The goals of this chapter are to:
- Evaluate the origin, composition, distribution, and succession of strata to determine past geologic events related to sedimentary environments and tectonic settings
- Apply Walther’s Law to marine transgression and regression
- Reconstruct sediment thickness to understand processes of deposition
- Create stratigraphic columns and correlations using multiple techniques
Stratigraphy is the area of geology that deals with sedimentary rocks and layers, and how they relate to geologic time; it is a significant part of historical geology. As you learned in Chapters 2 and 4, one of the primary goals of studying sedimentary rocks is to determine their depositional environment, and stratigraphy is no different.
Stratigraphy is mainly studied through outcrop observations, the collection of sediment cores, and seismic surveys. Sediment cores are mostly collected from the ocean floor by organizations like the International Ocean Discovery Program (IODP) using a dedicated ship for drilling (Figure 5.1a). Geologists can then study the collected sediment (Figure 5.1b) or send instruments into the drill hole to measure the geologic properties of the surrounding sediment. The cores are then stored at facilities around the world for scientists to request samples from (Figure 5.1c). Another way to study stratigraphy is to conduct seismic surveys sending sound waves into the ground and monitoring how the waves reflect back to the surface (Figure 5.2). These used to only be done in a line but now they are typically collected in grids to get three-dimensional data below the earth’s surface. Geologists use the patterns of reflectivity to help identify the rock types, deformation features, and where there might be water or petroleum and other characteristics in the subsurface. If a seismic survey is repeated over the same area, this gives another dimension of time.
Geologists subdivide stratigraphic columns into formations. Sometimes several formations are lumped together to form a group. Formations are defined as having an identifiable origin and relative age range that are distinctive, easily mapped with a set of characteristic petrographic, lithologic or paleontologic features (facies). If you want to propose a new formation or group, there are strict guidelines set up by the International Commission of Stratigraphy.
If you look closely at Figure 5.2, the sedimentary are flat and show layering in the Kumano Basin (top left side of the image or NW end) as well as to the right edge to the SE. Layering is flat and parallel to the ocean floor. This pattern implies it was deposited in relatively quiet water. Below these regions as well as in the middle of the seismic line, the layering is disrupted and non-continuous; this probably indicates that these sediments were deformed shortly after deposition and before they were lithified as the sediment was scraped off the down going oceanic crust. Towards the bottom, the seismic reflectors are very discontinuous as the oceanic crust has no internal structure. So, just the seismic character of the layering lets you know important tectonic and sedimentary history along this profile.
This exercise is an analogy for how geologists recover sediment cores. Instead of sediment though, we will use Play-Doh.
- Using an empty Play-Doh container (or another container), place layers of different colored Play-Doh; don’t use more than 10 layers. These layers of Play-Doh will act as our layers of sediment.
- Take a clear, plastic straw and push it down through the layers of your Play-Doh. This represents geologists drilling into layers of sediment.
- Pull the plastic straw out. You should see all your layers of “sediment” represented in the straw, which is what geologists recover when collecting a sediment core.
Depending on the type of coring that is done, geologists will split the sediment core in half. One half of the core will be sampled and studied by geologists, the other half will be archived and remain untouched. One of the first things geologists do on a recovered core is to describe the lithology. You are going to create a stratigraphic column of your Play-Doh core. A stratigraphic column is a graphical representation of layers of sediment and sedimentary rock and their characteristics.
- Measure and record the total length of your Play-Doh core in millimeters. ___________________
- Measure and record the thickness of each layer in your Play-Doh core in Table 5.1. Start with the upper-most layer.
Table 5.2 – Play-Doh “sediment” layer characteristics Layer Color Thickness (mm)
- Now let’s visualize this data by creating a stratigraphic column using Figure 5.3. Start with your uppermost layer, use its thickness to fill in the “Lithology” column. For example, if you measured a thickness of 15 mm, draw a horizontal line at the 15 mm mark. Color your layer so that it matches your Play-Doh. There, you just created your first layer.
