SEDIMENTARY ROCKS/Chert 59 pH in the wetter months, resulting in silica precipitation Silica is initially deposited as magadiite (hydrated sodium silicate), which is subsequently replaced by silica Thus, the controlling factors are evaporation and freshwater input to the lake In the Coorong type from South Australia, Mg-rich carbonate lakes acquire a high pH due to the seasonal activity of photosynthetic algae, resulting in the dissolution of silicates; with a seasonal reduction in pH, direct precipitation of mixed opal and cristobalite takes place The Cretaceous Uhangri Formation of southwest Korea was deposited in an alkaline lake surrounded by alkaline volcanics The sequence includes couplets of sandstone overlain by chert, and of laminated chert with black shale The sandstone/chert couplets were deposited following episodic influxes of fresh, less alkaline water The influxes carrying sand produced density-current underflows in the stratified lake, depositing sand followed by opaline silica, caused by the fall in pH due to the influx of freshwater The laminated cherts are interpreted to be the result of interflows causing silica precipitation The chert beds show soft-sediment deformation and injection features, indicating a gelatinous consistency for the deposited silica Thus, with regard to the feature of direct silica deposition, this example has similarities with the Coorong type Ancient deposits interpreted as belonging to the Magadi type are more common, and range in age from the Precambrian Reitgat Formation, Hartbeesfontain, South Africa, to the present day In typical examples, there is an association with contemporaneous volcanics, and evidence for evaporite minerals Chert of Hydrothermal Origin Silica-rich fluid expulsion from basins Basin marginal faults are commonly the site of chert deposition as veins and porosity-filling cement Chert is deposited as a result of the cooling of silica-rich water expelled from the basin and rapidly rising up marginal fault zones Silica is more soluble at high temperatures, and hence cooling results in silica precipitation Chert may seal a fault, and subsequent fault movement may result in new fractures, which themselves become sealed; the result is a chert-cemented and veined fault zone Cherts resulting from hydrothermal systems Hydrothermal systems associated with volcanic activity are seen today at Geysir in Iceland, Yellowstone National Park in the USA, and North Island, New Zealand At these, and many other localities, hot springs and geysers deposit large quantities of silica both in the subsurface and at the point of eruption, which may be on land or under water Silica is deposited from cooling waters that have dissolved silica from hot rocks at depth (Figure 11) In the subsurface, the result is the silicification of country rocks, particularly along fluid pathways such as faults Cherty rock may develop on a large scale in the subsurface above a hydrothermal system, resulting in chert cement and cherty veins The silica is initially deposited as amorphous silica, and this matures to chert with time, heat, and burial Hot springs and geysers bring hot water to the surface that cools rapidly on eruption, resulting in the instant deposition of amorphous silica in the form of sinter Sinter may form mounds around geyser vents, or the outflow from a hot spring may result in sinter terraces or a low-angle sinter outwash apron (Figure 12) Under water, sinter chimneys may form above vents as occurs in Lake Yellowstone The silica is deposited as highly porous amorphous opal-A, which is transformed to opal-CT, and later to chert, with a loss of porosity In New Zealand, the stages of mineral transformation are well documented The Umikiri sinter is up to 15 m thick, can be dated to between 27 000 and 200 000 years bp, and shows a preserved silica maturation stratigraphy of opal-CT to opal-C to quartz with depth, all original opal-A having already been converted to opal-CT Thus, the textural features associated with phase changes and solution–precipitation phenomena occur in a geologically short period of time in near-surface environments Probably the best-known fossil hot spring deposit is the Early Devonian Rhynie Chert of north-east Scotland (Figures 1A,B, and 13) The chert beds were deposited as sinters on a low-angle run-off apron from hot springs fed along a marginal fault to the Rhynie Basin of Old Red Sandstone The beds are up to 0.5 m thick, laterally non-persistent, and with interbedded shale and sandstone of an alluvial plain environment The chert is generally bluish to brown in colour, and is remarkable for the early terrestrial and freshwater biota it contains The plants in some beds are preserved in three dimensions, with perfect cellular preservation, with plant axes still in the position of growth (Figure 13) to a height of 15 cm This chert has yielded the most diverse terrestrial and freshwater arthropod fauna of any locality of similar age in the world The detail of preservation is remarkable, including germinating plant spores and even sperm in the process of release from the male fertile organ of a gametophyte plant Such features require virtually instant preservation, and point to a silica gel as the primary silica deposit The presence of framboidal pyrite and the preservation of organic matter suggest reducing conditions during silicification The textures within the Rhynie Chert are closely comparable with