Chapter 5: Stratigraphy

Learning Objectives

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

5.1 Introduction

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; 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 ship dedicated to 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 by sending sound waves into the seafloor or ground and monitoring how the waves reflect back to the surface (Figure 5.2). Previous seismic surveys were only collected 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 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 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. Sometimes several formations are lumped together to form a group. If you want to propose a new formation or group, there are strict guidelines set up by the International Commission of Stratigraphy.

 

Three pictures showing how marine geoscientists collect and store rock cores.
Figure 5.1 – A) The JOIDES Resolution, one of the research vessels used by the IODP. B) Scientists studying recovered core during Expedition 378 in the South Pacific. C) An IODP core repository at the University of Bremen in Germany; others are located at Texas A&M University in Texas, Rutgers University in New Jersey, and Kochi University in Japan. Image credit: A) Thomas Ronge and IODP, CC BY; B) Lindy Newman and IODP, CC BY; C) MARUM, CC BY-SA.

If you look closely at Figure 5.2, the sedimentary are relatively flat and show layering in the Kumano Basin (top left side of the image or NW end; highlighted in green in Figure 5.2b) 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, low energy water. Below these regions, and in the middle of the seismic profile, 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 downgoing oceanic crust. Towards the bottom, the seismic reflectors are very discontinuous as the oceanic crust has no internal structure. The of the layering lets you know important tectonic and sedimentary history along this profile.

 

Two images of the same cross-section of an accretionary prism offshore of Japan.
Figure 5.2 – Two seismic images of an accretionary prism near the Nankai trough offshore Japan. Uninterpreted (A) and interpreted (B) seismic data from IODP Expeditions 314, 315, and 316 for the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) in 2009. The interpretations on B show several packages of marine sediments overlying oceanic crust. The yellow line is the bottom simulating reflector (BSR). Image Credit: Moore et al., 2009 and IODP, CC BY.

Exercise 5.1 – Sediment Coring and Stratigraphic Columns

This exercise is an analogy for how geologists recover sediment cores. Instead of sediment, though, we will use Play-Doh.

  1. Using an empty Play-Doh container (or another container), place layers of different colored Play-Doh in the container; don’t use more than 10 layers. These layers of Play-Doh will act as our layers of sediment.
  2. Take a clear, plastic straw and push it down through the layers of your Play-Doh. This represents geologists drilling into layers of sediment.
  3. 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 while the other half will be archived and 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.

  1. Measure and record the total length of your Play-Doh core in millimeters. ___________________
  2. Measure and record the thickness of each layer in your Play-Doh core in Table 5.1. Start with the upper-most layer.
    Table 5.1 – Play-Doh “sediment” layer characteristics
    Layer Color Thickness (mm)
  3. 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. This will be your first layer.
  4. Now add the second layer to the column. If your second 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.
  5. 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?  ____________________
    A blank stratigraphic column for Exercise 5.1.
    Figure 5.3 – A blank stratigraphic column for Exercise 5.1.

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 s are subdivided by grain size, rounding and sorting. Also, how rocks weather is important in these exercises because their differences in weathering resistance 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; others use it for studying and to locate fluids (water or oil), or in the case of engineers, how much weight the sedimentary rock can hold before becoming unstable.

Exercise 5.2 – Grain Size

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 you need to be more accurate in the lab. Geologists use quantitative approaches with various particle size analyzers (instruments that can accurately measure grain size and shape), but the traditional way to measure grain size was to separate the sediment into different size ranges using sieves and then to measure 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

