Chapter 6: Sediment and Sedimentary Rocks Questions

Quartz is a common mineral in sandstone. Under certain circumstances, feldspar is common in sandstone, even though it normally weathers rapidly to clay. What conditions of climate, weathering rate, and erosion rate could lead to a feldspar-rich sandstone? Explain your answer.

Feldspar-rich sandstone forms in an arid, dry environment.

Describe with sketches how wet mud compacts before it becomes shale.

Mud/clay volume decreases as water is pushed out due to pressure from objects on top. Particles reorient and splitting surfaces in shale form parallel to mineral grains.

What do mud cracks tell us about the environment of deposition of sedimentary rock?

Sediment was exposed above water as water level dropped creating a very fine-grained sediment as it dries.

How does a graded bed form?

A single bed may have gravel at its base and grade upward through sand and silt to fine clay at the top. A graded bed may be deposited by a TURBIDITY CURRENT which is a turbulently flowing mass of sediment-laden water that is heavier than clear water and therefore flows down-slope along the bottom of the sea or a lake. Turbidity currents are underwater avalanches and are typically triggered by earthquakes or submarine landslides.

List the detrital sediment particles in order of decreasing grain size.

Boulder (256mm)

Cobble (64mm)

Pebble (2-64mm)

Sand (2mm)

Silt (1/16mm)

Clay (1/256mm)

How does a sedimentary breccia differ in appearance and origin from a conglomerate?

Conglomerate can be distinguish from breccia by the definite roundness of its particles.

Describe three different origins for limestone.

Biochemical (from remains of sea animals), inorganic (precipitated from solutions) and recrystallization (new crystals form in rock).

How does dolomite usually form?

Dolomite usually forms from the alteration of limestone by magnesium-rich solutions.

What is the origin of coal?

Origin from plant fossils in coal beds such as leaves, stems, tree trunk, and stumps with roots often extending into the underlying shales, so most coal formed right at the place where plants grew.

Sketch the cementation of sand to form sandstone.

a) Loose sand grains are deposited with open pore space between the grains. b) The weight of overburden compacts the sand into a tighter arrangement, reducing pore space. c) Precipitation of cement in the pores by groundwater binds the sand into the rock sandstone, which has a clastic texture.

How do evaporites form? Name two evaporites.

Evaporites form from the evaporation of seawater or a saline lake, such as Great Salt Lake in Utah. 2 types are Rock Gypsum, formed from the mineral gypsum & Rock Salt, composed of mineral called halite,.

Name the three most common sedimentary rocks.

Sandstone, Shale, & Limestone

What is a formation?

A formation is a body of rock of considerable thickness that is large enough to be mappable, and with characteristics that distinguish it from adjacent rock rock units.

Although a formation is usually composed of one or more beds of sedimentary rocks, Units of metamorphic and igneous rock are also called formations.

Explain two ways that cross-bedding can form.

By flow of air and water, sand grains move as migrating ripples and dunes.

Particles of sediment from 1/16 to 2 millimeters in diameter are of what size?

Sand

Rounding is

the grinding away of sharp edges and corners of rock fragments during transportation.

Compaction and cementation are two common processes of

lithification

Which is not a chemical or organic sedimentary rock?

shale

The major difference between breccia and conglomerate is

rounding of the grains

Which is not a type of sandstone?

travertine

Shale differs from mudstone in that

shale is visibly layered and fissle; mudstone is massive and blocky.

The chemical element found in dolomite not found in limestone is

Mg (Magnesium)

In a graded bed, the particle size

decreases upward

A body of rock of considerable thickness with characteristic that distinguish it form adjacent rock units is called a/an

formation

If sea level drops or the land rises, what is likely to occur?

a regression

Thick accumulations of graywacke and volcanic sediments can indicate an ancient

convergent boundary

A sedimentary rock made of fragments of preexisting rocks is

detrital.

Clues to the nature of the source area of sediment can be found in

the composition in the sediment.