Sediments, Sedimentary Rocks and Their Environments as seen in the National Parks

OBJECTIVES

• Know the processes that form sedimentary rocks
• Explore the variety of sediments and their characteristics
• Identify the common sedimentary rocks features and environments
• Be able to name the common sedimentary rocks
• Connect sediments and sedimentary rocks to the depositional environments
• Connect several National Park features to sedimentary rocks

MATERIALS

• • Samples of several sediments, Samples of common Sedimentary rocks: Shale, Mudstone, Sandstone/Arkose Sandstone, Conglomerate, Fossil Limestone, Coquina, Chalk, Chert/Flint, Lignite/bituminous coal, Rock Gypsum, Rock Salt. Various sediments.
  • Grain size chart, ruler, images of sediments and sedimentary environments

SEDIMENTARY ROCKS AND PROCESSES

INTRODUCTION

Sedimentary rocks are the pages in which Earths history is written. They contain powerful environmental indicators (glacial extent, shoreline locations), traces of life, and chemical signatures that can inform us about a wealth of subjects from the occurrence of ancient catastrophes (massive floods), overall climate, to the productivity of life.

The identification of sediments and sedimentary rocks is more than applying names, since each name is a loaded term that conveys information regarding its history, where it was formed, potentially when it was formed, and the processes that lead to its formation. Each sedimentary rock is a puzzle and by identifying the sediments within, how they are layered, the fossils within, and patterns in the rocks a geologist can reconstruct an entire paleoenvironment and ecosystem. Solving these puzzles is both an academic exercise to better understand the world around us as well as a tool for finding the resources that are important to our lives. Fossil fuels as well as many other natural resources are contained within sedimentary rocks such as coal, natural gas, salt, and the materials that go into wallboard or in the making of cement. Therefore, a better understanding of sediments and sedimentary rocks and how and where they are formed directly influences your everyday life.

LET’S RETURN TO THE ROCK CYCLE

Take note of the location on the rock cycle representing Sedimentary rocks and the processes that lead to them. In order to create sedimentary rocks, sediment must first form (Figure 1).

Figure 1. The Rock Cycle. Sediments are produced by the weathering, erosion, transportation and deposition of sediment such as clay and pebbles. Sedimentary rocks are formed when sediment is buries, compacted and cemented together. Image Steve Earle at opentextbc.ca.

MAKING SEDIMENT FIRST: Weathering of preexisting rocks

Sedimentary rocks are formed by the weathering, erosion, deposition, and lithification of sediments. Basically, sedimentary rocks are composed of the broken pieces of other rocks (sediment). The obvious place to start this lab is a discussion of how rocks are broken down, which is a process called weathering. There are two basic ways that weathering occurs in nature. First, rocks can be physically broken into smaller pieces (imagine hitting a rock with a hammer), which is called mechanical weathering (figure 2). Alternatively, rocks can be broken down and altered at the atomic level (imagine dissolving salt in a glass of water), which is called chemical weathering. There are multiple ways each type of weathering can occur. Therefore, both the rate that rocks breakdown and how, vary dramatically depending on the original rock and environment.

Figure 2. Mechanical weathering turns larger pieces of rocks into smaller pieces of rock. This increases the surface area and helps facilitate chemical weathering. Image CC BY S.A.4.0 International License by Johnson, Affolter, Inkenbrandt, Mosher at opengeology.org/textbook.

MECHANICAL WEATHERING

The most prevalent type of mechanical weathering is the collision, breaking, and grinding of rock by the movement of gravity, water, ice or air. Several common methods of mechanical weathering are: Frost Wedging, Thermal Expansion, Unloading or Exfoliation and Biological mechanical weathering. When sediments produced mainly by mechanical weathering turn into solid rock, they produce a classification of sedimentary rocks called Detrital or Clastic. They are made up of small bits of sediment compacted and cemented together.

Frost wedging occurs when water seeps into cracks in a rock and freezes (figure 3). Water has a unique property in that it expands when frozen, which puts pressure on the rock and can potentially split boulders. The repeated freezing and thawing cycles can break a significant volume of rock over time. Gravity can move the broken sediment downward into talus piles forming at the base of mountains. This frequently results in rockfalls as is seen in figure 4 from Yosemite National Park, California Half Dome and a rockfall from Zion NP, Utah.

Figure 3. Frost Wedging. Liquid water seeps into small fractures in a rock then expands when the water freezes. This mechanically breaks apart rocks. Image: https://opengeology.org/textbook/5-weathering-erosion-and-sedimentary-rocks/

Figure 4. Water, ice, snow melt, thermal changes and wind actively erode mechanically. Active rockfall from “Porcelain wall” west of Half Dome in Yosemite National Park, CA in June 2020. 1,040 cubic meters (nearly 3100 tons) in volume. Top right, Saddle Rock showing weathering and erosion at Scotts Bluff NM, in Nebraska. Rockfall on switchbacks in Zion NP, Utah. Image Morgan Newport https://www.nps.gov/yose/learn/nature/rockfall.htm. Right image NPS Mike Largehttps://www.nps.gov/zion/learn/nature/geology.htm

Thermal expansion weathering occurs when rocks are exposed to extreme heat and cold. This repeated expansion and contraction as the rock heats and cools can cause a rock to break. An example of this is if cold water is spilled on a hot light bulb it will shatter. This mechanical weathering is common in the extreme heat and cold found in desert environments and is seen when igneous rocks cool and shrink leaving shrinkage fractures (figures 5 & 6). Sharp angled sediments initially form. Gravity leaves piles of rock debris below. Jagged sharp edges to the rock debris are common.

Figure 5. Devils Tower NM Wyoming columnar basalts freeze/thaw as well as heat and cold weaken the rocks. They slide down due to gravity producing a talus slope at the base of the volcanic plug (right), which shrunk as the magma cooled. Notice the climber for scale. Images: NPFlynn author.