  1. Measure the starting weight of the entire sediment sample in grams. ____________________
  2. Now separate the sediment into different fractions using the set of sieves provided by your instructor.
  3. Measure and record the weight of each size fraction in the first two columns of Table 5.2.
  4. What weight percentage does each fraction make up? Fill in the third column of Table 5.2. 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 5.2 – Grain Size Analysis
    Size fraction range (μm) Weight (g) Weight Percent
  5. Take your weight percent data and plot it as a bar graph in Figure 5.4.
    Grain size graph to use in Exercise 5.2.
    Figure 5.4 – Grain size graph to use in Exercise 5.2.
  6. 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 5.3 contains percentages of sand, silt, and clay. Plot these percentages on the triangle graph in Figure 5.5 and record the descriptive term for the sediment size in the last column of Table 5.3.
    Table 5.3 – 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
    Triangular plot for sedimentary rocks with proportions of clay, silt, and sand.
    Figure 5.5 – Triangular plot for sedimentary rocks using proportions of sand, silt, and clay. Instructions for how to plot on a triangular plot are given with Figure 0.6. This plot is subdivided into 10 different areas that are used to assign a rock name to a sample. The abbreviations for each field are given on the figure. Image credit: Adapted from Krumbein, W.C., and Sloss, 1963 by VB Sisson CC BY-NC-SA.

Exercise 5.3 – Interpreting Outcrop Stratigraphy

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 (LIP) in the Atlantic ocean. The Triassic in this region is represented by the Lilstock Formation, which was 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.

Figure 5.6 – Triassic – Jurassic Boundary Section by NHM_Imaging on Sketchfab, CC BY-NC.

  1. Sketch a stratigraphic column of Figure 5.6. This is easy to do because the limestones and shales weather differently. Limestones tend to be more resistant to weathering than shales and thus make prominent layers.
    Sketch Area:
  2. Is there an unconformity in this stratigraphic column? Explain.
  3. What is the dominant lithology in the Triassic? ____________________
  4. What is the dominant lithology in the Jurassic? ____________________
  5. Can you explain why the lithology changed?

5.3 Facies and 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 that occur 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 sediments are 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 ), 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 will 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 ), 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).

Three periods of transgression of sea-level rise at the edge of the continent. Below are several stratigraphic columns.
Figure 5.7 – A time-lapse of sea-level rise, also called transgression. At each time interval, the sediment package that is deposited moves in the direction sea level is rising. The sediments are stacking on top of each other toward the coastline, and that pattern is called  onlapping. The red boxes in Time 3 represent sediment cores taken at those locations. By looking at the patterns in the sediment cores, you can tell that sea level was rising, and the coastline was to the right. Blue pattern represents limestone, gray pattern represents shale and yellow dotted pattern is for sandstone.  Image credit: Daniel Hauptvogel, CC BY-NC-SA.
Three periods of regression of sea-level fall at the edge of the continent. Below are several stratigraphic columns.
Figure 5.8 – A time-lapse of sea-level fall, also called regression. At each time interval, the sediment package that is deposited moves in the direction sea level is falling. The sediments are stacking on top of each other away from the coastline, and that pattern is called offlapping. The red boxes in Time 3 represent sediment cores taken at those locations. By looking at the patterns in the sediment cores, you can tell that sea level was falling and the coastline was to the right. Same patterns as in Figure 5.7. Image credit: Daniel Hauptvogel, CC BY-NC-SA.

Exercise 5.4 – Transgression and Regression

Identifying transgressive (rise) and regressive (fall) cycles in sea level is a key component of lithostratigraphy and sequence stratigraphy. Figure 5.9 is a basic stratigraphic column that contains evidence of transgression and regression.

  1. Shade in the facies environment box for each layer of sedimentary rock in the column.
  2. Identify where transgression and regression are taking place. You can mark this with vertical arrows on the side of the figure.
  3. Can you tell how much sea level has changed? Explain.
  4. Can you tell what direction the coastline was in? Explain.
    Stratigraphic column for Exercise 5.3 to identify marine transgression and regression..
    Figure 5.9 – Stratigraphic column for part of Exercise 5.4. The colors and patterns are similar to figures 5.7 and 5.8. You will need to fill in the columns on the right side of the figure. Image credit: Daniel Hauptvogel, CC BY-NC-SA.
  5. 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.
  6. Which layer pinches out? ____________________
  7. Explain why that layer pinched out.
  8. 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.

