Wednesday, January 26, 2011

Lecture 7: Stratigraphy Review, Weathering & Erosion

Lithostratigraphy and Chronostratigraphy (review)

Changes in depositional environments lead to variations in accumulation of sedimentary rock both laterally and vertically. If you look at ancient rocks and compare different stratigraphic columns, there are several ways you might "correlate" them. If you correlate different rock types, e.g. lithostratigraphy, you are marking regions with similar characteristics, but the sediments in each unit were not necessarily deposited at the same time. In contrast, if you correlate rocks that were deposited at the same time, e.g. chronostratigraphy, each unit often consists of more than one facies. This is obvious when you look at the distribution of depositional environments now. Different areas are accumulating different types of sediment at the same time; the beach is different than the offshore environment.

Lithostratigraphic correlations are easy because you can directly observe rock types, define facies, and match them between stratigraphic sections. Chronostratigraphic correlations can be VERY difficult because you have to have a time marker that tells you what deposits were synchronous. In real rocks, there are a number of tools that you can use to get correlations of various accuracy, including: fossils (biostratigraphy); magnetic properties (magnetostratigraphy); absolute ages of interbedded volcanic ash beds and basalt flows; some chemical properties such as isotopic ratios of certain elements in carbonates; geological instantaneous depositional events such as huge storms, tsunamis, meteorite impacts, etc.; and unconformities due to sea level falls and the geometry of sedimentary deposits (sequence stratigraphy). We will get back to all of these in more detail throughout the quarter, particularly near the end.

Walther’s Law is key for understanding the differences between lithostratigraphy and chronostratigraphy. Walther’s Law states that environments that are adjacent to each other are represented as vertical successions of facies in the rock record if there is no break in sedimentation (no unconformity). If sea level is rising relative to the shore line, the different depositional environments are migrating inland. This leads to different facies accumulating progressively inland as well. The most landward deposits might be river deposits and alluvial plain deposits, followed by marsh and then marine deposits. Vertically, you see the facies representing those depositional environments in the same order. At any given time, rocks are being deposited in all of the different environments.


Origins of Sediment
Sediment comes from the break down of rocks into smaller, transportable components. This occurs via two processes: physical weathering and chemical weathering. Physical weathering consists of breaking apart rocks and crystals. The results of physical weathering are smaller components of the same material that is being weathered. There is no change in composition. In contrast, chemical weathering consists of changing the composition of at least some components of the rock that is weathering. The sediment does not have the same composition as the original rock.

Physical Weathering:
Physical weathering occurs via:

1) Freeze-thaw action. Water in cracks expands when it freezes, putting force on the cracks. The cracks grow, and eventually crystals and pieces of rock break off into smaller components. Obviously, this process is most important in environments where temperatures cycle across the freezing point of water.

2) Salt crystal growth. When water evaporates, salts precipitate. If this happens in fractures in rock, the growth of the salt crystals can put pressure on the cracks, causing them to grow. This process is most important near oceans where rocks are exposed to lots of salt water spray and in arid environments where water evaporates rapidly.

3) Temperature changes. Minerals contract and expand as temperature decreases and increases, respectively, and different parts of the rock are heated different amounts. Those in direct sunlight expand as they heat, whereas the interiors and shaded areas do not. Differential expansion and contractions produces stresses which can result in cracks and physical weathering. This process is most important when temperatures change dramatically from day to night, a characteristic of many desert environments.

Physical weathering tends to produce mostly sand-sized sediment and larger grains because most of the fracturing occurs along mineral boundaries. Physical weathering of fine grained or finely crystalline rock can produce abundant very fine grains, but most of the sediment from these rock types consists of rock fragments (called lithic clasts).

Chemical Weathering:
Chemical weathering occurs via:

1) Dissolution of minerals. Some minerals like halite and other evaporites dissolve very easily in water. Other minerals, particularly silicates, do not dissolve easily. Carbonates are in between and dissolve in acidic waters. (Rain water has a pH of ~5.7 due to dissolved CO2, even without “acid rain” pollution.) The results of dissolution are ions in water that are transported downstream. Ions are not deposited until the water evaporates, they react with other minerals, or organisms use them to make shells. Often, only part of a rock dissolves, leaving sediment that can be transported by wind, water, etc.

