ESC102

Sedimentary RocksOur keys to the past

Sedimentary Rocks

Sedimentary rocks are rocks that form through the accumulation of sediment.

Divided into two major categories:

•Detrital or Clastic sedimentary rocks are rocks that are made of the weathered remains of other rocks.

•Chemical sedimentary rocks are produced either through life processes (biochemical/bioclastic) or through precipitation of minerals in water environments

Why is the study of sedimentary rocks important to understanding Earth history?

Investigating Sedimentary rocks

In order to decode past environment using sedimentary rock we need to pay particular attention to:

texturescompositionfossils (if present)thicknessrelationships to other rocks

Preliminary interpretations in the field- redish rocks may have been deposited on land- whereas greenish rocks are more typical of marine

deposits- (caution: exceptions are numerous)

Detrital Sedimentary RocksDetrital sedimentary rocks are identified based on

mineral composition and texture.

Mineral Composition:

• Common minerals include quartzes, feldspars, and clay minerals

Mineral composition is only used to determine source rocks it does not indicate the manner of transport or deposition

For example:

• Quartz sand may have been deposited

• in a river system

• on a beach or

• in sand dunes

Detrital Sedimentary Rocks

Sediment Type Sediment Description

blocks Angular particles greater than 256 mm in size.

boulders Rounded particles greater than 256 mm in size.

cobbles Rounded particles between 64 and 256 mm in size.

pebbles Rounded particles between 2 and 64 mm in size.

sand Particles between .0625 and 2 mm in size.silt Particles between 2µm and .0625 mm in size.clay Particles less than 2µm in size.

Texture refers to the grain size of the clasts that form the sedimentary rock

Texture as an environmental indicator

Grain size gives some indication of the energy conditions during transport and deposition

• The larger the clast the greater the energy needed to transport that clast.

• Sand and larger clasts require vigorous currents (fast water= high energy)

• Silt and clay are transported by very weak currents and and therefore only accumulate under low-energy conditions such as in lakes and lagoons

Textural characteristics

Sorting refers to the variation in size of particles making up sediment or sedimentary rocksIt results from processes that

selectively transport and deposit sediments of particular sizes

If the size range is not very great, the sediment or rock is well sorted

If they have a wide range of sizes, they are poorly sorted

Other textural characteristicsRounding is the degree to which detrital particles have their sharp corners and edges warn away by abrasion

Gravel in transport is rounded very quickly as the particlescollide with one another

Sand becomes rounded with considerably more transport

Chemical Sedimentary Rocks

Divided into two categories:

Chemical sedimentary rocks are produced through precipitation of chemicals in water environments as a result of changing environmental conditions ie. temperature, pH, oxidizing conditions

Biochemical sedimentary rocks are produced as a direct result of life processes

Chemical Sedimentary Rocks

Composition of chemical sedimentary rocks is more useful in revealing environmental informationLimestone is deposited in warm, shallow seas

although a small amount also originates in lakes

Evaporites such as rock salt and rock gypsum indicate arid environments where evaporation rates were high

Coal originates in swamps and bogs on land

Sedimentary Structures

Sedimentary structures are visible features that formed at the time of deposition or shortly thereafter

• Represent manifestations of the physical and biological processes that operated in depositional environments

• Structures seen in present-day environments or produced in experiments help provide information about depositional environments of rocks with similar structures

Sedimentary Structures

Sedimentary rocks generally have bedding or stratification (they are deposited in layers)

Beds are layers that are thicker than 1 cm common in rocks with coarser grains

laminations are layers less than 1 cm thick are common in mudrocks

Sedimentary StructuresGraded bedding occurs when sediment loaded currents (turbidity

currents) experience a relatively quick drop in velocity.

what will happen to the sediment as the current slows?

Will all sediment sizes behave the same way?

Sedimentary StructuresCross-bedding forms when layers come to rest at an angle to the surface upon which they accumulate as on the downwind side of a sand dune

Tabular crossbeds:shallow dips such as in this image are indicative of currents enter still bodies of water (Gilbert Delta). Steep foreset beds are indicative of eolian (wind) deposition

Trough cross bedding is produced by the downflow migration of lunate dunes in both subaqueous and subaerial environments

These herringbone crossbeds have foreset beds that dip in both directions. What may have caused this?

Sedimentary StructuresSmall-scale cross bedding creates ripple marks. These structures are common on bedding planes, especially in sandstone

• Current ripple marks form in response to water or wind currents flowing in one direction

• have asymmetric profiles allowing geologists to determine paleocurrent directions

• Wave-formed ripple marks result from the to-and-fro motion of waves

• symmetrical profile

Sedimentary Structures

When clay-rich sediments dry, they shrink and crack into polygonal patterns bounded by fractures called mud cracks

Mud cracks require wetting and drying to form so in what environments would you expect to find them?

Sedimentary Structures

Biogenic sedimentary structures include

• tracks• burrows• trails

These structures are also called trace fossilsExtensive burrowing by organisms is called bioturbationmay alter sediments so thoroughly that other structuresare disrupted or destroyed

Depositional Environments

A depositional environment is anywhere sediment accumulates

Three broad areas of deposition include

•continental

•transitional

•marine

each of these areas has several specific environments with their own physical, chemical, and biological processes