Sedimentary Rocks

Clues to the Past

Sedimentary Rocks• Form from sediment that’s Form from sediment that’s

been weathered or been weathered or precipitatedprecipitated

• Provide clues to past Provide clues to past environmentsenvironments

• Cover about 75% of Earth’s Cover about 75% of Earth’s surface.surface.

• Only rock type that contains Only rock type that contains fossilsfossils

ripples

Mud cracks/raindropsfossils

Types of Sedimentary Rocks

• DetritalDetrital

• ChemicalChemical

– Derived from weathering Derived from weathering of pre-existing rocksof pre-existing rocks

– Transported, deposited Transported, deposited and lithifiedand lithified

– Form from chemical Form from chemical components dissolved in components dissolved in waterwater

– precipitation from an precipitation from an aqueous solutionaqueous solution

Detrital Rock Samples

conglomerate

shale

sandstone

Chemical Rock Examples

Chemical and biochemicalChemical and biochemical

Halite Limestone

Sedimentary Textures

• Grain SizeGrain Size

• AngularityAngularity

• SortingSorting

Grain Size

pebbles

sandstone clay

silt

• Amount of energyAmount of energy

• Mode of transportationMode of transportation

• May be classified according to grain sizeMay be classified according to grain size

Amount of Energy

Mode of Transportation

Wind vs. WaterWind vs. Water

Which cannot transport large grains?Which cannot transport large grains? WindWind

Sorting

• Distance traveled - reworkedDistance traveled - reworked

• Depositional processesDepositional processes

• Measure of the variation in the range grain sizesMeasure of the variation in the range grain sizes- Well-sorted indicate prolonged wind or water actionWell-sorted indicate prolonged wind or water action

Depositional Processes

Landslide vs. RiverLandslide vs. River

HeadwatersHeadwaters

PlainsPlains

Which would produce a poorly-Which would produce a poorly-sorted rock?sorted rock? LandslideLandslide

What Type of Sorting Would This Produce?

Poorly sortedPoorly sorted

Angularity• Measure of how rounded the corners areMeasure of how rounded the corners are

- Rounded indicates far from sourceRounded indicates far from source

Depositional Environments

Angularity

rounded vs. angularrounded vs. angular

Depositional Environments

Mountain vs. beachMountain vs. beach(Near source) (far from source)(Near source) (far from source)

Where would you most likely find rounded sediments?Where would you most likely find rounded sediments?

BeachBeach

Sedimentary Structures• Represent past environmentsRepresent past environments

– Internal StructuresInternal Structures• StratificationStratification• Cross-beddingCross-bedding• Graded bedsGraded beds

– Surface StructuresSurface Structures• RipplesRipples• Mud cracksMud cracks• Rain DropsRain Drops• Sole marksSole marks

– Fossils Fossils

Internal Structures – Stratification

• Most obvious feature in sedimentary rocksMost obvious feature in sedimentary rocks• Layers visible to different color &/or textureLayers visible to different color &/or texture

– BedsBeds > 1 cm thick > 1 cm thick– LaminationsLaminations - < 1 cm thick - < 1 cm thick– Bedding planesBedding planes – upper and lower surfaces of layers – upper and lower surfaces of layers

Internal Structures– cross-bedding

• Formed from moving wind or waterFormed from moving wind or water• X-beds form below ripples and dunesX-beds form below ripples and dunes• Layering inclined at an angle – dips down in downcurrent Layering inclined at an angle – dips down in downcurrent

directiondirection• Tangential (curve) at base and truncated (cut) at topTangential (curve) at base and truncated (cut) at top• Good paleocurrent and paleo“up” indicatorsGood paleocurrent and paleo“up” indicators

Cross Bedding

Which way is paleo“up”?Which way is paleo“up”?

Which way is paleocurrent?Which way is paleocurrent?

DownDown

Right to leftRight to left

Internal Structures – Graded Beds

• Coarser grains at bottom and finer upwardsCoarser grains at bottom and finer upwards• May indicate paleo“up”May indicate paleo“up”

Where would you find graded beds?

• Turbidity currentsTurbidity currents– Rapid sediment-laden currentRapid sediment-laden current– Slow as it loses energySlow as it loses energy– Produce Produce turbiditesturbidites – graded beds that fine upwards – graded beds that fine upwards

Surface Structures - Ripple Marks

• Symmetrical – oscillating ripplesSymmetrical – oscillating ripples• Asymmetrical – rivers or streamsAsymmetrical – rivers or streams

• Undulations caused by wind or waterUndulations caused by wind or water

• Good paleocurrent indicatorGood paleocurrent indicator

Ripples

• Symmetrical vs.Symmetrical vs. asymmetricalasymmetrical

• Left-to-rightLeft-to-right or right-to-leftor right-to-left

Surface Structures - Mudcracks

Good paleoclimate and paleo“up” indicatorGood paleoclimate and paleo“up” indicator

• Polygonal cracks occur when surface mud dries and contractsPolygonal cracks occur when surface mud dries and contracts

Surface Structure - Raindrops

Good paleoclimate and paleo“up” indicatorsGood paleoclimate and paleo“up” indicators

• Produced by impact of raindrops on soft mudProduced by impact of raindrops on soft mud

Sole Marks – tool marks

• Sole marks are structures preserved at the bottom of surfacesSole marks are structures preserved at the bottom of surfaces• Tools (sticks, bones, rocks) carried by a current drags, bounce, Tools (sticks, bones, rocks) carried by a current drags, bounce,

skip along the bottom of channelskip along the bottom of channel• Parallel to direction of currentParallel to direction of current

Which way is paleocurrent?

top-to-bottomtop-to-bottomSide-to-side orSide-to-side or ??

Sole Marks – Flute Marks

• Erosion or scouring of bottom sedimentErosion or scouring of bottom sediment• Irregular bulbous shapeIrregular bulbous shape• Good paleocurrent and paleo-“up” indicatorsGood paleocurrent and paleo-“up” indicators

Which Way is Paleocurrent?

Left-to-rightLeft-to-right or right-to-left? or right-to-left? Are we looking at top or bottom? Are we looking at top or bottom? bottom bottom

Fossils

• Good indicator of type of Good indicator of type of environmentenvironment– Land vs. waterLand vs. water