Weathering & Sedimentary Rock

weathering

• disintegration and decomposition of rock by physical & chemical processes

• can be man made processes

physical weathering

mechanical processes that break rock into smaller pieces

chemical weathering

processes b which minerals in rock are either changed into other minerals or are dissolved

transport

movement of weathered rock material by water, wind, or ice

erosion

weathering + transport

deposition

• what happens to eroded rock material when transport stops

• most likely moving again & again

• final stop is the ocean (unless natural event occurs)

Chemical & Physical Weathering Relationship

Chemical & physical weathering reinforce each other

• greater chemical weathering, the weaker the rock

• weaker the rock, the easier it is to break

• more breakage, the greater the surface area

• greater surface area*, the greater the chemical weathering & greater weathering speed

*not that pieces are smaller, but more surface area

Weathering Rates

Rock Properties

• composition/mineralogy

  • hardest rocks to weather are silicates

• structure

  • if the rock has layers, it’s easier to weather

Climate

• moisture

  • wet/moist environments makes weathering better

• acidity

  • smog

• temperature

  • increase in heat makes greater weathering

Burial in soil

• bacterial action

Topography

• slope

Duration

Weathering Silicate Minerals

• both residual products & materials in solution are forms of sediment

• residual products will form clasts in clastic sedimentary rock & material in solution will form the cement

Four major categories of chemical weathering

• solution

• hydrolysis

• oxidation

• hydration

common agent of chem weathering is water

Four categories of physical weathering

• thermal expansion & contraction*

• wedging processes

• abrasion

• unloading or pressure release

*very seldom directly weathers rock; sets up cracks that the types of wedging processes require

Where does acid come from?

• carbonic acid from atmospheric CO₂

• sulfuric & hydrofluoric emitted by volcanic eruptions

• human activity: mining, burning fossil fuels

solution weathering

• dissolving minerals such as calcite by the action of weak acids

• common with carbonate minerals such as calcite

• how most caves form

hydrolysis

weak electrochemical process that is the most common way to weather feldspar minerals

Oxidation

minerals reacting with chemically active oxygen in the atmosphere (common result is rust)

hydration

• adding water, most common way to weather evaporite minerals

• when it rains, salt dissolves & the salt flat becomes a shallow lake of salty water

• as water evaporites, salt crystals form & salt flat reappears

thermal expansion & contraction

• when rocks are subjected to differing temperatures, minerals within the rock can expand & contract at different rates

• can weather rock, but seldom actually is primary way the rock weathers; great at creating cracks in & between the minerals in the rock

• cracks set stage for physical weathering processes that involve wedging

frost wedging

water seeps into cracks in rocks, freezes, & expands to break rock apart

biological wedging

rocks breaking by action of living organisms growing in their cracks

salt wedging

salt crystallizing within pores of stone, weathers out existing grains causing a seemingly regular pattern of holes in the rock

abrasion

aka rubbing; sediment is being transported by wind, but sediment transport can also be by either water or ice

unloading or pressure release

• only happens in plutonic igneous rock

• rock cools underground under pressure

• when exposed by erosion, rock cracks apart as pressure is released or overlying rock is removed by erosion or unloaded

• cracks fom parallel to the erosional surface, & rock slabs slide off or exfoliate over time

weathering & carbon cycle

• carbon is present in the atmosphere as carbon dioxide gas (CO₂)

• combines with water to form carbonic acid, which weathers rock & the runoff in the oceans leads to limestone formation in the ocean, which is occurring in addition to the formation of limestone by the death & burial of marine organisms

• subduction transports the marine limestone into the mantle

• volcanic eruptions return CO₂ gas to the atmosphere, as well as carbon emissions by human activities such as agriculture, fires, & animal husbandry

3 main types of sediment

• clastic (aka detrital)

• chemical

• organic or biogenic

clastic sediment

weathering, transport, & deposition of rock

chemical sediment

chemical precipitation from solution

organic sediment

secretions of organisms in water or undissolved remains of organisms

sediment size from large to small

1. boulder

2. cobble

3. pebble

4. granule

5. sand

6. silt

7. clay

(6-7 need magnifier; grouped together to refer as mud)

(1-5 visible to naked eye)

major types of sandstone

• quartz sandstone/quartz arenite

  • 90%+ quartz

• arkosic sandstone/arkose

  • mostly quartz & at least 25%+ feldspars

• graywacke/lithic sandstone

  • quartz, feldspar, mafic minerals, rock fragments & clay

Clastic Sedimentary rock names (large to small)

