quiz 3 Sedimentary Rocks, Rivers and Floods

List the three categories of sedimentary rock discussed in the powerpoint slides:

clastic, biogenic, chemical

The four properties of clastic sedimentary rocks that we study are:

grain size- how big are the clasts

grain composition - what are the clasts made of

grain roundness - are the clasts round or angular

grain sorting - are the clasts the same size of mixed sizes

Biogenic rocks that have visible fossil fragments are described as ? whereas biogenic rocks that are made of microscopic fragments or calcite crystals are described as ?

fossiliferous, muddy

Chemical sedimentary rocks are

minerals that grow out of water as the water evaporates

>25% of grains are feldspar

arkosic(immature)

large clasts (gravel or larger)

conglomerate

>90% of grains are quartz

quart-rich (mature)

medium clasts (smaller than gravel but large enough to see)

sandstone

>25% of clasts are rock fragments

lithic (immature)

Clasts are mostly clay minerals

clay-rich(mature)

small clasts (too small to see)

mudstone

Which conditions are necessary for the life that produces biogenic rocks to flourish? In other words, when you see a biogenic rock, what does it tell you about conditions when the rock formed?

warm water not too hot, shallow water, clear water, not too salty

Which conditions are necessary for chemical sedimentary rocks to form? In other words, when you see a chemical sedimentary rock, what does it tell you about conditions when the rock formed?

 

dry climate, stagnant, mineral-rich water, hot climate, supply of minerals

two cleavage planes, not at 90 degrees

calcite gypsum

clear to white color

calcite gypsum

salty taste

halite

can scratch with fingernail

gypsum

too hard to scratch with fingernail

calcite, halite

effervesces (fizzes) in hydrochloric acid

calcite

3 cleavage planes, all at 90 degrees

halite

clear to white but with impurites can be pink/red/orange/brown

halite

make(s) biogenic sedimentary rocks

calcite

make(s) chemical sedimentary rocks

gypsum, halite

clastic definition

made from clasts(fragments) of weathered rocks

Biogenic definition

made from shells of organisms

chemical definition

direct precipitation of minerals dissolved in water when water evaporates (also often called evaporites)

what affects grain size?

grain size = energy of the environment = gradient

so conglomerates come from steep mountains, mudstones come from still water in lakes or oceans

Describes MAXIMUM grain size, rivers carry smaller sediments as well but never bigger than maximum size

clastic composition

clastic rocks are classified according to how much quartz, feldspar, and rock fragments there are in the rock

Why are arkosic and lithic rocks compositionally immature but quartz and clay rich rocks are compositionally mature

immature not procces much must have been deposited near source

mature sediment was processed alot, deposited far from source or is product of many cycle of weathering

What affects grain roundness

“river rocks” as sediments are moved by water they bounce and smash into each other wearing away sharp edges. More round further from source?

What makes grains angular?

angular sediments never got the chance to have the sharp edges broken off, so these were either transported by glacier ice or never travelled far from source

what affects grain sorting?

from glacier will be poorly sorted ie mixed sizes

river sediment is medium to ewll sorted depending on how far the sediment travelled, more travel = better sorting

wind is best as sorting, large clasts aren’t moved, small clasts blown away as dust so only medium clasts are piled in dunes. wind moved sediment is always well sorted

The force of moving water plucks up and tears away material from the channel, including solid rock

scouring, breaking and lifting

Sediment carried by the river acts as sandpaper, scouring away material from the channel bottom

abrasion

Slightly acidic river water dissolves the rock of the river channel

dissolution

steep gradient

straight river

wide, shallow valleys

meandering river

high sinuosity

meandering river

transition from steep to gentle gradient (or too much sediment)

braided river

carries large, sand-sized, and mud-sized clasts, deposits large clasts

straight river

deep narrow valleys

straight river

primarily horizontal erosion

meandering river

caries coarser sand-sized and mud-sized clasts, deposits sand-sized clasts

braided river

carries finder sand-sized and mud-sized clasts, deposits finer sand-sized and mud-sized clasts

meandering river

low energy

meandering river

gentle gradient

meandering river

primarily vertical erosion

straight riverd

deep, narrow valleys

straight river

high energy

straight river

medium energy

braided river

low sinuosity

straight river

how can superposed drainage come to be?

change in river gradient

uplift of mountains around a river

differential weathering of rocks around a river

the base level of a river is

the elevation at the rivers mouth and

the lowest elevation a river can erode to

a rise in base level ____________

decreases a river's energy, leading to an increase in deposition

A drop in base level

increases a river's energy, leading to an increase in erosion

How horizontal erosion happens?

Current is fast at the outsides of the bends where erosion happens

current is slow at the insides of bends where deposition happens

eventually creates oxbow lake

How does superposed drainage come to be?

change in gradient over time

differential weathering and erosion (river was there before top layers eroded away)

uplift around river (river was there before mountain was raised)

drainage patterns

dendritic- treelike; equally resistant beds and gradual slope

trellis - ridges of resistant and soft beds

radial - outward from an isolated peak

rectangular - following perpendicular sets of joints

parallel - consisten, relatively steep slope

characteristics of seasonal flood

too much water from a long period of rain or snowmelt

relatively slow development and long duration

can linger for months before floodwaters recede

characteristics of a flash flood

relatively quick development and short duration

water comes too quickly to soak into ground, or for rivers to move away

can occur too quickly for a person to escape

Where does flooding occur in a meandering river system?

 

The flood fills the entire river valley

What can be done to protect against seasonal floods?

 

build dams to regulate water flow,  

build dikes and levees 

breach levees and flood less valuable areas to protect more valuable areas

What can be done to protect against flash floods?

 

get to high ground if it rains  

watch the weather forecasts and avoid flood-prone areas if rain is forecast  

get to high ground immediately if river flow increases or turns muddy

build flood control basins

What is monitored to predict flooding?

 

predicted precipitation 

current snowpack 

ground saturation  

current streamflow