Depositional Environments

Sedimentary rocks are made of pieces - parts of busted up rocks from some place else, or chemical bits that form minerals and rocks in place.

Conveniently, the basic principals of sedimentology are on display all around us today.

This page will explore how sediments form, move, and make rocks.

Sediments: What are they, and how do they form?
Sediments can include dirt, sand, mud, debris, boulders - whatever you've got that's ready to move and then sit around.

Consider a mountain of intrusive igneous granite, which has been sitting around eroding for a very long time.


 * Weathering breaks the granite down: freezing water breaks it apart; acid rain tears ions off some minerals.
 * Erosion turns granite chunks into smaller and smaller pieces: floods and snow and glaciers move big chunks of rock; rivers smash the pieces into smaller bits.
 * Clasts of rock, grains of mineral, and chemical elements of dissolved minerals all travel in water: rivers, springs, ponds, lakes, oceans.

It takes energy to erode and transport sediment.

If a pile of dead Camarasaurus jam up an old river, you might get some dinosaur fossils out of the deal!

Environments where erosion produces lots of sediment include:


 * river (smashes clasts into smaller and smaller bits)
 * exposed rock (beach, high mountains)
 * soil formation (plants that pull apart rocks)
 * volcanoes (those extrusive igneous basalt rocks get battered up!)
 * coral reefs (sea shells and abandoned coral homes fill with crystal)
 * Open-water oceans and lakes (microscopic life produces lots of crystal structure and lots of poop!)

Environments that transport lots of sediment include:


 * braided streams (often high in the mountains)
 * meandering rivers (on broad low plains)
 * beaches (the sand is always shifting up/down and side/side along a beach)
 * alluvial fans (the big sweep of jagged rocks that spill out of mountain canyons during flash-floods)
 * snowy volcanoes (melted snow + heavy gas + fizzy lava = bad news)
 * mountains that are too dang high

Any sediment that is above the "angle of repose" should move eventually!

Deposition: What, why, where?
What: Deposition is the process of setting sediment down.


 * Where and when an environment runs out of energy to move sediment, then the sediment will stop moving. Ta-da! A deposit!
 * (Ever come home from a long day and just drop all your stuff at the door and lay in bed? Same thing.)



Why (generally):


 * Confined flow causes erosion.
 * Unconfined flow allows deposition.
 * Examples:
 * A roaring river smashes rocks along; a quiet delta lets the rocks settle.
 * A thunderstorm flash flood sweeps rocks down a desert canyon; the water spills across the desert floor and lets the rocks settle.
 * Sea shells are swept off a high reef running around an island; they tumble down the slope and settle in quieter water.
 * If you zoom out, many settings have weathering, erosion, transport, and deposition all happening in a jumble.
 * Consider a river or an ocean beach.
 * Forming a rock from a sediment deposit requires a lot of things to go just right.

Where (generally):


 * Erosion happens high and fast.
 * Deposition happens low and slow.

Depositional Environments: What clues do they leave in rock?
Different environments collect different sediments.


 * A deep ocean will collect layers of quiet mud and dead micro-organisms.
 * A river bottom will collect alternating layers of sand, cobbles, and mud.
 * A reef-ringed island will collect layers of coral and sea shells.

Sediment layers can capture snapshots of flow patterns.


 * Water flowing along a beach can erode the sand and form ripples in the remaining sediment.
 * If deposition outpaces erosion, many layers of rippled sandstone can form.
 * Little animals leave burrows and trails in ocean mud.
 * If deposition outpaces erosion, crystals can form in the mud to record the little trails, and fossilize tiny dead animals.
 * Huge debris flows during flash flood landslides dump jumbles of huge boulders, big cobbles, and mud onto lower land.
 * If deposition outpaces erosion, the biggest clasts show how much energy the flow had, and the range of clast sizes and compositions shows the places the material came from.

Geologists use sedimentary structures like these to...


 * See which way was "up" on a layer or sample of sedimentary rock.
 * Decide the order of deposition events
 * Interpret change in habitats through time