10 describe how sediments are produced by weathering and erosion Ideas

KEY CONCEPTS

By the end of this chapter, students will be able to:

• Describe how water is an integral part of all 5.1 The Unique Properties of Water

  

  Water plays a role in the formation of most sedimentary rock. It is one of the main agents involved in creating the

  Polarity allows water molecules to stick to other substances. This is called

  The fact that water is attracted to itself leads to another important property, one that is extremely rare in the natural world—the liquid form is denser than the solid form. The polarity of water creates a special type of weak bonding called

  Even more critical for supporting life, water remains liquid over a very large range of temperatures, which is also a result of cohesion. Hydrogen 5.2 Weathering and Erosion

  Bedrock refers to the solid rock that makes up the Earth’s outer 5.2.1 Mechanical Weathering

  Mechanical Pressure Expansion

  

  Bedrock buried deep within the Earth is under high pressure and Frost Wedging

  

  Frost wedging, also called ice wedging, uses the power of expanding ice to break apart rocks. Water works its way into various cracks, voids, and crevices. As the water freezes, it expands with great force, exploiting any weaknesses. When ice melts, the liquid water moves further into the widened spaces. Repeated cycles of freezing and melting eventually pry the rocks apart. The cycles can occur daily when fluctuations of temperature between day and night go from freezing to melting.

  Root Wedging

  

  Like Salt Expansion

  

  Salt expansion, which works similarly to frost wedging, occurs in areas of high evaporation or near-5.2.2 Chemical Weathering

  

  Chemical Carbonic Acid and Hydrolysis

  

  Carbonic acid (H₂CO₃) forms when carbon dioxide, the fifth-most abundant gas in the Dissolution

  

  Dissolution is a

  The

  

  Oxidation

  

  Oxidation, the chemical reaction that causes rust in 5.2.3 Erosion

  

  Erosion is a mechanical process, usually driven by water, gravity, (see Chapter 10), wind, or ice (see Chapter 14) that removes sediment from the place of weathering. Liquid water is the main agent of erosion.

  

  5.2.4. Soil

  

  Soil is a combination of air, water, minerals, and organic matter that forms at the transition between

  These nitrogen-fixing bacteria absorb nitrogen from the atmosphere and convert it into nitrogen compounds. These compounds are absorbed by plants and used to make DNA, amino acids, and enzymes. Animals obtain bioavailable nitrogen by eating plants, and this is the source of most of the nitrogen used by life. That nitrogen is an essential component of proteins and DNA. Soils range from poor to rich, depending on the amount of humus they contain. Soil productivity is determined by water and nutrient content. Freshly created volcanic

  The nature of the

  O Horizon: The top horizon is a thin layer of predominantly organic material, such as leaves, twigs, and other plant parts that are actively decaying into humus.

  A Horizon: The next layer, called

  The United States governing body for agriculture, the USDA, uses a taxonomic classification to identify soil types, called soil orders. Xoxisols or laterite

  Not only is soil essential to 5.3 Sedimentary rocks

  Sedimentary rock is classified into two main categories: clastic and chemical. Clastic or detrital sedimentary rocks are made from pieces of bedrock, 5.3.1 Lithification and Diagenesis

  Lithification turns loose sediment grains, created by weathering and transported by erosion, into clastic

  5.3.2 Detrital Sedimentary Rocks (Clastic)

  Detrital or Grain Size

  

  Sorting and Rounding

  

  Sorting describes the range of grain sizes within

  Composition and provenance

  

  

  Geologists use Classification of Clastic Rocks

  

  

  Medium-grained rocks composed mainly of sand are called

  Fine-grained rocks include

  Rock types found as a mixture between the main classifications, may be named using the less-common component as a descriptor. For example, a rock containing some silt but mostly rounded sand and gravel is called silty conglomerate. Sand-rich rock containing minor amounts of clay is called clayey 5.3.3. Chemical, Biochemical, and Organic

  Chemical sedimentary rocks are formed by processes that do not directly involve mechanical weathering and erosion. Inorganic chemical

  

  Inorganic chemical sedimentary rocks are formed when Mineral sequence Percent Seawater remaining after evaporation Calcite 50 Gypsum/anhydrite 20 Halite 10 Various potassium and magnesium salts 5

  Table after.

  

  

  Calcium

  Cave deposits like stalactites and stalagmites are another form of chemical precipitation of calcite, in a form called

  

  Chert, another commonly found chemical

  Biochemical

  

  

  Limestone occurs in many forms, most of which originate from biological processes. Entire coral Organic

  

  Under the right conditions, intact pieces of organic material or material derived from organic sources, is preserved in the geologic record. Although not derived from sediment, this lithified organic material is associated with sedimentary Classification of Chemical Sedimentary Rocks

  

  In contrast to detrital sediment, chemical, biochemical, and organic sedimentary rocks are classified based on mineral composition. Most of these are monomineralic,

  Sedimentary rock identification chart

  5.4 Sedimentary Structures

  Sedimentary structures are visible textures or arrangements of sediments within a rock. Geologists use these structures to interpret the processes that made the rock and the environment in which it formed. They use 5.4.1. Bedding Planes

  

  The most basic sedimentary structure is bedding planes, the planes that separate the layers or strata in sedimentary and some volcanic rocks. Visible in exposed outcroppings, each

  As would be expected, bed thickness can indicate 5.4.2. Graded Bedding

  