Figure 6. Granitic boulders near the Wall Street mill trail in Joshua Tree National Park, California. Splits due to excessive heating and cooling result in sharp angled rocks. Image: https://www.usgs.gov/science-support/osqi/yes/national-parks/geology-and-ecology-joshua-tree-national-park

Unloading or exfoliation breakage can occur within rocks when they cool very quickly or experience extreme reduction in pressure due to erosion of overlying layers (figure 7). Many igneous rocks cool/solidify at great depths within the Earth. When they are uplifted and exposed to lower pressure they expand slightly. This expansion creates joints. The open spaces in the joints allow water to seep in and freeze and thaw, further expanding the cracks. Many large Igneous mountains such as the Granite in Yosemite NP’s El Capitan and Half Dome, the Sierra Nevada’s and the domes of Granite on Ryan Mountain in Joshua Tree National Park, CA.

Figure 7. Exfoliation fractures in granitic rocks in Yosemite NP, CA. Notice the layered appearance of the crystalline Igneous granite as it spalls due to the reduced overburden and pressure. Image CC BY NPFlynn.

Biological mechanical weathering from plants, animals, and humans can cause significant amounts of weathering. The small roots grow and expand into fractures. Over time rocks can be split apart by the expanding power of tree roots (figure 8). Biological weathering from organics such as lichen and mosses also chemically dissolve minerals.

Figure 8. Plant roots grow into small fractures and eventually expand to break apart large rocks. Yellowstone NP, Wyoming in Lamar Canyon. Image: NPS by Jim Peaco https://www.nps.gov/subjects/erosion/weathering.htm

The main products of mechanical weathering:

Lithic (rock) fragments: broken pieces of parent (pre-existing) rock. Mechanical weathering produces a wide range of sediments such as silt, sand, pebble sized particles (figure 9). They form a variety of sedimentary rocks such as Shale, Sand/Mudstones, Breccia and Conglomerates. The size of the fragments can be large and small.

Figure 9. Rock fragments are formed by combinations of weathering and produce a wide variety of sediment sizes. Left image shows angular rocks fragments from The Grand Tetons National Park, Wyoming. Right image rounded glacial deposits from Alaskas Exit Glacier near Kenai Fjords National Park. Images: CC BY NPFlynn.

SHAPE OF THE FRAGMENTS:

Rounding is the removal of sharp edges of rock fragments and resistant mineral grains as they grind against one another or the ground surface. Angular grains have not experienced as much abrasion as well-rounded grains. Therefore, the presence of rounded sediments indicates that the materials have been transporting for a greater amount of time than angular sediments (Figure 10). The rounding of the individual, and usually larger grains can be used to distinguish between the Detrital/Clastic Sedimentary rocks called Conglomerate (rounded) and Breccia (angular) (figure 11). Many National Parks contain sediments of varying rounding levels. Death Valley NP in California contains both angular and well-rounded sediments that have been turned into sedimentary rocks. Death Valley’s unique rugged environment contains sand dunes, craters, flood-carved canyons, lost lakes and volcanic activity. The variety of elevations from 11,049 feet at Telescope Peak to the lowest point in North America 282 feet below sea level along with its over 3-million-year history ensures dramatically distinct geologic setting.

Figure 10. The 4 stages of angular sediments to rounded sediments. More rounding indicates greater weathering. Image: http://publications.iodp.org/proceedings/352/102/figures/02_F05.png

Figure 11. Breccia angular rocks from Titus Canyon in Death Valley NP, CA (left) and Conglomerate rounded pebbles from Death Valley NP CA above Mesquite Springs. Image: Public Domain CCO license. https://commons.wikimedia.org/wiki/File:Conglomerate_Death_Valley_NP.jpg

Resistant mineral grains: some minerals are relatively stable at the Earth’s surface and are resistant to weathering processes, meaning they do not breakdown as easily as other minerals. Quartz is the most common resistant mineral. Because quartz is very resistant to weather processes it tends to remain as other minerals have turned into clays and been washed away. Therefore, sand grains (quartz) can build up to produce desert-like or beach environments. Quartz mineral grains are a main component of most Sandstones. Ions (elements with a charge) dissolved in groundwater can precipitate and cement particles together to form most Sandstones and Conglomerates. Many National Parks in Utah such as Bryce, Zion, Arches, and Canyonlands, have vast sandstone structures which were remnants of ancient inland seaways (figures 12 & 13). Great Sand Dunes NP & Pin Colorado is an example of the build-up of resistant quartz sand grains (figures 14 & 15). Great Sand Dunes NP & P preserves 30 square miles the remnants of erosion and loss of former wetlands that have been trapped by the prevailing winds against the Sangre de Cristo Mountain range.

Figure 12. Checkerboard Mesa showing cross-bedding from Zion NP Utah in the Navajo sandstone. Image: NPS Public Domain. https://www.nps.gov/media/photo/gallery-item.htm?pg=2122026&id=BC785694-155D-451F-67AFAEA2FABA3A41&gid=BCAE0D0A-155D-451F-67B35D5AE59F10C8

Figure 13. Arches NP in Utah showing vast sandstone structures. The resistant sand has been deposited and buried eventually turning the sand into sandstone. Differential weathering has led to certain layers existing while others have been removed by wind, and rain. Image NPFlynn author.

Figure 14. The formation and build-up of sand at Great Sand Dunes NP in Colorado. Images: NPS Illustration, public Domain and NPS Patrick Myers https://www.nps.gov/grsa/learn/nature/sanddunes.htm

Figure 15. Great Sand Dunes National Park and Preserve, Colorado. The Sangre de Cristo Mountains blocked the movement of the resistant sand grains allowing them to build great dunes. Image: NPFlynn author.

Clay: clay is formed by the chemical weathering/breakdown, specifically hydrolysis, of feldspar minerals. Besides being a mineral, the term clay also refers to sediment that is smaller than 1/256 mm (figure 16). The particles are so small that you can barely detect them as individual grains if you rub them between your fingers. Clay sized particles form the basis of Shale and Silt/Mud rocks. They can be found in a wide variety of colors (figure 17).