     

    Two stratigraphic columns for the correlation part of Exercise 5.3.
    Figure 5.10 – Two stratigraphic columns for part of Exercise 5.4. You will need to correlate the lithology between these two columns. Note, there are no unconformities in these columns. Image credit: Daniel Hauptvogel, CC BY-NC-SA.
  9. Imagine a scenario where you’re working as a , 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 s. 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.
  10. The best aquifers will be laterally continuous sand layers. Label the layer(s) that you think would be the best source of water.
  11. What is the minimum depth you need to drill to install a new water well? ____________________
Four stratigraphic columns for part of Exercise 5.3 as well the proposed location for a new water well.
Figure 5.11 – Four stratigraphic columns for part of Exercise 5.3 and the proposed location for a new water well. The height of the columns changes from core to core, representing a small hill. The horizontal scale is not the same as the vertical scale. Image credit: Joshua Flores and Daniel Hauptvogel, CC BY-NC-SA.

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 sediment chemistry, they could result from the facies changing. So, you need to have a good and accurate understanding of the facies.

Exercise 5.5 – Facies in a Coastal Environment

Part I

Coastal environments can have many sedimentary facies (Figure 5.12), and each has something different about it. You’ll notice that these facies are mainly sandstone or mudstone, which is not distinctive. Sedimentary rocks have other characteristics besides their particle size, including the abundance of fossils, sedimentary structures, and layering patterns of the strata. Use figure 5.12 below to match the environments to the lithofacies and descriptions for: , , , , , , and .

Coastal sedimentary environments and facies from offshore to beach and back bay.
Figure 5.12 – A model of coastal sedimentary environments and facies. Image credit: Carolina Ramón Dueñas, CC BY-NC-SA.
Table 5.4 – Lithofacies descriptions for some environments in Figure 5.12
Environment Lithofacies Description
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

Part II

You have collected a 13 m sediment core from the area. Your team has separated and described the sedimentary layers. Based on your team’s descriptions, identify the environment for each layer (using environments from Part I) and fill in the last column of Table 5.5.

Table 5.5 – Descriptions of layers in a sediment core for which you need to interpret the environment
Scale (m) Lithology Description Environment
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

Part III

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); otherwise, you can use Figure 5.13 to hand-draw your column. Indicate which facies of trace fossil is likely the cause of the bioturbation. Note: a stratigraphic column like this is exactly how geologists present lithologic data from a sedimentary core.

A detailed stratigraphic column that you need to fill out as part of Exercise 5.4
Figure 5.13 – A detailed stratigraphic column that you need to fill out as part of Exercise 5.4.

Part IV

Briefly describe the environmental history of this core, starting from the oldest layer.

Exercise 5.6 – Facies and Biostratigraphy

Remember that not all stratigraphic units are laterally continuous, which can make correlation difficult. In 1820, William Smith noticed that he would occasionally find fossils while digging canals in different parts of England. He noted that the types of fossils changed according to his location. Whenever he found fossils that matched, he assumed the locations were from the same layer and made maps detailing this. From the maps, he predicted the types of rocks and fossils found 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 . 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 where 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 see if you can find out information about the environment when the animals were alive. 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 your notes from Table 5.6 and the map of the stops in Figure 5.14 to see if you can find a new location to collect more fossils.

  1. Group the sedimentary rocks from your seven stops into five lithologic units. On the map in Figure 5.14, draw lines that separate the different units.
  2. Indicate which stop(s) are in each unit in Figure 5.15. Each stop can only belong to one unit.
  3. Create a geologic description of each rock unit based on your notes in Table 5.6.
  4. Shade in an area on your map that you would recommend looking for additional fossils.
Table 5.6 – Descriptions of the stops for Exercise 5.6
Stop Description
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.
Map showing various places (stops) that your cousin visited previously to collect fossils for his shop.
Figure 5.14 – Map showing various places (stops) that your cousin visited previously to collect fossils for his shop. Use this to draw a map showing the five different lithologies.

 

A stratigraphic column to fill out for Exercise 5.6.
Figure 5.15 – A stratigraphic column to fill out for Exercise 5.6.

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 to take physical property measurements. These measurements can be continuous or occur at specific intervals. Properties that are commonly measured are natural , , , 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 to easily correlate a section on one core to a section on another.

