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.