Sedimentary Rocks Clues to the Past. Sedimentary Rocks Form from sediment that’s been weathered or...
-
date post
20-Dec-2015 -
Category
Documents
-
view
215 -
download
0
Transcript of Sedimentary Rocks Clues to the Past. Sedimentary Rocks Form from sediment that’s been weathered or...
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