Physical Geology 15Ed Chapter 6 Testing Your Knowledge Sediment and Sedimentary Rocks

A body or rock of considerable thickness with characteristics that distinguish it from adjacent rock units is called a/an

A. Formation

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

A. The composition of the sediment

In a graded bed, the particle size

A. decreases upward

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

B. A regression

Thick accumulations of graywacke and volcanic sediments can indicate an ancient

B. Convergent boundary

The chemical element found in dolomite not found in limestone is

B. Mg

The major difference between breccia and conglomerate is

B. Rounding of the grains

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

B. Sand

Which is not a chemical or organic sedimentary rock?

B. Sandstone Sandstone is formed by the cementation of sand grains. Organic or biological sedimentary rocks are formed by living organisms. Chemical sedimentary rocks formed when conditions favor a chemical reaction.

Shale differs from mud stone in that

B. Shale is visibly layered and fissile; mud stone is massive and blocky

Rounding is

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

A sedimentary rock made of fragments of preexisting rocks is

C. Detrital

Name the three most common sedimentary rocks.

The three most common sedimentary rocks are shale, sandstone, and limestone. Shale is a fine grained sedimentary rock notable for its ability to spin into layers. Sandstones formed from sand deposited by rivers, wind, waves or turbidity currents. Limestone is a sedimentary rock composed mostly of calcite CaCO3.

What is a formation?

A convenient rock unit for mapping and classifying rock is a formation. A formation contains a variety of inter layer rocks. Larger formations are called groups whereas smaller formations are called members. One or more beds of sedimentary rock usually form a formation as well as units of metamorphic and igneous rock can also form formations.

How does a graded bed form?

A graded bed is a layer with a vertical change in particle size, usually from coarse grains at the bottom of the bed to progressively finer grains toward the top. A graded bed may be deposited by turbidity current.

Sketch the cementation of sand to form sandstone.

Cementation of sand grains forms sandstone. Any deposit of sand can lithify to sandstone. Rivers deposit sand in their channels, and wind piles up sand into dumes. Waves deposit sand on beaches and in shallow water. Deep sea currents spread sand over the sea floor. Thereby sandstone show a great deal of variation in mineral composition, degree of sorting, and degree of rounding.

How does dolomite usually form?

Dolomite is a sedimentary rock and chemical formula of dolomite is CaMg(CO3)2. It usually forms form the alteration of limestone by magnesium-rich solutions. Most commonly dolomite is formed in dry areas.

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

Dried polygonal pattern of cracks which has very fine-grained sediments are called mud cracks. It is exposed above water, since it needs air for drying. In lake-bottom sediment and in marine sediment we can see mud cracks.

Quartz is a common mineral in sandstone. Under certain circumstances, feldspar is common in sandstone, even through 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.

Dry, cold climate helps in slow weathering rate is one of the condition which lead to feldspar-rich sandstone. Quick erosion rate and fast weathering rate also plays a major role in formation of feldspar-rich sandstone.

What is the origin of coal?

Coal is a major fuel which is a consolidated plant material. It is a sedimentary rock that forms from the compaction of plant material that has not completely decayed. Tropical climate or shallow swamps are the likely environments for deposition. The plant fossils in coal beds include leaves, stems, tree trunks, and stumps with roots often extending into the underlying shales, so apparently most coal formed right at the place where the plants grew.

Explain two ways that cross-bedding can form.

Cross beds form because i flowing air and water, sand grains move as migrating ripples and dunes. Sand is pushed up the shallow side of ripple to the crest, where it then avalanches down the steep side, forming a cross bed. Cross bed forms one after the other as the ripple migrates downstream. Any environment favors the formation of cross bed in which a fluid flows over a bed with mobile material. Cross bed is seen in stream deposits and in tidal areas.

Compaction and cementation are two common processes of

D. Lithification

Which is not a type of sandstone?

D. Travertine Travertine is not a type of sandstone. Sandstone with more than 25% of the grains consisting of feldspar is called arkose. Quartz sandstone is sandstone in which more than 90% of the grains are quartz. Greywacke is a type of sandstone in which more than 15% of the rock's volume consists of fine-grained matrix.

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

Fine grained rocks such as slate undergo pronounced compaction as they lithify. The figure below shows the role of compaction in the lithification of shale from wet mud. Before compaction 80% of the volume of the wet mud has been pore space filed with water arranging the clay minerals randomly within the mud. Pressure from overlying material packs the sediment grains together and reduces the overall volume by squeezing water out of the pores. Due to the pressure the clay minerals are reoriented perpendicular, becoming parallel to one another.

How do evaporites form? Name two evaporites.

Rocks that are formed from the remain precipitate of the solution are called evaporates. They form from the evaporation from the seawater or saline lake. Rock gypsum formed from the mineral gypsum is a common evaporate and the other one is rock salt. All evaporates have a crystalline structure.

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

Sedimentary breccias are a coarse-grained sedimentary rock formed by the cementation of coarse, angular fragments of rubble. It is a talus slope deposit that forms at the base of a steep rock cliff that is being mechanically weathered. Coarse-grained sedimentary rock formed by the cementation of rounded gravel is a conglomerate. It is rounded particles which may have not traveled far and transported only a short distance by a river or a wave.

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

The most sedimentary rocks are detrital sedimentary rocks formed from cemented sediment grains that are fragments of preexisting rocks. Gravel, sand, silt, and clay are the detrital sedimentary particles in the decreasing order of size.

Describe three different origins for limestone.

Three different origins for limestone are inorganic, Biochemical and re-crystallized in nature. Inorganic limestones are precipitated directly as the result of inorganic processes, by precipitation of calcite from water. Biochemical limestones are precipitated through the actions of organisms which are formed on continental shelves in warm, shallow seawater, and made up of skeletal remains. Re-crystallized limestones are formed by re-crystallization of calcite due to increase in temperature and pressure.