Sedimentary Rocks

Bradley Deline

INTRODUCTION

We are particularly interested in the history and events that occur on the surface of the Earth both because it is easier to directly observe and test, and has direct relevance to our lives and our own history. Sedimentary rocks are the pages in which history is written, since they contain powerful environmental indicators, traces of life, and chemical signatures that can inform us about a wealth of subjects from the occurrence of ancient catastrophes to the productivity of life.

The identification of 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 a set of rocks, how they are layered, the fossils within, and patterns in the rocks a geologist can reconstruct an entire environment and ecosystem. Solving these puzzles is both an academic exercise to better understand the world around us as well as a tool for find- ing the resources that are important to our lives. In particular, fossil fuels as well as many other natural resources are, or are contained within, sedimentary rocks such as coal, natural gas, petroleum, salt, and the materials that go into wallboard or in the making of cement. Therefore, a better understanding of sedimentary rocks and how and where they are formed directly influences your everyday life.

Learning Outcomes

After completing this chapter, you should be able to:

• Describe how erosion and weathering relate to the formation of sedimentary rocks
• Identify sedimentary rocks and their features
• Describe the formation and history of different types of sedimentary rocks

• Describe the formation of different sedimentary structures
• Discuss the distribution of sedimentary rocks, sedimentary structures, and types of weathering in varying sedimentary environments

Key Terms

• Beach Depositional Environments
• Biochemical Sedimentary Rocks
• Chemical Sedimentary Rocks
• Chemical Weathering
• Clastic Sedimentary Rocks
• Continental Environments
• Deep Marine Depositional Environments
• Deltas
• Eolian Depositional Environments
• Erosion
• Fluvial Depositional Environments
• Glacial Depositional Environments
• Lacustrine Depositional Environments
• Marine Environments
• Maturity
• Mechanical Weathering
• Organic Sedimentary Rocks
• Reef Depositional Environments
• Sedimentary Structures
• Shallow Marine Depositional Environments
• Tidal Mudflat Depositional Environments
• Transitional Environments

WEATHERING AND EROSION

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. The obvious place to start this chapter 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. 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 and, therefore, both the rate that rock breaks down and how it breaks down vary dramatically depending on the area and environment.

The most prevalent type of mechanical weathering is the collision, breaking, and grinding of rock by the movement of a fluid, either water or air. The size of the carried sediment depends on the type of fluid and speed of the movement. A fast fluid (like a rapid flowing river) can carry large particles and cause immense amounts of weathering while a slow fluid (like a calm stream) would hardly cause

any weathering. The density of the fluid also controls the size of particle that can be transported, for instance denser fluid (like water) can carry larger particles than less dense fluid (like air). Another common method of mechanical weathering is called frost wedging, which occurs when water seeps into a crack in the rock and freezes. Water has a unique property in that it expands when frozen, which puts pressure on the rock and can potentially split boulders. The addition and subtraction of heat or pressure can also cause rock to break, for instance spilling cold water on a hot light bulb will cause it to shatter. This breakage can also occur with rocks when they cool very quickly or immense pressure is released. Finally, plants, animals, and humans can cause significant amounts of weathering. These sediments then undergo erosion, which is the transport of sediment from where it is weathered to where it will be deposited and turned into a rock.

Rocks can also be chemically weathered, most commonly by one of three processes. The first, which you are probably familiar with, is called dissolution. In this case, a mineral or rock is completely broken apart in water into individual atoms or molecules. These individual ions can then be transport with the water and then re-deposited as the concentration of ions increases, normally because of evaporation. Chemical weathering can also change the mineralogy and weaken the original material, which again is caused by water. A mineral can undergo hydrolysis, which occurs when a hydrogen atom from the water molecule replaces the cation in a mineral; this normally alters minerals like feldspar into a softer clay mineral. A mineral can also undergo oxidation, which is when oxygen atoms alter the valence state of a cation, this normally occurs on a metal and is commonly known as rusting.

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 surface area that is exposed to chemical weathering. Therefore, environments with multiple types of weathering can erode very quickly. As you go through the following sections (on rocks and environments) think about the types of weathering required to make the sediment that will then make up different types of sedimentary rocks as well as what types of weathering you would expect to occur in different environments.

