Name: Princeton Chert Location: British Columbia, North America Age: Middle Eocence (49 mya)

Geological Setting and Age

Figure 3. The Earth during the Eocene epoch (NA: North America; E: Europe) (courtesy of UCMP)

The Princeton Chert has been dated as Middle Eocene (48.7 mya; Figure 3), through the use of radiometric techniques (Stockey, 2001). The Eocene represents the most recent time of greenhouse climates (Pearson et al., 2001). Indeed, Eocene environments were characterised generally by high annual temperatures, high wetness, and tropical or subtropical climates with a dry-wet cycle. This is shown by extensive limestone and lignite (poor-quality coal deposit) formations in North America and Middle Europe (Pearson et al., 2001).

The chert is part of the Allenby Formation, which has also yielded pollen and spores, mammals and fishes, all of which confirm the mid Eocene age (Stockey, 2001). The fossils and sediments indicate that the deposit preserves an aquatic floral community that grew in tropical to subtropical conditions (Pigg and Stockey, 1996). A few of the thinner chert layers grade into volcanic ashes across the outcrop (Figure 4), and it has been suggested that the fossil plants and animals were permeated with silicic acid due to adjacent volcanic activity. Mineral-charged water then seeped into low-lying basins, and thus surrounded the plants and their debris - where they grew - either in a small pond or lake environment (Little and Stockey, 2002). Preservation and hardening of the rock matrix may have been very quick, as shown by the fine preservation of the encased organic material. This may have occurred many times at the site, whereby a sedimentary basin allowed peat matrial to build up after entombment (Stockey et al., 1998).

Figure 4. View of Princeton Chert outcrop, demonstrating the gradation of the layers into volcanic ashes (left) across the outcrop (courtesy of RA Stockey)