Chapter 6: Sedimentary Rocks

Sediment(s)

The collective name for loose, solid particles of mineral that originate from:

1. Weathering and erosion of preexisting rocks (detrital sediments)

2. (Chemical) precipitation from solution, including secretion by organisms in water (chemical sediments) (pg. 138)

(3. and sometimes by organisms pg. 165)

Gravel, Sand, Silt, Clay

All particles defined by grain size (pg. 165)

Sedimentary Rocks

-Formed from (1) eroded mineral grains, (2) minerals precipitated from low-temperature solution, or (3) consolidation of the organic remains of plants (pg. 142)

-Usually found in beds separated by bedding planes b/c the original sediments are deposited in horizontal layers (pg. 165)

How to sedimentary rocks form?**

LITHIFICATION of sediment, CRYSTALLIZATION from solution, or CONSOLIDATION of remains of organisms

Three Types of Sedimentary Rocks**

1. Detrital

2. Chemical

3. Organic

Physical Weathering**

-Processes that disintegrate solid rock into smaller pieces resulting in loose particles (clasts) of rock

Chemical Weathering**

-The reaction of minerals in rock to create new compounds and ions in solution

Erosion

Where moving water, ice, or wind loosens and removes material; takes place at earth's surface where rocks are exposed to air and water

Transport

-When sediment moves some distance by gravity, wind, water , or ice before coming to rest and settling into layers (pg. 139)

Deposition

-When transported material settles or comes to rest; occurs when the argent of transport loses energy and can no longer transport its load (pg. 140)

Burial

-The rock being buried underground

Diagenesis**

-The physical and chemical changes occurring during the conversion of sediment to sedimentary rock.

Lithification

-The general term for processes (compaction, cementation) that convert loose sediment into sedimentary rock (pg. 141)

Composition

-How reactive it is when it's being transported (it might be hard or easy to dissolve)

-High temp formation = easier to break down

-Governed by the rates of chemical weathering, mechanical weathering, and erosion; during transportation, grains can become rounded and sorted (pg. 165)

Texture

Current**

-The form and speed of transportation

-The stronger the current, the larger the material that can be transported

-Different current speeds causes sorting

Sorting

-The process by which sediment grains are selected and separated according to grain size (or grain shape or specific gravity) by the agents of transportation, especially by running water (pg. 139)

Rounding

-The breaking down of sediment, making it more round

-The grinding away of sharp edges and corners of rock fragments during transportation (pg. 139)

Abrasion

Physical Rounding

Dissolution

Chemical Rounding

Aeolian**

-Desert environment

-Means "wind derived"

Fluvial**

-Ephemeral streams deposit poorly-sorted sediments on flatland in front of mountains in arid climates

-Seasonal

Channel**

-Conglomerate and sands with unidirectional cross-beds

-Current ripples

Floodplain**

-Thin-bedded shale w/ mud cracks

-Fossil tracks

Deltas**

-Marine or non-marine

-Rivers empty into a sea, lake or other still water

-Wide variety of sub-environments, but material is finer further from the mouth of river

-Swampy areas w/ terrestrial fossils, coals

-Further from the mouth: marine fossils

Coastline**

-Interface between marine and terrestrial conditions

-Very ephemeral

Cross-Bedding

-Inclined beds deposited by currents of wind or water; caused by migration of grains that move as ripples or dunes; indicative of paleo-current directions

-Usually in sandstone in a thicker bed of rock, consisting of a series of thinner, inclined beds (pg. 153)

Graded Bedding

-Bed that has vertical change in particle size; can decrease in grain size (fining upward); can increase in grain size (coarsening upward)

-A layer w/ a vertical change in particle size, usually coarse grains at the bottom of the bed to progressively finer grains toward the top (pg. 157)

-Forms as coarse particles fall from suspension before fine particles due to decreasing water flow velocity in a turbidity current (pg. 165)

Turbidite

CHECK FIRST DEFF

-Graded bedding caused by deep sea gravity flow; coarsest particles concentrated at base; increasingly finer particles toward the top of the bed

-A turbulently flowing mass of sediment-laden water that is heavier than clear water and therefore flows downslope along the bottom of the sea or a lake (underwater avalanches) (pg. 157)

Mud Cracks

-Dried out mud reduces volume -> shrinks and cracks; paleo-environment and paleo-climate; must have sub-aerial exposure of mud

-A polygonal patten of cracks formed in very fine-grained sediment as it dries (pg. 157)

-Form in drying mud (pg. 165)

Compaction

-A decrease in volume and porosity of a sediment pile that occurs when the grains are squeeze together by the weight of overlaying sediments -> pushes out air and water (pg. 141)

-Sometimes this is all a rock needs

-Driven by burial and gravity

Cementation

-Minerals precipitated in the pores binds grains together -> precipitation or addition of new minerals cements sediment particles

-Cement partially or completely fills the pores, reducing the total amount of pore space further -> the loose sand forms a hard, coherent sandstone (pg. 141)

Siliciclastic (Detrital) Sediments/Sed Rocks

-Derived from weathering or existing rock

-Most prominent

-Formed from cemented sediment grains that are fragments of preexisting rocks; eroded mineral grains (pg. 142)

-Form mostly by compaction and cementation of grains; matrix can partially fill pore space of clastic rocks (pg. 165)

Biochemical Sediments/Sed Rocks

(Subtype of Chemical)

CHECK DEFF

-Formed by organisms who take elements and compounds directly from water to make their shells (forams, clams, etc.)