IDENTIFYING SEDIMENTARY ROCKS

The classification of sedimentary rocks is largely based on differentiating the processes that lead to their formation. The biggest division in types of sedimentary rocks types is based on the primary type of weathering that leads to the material building the sedimentary rock. If the rock is largely made from broken pieces (called clasts) of rock that have been mechanically weathered the rocks are referred to as Detrital or Clastic Sedimentary Rocks. Simply put, these are rocks that are composed of the broken pieces of other rocks. In this case, the mineralogy of the clasts is not important, but instead we need to note the properties of the sediment itself. Alternatively if the rock is largely the product of chemical weathering the classification is then based on the composition of the material as well as the processes involved in the materials precipitation from solution. Chemical Sedimentary Rocks form from the inorganic precipitation of minerals from a fluid. If the ions present within a fluid (water) become very concentrated either by the addition of more ions or the removal of water (by freezing or evaporation), then crystals begin to form. In this case the identification of the type of sedimentary rocks is based on the minerals present. If organisms facilitate the precipitation of these minerals from water we refer to the rocks as Biochemical Sedimentary Rocks. An example of biochemical precipitation is the formation of skeletal minerals in many organisms: from starfish and clams that grow calcite, to sponges that grow silica-based material, to humans that have bones made of hydroxyapatite. In many cases it is hard to differentiate whether a mineral was formed organically or inorganically, so in the current lab we will mostly group these two types of sedimentary rocks together. Rocks can also be formed from the carbon-based organic material produced by ancient life and are called Organic Sedimentary Rocks. Now we can discuss the identification and formation of particular sedimentary rocks.

Clastic Sedimentary Rocks

Weathering and erosion occur normally in areas that are at high elevation, such as mountains, while deposition occurs in lower areas such as valleys, lakes, or the ocean. The sediment is transported from the area of erosion to area of deposition by ice, water, or air. Not surprisingly, the sediment changes during its journey and we can recognize the amount of change and the distance the material has trav- eled, and the transport mechanism, by looking at its maturity (Figure 10.1). Maturity is defined as the texture and composition of a sedimentary rock resulting from varying amounts of erosion or sedimentary transport. Imagine a mountain composed of the igneous rock granite and let us explore how the sediment from this mountain changes as it makes the long distance trek via river to the ocean. The first process is just breaking the rock down into smaller pieces mechanically, which creates sediment that has large and small pieces, the pieces are jagged, and all of the minerals remain. The sizes of clasts in these rocks can range from large boulders, to cobbles, to pebbles, to the smallest particles, clay. As this sediment is transported in the river the pebbles collide with other pebbles and the rocks get smaller and the sharp edges are broken off. Also, as the slope of the land decreases the river slows leaving behind the large boulders and cobbles while carrying away the smaller particles. This results in sediments further from the source to be more uniform in size, which is a process called sorting. Chemical weathering also occurs, altering the feldspars into clay-sized particles. The end result of this process is the granite reduced from boulders and cobbles close to the mountain, to pebbles in the rivers, and finally to pure and uniform quartz sand at the beach and miniscule clay grains on the ocean floor. Therefore, different clastic rocks are found in different areas and have traveled different distances.

In this lab, we will look at three types of clastic rocks (Figure 10.1, Table 10.1), conglomerate, sandstone, and shale. Conglomerate is an immature sedimentary rock (rock that has been transported a short distance) that is a poorly sorted mixture of clay, sand, and rounded pebbles. The mineralogy of the sand and pebbles (also called clasts) can vary depending on its source. These rocks would be found on the continent in a several types of deposits such as ancient landslides or pebble beds in rivers. Sandstone is defined as a clastic sedimentary rock that consists of sand-sized clasts. These clasts can vary from jagged to rounded as well as contain- ing many minerals or just quartz. Therefore, sandstone ranges from being relatively immature to mature which makes sense because we can find layers of sand associated with mountain rivers to pure white quartz beaches. Last we have shale, which is composed of clay particles and has a finely layered or fissile appearance. This extremely mature sedimentary rock is made from the smallest particles that can be carried by wind or barely moving water and can be found thousands of miles away from the original source.

Figure 10.1 | Maturity in clastic sedimentary rocks showing how the sediments change as they are eroded further from their source.

