Geologic Time in the National Parks

Relative Age Dating

OBJECTIVES

• Understand and apply the basic principles used to determine relative ages
• of geological rock sequences
• Understand the role of fossils in determining the age of rock units
• Identify and describe how unconformable boundaries are formed
• Identify geologic time principles in several National Park: Grand Canyon NP New Mexico, Capital Reef NP, Utah, Acadia NP, Maine

MATERIALS

• Ruler, Calculator and a Pencil

INTRODUCTION

Time is the dimension that sets geology apart from most other sciences. Geological time is vast – 4.6 billion years for Earth. Even though most geological processes are very, very slow, the vast amount of time that has passed has allowed for the formation of extraordinary geological features, such as the Grand Canyon in Arizona, the extensive Appalachian Mountains and the Black Canyon of the Gunnison in Colorado. Scientists such as Aristotle (384-322 BCE) connected observations that seashell fossils found on the beach were similar to shell fossils found in rocks, proposing that the sea and land changed positions over long periods of time. The quest to understand the changes and dynamic nature of our home planet has led scientists to develop many methods of understanding the vast amount of geologic time. We can tell the relative ages of rocks (whether one rock is older than another) based on their spatial relationships; we can use fossils to date sedimentary rocks because we have a detailed record of the evolution of life on Earth; and we can use a range of isotopic techniques to determine the absolute age (in millions of years) of igneous and metamorphic rocks. In this lab, we will explore the relative age dating methods and look at several National Parks that are helpful in understanding geologic time.

PRINCIPLES OF RELATIVE DATING

Relative dating is the process of placing geological events in sequential order. For example, determining that one rock unit is older or younger than another. This type of age dating does not tell you how old a unit is numerically, only its order relative to other units. Many of the principles were developed in the late 17^th century by Danish physician Nicholas Steno (figure 1). Steno observed rocks in the area of Italy and developed many of the principles of modern stratigraphy. Many of these principles seem like common sense but they implied that the Earth was much older than was believed at the time and that it is a dynamic system that underwent and continues to undergo changes both small and large.

Figure 1. Nicolas Steno circa 1670 CE (Danish born Niels Steensen). He was among the first scientists to establish the theoretical basis for stratigraphy and therefore stating that the Earth was much older than originally believed and in a constant state of change. He later became a Catholic Bishop. Image opengeology.org CC BY S.A. 4.0. Original painting unsigned but attributed to court painter Justus Sustermans.

Principle of Superposition

The Principle of superposition states that in an undeformed sequence of sedimentary rocks the oldest rocks will be at the bottom of the sequence while the youngest layer will be on top (figure 2). Imagine a pile of newspapers or mail being piled one on top of the other, day after day. Sedimentary rocks are also deposited one on top of the other. Rock layers are frequently referred to as Strata. The Grand Canyon NP, Arizona is a classic example of rocks in superposition.

Figure 2. Undeformed layer of sedimentary rock called strata in the Grand Canyon NP, Arizona. The oldest layer is at the bottom while the youngest layers are on top. Image: The Commons project www.nps.gov/grca/parkmgmt/upload/get-married-grca.pdf Michale Quinn https://www.flickr.com/photos/grand_canyon_nps/13157900264/in/album-72157625927050687/

Principle of Lateral Continuity

The principle of lateral continuity states that layers within a depositional sequence will be continuous in all directions until they thin out at the edges or are stopped by a physical topographic barrier (figure 3). Therefore strata cut by a canyon are continuous across the canyon. The missing canyon section was caused by erosion and uplift after the original layers were deposited (figure 4).

