Sedimentary Rocks Clues to the Past

Sedimentary Rocks Form from sediment that’s been weathered or precipitatedForm from sediment that’s been weathered or precipitated Provide clues to past environmentsProvide clues to past environments Cover about 75% of Earth’s surface.Cover about 75% of Earth’s surface. Only rock type that contains fossilsOnly rock type that contains fossils ripples Mud cracks/raindrops fossils

Types of Sedimentary Rocks DetritalDetrital Chemical Chemical – Derived from weathering of pre-existing rocks – Transported, deposited and lithified – Form from chemical components dissolved in water – precipitation from an aqueous solution

Detrital Rock Samples conglomerate shale sandstone

Chemical Rock Examples Chemical 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. Water Which cannot transport large grains? Wind

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 action

Depositional Processes Landslide vs. River Headwaters Plains Which would produce a poorly- sorted rock? Landslide

What Type of Sorting Would This Produce? Poorly sorted

Angularity Measure of how rounded the corners areMeasure of how rounded the corners are -Rounded indicates far from source

Depositional Environments

Angularity rounded vs. angular

Depositional Environments Mountain vs. beach (Near source) (far from source) Where would you most likely find rounded sediments? Beach

Sedimentary Structures Represent past environmentsRepresent past environments –Internal Structures StratificationStratification Cross-beddingCross-bedding Graded bedsGraded beds –Surface Structures RipplesRipples Mud cracksMud cracks Rain DropsRain Drops Sole marksSole marks –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 –Beds > 1 cm thick –Laminations - < 1 cm thick –Bedding planes – 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 directionLayering inclined at an angle – dips down in downcurrent direction 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 paleocurrent? Down Right 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 current –Slow as it loses energy –Produce turbidites – 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.asymmetrical Left-to-rightLeft-to-right or right-to-left

Surface Structures - Mudcracks Good 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” 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, skip along the bottom of channelTools (sticks, bones, rocks) carried by a current drags, bounce, skip along the bottom of channel Parallel to direction of currentParallel to direction of current

Which way is paleocurrent?top-to-bottom Side-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-right or right-to-left? Are we looking at top or bottom? bottom

Fossils Good indicator of type of environmentGood indicator of type of environment –Land vs. water