Diamictite is a poorly sorted or non-sorted terrigenous non-calcareous sedimentary rock. Diamictite is composed of widely variable clast sizes from clay to boulders.
Diamictite is a nongenetic term (purely descriptive, no formation mechanism is implied). Most diamictites are glacial in origin (tillite), but some are volcanic (lithified lahars), submarine (turbidites), mass wasting breccias, etc
Matrix-supported conglomerates, also called diamictites, exhibit a disrupted, matrix-supported fabric; they contain 15 percent or more (sometimes as much as 80 percent) sand-size and finer clastic matrix. The coarse detrital clasts “float” in a finer-grained detrital matrix.
They actually are mudrocks in which there is a sprinkling of granules, pebbles, cobbles, or boulders, or some combination of them. Accordingly, they are sometimes referred to as conglomerate mudstones or pebbly mudstones.
Although matrix-supported conglomerates originate in a variety of ways, they are not deposited by normal currents of moving water. Some are produced by submarine landslides, massive slumping, or dense, sediment-laden, gravity-driven turbidity flows. Matrix-supported conglomerates that can be definitively related to such mechanisms are called tilloids.
Tilloids commonly make up olistostromes, which are large masses of coarse blocks chaotically mixed within a muddy matrix. The terms till (when unconsolidated) and tillite (when lithified) are used for diamictites that appear to have been directly deposited by moving sheets of glacial ice. Tillites typically consist of poorly sorted angular and subangular, polished and striated blocks of rock floating in an unstratified clay matrix.
The clasts may exhibit a weak but distinct alignment of their long axes approximately parallel to the direction of ice flow. Tillites are notoriously heterogeneous in composition: clasts appear to be randomly mixed together without respect to size or compositional stability. These clasts are derived mainly from the underlying bedrock. Extremely coarse, far-traveled blocks and boulders are called erratics.
Other rarer diamictites, known as laminated pebbly (or cobbly or bouldery) mudstones, consist of delicately laminated mudrocks in which scattered coarser clasts occur. Laminations within the muddy component are broken and bent. They are located beneath and adjacent to the larger clasts but gently overlap or arch over them, suggesting that the coarse clasts are dropstones (i.e., ice-rafted blocks released as floating masses of ice melt).