SILICICLASTICSILICICLASTICSEDIMENTARY ROCKSSEDIMENTARY ROCKS

Prepared by Dr. F. ClarkPrepared by Dr. F. Clark

Department of Earth and Atmospheric Department of Earth and Atmospheric Sciences, University of AlbertaSciences, University of Alberta

August 06August 06

INTRODUCTION - TEXTURESINTRODUCTION - TEXTURES

Just as with igneous rocks, the textures of siliciclastic Just as with igneous rocks, the textures of siliciclastic sedimentary rocks are involved in their sedimentary rocks are involved in their classification. As a first pass, the rock name classification. As a first pass, the rock name depends on the grain size, but other aspects of depends on the grain size, but other aspects of texture, namely shape and arrangement, are texture, namely shape and arrangement, are factors in further refinement of the name. In gross factors in further refinement of the name. In gross terms, three grain sizes, namely 2 mm, 1/16 terms, three grain sizes, namely 2 mm, 1/16 (0.0625) mm, and 1/256 (0.0039) mm, divide (0.0625) mm, and 1/256 (0.0039) mm, divide grains, and thus siliciclastic sedimentary rocks, into grains, and thus siliciclastic sedimentary rocks, into four size classes. Those four size classes four size classes. Those four size classes correspond to conglomerate, sandstone, siltstone, correspond to conglomerate, sandstone, siltstone, and claystone, in order from coarsest to finest and claystone, in order from coarsest to finest grain sizes.grain sizes.

Conglo-Conglo-merate.merate.A significant A significant proportion of proportion of grains (some grains (some sources sources suggest over suggest over 30%) is 30%) is larger than 2 larger than 2 mm in mm in diameter.diameter.

Loose sediment with this grain size characteristic is referred to Loose sediment with this grain size characteristic is referred to as gravel. In the sample above, the larger gravel-sized grains as gravel. In the sample above, the larger gravel-sized grains [yellow arrows] constitute the framework, whereas the smaller, [yellow arrows] constitute the framework, whereas the smaller, sand- and silt-sized grains constitute the matrix [blue arrows].sand- and silt-sized grains constitute the matrix [blue arrows].

Conglo-Conglo-merate?merate?Perhaps no Perhaps no more than more than 15% of the 15% of the grains grains exceed the 2 exceed the 2 mm lower mm lower size limit; size limit; most of the most of the grains are grains are sand-sized.sand-sized.This sample shows that the distinction between the size classes This sample shows that the distinction between the size classes can be somewhat arbitrary. A subtle change in the current which can be somewhat arbitrary. A subtle change in the current which produced the lamination [green arrow] in these finer gravels, and produced the lamination [green arrow] in these finer gravels, and from which this sediment was deposited, could have resulted in all from which this sediment was deposited, could have resulted in all grains being sand [purple arrows] or gravel [yellow arrows].grains being sand [purple arrows] or gravel [yellow arrows].

Conglomerate vs. BrecciaConglomerate vs. Breccia

In more traditional usage, the term conglomerate applies to In more traditional usage, the term conglomerate applies to those rocks with rounded clasts (left), whereas those rocks those rocks with rounded clasts (left), whereas those rocks with more angular grains (right) are referred to as breccia. with more angular grains (right) are referred to as breccia. The angularity of the grains on the right specimen is not The angularity of the grains on the right specimen is not pronounced, so the term breccia might not be appropriate in pronounced, so the term breccia might not be appropriate in this case.this case.

Conglomerate vs. DiamictiteConglomerate vs. Diamictite

In more current usage, the term conglomerate applies to those In more current usage, the term conglomerate applies to those rocks that are grain-supported, such as on the left; framework rocks that are grain-supported, such as on the left; framework grains are in contact [light blue arrows]. It is said to have an grains are in contact [light blue arrows]. It is said to have an intact framework. On the right, the rock is matrix-supported intact framework. On the right, the rock is matrix-supported [dark blue arrows] and the grains are not touching. This is [dark blue arrows] and the grains are not touching. This is called a diamictite, and is said to have a dispersed framework.called a diamictite, and is said to have a dispersed framework.

DiamictiteDiamictite..The high The high matrix matrix content [blue content [blue arrows] is arrows] is best seen best seen where where framework framework grains have grains have been plucked been plucked [yellow [yellow stars].stars].The large amount of matrix is sufficient to form durable external The large amount of matrix is sufficient to form durable external molds of missing framework grains. Such high matrix content is molds of missing framework grains. Such high matrix content is commonly associated with glacial activity, which does not commonly associated with glacial activity, which does not selectively remove the finer matrix grains, or flood episodes.selectively remove the finer matrix grains, or flood episodes.

