Because it is composed of light colored minerals, sandstone is typically light tan in color. Other elements, however, create colors in sandstone. The most common sandstones have various shades of red, caused by iron oxide rust.
In some instances, there is a purple hue caused by manganese. Loosely consolidated material can be mined using a high-pressure hydraulic monitor. Once drained, the sand reports to a plant. The greater the outcrop area of the source that produces the lithic fragment, the more likely it is to occur in sediment derived from that source. Location and relief of the drainage basin.
If the source is located close to the depositional basin, lithic fragments derived from the source are more likely to occur in the sediment. If the source area has high topographic relief, rates of erosion will be higher, and lithic fragments derived from the source will be more likely to occur in the sediment.
Stability of the rock fragment in the sedimentary environment. Fragments of mudrocks are relatively rare due to their mechanical weakness during transport. Similarly fragments of gabbros are rare in sandstones because the minerals they contain are chemically unstable in the sedimentary environment.
Because sandstones are usually cemented together with calcite or hematite, sandstone fragments break down easily during transport. The minerals that occur in granites, however, are more stable under conditions present near the Earth's surface, and thus granitic fragments are more common in sandstones. Volcanic rock fragments, with the exception of crystalline rhyolites, are generally unstable, but may occur if factors 1, 2 and 4 are favorable.
Size of the crystals in the fragments. In order to be present in a sandstone as a lithic fragment, the grain size of the minerals in the lithic fragment must be smaller than the grain size of the sediment.
Thus, granitic fragments will be expected to be rare, except in coarse sands, and volcanic and fine-grained metamorphic fragments will be expected to be more common. Accessory Minerals. Since it is possible that any mineral could be found in a sand or sandstone depending on the degree of mineralogical maturity, a variety of other minerals are possible.
Some of these can be useful in determining provenance of the sand. Thus the accessory minerals are usually referred to as heavy minerals. This is convenient because if the sandstone can be desegregated, then the heavy minerals can easily be separated from the quartz and feldspar on the basis of density.
The heavy minerals can be divided into three groups, as shown in the table below. Using this list, provenance of the sand can sometimes be determined to be from an igneous source or a metamorphic source.
Provenance of Accessory Minerals in Sandstones. Aegerine Augite Chromite Ilmenite Topaz. Mostly from Colorado River of Texas. Mexican Province - There are few studies of these sands, but they are expected to be similar to Rio Grande Province, reflecting a volcanic source. Glauconite occurs as green or brown sand-sized pellets in some quartz arenites, although sometimes the glauconite pellets make up a substantial portion of the rock.
Glauconite has the chemical formula - K,Na,Ca 1. The pellets are thought to originate as fecal pellets. They commonly occur in sands deposited in shallow water up to 2, m and are most common in Cambro-Ordovician and Cretaceous marine rocks, times when sea level was unusually high and the continents were flooded with epiric seas.
Because glauconite contains K, the sands can sometimes be dated by the K-Ar method of radiometric dating. Tectonics and Sandstone Compositions The main factor that creates the basins necessary to form clastic sedimentary rocks is tectonics. Once a basin is formed, the area surrounding the basin will shed its erosional debris and the sediment transported and deposited could form a sandstone.
Clues to the tectonic setting in which the basin formed may be left in this accumulated sediment. Quartz Cement. Quartz cement is most common in nearly pure quartz arenites. Such rocks generally only form in environments of high energy currents, such as beach deposits, marine bars, desert dunes, and some fluvial sandbars. Thus, it appears that most of the quartz cement is derived from the sands themselves or quartz sands in other parts of the section.
Calcite Cement. Calcite is the most common cement in sandstones, although when present, is doesn't tend to fill all pore spaces completely, but occurs as patchy cement. Calcite is soluble in surface waters, therefore calcite cemented sandstones often have their cement partially dissolved. Dissolution of the calcite cement results in secondary porosity. Carbonate ion also occurs in abundance in surface and groundwater, but is either derived by dissolution of carbonate minerals, or from bicarbonate ion HCO 3 - that results from dissolution of CO 2 gas in the atmosphere by H 2 O.
Hematite Cement. Only small amounts of hematite coating a mineral grain or rock surface is sufficient to give a red colored stain. Once the hematite precipitates it is very insoluble in water unless the water becomes highly reduced. Thus, the presence of hematite cement indicates an oxidizing environment during diagenesis.
Other Cements. Other cement forming diagenetic minerals can occur under special circumstances. H 2 O can from if the fluids are oxidizing and rich in sulfur. Authigenesis is when new minerals are crystallized in the sediment or rock during diagenesis.
The cross beds are probably large scale trough type resulting from the migration of large ripples. Antietam formation, Cambrian, Virginia. Tectonic Association Pure quartz sandstones, especially when abundant, form in areas of great tectonic stability.
This is because if any clastic sourcelands are available they will weather to produce at least shales, and possibly feldspars and lithics.
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