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Depositional Environments - General Geology - Lecture Notes, Study notes of Geology

Aligarh Muslim UniversityGeology

In these Lecture notes the professor has tried to elaborate the following points Depositional Environments, Sedimentary Rocks, Glacial Environment, Past Distributions, Sub-Discipline of Geology, Sedimentary Structures, Characteristic, Rock Color, Type, Sediment Present

Typology: Study notes

2012/2013

Uploaded on 07/18/2013

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14. Sedimentary Rocks II (p. 208-217)
Depositional Environments
Sedimentary rocks can tell us a lot about the type of en vironment in which they formed, such as a river, a
beach, or a glacial environment, and the past distributions of such environments. These types of studies
of sedimentary depositional environments form a sub-discipline of geology called:
________________________.
Sedimentary Structures
Sedimentary structures provide clues about the type of depositiona l environment a particular
sedimentary rock formed in.
Sedimentary structures are the ________________________________________ that form during or
after deposition as a result of phys ical or biological processes in the depositional environment.
In other words, they are generally so me form of observable characteristic of the rock appearance,
EXCLUDING rock color or the type of sedi ment present.
Types of structures :
1) Layers within sed imentary rocks form a type of sedimentary structure called:
__________________ (or stratification)
How thick are beds? _________________________
Beds represent a more or less continuous episode of deposition of sediment. They are separated by flat
surfaces called ________________________ along which the rock tends to part or break.
Bedding planes represent one of the following two possibilities:
(1) ___________________________________________________________
(2) ___________________________________________________________
At some later time after a bed has formed, a new layer of sediment is deposited on top of it, forming a
new bed.
2) Sometimes we can see gradual changes in the size of sediment grains from the bottom to the top of
an individual bed. This is a type of sedimentary s tructure called:
_______________________________
What size sediment is at the bottom of a graded bed ? ____________ or ____________
What size sediment is at the top of a graded bed? ____________
The reason this happens is because of a gravitational effect in flowing water. The larger, heavier clasts
are hard for the water to carry, so they are the first to sink to the bottom when the water slows down and
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14. Sedimentary Rocks II (p. 208-217)

Depositional Environments Sedimentary rocks can tell us a lot about the type of environment in which they formed, such as a river, a beach, or a glacial environment, and the past distributions of such environments. These types of studies of sedimentary depositional environments form a sub-discipline of geology called: ________________________. Sedimentary Structures Sedimentary structures provide clues about the type of depositional environment a particular sedimentary rock formed in. Sedimentary structures are the ________________________________________ that form during or after deposition as a result of physical or biological processes in the depositional environment. In other words, they are generally some form of observable characteristic of the rock appearance, EXCLUDING rock color or the type of sediment present. Types of structures:

  1. Layers within sedimentary rocks form a type of sedimentary structure called: __________________ (or stratification) How thick are beds? _________________________ Beds represent a more or less continuous episode of deposition of sediment. They are separated by flat surfaces called ________________________ along which the rock tends to part or break. Bedding planes represent one of the following two possibilities: (1) ___________________________________________________________ (2) ___________________________________________________________ At some later time after a bed has formed, a new layer of sediment is deposited on top of it, forming a new bed.
  2. Sometimes we can see gradual changes in the size of sediment grains from the bottom to the top of an individual bed. This is a type of sedimentary structure called:

What size sediment is at the bottom of a graded bed? ____________ or ____________ What size sediment is at the top of a graded bed? ____________ The reason this happens is because of a gravitational effect in flowing water. The larger, heavier clasts are hard for the water to carry, so they are the first to sink to the bottom when the water slows down and

loses energy. As the speed of the water decreases, progressively smaller clasts settle down on top of the larger clasts, forming a graded bed. Graded bedding is commonly related to these two causal factors: (1) ___________________________ (form when the water slows down); and (2) ___________________________ (due to submarine landslides)

  1. In many sedimentary rocks, the beds will appear to have fine layers inside them cutting from the top to the bottom of the beds at an angle to the bedding planes. This type of sedimentary structure is called:

In what three environments is cross-bedding common in sandy sediment?

Cross-bedding forms as a result of flowing water or wind that topples sediment over the edges of dunes or underwater ripples. As a result, the dune or ripple creeps forward, with cross-beds forming on the front slope because of the sediment grains sliding down. Cross-beds are always inclined downwards in the direction of the flow of the wind or water current that formed them. For this reason, cross-beds can tell us about ancient water or wind flow directions, which we call ________________________. Make a sketch of a cross-bed:

  1. The rippled appearance of sandy sediment along the bottom of river beds are like tiny underwater sand dunes created by flowing water. These ripples sometimes get preserved in sedimentary rocks, where they are called:

They can also form on top of sand dunes in deserts by wind currents. Which type of ripple shape forms where there is a more or less constant water or wind flow direction? asymmetric or symmetric? For this reason, asymmetric ripple marks are also called ________________________. e.g., environment where asymmetric ripples form: ____________________ Such ripple marks can therefore indicate the paleocurrent direction (i.e., ancient flow in a consistent direction).

rock, you know the rock must have formed during the interval of Earth's history during which that particular species existed on Earth. Sedimentary Bed Variability Why do differences occur from one bed to the next in sedimentary rocks (e.g., different colors, grain sizes, sorting and rounding, and bed thicknesses)? _______________________________________________________. The types of changes that can occur in depositional environments over time include:

  • rise and fall of sea level
  • rivers changing course
  • glaciers advancing or retreating
  • lakes swelling or evaporating
  • deserts migrating and growing
  • tides moving in and out Whenever a particular type of sediment is distinguishable from another type of sediment that formed in a different depositional environment, we say that each sediment type represents a particular type of _________________________________. What are the different types of sediment characteristics that we can use to distinguish one sedimentary facies (i.e., formed in different environments) from another?

Because depositional environments have a limited lateral extent on the Earth’s surface, we see lateral variations in the sedimentary facies of the sedimentary rocks that are ultimately produced. For example, right near the beach, the high energy of the waves produces well sorted quartz sand grains. Slightly deeper in the water, finer sediment like silt and clay sinks to the ocean floor. In deep water, bioclastic carbonate sediment may be collecting on the ocean floor. List the three different facies types associated with these three environments and the three types of sedimentary rocks they will create: FACIES ROCK TYPE

These three facies can therefore tell us about the ___________________________ as well the fact that the water must get _____________________________________.

Sediment Types in Depositional Environments We can divide sediment up into two categories based on depositional environment:

  • ________________: forms on land Examples: ______________________________________________________
  • ________________: forms in the ocean Examples: ______________________________________________________