7.2: Siliceous Sedimentary Rocks

As the name implies, siliceous sedimentary rocks are enriched in SiO₂. 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.

Sources of dissolved silica

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. 

Figure \(\PageIndex{1}\): Schematic view of the silicon cycle in the modern world ocean (input, output, and biological silicon fluxes). The white arrows represent fluxes of net sources of dissolved silicic acid (dSi) and/or of dissolvable amorphous silica (aSi) and of dSi recycled fluxes. Orange arrows correspond to sink fluxes of silicon (either as biogenic silica or as authigenic silica). Green arrows correspond to biological (pelagic) fluxes. All fluxes are in teramoles of silicon per year (Tmol Si yr⁻¹). Figure from Tréguer et al. via Wikimedia Commons; CC BY SA 4.0.

 

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 from diagenetic replacement when primary components of the rock are replaced by micrcrystalline quartz.

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.

Figure \(\PageIndex{1}\): Outcrop photographs of siliceous sedimentary rocks.  A) Gray, tan, and black Bedded chert in the Permian Phosphoria Formation (James St. John via Wikimedia Commons, CC BY 2.0). B) Black chert nodules in lthe Devonian Delaware limestone (James St. John via Wikimedia Commons, CC BY 2.0). C) Hematite, red chert (“jasper”), and specular hematite in the Precambrian Negaunee Iron-Formation of Michigan (James St. John via Wikimedia Commons, CC BY 2.0)  D) A nodular silcrete paleosol in the Pennsylvanian Waddens Cove Formation of Nova Scotia (Michael C. Rygel via Wikimedia Commons, CC BY-SA 3.0).