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Origin and diagenesis of Quaternary travertine shrub fabrics, Rapolano Terme, central Italy

Origin and diagenesis of Quaternary travertine shrub fabrics, Rapolano Terme, central Italy ABSTRACT Millimetre to centimetre sized arborescent shrub‐like calcite precipitates are common constituents of hot water travertine shallow pool deposits of Quaternary age at Rapolano Terme, Tuscany, Italy. In the presently forming travertines, the shrubs consist of apparently random associations of (i) micrite aggregates and (ii) subhedral to euhedral rhombic spar crystal aggregates. In thin section, the micrite aggregates appear dark and the spar‐rhomb aggregates light, giving the shrubs a mottled appearance. Travertines are basically produced by CaCO3 precipitation due to degassing and evaporation of the spring waters, although biological influence may also stimulate precipitation. The formation of masses of erect shrubs, rather than dense crystal crusts that form on slopes, is probably due to limited water flow in the pool environments. Microbes, including bacteria and diatoms, are important influences on shrub microfabric and external shape. The micrite aggregates are associated with bacteriform bodies, seen as tiny rods and spheres. The micrite precipitates around these bodies and in adjacent biofilm. Spar‐rhomb precipitation appears to be external to the biofilm, and may be related to the presence of diatoms which are locally closely associated with the spar‐rhombs, although an essentially inorganic origin, particularly for the more euhedral rhombs, cannot be ruled out. In the older Quaternary travertines, the original microfabric of the shrubs has been diagenetically altered. The original mottled appearance of the shrubs has become uniformly dark and micritic, and the evidence for the dual micritic and spar‐rhomb origin of the shrubs is obscured or destroyed. Spar‐micritization of the shrubs is probably due to abiotic, and locally biotic, dissolution. Previous studies did not recognize the diagenetic micritization and attributed shrub formation entirely to bacterial activity. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Sedimentology Wiley

Origin and diagenesis of Quaternary travertine shrub fabrics, Rapolano Terme, central Italy

GUO, LI; RIDING, ROBERT
Sedimentology , Volume 41 (3) – Jun 1, 1994
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References (65)

Publisher
Wiley
Copyright
Copyright © 1994 Wiley Subscription Services, Inc., A Wiley Company
ISSN
0037-0746
eISSN
1365-3091
DOI
10.1111/j.1365-3091.1994.tb02008.x
Publisher site
See Article on Publisher Site

Abstract

ABSTRACT Millimetre to centimetre sized arborescent shrub‐like calcite precipitates are common constituents of hot water travertine shallow pool deposits of Quaternary age at Rapolano Terme, Tuscany, Italy. In the presently forming travertines, the shrubs consist of apparently random associations of (i) micrite aggregates and (ii) subhedral to euhedral rhombic spar crystal aggregates. In thin section, the micrite aggregates appear dark and the spar‐rhomb aggregates light, giving the shrubs a mottled appearance. Travertines are basically produced by CaCO3 precipitation due to degassing and evaporation of the spring waters, although biological influence may also stimulate precipitation. The formation of masses of erect shrubs, rather than dense crystal crusts that form on slopes, is probably due to limited water flow in the pool environments. Microbes, including bacteria and diatoms, are important influences on shrub microfabric and external shape. The micrite aggregates are associated with bacteriform bodies, seen as tiny rods and spheres. The micrite precipitates around these bodies and in adjacent biofilm. Spar‐rhomb precipitation appears to be external to the biofilm, and may be related to the presence of diatoms which are locally closely associated with the spar‐rhombs, although an essentially inorganic origin, particularly for the more euhedral rhombs, cannot be ruled out. In the older Quaternary travertines, the original microfabric of the shrubs has been diagenetically altered. The original mottled appearance of the shrubs has become uniformly dark and micritic, and the evidence for the dual micritic and spar‐rhomb origin of the shrubs is obscured or destroyed. Spar‐micritization of the shrubs is probably due to abiotic, and locally biotic, dissolution. Previous studies did not recognize the diagenetic micritization and attributed shrub formation entirely to bacterial activity.

Journal

Sedimentology – Wiley

Published: Jun 1, 1994

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