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

Dibenedetto, S. and Grotzinger, J. (2005) Geomorphic evolution of a storm-dominated carbonate ramp (c. 549 Ma), Nama Group, Namibia. Geological Magazine, 142 (5). pp. 583-604. ISSN 0016-7568. doi:10.1017/S0016756805000890.

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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.

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Grotzinger, J.0000-0001-9324-1257
Additional Information:© 2005 Cambridge University Press. Received 25 April 2004; accepted 5 April 2005, published online October 4 2005. The Geological Survey of Namibia is gratefully acknowledged for providing a field vehicle and logistical support for many years of research in Namibia. Wolf Hegenberger, Charlie Hoffman and Roger Swart are thanked for help in providing an introduction to the geology of Namibia, and for providing helpful guidance and advice. Roger Swart and NAMCOR are thanked for providing the Landsat TM image in Figure 2. Special thanks go to Rob and Marianne Field and Roy Magson for access to their farms, Zebra River and Donkergange, and for their hospitality. Support for this research was provided by NSF grants EAR-9904298 and EAR-0001018. Nic Beukes and Roger Swart reviewed the manuscript and suggested helpful way to improve it.
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Subject Keywords:carbonate; geomorphology; platform; ramp; Proterozoic
Issue or Number:5
Record Number:CaltechAUTHORS:20141017-104450994
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Official Citation:S. DIBENEDETTO and J. GROTZINGER (2005). Geomorphic evolution of a storm-dominated carbonate ramp (c. 549 Ma), Nama Group, Namibia. Geological Magazine, 142, pp 583-604. doi:10.1017/S0016756805000890.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:50491
Deposited By: Tony Diaz
Deposited On:17 Oct 2014 18:10
Last Modified:10 Nov 2021 18:56

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