Chapter 6: Sediments and Sedimentary Rocks

Main types of clastic sedimentary rocks

***Know the difference between Breccia, Conglomerate, and Coal Rocks - Breccia = dominated by angular, pebble size and larger - Conglomerate = dominated by round clasts, pebble size and larger

What is a clast?

- A clast is a fragment of rock or mineral, ranging in size from less than a micron1 (too small to see) to as big as an apartment block. - Most clasts that are smaller than sand size (<1/16 mm) are made of clay minerals. Most clasts larger than sand size (>2 mm) are actual fragments of rock, and commonly these might be fine-grained rock like basalt or andesite, or if they are bigger, coarse-grained rock like granite or gneiss.

Where does most limestone form?

- Almost all limestone forms in the oceans, and most of that forms on the shallow continental shelves, especially in tropical regions with coral reefs. - Limestone also accumulates in deeper water, from the steady rain of the carbonate shells of tiny organisms that lived near the ocean surface. The lower limit for limestone accumulation is around 4,000 m. Beneath that depth, calcite is soluble so limestone does not accumulate

What is the difference between clastic and chemical sedimentary rocks?

- Clastic sedimentary rocks are mainly composed of material that has been transported as solid fragments (class) - Chemical sedimentary rocks are mainly composed of material that has been transported as ions in solution.

What is cross-bedding?

- Cross-bedding is bedding that contains angled layers and forms when sediments are deposited by flowing water or wind. - Cross-bedding is a very important sedimentary structure to recognize because it can provide information on the direction of current flows and, when analyzed in detail, on other features like the rate of flow and the amount of sediment available.

What is Deposition and where does it takes place?

- Deposition is the geological process in which sediments, soil and rocks are added to a landform or land mass. - Deposition takes place where the conditions change enough so that sediments being transported can no longer be transported (e.g., a current slows).

Describe the 6 main grain-size categories?

. In general, a boulder is bigger than a toaster and difficult to lift. There is no upper limit to the size of bolder 2. A small cobble will fit in one hand, a large one in two hands. 3. A pebble is something that you could throw quite easily. 4. The smaller ones known as granules — are gravel size, but still you could throw one. But you can't really throw a single grain of sand 5. Silt is essentially too small for individual grains to be visible, and while sand feels sandy to your fingers, silt feels smooth to your fingers but gritty in your mouth

Geologists have discovered that accumulation of sediments and sedimentary rocks take place according to what Geological principles?

1. The Principle of Original Horizontality 2. The Principle of Superposition 3. The Principle of Inclusion 4. The Principle of Faunal Succession

What is a formation?

A formation (a stratigraphic unit) is a series of beds that is distinct from other beds above and below, and is thick enough to be shown on the geological maps that are widely used within the area in question

What is Bedding and indication of?

Bedding is an indication of changes in depositional processes that may be related to seasonal differences, changes in climate, changes in locations of rivers or deltas, or tectonic changes.

What is Bedding?

Bedding is the separation of sediments into layers that either differ from one another in texture, composition, color, or weathering characteristics, or are separated by partings — narrow gaps between adjacent beds (Figure 6.19).

When does Burial occur?

Burial occurs when more sediments are piled onto existing sediments, and layers formed earlier are covered and compacted.

What is Cementation?

Cementation involves ions carried in groundwater chemically precipitating to form new crystalline material between sedimentary grains. The new pore-filling minerals forms "bridges" between original sediment grains, thereby binding them together.

What are chemical sedimentary rocks dominated by?

Chemical sedimentary rocks are dominated by components that have been transported as ions in solution.

Why is coal classified with clastic rocks?

Coal is classified with the clastic rocks for two reasons: first, because it is made up of fragments of organic matter; and second, because coal seams (sedimentary layers) are almost always interbedded with layers of clastic rocks, such as mudrock or sandstone. In other words, coal accumulates in environments where other clastic rocks accumulate.

How do evaporites form?

Dissolved salts are washed into seas or inland lakes. The water in these sea/lakes evaporates, salt deposit are left behind. The salts eventually reach saturation levels and start to crystallize.

What is Lithification?

Lithification is what happens — at depths of hundreds to thousands of metres — when those compacted sediments become cemented together to form solid sedimentary rock.

When do mud cracks form?

Mud cracks form when a shallow body of water (e.g., a tidal flat or pond), into which muddy sediments have been deposited, dries up and cracks (Figure 6.24). This happens because the clay in the upper mud layer tends to shrink on drying, and so it cracks because it occupies less space when it is dry.

What lead to the accumulation of Banded Iron Formation?

Photosynthetic bacteria (i.e., cyanobacteria, a.k.a. blue-green algae) consume carbon dioxide from the atmosphere and use solar energy to convert it to oxygen. These bacteria first evolved around 3500 Ma, and for the next billion years, almost all of that free oxygen was used up by chemical and biological processes, but by 2400 Ma free oxygen levels started to increase in the atmosphere and the oceans. Over a period of 600 million years, that oxygen gradually converted soluble ferrous iron (Fe2+) to insoluble ferric iron (Fe3+), which combined with oxygen to form the mineral hematite (Fe2O3), leading to the accumulation of BIFs. After 1800 Ma, little dissolved iron was left in the oceans and the formation of BIF essentially stopped.

What is the importance of Ripples as it pertains to cross-bedding?

Ripples, which are associated with the formation of cross-bedding, may be preserved on the surfaces of sedimentary beds. Ripples can also help to determine flow direction as they tend to have their steepest surface facing down flow.

What are some of the most common sedimentary rocks?

Some of the most common sedimentary rocks are limestone, chert, and banded iron

What does The Principle of Faunal Succession state?

The principle of faunal succession states that there is a well-defined order in which organisms have evolved through geological time, and therefore the identification of specific fossils in a rock can be used to determine its age.

What does The Principle of Inclusion state?

The principle of inclusions states that any rock fragments in a sedimentary layer must be older than the layer. For example, the cobbles in a conglomerate must have been formed before the conglomerate.

What is the Principle of Original Horizontality?

The principle of original horizontality states that sediments accumulate in essentially horizontal layers. The implication is that tilted sedimentary layers observed to day must have been subjected to tectonic forces.

What is The Principle of Superposition?

The principle of superposition states that sedimentary layers are deposited in sequence, and that unless the entire sequence has been turned over by tectonic processes, the layers at the bottom are older than those at the top.

What determines the rate of settling?

The rate of settling is determined by the balance between gravity and friction, as shown in Figure 6.3.

After the first two steps in the transformation of existing rocks into sedimentary rocks, what are the remaining steps in the formation of sedimentary rocks?

The remaining steps in the formation of sedimentary rocks are transportation, deposition, burial, and lithification (Figure 6.2).

What is Transportation?

Transportation is the movement of sediments or dissolved ions from the site of erosion to a site of deposition; this can be by wind, flowing water, glacial ice, or mass movement down a slope. - Professor also calls this erosion...wtf

What are the first two steps in the transformation of existing rocks into sedimentary rocks?

Weathering and Erosion