Figure 16. Large clay deposits (left). Right image shows the rock shale formed from the finest size clay grains which are typically 1/256^th of a mm in size. Shale also shows a fissile splitting texture. Image (left): By Siim Sepp - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=328890. Right image:https://ohiodnr.gov/wps/portal/gov/odnr/discover-and-learn/rock-minerals-fossils/common-rocks/shale

Figure 17. The Mancos Shale in Capitol Reef NP, Utah. Image: Public Domain https://en.wikipedia.org/wiki/Mancos_Shale#/media/File:Mancos_Shale_badlands_in_Capitol_Reef_NP.jpg

Let’s Explore Sediments

Sediment Maturity

Detrital sediments may be characterized as mature, or immature based on their mineral composition and textural aspects (Figure 18). Compositional maturity is determined by the relative abundance of soft, easily weathered materials in a sediment. For example, compositionally mature sands are dominated by quartz grains, whereas immature sands may contain large amounts of feldspar, shells or lithic fragments. Textural maturity considers the degree of sorting of the sediment and the shape of the constituent grains. A texturally mature sand will be well sorted (possessing little variation in grain size), and the grains will be well rounded (but not necessarily spherical), with no sharp edges. Sediments that are compositionally and texturally mature are highly weathered and have typically been transported a significant distance from their point of origin, so the concept of maturity is closely tied to the processes of sediment formation and deposition. When used in conjunction with mineral composition data, maturity assessments can help refine the geographic origins, or provenance, of sediments or sedimentary rocks, in essence where did the sediments come from.

Figure 18. Aspects of textural and compositional maturity in sands and sandstones. Created by TSMeyers.

TASK: Your instructor has several containers of sediments. Use the samples, a ruler and the hand lens magnifiers to measure and draw the different sediments. Answer the following questions about each sample: Use the circles to draw some of the sediments.

Are the grains well or poorly sorted? – What does this tell you?

What is the range of sizes of the individual sediments?

What is the average size?

Does this seem like a high energy or low energy system, based on the grain sizes? Explain.

Describe the shape of the grains. – What does this tell you?

Are there many different minerals?

Do you see any organic materials such as shells etc.?

What is the dominant weathering (mechanical or chemical)? Explain.

Write a statement connecting the answers to the questions above postulating a weathering and transportation environment. Comment on overall Maturity.

SAMPLE 1. SAMPLE 2.

SAMPLE 3. SAMPLE 4.

TAKE A LOOK AT THE SEDIMENTARY ROCKS PROVIDED. IDENTIFY ANY THAT SEEM TO BE MADE OF MECHANICALLY WEATHERED SEDIMENTS. ARE THEY MADE OF BITS AND PIECES OF OTHER ROCKS? PUT THEM INTO A PILE. Let’s sort them by size and shape of the particles and add names.

YOUR INSTRUCTOR WILL GUIDE YOU IN NAMING THESE DETRITAL SEDIMENTARY ROCKS.

CHEMICAL WEATHERING

Rocks can also be chemically weathered, most commonly by one of three processes: Dissolution, Hydrolysis and Oxidation. Chemical weathering is different from Mechanical weathering in that the original rock is broken down into its chemical elements. This usually involves water and oxygen. When chemically weathered materials reform into a solid rock, they form a group of sedimentary rocks called Chemical or Evaporite Sedimentary rock. If significant amounts of biological materials such as fossil shells or organic material is present a group of sedimentary rocks called Bio-Chemical Sedimentary rocks is formed.

Dissolution. A mineral or rock is broken apart by water into individual atoms or molecules. You are familiar with this process if you dissolve sugar in a hot beverage. The individual ions, such as sodium (Na), Calcium (Ca), and others can then be transported with the water and then redeposited/precipitated as the concentration of ions increases, normally because of evaporation of the water. Rock salt (Bonneville Salt Flats State Park, Utah) (figure 19) and rock gypsum (White Sands NM in New Mexico) and some tufa limestones (Mono Lake Volcanic Field, CA) form this way (figure 20). The Bonneville salt flats are produced by the evaporation of super saturated saline sea waters evaporating, as well as the White sands (gypsum) found at the White sands National Monument in New Mexico. There are many National Parks that preserve ancient lakes, seaways, or coastal environments. Many Parks such as White Sands NM in New Mexico preserves not sand (which is primarily quartz) as the name states but gypsum (a calcium sulfate). Chemical weathering can change the rocks and weakens original material sometimes producing dissolution caves and caverns that redeposit dissolved calcite into spectacular cave formations such as in Carlsbad Caverns NP, New Mexico (figure 21).

Figure 19. Bonneville Salt Flats State Park, Utah. Large Halite (salt) deposits result from the evaporation of salt water inland. Image: NPFlynn author

Figure 20. White gypsum dunes of the White Sands National Monument in New Mexico, (left). Right image is the formation of tufa towers of limestone forming from the highly alkaline soda lake in Mono Lake Volcanic Field within the Mono Basin National Forest Scenic area in Ca, Inyo Valley Caldera. Images: CC BY NPFlynn.

Figure 21. Dissolution and cave formations are a chemical sedimentary weathering process. Carlsbad Caverns National Park New Mexico Dolls’ Theater. Image NPS Peter Jones. https://www.nps.gov/media/photo/gallery-item.htm?pg=1828436&id=FCFBD14C-155D-451F-67CA1AD30BFE330E&gid=FC3F3C1D-155D-451F-678F10C79C37264B

Hydrolysis occurs when a hydrogen atom from a water molecule replaces the cation in a mineral; this normally alters minerals like feldspar into softer clay minerals such as kaolinite (figure 22). The clay minerals are often very dull in luster. Minerals such as Quartz are very resistant to the chemical weathering of hydrolysis, therefore they may still appear vitreous and glassy.

Figure 22. Unweathered granite (left) and weather by hydrolysis (right) Notice the chalky dull appearance of the feldspars as they become the clay mineral kaolinite. Image: https://opentextbc.ca/physicalgeology2ed/Steve Earle.

Oxidation is when oxygen atoms alter the valence state of a cation, this normally occurs in metal bearing minerals such as magnetite and iron rich minerals. This is commonly known as rusting (figure 23). The common yellow-red streaks or surface veneer is evidence of this chemical weathering process.