Sand-Sand-stone.stone.The highest The highest proportion of proportion of grains lie in grains lie in the range the range between 2 between 2 mm and 1/16 mm and 1/16 mm. These mm. These rocks are rocks are also called also called arenites.arenites.

The yellow arrows point to individual grains which show up The yellow arrows point to individual grains which show up slightly darker than their neighbouring grains. Virtually all the slightly darker than their neighbouring grains. Virtually all the grains are of the stable silicate mineral quartz, and so this is a grains are of the stable silicate mineral quartz, and so this is a quartz sandstone or quartz arenite.quartz sandstone or quartz arenite.

More Quartz SandstonesMore Quartz Sandstones

The sample on the left shows lamination, parallel to the green The sample on the left shows lamination, parallel to the green arrow, reflecting subtle changes in colour due to trace amounts arrow, reflecting subtle changes in colour due to trace amounts of stain in the cementing material that holds the grains of stain in the cementing material that holds the grains together. Lighter coloured layers, lacking the stain, are together. Lighter coloured layers, lacking the stain, are highlighted by light blue arrows. Yellow arrows point to highlighted by light blue arrows. Yellow arrows point to individual grains. The right sample shows the uniform light individual grains. The right sample shows the uniform light appearance of many quartz arenites.appearance of many quartz arenites.

Lithic Lithic Arenite.Arenite.Lithic (from Lithic (from the Greek the Greek lithoslithos, , meaning meaning stone) stone) sandstone or sandstone or arenite is arenite is characterized characterized by abundant by abundant rock rock fragments.fragments.Because of the very high mechanical and chemical stability of quartz, Because of the very high mechanical and chemical stability of quartz, it will also usually be abundant in lithic sandstone. As a result, lithic it will also usually be abundant in lithic sandstone. As a result, lithic arenites characteristically have a “salt-and-pepper” look to them. arenites characteristically have a “salt-and-pepper” look to them. This example has traces of woody plant fossil matter [brown arrows].This example has traces of woody plant fossil matter [brown arrows].

Lithic Arenites/SandstonesLithic Arenites/Sandstones

These examples are from the Belly River Formation of Cretaceous These examples are from the Belly River Formation of Cretaceous age (on the order of 80 million years old), from the Foreland age (on the order of 80 million years old), from the Foreland Basin of Western Canada. In these classic salt-and-pepper lithic Basin of Western Canada. In these classic salt-and-pepper lithic arenites, the “pepper” is sand-sized grains of chert. Although arenites, the “pepper” is sand-sized grains of chert. Although chemically the same as quartz, chert is classified as a lithic grain chemically the same as quartz, chert is classified as a lithic grain or rock fragment by most sedimentologists, rather than as a or rock fragment by most sedimentologists, rather than as a variation of quartz.variation of quartz.

SiltstoneSiltstone..Silt-sized Silt-sized grains are by grains are by definition definition between 1/16 between 1/16 and 1/256 and 1/256 mm.mm.

This sample from the Spray River Group of Western Canada is This sample from the Spray River Group of Western Canada is quarried near Canmore as “Rundle Rock”, and is used as a facing quarried near Canmore as “Rundle Rock”, and is used as a facing stone in construction, especially common on upscale homes. Clearly, stone in construction, especially common on upscale homes. Clearly, individual grains are barely discernible without magnification.individual grains are barely discernible without magnification.

Lamination and Bedding in SiliciclasticsLamination and Bedding in Siliciclastics

These two views of the Spray River siltstone illustrate lamination These two views of the Spray River siltstone illustrate lamination [parallel to green arrows], which is basically a synonym for [parallel to green arrows], which is basically a synonym for layering. This characteristic of many sedimentary rocks is layering. This characteristic of many sedimentary rocks is produced by discontinuities (e.g. grain size, grain type, colour) in produced by discontinuities (e.g. grain size, grain type, colour) in sedimentation. Discreet units of sediment are bounded by sedimentation. Discreet units of sediment are bounded by bedding planes [blue arrows]; the layers are called beds if they bedding planes [blue arrows]; the layers are called beds if they exceed 1 cm in thickness.exceed 1 cm in thickness.

