Chapter 4: Sedimentary Structures

Learning Objectives

The goals of this chapter are to:

  • Identify types of sedimentary structures
  • Explain how sedimentary structures form
  • Interpret paleoenvironments using sedimentary structures

4.1 Introduction

Sedimentary structures are features that form in sediment as it is being deposited. These structures are typically an indication of what the sedimentary environment was like. When looking at structures in sedimentary rocks, they can often be identified by observable patterns in the sedimentary bedding or distinct shapes within the sediment. Basically, if the sedimentary rock doesn’t look uniform and has a distinctive feature, there’s a good chance it’s a sedimentary structure.

Exercise 4.1 – Observing Sedimentary Structures

Part A
For this exercise, your instructor will provide you with a set of sedimentary structures. Look at the samples closely and use a hand lens if necessary. Create a sketch of each sedimentary structure in the blank space below, focusing on what you think are the most important characteristics of the sample. Then, in full sentences, describe the structure you just sketched. If it helps, pretend you are describing what the structure looks like to someone who can not see it.

Structure #1 Sketch:
Description:
Structure #2 Sketch:
Description:
Structure #3 Sketch:
Description:
Structure #4 Sketch:
Description:
Structure #5 Sketch:
Description:

Part B
Now put your observational and descriptive skills to the test. Read your descriptions to your classmates and see if they can identify which samples you are talking about.

4.2 Types of Sedimentary Structures

The simplest sedimentary structure is , which is layering that can be observed in sedimentary rocks (Figure 4.1). Layers of sediment that are thicker than 1 cm are called and layers thinner than 1 cm are called . Laminations are typically composed of fine-grained silt and clay-sized sediment. Structures can be more complex like the wavy pattern seen in ripple marks (Figure 4.2) or chaotic looking patterns in (Figure 4.3).

This image shows slightly tilted layering in sedimentary rock downward to the right.
Figure 4.1 – Titled sedimentary beds from Morro Solar near Lima, Peru. Thicker layers are most likely sandstone and thinner layers are shale. Layering tilts down to the right. Image credit: Miguel Vera León, CC BY.
This image shows ripple marks, which are wavy structures in sedimentary rocks.
Figure 4.2 – Ripples caused by waves. This is a view from the top surface and not the side. This rock is Permian in age from Nomgon, Mongolia. Image credit: Matt Affolter, CC BY-SA.
This image shows an example of cross-bedding, which looks like layers of sedimentary rocks dipping in different directions.
Figure 4.3 – Cross-bedding in sandstone seen on a cliff face in Zion National Park, Utah. You can figure out the scale of the image by looking for trees or other vegetation. Image credit: NOAA, Public Domain.

Sedimentary structures provide a lot of information about the environment in which they formed, including processes that were occurring when sediment was deposited, the environment of deposition, the direction sediment was traveling, and/or the mechanism for transporting the sediment (wind, water, or ice). Some sedimentary structures also help you determine which side of the rock was originally facing upwards, called . When outcrops have overturned rocks (rocks that have been tilted so far they are upside down), sedimentary structures can be used to tell which way was originally facing up.

Each structure tells a story that geologists use to interpret Earth’s history. For this chapter, only a few of them are discussed: dunes and ripple marks, cross-bedding, graded bedding, mudcracks, raindrop imprints, sole marks, and s and .

Dunes and Ripple Marks

As water or wind moves across sediment, it can shape the grains into wavy patterns called dunes (>10 cm) and ripples (<10 cm). , like those seen in Figures 4.2 and 4.4, are formed by the back-and-forth flow of water over sediment. These types of ripples are formed in the shallow marine environment where the back-and-forth motion of waves, or even tides, shape the sediment at the bottom of the ocean. These ripples have symmetrical limbs, meaning that both sides of the ripple dip at about the same angle. This video on symmetrical ripples can help you see how this process works. Symmetrical ripple marks are most commonly found in sandstones from shallow marine environments.

Four images of symmetrical ripple marks. The limbs of the ripples dip at similar angles.
Figure 4.4 – Examples of symmetrical ripple marks with limbs at the same angle on either side of the structure. A) Modern symmetrical ripples from the Bahamas. B, C, and D) Symmetrical ripples in Devonian-Missippian age sandstone from Ohio, USA. These are all views from the top. Image credits: James St. John, CC BY.