Figure 23. Sandstone located at Temple University’s Ambler campus contains iron bearing minerals that oxidize (rust) to produce a red-orange rusty staining limonite/hematite (reddish orange form) (left). Right image shows the black desert varnish from oxidized manganese in sandstones Mesa Verde National Park, Colorado. Images: NPFlynn.

Chemical and mechanical weathering can work together to increase the overall rate of weathering. Chemical weathering weakens rocks making them more prone to breaking physically, while mechanical weathering increases the surface area of the sediment, which increases the area exposed to chemical weathering (figure 24). Therefore, environments with multiple types of weathering can erode very quickly.

Figure 24. A combination of mechanical (small pebbles rotating in moving water) and chemical weathering produces these solution pockets in the soft sandstone at Capital reef National Park Utah. This unique pocket formation is called Tafoni. Image public domain https://www.nps.gov/media/photo/gallery-item.htm?pg=2189358&id=68DF6B38-155D-451F-674DACCB5A959BD2&gid=68D0E3D7-155D-451F-670C6155E53196CC

Biological and Chemical Material:

Many weathered sediments occur in marine environments, such as the ocean or terrestrial environments such as lakes or swamps. As a result, biological organisms are often a small or large part of some sedimentary rocks. If you can identify biological remains such as shells this is a strong indicator of a sedimentary rock since living organisms could not survive in igneous/magma conditions such as a volcano. Common biological materials that combine create the classification of Biochemical sedimentary rocks. Fossils and shell fragments when mixed with limestone material can produce fossil Limestones. Limestone can also form from non-biological marine conditions and may not contain fossils. Limestones are the most common sedimentary rock and have a wide variety of colors and overall appearances. Guadalupe Mountains NP in Texas was once (240-280 million years ago) part of an enormous land mass located near the equator with three arms of inland seas where great coral reefs (mostly sponges and algae) existed (figure 25). As the seas evaporated minerals precipitated and bands of slats and muds filled the basin and covered the reef. The Guadalupe Mountains are recognized as the best-preserved Permian-aged fossil reef in the world. When shell fragments dominate the rock, they can produce a rock called Coquina (figure 26).

Figure 25. Permian aged El Capitan at Guadalupe Mountains NP in Texas. Image: NPS D. Buehler https://www.nps.gov/im/chdn/gumo.htm

Figure 26. Images of fossiliferous limestone and brachiopod fossils on top. Bottom images show a coquina with cemented shells fragments. Images Roger Weller Cochise college. http://skywalker.cochise.edu/wellerr/GEO/sedimentary/sedimentary-list.htm

Powdered carbonate coccoliths are microscopic marine organisms composed of calcium carbonate microfossils which dominated some marine environments in the late Mesozoic era (figures 27 & 28). Their outer layers build up and form massive chalk layers. This material is used to make the common chalk used for drawing on sidewalks.

Figure 27. Coccolithopore while still alive. The microplates shed as the organism grows. There are mostly a low magnesium calcium carbonate. Image:

https://en.wikipedia.org/wiki/Coccolith#/media/File:Gephyrocapsa_oceanica_color.jpg

Photo by NEON ja, colored by Richard Bartz, CC BY-SA 2.5 <https://creativecommons.org/licenses/by-sa/2.5>, via Wikimedia Commons

Figure 28. Sedimentary rock formed from coccoliths. The Bio-chemical sedimentary rock Chalk is formed. Image http://skywalker.cochise.edu/wellerr/GEO/sedimentary/sedimentary-list.htm

Terrestrial organic material from swamps produces a common material such a peat (figure 29). When this material compacts in an oxygen poor environment it can produce a low-quality type of coal called Lignite (figure 30). Coal primarily comprised of hydrogen and carbon and is otherwise known as a fossil fuel.

Figure 29. The terrestrial formation of organic peat and then sedimentary coal into bituminous coal. Image: https://openpress.usask.ca/app/uploads/sites/29/2017/05/Formation-of-coal.png Steve Earle CC BY 4.0

Figure 30. Dull black lignite coal. It is a low-quality sedimentary coal and is soft. Image: https://www.usgs.gov/media/images/lignite-coal-0 lignite coal donna pizzarelli USGS Public Domain.

TASK: Look at the sediments provided. Look for evidence of chemical weather, organic origins of material or precipitated/evaporated. Use the circles to draw the sediments. Answer the questions.

What biological material can you identify?

What does this material tell you about the formation of these sediments?

Can you connect this to a specific chemical weathering process? Explain.

SAMPLE 1. SAMPLE 2. SAMPLE 3.

Connect the biological organic and evaporite/precipitate rocks to the sediments.

YOUR INSTRUCTOR WILL GUIDE YOU IN NAMING THE SAMPLES.

Transportation and Deposition of Sediment – The story that it tells.

Weathering products that are eroded from their source can be transported by moving water, ice (glaciers), or wind to a new location. The size of the carried sediment depends on the speed/energy and type of material carrying the sediment (figure 31). A fast fluid/high energy medium (like a rapidly flowing river) can carry a large amount and large sized particles and cause significant amounts of weathering. A slow fluid (like a calm stream) would hardly cause any weathering and carry only the smallest sized particles. For example, a quietly slow-moving stream may appear clear due to the low amount of carried sediments as it has likely deposited most larger particles already. The density of the fluid also controls the size of the particles that can be transported, for instance denser fluids (like water or glaciers) can carry larger particles (boulders) than less dense fluid (like air). The shape, round or angular also indicates if the sediment has been transported to a nearby deposition or transported a great distance. The sorting of the grains can also tell us about the variation in the transportation and deposition of the sediments. If the grains are all a similar size (all very small such as clay or sand) the energy was consistent or stable in energy. If there is a wide variety of sizes such as pebbles mixed with mud this indicates a variety of energy during the transportation and deposition process. The three main sedimentary textures that tell us about the transport history of sediments are listed below:

Grain size of a sedimentary rock can be interpreted to indicate several things. The energy of the environment at the time of deposition. The higher the energy (e.g. the faster the water), the larger the grain size that can be moved. Lower energy environments encourage deposition of transported grains. Clay and silt sized particles will settle out of transportation only in the lowest energy environments such as lakes or deep marine setting, producing Shales. A rock made of mostly sand sized particles produces Sandstones. The grain size of sediment generally decreases as it gets farther from the source area due to breakage, abrasion, or chemical weathering. The Wentworth scale is a chart of grain sizes of sediment.