Exercise 5.7 – Physical Property Correlation

Figure 5.16 below shows the natural gamma radiation logs for two cores recovered by the International Ocean Discovery Program Expedition 369 from the southwestern coast of Australia in the fall of 2017. These two cores were only 20 meters away from each other. The unit for natural gamma radiation is counts per second (CPS). You don’t need the details behind this unit’s meaning, but the higher the number, the more clay particles there are in the sediment (more shale-like). Geologists use this property to determine and compare lithology; sand versus clay and mud. This property is also used extensively both in oil and gas exploration and hydrogeology.

 

  1. 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.
  2. How many layers did you correlate? _______________
  3. Compare your answer to c to your classmates. Did you have the same number of layers? Explain any differences.
Natural gamma radiation logs for two holes as part of IODP Expedition 369
Figure 5.16 – Natural gamma radiation logs for two holes as part of IODP Expedition 369. The data is from IODP.

Side note: You might be wondering why the IDOP collects sediment cores near 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 several reasons that could cause incomplete recovery of sediment. So, the IODP collects a few cores close to each other and slightly offset to ensure 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.

Exercise 5.8 – Paleomagnetic Properties to Determine Depositional Age

Geologists also analyze magnetic properties of sediment cores 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 while they are being deposited too. So, paleomagnetic properties in sediment cores can be correlated to known changes in Earth’s magnetic field to determine the sediment’s depositional age.

We currently have normal polarity, where the magnetic north pole and the geographic north pole are in the Arctic. During periods of reversed polarity, the magnetic north pole moves to the geographic south pole in the Antarctic. These magnetic reversals are a natural process on Earth, and geologists have developed a timescale of these reversals called the Geomagnetic Polarity Time Scale (GPTS).

  1. Use the inclination data from an IODP sediment core in Figure 5.17 to determine periods of normal and reversed polarity. Normal polarity is a positive number, and reversed polarity is a negative number. Fill in the blank polarity column by coloring in the normal polarity intervals in black. Reversed polarity intervals should remain white.
  2. 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.
  3. What is the age range of the missing sediment in the core gap? _________________________
Inclination data from an IODP drill site to correlate with paleomagnetic data for exercise 5.8
Figure 5.17 – Inclination data (blue dots) from an IODP drill site from Expedition 352. There is a core gap between 38 m and 42 m (gray bar), which means the sediment there was not recovered during the drilling process. Gaps in sediment cores are frequent and can happen for a variety of reasons.  Data from IODP

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 over previous ones.

Sequence stratigraphy comes with a slew of new vocabulary words described below and shown in Figure 5.18.

  • Base Level – commonly the same as sea level but changes due to basin size and tectonics
  • Clinoform – sedimentary strata that are tilted, typically in an S-shaped curve
  • 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
  • Maximum flooding surface – a surface that marks the transition from a transgression to a regression
  • Onlap – shallowly dipping, younger strata next to more steeply dipping, older strata; indicative of sea-level rise
  • Sequence Boundary – often an unconformity formed by erosion that separates different depositional cycles
  • Unconformity – surface separating older from younger rocks and representing a gap in the geologic record
Line drawing of simplified seismic reflections seen in a stratigraphic sequence.
Figure 5.18 – Seismic patterns for a stratigraphic sequence. The lines are seismic reflections.  Their shape and how they terminate (shown with the arrowheads) at different locations indicate different geological processes. Image credit: adapted from Vail et al. (1977) by Virginia Sisson and Carolina Ramon Dueñas, CC BY-NC-SA.

As you look at this figure, there are several things to note. First, 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. New sedimentary layers will mimic the underlying layers until the underlying surface’s angle exceeds that of the maximum angle of repose. The angle of repose is the angle at which loose sediments will cascade or landslide down the slope, and it varies depending on the sediment; sandstone is about 30°, and shale is higher than 40°. So, the slope of the clinoforms reflects the underlying sloping strata. Continental margins are where you can commonly find sloping surfaces.