-Deposited by ORGANIC precipitation of minerals from solution (Ex: cementation of broken seashell -> limestone) (pg. 142)

-Ex: Limestone, Chert (pg. 149)

Chemical Sediments/Sed Rocks

CHECK DEFF

-Sediments form from supersaturated fluids

-Precipitation

-Deposited by INORGANIC precipitation of minerals from solution (Ex: seawater evaporates -> rock salt) (pg. 142)

-Ex: Limestone, Dolomite, Evaporites (rock salt, rock gypsum) (pg. 149)

Organic Sediments/Sed Rocks

CHECK DEFF

-Rocks that are composed of organic carbon compounds (Ex: compression of plant remains -> coal) (pg. 142)

Conglomerate

-Siliciclastic

-Rounded Clasts

-A coarse-grained sedimentary rock formed by the cementation of rounded gravel (pg. 142)

-Forms from sediment grains transported a short distance by a river or waves (pg. 165)

Breccia

-Siliciclastic

-Angular Clasts

-A coarse-grained sedimentary rock formed by the cementation of coarse, angular fragments of rubble (pg. 142)

Sandstone

-Formed by the cementation of sand grains; any deposit of sand can lithify into sandstone (pg. 143)

-Formed by sand deposited by a river, wind, waves, or turbidity currents (pg. 165)

Shale

-A fine-grained sedimentary rock notable for its ability to split into layers (fissility) (pg. 143)

-Forms from river, lake, or ocean mud (pg. 165)

Siltstone

-A rock consisting mostly of silt grains

-Coarser grain than shale

Chemical

Carbonates

Evaporites

-Evaporation or supersaturated fluid, minerals precipitate out in "situ"

-Calcium sulfate, sodium chloride

-Rocks formed from crystals that precipitate during evaporation of water; form from the evaporation of seawater or a saline lake (pg. 152)

What are the 6 surface processes and how do they interact to produce a sedimentary rock? Are these important for chemical rocks?

1. Weathering

2. Erosion

3. Transportation

4. Deposition

5. Burial

6. Diagenesis and Lithification

What is the difference between physical and chemical weathering? How do they break down a rock into sediment?

-Physical Weathering: processes that disintegrate solid rock into smaller pieces resulting in loose particles (clasts) of rock

-Chemical Weathering: the reaction of minerals in rock to create new compounds and ions in solution

How does current strength and distance from the source impact transportation of sediments? Sorting? Rounding? Composition?

-Current strength: the form and speed of the transport

-The stronger the current, the larger the material it's able to carry

-Different speeds cause different sorting

-Faster speeds -> more rounding

-Deposition starts when transport stops

-Sorting varies w/ distance from the source and current velocity

-More distance -> more round (depending on rocks hardness, cleavage, and composition)

-How reactive it is when it's being transported

What determines how well rounded a sediment becomes?

-Hardness

-Cleavage

-Composition

What determines a rocks maturity? How does this vary away from the source? What rocks types represent different maturity? Does this matter for chemical rocks?

-Texture and composition

-Texturally mature: fine grained, well sorted, well rounded

-Compositionally mature: quartz

-Mature: quartz

-Immature: lithic grains

What is the main difference between siliciclastic rocks and chemical/biochemical rocks?

Siliciclastic is formed from existing rocks; Siliciclastic is classified by grain size and chemical/biochemical is classified by the main mineral component (composition)

In what kind of EOD (environment of deposition) would you expect to see strong/medium/weak currents. What would the sediment size be?

Strong currents: large sediments (coarse)

Medium currents: medium sediments

Weak currents: small sediments (fine)

What do sedimentary structures tell us? What are some examples? In what EOD would you find each?

Sedimentary Structures: features that formed at the time of deposition

-They tell us features that formed at the time of deposition

-Bedding: reefs

-Ripples: channels

-Cross bedding: alluvial, channel, dunes beach barrier island, delta, shelves

-Graded bedding:

-Turbidite: deep marine

-Mudcracks: floodplain*, (playa) lakes

-Fossils: delta, lagoon, (playa) lakes, dunes beach barrier island, shelves, reefs, floodplain*