Figure 31. Wentworth scale of grain size. Particle size terms such as clay, silt, sand, pebbles, cobbles and boulders are connected directly to the energy necessary to transport and deposit them (settling velocity). Image: usgs.gov  http://pubs.usgs.gov/of/2006/1195/htmldocs/images/chart.pdf

WIND TRANSPORTED AND DEPOSITED Environments – Aeolian (or Eolian)

Aeolian (or eolian) processes involve the erosion, transport, and deposition of sedimentary material by wind. Wind is not nearly as effective as water when it comes to erosion and transport of sediment, but it still has the ability to shape the landscape in arid environments. Aeolian deposits include sand dunes, a common feature in many desert environments, as well as loess deposits, accumulations of silt that are often blown many miles from their original source area (Figure 32). Thick blankets of loess, in some

places over 200 feet thick, cover large parts of the central U.S. and the Midwest. These sediments initially collected in glacial outwash plains at the edge of the Laurentide Ice Sheet during the Last Glacial Maximum, and the finer particles were winnowed from these glacial deposits and spread across the continent by wind. Wind also produces erosional features, such as desert pavements, rocky surfaces composed of coarse sedimentary material left behind following the removal of fine-grained sediment (Figure 33). Over long periods of time, abrasion by windblown particles can even polish the surface of rocks or sculpt larger landforms into streamlined shapes called yardangs (Figure 34).

Figure 32. Sand dunes in Great Sand Dunes National Park, Colorado (left), and thick loess deposits along the Natchez Trace Parkway in Mississippi (right). Images: https://www.nps.gov/articles/checking-great-sand-dunes-vital-signs.htm NPS.gov. https://www.nps.gov/places/loess-bluff.htm

Sand Dunes

Aeolian processes play a key role in the formation of several National Parks. Ancient dune deposits compose a major part of the rocks preserved in Arches and Zion National Parks, Utah, and modern dune fields are marquee attractions at Death Valley in California, Great Sand Dunes in Colorado, and White Sands National Park in New Mexico. Modern aeolian deposits are also present in parts of Grand Canyon National Park in Arizona and Guadalupe Mountains National Park in west Texas, as well as National Seashores like Assateague Island along the coast of Maryland and Virginia, Cape Cod in Massachusetts, and Padre Island in south Texas (figure 33).

Figure 33. Desert pavement in Death Valley National Park produced by wind erosion (left), and yardangs in White Sands National Park (right). Images: Daniel Mayer, GNU FDL https://upload.wikimedia.org/wikipedia/commons/4/48/Desert_pavement_on_alluvial_fan_of_Hanaupah_Canyon_in_Death_Valley_NP.JPG (left), and Ken Redeker, Public Domain (right). https://commons.wikimedia.org/wiki/File:Yardangs_in_dunes,_White_Sands_National_Park,_New_Mexico,_United_States.jpg

In contrast to clay and silt-size particles that can be carried aloft for great distances, wind moves sand grains via a process called saltation. Sand particles hop across the surface, traveling a short distance through the air before falling back to the ground. In this fashion, sand migrates up the gently sloped, windward side of a dune and then avalanches down the steeper leeward face (Figure 34). As this process repeats for numerous sand grains, the dune structure itself migrates across the landscape in the direction of the prevailing winds.

Figure 34. Diagram illustrating how sand particles move across a dune. Image: USGS. https://pubs.usgs.gov/gip/deserts/eolian/

TASK: Interpreting ancient dune deposits in Arches NP, Utah.

In contrast to the large modern dune fields found in Great Sand Dunes NP in Colorado and White Sands NP in New Mexico, some National Parks preserve the lithified remains of ancient dune systems. Arches National Park, located in southeastern Utah, contains two thick aeolian sandstones known as the Navajo and Entrada formations. These rock units were deposited in massive sand seas called ergs that covered much of the western United States during the Middle Jurassic (174–164 Ma). These Jurassic ergs contained some of the largest sand dunes ever found in the rock record. The Navajo Sandstone (Figure 35), which can be as much as 550 feet thick in some parts of the park, is characterized by cross-bed sets 15 to 25 feet thick. The angled surfaces, or foresets, visible in the cross-bedding represent the slipface of a dune as it migrated across the land surface in the direction of the prevailing wind. The Entrada Formation, 200–350 feet thick, is the primary arch-forming unit at Arches.

Figure 35. Crossbedding in the Navajo Sandstone in Cottonwood Cove, Arches National Park. Image: John Fowler, CC-BY 2.0, https://commons.wikimedia.org/wiki/File:Cottonwood_Cove_(6964540853).jpg

Based on the orientation of the foresets in Figure 35 above, was the prevailing wind direction to the left or right of the page? How do you know?

Is there any evidence that the direction of the prevailing winds changed significantly during deposition of the units in Figure 35?

How many cross-bedded units do you count in Figure 35? What are the sharp divisions between the units, and what do they represent?

Figure 36. Crossbedding in the Navajo Sandstone in Arches National Park. Image: Adrienne Fitzgerald. NPS. https://www.nps.gov/zion/learn/nature/navajo.htm

Based on the orientation of the foresets in Figure 36 above, was the prevailing wind direction to the left or right of the page? How do you know?

Trace the foresets and the upper and lower contacts of the cross-bedded layer in Figure 36. Use a protractor to determine the approximate angle of the foresets relative to the lower contact.

The foresets in cross-bedded aeolian deposits are usually inclined at relatively steep angles (~30°), however some deposits have lower angles of repose when the dune crest is curved, resulting in oblique airflow relative along the edges of the dune. Based on your measurements of the foresets in Figure 36, were these dunes straight crested or crescent shaped?