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 distinguish a clinoform from a sequence boundary.

Exercise 5.9 – Sequence Stratigraphy from a Seismic Image

In this exercise, you will work on determining whether sea level influenced the deposition of 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).

Geologic map of part of the National Petroleum Reserve of Alaska (NPRA) west of the Colville River
Figure 5.19. Geologic map of part of the National Petroleum Reserve of Alaska (NPRA) west of the Colville River. You may wonder why all the exposure is mapped as  Quaternary deposits and not Cretaceous.  This is because glaciation and formation of tundra has covered all of the rock exposures.  They are only exposed along rivers and seaside cliffs as well as buried beneath the tundra. This is one reason you need to use a seismic line to do this exercise. Image credit: adapted from the geologic map of Payne (1951) with the location of a seismic line from the U.S. Geological Survey NPRA data archive by Virginia Sisson, CC BY-NC-SA.

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.

Simplified stratigraphic section for the Cretaceous sedimentary sequence in northern Alaska
Figure 5.20. Simplified stratigraphic section for the Cretaceous sedimentary sequence in northern Alaska. Image credit: adapted from Mull et al. (2003) by Virginia Sisson, CC BY-NC-SA.

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.

  1. 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
  2. Use the ages you determined in a and the sea-level record for the Eon in Figure 5.21 to determine if sea level was rising, falling, or not changing when these three formations were deposited. You might wonder how to do this since this figure has three different sea-level curves. It’s up to you to choose one of these to answer this question. Indicate which curve you used to answer this question
    Three proposed sea-level curves for the Phanerozoic era.
    Figure 5.21. Three proposed sea-level curves for the Phanerozoic Eon. The gray area is a compilation of curves proposed by researchers at ExxonMobil. The dashed purple curve considered global ice volumes without tectonic influences on accommodation space. The red curve used a geodynamic model combined with sea level data. Image credit: simplified by Virginia Sisson CC BY-NC-SA from Vérard et al. (2015).
  3. In this exercise, you will identify the age progression of deposition and 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 and the angle at which they touch another reflector; if it is at a high angle, this is likely to be either onlap or downlap.
    Seismic section across the Colville basin, Arctic Alaska for exercise 5.9
    Figure 5.22 Seismic cross-section (line 16-81b) across Cretaceous sediments of the northeastern National Petroleum Reserve of Alaska (NPRA). These sediments filled the Colville foreland basin about 6000′ deep. This view is looking to the north so that the left side is to the west, whereas the right side is to the east. Figure 5.19 shows the location of this seismic line. Blue is for positive amplitude, black for negative amplitude, and white is for no amplitude difference. Image credit: SEPM exercises for sequence stratigraphy with data from U.S. Geological Survey NPRA data archive public domain

     

  4. Using the seismic features as your guide, which direction would you go to find a marine basin? Explain your reasoning.
  5. Based on the seismic section, do you think that sea level was rising or falling during the deposition of this sequence? Explain your reasoning.
  6. 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 of 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 (470 to 440 Ma), (420 to 380 Ma), and (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 increase the weight on the lithosphere. You may have learned about 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.

Exercise 5.10 – Convergent Tectonics in a Sedimentary Record

The Catskill clastic wedge (Figure 5.23) is a sediment package 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 mountain building process, there is a return to a stable passive margin sequence ranging from fine sediment deposition to shallow marine carbonates.

Continental collision can result in formation of the inland Kaskaskia sea and deposition of sediment in foreland basins
Figure 5.23 Continental collision can result in the formation of inland seas, such as the Kaskaskia Sea and foreland basins. The brown represents the collision of North America with the Avalon terrane. Image credit: Virginia Sisson, CC BY-NC-SA.