SiltstoneSiltstone..At these fine At these fine grain sizes, grain sizes, individual individual grains can grains can barely be barely be detected detected even with a even with a hand lens, hand lens, and only if and only if they are they are coarse silt.coarse silt.The next grain size working down from silt is clay, less than 1/256 The next grain size working down from silt is clay, less than 1/256 mm. It is not generally practical, even with significant magnification, mm. It is not generally practical, even with significant magnification, to distinguish between fine silt- and clay-sized grains. This practical to distinguish between fine silt- and clay-sized grains. This practical limitation gives rise to the two siliciclastic rock types that follow.limitation gives rise to the two siliciclastic rock types that follow.

MudstonMudstone.e.This term This term embraces embraces rocks with rocks with grain sizes grain sizes less than less than 1/16 mm, 1/16 mm, and what is and what is called blocky called blocky fracture, fracture, without without distinguishindistinguishing silt vs. clay.g silt vs. clay.The sample above is relatively thick, bounded above and below The sample above is relatively thick, bounded above and below by bedding planes [blue arrows], and has broken along irregular by bedding planes [blue arrows], and has broken along irregular failure surfaces unrelated to bedding [purple arrows] into the failure surfaces unrelated to bedding [purple arrows] into the three pieces, producing relatively thick, irregular chunks of rock.three pieces, producing relatively thick, irregular chunks of rock.

MudstonMudstone.e.At > 1 cm, At > 1 cm, the the mudstone mudstone slab slab constitutes a constitutes a bed, whose bed, whose upper and upper and lower lower bounding bounding surfaces surfaces [blue arrows] [blue arrows] are bedding are bedding planes.planes.The tendency of mudstones is to break along fracture surfaces The tendency of mudstones is to break along fracture surfaces [purple arrow] unrelated to both bedding [blue arrows] and [purple arrow] unrelated to both bedding [blue arrows] and lamination [green arrow]. Our understanding is that mudstones lamination [green arrow]. Our understanding is that mudstones do this because flat or platy grains are not aligned parallel to do this because flat or platy grains are not aligned parallel to each other and the lamination.each other and the lamination.

Shale.Shale.The term The term shale is shale is applied to applied to those rocks, those rocks, with grains with grains less than less than 1/16 mm, 1/16 mm, that are that are fissile, or fissile, or split into thin split into thin sheets, sheets, without without regard to silt regard to silt vs. clay.vs. clay.Again, we may not be able to distinguish siltstones from claystones Again, we may not be able to distinguish siltstones from claystones proper, so we classify the rock according to a gross textural proper, so we classify the rock according to a gross textural characteristic, namely how it breaks or splits. Our understanding is that characteristic, namely how it breaks or splits. Our understanding is that fissile rocks owe their character to parallel alignment of platy grains.fissile rocks owe their character to parallel alignment of platy grains.

Fissility ExpandedFissility Expanded

In these two views of a shale, we see bedding planes [blue arrows] In these two views of a shale, we see bedding planes [blue arrows] being exploited as planes of weakness [yellow arrows] that being exploited as planes of weakness [yellow arrows] that make this rock fissile. It must be pointed out that the parallel make this rock fissile. It must be pointed out that the parallel alignment of mineral grains that produces these planes of alignment of mineral grains that produces these planes of weakness occurs at the time of deposition, unlike the parallel weakness occurs at the time of deposition, unlike the parallel alignment that produces slaty cleavage in certain similar alignment that produces slaty cleavage in certain similar metamorphic rocks, in response to stress.metamorphic rocks, in response to stress.

Fissility – Not Always PlanarFissility – Not Always Planar

These views of a shale illustrate that the lamination of a shale, the These views of a shale illustrate that the lamination of a shale, the bedding planes of that shale [blue arrows], and the resulting bedding planes of that shale [blue arrows], and the resulting fissility [purple arrows] are not necessarily planar. The sea or fissility [purple arrows] are not necessarily planar. The sea or lake bottom is often characterized by an irregular surface that is lake bottom is often characterized by an irregular surface that is referred to as a bedform (ripples and dunes are examples). referred to as a bedform (ripples and dunes are examples). Bedform development is controlled by the interplay of sediment Bedform development is controlled by the interplay of sediment and waves or currents.and waves or currents.

Colour as Environmental IndicatorColour as Environmental Indicator

The different colours of these shale samples tell us something The different colours of these shale samples tell us something about the conditions at their environment of deposition. The about the conditions at their environment of deposition. The black colour of the left specimen is due to preserved organic black colour of the left specimen is due to preserved organic matter in an anoxic or anaerobic environment, whereas the matter in an anoxic or anaerobic environment, whereas the red sample on the right reflects oxidizing conditions that red sample on the right reflects oxidizing conditions that have turned the iron content red.have turned the iron content red.