LOOK AT THE ROCK SAMPLES PROVIDED THAT SHOW CROSS BEDDING. MEASURE THE THICKNESS AND THE ANGLE ON THE ROCK.

TASK: Provenance and maturity in White Sands NP, New Mexico.

White Sands National Park, located in south central New Mexico, was initially established as a National Monument in 1933, eventually becoming a National Park in 2019. The sand dunes in White Sands are unique due their composition. Whereas most inland dune fields are composed of quartz sand, the sand at White Sands is made of gypsum (hydrous calcium sulfate – CaSO₄ • 2H₂O). The park encompasses approximately 40% of the dune field, with the rest located in the surrounding White Sands Missile Range. White Sands lies in a basin at the foot of the San Andres Mountains (Figure 37), which are composed primarily of marine limestones. At the end of the Late Glacial Maximum, approximately 11,000 years ago, a large lake formed in the basin. As the regional climate warmed and became increasingly arid, the lake dried up, developing into an extensive playa, or dry lakebed, that is today known as Alkali Flat. A smaller, ephemeral lake called Lake Lucero currently occupies the basin. Precipitation and snowmelt periodically fill Lake Lucero, but the water eventually evaporates, leaving behind abundant evaporite deposits that precipitate as a crust on the lakebed (Figure 38).

Figure 37. Aerial view of White Sands National Park with sand blowing to the east. Image: NASA Earth Observatory. https://earthobservatory.nasa.gov/images/77775/white-sands-dust-storm

Figure 38. Evaporite deposits with large selenite crystals in Lake Lucero, White Sands National Park, New Mexico. Image: NPS. https://www.nps.gov/whsa/learn/gypsum.htm

Based on the description of the geologic setting of White Sands provided above and shown in Figure 38, what is the source of the gypsum sands in the park?

Consider the evaporite deposits in Lake Lucero. What is the source of the calcium and sulfur that form mineral crust on the lakebed? In what modern environment are these elements found in abundance?

Examine the gypsum sand grains in Figure 39. How would you characterize the characterize the compositional and textural maturity of this sediment? Provide details to support your interpretation.

Figure 39. Sand grains from White Sands National Park, New Mexico. Image: Wilson44691 - Own work, Public Domain, https://commons.wikimedia.org/.

GLACIAL EROSION AND DEPOSITS https://www.nps.gov/subjects/geology/glacial-landforms.htm

Glaciers currently cover about 10% of the land surface today. This percentage is rapidly declining as anthropogenic climate change is warming the Earth. Glaciers are classified as either: Alpine or Continental. Alpine glaciers originate in high mountainous National Parks such as Kenai Fjords NP in Alaska, Glacier NP in Montana, North Cascade and Olympic NP in Washington (figure 40). Continental glaciers are typically larger ice sheets covering flatter landscapes and flow outward from where the greatest volume/thickness of snow and ice accumulates. Glaciers by definition move under their own weight (of snow and ice) and are therefore a dynamic changing system that erodes, carries and deposits large volumes of material.

Figure 40. The Eel Glacier on Mt. Anderson (Olympic National Park, Washington) is an excellent example of a cirque glacier. The bare ice in the glacier's ablation zone appears bright because it reflects the sunlight much more efficiently than the dull snow around it. Image: NPS Photo/Janis Burger https://www.nps.gov/articles/cirqueandalpineglaciers.htm

As glaciers move downhill due to gravity (figure 41), flowing (plastically), slipping (basal is the dominant movement) and grinding over rocks it carves steep sided flat bottomed U-shaped valleys. Rocks that have been eroded by heavy glacial ice often show horizontal and parallel gouge marks called striations (figure 42) and unique deposition and is very distinctive. It can be extremely poorly sorted, containing all sizes of sediment from clay/flour sized particles to car sized boulders. This debris is called glacial till (figure 43) and forms large land structures called moraines when deposition occurs on the land, as opposed to depositing in the water where it produces unique dropstones.

Figure 41. This diagram shows how glaciers can erode bedrock. Abrasion involves scratching the bedrock with debris in the basal ice. Plucking is removal of entire chunks of rock. Image: Courtesy of Rocky Mountain National Park. https://www.nps.gov/articles/howglacierchangethelandscape.htm

Figure 42. Glacial striations produced by passing glacial bedload abrading rocks. Notice that the gouge marks are parallel and indicate the direction of movement of the glacier. This glacial deposit is from Alaska’s Turnagain arm area near Byron and Portage glaciers. Image: Npflynn

Figure 43. A road cut through a moraine in Yellowstone National Park, Wyoming exposes the glacial till. Notice the very large, rounded pebbles and boulders mixed with medium and small sized particles. The individual rocks will also vary in their type. Image NPS John Good. https://www.nps.gov/articles/glacialtillandglacialflour.htm

When glaciers on the land melt they can no longer transport very large particles. Ice can carry a greater volume and size of sediment than liquid water. Large boulder sized rocks will deposit at the maximum extent, the terminus of the glacier. The terminus is called a moraine and creates a unique depositional environment (figure 44). Terminal moraines produce long linear hills or mounds that indicate the last location of the glacier. The melted glacial water carries the finest and small sized sediments away creating a milky and cloudy stream (figures 45 & 46). Rocks in the outwash plane are covered with the fine-grained debris.

Figure 44. A newly formed lateral moraine beside Andrews Glacier in Rocky Mountain National Park, Colorado. Note the variety of sediment sizes and the curved U-shape to the valley. Image: courtesy of Rocky Mountain National Park. https://www.nps.gov/romo/lateral_moraine.htm

Figure 45. Exit glacier near Kenai Fjords National Park, Alaska. The rocks in the outwash plain are covered with fine grained debris. Image: Npflynn

Figure 46. The outwash plain in front of the Red Glacier in Lake Clark National Monument, Alaska ends far away from the glacier and is characterized by braided rivers and small ponds. Finer particles are carried farther from the terminus. Notice Moraines north of the outwash plain. Image: NPS Photo

https://www.nps.gov/articles/outwashplainsandeskers.htm?utm_source=article&utm_medium=website&utm_campaign=experience_more&utm_content=small

FIND THE MORAINE AND ESKAR OUTWASH PLANE IN THE IMAGE ABOVE.