Figure 3. The layers on either side of the gap would have originally been deposited at the same time. Note the dashed lines across the gap. Time and erosion have caused the gap. Therefore the layers where originally continuous across laterally. Image: Woudloper, Public domain, via Wikimedia Commons. https://commons.wikimedia.org/wiki/File:Principle_of_horizontal_continuity.svg

Figure 4. The sedimentary sequences in the Grand Canyon NP, Arizona showing vast lateral continuity.  The Colorado river cut through the top, newer/younger sequences to expose deep older layers at the bottom, along with considerable tectonic uplifting. Image CC BY S.A. 3.0 International License, John Kees Commons.Wikimedia.org. <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

Principle of Original Horizontality

The principle of original horizontality states that undeformed sedimentary rock is deposited horizontally. The deposition of sediment is controlled by gravity and on the whole sediment will deposit in relatively flat layers. Therefore, if sedimentary rock layers are tilted or folded it must first have been horizontal and folded or tilted later. This makes the layers older than the folding or tilting of them (figures 5 and 6).

Figure 5. Capital Reef NP, Utah sedimentary rocks are uplifted. Erosion has created the impressive cross section of 19 different rock formations. The tilting of the layers must be younger or more recent than the deposition of the layers. In essence the rocks needed to be present before they could be uplifted. Image https://www.nps.gov/care/learn/nature/geology.htm Ron Blakey

Figure 6. Originally horizontal layers left. Colorado Plateau southeastern Utah in Glen Canyon National Recreation Area. Tilted layers of sandstone and siltstone, from The Scenic Drive in Capitol Reef NP, Utah right. Image (left) By Matt Affolter (QFL247), CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=14715935 Right: Wiki Commons By Jonathan Wilkins, CC BY-SA 2.0, https://commons.wikimedia.org. https://www.nps.gov/care/planyourvisit/scenicdrive.htm NPS/Chris Roundtree

Principles of Cross-cutting and Intrusion

The Principle of cross-cutting states that when two geologic features intersect, the one that cuts across the other is younger (figure 7). This principle relates to faults that cut existing rocks being younger than the rocks they cut. This principle also includes igneous rock intruding into or cutting across another rock layer. The layers must have been present for the fault or igneous rock to intrude or cut across. The cross-cutting and intrusion principles explain that any alteration (fracturing, new rocks into older rocks) occurs more recently than the existing rocks that they alter.

Figure 7. Dark igneous intrusion cuts across paler rock. The dark igneous diabase rock is younger than the paler pink granite rock that it intrudes across in Acadia NP, Maine. Image https://www.nps.gov/subjects/geology/igneous.htm Georgia Hybels

LET’S PRACTICE APPLYING A FEW PRINCIPLES – What principle is shown in these four images? Explain.

Figure 8. Identify the principle in the image https://geologypics.com/search-images/?_sf_s=intrusion%20national%20park MarliMiller

Figure 9. Identify the principle (s) shown in the image. Thors Hammer, Bryce Canyon NP Utah Public Domain https://www.nps.gov/media/photo/gallery-item.htm?pg=5169377&id=634FBCE2-155D-451F-6746F5FAC26ACDB5&gid=634FBC94-155D-451F-679EB72AA3CF3B42 Credit NPS.

Figure 10. Identify the principle shown in the figure. Death Valley NP, CA. https://geologypics.com/search-images/?_sf_s=national%20park Geology Pics by Marli Miller freely downloadable for instructional purposes

Figure 11. Identify the principle(s) shown in the figure. Image: Petrified Forest NP, Arizona. Tepees area

By Finetooth - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=11931890

Age sequence the 5 parts in the image – Include proofs.

Figure 12. Cartoon image representing Igneous dike D, Sedimentary layers A, B and C and fault E. Image CC BY-SA 3.0 Kurt Rosenkrantz. http://cafreetextbooks.ck12.org/science/CK12_Earth_Science_rev.pdf (page 420) If the above link no longer works, visit http://www.ck12.org and search for CK-12 Earth Science., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=11017657

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A FEW MORE PRINCIPLES

Principle of Inclusions

The principle of Inclusions represents the relative age relationship between small weathered bits of rock material that are included within the boundaries of other rocks. A rock fragment that is included within another rock unit must be older than the rock in which it is included. In other words, the small fragment must have existed before it could be surrounded by the younger rock (figures 13 & 14).

