1. Sedimentary rocks have highly diverse origins and are difficult to classify,
but only three are very common: shale, sandstone and limestone. Seventy five percent of continental
surfaces are covered by sedimentary rocks.
2. Sediment is unconsolidated particles of either preexisting rocks or chemical
precipitates. It is classified by size: gravel> 2 mm< sand> 1/16
mm<"mud", without regard to composition, although most grains
of"mud" size are clay minerals.
3. Rounding (grinding away sharp edges) and sorting (separation by size)
occur during transportation, usually by streams. Size decreases downstream
in a river. Deposition occurs when agents of transportation lose their energy.
Preservation of sediments requires their burial and is favored in subsiding
basins.
4. Lithification converts loose sediment to sedimentary rock, usually by
compaction (reduces pore space) and cementation (fills remaining pore space).
These rocks have a clastic texture.
5. Not all sedimentary rocks form from sediment. Some form through crystallization
of minerals from solution (example, calcite). These rocks have a crystalline
texture, but that texture can also result from recrystallization that has
destroyed an originally clastic texture.
6. A section on types of sedimentary rocks expands the discussion of clastic
versus crystalline textures, and includes organic rocks as well. Clastic sedimentary
rocks are classified by grain size and composition. Breccia (angular) and
conglomerate (rounded) (the term "till" is deleted in 9th edition) have a gravel
fraction. Sandstones contain sand-size grains, distinguished as quartz sandstone
(>90% quartz), arkose (>25% feldspar), and graywacke (>15% matrix
= silt and clay). Graywackes result from deposition by turbidity currents.
Lithified silt forms siltstone, while combinations of silt and clay form shale.
Predominately clay-size particles form claystone and mudstone.
7. Limestone is composed mostly of calcite through the action of organisms
or as an inorganic precipitate. Varieties include coquina (cemented shells),
bioclastic limestone (coarse-grained fossils), chalk (very fine-grained bioclastic
limestone), oolitic limestone (small spheres of calcite), tufa and travertine
(crystalline precipitates) and recrystallized limestone (original texture
lost). Dolomite is a mineral CaMg(CO3)2
and a rock (sometimes called dolostone) that occurs as a replacement of limestone
and destroys its original texture.
8. Chert is a fine-grained, sedimentary rock composed almost entirely of
silica. It
can be a replacement, inorganic precipitate, or bioclastic, and may be recrystallized.
Evaporites are sedimentary rocks formed from evaporation of seawater. They
have crystalline textures and include rock gypsum and rock salt. Coal forms
from consolidation of plant material, originally as peat. Compaction transforms
peat to coal and several varieties are recognized. Organic material preserved
in marine muds change to oil and natural gas through increased heat and pressure
provided by burial.
9. Sedimentary structures form before lithification. Horizontal bedding planes
are the most common feature of sedimentary rocks and reflect original horizontality
and superposition. Cross bedding is inclined and most common in sandstones
as a reflection of wind or water currents. Graded bedding exhibits a vertical
change in grain size and characterizes turbidity current deposition. Mud cracks
require air-drying of very fine-grained sediments. Ripple marks are either
symmetric (waves) or asymmetric (currents) and form in any clastic rock. Fossils
are traces of plants or animals buried by sediment and preserved as unaltered
original material, replacements, molds or carbon films. Fossils may occur
in any sedimentary rock type, but are most common in limestones.
10. Formations are bodies of rock recognized as a convenient means to map,
describe and interpret the geology of a region. The first name is a geographic
location where it is well exposed, and the second name is its rock type. The
bounding surfaces are called contacts.
11. Source area of the sedimentary rock is determined by the composition
of its grains (for example feldspar, quartz and mica indicate a granitic source).
Sedimentary deposits thin away from their source, and sedimentary structures
may help determine direction of current flow.
12. Continental environments include glacial environments, alluvial fans,
river channels, that usually have a gravel component. Flood plains and lakes
usually develop shales. Dunes have high angle cross-bedding. Shallow marine
environments include deltas (usually with thick siltstone and shale, cut by
sandstone channels), beaches and barrier islands (well sorted, quartz sandstone),
dunes (high angle cross bedding), lagoons (shales), shallow marine shelves
(widespread sandstone, siltstone and shale), and reefs (massive limestone
cores). Deep marine environments receive deposition from turbidity currents.
13. The distribution of sedimentary rocks may be controlled by plate tectonics.
Convergent boundaries accumulate thick clastic deposits in sedimentary basins.
Turbidity currents dominate forearc basins, while sediments derived from rising
mountains fill backarc basins. These deposits may now be found in mountains
marking those plate boundaries. Transform boundaries may have organic rich
deposits, while diverging boundaries form rift valleys with gravels, lake
deposits and evaporites.
