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Probable calcified metaphytes in the latest Proterozoic Nama Group, Namibia: origin, diagenesis, and implications

Published online by Cambridge University Press:  14 July 2015

S. W. F. Grant ,
A. H. Knoll  and
G. J. B. Germs
S. W. F. Grant
Affiliation:
1Botanical Museum, Harvard University, Cambridge, Massachusetts 02138
A. H. Knoll
Affiliation:
1Botanical Museum, Harvard University, Cambridge, Massachusetts 02138
G. J. B. Germs
Affiliation:
2J. C. I. Research Unit, P.O. Box 976, 1760 Randfontein, South Africa
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Abstract

Samples from the Huns Limestone Member, Urusis Formation, Nama Group, at two adjacent localities in southern Namibia contain thin foliose to arched, sheet-like carbonate crusts that are 100–500 µm thick and up to 5 cm in lateral dimension. Morphologic, petrographic, and geochemical evidence supports the interpretation of these delicate crusts as biogenic, most likely the remains of calcified encrusting metaphytes. The original sediments of the fossiliferous samples contained aragonitic encrusting algae, botryoidal aragonite cements, and an aragonite mud groundmass. Spherulites within the precursor mud could represent bacterially induced mineral growths or the concretions of marine rivularian cyanobacteria. Original textures were severely disrupted during the diagenetic transition of aragonite to low-magnesian calcite, but some primary structures remain discernible as ghosts in the neomorphic mosaic. Gross morphology, original aragonite mineralogy, and hypobasal calcification indicate that the crusts are similar to late Paleozoic phylloid algae and extant peyssonnelid red algae. Structures interpreted as possible conceptacles also suggest possible affinities with the Corallinaceae.

Two species of Cloudina, interpreted as the remains of a shelly metazoan, are also known from limestones in the Nama Group. It is possible, therefore, that skeletalization in metaphytes and animals arose nearly simultaneously near the end of the Proterozoic Eon.

Type
Research Article
Information
Journal of Paleontology , Volume 65 , Issue 1 , January 1991 , pp. 1 - 18
Copyright
Copyright © The Journal of Paleontology 

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