1. conglomerate

2. breccia

3. sandstone

4. mudstone

5. shale

6. siltstone

7. claystone

changes caused by transport

• size

• roundness/angularity

• sorting

size

• longer/further transport = smaller size

• major way sediment is classified & basis for naming clastic sedimentary rock

roundess/angularity

longer/further transport = more rounded & less angular (less pointy)

sorting

longer/further transport = better sorted the sediment

3 types of sedimentary rock

• clastic or detrital

• chemical

• organic or biogenic

Limestone is a sedimentary rock that occurs as all 3 types

clastic or detrital sedimentary rock

formed by compaction & cementation of sediment

chemical sedimentary rock

precipitated from solution; frequently these rocks are carbonates (contain CO₃) & part of CO₂ atmospheric cycle or evaporates

organic or biogenic sedimentary rock

formed by the actions of living organisms or by compaction & cementation of dead ones

parts of sedimentary rock

• clasts

  • large material

• matrix

  • if clasts in rock are poorly sorted, the matrix are the smaller clasts

• cement

  • minerals that have grown between clasts & hold rock together

• pore space

• composition

  • origin of sediment

• texture & structuring

  • transport & depositional history

sedimentary rock patterns

• strata - bedding - layers

• graded bedding

• paleocurrent indicators

  • cross bedding

  • ripple marks

• geopetal structures

  • bioturbation

  • mud cracks

  • rain drop impressions

strata - bedding - layers

adjacent rock layer are distinct from others layers by their physical properties

graded bedding

layer showing a change in grain size from coarse at bottom to fine at top

cross bedding

layers are at an angle to the main bedding plane (indicates direction of wind or water flow)

ripple marks

wavy features of sand or other sediments (formed by wind or water)

bioturbation

mixing of sediment by living organisms

mud cracks

cracks form when wet, clay rich sediment dries out

rain drop impressions

small circular pits with raised edges by impact of rain drops

depositional environments

divided into 3 main categories, associated with distinctive physical or chemical indicators

depositional environment: continental or terrigenous environments

• streams (fluvial)

• lakes (lacustrine)

• alluvial (flood areas)

• desert (evaporite & aeolian)

• glacial

include both areas of deposition, areas of erosion, & areas where deposition & erosion occur depending upon circumstances; areas where either deposition or erosion can occur are areas where work occurs

depositional environment: coastal or transitional environments

• lagoons

• deltas

• beaches (littoral)

• tidal flats

include both areas of deposition, areas of erosion, & areas where both deposition & erosion occur depending upon circumstances; areas where either deposition & erosion can occur are areas where work occurs

depositional environment: marine environments

• reefs

• continental margin or near shore

  • comparatively shallow water

• benthic (deep water)

almost always depositional; oceans are ultimate depositional basins (exception of earthquakes)

continental or terrigenous environment: fluvial (streams)

• most landscapes seen on Earth are caused by streams

• when water flows fast, stream tends to pick up & transport sediment (erosion)

• as water slows, stream tends to deposit sediment

continental or terrigenous environment: lacustrine (lakes)

• predominately depositional

• sediments get deposited on lake bottom

continental or terrigenous environment: alluvial (floods)

• predominately depositional

• can be dry water course

• can move bigger sediments if water flow is fast

continental or terrigenous environment: desert

• can vary

• evaporite environments tend to be depositional

• aeolian environments have wind & tend to be erosional & depositional

continental or terrigenous environment: glacial

both depositional & erosional with work done by ice

coastal or transitional environments: lagoons & wetlands

• primarily depositional

• important as nurseries for many marine life

• separates shallow body of water from a larger body of water

coastal or transitional environments: deltas

• primarily depositional

coastal or transitional environments: littoral (beaches)

both erosional & depositional because of work done by waves

coastal or transitional environments: tidal flats

both erosional & depositional because of work done by waves

marine environment: reef

• usually depositional

• when sea levels fall, may become erosional because of work done by waves

marine environment: continental margins (near shore)

• comparatively shallow water-continental shelf & continental slope

• almost always depositional

• may be erosional when earthquakes start underwater landslides or when falling sea level helps create submarine canyons 

marine environment: benthic (deep-water)

almost always depositional

Facies

• patterns in deposition

• no depositional environment exists in a vacuum; eventually changes into a different depositional environment or an erosional environment

• sedimentary facies would include all physical, chemical, & possibly biological properties of rock including examination of these change with respect to adjacent rocks

• patterns are useful in determining both the depositional environment & figuring out the paleogeography of an area

sequences

rise and fall of sea level effects coastal depositional patterns

transgressive sequences

rising sea level - vertical sequence fines up aka onlap

regressive sequences

falling sea level - vertical sequence fines down aka offlap

Rock Naming Basics

• rocks get both first & last names

• when layer or unit of rock is identified, it’s called a “mappable unit”

  • layer is different enough from the rocks above it and below it to be easily identified as different

• if this mappable unit is made of the same type of rock; its last name is the type of rock

  • ex: Tapeats Sandstone is therefore a Sandstone

• if this mappable unit is a combination of different types of rocks, then its last name is “formation”

• multiple layers are described as “group”

• rock’s first name comes from local geography or folklore