Sedimentary Rocks, Fossils & Relative Dating Techniques.
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Transcript of Sedimentary Rocks, Fossils & Relative Dating Techniques.
![Page 1: Sedimentary Rocks, Fossils & Relative Dating Techniques.](https://reader035.fdocuments.in/reader035/viewer/2022081512/56649ebc5503460f94bc5c05/html5/thumbnails/1.jpg)
Sedimentary Rocks, Fossils & Relative Dating Techniques
![Page 2: Sedimentary Rocks, Fossils & Relative Dating Techniques.](https://reader035.fdocuments.in/reader035/viewer/2022081512/56649ebc5503460f94bc5c05/html5/thumbnails/2.jpg)
How do geologists know about events like the extinction of the
dinosaurs and what the climate of earth was like in the past?
• By studying the rock record!• Fossils, types of rocks, sequence of rocks,
radioactive minerals in rocks all tell a history of earth
• Why do we care?
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What are the 3 major types of rocks?
• Sedimentary– Bits of rock, mineral or organic material deposited
by/in fluids like water, air or ice. Usually cemented together by mineral “glue”
• Igneous– Minerals that grew together out of hot molten rock
and cooled into a solid rock• Metamorphic– Rocks formed when heat and or pressure changed the
minerals over time into a new and different form
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How are sedimentary rocks formed?
• Depositioncompactioncementation– Sediment (bits of mimeral, rock, or oganic matter)
is deposited in layers – Over time, the layers of sediment get buried.– weight of the layers of sediment compacts it
together– Often, water flows through carrying dissolved
minerals and deposits mineral “glue” in the holes that cements it together
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• Conglomerate• Large roundish
peobbles in a smaller grained cement. You can always see the individual pebbles
• #4• Very fast moving rivers,
active wave coastlines
Clastic Sedimentary Rocks
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Clastic Sedimentary Rocks
• Sandstone• “sand”sized grains
“cemented” together. Sometimes hard to see individual grains but feels rough like sandpaper.
• #3• Forms in: medium fast
moving rivers, typical coastlines, deserts with sand dunes
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Other Sedimentary Rocks
• If not made from from broken up bits of rock and mineral they are called NON-clastic
• Made from remains of living things (leaves, shells etc.) or precipitated minerals from water (like salt)
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Clastic Sedimentary Rocks
• Shale• “mud” sized particles that you
can not see with your naked eye. Feels smooth to the touch. Great for storing fossils
• #5• Forms in slow moving
streams, just off-shore of the waves of the ocean, at the bottom of swamps and lakes on land
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What are fossils• Preserved remains of living things– Shells, footprints, bone shaped rocks, whole insect
• Can be formed in many ways, typically:– Organism dies near/under water – Body does not decompose fully before being covered by
sediment (or leaves hard parts)– Water flows through and changes the body parts into
mineral material by replacing each cell, turning it to rock.
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What type of rocks have fossils?• Sedimentary rocks are the best
preservers of fossils, sometimes they will survive the heat and pressure and be in metamorphic rocks.
• Ice, hardened sap (amber), ash
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What conditions favor fossil formation?
• Presence of hard parts• Rapid burial – beneath sediment or in some other material like
sap or ice or ash that preserves the organism from decay due to exposure to air and microbes
– Ocean, swamp, lake are ideal
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Echinodermata: Crinoid
• A filter feeding, shallow sea-living, plant-like animal!
• Paleozoic
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Anthozoa: Heterophrentis
• “Horn coral”• Bottom of shallow ocean
living plant-like animal…like a sea anemony
• Soft tenticals that organism used to sweep food into its mouth not preserved
• Each wrinkle represents 1 day!
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Trilobita: Phacops Rana
• “trilobite”• Shallow sea dwelling
arthropod with a hard exoskeletan (like a lobster)
• From 1 mm-1m lots of types
• Some swam, some crawled
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Brachiopoda: Mucrospirifer mucronatus
• shallow sea floor dwelling
• Fed on tiny organic “snow” floating in ocean water
• paleozoic
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Sedimentary rocks are useful to geologists trying to figure out earth
history because of 2 things…
• They contain fossils! – Can tell about life and environment on earth
• They are laid down in layers in a predictable way with the oldest at the bottom– Can help to sequence events on earth
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Relative Dating Principles
• Sequencing events…older than, younger than• Principle of superposition– Oldest rocks/events are on the bottom
• Principle of original horizontality– Sedimentary rocks are always flat layers when formed,
if not flat now…plate shifting has changed them• Principle of cross-cutting relationships– If igneous rocks or faults go across sedimentary rocks
the igneous rocks or faults are younger than the rocks they cut across.
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Note: H=cooled magma/igneous rockblack linebetween I and B=fault1. I deposited, compacted cemented2. B3. F4. Tectonic activity: faulting pushes
rock layers on left up and on right down
5. Erosion of rocks at surface6. M7. R8. H is formed as magma melts its
way through cracks or weak spots in rock layers.