 

Complete the following questions about the Catskill clastic wedge:

    1. Figure 5.24 is a roadside outcrop of the Brallier Formation in Pennsylvania, composed of shale and fine sandstone layers. Provide a sketch of this outcrop and identify the layers of sandstone vs. the layers of shale.
      Outcrop of Devonian Brallier Formation on the north side of the Pennsylvania Turnpike, Pennsylvania
      Figure 5.24 – Outcrop of Devonian Brallier Formation on the north side of the Pennsylvania Turnpike, central Bedford County, near Mile Marker 138 in Pennsylvania. Sandstone beds near the bottom of the photo are possible turbidites. Image credit: Wikimedia user Jstuby, CC0 Public Domain, photo taken August 2011.
      Sketch Area:
    2. Figure 5.25 is a roadside outcrop of the Hampshire Formation in Pennsylvania, composed of sandstone and fine-grained siltstone. Provide a sketch of this outcrop.
      Point bar deposits in the Hampshire Formation near North Bend, Pennsylvania
      Figure 5.25 – Point bar deposits in the Hampshire Formation near North Bend, Pennsylvania. Image credit: Wikimedia user Rygel, M. C., CC BY-SA.
      Sketch Area:
    3. What sedimentary structure is shown in Figure 5.25, and can you tell in which direction sediment was being transported?
    4. Figure 5.26 below contains the sequence of sedimentary rocks associated with the Catskill classic wedge. Based on the rock descriptions, identify the depositional environment for each stratigraphic unit and which orogenic phase it represents.
Stratigraphy of the Catskill clastic wedge with simplified lithology for each unit.
Figure 5.26 – Stratigraphic units of the Catskill clastic wedge and the dominant lithology of each unit. Image credit: adapted by Carlos Andrade and Virginia Sisson CC BY-NC-SA.

Another dynamic aspect of plate tectonics is continental rifting, a process in which a continental mass splits 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. The Recôncavo basin in Brazil near the city of Salvador (Figure 5.27) 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:

  1. Before rifting begins, continental processes such as fluvial, eolian, and lacustrine deposition dominate the sediment deposition.
  2. Then as the rifting begins, igneous activity also begins. In continental rifts, this is usually bimodal (rhyolite and basalt) volcanism. Rifting also causes normal faulting of the continent forming 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 various shallow marine species.
  3. As the shallow seas and lakes are evaporated, massive salt and anhydrite (evaporites) deposits are formed.
  4. Rifting completely separates the continent into two new landmasses, which start drifting apart and are marked by a “passive margin system”. Tectonic activity is no longer present, and the sediments are increasingly deeper marine.
Image with the geographic location of The Recôncavo basin
Figure 5.27 – The Recôncavo basin covers 11,500 km2 and is a prime source for petroleum. Since 1939, 225 wells have been drilled. By 2012, this basin had produced over 1.6 billion barrels of oil and 70 billion km3 of gas. Image credit: simplified by Virginia Sisson CC BY-NC-SA from Magnavita et al., 2005  with insert map of Brazil from Wikimedia Commons user Addicted04 CC-SA

Exercise 5.11 – Continental Rifting in a Sedimentary Record

Part I – Stratigraphic Evidence of Continental Rifting

  1. Determine the depositional environments for the different lithologies in the northeastern Brazilian Recôncavo Basin’s stratigraphic column in Figure 5.28 below.
  2. Identify the 4 stages of rifting (described above Figure 5.28) 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.
    Simplifed stratigraphy of the the Brazilian Recôncavo Basin during the Jurassic and Cretaceous
    Figure 5.28 – Stratigraphy of the Brazilian Recôncavo Basin with simplified lithology. Image credit: Carlos Andrade and Virginia Sisson CC BY-NC-SA.
  3. Critical Thinking: What will the sedimentary environment be in this part of Brazil after rifting has stopped?
  4. 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

 

Sandstones and fossiliferous shales from the Dom João Formation on the southwestern side of the Recôncavo basin at Praia da Pedra Mole
Figure 5.29 – Sandstones and fossiliferous shales from the Dom João Formation on the southwestern side of the Recôncavo basin at Praia da Pedra Mole (translates approximately to soft rock beach) near Barra do Paraguaçu. These geologists have a rock hammer on the outcrop, and one is about to take a picture. They are standing among large blocks with distinctive white spots, which are oyster shells. Image credit: Luzia Brito via Google Earth.

The following questions refer to Figure 5.29.