Water moving in one direction, like a river, can produce asymmetrical ripple marks. The limbs on these ripples are not equal, with one side that is more shallow and one side that is steeper. These types of ripple marks can tell you which direction the river was flowing because sediment moves up the shallow side of the ripple and gets deposited on the steep side (Figures 4.5 and 4.6). The deposition on the steep side of the ripple allows the ripple to move in the same direction that water is flowing, as shown in this video. Wind can also create asymmetrical ripple marks at different scales. Ripple marks at smaller scales can usually be found along a beach. Large-scale ripple marks are called dunes, and are common in deserts and some coastal environments.

This image shows how sediment grains move to form asymmetrical ripple marks and cross-bedding.
Figure 4.5 – How asymmetrical ripples form. The red circles represent grains of sediment that move up a shallow side and fall down a steep side to form cross-bedding. Image credit: Wikimedia user Nwhit, CC BY-SA.
Four examples of asymmetrical ripple marks.

Figure 4.6 – Examples of asymmetrical ripple marks. The limbs of the ripples dip at different angles; one shallow and one steep. Three views (A, C and D) are from the top and B is from the side. A) Modern asymmetrical ripples from the Bahamas. B and C) Asymmetrical ripples in sandstone from Colorado, USA. D) Precambrian asymmetrical ripples in quartzite from Wisconsin. Image credits: James St. John, CC BY.

3D Image 4.1 – Asymmetrical Ripples

Cross-Bedding

The top layer of a ripple or dune is not always preserved in the rock record, so it is rare to find ripples like those seen in Figures 4.3 and 4.7. Dunes and ripples are constantly moving, and as one passes and deposits its sediment, another follows right behind it to deposit more sediment on top. What geologists typically find in the rock record is the deposited sediment from the steep side of a series of ripples or dunes. The deposition of the steep side of several dunes or ripples creates a sedimentary structure called cross-bedding (Figure 5). One of the most important pieces of information that cross-bedding gives geologists is the direction that wind or water was moving. The steep side of a ripple always angles downward toward the direction the water or wind was moving, as shown by the blue lines in Figure 5b. There are many different types of cross-bedding and each form in a similar way.

Both images are of red sedimentary rocks with cross-bedding from the migration of dunes. The lower image has markings showing the dunes and cross-beds.
Figure 4.7 – Cross-bedding from ancient sand dunes in Coyote Gluch, part of the Canyons of the Escalante, Utah. The upper image is uninterpreted, the lower image shows interpretations of four dunes as yellow dashed lines and the cross-beds in blue. Image credit: G. Thomas, Public Domain.

Sedimentary structures are not limited to Earth since similar features have been found on Mars, Venus, and Titan, Saturn’s largest moon. Figure 4.8 shows cross-bedding from Mars, and it looks very similar to the wind-blown sand outcrops commonly found in the southwestern U.S. (see Figure 4.7). Do you think the scale is similar between these two images? The size of the cross-bedding can help to determine if these formed in water or air (). Smaller ripples form in water while larger ones form in terrestrial dunes.

Cross-bedding outcrop at Whale Rock, Mars.
Figure 4.8 – An outcrop of cross-bedded sandstone on the lower slope of Mars’ Mount Sharp. The direction of sediment transport is interpreted as sediments carried by currents moving down the deltas and into deeper lake water. This photograph was taken by NASA’s Mars Rover Curiosity on August 27, 2015, using its mast camera. This area is now known as Whale Rock in the Pahrump Hills and far from where Curiosity found evidence of [pb_glossary id="2237"]delta[/pb_glossary] deposits where a stream entered a lake.  Image credit: NASA/JPL-Caltech/MSSS.

Graded Bedding

Graded bedding is a common sedimentary structure where a change in grain size can be observed within a single sedimentary bed (Figure 4.9). At the bottom of the bed are mainly coarse particles which get progressively smaller as you move vertically up the bed. Graded beds generally represent depositional environments in which transport energy decreases over time, like the changing water velocity in a river. However, these beds can also form during rapid depositional events, most commonly from turbidity currents. Turbidity currents are essentially underwater avalanches of sediment that move downslope, usually starting at the edge of the continental shelf and flowing down the continental slope. The sediment deposited from a turbidity current is called a , which often has graded bedding with the coarsest particles at the bottom of the bed and the smallest at the top.

