GeologyRocks

Member

A division of a formation, generally of distinct lithologic character and of only local extent.

Micrite

Micro-crystalline carbonate mud.

Olistostrome

An olistostrome is a sedimentary rock formation that comprises a chaotic assemblage of blocks of preexisting rocks, which can be of considerable size, in a finer grained matrix. Olistostromes are the result of submarine slumping or gravity sliding on an unstable shelf. A mélange is a similar rock unit but has a tectonic origin. The term olistostrome is derived from olio - a dish of many ingredients or stew; a mixture of heterogeneous elements; hodgepodge. and stroma - a bed covering (Greek equivalent of the Latin stratum).

Ooid

A spherical grain of calcium carbonate, either aragonite or calcite. See the Ooid Formation tutorial for more details of their formation and origin.

Sandstone

A clastic rock composed of sand-sized grains, i.e. between 0.625 and 2mm. The term usually implies a quartz rich composition, although this is not strictly necessary.

Sole Structures

Found in sedimentary rocks, a sole structure or sole mark is a cast found on the base of the rock unit. They are often well developed when a coarse grained rock overlies a fine grained rock. Sole structures can be formed by sticks or stones being carried by a current or by pebbles allowing a vortex to erode the underlying rock in places, forming a depression.

Sparite

Crystalline carbonate. Also known as sparry calcite.

Stratigraphy

The study of layered sedimentary or metamorphic rocks and how they related to each other, particularly their ages.

Turbidite

A deposit formed by a turbidity current. Turbidity currents are turbid, sub-aqueous, density flows driven by gravity. They can form on slopes of less than 1 degree and can flow uphill as they can travel at 100kph (60mph). They form when a unstable build-up of sediment is triggered to collapse due to a storm, earthquake or other triggering event.

Unconformity

A break in the stratigraphic record which represents a period of no sediment deposition. The interval of geologic time not represented is called a hiatus.

There are three types of unconformities: disconformity, nonconformity, and angular unconformity.