2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 2
Presentation Time: 1:45 PM


FOLK, Robert L., Department of Geological Sciences, Univ of Texas, Austin, Austin, TX 78712, rlfolk@mail.utexas.edu

Many previous workers over the past century have documented the occurrence of filamentous microbes 1-10 µm in diameter in ancient cherts. Despite an overwhelmingly biologic appearance these objects are still controversial and even construed as inorganic “artifacts” by some workers. This paper concentrates on cherts containing yet more contentious “organisms” in the typical size range of 0.03-0.1 µm: the nannobacteria. They are often visible in freshly broken siliceous specimens but can be brought out more clearly with a slight etch in HF. Some chert specimens contain no nannobacteria and are therefore inorganic in origin; in other specimens the organisms form discrete colonies, and in other examples they are as closely packed as beans in a bag and form the bulk of the rock. In the latter they must play an important role in the precipitation of silica.

The size of the nannobacterial cells in cherts from Archaean to Recent is the same as their size in limestones, phosphates, pyrite, clay minerals, etc., as well as in more exotic locales such as petrified wood, human arterial plaque (hydroxyapatite) and the Martian meteorite. Such a constant size range over such a wide variety of chemical environments and ages indicates they are minute organisms rather than inchoate inorganic nuclei. The clincher is that nannobacteria have been cultured by microbiologist R. J. C. McLean and grown on dead leaves, where they form colonies of 30-70 nm silica spheroids.

Cristobalite lepispheres, common precursors of chert, are composed of blades, which in turn are made of a single layer of 30-40 nm nannobacterial cells. Chalcedonic infills of fossil tests are composed of an arbor vitae-like arrangement of silica blades, likewise composed of 30 nm balls in the form of planar dendrites. Precious opal cement in sandstone from Leesville, La. consists of 50-100 nm balls with areas of precise geometrical stacking. Such regular stacking patterns are characteristic of some viruses. Whether produced by viruses, nannobacteria, or “normal-sized” microbes, there is a very important interaction between microbiologist effects and the precipitation of SiO2.