- Now add the second layer to the column using the thickness of that. If your next layer is 10 mm, start where the first layer ended and then measure 10 mm down from there. Finish the stratigraphic column with all of your layers.
- If the sediment at the bottom of your column was deposited 100,000 years ago, what is the sedimentation rate of your core in mm/yr? ____________________
Before you begin the rest of the exercises, you may want to review what you know about classifying sedimentary rocks (Chapter 2) and sedimentary structures (Chapter 4). Remember that clastic rocks are subdivided by grain size, rounding and sorting. Also, how rocks weather is important in these exercises as how resistant they are to weathering makes it easier to see the different stratigraphic units. For example, shales weather easily compared to sandstones and limestones.
5.2 Grain Size
One of the simplest analyses done on sediment is a grain size analysis. Remember, clastic sedimentary rocks are classified based on their grain size, and it can tell you a lot about the transport and depositional history of the sediment. Some geologists use grain size to help determine the geologic history of the sediment, some use it for studying porosity and permeability to locate fluids (water or oil), or in the case of engineers, how much weight the sedimentary rock can hold before becoming unstable.
You can try to visually estimate the distribution of grain size in a sample of sediment, but that can lead to significant error if you over- or underestimate something. Visual estimates are best for field measurements, but in the lab, you need to be more accurate. Geologists use quantitative approaches with various particle size analyzers (instruments that can accurately measure grains size and shape), but the traditional way to measure grain size was to separate the sediment into different size ranges using sieves and then measuring their weights. Computerized particle sizers are probably not available for you to use in your lab, so let’s do a grain size analysis on a sample of sediment using sieves.
Note: Grain sizes smaller than 63 μm need to be wet-sieved and dried
- Measure the starting weight of the entire sediment sample in grams. ____________________
- Now separate the sediment into different fractions using the set of sieves provided by your instructor.
- Measure and record the weight of each size fraction Table 5.3.
- What weight percentage does each fraction make up; fill in the table? Weight percent is the percentage of the entire weight of the sediment that each size fraction takes up. Take the weight of each fraction and divide it by the total weight, then multiply by 100.
Table 5.3 – Grain Size Analysis Size fraction range (μm) Weight (g) Weight Percent
- Take your weight percent data and plot it as a bar graph in Figure 5.4.
- Another way geologists use sediment grain size is to classify and describe the sediment. Do you remember the from Chapter 0? Geologists use ternary plots to compare sand, silt, and clay content of the sediment. Table 5.4 contains percentages of sand, silt, and clay, plot them on the triangle graph in Figure 5.5, and record the descriptive term for the sediment size in the last column of Table 5.4.
Table 5.4 – Particle size percentages to plot on Figure 5.5 Sand % Silt % Clay % Sediment Description 55 23 22 18 27 55 40 48 12 23 7 70
The boundaries between different geologic time periods are often easy to find in the stratigraphic record as a change in sediment type. Sometimes these correspond to major extinction events. One of these occurs at the end of the and beginning of the time periods (201.3 Ma). At this time about 25-30% of all marine species died out. The cause for the extinction is debatable with some geoscientists relating it to increased CO2 in the atmosphere from volcanic eruption of a large igneous province (LIP) in the Atlantic ocean. The Triassic in this region is represented by the Lilstock Formation deposited in a warm tropical ocean that had abundant marine life. Above this is the Blue Lias Formation, well known for its fossil ammonites.
Below is an image created by stitching together many images with software that renders them in three dimensions. Once the image is loaded, you can use your mouse to rotate, tilt and zoom in on the figure. This image has several numbers that you can click on highlighting some of the features at the Triassic-Jurassic boundary near Lyme-Regis in southern England. There are also two tape measures draped across the outcrop that you can use to get a sense of the scale of this sea cliff exposure. These are both in inches and feet units.