This images show normal grading. The left side of the figure is a sketch and the right a rock sample.
Figure 4.9 – A sketch and an example of graded bedding. The left side of the figure is a sketch of graded bedding showing larger grains at the bottom and getting finer towards the top. The right side of the figure is a sample of graded bedding. Image credits: Sketch from Mike Clark, CC BY-SA; example from James St. John, CC BY.

Mudcracks

Mudcracks, also called desiccation cracks, form when wet sediment, typically clay-rich, dries out (Figure 4.10). Clay minerals expand when they get wet and shrink when they dry out. As the sediment shrinks, cracks can develop, which form polygons on the surface of the mud. Today, you can find plenty of modern mudcracks along the margins of rivers or in desert valleys that periodically get inundated with floods. After a mudcrack forms, it can be filled in with new sediment.

Mudcracks are typically wider at the top of the crack and get progressively smaller toward the bottom of the crack. Because of this pattern, mudcracks can be a good if you are able to see a cross-section view of the crack.

These images show mudcracks. The left image shows modern dried up clay-rich sediment. The right image has ancient cracks.
Figure 4.10 – A) A modern example of large mudcracks in a dried up river bed in the Rio San Juan, Argentina. B) An ancient example of mudcracks with sediment filling in the cracks from Maryland. Image credits: A) Daniel Hauptvogel, CC BY-NC-SA; B) James St. John, CC BY.

Mars also has mudcracks (Figure 4.11), one of the pieces of evidence that indicates the red planet used to have liquid water on its surface. These were found in Gale crater in an exposure of Murray Formation mudstone on lower Mount Sharp. The white material in the cracks may be a form of calcium sulfate, either anhydrite or gypsum. This is a guess since Curiosity rover cannot test the mineral hardness.

Composite photos of mudcracks seen in Gale Crater, Mars.
Figure 4.11 – This photograph of mudcracks was taken by NASA’s Mars Rover Curiosity on December 31, 2016. The view spans about 4 feet (1.2 meters) left-to-right and combines three images taken by the Mars Hand Lens Imager (MAHLI) camera. The cracks may have formed more than 3 billion years ago. Image credit: NASA/JPL-Caltech/MSSS.

Raindrop Impressions

Raindrop impressions are small, concave imprints made by rain when it falls on soft sediment (Figure 4.12). The impression or small crater is made from the force of the raindrop falling onto the sediment, which make these structures good way-up indicators. If you were to see only the bottom of the impression, it would look like a raised bump (convex)

Raindrop impressions tend to be found in fine-grained rocks like siltstones and shale but not in coarser-grained sandstones. The impressions likely represent the end of a rainstorm as rain is letting up because any previously formed impressions would be destroyed by subsequent rainfall. That’s why most raindrop impressions are very scattered rather than occurring all over the surface. Then the impressions need to be filled in with sediment, before the next rainstorm, in order to be preserved.

These three images show raindrop impressions, which are tiny concave and convex circles on the surfaces of these rocks.
Figure 4.12 – A) Overhead view of modern raindrop impressions (and mudcracks) from Argentina. B and C) Ancient raindrop impressions from the Lower Permian, Upper Pecos Valley, New Mexico. B is an overhead view, C is the bottom-up view, showing the convex, underside of raindrop impressions (not the same sample as B). Image credits: A) Daniel Hauptvogel, CC BY-NC-SA; B and C) James St. John, CC BY.

Sole Marks

Sole marks is a broad term that describes a number of different sedimentary structures that appear as impressions or grooves in sediment, including flute casts, tool marks, groove casts, and load casts. Typically the cast of the marking (the raised bump) is preserved at the bottom of a sedimentary bed, hence the term “sole” mark, and the mold side (the impression) is filled with sediment. This makes sole marks good way-up indicators since the cast side is facing down.

Flute casts are common structures created by (Figure 4.13). The movement of these sediment avalanches underwater can scour the ocean floor, creating an elongated impression. Flute casts are usually closely spaced and can be stacked on top of one another. Not only can they tell you which way is up, but they can also tell you which way the current was flowing. The tapered end of the flute cast points in the direction of flow.

The elongated tube structures are flute casts. The start deeper on one side and become more shallow on the other.
Figure 4.13 – Flute casts from the central Alps, Switzerland. The view is from the underside of the rock. Image credit: Chris Spencer, CC BY-NC-ND.

Tool marks are made when an object, such as a stick, is dragged across sediment by a current and leaves behind what looks like scratches in the soft sediment (Figure 4.14). The elongated scratches can be used as an indicator of the paleocurrent.

