GeologyRocks

Evaporite

A chemical sediment or sedimentary rock that forms by precipitation from evaporating waters. Gypsum, halite, nitrates and borates are examples of evaporite minerals. The minerals precipitate out in reverse order of their solubility. For sea water this is:

1. Calcite (CaCO₃) and dolomite (CaMg(CO₃)₂)
2. Gypsum (CaSO₄-2H₂O) and anhydrite (CaSO₄).
3. Halite (i.e. common salt, NaCl)
4. Potassium and magnesium salts (sometimes referred to as Bittern Salts)

Flint

Microcrystalline quartz (SiO₂) found in chalk. Chert is the equivalent in any other rock, i.e. flint is chert.

Fluvial

Pertaining to a river or stream. A fluvial sandstone is a sandstone that was deposited in a river.

Folk's Classification

Folks classification of limestones based on the matrix/cement and the main lithological components. For example, a limestone with a micrite matrix containing mainly ooids would be Oomicrite. If the cement were sparite instead, then it would be an Oosparite. Folk used four main clast types: ooids, bioclast, peeloid and intraclast.

Formation

A formation is a group of beds used in stratigraphy. This is the smallest unit on a geological map. Formations are grouped together into a group. See also Member.

Gravel

A sediment grain with a diameter of 2-20mm

Group

A grouping of formations used in stratigraphy.

Limestone

A rock composed mainly of calcium carbonate CaCO₃. Limestones comprise a large variety of rocks including carbonate muds and oozes, chalks, ooliths and bioclastic limestone. They form in a variety of environments from deep sea (but above the carbonate compensation depth), temperate seas, tropical sea, lakes and rivers. Most caves are found in limestones due to erosion by acidic water. Limestones erode into karst topography.

Lithification

The process of turning loose sediment into a sedimentary rock. This is done by heat, pressure and time. This process can take from years to millions of years. Hardgrounds are formed in inter-tidal carbonates and can form in years. Usually this process is a slow one. If this process continues, metamorphism occurs.

Melange

A melange is formed in the accretionary wedge as sediment and oceanic crust is scraped off the descending plate in a subduction zone. The melange comprises a strongly brecciated unit with large blocks of pre-existing rocks in a deformed fine grained matrix. Olistostromes are similar units but have a gravity sliding origin rather than a tectonic one. For a melange to be described as such it must be of mapable size, contain 'exotic' clasts (i.e. not derived from immediately adjacent units) and be matrix supported.