Figure 13. The principle of inclusion states that the small inclusions (bits of other rocks) are older than the larger rock surrounding them. Sierra Nevada granites inside of Sequoia NP California. Image courtesy author NPFlynn.

Figure 14. An older inclusion (Xenolith) from Cadillac mountains Acadia NP Maine. Image: CC BY SA NC 3.0 Vertebrate Paleontology Lab by Alton Dooleyhttps://vmnhpaleontology.files.wordpress.com/2011/07/2009-05-12b.jpg

Principle of Faunal Succession and Index Fossils

William “Strata” Smith, worked as a surveyor in the coal-mining and canal-building industries in southwestern England in the late 1700s and early 1800s (figure 15). While doing his work, he had many opportunities to look at the Paleozoic and Mesozoic aged sedimentary rocks of the region, and he did so in a way that few had done before. Smith noticed the textural similarities and differences between rocks in different locations, and more importantly, he discovered that fossils could be used to correlate rocks of the same age. He noticed that organisms (fossils) appear and disappear from the rock record, indicating appearance, change, and extinction. Therefore, rocks of the same age where likely to contain the same type of fossil (index fossil). Smith is credited with formulating the Principle of faunal succession (the concept that specific types of organisms lived during different time intervals and there was a successive change of fossil type over time), and he used it to great effect in his monumental project to create a geological map of England and Wales, published in 1815. For more on William Smith, including a large-scale digital copy of the famous map (figure 65), see http://en.wikipedia.org/wiki/William_Smith_%28geologist%29.

Figure 15. William ‘Strata’ Smith (1769-1839). Credited with creating the first detailed nationwide geologic map. He eventually received recognition and became known as the “Father of English Geology”. His work influenced the work of Charles Darwin and many others. Award winning author Simon Winchester wrote his story in “The Map that Changed the World”. Image: Public Domain Portrait by Hugues Fourau https://en.wikipedia.org/wiki/William_Smith_(geologist)

Figure 16. William Smith’s initial geologic stratigraphic map of England. He was the first stratigrapher to create and use this type of mapping. Image wiki Commons released to the Public Domain. British Library.

earthobservatory.nasa.gov/Features/WilliamSmith

Although Smith did not put any dates on these maps — because he did not know them — he was aware of the principle of superposition (the idea, developed much earlier by the Danish theologian and scientist Nicholas Steno) that young sedimentary rocks deposit on top of older ones, he knew that this diagram represented a stratigraphic column. And because almost every period of the Phanerozoic is represented along that section through Wales and England, it is a primitive geological time scale.

Some fossils, more than others, are particularly useful in telling time, these are called Index Fossils (Figure 17). These are organisms that we are likely to find because they were abundant when they were alive and were likely to become fossilized. They often likely have a wide geographic range so that they can correlate rocks over a large distance. However, they should have a short geologic existence so that we can precisely narrow their life span. The use of animal and plant fossils in combination with other relative age principles can help scientist build a precise time sequence.

Figure 17. Fossil succession showing correlation among strata (rock layers) and Index fossils. These fossils can be used to correlate rocks that may be far apart in distance. Image opengeology.org/textbook. CC BY S.A.

If we can identify a fossil to the species or at least genus level, and we know the period that the organism lived, we can assign a range of time to the rock. That range might be several million years, especially if that organism lived for a long period of time. If a specific organism is found, it can be postulated that the rocks are of that specific time frame. If a rock unit contains several fossils, we can determine the range of time that all the fossils lived simultaneously (figure 18). This may narrow down the time range considerably. So instead of needing only species that lived for a short period of time, we can use the overlap of several organisms to determine the timeframe of the rock.

Figure 18. Species A, B, C, and D are four distinct species of fossils. Each lived during the age range indicated by the different colored bars. If all 4 fossils are found in a single rock unit that unit can only have been deposited between 7.0 and 8.3 million years ago as this is the only overlap between all four fossils. Image CC BY S.A. S Earle. opentextbc.ca. https://opentextbc.ca/physicalgeology2ed/chapter/8-3-dating-rocks-using-fossils/ is licensed under: CC BY 4.0

LET’S PRACTICE IDENTIFYING INDEX FOSSILS.

