7.2: Siliceous Sedimentary Rocks
As the name implies, siliceous sedimentary rocks are enriched in silica . We will avoid the numerous mineral and gem names applied to siliceous material (jasper, flint, agate, chalcedony, opal, etc.) and instead use the terms microcrystalline quartz or chert to refer to any/all of them.
Formation of siliceous sediments and sedimentary rocks
Ultimately, all dissolved silica can trace its origin back to the weathering and/or dissolution of existing silicate rocks or input from hydrothermal sources. Once in solution, there are several possibilities for the formation of siliceous sediments:
1. Direct precipitation from water has been reported in modern nonmarine settings and, although not active today, the abundance of chert in Precambrian banded iron formations suggests that this process was active in the ocean in the geologic past.
2. Biogenic silica forms when organisms such as sponges, radiolarians, and/or diatoms extract silica from the water to build their skeletons.
Common siliceous sedimentary rocks
Bedded cherts form in the deep ocean as siliceous skeletons rain down from shallow waters. Although siliceous organisms occur across the entirety of the ocean, their contribution is generally overwhelmed by calcareous skeletal material. However, in the deep ocean (below the carbonate compensation depth) or in areas where upwelling occurs, the water chemistry can result in the accumulation of siliceous oozes and environmental conditions that are not favorable for carbonate deposition.
Nodular cherts form in carbonates and other sedimentary rocks when primary components of the rock are replaced by microcrystalline quartz during diagenesis.
Banded Iron Formations (BIFs) are largely Precambrian deposits that are composed of alternating layers of chert and iron oxides (typically hematite or magnetite). Although the exact mechanism for BIF formation is not well understood, most workers agree that they reflect profoundly different ocean chemistry during the Precambrian (less oxygen) that allowed for upwelling of silica and iron-rich waters and regular changes in oxygen levels that resulted in the precipitation of iron oxides. Beyond that, there is ongoing discussion of the nature of diagenetic/metamorphic overprinting of iron-bearing phases and the direct/indirect involvement of microorganisms beyond the production of oxygen.
Silcretes are resistant duricrusts formed at or near the sediment surface by the accumulation of silica through pedogenic processes, groundwater precipitation, or evaporation in ephemeral lakes.