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Microbial–metazoan reefs of the terminal Proterozoic Nama Group (c. 550–543 Ma), Namibia

Grotzinger, J. P. and Adams, E. W. and Schröder, S. (2005) Microbial–metazoan reefs of the terminal Proterozoic Nama Group (c. 550–543 Ma), Namibia. Geological Magazine, 142 (5). pp. 499-517. ISSN 0016-7568. https://resolver.caltech.edu/CaltechAUTHORS:20141009-154142776

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Abstract

Thrombolite and stromatolite reefs occur at several stratigraphic levels within the terminal Proterozoic Nama Group (c. 550–543 Ma) of central and southern Namibia. The reefs form integral parts of several carbonate platforms within the Nama Group, including the Kuibis platform of the northern Nama Basin (Zaris subbasin), and Huns platform (Witputs subbasin) of the southern Nama Basin. The reefs are composed of both thrombolites and stromatolites that form laterally continuous biostromes, isolated patch reefs, and isolated pinnacle reefs ranging in scale from a metre to several kilometres in width. In the majority of cases, the reefs occur stratigraphically as an integral facies within the transgressive systems tracts of sequences making up the Kuibis and Huns platforms. This suggests that a regime of increasing accommodation was required to form well-developed reefs, though reefs also occur sporadically in highstand systems tract settings. Within a given transgressive systems tract, a regime of increasing accommodation through time favours the transition from sheet-like biostromal geometries to more isolated patch and pinnacle biohermal geometries. Similarly, increasing accommodation in space, such as a transect down depositional dip, shows a similar transition from more sheet-like geometries in updip positions to more isolated geometries in downdip positions. Reefal facies consist of thrombolitic domes, columns and mounds with well-developed internal clotted textures, in addition to stromatolitic domes, columns and mounds, with crudely to moderately well-developed internal lamination. Stromatolites are better developed in conditions of relatively low accommodation, and updip locations, under conditions of higher current velocities and greater sediment influx. Thrombolites are better developed in conditions of relatively high accommodation and low sediment influx. Both types of microbialites are intimately associated with the first calcifying metazoan organisms, which may have attached themselves to the sediment surface or otherwise lived within sheltered depressions within the rough topography created by ecologically complex mats. The appearance of thrombolitic textures during terminal Proterozoic time is consistent with colonization of cyanobacterial mats by higher algae and metazoans, which would have been an important process in generating clotted textures. Fabrics in the Nama thrombolites are well preserved and show evidence of thrombolitic mesoclots being overgrown by fibrous marine carbonate, interpreted as former aragonite. This was followed by emplacement of geopetal micrite fills, and precipitation of dolomite as an isopachous rim cement, followed by occlusion of remaining porosity by blocky calcite spar.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1017/S0016756805000907DOIArticle
http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=351103&fileId=S0016756805000907PublisherArticle
ORCID:
AuthorORCID
Grotzinger, J. P.0000-0001-9324-1257
Additional Information:© 2005 Cambridge University Press. Received 6 April 2004; accepted 31 March 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 8. 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. Maurice Tucker and Rachel Wood are acknowledged for helpful reviews of the manuscript.
Funders:
Funding AgencyGrant Number
NSFEAR-9904298
NSFEAR-0001018
Subject Keywords:carbonate; reef; Ediacaran; thrombolite; Proterozoic
Issue or Number:5
Record Number:CaltechAUTHORS:20141009-154142776
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20141009-154142776
Official Citation:J. GROTZINGER, E. W. ADAMS and S. SCHRÖDER (2005). Microbial–metazoan reefs of the terminal Proterozoic Nama Group (c. 550–543 Ma), Namibia. Geological Magazine, 142, pp 499-517. doi:10.1017/S0016756805000907.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:50321
Collection:CaltechAUTHORS
Deposited By: Jason Perez
Deposited On:13 Oct 2014 23:24
Last Modified:03 Oct 2019 07:22

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