5.3 Facies and Lithostratigraphy
Lithostratigraphy is a sub-discipline of stratigraphy that deals with the type of rock and depositional environment of sediments and sedimentary rocks. One of the fundamental concepts in stratigraphy is Walther’s Law (Johannes Walther,1860-1937), which states that depositional environments vary in both space and time such that “the facies that occur conformably next to one another in a vertical section of rock will be the same as those found in laterally adjacent depositional environments” (Walther, 1894). That’s a pretty dense statement, so let’s break it down. First, let’s tackle two definitions; facies and conformably. A facies is a characteristic rock that represents a certain depositional environment. These characteristics include physical, chemical, and biological aspects. In geology, sedimentary layers are said to lie conformably when there is no unconformity between them. And remember, sediment is deposited in continuous, horizontal layers.
Now for the rest of the law. In the marine environment, sand is deposited close to shore, silt and clay further away, and maybe limestone very far away (Figure 5.7). All three of these types of sediments are being deposited at the same time and form a single, continuous layer, as in Time 1 in Figure 5.7. The boundary between them is gradational, meaning that the transition from sand to silt, or silt to clay, or clay to limestone is a gradual transition rather than an abrupt boundary (That’s what the zig-zag lines in the figure mean). This gradual transition keeps the single-layer continuous and conformable. Now, what if sea level were to rise (called transgression), as in Time 2 in Figure 5.7? As the next layer of sediment is deposited, all types of sediment are shifted toward the coastline following the sea level rise. If sea level continues to rise, the next sediment layer would shift even further, as in Time 3 of Figure 5.7. Looking at Time 3 in Figure 5.7, the shale at the bottom is overlain by limestone, and that limestone is the same facies as the limestone to the left of that original shale. The same could be said about the sandstone and shale. That’s what Walther’s Law means, vertical changes in the succession of sedimentary rocks reflect lateral (horizontal) changes in the environment. If sea level were to lower (called regression), the facies would move in the other direction, as in Figure 5.7 (Note: if sea level drops enough to expose sediment, erosion will take place).
Identifying transgressive (rise) and regressive (fall) cycles in sea level is a big component of lithostratigraphy and sequence stratigraphy. Figure 5.9 is a basic stratigraphic column that contains evidence of transgression and regression.
- Shade in the facies environment box for each layer of sedimentary rock in the column.
- Identify where transgression and regression are taking place. You can mark this with vertical arrows on the side of the figure.
- Can you tell how much sea level has changed? Explain.
- Can you tell what direction the coastline was in? Explain.
- Sedimentary layers are laterally continuous, and they transition into other sediment types. This means that a facies can “pinch out” over horizontal distances. Complete the lithologic correlation in Figure 5.10 below of two sediment cores.
- Which layer pinches out? ____________________
- Explain why that layer pinched out.
- The addition of a second core now lets you interpret the direction of the coastline. Was the coastline to the right or the left? Explain your reasoning.
- Imagine a scenario where you’re working as a hydrogeologist, a type of geologist that deals with water. Your company has assigned you to locate the best source of groundwater for drilling a new water well. Groundwater is contained in rocks with a lot of pore space between the grains. In this area, sandstones make the best aquifers. You have found stratigraphic columns for the rocks in the area from wells drilled on nearby properties. The site you are investigating is located between cores 2 and 3. Use this data to create a cross-section to help you understand the thickness of the various layers and potential reservoirs for groundwater at the new well location. To do this, connect layers with similar lithology, and be careful of layers pinching out. The first layer of limestone is already completed for you.
- The best aquifers will be laterally continuous sand layers. Label the layer(s) that you think would be the best source of water.
- What is the minimum depth you need to drill to install a new water well? ____________________
Determining facies is a very important part of stratigraphy, and many other interpretations of data from sediments and sedimentary rocks rely on an accurate facies analysis. For example, if you are looking at changes in the chemistry of sediment, the changes could be the result of the facies changing. So, you need to have a good and accurate understanding of the facies.
