B Stromatolites

The observations have shown that consideration must be given to several different processes of carbonate deposition and/or silica or iron oxide deposition in contact which such bacterial mats. Obviously some important lithification processes take place within the decay zone below the active photosynthetic zone. In most of the cases where lithification was observed there, it was carbonate lithification of a type not related to the photosynthetic depletion of C02. Different filamentous and coccoid cyanobacteria can become more or less lithified depending on slime production, mobilization, outer morphology and microenvironments. 

According to observations on recent occurrences, a relatively high percentage of approximately 2 to 4% organic carbon of the initial concentration of 16 to 18% organic carbon is preserved within the final rock generated from the lithified mat. A central topic in this context is the tracing of molecules which survive the decompo- 

Capsular slimes and sheaths are a very conspicuous feature of microbial mats. Boon (1984) 521 has succeeded in isolating a few milligrams of sheath tubes from 

---

Figure 3 (a) The surface of the 3 Ga Earth, Steep Rock, NW Ontario, Canada. The hill-face is very close to a 3 Ga unconformity surface, and the rocks (granitoids and mafic dikes) exposed on the hill-face are immediately helow the unconformity. Above them are assorted sediments, including thick stromatolitic limestones, (b) Stromatolitic limestone, Steep Rock, Ontario, Canada (ca. 3 Ga old). The palaeohorizontal surface dips 70°. Stromatolitic domes are up to 4-5 m long and 2 m high. 

---