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
Monday, February 11, 13
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
Monday, February 11, 13
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.
Monday, February 11, 13
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
Monday, February 11, 13
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.
Monday, February 11, 13
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.
Monday, February 11, 13
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
Monday, February 11, 13
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
Monday, February 11, 13
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)
Monday, February 11, 13
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
Monday, February 11, 13
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
Monday, February 11, 13
Clastic Sedimentary Rocks Texture refers to the grain size of the individual clasts that
form the sedimentary rockWith increasing transport, average grain size decreases.
Monday, February 11, 13
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.
Monday, February 11, 13
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.
Monday, February 11, 13
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
Monday, February 11, 13
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
Monday, February 11, 13
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.
Monday, February 11, 13
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
Monday, February 11, 13
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
Monday, February 11, 13
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?
Monday, February 11, 13
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?
Monday, February 11, 13
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?
Monday, February 11, 13
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?
Monday, February 11, 13
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
Monday, February 11, 13
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
Trough cross bedding is produced by the downflow migration of lunate dunes in both subaqueous and subaerial environments
Monday, February 11, 13
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
These herringbone crossbeds have foreset beds that dip in both directions. What may have caused this?
Monday, February 11, 13
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
Monday, February 11, 13
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
Monday, February 11, 13
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?
Monday, February 11, 13
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?
Monday, February 11, 13
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
Monday, February 11, 13
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
Monday, February 11, 13
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
Monday, February 11, 13
Monday, February 11, 13