Coastal environments can have many sedimentary facies (Figure 5.11), and each has something distinctive about it. You’ll notice that these facies are mainly sandstone or mudstone, which is not distinctive. Distinctive characteristics in these facies include the abundance of fossils, sedimentary structures, and layering patterns of the strata. Use figure 5.11 below to match the environments to the lithofacies and descriptions for: Backshore, Dunes, Foreshore, Bay/Lagoon, Marsh, Nearshore, and Overwash.
|Sandstone||Coarse sand with abundant shell fragments|
|Sandstone||Medium to fine sand and rare shell fragments|
|Sandstone||Medium to fine sand and no shell fragments|
|Sandstone||Fine to very fine sand, root structures, and cross-bedding|
|Sandstone with mudstone||Horizontal bedding, root structures, and shell fragments|
|Mudstone||Bioturbation, root structures, and peat|
|Mudstone||Bioturbation, root structures, and shell fragments|
You have collected a 13 m sediment core from the area. Your team has separated and described the sedimentary layers. Based on the descriptions from your team, identify the environment for each layer (using environments from Part I).
|0.00 – 0.50||Sandstone||Coarse sand with abundant shell fragments|
|0.50 – 1.20||Sandstone||Medium sand with occasional shell fragments|
|1.20 – 1.90||Sandstone||Coarse, well-sorted sand with abundant shell fragments|
|1.90 – 2.20||Sandstone||Medium-grained sand with rare shell fragments|
|2.20 – 3.80||Sandstone||Medium-grained sand with rare shell fragments|
|3.80 – 4.80||Sandstone||Medium to fine sand, no shell fragments|
|4.80 – 5.60||Sandstone||Fine sand, well-sorted, cross-bedded with occasional root structures|
|5.60 – 7.00||Sandstone||Medium to fine sand, no root structures or shell fragments|
|7.00 – 7.45||Sandstone||Fine sand, well-sorted with cross-beds|
|7.45 – 8.05||Sandstone interbedded with mudstone||Unconformity at the base, alternating poorly sorted sand and 1 to 10 cm thick beds of silty clay with bioturbation. Root structures and shell fragments throughout.|
|8.05 – 8.60||Mudstone||Silty clay, moderate bioturbation, root structures, shells, and peat|
|8.60 – 9.20||Sandstone interbedded with mudstone||Alternating medium- to coarse-grained sand and 1 to 10 cm thick beds of silty clay with root structures|
|9.20 – 10.80||Mudstone||Silty clay, moderate bioturbation, root structures, shell fragments, and peat|
|10.80 – 13.00||Mudstone||Dark gray silty clay, intense bioturbation, and shells|
Use your information from Part II to construct your stratigraphic column. If you have computers available, you can do this digitally through the free program Sedlog (Some geologists also use another free program, PSICAT). Note: a stratigraphic column like this is exactly how geologists present lithologic data from a sedimentary core. Indicate which facies of trace fossil is likely the cause of the bioturbation.
Briefly describe the environmental history of this core starting from the oldest layer.
Remember that not all stratigraphic units are laterally continuous, which can make correlation difficult. In 1820, William Smith noticed that while digging canals in different parts of England, he would occasionally find fossils. He noted that the types of fossils changed with respect to his location. Whenever he found fossils that matched, he assumed the locations were from the same layer and eventually made maps detailing the location of the layers and predicting the types of rocks and fossils they would find at different locations while digging more canals. This made it easy to avoid “problem” rock layers and predict the best way to dig a new canal.
The branch of stratigraphy that uses fossils to determine depositional environments and ages of sediment is called biostratigraphy. In this exercise, you will use a combination of lithostratigraphy and biostratigraphy.
Background: Your cousin has a small fossil shop that has become very popular among tourists, but the location your cousin has been collecting fossils from seems to be running out. You have been asked to help find the best place to collect more fossils and to see if you can find out information about what the environment was like when the animal lived. Your cousin told you there are 5 different lithologies in the area. You have gone to several locations and described the geology at each.
Use the following map and your notes about what you observed to see if you can find a new location to collect more fossils.
- Use the clues in your notes to figure out the extent of the rock layers in the area.
- Group the sedimentary rocks from your seven stops into five lithologic units. On the map, draw lines that separate the different units. Indicate which stop(s) are in each unit, and on the graph, fill in the stops for each unit.
