Diamictites

Diamictite is a descriptive term for a clastic sedimentary rock defined by a highly heterogeneous mix of particle sizes, from clay to boulders, in a muddy or sandy matrix. The fabric is typically poorly sorted and may be massive or only weakly bedded; clasts can be angular to subrounded and may show glacial markings. Because it is a texture-driven label rather than a strict facies, diamictite encompasses a range of deposits produced by different processes.

Diamictites include both clast-supported and matrix-supported varieties. In many cases the coarse fraction is the dominant component (tillites or lodgement tills), with interstitial clay or silt forming the matrix. However, glaciomarine diamictites, debris-flow diamictites, and turbidites may also be described as diamictites when they exhibit a broad grain size distribution and poor sorting.

Historically, diamictites are frequently interpreted as glaciogenic, formed by subglacial or proximal glacial processes such as lodgement tills, debris flows, or sliding under ice, with meltwater or glacier-proximal environments contributing to the matrix. Non-glacial mechanisms can also generate diamictites, so lithology alone does not uniquely identify a glacial origin. In the geological record, diamictites are common in Neoproterozoic successions and have been associated with large-scale ice ages, though not all are tied to Snowball Earth events.

As a descriptive term, diamictite often requires additional sedimentological, isotopic, and paleontological evidence to determine origin and age. Distinguishing between tillites (lithified glacial tills) and diamictites that formed by other processes is a common interpretive step for field scientists.