Presentation on theme: "Sedimentary Rocks Sediments form layers, called strata."— Presentation transcript:
Sedimentary Rocks Sediments form layers, called strata
6_11 Ions weather out of rock, are transported by groundwater to sediment layers below Ion-rich ground- water Ions transported to lake or ocean Dissolved ions precipitate to form cement between sediment grains Water enters pore spaces between sediment grains Sediment grains moved to ocean by streams LITHIFICATION Compaction and Cementation
6_5 Particles are large and irregular, and consist of a variety of lithologies, including the least resistant. Particles are mid-sized and of intermediate sphericity, and include resistant and nonresistant lithologies. Particles are small and nearly spherical, and consist mainly of the most resistant lithologies, such as quartz. Character of detrital sediments depends on time, distance, and energy. For example, in streams: HIGHLANDS LOWLANDS NEAR-COASTAL Sedimentary environment determines roundness sorting, mineral diversity
Examination of minerals present and their sorting, angularity, size and cements reveal the distance from the source and local chemistry
Remember: sand – 2mm Gravels coarser Muds less
Remember: sand – 2mm Gravels coarser Muds less
500 mm =.5 mm log base 2 of.5 is x -1 is 1 /08/log-calculator.html /08/log-calculator.html
Dott: % mud matrix
Sparry cement: clear, relatively coarse-grained calcite in the interstices of any sedimentary rock Micrite is a term used to describe lime mud, carbonate of mud grade. The term is also used in the Folk classification to describe a carbonate rock dominated by fine-grained calcite. Folk: bound by mud or precipitated calcite
restrain or regulate When bound :
Maturity Blatt and Tracy page 232 Recall that weathering makes smaller grains, and chemical weathering, especially hydrolysis, dissolves feldspars and micas leaving clays, metal ions, and dissolved silica. Quartz is not effected except for fragmentation and rounding. Thus a granite or granodiorite, or compositionally equivalent metamorphic rock, leaves behind quartz and clay. Ions and molecules in aqueous solution are the raw materials fro cements
Transport The clays are FLAT microcrystals and spend much of their time in suspension. Most end in deep marine sediments over the long run. DEMO sheet of paper Quartz fragments generally travel in the bed load, so they are deposited, and frequently buried, in a variety of non-marine and marine environments, preserving less common environments Without rift valleys, we wouldn’t know much about our own past.
Angularity Using these ratios then you can apply a name according to Folk (1974): Under 0.60 very elongate 0.60 to 0.63 elongate 0.63 to 0.66 sub-elonate 0.66 to 0.69 intermediate shape 0.69 to 0.72 sub-equant 0.72 to 0.75 equant Over 0.75 very equant
Grains become more rounded with time in transport
Fine sediments need still water to settle
What fills the spaces between grains?
Graywacke with matrixSandstone with hematite cement
Compaction (some dewatering) and cementation
Micritic – very fine grained; Acicular – needle like; Drusy - aggregate of minute crystals coating a surface poikilotopic a crystalline sedimentary rock having multisized crystals, the larger of which enclose small ones, often of a different mineral
Cements often control appearance
Sometimes Quartz from volcanic rocks (e.g. from Rhyolites) does NOT have undulatory extinction because it was not strained One crystal in the grain
Two or more crystals in the grain
Chert: many tiny “cryptocrystals” in the grain Chert grain - The subangular grain on the left is a chert fragment. Internally it is made up of a mosaic of very small crystals ('cryptocrystalline') of quartz. As you rotate this grain under polarized lights, there is a shimmering or flickering effect as each subgrain of quartz goes in and out of extinction
Blue filter light shining through gaps with no cement Low cement SS are often very friable
Here the cement completely fills gaps between grains
Chalcedony: microcrystalline Quartz
Quartz Overgrowths- In this quartzite the individual quartz grains are cemented together with a quartz (silicate) cement. Note that cement that precipitated onto a quartz grain grew in 'optical continuity' with its substrate. For example, the right quartz grain with the 'q' label seems to be a big white blob. However, there is a faint dark line that outlines where the original, rounded, grain boundary is. The outer most part of this grain is composed of quartz overgrowth that is also white in this photo. If you rotated this slide under polarized light you'd see that the entire grain, the ghost and its overgrowth, go extinct together.
Two phases of cementation - This is a quartz sandstone, seen under plain light. The quartz grains have quartz overgrowths (labeled 'o'), reflecting a first phase of cementation, and then, later, a period of carbonate cementation (c) seen as a brownish material filling the pore space.
Calcite cemented sandstone (specifically, a quartz arenite). Well-rounded quartz grains (Q) have euhedral quartz overgrowths beyond their original grain edges (arrows). Calcite (C) has filled remaining pore space. Note very high interference color of the Calcite.
Calcite corrosion of Quartz - The light yellowish blue colored material here is calcite cement. The black and speckled black blobs are quartz grains. The edges of the quartz grains are corroded and filled in with the calcite cement. Note that calcite tends to precipitate under high pH conditions, but that quartz tends to dissolved under these conditions. So with high pH conditions this type of erosion of the quartz grains with cementation of calcite can occur.
Volcanic lithic grain (Lv) - This grain, seen under plain light, is nicely rounded and internally you can see many, euhedral (i.e. with sharp angular crystal faces) microphenocrysts of plagioclase (note the twinning on these euhedral grains). This is a classic Lv grain, probably from a volcanic arc.
R = rock fragments Folk Classification
Embayed plagioclase and calcite cement - The plagioclase feldspars here (note twinning) do not have the nice perfectly round, smooth shape that they probably had originally. Instead the grains have been 'embayed' (partially dissolved away) and calcite cement (bright bluish mosaic) has filled in the embayment. This happens under high pH conditions, where calcite is precipitated and silicates are dissolved. Imagine there were high pH conditions (leading to dissolving the feldspar grains) but no carbonate rich ground water around (so no cement). You can see how this would develop porosity in the sandstone while it was buried underground. Such 'secondary porosity' can be abundant and makes those seeking hydrocarbons quite excited.
Litharenites are sandstones with a significant (>5%) component of lithic fragments, symbol R or RF.Lithic sandstones are usually associated with one specific type of lithic fragment (i.e., igneous, sedimentary, or metamorphic). Lithic sandstones form in a wide variety sedimentary depositional associated with active margins. This tectonic setting provides the source of the lithic fragments, either through arc volcanism, thin-skinned faulting, or continental collisions.
This can also occur when minerals are part of a sedimentary rock.