Using figure 19, if a sedimentary rock layer contains the three foraminifera fossils:

Uvigennamminus Jankoi, AND Geosella Rugosa AND Caudammina gigantea,

Draw in the horizontal age range bar on the diagram.

What is the Period and Age/Epoch of this rock unit?

Figure 19. Foraminifera figure is from the Geologic Timescale Foundation. https://timescalefoundation.org/. Modified by Shelley Jaye https://opengeology.org/historicalgeology/geologic-time/#Uniformitarianism

LET’S PRACTICE TOGETHER.

Sequence the 10 units in order from oldest to youngest. Note: I is the surface, G and H are igneous rocks, A is a fault all other layers are sedimentary.

Figure 20. Image CC BY SA 4.0 international original by Jonathan R Hendrickshttps://opengeology.org/historicalgeology/geologic-time/

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IF IGNEOUS INTRUSION G IS 120 MILLION YEARS OLD, WHAT AGE INFORMATION CAN YOU SAY ABOUT FAULT A. ____________________________________

Figure 21. A map marking the many National Parks with significant fossil resources. Image: https://www.nps.gov/subjects/fossils/fossil-parks-list.htm

UNCONFORMITIES

The many previous stratigraphic principles relate to physical layers and units of rocks. One other tool that can be useful in building relative time sequences is what is missing from a sequence of rocks. Unconformities are boundaries between rock layers. They are surfaces that represent significant weathering and erosion which results in missing or erased time. They also often represent plate tectonic forces such as mountain building and uplift. As a result, significant amounts of geologic time are often necessary to produce the three unconformable boundaries: Disconformity, Nonconformity, and Angular Unconformity (Table 1). Unconformities represent interruptions in the process of sedimentary rocks. They are boundaries not specific rock units and frequently represent significant amounts of geologic time where individual rock layers are not deposited (figure 22). If the rocks above and below are parallel to each other, but show missing strata the surface between them is a disconformity. This boundary occurs when sedimentary layers are deposited and then removed by erosion and then new sedimentary rocks form on top.

If two different rock types are in contact with one another (for example sedimentary rocks above and igneous rocks below), the boundary it is called a nonconformity. For example, this results when an igneous rock is uplifted and exposed at the surface then covered with sedimentary rock. If the rocks above and below the boundary are both sedimentary but they have different orientations, (they are not parallel to each other) the boundary is an angular unconformity (figure 23). This occurs when sedimentary rocks are folded or tilted then eroded and new sedimentary rocks deposit on top.

Table 1. Unconformities and their description with image. Image CC BY S.A. S. Earle opentextbc.ca

Figure 22. Sequence of geologic events producing the three unconformities. Follow the arrows from left to right as the processes of uplift and erosion result in unconformable boundaries. Image CC BY S.A. S. Earle opentextbc.ca

Figure 23. The detailed evolution of an Angular Unconformity in four stages. 1) Deposition of sedimentary rocks; 2) uplift and folding of the sedimentary layers; 3) erosion; 4) resumed deposition on top of the folded layers. Image: Utah Geological Survey, Public Domain https://geology.utah.gov/map-pub/survey-notes/glad-you-asked/unconformity

LET’S PRACTICE: Using the photos below, draw in the Unconformable boundary. Then name the type of unconformity and explain its possible formation.