  1. What sedimentary environment do you find oysters?
  2. Why is the rock in Figure 5.29 black?
  3. What is the sedimentary environment of the overlying buff-colored layers?
  4. How does this change in sedimentary environments fit with the tectonic setting of the Recôncavo Basin?
    Dom João Formation along BR 101 about 14 km southwest of Algoinhas, Brazil
    Figure 5.30 – Dom João Formation along BR 101, about 14 km southwest of Algoinhas, Brazil. The red color is caused by two processes; one is intense tropical weathering creating red soil (the left side of the outcrop is almost completely converted into red soil as is a layer at the top of the outcrop), and two, hematite oxidation during deposition of these sandstones and shales. The bedding in this outcrop is offset by a fault along the yellow line. Image credit: Google Street View.
  5. What type of fault is present in Figure 5.30? It may help you to draw arrows on either side of the fault showing the sense of motion. ____________________
  6. What is the age of the fault (remember to look at the stratigraphic column for the age of the formation)? ____________________
  7. Is this fault related to continental rifting? How does this help you determine the depositional environment?

 

Exercise Contributions

Daniel Hauptvogel, Virginia Sisson, Carlos Andrade, Joshua Flores, Melissa Hansen

References

Krumbein, W.C., and Sloss, 1963, Stratigraphy and Sedimentation (2nd edition), W.H. Freeman and Co., 660 p.

Magnavita, L., Da Silva, R.R., and Sanches, C.P., 2005, Field trip guide of the Recôncavo basin, NE Brazil, April 2005, Boletim de Geociencias – Petrobras, 13(2), 301-333.

Miller, K.G., Kominz, M.A., Browning, J.V., Wright, J.D., Mountain, G.S., Katz, M.E., Sugarman, P.E., Cramer, B.S., Christie-Blick, N., and Pekar, S.F., 2005, The Phanerozoic Record of Global Sea-Level Change, Science, 310, 1293-1298. doi: 10.1126/science.1116412

Moore, G.F., Park, J.-O., Bangs, N.L., Gulick, S.P., Tobin, H.J., Nakamura, Y., Sato, S., Tsuji, T., Yoro, T., Tanaka, H., Uraki, S., Kido, Y., Sanada, Y., Kuramoto, S., and Taira, A., 2009. Structural and seismic stratigraphic framework of the NanTroSEIZE Stage 1 transect. In Kinoshita, M., Tobin, H., Ashi, J., Kimura, G., Lallemant, S., Screaton, E.J., Curewitz, D., Masago, H., Moe, K.T., and the Expedition 314/315/316 Scientists, Proc. IODP, 314/315/316: Washington, DC (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/​iodp.proc.314315316.102.2009

Mull, C.G., Houseknecht, D.W., and Bird, K.J., 2003, Revised Cretaceous and Tertiary Stratigraphic Nomenclature in the Colville Basin, Northern Alaska: U.S. Geological Survey Professional Paper 1673, 51 p.

Payne, T.G., 1951, Geology of the Arctic slope of Alaska: U.S. Geological Survey Oil and Gas Investigations Map 126, 3 sheets, scale 1:1,000,000.

Picard, M.D., 1971, Classification of fine-grained sedimentary rocks, Journal of Sedimentary Petrology, 41, 179-195. https://doi.org/10.1306/74D7221B-2B21-11D7-8648000102C1865D

Vail, P.R., Mitchum, R.M., and Thompson, S., 1977, Seismic stratigraphy and global changes of sea level, part 3: Relative changes of sea level from coastal onlap, in C.E. Clayton, ed., Seismic stratigraphy – applications to hydrocarbon exploration: Tulsa, Oklahoma, American Association of Petroleum Geologists Memoir 26, 63–81.

Vérard, C., Hochard, C., Baumgartner, P.O., and Stampfli, G.M., 2015, 3D paleogeographic reconstructions of the Phanerozoic versus sea-level and Sr-ratio variations, Journal of Palaeogeography, 4, 64-84. doi:10.3724/SP.J.1261.2015.00068

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