Author: Bradley Deline Source: Original Work License: CC BY-SA 3.0

Table 10.1 | Classification of Sedimentary rocks. Author: Bradley Deline Source: Original Work License: CC BY-SA 3.0

Biochemical and Chemical Sedimentary Rocks

As mentioned before, biochemical and chemical sedimentary rocks either precipitated directly from water or by organisms. The most recognizable chemical sedimentary rocks are evaporites. These are minerals that are formed by the precipitation of minerals from the evaporation of water. You have already examined multiple examples of these minerals/rocks in a previous lab, such as halite and gypsum. In this current lab, we will focus on siliceous and carbonate biochemical sedimentary rocks. Chert is a rock composed of microcrystalline varieties of quartz, and thus it has properties that are associated with quartz itself, such as conchoidal fracturing and hardness greater than glass. Chert is often formed deep in the ocean from silicious material that is either inorganic (silica clay) or biologic (skeletons of sponges and single-celled organisms) in origin. Carbonates are one of the most important groups of sedimentary rocks and can result in distinctive landscapes (karst) and human hazards (sinkholes). Limestone is a sedimentary rock composed of the carbonate mineral calcite and can vary greatly in its appearance depending on how it is formed, but can easily be identified by its chemical weathering. Limestone composed of the mineral calcite undergoes dissolution in acids, in other words it effervesces dramatically when we apply dilute HCl. As with chert, limestone can be formed inorganically from a supersaturation of calcium and carbonate ions in water in varying environments from caves to tropical beaches. Limestone that consists of crystals of calcite or microcrystalline masses of calcite is called crystalline limestone. Alternatively, limestone can be formed biologically with the most striking example called a coquina, which are rocks made exclusively of fragmented carbonate (calcite or its polymorph aragonite) shells or coral. Lastly, we have dolostones, which are made from crystals (large or microscopic) of the mineral dolomite and are a carbonate that only weakly reacts to dilute HCl; you can scratch and powder dolostone to increase the surface area to see the reaction with acid. Dolostone is formed by the inorganic chemical alteration of limestones, therefore they are classified as chemical rather than biochemical sedimentary rocks.

Organic Sedimentary Rocks

Organic compounds are materials that contain a significant amount of the element carbon and are often associated with life. Organic sedimentary rocks are, therefore, rocks that consist mostly of carbon and are associated with significant biological activity. Other sedimentary rocks such as limestone and shale can contain carbon, but at much lower concentration (though shale can appear black from their carbon content). The most common organic sedimentary rock is coal, which is a very low density (light) black rock that has a dusty (sooty) or shiny appearance. It also produces a dark gray streak that can be seen both on a streak plate or a piece of paper. Coal is formed from the preservation and compaction of abundant plant material often in areas where oxygen is lacking, such as a swamp.

SEDIMENTARY STRUCTURES

Sedimentary rocks often show distinctive patterns that are unrelated to their type of rock, yet reflect events or conditions during deposition and are called sedimentary structures. These patterns in the rocks can be very informative to geologists attempting to reconstruct the environment in which a sedimentary rock was formed. Imagine wind blowing steadily along a beach; this wind pushes the sand into dunes that can be preserved in the rock record, informing us about the strength and direction of the wind along with the rock type.

Figure 10.3 | Cross-section of a ripple showing the area of erosion and deposition.

Author: Bradley Deline Source: Original Work License: CC BY-SA 3.0

Examples of sedimentary structures are given in Table 10.2, but let us discuss them in more detail. The sedimentary structures that most students are familiar with are ripples and dunes. We are familiar with seeing dunes at the beach or in deserts or smaller ripples in mud puddles. In each case, these ripples are formed from either a wind or water current. Ripples form by the current pushing sediment into a pile; on the down-current side the sediment is shadowed and protected from the wind or water current. This means that there is erosion on the up-current side making a shallow slope and deposition on the steeper, down-current side. Therefore, if you cut a ripple in half and look at it in prospect you can see inclined layers of sediment building up on the steep down current side of the ripple. We can often see multiple layers of beds consisting of these inclined layers, which represent multiple generations of migrating ripples or dunes that are called cross-beds. Both ripples and cross-beds can indicate the presence and direction of the current in an environment.

Table 10.2 | Descriptions of common sedimentary structures. Images provided by (from top to bottom) Grand Canyon National Park, Zion National Park, B. Deline, James St. John, Grand Canyon National Park, and B. Deline. Author: Bradley Deline Source: Original Work License: CC BY-SA 3.0

We can also see structures that indicate changes in the strength of a current. Imagine a fast moving current carrying a variety of sediment sizes; if the current slows it will no longer be able to carry the largest particles and they will be deposited first. Then as the current continues to slow progressively smaller particles are deposited on top of the bigger particles, forming a sedimentary deposit called a graded bed. This graded bed is a sedimentary layer with larger clasts on the bottom and smaller clasts on the top. These types of beds can be the result of floods in a river or storms in the ocean. We can also observe features that are pretty self-explanatory such as casts of mud cracks (covered and preserved cracks that are the result of the drying of wet mud), and rain drop impressions (covered and preserved impacts of rain drops in soft mud). In both of these cases, the sedimentary structure tells us about the sediment, the water content, and its exposure at the surface above water level.