FLUVIAL SYSTEMS: TRANSPORT AND DEPOSIT SEDIMENTS https://www.nps.gov/subjects/geology/fluvial-landforms.htm

Running water is critical for life on Earth. Water also plays a critical role in the rock cycle as it plays a major role in the weathering and transportation of sediment, sculpting many unique features. While the majority of the water on Earth is located in the oceans (approximately 97%), surface water in the form of lakes, stream, marshes and swamps represent just over 1% of the water in the hydrologic cycle (figure 47). This surface water has tremendous energy to erode, sculpt, transport and eventually deposit significant amounts of surface material as the water makes its way back to the oceans (figure 48). The energy from the sun is the primary energy source for the hydrologic cycle with downward gravity adding in moving water back to the oceans.

Figure 47. Column one represents the global water percentages on Earth. Image: U.S. Geological Survey, Water Science School. Data source: Igor Shiklomanov’s chapter “World fresh water resources” in Peter H. Gleick (editor), 1993, Water in Crisis: A Guide to the World’s Fresh Water Resources. https://www.usgs.gov/special-topics/water-science-school/science/fundamentals-water-cycle

Figure 48. The Hydrologic cycle. All water cycles between the oceans and the atmosphere with surface water acting as an in between stage. Image: Public Domain

https://www.usgs.gov/special-topics/water-science-school/science/fundamentals-water-cycle

Rivers and streams may not seem like a fundamental part of our National Parks, but they are responsible for the geological formations that characterize a number of parks. The Grand Canyon, Arizona, Black Canyon of the Gunnison, Colorado, Chaco Canyon, Arizona, and New River Gorge, West Virginia (one of the newest additions to the National Park system), were carved by rivers. Streams deposited the sediments that would eventually form many of the rocks exposed in Bryce Canyon and Zion National Park, Utah. Streams also play a major role in a variety of other parks, including Big Bend, Texas, Denali, and The Gates of the Arctic National Park, Alaska, to name a few (figure 49).

Figure 49. Big Bend NP, Texas. A dramatic image of the power of water to carve a landscape. Image: NPS Ann Wildermuth https://www.nps.gov/subjects/geology/fluvial-landforms.htm

Streams and the sediments they deposit vary according to local environmental differences. Near the headwaters, where streams start high in the mountains (Figure 50), they flow across solid bedrock rather than soft sedimentary substrates. Mountain streams are fast-rushing with turbulent flow patterns and waterfalls developed at major changes in elevation, and over time they can carve deep V-shaped canyons in the bedrock beneath them. Farther from a river’s mountainous source area, channels cut across gently sloping plains of sediment that is both transported and deposited by fluvial processes. Braided streams, composed of multiple interwoven channels, develop in areas with somewhat gently sloping terrain, coarse sedimentary substrate, and intermittent rainfall patterns. Meandering streams, characterized by their sinuous channel morphology, develop in relatively flat areas with an abundance of fine-grained sediment and a regular supply of water. These streams develop muddy floodplains and natural levees adjacent to their channels. Over time, the loops in meandering channels become more pronounced as sediment is deposited from one side of the channel and deposited on the other. The inside of the channel loop where deposition occurs is called the point bar, and the outside of the loop where sediment is removed by erosion is called the cut bank. As the point bar builds out into the channel and the cut bank retreats further into the floodplain, the channel morphology becomes increasingly convoluted and can inhibit rapid water flow. During flood periods when a stream’s discharge increases significantly, the water may erode a new channel segment that cut through old point bar deposits and bypass elaborate channel loops. The old, bypassed meanders, now cut off from the main channel, develop into crescent shaped oxbow lakes that flank the new, straighter river course.

Figure 50. A cross-section of the three maturation levels of streams as the waters carve and them deposit on their path back to the ocean. Image NPS Trista L. Thornberry-Ehrlich Colorado State University. https://www.nps.gov/subjects/geology/fluvial-landforms.htm

A floodplain is the relatively flat surface adjacent to the river or stream. During floods, when the stream overflows its banks, water flows over the floodplain and deposits sediment (figure 51). Through fluvial processes, streams construct floodplains that accommodate their maximum flood capacity. Geomorphic features of the floodplain include:

• Natural Levees—River may be immediately flanked by a buildup of sediment that forms natural levees. These provide some defense against flooding but are occasionally breached in areas producing flood-plain splays—coarse fan-shaped deposit of sediment created during high flow events.
• Oxbows and oxbow lakes—See below, features of a Meandering Stream Channel.
• Point Bars—See below, features of a Meandering Stream Channel.
• Terraces

Figure 51. Chaco Canyon channel featured in Chaco Culture National Historical Park, New Mexico represents many of the erosional and depositional features of running water. Geologic report Image Trista L. Thornberry-Ehrlich Colorado state university. https://www.nps.gov/subjects/geology/fluvial-landforms.htm

Figure 52. A well-developed meandering stream in the Gates of the Arctic National Park and Preserve, Alaska. Notice the eroded outer banks and the deposition on the slower inner banks. Image: NPS photo https://www.nps.gov/articles/meandering-stream.htm

ON THE IMAGE ABOVE CIRCLE/LABEL AS MANY STREAM FEATURES AS YOU CAN.

BEACHES and COASTLINES

There are currently 88 ocean, coastal and great lake park units within the National Park Service. These spectacular places range from coral reefs to kelp forests to estuaries to sand dunes to maritime and military heritage parks. A few Beaches and Coastal parks range from Saint Croix Island International Historic Site on the Maine/Canada border to Dry Tortugas in Florida on the Atlantic coast to the War in the Pacific National Historical Park National Park on Guam and the American Samoa’s to the far western Pacific Ocean. Beaches and coastlines host a wide variety of erosional and depositional systems. Canaveral National Seashore, Florida, Cape Cod National Seashore, Massachusetts (figure 53), and Assategue Island National Seashore in Maryland are a few examples of national parks sculpted by coastal environments.