- Create a geologic description of each rock unit based on your notes.
- Shade in an area on your map that you would recommend looking for additional fossils.
|Stop 1||This is where your cousin has been harvesting fossils, specifically a shell that looks similar to a clam. Fossilization has made it have a variety of colors that are dazzling to the eye.|
|Stop 2||You’ve seen lots of fossil imprints of what look to be leaves and branches. You don’t see any sedimentary structures and the rock is very fine-grained.|
|Stop 3||This unit is dark shale and has some fossil fish in it. A local geologist has told you that this is the oldest unit in the area.|
|Stop 4||Cross bedding!! No fossils, but you were fascinated by beautiful cross-bedded sandstone from what look to have been oscillatory waves.|
|Stop 5||You have found another shale. There aren’t as many fossil fish here, but you have seen a few smaller ones. You try to dig one out, but they keep breaking apart.|
|Stop 6||So many shell pieces. Unfortunately, everything here is broken up. You think you have seen parts of the desired fossil, but everything has been broken to bits and mixed together.|
|Stop 7||More leaves and branches. There are so many imprints of leaves here it’s hard to distinguish all the different plants that must have been growing here.|
5.4 Physical Properties
When geologists recover sediment cores, they run an assortment of analyses on the core itself and the hole it came from. After the drilling is completed, geologists will lower an instrument into the hole that will take physical property measurements. These measurements can be continuous or occur at specific intervals. Properties that are commonly measured are natural gamma radiation, resistivity, conductivity, density, porosity, and others. These properties are used to give geologists as much information about the rocks or sediment as possible. Physical properties can also be used to correlate sediment cores to each other. Geologists make these correlations so that when they research the cores over the next few years, they can easily correlate a section on one core to a section on another.
Figure 5.16 below shows the natural gamma radiation logs for two cores that were recovered by the International Ocean Discovery Program Expedition 369 from the southwest coast of Australia in the fall of 2017. These two cores were only 20 meters apart. The unit for natural gamma radiation is counts per second (CPS), which really doesn’t mean anything to you, but the higher the number the more clay particles there are in the sediment. So geologists can use this property to determine and compare lithology. It is also used in oil and gas exploration.
Correlate the two sediment cores to each other using the natural gamma radiation. Draw dashed lines across both columns that match patterns to each other.
Side note: You might be wondering why the IDOP collects sediment cores in close proximity to each other. The reason for this is that the drill rig is only capable of collecting 1.5 m of core at a time, and when there is a section break every 1.5 m, some of the sediment is actually lost where one section ends and the next one starts. Additionally, there are a number of reasons that could cause the sediment to not be completely recovered. So, the IODP collects a few cores in close proximity to each other that are slightly offset to make sure they can get a complete record of the sediment. For example, the first section of the first drill site will start at the seafloor and collect sediment every 1.5 m, but the first section at the second drill site will only go down 1.0 m and then the rest will be every 1.5 m.
Magnetic properties of sediment cores are also analyzed by geologists (magnetostratigraphy) for correlation, but they are also used to help determine the age that the sediment was deposited. Do you remember how magnetic minerals in igneous rocks align themselves to Earth’s magnetic field as the mineral crystallizes? Well, sediments can align themselves with Earth’s magnetic field as they are deposited. So, the paleomagnetic properties in sediment cores can be correlated to known changes in Earth’s magnetic field to determine the age of the sediment.
Normal polarity is the magnetic field of today, where the magnetic north pole and geographic north pole are in the Arctic. During reversed polarity, the magnetic north pole moves to the geographic south pole in the Antarctic. Magnetic reversals are a natural process on Earth, and geologists have developed a timescale of these reversals called the Geomagnetic Polarity Time Scale (GPTS).
- Using the inclination data from an IODP sediment core in Figure 5.16 to determine when there was normal and reversed polarity. Normal polarity are positive numbers, reversed polarity are negative numbers. Fill in the blank polarity column by coloring in the normal polarity intervals in black. Reversed polarity intervals will remain white.
