sedimentary rocks - this old earththisoldearth.net/esc102_lectures/06.pdf · clastic sedimentary...

ESC102

Sedimentary RocksOur keys to the past

Monday, February 11, 13

Sedimentary RocksSedimentary rocks are rocks that form through the accumulation of sediment and the process of lithification.

Lithification occurs after deposition and involves compaction and cementation.

Sedimentary rocks divided into two major categories:

•Detrital or Clastic sedimentary rocks are rocks that are composed 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


Clastic Sedimentary Rocks Cement – Minerals that fill sediment pores.

Fluids with dissolved solids flush through pore system. Dissolved ions slowly crystallize and fill pores.

Cementation varies from weak to strong. Common cements:

QuartzCalciteHematiteClay minerals


Sedimentary Cover Earth is covered by a thin ‘veneer’ of sediment. The veneer caps igneous and metamorphic “basement.” Sediment cover varies in thickness from 0 to 20 km.

Thinner (or missing) where ig and meta rocks outcrop.Thicker in sedimentary basins.


Sedimentary Rocks Sediments are the building blocks of sedimentary rocks. Four classes:

Clastic – Made from weathered rock fragments (clasts). Biochemical – Cemented shells of organisms.Organic – The carbon-rich remains of plants.Chemical – Minerals that crystallize directly from water.

ChemicalClastic OrganicBiochemical


Clastic Sedimentary Rocks Clastic sedimentary rocks reflect several processes.

Weathering – Generation of detritus via rock disintegration.Erosion – Removal of sediment grains from rock.Transportation – Dispersal by wind, water, and ice.Deposition – Settling out of the transporting fluid. Lithification – Transformation into solid rock.


Lithification – Transforms loose sediment into solid rock.Burial – More sediment is added onto previous layers.Compaction – Overburden weight reduces pore space.

Sand – 10 to 20%Clay – 50 to 80%

Cementation – Minerals grow in pores, “gluing” sediments.

Clastic Sedimentary Rocks


Investigating Sedimentary rocks

In this class you will learn how to decode past environmentusing sedimentary rock.

To do this we must pay particular attention to:texturescompositionfossils (if present)thicknessrelationships to other rocks

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

(caution: exceptions are numerous)


Clastic Sedimentary RocksClastic sedimentary rocks are identified based on

mineral composition and texture.

Mineral Composition:

• Common minerals clasts include quartz, feldspars, and clay minerals Mineral composition can only be used to determine the source rocks

and physical/chemical properties of the depositional environment.For example:

• Quartz sand may have been deposited

• in a river system

• on a beach or

• in sand dunes


Clastic Sedimentary Rocks Texture refers to the grain size of the individual clasts that

form the sedimentary rockWith increasing transport, average grain size decreases.


Clastic 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.


Clastic Sedimentary Rocks Rounding– Indicate degree of transport.

Fresh clasts are usually angular and non-spherical. Grain roundness and sphericity increases with transport.

Well-rounded – Long transport distances.Angular – Negligible transport.


Clastic Sedimentary RocksSorting refers to the variation in size of particles making up sediment or sedimentary rocks

It results from processes that selectively transport and deposit sediments of particular sizesIf the size range is not very great, the sediment or rock is well sorted- wind produces very well sorted sediments

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


Texture as an environmental indicator

Clast size allows us to determine the energy conditions that existed at the time the sediment was deposited.

• 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


Environmental Analysis

Rock A Rock BBased on textural clues answer the following question about each sediment:

1. Was the sediment deposited near or far from its source? Explain your reasoning.

2. Was the sediment deposited in a high or low energy environment? Explain your reasoning.

3. Was the sediment deposited by wind, water, or gravity? Explain your reasoning.


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 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 StructuresWhen 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?



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


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


Depositional EnvironmentsA 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


sedimentary rocks - this old earththisoldearth.net/esc102_lectures/06.pdf · clastic sedimentary...

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