Sedimentary Environments, Rocks, and Structures

Lab 2: Sedimentary Environments, Rocks, and Structures

Sedimentary rocks account for a negligibly small fraction of Earth’s mass, yet they are commonly encountered because the processes that form them are ubiquitous in the near-surface environment. Thus, they preserve the history of that portion of the planet that is most familiar. Sedimentary rocks indicate paleoenvironment, i.e. ancient climates and ecology. Sometimes they provide the only remaining evidence of former mountain ranges or shallow seas. Sedimentary rocks are also essentially the only type of rock that contains fossils, which not only are indicative of previous environments, but also are crucial in dating and correlating rock units. Sedimentary rocks also provide a record of previous geologic hazards such as seismic events, volcanic eruptions (ash deposits), storms, and fluctuations in climate. Furthermore, key economic natural resources involve sedimentary rocks. Resources such as coal, oil, natural gas, gypsum, aggregate (sand and gravel), and salt are all found within sediments.

Sedimentary Structures

Sedimentary structures such as stratification (layering), ripple marks, cross-bedding, and mudcracks can be preserved in sedimentary rocks. These structures provide important information about depositional environments such as flow direction, climate (arid, semi-arid, or humid) and setting (e.g. fluvial, lacustrine, or marine). These structures also may indicate which direction was originally up within the rock. Tectonic forces can fold and overturn rocks, so establishing the original orientation is not always easy, but is often useful.

Bedding

Sedimentary rocks will often be deposited in discrete layers, which leads to a particularly important sedimentary structure called bedding. Bedding layers can range in thickness from millimeters to tens of meters. Typically, though not always, bedding is originally horizontal in orientation; tilted bedding indicates that the rock has been deformed in some way.

Lithification

Sedimentary rocks start out as loose sediment. To become a sedimentary rock, the sediments must be lithified, which involves compaction and cementation. Compaction occurs through pressure via deep burial. Water is removed and the grains are packed tightly together. During cementation, minerals such as quartz, calcite, or hematite precipitate out of water and fill the spaces between the clasts, locking them together. The term friable describes a poorly cemented rock that falls apart easily. Note: rocks can become friable either because they were never cemented thoroughly, or because the cement has been re-dissolved and removed.

Depositional Environments

A sedimentary environment is a geographic location characterized by a particular combination of geologic processes and environmental conditions. Geologic processes include the currents that transport and deposit sediments (water, wind, or ice) and the plate tectonic settings that affect sedimentation. For example, the geologic processes of a beach environment include the dynamics of waves crashing against the shore, shoreline currents, and the distribution of sediments on the beach. Environmental conditions include the kind and amount of water (ocean, lake, river, arid land), the landscape (lowland, mountain, coastal plain, shallow ocean, deep ocean), and biological activity.

