diamictite

Diamictite is a clastic sedimentary rock characterized by poor sorting and a wide range of grain sizes within a single bed, from clay to boulders, embedded in a finer matrix. The material is often mixed and may be matrix-supported, with no uniform grain-size trend across the rock.

Textures and features: Diamictites are typically massive or weakly layered; clasts can be angular to subrounded and poorly sorted. The matrix is usually clay- or silt-sized. In some beds, dropstones and other ice-rafted debris features indicate an ice-contact depositional setting; in others, diamictites form from debris flows or turbidity currents.

Origins: The term covers a broad range of depositional processes. Many diamictites are glaciogenic and interpreted as subglacial till or glacimarine deposits, whereas others result from non-glacial processes such as debris flows, submarine landslides, or bottom currents. When derived from glacial till, lithified rocks are called tillites; diamictite remains the broader term when the provenance is not certain.

Geologic significance: Diamictites are widespread in Precambrian sequences and are used as indicators of past glaciation and related climate episodes, including Cryogenian Snowball Earth events. They are useful for understanding paleoenvironments and correlations between continents. See also: tillite, glaciomarine deposit, debris-flow deposit.

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