2) Alteration of minerals. Silicates do not dissolve very easily, but they do react with water to form new minerals. Feldspars react with water to form clay minerals and ions, olivine reacts with water and O2 to form oxides, clay minerals and ions, pyrite reacts with water and O2 to form oxides and sulfate ions. Iron oxides, such as hematite, are commonly red, giving weathered rocks a rusty hue. Alteration of minerals is one of the main sources of clay minerals and mud-sized grains.

Mineralogy of Weathered Rocks
Sediments that have been subjected primarily to physical weathering have a mineralogy that is similar to that of the parent rock. If the sediments have been subject to extensive chemical weathering, it is much harder to characterize the source rocks because the composition has changed extensively. Overall, the composition of the resulting sediment depends on the mineralogy of the rock, how it is transported, and the weathering environment.

Some minerals alter more quickly than others. Quartz is difficult to dissolve and is hard, so it commonly lasts through both chemical and physical weathering and is the most common composition of sand on Earth. In contrast, minerals like Ca-feldspar and olivine react to form new minerals quickly. They are substantially less common in sediments. Thus, mafic rocks (which contain Ca-feldspar, olivine and pyroxenes) tend to alter to clay minerals very easily and produce little sand and abundant mud. In contrast, granites (quartz, K-feldspar, Na-feldspar, mica) contains minerals that react more slowly and tend to produce sand-sized grains, especially quartz.

The following lists minerals from most reactive (rarely found in sediments) to least reactive (common in sediments): Olivine, Ca-feldspar, Pyroxene, Amphibole, Na-feldspar, Biotite, K-feldspar, Muscovite, and Quartz

The other main control on sediment mineralogy is the hardness of the grains. During transport, grains hit each other. Softer grains tend to be damaged when they collide with harder grains, and this damage can cause them to break into smaller grains. Thus, soft grains become smaller very quickly when they are transported with hard grains. Quartz is the most common mineral in sandstones because it is hard and unreactive. Clay minerals are also very common because they are too small to damage much during collisions and they are the product of the alteration of other minerals.

Controls on Weathering
The extent and style of weathering is mainly controlled by climate. Water is extremely important, even for physical weathering. The more water present, the faster weathering occurs. Temperature is also important, as discussed for physical weathering. Warmer temperatures also promote faster reactions, so chemical weathering is more effective in warm climates. Thus, warm, humid climates tend to have the most rapid weathering (and poor outcrop). Finally, vegetation has a strong influence on weathering. Plants tend to increase the extent of chemical weathering by producing organic acids which help break down rocks into soil through both dissolution and alteration. They also help soil retain moisture, increasing the availability of water for weathering.

Once sediment is produced by weathering, it is available for transport. The two main forces in erosion are gravity and fluid flow. Gravity pulls sediment down steep slopes through creep, rock or debris falls, landslides and slumps. These processes are really important for the hills in coastal California where there is enough water for extensive weathering, but there is little runoff of water most of the time. Fluid flow is what we talk about most, e.g. glacial erosion of sediment, wind blown sediment, and mostly water flow. Flowing water is the biggest influence in erosion because it is very common and effective at transporting sediment.

Erosion by water occurs when water is flowing across a surface and the flow is capable of transporting more sediment than is currently moving as bedload. This is called the sediment transport “capacity”. A certain number of grains of a certain size can be picked up by the Bernouli effect for a given flow. If there are too many grains, they start colliding and and the characteristics of sediment transport change. Grains are directed back toward the bed and up into the flow. Eventually, more go back to the bed and are deposited, leaving fewer grains in the flow even at high flow speeds because there are more grains than the transport capacity of the flow.* In contrast, if there is a shortage of grains of a size that can be moved by the flow, e.g. the flow is moving all of grains present, any new grains will be eroded off the bed as soon as they are available. The flow then has excess transport capacity.

* Think about dumping a truck load of fine sand into a fast moving river, it takes time to move all that sediment even if the flow speed is theoretically fast enough to erode fine sand.

One of the most common times for a flow to have excess transport capacity is when the flow is speeding up. We know from the Hjulstrom diagram that faster flows transport larger grains. They can also transport more grains. Thus, water flowing from a shallower slope to a steeper slope commonly speeds up, has excess capacity and erodes sediment. When it slows down, sediment is deposited. In floods, the water speeds up, erodes sediment, and transports it. As the flood ends, the water slows down and deposits the excess sediment. In general, erosion occurs when flows are speeding up or when they go from an environment with low sediment (e.g. a dam spillway) to an environment with more sediment (e.g. a river bed).

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