Silcrete

Silcrete is an indurated (resists crumbling or powdering) soil duricrust formed when surface soil, sand, and gravel are cemented by dissolved silica. The formation of silcrete is similar to that of calcrete, formed by calcium carbonate, and ferricrete, formed by iron oxide. It is a hard and resistant material, and though different in origin and nature, appears similar to quartzite. As a duricrust, there is potential for preservation of root structures as trace fossils.

There are two types of silcrete that form in nature microcrystalline quartz and lacustrine silcretes. The microcrystalline forms in arid environments with increased levels of pH and are characterized by alterations of silicate cappings. These cappings are related to translocation of fine material and the silicification of those materials which occurs in phreatic fluctuations causing silica precipitation. Because of this formation the deposits in microcrystalline quartz silcrete have regular fine grained bands that appear in the rock. These types of silcretes form in environments that transition from wet to dry conditions. Lacustrine silcretes form in environments that have decreases in pH. They contain opal precipitation due to changes in pore fluid salinity these changes result in irregular opal nodules forming in the rock formation. This collection of opal nodules that form in this type of silcrete create irregular deposits of opal in the rock formation during the silicification process. It is most common for lacustrine silcretes to form in places that go from dry to wet conditions.

Silcrete is common in the arid regions of Australia and Africa often forming the resistant cap rock on features such as the breakaways of the Stuart Range of South Australia. Silcrete can be found at a lesser extent throughout the world especially England (e.g. Hertfordshire puddingstone and sarsen stone), and France. In the Great Plains of the United States, polished silcrete cobbles are locally common on the surface and in river gravels east of the outcrops of the Ogallala Formation.