Sedimentary Rocks, Fossils & Relative Dating Techniques

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?

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

How are sedimentary rocks formed? Deposition  compaction  cementation – 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

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

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

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)

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

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.

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

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

Echinodermata: Crinoid A filter feeding, shallow sea-living, plant-like animal! Paleozoic

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!

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

Brachiopoda: Mucrospirifer mucronatus shallow sea floor dwelling Fed on tiny organic “snow” floating in ocean water paleozoic

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

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.

Note: H=cooled magma/igneous rock black linebetween I and B=fault 1.I deposited, compacted cemented 2.B 3.F 4.Tectonic activity: faulting pushes rock layers on left up and on right down 5.Erosion of rocks at surface 6.M 7.R 8.H is formed as magma melts its way through cracks or weak spots in rock layers.