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Facies and sequence controls on the appearance of the Cambrian biota in southwestern Mongolia: implications for the Precambrian–Cambrian boundary

Published online by Cambridge University Press:  01 May 2009

J. F. Lindsay
Affiliation:
Australian Geological Survey Organisation, P. O. Box 378, Canberra, ACT, 2601, Australia
M. D. Brasier
Affiliation:
Department of Earth Sciences, Parks Road, Oxford University, Oxford OX1 3PR, UK
D. Dorjnamjaa
Affiliation:
Geological Institute, Peace Avenue, Ulaan Baatar, P. O. Box 863, People's Republic of Mongolia
R. Goldring
Affiliation:
PRIS, University of Reading, Whiteknights Park, Reading RG6 2AD, UK
P. D. Kruse
Affiliation:
Northern Territory Geological Survey, Darwin, NT, 0801, Australia
R. A. Wood
Affiliation:
Department of Earth Sciences, Downing Street, University of Cambridge, Cambridge CB2 3EQ, UK

Abstract

Neoproterozoic–Cambrian rocks of the Zavkhan Basin (Govi-Altay, western Mongolia) comprise large-scale alternations of siliciclastic- and carbonate-dominated units (cf. ‘Grand Cycles’). Analysis of such depositional sequences near the base of the Cambrian confirms that the distribution of trace fossils, small shelly fossils and calcimicrobial structures was strongly controlled by ecology and taphonomy, corresponding to specific points in a sea-level cycle. Evolution of the Cambrian biota is thus viewed through aseries of narrow time windows, once only for each depositional cycle. Correlation of the Precambrian–Cambrian boundary level on the basis of the first appearance of the Phycodes pedum assemblage is also fraught with difficulty, since stratigraphic resolution may be limited to a single sea-level cycle(c. 1–5 Ma). It is suggested that, in many cases, basin analysis will need to be undertaken before this boundary can be drawn.

Type
Articles
Copyright
Copyright © Cambridge University Press 1996

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