Clastic Sedimentary Rocks

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Published on January 26, 2009

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Clastic Sedimentary Rocks : Clastic Sedimentary Rocks The specification states that you need to be able to: Identify, describe and explain the origin of clastic sedimentary rocks using observation of grain size, grain shape, degree of sorting, colour, mineral composition and texture. Breccia Conglomerate Sandstones Orthoquartzite Arkose Greywacke Micaceous sandstones Desert sandstones Mudstone Clay Shale Clastic Sedimentary Rocks : Clastic Sedimentary Rocks How might you define a clastic sedimentary rock? Made of clasts. What is a clast? A piece of a pre-existing rock. Slide 3: Strictly the length of its intermediate axis determines the size of a sedimentary grain. With large grains, such as pebbles, this can be easily measured. However, for finer grained sediments, such as sands, it would be a very difficult process. An easier method is by visual comparison with sand of known grain sizes. The grain sizes can be printed on a "grain size comparator”. GRAIN SIZE: Slide 4: Sediment grain size is described according to the "Wentworth Scale" . Coarse >2 mm Rudaceous Medium 1/16 - 2 mm Arenaceous Fine < 1/16 mm Argillaceous GRAIN SIZE: Slide 5: The term used to describe the overall shape of a grain is sphericity (how like a sphere it is). Another, and separate, aspect of grain shape is the degree of roundness. This relates to the extent to which originally angular edges and corners have been rounded off. GRAIN SHAPE: Slide 6: The range of terms used to describe the rounding of grains is : very angular Angular subangular subrounded rounded well rounded GRAIN SHAPE: Slide 7: Very often, sediments contain grain sizes belonging to several different classes on the Wentworth Scale. Sorting is an expression of the overall grain size of sediment. If most of the grains in sediment are of a similar size the sediment is well sorted. SORTING: Slide 8: If the range of grain sizes is great then the sediment is poorly sorted. Poorly sorted sediments were usually deposited rapidly whilst well sorted have taken time to be deposited. SORTING: Maturity in sedimentary rocks : Maturity in sedimentary rocks This can be divided into: Textural Maturity Compositional Maturity If a rock is texturally mature it will be well sorted with well rounded clasts. What does this suggest about it’s formation? Texturally immature will be poorly sorted with angular clasts. If a rock is compositionally mature it will only contain quartz. Why will it only contain quartz? Compositionally immature will have a wide range of clast compositions. Breccia : Breccia Coarse grained > 2mm. Very poorly sorted. Angular. Therefore texturally? Immature. Also tends to have various clast compositions and so is compositionally? Immature. Poor sorting suggests what? Angularity of clasts suggests what? Suggest environments of deposition. Conglomerate : Conglomerate Coarse grained > 2mm. Very poorly sorted. Rounded clasts. Therefore texturally? Immature. Also tends to have various clast compositions and so is compositionally? Immature. Poor sorting suggests what? Rounded clasts suggests what? Clast size suggests what? Suggest environments of deposition. Sandstones : Sandstones These are medium grained arenaceous rocks (1/16th – 2 mm). There are various kinds that you need to know: Orthoquartzite Arkose Greywacke Micaceous sandstones Desert sandstones Sandstones: Orthoquartzite : Sandstones: Orthoquartzite These are just very pure sandstones just made of quartz. They are therefore compositionally? Mature. How could they form? Lots of transport Desert conditions. Parent rocks were quartz rich. Sandstones: Arkose : Sandstones: Arkose Arkoses contain > 25% feldspar (mainly potash-feldspar). They are commonly reddish and cross-bedded. They have variable textural maturity. They commonly are derived from granite or gneiss. Semi-arid environments favour feldspar preservation (low chemical weathering), so they are commonly formed in warm and cool deserts. Sandstones: Greywacke : Sandstones: Greywacke Greywackes are texturally immature with > 15% matrix. They are also compositionally immature. Greywackes have a diverse mineralogy; they may contain quartz, feldspar, mica, clay minerals, mafic minerals, rock fragments (volcanic, metamorphic), with a matrix of carbonates, clay, pyrite, and organics. Greywacke Formation: : Greywacke Formation: The poor sorting suggests rapidly changing energy conditions and rapid deposition. The classic mechanisms are turbidity flows. These are rapid debris flows down the continental slope onto the abyssal plain. As the flow slows down sediment is deposited, coarse first and then fine last. This produces graded bedding. Sandstones: Micaceous Sandstone : Sandstones: Micaceous Sandstone These are sandstones that contain micas. Why are they unlikely to form in desert conditions? Because the micas will be blown (winnowed) out of the system. Therefore they form in water formed deposits especially rivers. They are usually close to the original rock that provided the mica. Why? Because mica usually breaks down rapidly so must be close. The mica often allows the rock to split (friable). Dune Sandstones : Dune Sandstones As you know desert sands are transported by wind (can be called aeolian deposits). They typically form sand dunes. The windward slope is lower angled <25° and the dip slope is < 40°. Sand Dunes : Sand Dunes Sand is driven up the windward side and then falls down the dip slope. This moves sand from the windward side and makes the dune migrate. Former positions of the dip slope are marked by “cross bedding”. This is one of the typical features of desert sandstone deposits. Cross bedding : Cross bedding The top of the cross bed is cut across by the bed above. The base of the cross- bed curves into the bed below. This allows you to work out “way up”. Desert Sandstones : Desert Sandstones As well as having cross bedding desert sandstones have the following features linked to their transport by wind. Well sorted Well rounded Frosted “Millet seed” texture They will also be red/brown/orange in colour. Sandstones: Desert Sandstones : Sandstones: Desert Sandstones Therefore desert sandstones are: Texturally? Mature. And compositionally? Mature. Mudstone : Mudstone The constituents of these rocks are varied: "Rock Flour" comes from glacial erosion or abrasion by running water or wind. New "Clay Minerals" can be produced by chemical weathering of feldspars and ferromagnesian minerals. Such fine grained sediments settle out only in low energy environments which exist in: currentless lagoons Lakes estuaries or in the oceans beneath the wave base. As the original mud deposit has water squeezed out it changes to a sticky compact clay and then to a hard mudstone or laminated shale. Clay : Clay By definition: “A smooth, earthy sediment or soft rock composed chiefly of clay sized particles and a significant content of Clay Minerals.” A good example is the London Clay. Shale : Shale Definition: A fine grained sedimentary rock that is fissile (can be split easily). A bit like slate but much softer. The clay particles have been aligned by compaction.

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