- Correlate your polarity column to the GPTS to determine depositional ages for this sediment core. You can do this by drawing lines that match your polarity intervals of the sediment core to the polarity intervals of the GPTS.
- What is the age range of the missing sediment in the core gap? _________________________
5.5 Seismic and Sequence Stratigraphy
So far, layers on top of one another have been interpreted as being younger than the layers beneath them. In sequence stratigraphy, age is interpreted from lateral variations of rocks. As sea level rises or lowers (or changes in ) deposition migrates landward or seaward. This can be seen in echelon shaped groups () that appear to (lay somewhat on top of) each other as seen in diagram A. These form as land migrates seaward, building these clinoforms on top of one another. A rapid sea-level rise will begin depositing new clinoforms on top of previous ones. In contrast, a rapid sea-level drop will deposit clinoforms that abut the previous ones.
Since a picture is worth a thousand words, Figure 5.18 will help you learn the vocabulary associated with sequence stratigraphy. Terms to know:
- Clinoform – sedimentary strata that are tilted, typically in an S-shaped curve
- Sequence Boundary – often an unconformity formed by erosion that separates different depositional cycles
- Onlap – shallowly dipping, younger strata next to more steeply dipping, older strata; indicative of sea-level rise
- Downlap – steeply dipping younger strata against a surface or underlying shallowly dipping strata; indicative of sea-level fall
- High Stand – sea level rise and relative increase of onlapping sediment packages
- Low Stand – sea level falls
- Unconformity – surface separating older from younger rocks and representing a gap in the geologic record
- Base Level – commonly the same as sea level but changes due to basin size and tectonics
- Maximum flooding surface – a surface that marks the transition from a transgression to a regression
As you look at this figure there are several things to note. One is that the clinoforms are not flat. Why, you might ask yourself. Well, sediments that are deposited on flat surfaces are flat-lying whereas those that are deposited on shallowly dipping surfaces are shallowly dipping. Layers will mimic the underlying layers until the angle of the underlying surface exceeds that of the maximum angle of repose. This is the angle at which loose sediments will cascade or landslide down the slope. The angle of repose varies depending on the sediment and can vary from 0° to 90°; sandstone is about 30° and shale is higher than 40°. So, the slope of the clinoforms reflects the underlying sloping strata. The continental margins are where you might commonly find sloping surfaces. So, clinoforms indicate that you are in a marine continental margin.
Look carefully at the top sequence boundary. Does it look like a clinoform? Sometimes sequence boundaries look similar to clinoforms as the transition between depositional cycles can be very gradual without significant erosion. You have to look to see if there is onlap of the next strata to be able to distinguish a clinoform from a sequence boundary.
In this exercise, you will work on determining whether sea level influenced the deposition of Cretaceous sediments as well as the different features associated with sequence stratigraphy. To do this, you will investigate a stratigraphic section in the Colville Basin, Arctic Alaska (Figure 5.19).
Long before Alaska was granted statehood, oil was found seeping from the ground in the Arctic. This oil helped lead to the creation of the National Petroleum Reserve of Alaska (NPRA – an area the size of Indiana). This area is also known for its coal; some have estimated that this region has ~40% of the U.S. reserves. Arctic Alaska has provided much of the energy resources for the rest of the United States for many years. To help define where the oil was located and how it formed, the U.S. Geological Survey conducted many seismic surveys (often called lines) across the region. We will look at one line for this exercise. Figure 5.20 shows the Cretaceous formations in this section: Pebble Shale unit, Torok Formation, and Nanushuk Formation.
Before you analyze the seismic section, you should know how sea level is changing worldwide. If you know if the sea level was rising or falling during sediment deposition, this will let you know if you are in a highstand or lowstand.
- We don’t know the exact age of these sediments, so first you must determine the absolute age of these formations using Figure 3.1. Fill out the ages in Table 5.8.
