Gravitationally banded ('Uruguay-type') agates in basaltic rocks - where and when?


Authors: Petránek J

Published in: Bulletin of Geosciences, volume 79, issue 4; pages: 195 - 204; Received 28 June 2004; Accepted in revised form 30 September 2004;

Keywords: agates, gravitational banding, adhesional banding, basalts,

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The terms gravitational and adhesional banding of agates, as opposed to Uruguay, concentric, common, normal, and fortification banding, are recommended toward alleviating confusion and because they bear clear, easily distinguishable genetic implications. Gravitational banding results from the deposition of coagulated silicic acid by the force of gravity (i.e. like sedimentation), whereas adhesional banding forms by the adhesion of silica to the walls of vesicles. Gravitational banding is distinguishable from adhesional banding by greater band thickness and less distinct boundaries. Gravitational banding occurs especially in continental flood basalts (plateau basalts). Because of the enormous size of these flows, the increased terrestrial thermal flow was sustained, while the temperature of percolating fluids, mostly of meteoric origin, was raised mostly during the early stages of chemical weathering. The least stable constituents of the basalts are significantly affected by these conditions, and supply much monomeric silicic acid capable of diffusion. Thus, gravitational banding is most often confined to the bottom of agate accumulations.
The main factors that encourage the generation of gravitational banding are: (1) considerable lava thickness and prolonged thermal flow, (2) abundance of mineral constituents that decompose relatively easily, (3) increased carbon dioxide in the atmosphere and in the fluids circulating within the volcanics, (4) warm climate, and (5) sufficient precipitation.
The quantity of rainfall is of considerable importance, as shown by agates in the Permian mafic lavas of the Czech Republic and Germany, and in Triassic flows in Morocco. In the present article, it is suggested that the generation of both gravitational and adhesional banding is largely controlled by climate.