Header image  
A Marine Focus
Faunal Effects: The Chert Gap

Chert is composed predominantlyof silica. It originates, for a large, part from marine organisms that construct their skeletons of silica which they procure from seawater. An important organism such as this is the radiolarian, a single-celled eukaryote. These eukaryotes form tiny skeletons of opal, a form of silica. Upon death, radiolarian skeletons rain down upon the seafloor. If the accumulation is sufficient, the radiolarian ooze thick enough, these sediments may become compacted to form chert. Glass sponges also contribute to chert production. During the Early Triassic, there is a sudden chert gap; unusual because it follows a major episode of chert deposition, thirty million years long, in the Permian. (Murchey and Jones, 1992) (Beauchamp and Baud, 2002)

The early Triassic chert gap is seen, in the Sverdrup Basin of the Canadian Arctic, to be marked with finely laminated, pyrite rich siltstone. There are also small beds, up to two centimeters, of mud carbonate containing dissolved sponge spicules. From this it is inferred that chert deposition or its preservation has ceased. The dissolved sponge spicules suggest that ocean temperatures had warmed. The presence of pyrite reveals the anoxic nature of the water. The thinly laminated beds show a lack of bioturbation. (Beauchamp and Baud, 2002)

The chert gap of the Early Triassic lasted for between eight and ten million years. For the radiolarians, the gap was as long as thirteen million years. There is no chert in the Sverdrup Basin, even as small nodules. Indeed, no chert appears anywhere on Earth until the middle Triassic. (Raki, 2000)