Table 5.8 – Worksheet for Exercise 5.9a Rock Formation Absolute Age Range Nanushuk Formation Torok Formation Pebble Shale unit
- Using the ages you determined in a and Figure 5.21 to determine if sea level was rising, falling, or not changing when these formations were deposited. You might wonder how to do this since this figure has three different sea-level curves. Well, it is up to you to choose one of these to answer this question. Be sure indicate which curve you used to answer this question
- In this exercise, you will identify the age progression of deposition as well as the different features associated with sequence stratigraphy. In the seismic image, identify as many features as you can. Number them in order of depositional age with “1” being the oldest. You should be able to identify at least 10 features. Hint: There is at least one unconformity. Pay attention to the angles at the beginning and end of your features as well as the angle at which they touch another reflector; if it is at a high angle, this is likely to be either onlap or downlap.
- Using the seismic features as your guide, which direction you would go to find a marine basin? Explain your reasoning.
- Based on the seismic section, do you think that sea level was rising or falling during the deposition of this sequence? Explain your reasoning.
- Is your answer to e the same or different from your answer to b, which used the stratigraphic section and the global sea-level curve? If it’s different, explain what might cause the discrepancy. Or if your answers are the same, how does this strengthen your interpretation?
All the previous exercises were aimed at you understanding changes in stratigraphy due to environmental conditions and processes. Another component to stratigraphy is the tectonic setting for the sedimentary basin. So, which two tectonic settings are important to consider? The next two exercises will address both convergent and divergent plate boundaries in terms of stratigraphic change. Transform margins may have overlapping faults that create subsidence and small .
The Appalachian/Caledonide Mountains formed during three separate tectonic events over a period of 220 million years. These events were the Taconic (470 to 440 Ma), Acadian (420 to 380 Ma), and Alleghenian (350 to 250 Ma) orogenies. Between each of these tectonic events, the mountains eroded as they were uplifted. One of the concepts you need to know for this exercise is what happens when you put a load on the lithosphere. You may have learned about isostasy in Physical Geology and how the lithosphere responds like an iceberg melting in the ocean. Well, during continental collision, the added weight of the continental mass will cause the lithosphere to bend creating a or foredeep basin. For example, the Persian Gulf is a basin formed from the collision of the Asian and Arabian plates creating the Zagros Mountains. These sedimentary basins often form shallow (30-100 m) repositories for continentally derived sediments. If the area beside the continental collision is wide, it will form a shallow inland ocean (sometimes called an epeiric or epicontinental sea), such as Baffin Bay in northern Canada.
The Catskill clastic wedge (Figure 5.23) is a package of sediment that was sourced from the Appalachian Mountains during the Acadian Orogeny. This sediment package extends from New York through Virginia and West Virginia and is part of the larger Appalachian Basin. In western Virginia and eastern West Virginia, the Catskill clastic wedge contains evidence for three phases of the Acadian Orogeny:
Phase 1 – A rapid drop of the sea-floor caused by the massive weight of the mountains. The load causes the lithosphere to bend, creating a concave basin in front of the uplifting mountains. It is characterized by a sudden change to deep-water sediments such as shales.
Phase 2 – As the mountains erode, coarse sediment is deposited. These can be deposited in fluvial (rivers) or near-shore marine depositional settings.
Phase 3 – At the end of the tectonic event, at the end of the mountain building process there is a return to a stable passive margin sequence ranging from fine sediment deposition to shallow marine carbonates.
Complete the following questions about the Catskill clastic wedge:
- Figure 5.24 is a roadside outcrop of the Brallier Formation in Pennsylvania, which is composed of shale and fine sandstone layers. Provide a sketch of this outcrop and identify the layers of sandstone vs the layers of shale.
- Figure 5.25 is a roadside outcrop of the Hampshire Formation in Pennsylvania, which is composed of sandstone and fine-grained siltstone. Provide a sketch of this outcrop.
- What sedimentary structure is shown in Figure 5.20 and can you tell in which direction sediment was being transported?
- Figure 5.26 below contains the sequence of sedimentary rocks associated with the Catskill classic wedge. Based on the description of the rocks, identify the depositional environment for each stratigraphic unit and which orogenic phase it represents.