Related to sedimentary structures are trace fossils, which are patterns in the rocks that are caused by the activity of organisms. These can occur in many different ways and can indicate many different aspects about the environment depend- ing on the trace and the identity of the tracemaker. Traces can be terrestrial such as footprints, burrows or dens, or the traces of roots, which can inform us about the climate, ecosystem, and the development of soils. Traces can also be found in freshwater and marine environments, such as burrows, borings, footprints, or feeding traces, which tell us about the sediment, chemistry of the poor water, and the life that lived within it.

DEPOSITIONAL ENVIRONMENTS

A depositional environment is the accumulation of chemical, biological, and physical properties and processes associated with the deposition of sediments that lead to a distinctive suite of sedimentary rocks. Sedimentary environments are interpreted by geologists based on clues within such as rock types, sedimentary structures, trace fossils, and fossils. We can then compare these clues to modern environments to reconstruct ancient environments. We can break up the numerous depositional environments found on earth into common environments we find on land (continental environments), in the ocean (marine environments), and at the interface between the two (transitional environments). As each of the environments is briefly described, think about: 1) the image that best depicts the depositional environment in Figure 10.4, 2) the types of sediments and sedimentary rocks most likely to occur in that environment, 3) the maturity of those sediments if clastic, and 4) the types of sedimentary structures that may be present.

Continental Environments

There are many different environments on the continents, but again we are limited to those that are dominated by the deposition rather than the erosion of

sediments. Erosion occurs in high altitude areas and although continents are over-all topographically elevated compared to the oceans, there are several different areas on the continent where we get distinctive depositional properties. Continental depositional environments are dominated by clastic sedimentary rocks, largely because of their proximity to the source of the sediments.

Glacial depositional environments are controlled mostly by the weather- ing and erosion by glaciers and glacial meltwater. Glaciers most commonly occur in areas that are both high elevation and/or high latitudes. Glaciers are fairly slow moving (centimeters a day) and normally travel short distances from their source, but they can cause immense mechanical weathering. Glaciers grind and bulldoze rock and create piles of poorly sorted sediment called moraines. Glaciers also produce a significant though fluctuating amount of melt water, which flows through the moraines building a system of braided rivers. These rivers carry the small sediments further from the end of the glaciers into an area called the outwash plain, which consists of poorly sorted sediment.

We have spent a significant amount of time in chapter five discussing rivers and how they erode, transport, and deposit sediment. Sediments that are deposit- ed through the action of rivers are referred to as fluvial depositional environments. The gradient and discharge of a river can greatly control the shape of the river, how it flows, and how it deposits sediment. Rivers alter sediment both chem- ically and physically as well as sort the sediment since there is a limit to the size or particles a river can carry. Within a meandering river we see several different types of sediments, from the pebbles and stones within the river channel to sandy point bars along the outer edge of the meander where the water slows. In addition, we see multiple types of sedimentary structures related to the flow of the river as well as those related to flood events.

We also have sediments deposited within lakes, which are called lacustrine depositional environments. Unlike rivers, lakes do not have rapidly flowing water and thus there is significantly less movement of sediment. The sediment that ac- cumulates in lakes can come from several sources including rainwater carrying sediment into the lake from the shores, rivers that flow into the lake, and sediment that is transported by the wind. Once the sediment reaches the lake it remains undisturbed so we see thin layers of fine sediment, with varying amounts of trace fossil.

Lastly, there are eolian depositional environment, which are dominated by currents of wind rather than water. Since air is less dense than water, only smaller particles can be transported. In addition, wind is not restrained within distinctive channels like water and, therefore, the features of eolian deposits are more widespread than those of fluvial deposits. Certain areas have predominant wind patterns, such that the wind is fairly consistent in direction and strength, which can generate significant sedimentary structures. When water is present within these environments it is often in the form of seasonal lakes that undergo significant evaporation and sometimes leaving behind chemical sedimentary rocks (evaporite deposits).

Transitional Environments

The interface between the continents and oceans are complicated areas that can be influenced by rivers, ocean currents, winds, waves, and tides. In addition, the sediments that are present in these areas are a mixture of materials derived from the continents (clastic and organic) and those from the ocean (chemical and biochemical). Finally, with the abundant currents (both air and water), sediments influenced by the elements, and abundant life these areas, which results in abundant sedimentary structures and trace fossils. However, we can distinguish transitional areas that are dominated by different forces such as tides, ocean currents, and rivers.