  5.4.3. Flow Regime and Bedforms

  

  In fluid systems, such as moving water or wind, sand is the most easily transported and deposited sediment grain. Smaller particles like silt and clay are less movable by fluid systems because the tiny grains are chemically attracted to each other and stick to the underlying Flow Regime (part) Bedform Description Lower (lowest) Plane Beds

  

  Plane beds created in the lower Ripples

  

  Ripples are known by several names:

  First scientifically described by Hertha Ayrton, ripple shapes are determined by flow type and can be straight-crested, sinuous, or complex. Asymmetrical ripples form in a unidirectional flow. Symmetrical

  Dunes

  

  Dunes are very large and prominent versions of

  Dunes are the most common sedimentary structure found within channelized flows of air or water. The biggest difference between

  

  Another Antidunes

  

  Antidunes are so named because they share similar characteristics with dunes, but are formed by a different, opposing process. While 5.4.4. Bioturbation

  

  5.4.5. Mudcracks

  

  5.4.6. Sole Marks

  

  

  Formed similarly to

  Load 5.4.7. Raindrop Impressions

  

  Like their name implies, 5.4.8. Imbrication

  

  5.4.9. Geopetal Structures

  

  Geopetal structures, also called up-direction indicators, are used to identify which way was up when the sedimentary rock layers were originally formed. This is especially important in places where the rock layers have been deformed, tilted, or overturned. Well preserved mudcracks,

  • Vugs: Small voids in the rock that usually become filled during diagenesis. If the void is partially filled or filled in stages, it serves as a permanent record of a level bubble, frozen in time.
  • 5.5 Depositional Environments

    

    The ultimate goal of many stratigraphy studies is to understand the original depositional environment. Knowing where and how a particular sedimentary rock was formed can help geologists paint a picture of past environments—such as a mountain glacier, gentle Location Sediment Common Rock Types Typical Fossils Sedimentary Structures 5.5.1. Marine

    Marine depositional environments are completely and constantly submerged in seawater. Their depositional characteristics are largely dependent on the depth of water with two notable exceptions, submarine fans and turbidites.

    Abyssal

    

    Abyssal sedimentary rocks form on the abyssal plain. The plain encompasses relatively flat

    The third sediment type is pelagic clay. Very fine-grained clay particles, typically brown or red, descend through the water column very slowly. Pelagic clay deposition occurs in areas of remote open ocean, where there is little plankton accumulation.

    

    Two notable exceptions to the fine-grained nature of abyssal Continental Slope

    

    Continental slope deposits are not common in the rock record. The most notable type of continental slope deposits are contourites. Contourites form on the slope between the continental shelf and deep Lower shoreface

    

    The lower shoreface lies below the normal depth of wave agitation, so the Upper shoreface

    

    The upper shoreface contains sediments within the zone of normal wave action, but still submerged below the beach environment. These 5.5.2. Transitional coastline environments

    

    Transitional environments, more often called Littoral

    

    The littoral zone, better known as the beach, consists of highly weathered, homogeneous, well-sorted sand grains made mostly of Tidal Flats

    

    Reefs

    

    Reefs, which most people would immediately associate with tropical coral reefs found in the oceans, are not only made by living things. Natural buildups of sand or rock can also create

    Most reefs, now and in the geologic past, originate from the biological processes of living organisms. The growth habits of coral

    Sediment found in coral reefs is typically fine-grained, mostly carbonate, and tends to deposit between the intact coral skeletons. Water with high levels of silt or clay particles can inhibit the

    Lagoon

    

    Deltas

    

    

    Deltas form where rivers enter lakes or oceans and are of three basic shapes:

    Deltas are organized by the dominant process that controls their shape: tide-dominated, wave-dominated, or river-dominated. Wave-dominated deltas generally have smooth coastlines and beach-ridges on the land that represent previous shorelines. The Nile River 5.5.3. Terrestrial

    Terrestrial Fluvial

    

    Fluvial (river) systems are formed by water flowing in channels over the land. They generally come in two main varieties:

    Braided fluvial systems generally contain coarser sediment grains, and form a complicated series of intertwined channels that flow around gravel and sand bars (see Chapter 11, Water).

    Alluvial

    

    A distinctive characteristic of Lacustrine

    

    Lake systems and deposits, called lacustrine, form via processes somewhat similar to Paludal

    Paludal systems include bogs, marshes, swamps, or other wetlands, and usually contain lots of organic matter. Paludal systems typically develop in coastal environments, but are common occur in humid, low-lying, low-Aeolian

    

    Aeolian, sometimes spelled

    Compacted layers of wind-blown Glacial

    

    Glacial sedimentation is very diverse, and generally consists of the most poorly-sorted sediment deposits found in nature. The main clast type is called 5.5.4. Facies

    In addition to

    5.5 Did I Get It?

    Use this quiz to check your comprehension of this section. Click directly on the answer button, not on the answer bar.

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    1. Shoreface

    2.

    3. Aeolian

    4. Alluvial

    5.

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    1. Alluvial

    2. Aeolian

    3. Paludal

    4.

    5.

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    3. On the dynamic Earth, shifting environments over time cause facies distributions that allow interpretation of ancient shifting shorelines. In the Middle Cambrian strata of the Grand Canyon and surrounding regions, 1. East to west

    2. North to south

    3. Southeast to northwest

    4. East to west

    5. Northwest to southeast