5.4: Diamictites, Pebbly Sandstones, and Outsized Clasts
- Page ID
- 26416
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Diamictites
Diamictites are poorly-sorted, generally matrix-supported conglomerates/breccias with 15-50% matrix that is a mixture of sand and mud. The presence of mud is important because if purely a product of bedload transport, the silt and clay grains should have been swept downstream by a current that was moving fast enough to transport gravel. Diamictites are important because they can only form in one of three ways: debris flows (slurry of sediment and water), glacial deposits (transport by viscous glacial ice), or bioturbation (biological mixing of discrete beds of mud/sand/gravel).
Pebbly Sandstones and Sandstones/Mudrocks with Outsized Clasts
In both of these cases, the eye is drawn to the coarse fraction, even if it represents only a small percentage of the rock. Although really a subset of mudrocks, we will discuss pebbly sandstones and mudstones here because our eyes are typically drawn to the coarse-grained fraction of the rock.
Pebbly sandstones typically occur in situations where flows are occasionally fast enough to transport gravel and winnow away sand but then slow to allow for the deposition of sand atop and between the lags of coarser-grained materials. These deposits are common along the base of channels or along scours or other erosional surfaces within sandstones. If the clasts are intraformational, its likely that they were locally derived and that erosion incorporated robust clasts derived from adjacent un- or partially-lithified sediments (ex: a river eroding its banks).
Video \(\PageIndex{1}\): Video of coarse sand transportation and the transport/deposition of pebbles within sandy sediment.
Mudstones and sandstones with exceptionally large clasts (outsized clasts) require a bit more explanation because, once again, there is a problem with hydrodynamics. Although bioturbation is possible, large, outsized clasts actually deposited within mudrocks or sandstones can typically only be attributed to ice rafting, rafting by tree roots, gastroliths derived from certain types of vertebrates or volcanic bombs. Context and close examination of the clasts might reveal which of the four options is most likely.
Figure \(\PageIndex{1}\): Photographs of diamictites, pebbly sandstones and mudstones. A) Tertiary debris flow deposit, California (Wilson44691 via Wikimedia Commons; public domain). B) Poorly-sorted glacial till outside of Whitefish, Montana. C) Former layers of mud, sand, and gravel have been completely homogenized to form this Permian diamictite. D) A coarse-grained pebbly sandstone where gravel is hydrodynamically concentrated along bedding planes. E) Although these mineralized mud-chip rip-up clasts in a sandstone are much bigger than any other clasts in this formation, they were locally derived from underlying mudrocks and don't require any special explanations. F) Glacial dropstone in a deep-marine black shale. Images B-F from Michael C. Rygel via Wikimedia Commons; CC BY-SA 3.0 or CC BY-SA 4.0.