The skinny, elongated structures on this image are tool marks. Most are oriented up-down.
Figure 4.14 – Tool marks from Banff National Park. The view is from the underside of the rock. Image credit: Callan Bentley, used with permission.

Groove casts are raised parallel ridges (Figure 4.15). They are spaced closely together, often appearing in sets of 2 and 3, but do not occur on top of one another like flute casts. Interpreting the paleocurrent from groove casts can be difficult because the marking is often symmetrical. Without the addition of other paleocurrent evidence, you may only be able to narrow down the paleocurrent to two directions that are 180° apart.

This image shows groove casts, which are elongated impressions in the rock. These are oriented up-down.
Figure 4.15 – Groove casts. The yellow arrow indicates the direction of transport. This arrow is double ended as there is no indication if the water was flowing up or down the river before it was tilted and exposed. The view is from the underside of the rock. Image credit: Brian Ricketts, CC BY.

Load casts form when dense, sandy sediment is deposited on less dense, water-saturated sediment, usually silt or clay (Figure 4.16). The dense sand load pushes into the soft layer below, creating bulb-like impressions.

This rock shows circular structures that are load casts.
Figure 4.16 – Load casts in arkose sandstone from the Aquitaine Basin near Nontron, France. The view is from the underside of the rock. Image credit: Rudolf Pohl, CC BY-SA.

Exercise 4.2 – Identifying Sedimentary Structures

Your instructor will pass around examples of different sedimentary structures. Now that you know what to look for in these structures, create a detailed sketch of each one. It may help to sketch the structures from several angles. Identify the sedimentary structures and complete any of the relevant information about them.

Within the sketch area for each structure, provide answers to the following questions:

  • What type of sedimentary rock is this sedimentary structure in?
  • If your structures can provide the paleocurrent, indicate the direction on your sketch.
  • If your structures are way-up indicators, indicate which way is up.
Structure #1 Sketch:
Structure #2 Sketch:
Structure #3 Sketch:
Structure #4 Sketch:
Structure #5 Sketch:
Structure #6 Sketch:
Structure #7 Sketch:

4.3 Sedimentary Structures and Paleoenvironments

As you may have guessed, sedimentary structures are incredibly useful for determining what were like. By combining sedimentary structures and the surrounding geology, a geologist could describe a pretty accurate picture of what the environment was like when these sediments were being deposited.

Exercise 4.3 – Summarizing Sedimentary Structures and Paleoenvironments

Using the information you have learned about sedimentary structures, complete Table 4.1.

Table 4.1 – Worksheet for Exercise 4.3
Structure Type of Rock it Forms in Environment Description
Symmetrical Ripples
Asymmetrical Ripples
Cross-bedding
Graded Bedding
Mudcracks
Raindrop Imprints
Sole Marks

 

Exercise 4.4 – Interpreting Paleoenvironments using Sedimentary Structures and Sedimentary Rocks

Geologists don’t only focus on a single rock outcrop to interpret the paleoenvironment of a region, they look at many outcrops so they can see how an environment changes across a region. Figure 4.17 below is a map of an area where sedimentary rocks and structures were described by a geologist. It is divided up into 4 zones with the following descriptions of the rocks and sedimentary structures:

  1. Sandstone with large scale cross-bedding and very well rounded sand grains.
  2. Sandstone and mudstone with bedding toward the east and mudcracks toward the west.
  3. Fine sandstone with symmetrical ripple marks.
  4. Shale with lots of plankton fossils and fine laminations.
Map to be used in Exercise 4.4
Figure 4.17 – Map for Exercise 4.4.

Answer the following questions:

  1. Come up with a brief depositional environmental interpretation for each zone.
    1. Zone A:
    2. Zone B:
    3. Zone C:
    4. Zone D:
  2. In what direction was the ocean in this paleoenvironment? ____________________
  3. Below are descriptions of two different environments. Which one is a and which is a swamp/wetlands?
    1. Mostly sand deposits with some sediment beds having a layer of gravel on the bottom and cross-bedding. ____________________
    2. Mostly silt and clay deposits with root structures, lots of bioturbation, and many coal layers. ____________________

 

