Sedimentary Rocks
Classes of Sedimentary Rocks
Chemical Sedimentary Rocks (non-clastic) Organic Sedimentary Rocks (non-clastic) Clastic Sedimentary Rocks
Evaporites
Minerals left behind after evaporation
Well-sorted sediments
All grains are roughly the same size and shape
Massive Beds
Bed that have no internal structures, form when similar sediment is deposited for long periods of time or when a large amount of sediment is deposited at one time
Ripple marks
Caused by action of wind or water on sand or river bed
Two main processes convert loose sediment to sedimentary rock
Compaction Cementation
Sedimentary Rock Features
Depositional Environment Stratification Beds Massive Beds Cross-beds Graded bedding Reverse grading Ripple Marks Mud Cracks Fossils Concretions
Mud cracks
Form when muddy deposits dry and shrink; river's flood plain, dry lake bed
Sedimentary Rocks
Formed either by rock fragments or organic matter being bound together or by chemical precipitation
Chemical Sedimentary Rocks (non-clastic)
Forms from minerals that were once dissolved in water
Organic Sedimentary Rocks (non-clastic)
Forms from the remains of living things
Poorly-sorted sediments
Grains of many different sizes
Stratification
Layering of sedimentary rock; occurs when the conditions of sediment deposition
Sediment
Loose fragments of rock, minerals and organic material that result from natural processes, including the physical breakdown of rocks
Concretions
Lumps of rock that have a composition that is different from that of the main rock Form when minerals precipitate from fluids and build up around a nucleus
Cementation
Process in which sediments are glued together by minerals that are deposited by water As water moves through the sediment, minerals precipitate from the water, surround the sediment grains, and form cement that holds the fragments together.
Fossils
Remains or traces of ancient plants and animals are preserved in sedimentary rock
Clastic Sedimentary Rocks:
Rounded, pebble-sized fragments that are held together by cement
Characteristics of Clastic Sediments
Sediments are transported by four main agents: water, ice, wind, effects of gravity The speed the agent of erosion moves affects the size of sediment particles that can be carried and the distance that the particles will move.
Depositional Environment
Setting in which sediment is deposited Rivers, deltas, beaches, oceans
Cross-beds
Slanting layers; generally form in sand dunes or river beds
Reverse grading
Smallest grains are on the bottom; mudflows
Beds
Stratified layers, vary in thickness depending on the length of time during which sediment is deposited and how much sediment is deposited
Compaction
The process in which sediment is squeezed and in which the size of the pore space between sediment grains is reduced by the weight and pressure of overlapping layers.
Angularity
The shape of the fragments When particles first break from the source rock, they tend to be angular and uneven. Particles that have moved long distances from the source tend to be more rounded and smooth. The farther the sediment travels from its source, the finer and smoother the particles.
Characteristics of sedimentary rock are determined by
The source of the sediment The way the sediment was moved The conditions under which the sediment was deposited
Sorting
The tendency for currents of air or water to separate sediments according to size Well sorted, poorly sorted or somewhere in between
Graded bedding
Various sizes and kinds of materials are deposited within one layer; largest grains on bottom
Evaporation
When water evaporates, the minerals that were dissolved in the water are left behind. Eventually, the concentration of minerals in the remaining water becomes high enough to cause the minerals to precipitate out of the water.