- Figure 5.24 is a roadside outcrop of the Brallier Formation in Pennsylvania, which is composed of shale and fine sandstone layers. Provide a sketch of this outcrop and identify the layers of sandstone vs the layers of shale.
Another dynamic aspect of plate tectonics is a process we call “continental rifting”, a process in which a continental mass split apart and two separate new landmasses form and drift away from each other. After the last orogeny in the Appalachian mountains, Pangea broke apart into our current continent configuration through continental rifting processes. One example of this continental rifting was responsible for the present geologic setting for the eastern margin of theSouth American continent.
The Recôncavo basin in Brazil, near the city of Salvador, is the southernmost basin in northeastern Brazil created by continental rifting of South America and Africa. This rifting process is well recorded in the eastern Brazilian margin, and shows different stages of this process:
- Beginning of the cycle, before rifting, continental processes such as fluvial, eolian and lacustrine deposition dominate the sediment deposition.
- Then as the rifting begins, igneous activity begins. In continental rifts, these are usually bimodal (rhyolite and basalt) volcanics. Rifting also causes normal faulting of the continent forming grabens and half grabens resulting in lacustrine (lake) deposits. In some areas, very high energy processes dominate. As the continent starts to split apart, the ocean water will flood the depression caused by the faulting, creating shallow-water seas, which sometimes host a variety of shallow marine species.
- As the shallow seas and lakes are evaporated at high rates, massive salt and anhydrite (evaporites) deposits are formed
- Rifting completely separates the continent into two new landmasses, which start drifting apart from each other and are marked by a “passive margin system”. Tectonic activity is no longer present, and the sediments are increasingly deeper marine.
Part I – Stratigraphic Evidence of Continental Rifting
- Determine the depositional environments for the different lithologies in the stratigraphic column of the northeastern Brazilian Recôncavo Basin in Figure 5.28 below.
- Identify the 4 stages of rifting described at the beginning of this exercise and mark them 1, 2, 3, and 4 in the right-most column in Figure 5.28. Mark your boundaries between the stages by drawing a thick line at the top of the formation that ends each stage.
- Critical Thinking: What will the sedimentary environment be in this part of Brazil after rifting has stopped?
- Critical Thinking: Today, the Atlantic Ocean marks the rift boundary between Brazil and Africa. At what stage did the rifting stop in the Recôncavo Basin? Why?
Part II – Outcrop Interpretations
- What sedimentary environment do you find oysters?
- Why is this rock black?
- What is the sedimentary environment of the overlying buff-colored layers?
- How does this change in sedimentary environments fit with the tectonic setting of the Recôncavo Basin?
- What type of fault is this? It may help you to draw arrows on either side of the fault showing the sense of motion. ____________________
- What is the age of the fault (remember to look at the stratigraphic column for the age of the formation)? ____________________
- Is this fault related to continental rifting? How does this help you determine the depositional environment?
Daniel Hauptvogel, Virginia Sisson, Carlos Andrade, Joshua Flores, Melissa Hansen
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a seismic reflector may represent a change in lithology, a fault or an unconformity
a graph that depicts the ratios of the three variables as positions in an equilateral triangle
a geologic period that spans 50.6 million years from the end of the Permian Period 251.9 Ma, to the beginning of the Jurassic Period 201.3 Ma. This is the first and shortest period of the Mesozoic Era
a geologic period that spanned 56 million years from the end of the Triassic Period 201.3 Ma to the beginning of the Cretaceous Period 145 Ma. This constitutes the middle period of the Mesozoic Era, also known as the Age of Reptiles
the space that is available for the deposition of sediments. In river systems, changes in accommodation are controlled by the river gradient, water discharge rates and sediment supply.
these sedimentary layers are timelines that represent a moment in geological time
pattern of reflections in seismic data that occur during periods of transgression (sea level rise)
a basin formed between two adjacent strike-slip faults in a region where subsidence generates accommodation space for the deposition of sediments
structural basin that develops adjacent and parallel to a mountain belt formed from the mass created by crustal thickening that causes the lithosphere to bend creating accommodation space