SILICICLASTICSEDIMENTARY ROCKS Prepared by Dr. F. Clark Department of Earth and Atmospheric Sciences, University of Alberta August 06
INTRODUCTION - TEXTURES Just as with igneous rocks, the textures of siliciclastic sedimentary rocks are involved in their classification. As a first pass, the rock name depends on the grain size, but other aspects of texture, namely shape and arrangement, are factors in further refinement of the name. In gross terms, three grain sizes, namely 2 mm, 1/16 (0.0625) mm, and 1/256 (0.0039) mm, divide grains, and thus siliciclastic sedimentary rocks, into four size classes. Those four size classes correspond to conglomerate, sandstone, siltstone, and claystone, in order from coarsest to finest grain sizes.
Conglo-merate.A significant proportion of grains (some sources suggest over 30%) is larger than 2 mm in diameter. Loose sediment with this grain size characteristic is referred to as gravel. In the sample above, the larger gravel-sized grains [yellow arrows] constitute the framework, whereas the smaller, sand- and silt-sized grains constitute the matrix [blue arrows].
Conglo-merate?Perhaps no more than 15% of the grains exceed the 2 mm lower size limit; most of the grains are sand-sized. This sample shows that the distinction between the size classes can be somewhat arbitrary. A subtle change in the current which produced the lamination [green arrow] in these finer gravels, and from which this sediment was deposited, could have resulted in all grains being sand [purple arrows] or gravel [yellow arrows].
Conglomerate vs. Breccia In more traditional usage, the term conglomerate applies to those rocks with rounded clasts (left), whereas those rocks with more angular grains (right) are referred to as breccia. The angularity of the grains on the right specimen is not pronounced, so the term breccia might not be appropriate in this case.
Conglomerate vs. Diamictite In more current usage, the term conglomerate applies to those rocks that are grain-supported, such as on the left; framework grains are in contact [light blue arrows]. It is said to have an intact framework. On the right, the rock is matrix-supported [dark blue arrows] and the grains are not touching. This is called a diamictite, and is said to have a dispersed framework.
Diamictite.The high matrix content [blue arrows] is best seen where framework grains have been plucked [yellow stars]. The large amount of matrix is sufficient to form durable external molds of missing framework grains. Such high matrix content is commonly associated with glacial activity, which does not selectively remove the finer matrix grains, or flood episodes.
Sand-stone.The highest proportion of grains lie in the range between 2 mm and 1/16 mm. These rocks are also called arenites. The yellow arrows point to individual grains which show up slightly darker than their neighbouring grains. Virtually all the grains are of the stable silicate mineral quartz, and so this is a quartz sandstone or quartz arenite.
More Quartz Sandstones The sample on the left shows lamination, parallel to the green arrow, reflecting subtle changes in colour due to trace amounts of stain in the cementing material that holds the grains together. Lighter coloured layers, lacking the stain, are highlighted by light blue arrows. Yellow arrows point to individual grains. The right sample shows the uniform light appearance of many quartz arenites.
Lithic Arenite.Lithic (from the Greek lithos, meaning stone) sandstone or arenite is characterized by abundant rock fragments. Because of the very high mechanical and chemical stability of quartz, it will also usually be abundant in lithic sandstone. As a result, lithic arenites characteristically have a “salt-and-pepper” look to them. This example has traces of woody plant fossil matter [brown arrows].
Lithic Arenites/Sandstones These examples are from the Belly River Formation of Cretaceous age (on the order of 80 million years old), from the Foreland Basin of Western Canada. In these classic salt-and-pepper lithic arenites, the “pepper” is sand-sized grains of chert. Although chemically the same as quartz, chert is classified as a lithic grain or rock fragment by most sedimentologists, rather than as a variation of quartz.
Siltstone.Silt-sized grains are by definition between 1/16 and 1/256 mm. This sample from the Spray River Group of Western Canada is quarried near Canmore as “Rundle Rock”, and is used as a facing stone in construction, especially common on upscale homes. Clearly, individual grains are barely discernible without magnification.
Lamination and Bedding in Siliciclastics These two views of the Spray River siltstone illustrate lamination [parallel to green arrows], which is basically a synonym for layering. This characteristic of many sedimentary rocks is produced by discontinuities (e.g. grain size, grain type, colour) in sedimentation. Discreet units of sediment are bounded by bedding planes [blue arrows]; the layers are called beds if they exceed 1 cm in thickness.
Siltstone.At these fine grain sizes, individual grains can barely be detected even with a hand lens, and only if they are coarse silt. The next grain size working down from silt is clay, less than 1/256 mm. It is not generally practical, even with significant magnification, to distinguish between fine silt- and clay-sized grains. This practical limitation gives rise to the two siliciclastic rock types that follow.
Mudstone.This term embraces rocks with grain sizes less than 1/16 mm, and what is called blocky fracture, without distinguishing silt vs. clay. The sample above is relatively thick, bounded above and below by bedding planes [blue arrows], and has broken along irregular failure surfaces unrelated to bedding [purple arrows] into the three pieces, producing relatively thick, irregular chunks of rock.
Mudstone.At > 1 cm, the mudstone slab constitutes a bed, whose upper and lower bounding surfaces [blue arrows] are bedding planes. The tendency of mudstones is to break along fracture surfaces [purple arrow] unrelated to both bedding [blue arrows] and lamination [green arrow]. Our understanding is that mudstones do this because flat or platy grains are not aligned parallel to each other and the lamination.
Shale.The term shale is applied to those rocks, with grains less than 1/16 mm, that are fissile, or split into thin sheets, without regard to silt vs. clay. Again, we may not be able to distinguish siltstones from claystones proper, so we classify the rock according to a gross textural characteristic, namely how it breaks or splits. Our understanding is that fissile rocks owe their character to parallel alignment of platy grains.
Fissility Expanded In these two views of a shale, we see bedding planes [blue arrows] being exploited as planes of weakness [yellow arrows] that make this rock fissile. It must be pointed out that the parallel alignment of mineral grains that produces these planes of weakness occurs at the time of deposition, unlike the parallel alignment that produces slaty cleavage in certain similar metamorphic rocks, in response to stress.
Fissility – Not Always Planar These views of a shale illustrate that the lamination of a shale, the bedding planes of that shale [blue arrows], and the resulting fissility [purple arrows] are not necessarily planar. The sea or lake bottom is often characterized by an irregular surface that is referred to as a bedform (ripples and dunes are examples). Bedform development is controlled by the interplay of sediment and waves or currents.
Colour as Environmental Indicator The different colours of these shale samples tell us something about the conditions at their environment of deposition. The black colour of the left specimen is due to preserved organic matter in an anoxic or anaerobic environment, whereas the red sample on the right reflects oxidizing conditions that have turned the iron content red.