Weathering Behavior: Small particle size and poor cementation leads to rapid physical and chemical weathering. The image below shows weathering of sandstone and shale. The steep cliffs are made up of weathering-resistant sandstone, while the slope at the base of the cliff is composed of rock units containing a larger abundance of shale.
Impact on Soils: Rapid disintegration generally leads to deep soils, high in clay-size particles, so slow permeability for water. How it Forms: Shale forms by deposition of sediment in low-current environments, such as lakes or along ocean shores in deep water not affected by waves.
Distinguishing Features: Relatively soft, can easily be scratched with a nail or pocket knife, reacts to hydrochloric acid fizzes , often contains fossils. Weathering Behavior: Strongly affected by chemical weathering, leading to rounded edges see image below. Impact on Soils: Abundance of calcite makes for alkaline soils, which do not acidify rapidly. How it Forms: In shallow oceans or lakes, by deposition of animal shells or by precipitation from solution.
Sandstone Properties. Type: Sedimentary Distinguishing Features: Made up of relatively coarse particles, which can often be seen without a hand lens; feels gritty, like sandpaper; sedimentary structures can be seen, such as layering or fossils. Main Minerals: quartz, some 'dirty' sandstones contain feldspars, muscovite Weathering Behavior: Can erode easily depending on the mineral composition of the cement which holds single grains together; affected by chemical and physical weathering; individual grains often very resistant to weathering i.
How it Forms: Deposition of sediment on beaches, sand dunes, stream valleys. Shale Properties. As the marine environment dries during various epochs of climate change, the sedimentary rock is left behind. The short answer? Siltstones and shales form in environments where water is quite still and calm, as in lagoons, ponds or puddles, or offshore in lakes and oceans. The silt and clay particles are so small that they easily float if there are any currents.
When the water is very still, the particles settle out to form the layers that eventually become siltstone or shale. Siltstone and shale, two types of sedimentary rock called clastic rock, form from "clasts" -- that is, fragments of other rocks or minerals. When the rock fragments are buried and compacted, they form sedimentary layers. In the case of siltstone and shale, the clasts are tiny silt and clay particles.
Over time, the buried sediment becomes cemented and forms sedimentary rock. Geologists can date sedimentary rocks relative to each other, because older rock is buried beneath younger rock. Clastic sedimentary rocks are deposited in three ways: by water, glaciers and wind. Although siltstone and shale are similarly formed in water, identifying siltstone and shale requires distinguishing between silt and clay particles.
Silt and clay are both tiny particles that have weathered away from rocks and minerals. Silt is intermediate in size between the larger grains of sand and the smaller clay particles. To be classified as silt, the particles must be smaller than. Aspects of the rock such as the rock type, the position of sedimentary structures, and sequencing of the rocks tell geologists about overall changes in the environment in an area Heinz and Aigner, Geologists can also look at sea level changes on a global scale.
To do this, geologists look at major tectonic changes in the world. Opening and closing of geologic basins and the orientation of the continents can cause rocks that typically form in a tropical environment to be found at the top of mountains. These sedimentary relationships build connections between lateral plate motion and vertical tectonic uplift or subsidence. From these spatial relationships, geologist can interpret global changes in sedimentary environments.
When geologists and their students look at an outcrop, they may have substantial differences in their assumptions about the relationship between the rocks and the time they represent. Experts may have a sense that a conglomerate, or sedimentary rock of mixed size grains, may be deposited during a geologically-instantaneous flooding event, while a shale, composed of clay particles, may take thousands of years to accumulate in a quiet lagoon environment.
In contrast, their students may assume that the rocks in the idealized outcrop of Figure 1 took an equal amount of time to form because they are the same thickness Cheek, It appears this conceptual understanding can be improved with a targeted intervention Cheek, , but students struggle to understand the time scale on which many geologic events occur.
Drawing from Montagnero's model of diachronic thinking, Dodick and Orion discuss how students must engage in Temporal Organization to put geologic strata in order, then build Interstate Linkages to relate geologic strata based on cause and effect.
Finally, all of the information must be integrated in Dynamic Synthesis which links the pieces as a whole process of change. This model is useful for thinking about how students build a systems-level conceptions of depositional processes. Yet, students' mental models will likely be fraught with errors in temporal reasoning if they do not understand depositional rates. Further, this type of thinking does not take into account other spatial errors associated with the integration of space and time required to understand depositional systems.
Transgression and regression, as depicted in most lower level textbooks, represent sedimentary deposition as a process where time may be drawn as a horizontal line across the diagram. This diagram assumes that there is an endless, continuous sediment source concurrent with changing sea level. Catuneanu, As students progress to higher level geology tasks, it becomes increasingly important that they are able to integrate temporal and spatial components of a problem.
Early on, these tasks relate to integrating a process with an outcrop Figure 1 and 2. In advanced undergraduate and graduate coursework, students are introduced to sequence stratigraphy. This may be their first real geologic example where the simple stratigraphic rules, or Steno's Laws, become more nuanced. Steno's Law of Original Horizontality posits that sediments are typically deposited into flat layers.
For example, in Figure 3, sea level transgression or rise and regression or fall will lead to a shift in sedimentary facies onshore and offshore. But this diagram is a bit mis-leading because it assumes that there's an endless sediment supply coming into the basin from the continent, which is typically not the case.
More commonly, the sediment source fluctuates as relative sea level fluctuates, at times sediment supply out paces sea level rise and vice versa Figure 4a. The depositional basin will fill with sediments and the shoreline will migrate. Deposition in a marine environment typically displays more complex geometry, like that depicted here, where the sedimentary base either a fills as sea level rises, b fills as sea level remains the same, or c shifts as sea level falls.
Mitchum, As a result, the geometry becomes quiet complex as there are several variables interacting i. The important point is that in Figure 4, a horizontal line does not accurately represent time.
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