GeologyRocks

Angular unconformity

An unconformity where horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers that are at an angle to the overlying horizontal layers.

Hutton's unconformity at Siccar Point is a classic example where Devonian sediments are deposited on Silurian turbidites that are tilted at 70°.

Bed

A centimetre to decimetre layer in a stratigraphy. Beds can be massive or contain structures, such as cross bedding, ripple marks or laminations. Beds are grouped into formations.

Bouma Sequence

The five part deposit produced by a waning turbidite flow. The deposit is split into:

• Structureless massive sand with gravel-rich erosional base.
• Parallel bedded sands produced by the upper flow regime.
• Cross laminated sands with ripple marks.
• Deposited in the lower flow regime.
• Laminated silts.Fine pelagic muds.

A typical Bouma sequence is illustrated below, although all five units may not be developed in each flow:

Breccia

A clastic rock composed of a matrix and angular clasts. A fine example of a breccia are the Devonian deposits at Siccar Point.

Carbonate

Carbonates are compounds in which one or more metallic or semi-metallic elements have combined with the carbonate radical (CO₃). It also refers to a sediment or a sedimentary rock formed by precipitation of carbon from an aqueous solution of carbonates (typically calcium, magnesium, or iron). Limestone is a carbonate rock. An common example of a carbonate mineral is calcite.

Chalk

A fine grained, white calcium carbonate composed of coccoliths. Common in the Cretaceous of western Europe.

The White Cliffs of Dover are a classic example of chalk cliffs.

Chert

Microcrystalline silica (SiO₂). Flint is the best known example.

Conglomerate

A clastic rock composed of a fine matrix and rounded class. This is in contrast to a breccia.

Disconformity

An unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition.

Dunham Classification

A classification of limestone based on depositional textures. The classification consists of mudstone (<10% grains, >90% mud), wackestone (>10% grains, mud supported), packstone (grain supported, but contains mud), grainstone (grain supported, no mud) and boundstone (grains bound in situ, i.e. reef).