Shorelines that are influenced by strong daily tidal currents are called tidal mudflat depositional environments. Tides are currents that are the result of the gravitational forces exerted by the moon and the rotation of the earth. Shore- lines that have strong tidal currents as well as seafloors with low gradients can have large areas that are submerged during high tide and exposed to air during low tide. These areas often have smaller particles than a normal shoreline since the tidal currents can pull marine sediments into the area. In addition, the strong bidirectional currents, daily drying out, exposure to the elements, and abundant life create abundant indicators of these environments.

Shorelines that are dominated by ocean currents are called beach depositional environments. Shorelines have constant wind on and off shore that are the result of the difference in the way the land and water heat and cool through the day. These winds produce the waves that are iconic at the beach, but as these waves move onto shore they curve, mimicking the shape of the shore and result in a cur- rent that runs parallel to the shore itself. This current carries and deposits sand along the beach. In addition, the wind can also produce dunes, which promote a di- verse and complicated ecosystem. Within beach depositional environments there are areas where rivers flow into the ocean, which are called Deltas. As a river that is carrying material empties into the ocean the water slows and deposits sediment. Most of the sediment is deposited at the mouth of the river with some spilling out into the surrounding areas building a distinctive fan of sediment. Since the sediment is coming from the river, the delta is largely a thick sequence of clastic sediment showing indications of the strong flow of the river.

Marine Environments

Marine depositional environments differ in multiple ways, but the controlling factors in the rocks that are produced is related to the proximity and supply of continental sediment, the water depth, and the community of organisms that live in the area. The further an environment is from the shore the less clastic sediment will be present and the area will have a higher concentration of the chemical and biological sedimentary rocks that are formed within the ocean. In addition, some organisms in the right environmental conditions can produce huge amounts of skeletal material.

Shallow marine depositional environments are areas that are close to shore, but always submerged. These areas have a significant amount of mature clastic sediment along with marine algae (like sea grass) as well as skeletal material from animals like coral, echinoderms (sea urchins and sand dollars), and mollusks (clams and snails). These areas can have a significant difference in their energy level from very shallow areas influenced by waves to deeper areas only influenced by large storms. A better understanding of the relative depth can often be determined based on the sedimentary structures as well as the community of organisms and types of trace fossils, which can be very sensitive to depth.

In warm tropical shallow water area we often find reef depositional environments. Reefs are formed through the growth of coral colonies building a large three-dimensional structure built from calcite skeletons. Corals can grow in many different marine environments, but they can only produce reefs when their symbiotic algae that live within their tentacles can photosynthesize effectively, resulting in more energy for the coral to grow faster. Reefs also create a barrier between shallow water environments and ocean currents producing shallow, low-energy environment called lagoons. Lagoons can have thin layers of fine sediment that we would expect in quiet water along with chemical sedimentary deposits that are the result of evaporation.

Most of the ocean consists of deep marine depositional environments. These areas are beyond the reach of most clastic sediment other than the dust carried by the wind. Therefore, the sediment is being produced chemically and biologically within the ocean. The largest source of sediment in these deep settings is skeletal material from some of the smallest marine organisms. Multiple types of single-celled organisms can produce shells composed of either silica or calcite. These shells are mostly produced in the surface waters that are bathed in sunlight permitting photo- synthesis. When the organism dies, these shells then rain down into deeper water; this slow accumulation of sediment produces fine layers of biochemical sedimentary rocks. In some cases, these shells are dissolved or altered before they reach the bottom (which can be miles away) and are precipitated as chemical sedimentary rocks. Obviously, there is not a clear boundary between shallow and deep-water environments given the gradient of the ocean floor. The deep marine depositional environment is normally thousands of feet deep and beyond the influence of even large storms.

Figure 10.4 | Modern examples of the discussed depositional environments.

Photo A Author: Kelsey Roberts, USGS Source: Flickr License: Public Domain

Photo B Author: Alan Cressler Source: Flickr License: CC BY 2.0

Photo C Author: Kim Keating, USGS Source: Flickr License: Public Domain

Photo D Author: Deborah Bergfeld, USGS Source: Flickr License: Public Domain

Photo E Author: Paul Hutchinson Source: Wikimedia Commons License: CC BY-SA 2.0

Photo F Author: James St. John Source: Wikimedia Commons License: CC BY 2.0

Photo G Author: NOAA Source: Wikimedia Commons License: CC BY 2.0

Photo H Author: Peter Dowley Source: Wikimedia Commons License: CC BY 2.0

Photo I Author: Diueine Monteiro Source: Wikimedia Commons License: CC BY-NC-SA 2.0