Overview of the main topics you will encounter in Unit 9.
Nice Bryce: Stories in Sediment
Weathering changes large rocks into small pieces and salts.
After being transported, these small pieces and salts are deposited as sediment.
Sediment is slowly changed to sedimentary rock.
Transformation is NOT magic, but happens because:
Hard-water deposits cement grains together;
Squeezing compacts grains;
Recrystallization as new minerals grow creates interlocking grains.
Classy classification
Clasts (another name for pieces) make clastic rocks; dissolved salts make precipitates (rock salt, Death Valley borax).
Limestone is both--precipitated as shells, which are clasts.
Subclassify clastic rocks by size:
Clay (tiny) makes claystone, also called shale;
Silt (small) makes siltstone, sand (bigger) makes sandstone;
Cobbles (still bigger) make cobblestone, boulders (even bigger) make boulderstone, both often called conglomerate.
Environment is Evident
Clues in the rock tell the environment in which the sediment was deposited. For example:
Sand dunes, lizard tracks? Desert
Quiet-water muds, fish fossils? Lake
Corals and shells? Coastal Reef
Takes lots of study to know the rocks that different environments produce, but now is well-known.
May I Take Your Order?
Something must exist before it can be moved or cut; a clastic rock is younger (that is, formed more recently) than the clasts it is made of, and a fault is younger than the rocks that it cuts.
Layers on top are younger than those below (Principle of Superposition).
After being hardened by hard-water deposits, etc., layers may be stood up or turned over; however, the rocks contain many "up" indicators that tell us which way was right-side up when the sediment was deposited, so we can learn whether it was turned over.
Getting Into "Up" Indicators
Mud cracks, footprints, raindrop imprints go down into mud.
Tops of slightly slanting sand-dune layers are eroded by wind.
Shells on a beach are typically flipped into the stable hollow-side-down position.
Bubbles rise toward the tops of lava flows.
Nothing Succeeds Like Succession
Using these rules, we can put rocks in order from oldest to youngest.
Remarkably, this puts fossils in order, so the more similar in age, the more similar in type—we call this the "Law" of Faunal Succession.
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