Chapter 6 Flashcards
What are the minimum and maximum sizes of sand grains?
Sand grains range in size from 1/16 mm to 2 mm.
How can you easily distinguish between a silty deposit and one that has only clay-sized material?
Both silt and clay feel smooth between your fingers, but only clay feels smooth in your mouth.
What factors control the rate at which a clast settles in water?
The key factor is particle size (not density). Settling velocity is controlled by the friction around the grain holding it up and the gravitational force pushing it down. The gravitational force is proportional to the grain volume and the friction is proportional to the surface area.
The material that makes up a rock such as conglomerate cannot be deposited by a slow-flowing river. Why not?
Conglomerate cannot be deposited by a slow-flowing river because clasts larger than 2 mm are not transported by slow-moving water.
Describe the two main processes of lithification.
Sediments are buried beneath other sediments where, because of the increased pressure, they become compacted and dewatered. With additional burial they are warmed to the point where cementing minerals can form between the grains (less than 200˚C).
What is the difference between a lithic arenite and a lithic wacke?
Lithic arenite has less than 15% silt- and clay-sized particles, while a lithic wacke has more than 15%. Both have more than 10% rock fragments and more rock fragments than feldspar.
How does a feldspathic arenite differ from a quartz arenite?
Feldspathic arenite has more than 10% feldspar and more feldspar than rock fragments. Quartz arenite has less than 10% feldspar and less than 10% rock fragments. Both have less than 15% silt and clay.
What can we say about the source area lithology and the weathering and transportation history of a sandstone that is primarily composed of rounded quartz grains?
Source area lithology: rock that contains quartz (such as granite or sandstone), strong weathering to remove feldspar, long fluvial transportation to round the grains.
What is the original source of the carbon that is present within carbonate deposits such as limestone?
The carbon within carbonate deposits such as limestone originally comes from the atmosphere.
What long-term environmental change on Earth led to the deposition of banded iron formations?
Most of Earth’s banded iron formations formed during the initial oxygenation of the atmosphere between 2.4 and 1.8 Ga because iron that had been soluble in the anoxic oceans became insoluble in the oxidized oceans.
Name two important terrestrial depositional environments and two important marine ones.
Terrestrial depositional environments: rivers, lakes, deltas, deserts, glaciers. Marine depositional environments: continental shelves, continental slopes, deep ocean.
What is the origin of a foreland basin, and how does it differ from a forearc basin?
A foreland basin forms in the vicinity of a large range of mountains where the weight of the mountains depresses the crust on either side. A forearc basin lies between a subduction zone and the related volcanic arc.
Explain the origin of (a) bedding, (b) cross-bedding, (c) graded bedding, and (d) mud cracks.
Bedding forms where there is an interruption or change in the depositional process, or a change in the composition of the material being deposited.
Cross-bedding forms in fluvial or aeolian environments where sand-sized sediments are being moved and ripples or dunes are present.
Graded bedding
Mud cracks form where fine-grained (silt or clay) sediments are allowed to dry because the level of a lake decreases.
Under what conditions is reverse graded bedding likely to form?
Reverse graded bedding forms during gravity flows, such as debris flows.
What are the criteria for the application of a formation name to a series of sedimentary rocks?
A formation is a series of beds that is distinct from other beds above and below it, and is thick enough to be shown on the geological maps that are widely used within the area in question.
