialites (Saylor et al, 1998;Grotzinger and Miller, 2008). The ubiquitous distribution of trough-bedded coarse grainstones, rip-up clasts, and wave-scoured surfaces within these units indicates that they were regularly reworked by intense storm activity (DiBenedetto and Grotzinger, 2005). See the GSA Data Repository 1 for a detailed stratigraphic description and discussion of age constraints.…”

“…Such an ocean stratifi cation model, however, is not applicable to the Nama Group carbonates because sedimentological evidence indicates that these carbonates were deposited in a shallow, storm-dominated ramp environment (Saylor et al, 1998;DiBenedetto and Grotzinger, 2005), which would have been well mixed.…”

Deep-Water Incised Valley Deposits at the Ediacaran-Cambrian Boundary in Southern Namibia Contain Abundant Treptichnus Pedum

“…Previous work has documented the sequence stratigraphy, carbonate sedimentology, and geomorphic evolution of carbonate ramps and siliciclastic-dominated packages within the subgroup (Germs, 1983;Saylor et al, 1995;Saylor and Grotzinger, 1996;Saylor, 2003;DiBenedetto and Grotzinger, 2005;Grotzinger and Miller, 2006). Carbonate lithologies are best developed on the western margin of the basin, in the depocenter where siliciclastic input was insufficient to fill the accommodation.…”

Facies, stratigraphy, and evolution of a middle Ediacaran carbonate ramp: Khufai Formation, Sultanate of Oman

“…The origin of Precambrian mudstone is debated with possible explanations ranging from direct precipitation from the water column, precipitation as stromatolite laminae and subsequent disaggregation, or disintegration of algal skeletons similar to today (Grotzinger, 1989;Knoll and Swett, 1990;Tucker and Wright, 1990;Fairchild, 1991;Sumner and Corcoran, 2001;Dibenedetto and Grotzinger, 2005). Large accumulations of mudstone occur in Proterozoic carbonate platforms in a variety of depositional environments (Grotzinger, 1986a(Grotzinger, , 1989Knoll and Swett, 1990;James, 1994, 1996).…”