Figure 53. Marconi Beach Cape Cod National Seashore, left. Active waves energy sculpting the coast. Image: NPS public Domain Brittani Connell public Domain (left). Right image: Public Domain https://www.nps.gov/media/photo/gallery-item.htm?pg=6223530&id=C0FEE5F1-155D-451F-67EF853372D4EEAA&gid=75E67DCE-78B9-4E4E-97FE-DB40B675075E

Indicators of Energy and Deposition Environment

Fast, turbulent waters are high energy environments, and calm waters are low energy environments. Higher energy environments produce large, poorly-sorted sediments, especially if the energy/movement changes suddenly. Whereas low energy environments produce small, rounded, well-sorted sediments. Well sorted sediments, mostly one size range indicates a stable level of energy, not a varying energy level. DEPOSITION: Sediment is deposited when transporting agents, such as running water, glacial ice, or wind, lose energy and can no longer transport the sediment load. Deposition also refers to the accumulation of chemical or organic sediment, such as calcium carbonate (CaCO3), clamshells on the sea floor, or plant material in a swamp. Sediments are deposited in layers on top of one another, which packs loose sediment grains tightly together (compaction). Compacted sediment can be hardened even further by the precipitation of cement (ions dissolved in circulating groundwater) in the pore space between the grains (figure 54). Common cements are calcite (CaCO3), silica (SiO2), and iron oxides.

Figure 54. Deposition of sediment followed by the compaction and cementation of the grains into a hardened lithified sedimentary rock. Image: CC BY 4.0 Karla Panchuk. https://opentextbc.ca/physicalgeology2ed/

ROCK COLOR AND SEDIMENTARY ENVIRONMENTS

Sediments accumulate in depositional environments such as alluvial fans, river channels, flood plains, deltas, lakes, desert valleys, beaches, shallow marine, and the deep-sea floor. An important task of a geologist who studies sedimentary rocks is to interpret the ancient environment in which the rock formed. By making detailed observations, a geologist can read the many clues that tell the depositional story of a rock (figure 55 - 57).

TASK: Look at the Sedimentary rocks provided. Focus on the color of the samples and see if you can interpret a possible sedimentary environment.

Figure 55. A visual representation of many terrestrial and marine depositional environments. Image CC BY S. Earle opentextbc.ca/geology.

Table 1. Properties to interpret the depositional environment of and formation of sedimentary rocks.

TAKE A LOOK AT YOUR SEDIMENTARY SAMPLES. CAN YOU IDENTIFY ANY POSSIBLE ENVIRONMENTS?

USE the FIGURES/IMAGES/TABLES within the lab and provided by your instructor to help create the story of the formation of your sedimentary rocks.

Figure 56. This chart is another way to look at some of the information listed in Table 1.

Figure 57. Common depositional environments and their corresponding sedimentary rocks.

LET’S PRACTICE THE CLASSIFICATION OF DETRITAL OR CLASTIC SEDIMENTARY ROCKS

Sedimentary rocks made from sediments such as bits and pieces of weathered rocks are classified as Detrital or Clastic. They are classified based on the size of the grains, the sorting of the grains, and the rounding of the grains. Each of these components indicate the weathering, transportation and depositional environments. Try to identify the Shales, Silt/Mudstone, Sandstones, Conglomerate and Breccia.

Next to the rock name write a few characteristic features and how they formed:

Shale

Silt/mudstone

Sandstone

Conglomerate

LET’S PRACTICE THE CLASSIFICATION OF CHEMICAL, BIOCHEMICAL, ORGANIC SEDIMENTARY ROCKS

This classification of sedimentary rocks form from chemical weathering processes. Many are formed from the evaporation of marine waters to produce rocks such as Rock salt and Rock gypsum. Biochemical sedimentary rocks form from marine organisms that extract chemicals components from water to build shells and other body parts. Rocks such as Fossil limestone (which can also be inorganic), Chalk, Coquina, and Chert/Flint. Organic matter can build up in layers of meters and meters of organic debris to form terrestrial coal seams.

Next to the rock name write a few characteristics that will help you identify each sample:

Rock Salt

Rock Gypsum

Fossil Limestone

Chalk

Coquina

Chert/Flint

Lignite Coal

Using the Charts try to distinguish between Clastic/Detrital and Biological/Chemical Sedimentary Rocks. Then try to name them. Look carefully at the sediment size, shape & sorting. Can you connect a depositional environment to each of the samples?

TRY A FEW QUESTIONS:
1. How are Breccia and Conglomerate the same but different?

2. How would you identify Coquina from Chalk?

3. What environmental conditions lead to the formation of Shale vs Sandstone?

4. Rock salt can form in what type of geologic setting?

5. If you find coal measures/seams what can this tell you about the paleoenvironment of that area?

6. Why are Sedimentary rocks important?

7. Explain how you would distinguish an Igneous from a Sedimentary rock. Name a few.

8. Identify the following images as type of rock, details of their environment, and components. Write a brief statement of what this rock can tell you about its formational environment. (Images: first 2- CC BY S. Earle, 4^th image openpress.org)

https://www.wikiwand.com/en/Coquina. https://opengeology.org/textbook/wp-content/uploads/2017/02/XBedsZion.jpg

USE THE DEPOSITIONAL ENVIRONMENT IN THE IMAGES BELOW TO WRITE A BRIEF STATEMENT ABOUT THE TYPES OF SEDIMENT/ROCK THAT WOULD DEPOSIT. Note the different energy levels.

https://en.wikipedia.org/wiki/Death_Valley_National_Park#/media/File:Sand_Dunes_in_Death_Valley_National_Park.jpg Image CC BY -SA 4.0 Broken Inaglory

By CeeGee - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=41540102. https://en.wikipedia.org/wiki/%C4%B0%C4%9Fneada_Floodplain_Forests_National_Park#/media/File:%C4%B0%C4%9FneadaFloodplainForestsNP_(11).JPG