Figure 24. Draw the unconformable boundary on the image from Death Valley NP, California. Image: https://geologypics.com/search-images/?_sf_s=unconformity%20national%20park Marli Miller

Figure 25. Draw the unconformable boundary on the image from Pikes National Forest, Colorado. Pink rock on the bottom is the famous Pikes Peak Granite (igneous). Image: By James St. John - https://www.flickr.com/photos/47445767@N05/49270380632/, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=85214138. https://en.wikipedia.org/wiki/Pikes_Peak_granite#/media/File:Precambrian-Cambrian_nonconformity_(Sawatch_Sandstone_over_Pikes_Peak_Granite;_Ute_Trail,_Manitou_Springs,_Colorado,_USA)_3.jpg

GEOLOGIC TIME SCALE

The Geologic Time scale developed over many centuries as scientists attempted to understand the complexities and the vastness of time seen in the rocks of the Earth. This time scale was constructed by lining up in order, rocks that had particular features such as rock type (lithology), environmental indicators (paleostratigraphy, geomorphology), chemical signatures (isotope geochemistry), or fossil (paleontology). Scientists in the 1700’s attempted to create a geologic time scale that could be applied to rock sequences around the world. Rocks were initially divided into four categories: Primary, Secondary, Tertiary and Quaternary. Once the identification of unique fossils found in specific rock layers (strata) was understood geologists such as William Smith, George Cuvier, Charles Lyell and others were able to divide the earths layers more precisely. During the 1800’s mostly British and European geologists began naming time units that reflected a dominant location and rock type. For example, the “Devonian” was named for the English county of Devon and the “Jurassic” was named by a French geologist for the marine limestone exposures in the Jura mountains. The first global geologic time scale was published in the mid 1800’s and standardized the Eras of Paleozoic (old life) and Mesozoic (middle life). It wasn’t until the early 1900’s when the discovery of radioactivity lead to the use of radiometric age dating (Absolute age dating) that actual numerical ages could be applied to the existing Geologic time scale.

The current time scale is divided into increasingly smaller units and subunits (figure 26). You will notice that the duration of time in each unit is not the same. Each of the divisions are based on unique features found in the rock units and divided by specific events. Recall that the time scale was developed long before numerical dates were determined. The primary division is an Eon: Hadean, Archean, Proterozoic and Phanerozoic. Eons are divided into Eras, then periods, epochs and ages. Current understanding of the age of the Earth places Earth’s age at 4.54 Billion years old (figure 27).

Figure 26. The 4.6 billion years of the Earth’s geologic time are subdivided four large Eons. Note that each Eons has a different proportion of the overall all time. Image:

https://opengeology.org/historicalgeology/geologic-time/ Jonathan R. Hendricks.  https://www.digitalatlasofancientlife.org/learn/geological-time/geological-time-scale/. CCBY SA 4.0 International License.

Figure 27. The Geologic Time Scale. Note: not to scale as the Precambrian, which represents over 4 billion years is compressed. Image public domain nps.gov. https://www.nps.gov/subjects/geology/time-scale.htm

LET’S PRACTICE A FEW:

1. Age sequence the layers in each of the images below from oldest at the bottom to youngest on top.
2. Explain the Stratigraphic principle, such as cross cutting or superposition for each layer in the sequence.
3. Identify the unconformities by specific type. Explain.

Image (figure 28) has 8 sedimentary layers, 2 unconformities (G_u and S_u), and R_D is a cross-cutting dike, M_(tilting) is the tilting episode.

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Figure 28. Practice sequencing this geologic structure. Image: Daniel Hauptvogel, CC BY-NC-SA.

https://uhlibraries.pressbooks.pub/historicalgeologylab/chapter/chapter3-geologic-time/

Figure 29. G is metamorphic, M_D is an igneous dike cross-cutting, all others are sedimentary layers. P_F is a fault There are 3 unconformities L_u, B_u, H_u

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If Igneous unit M_d is determined to be 50 million years old, what can you say about the age of unit K._______

Image: Daniel Hauptvogel, CC BY-NC-SA.

https://uhlibraries.pressbooks.pub/historicalgeologylab/chapter/chapter3-geologic-time/

Figure 30. G and I are Igneous, D is a fault, there is one unconformity and one inclusion (I is included in C). Image CC BY SERC.Carleton.edu https://serc.carleton.edu/details/images/3677.html

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