4.4 Trace Fossils & Bioturbation

Organisms borrow and move through sediment on the ocean floor and the bottoms of lakes and rivers; this is called bioturbation. When organisms disturb the sediment by borrowing, their borrows can be preserved when the sediment hardens into rock. In most cases, the borrows will fill with new sediment, but the outline is preserved. It is difficult to assign a specific organism to the creation of a single borrow. Instead, geologists look at different borrows that tend to occur together in the rock record in order to classify them, a branch of study known as . Each ichnofacies is named after the most common trace fossil in the facies. Determining which ichnofacies the trace fossil fits into can tell you about the environment in which the organism lived, including water depth, salinity, energy and turbidity, and what the substrate was like. Generally, vertical borrows were formed in shallow water environments while horizontal burrows in deeper water environments. Table 4.2 contains a list of ichnofacies and Figures 4.18-4.25 are images of them.

Table 4.2 – Common ichnofacies
Ichnofacies Substrate Paleoenvironment Description Image*
Scoyenia Sandstones, may be associated with red beds Terrestrial, freshwater, low energy Horizontal, curved, and rope-like burrows. Unbranching, but can cross each other. Occasional vertical burrows.
This image shows Scoyenia burrows and the markings cross one another.
Figure 4.18 – Scoyenia burrows from the Grand Canyon. Image credit: National Park Service, Public Domain.
Psilonichnus Variable grain size, sand Coastal, barrier islands, deltas, estuaries, lagoons, bays Vertical burrows with J, Y, or U shapes. Can also have vertebrate footprints.
A single vertical burrow is shown in the center of this image.
Figure 4.19 – A single Psilonichnus burrow from Holocene beach dunes, San Salvador Island, Bahamas. Cross-bedding is also present. Image credit: James St. John, CC BY.
Trypanites Hard rock, carbonate, shells Coastal cliffs, reefs, beachrock Closely-spaced straight, or slightly curved, vertical burrows.
This rock has numerous circular holes, which are the Trypanites burrows.
Figure 4.20 – Trypanites burrows in Ordovician limestone from Kentucky. Image credit: James St. John, CC BY.
Glossifungites Firm but not lithified sediment Shallow marine, marginal marine, delta, estuary Three-dimensional network of cylindrical burrows and individual, vertical, teardrop-shaped burrows.
This rock has criss-crossing burrows that represent the Glossifungites ichnofacies.
Figure 4.21 – Glossifungites burrows in sandstone from Lima, Peru. Image credit: Miguel Vera León, CC BY.
Skolithos Sand Beaches, tidal flats, shallow marine, above wave base Straight, vertical, burrows that do not branch or cross. Can be slightly angled and J-shaped.
This yellow sandstone has four vertical Skolithos burrows.
Figure 4.22 – Skolithos burrows in sandstone from western Maryland. Image credit: James St. John, CC BY.
Cruziana Sand and silt Mid to distal continental shelf, below wave base, but above storm wave base Horizontal and vertical burrows from a wide variety of organisms.
This rock has dual-lobed tracks that are part of the Cruziana ichnofacies.
Figure 4.23 – Cruziana burrows from Portugal. Image credit: Wikimedia user CorreiaPM, Public Domain.
Zoophycos Sand and silt Deep water, base of continental shelf, may be associated with turbidites A series of horizontal burrows curving away from a central point.
The spiraling marks on this rock are Zoophycos trace fossils.
Figure 4.24 – Zoophycos burrows from the Swiss Alps. Image credit: Chris Spencer, CC BY-NC-ND.
Nereites Silt and clay Deep water, open ocean Meandering and spiraling horizontal trails or burrows.
The tightly curving markings on this rock are the Nereites trace fossils.
Figure 4.25 – Nereites trace fossils. Image credit: Wikimedia user Richdebtomdom, CC BY-SA.

*No OER images are available to summarize each ichnofacies, so these are single examples of an ichnofossil that belongs to the ichnofacies. You can find depictions of each facies here and an extensive list of ichnofossils here.

Exercise 4.5 – Determining Ichnofacies

Your instructor will provide you with some samples of ichnofacies or ichnofossils. Sketch each sample, paying special attention to the details of each ichnofossil. Identify the ichnofacies of each sample and the type of sedimentary rock. Based on those identifications, give a brief description of the environment.

Trace Fossil #1 Sketch:
Trace Fossil #2 Sketch:
Trace Fossil #3 Sketch:
Trace Fossil #4 Sketch:
Trace Fossil #5 Sketch:

Exercise Contributions

Daniel Hauptvogel, Virginia Sisson, Carlos Andrade

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