Location Depositional Characteristics Rock Type(s) Formed Environment Lakes Lake deposits are generally low-energy shale, siltstone (Lacustrine) environments where fine-grained sediments are deposited in thin layers. Lakes that freeze over seasonally may develop varves: alternating layers of light (coarser) and dark (finer) sediments. Large lakes can also have higher-energy, sandy beaches. Glacial Glacial deposits form along the margins of and Conglomerate and sometimes beneath glacial ice. Because ice can transport any breccia size grain (unlike water or wind), deposits are If glacial origin is known, the typically very poorly-sorted rock is called a “tillite” Alluvial Fan Alluvial fans are fan-shaped wedges of sediment Conglomerate, sandstone, and deposited along the margin of a steep slope. sometimes breccia They often contain a lot of coarse-grained, moderately to poorly-sorted sediment. Continental Rivers and Streams Rivers and streams typically deposit medium- to Sandstone, siltstone, some Environments (Fluvial) coarse-grained (sand to cobble-sized) sediment in conglomerate their channels. River sediment is often moderately- to well-sorted. Floodplains Floodplains are relatively low-energy Siltstone, shale environments where finer (clay, silt, fine sand) sediments are deposited in well-defined layers. These plains are only periodically wet and when they dry out mudcracks often develop. Swamps, Marshes, or Swamps are typically rich in organic material that Coal Bogs is buried and compressed to form coal Evaporite Basins Shallow basins in arid regions and lagoons may Rock gypsum, Rock salt, become supersaturated and precipitate evaporite crystalline limestone minerals. Desert (Eolian) Eolian environments are arid and typically have Sandstone, siltstone winds that transport and sort medium- and fine- grained sediment (sand to silt). Eolian sediments are often well-sorted and show well-developed cross-bedding. Deltas Deltas form where rivers and streams enter larger Sandstone, siltstone, shale bodies of water. They often contain fluvial-type Shoreline deposits as well as swampy environments. Environments Beaches Beaches occur on the margins of large bodies of Sandstone water. They generally contain deposits of well- sorted, medium-grained sediment with planar bedding. Shallow marine or Shallow marine environments are formed on the Sandstone, siltstone, shale, continental shelf margins of continents, on the continental shelf. diatomite, oolitic limestone, Associated deposits are typically medium- to chalk fine-grained and well sorted. In warm (sub- (Depends on supply of clastic tropical to tropical) environments these sediments and chemical sediments) may be calcite-rich. Deep marine Deep marine areas receive relatively little clastic Chert, shale Marine sediment. Common deep marine sediments are Environments either very fine-grained or microcrystalline (from recrystallization of microscopic silica-producing organisms). Reefs Organic structures composed of calcium Limestone (fossiliferous) carbonate-secreting organisms (i.e. coral) built up on continental shelves or oceanic volcanic islands

Figure 7.2 Depositional Environments

Clastic Sedimentary Rocks

Clastic sediments are made of particles of mineral or rock fragments, known as clasts, that have been weathered from preexisting rock and transported by gravity, water, ice, or air. Chemical weathering involves the dissolution or decomposition of these minerals, whereas mechanical weathering consists of processes such as abrasion and cracking that do not change the mineral content of the material. During transport, clasts are abraded and become increasingly rounded (smooth surfaced) and equidimensional (spherical). Chemically and mechanically stable minerals, such as quartz, survive this transportation better than the less stable minerals and are therefore concentrated in sediments that have been transported for long times and distances. Transportation also tends to segregate particles by size. High-energy environments (e.g. rivers, coasts) can transport large clasts, while low energy environments (deep ocean) can transport only small clasts. Different transport processes can be more or less selective about which grain sizes are moved, therefore the degree of sorting can be indicative of the transport medium. For example, glacial ice is not selective at all and can move the widest range of clasts (tiny clay-sized particles to boulders the size of buildings), while wind typically moves grains sized within the narrow range of silt to fine sand. A sediment with clasts of uniform size is known as well-sorted, while one containing a wide range of clast sizes is poorly-sorted. Sediments that consist primarily of well-sorted, rounded, and spherical quartz grains indicate that the material has been subjected to long or repeated periods of transport and is designated as mature. On the other hand, sediments that consist of various minerals and rock fragments that are angular, non- spherical, and poorly-sorted are indicative of sediments that have not been transported far and are called immature. Factors relating to maturity are outlined in the following tables and figures:

Stability of common minerals under surficial weathering conditions. Note that there is a relationship between this series and Bowen’s reaction series. The minerals formed at highest temperatures and pressures are the least stable, while those formed at lower temperatures and pressures (closer to surface conditions) are more stable.

Most Stable Fe Oxides Al Oxides Quartz Clay minerals Muscovite Potassium Feldspar Biotite Sodium-rich Plagioclase Amphibole Pyroxene Calcium-rich Plagioclase Least Stable Olivine

Degrees of rounding and sphericity. The degree of rounding is often a function of transport duration; the longer a clast is in transport, the more rounded it will become. The degree of sphericity also depends somewhat on transport duration, though some mineral grains start out more equidimensional than others. Quartz, for example, often is found in nearly equidimensional grains in granite, while amphibole and feldspar crystals are elongated.

Guide to grain size. Note that (a) A conglomerate may instead be called a breccia if its clasts are angular. (b) Sand can be further divided into fine sand (1/16 to about 1/8 mm), medium sand (1/8 to 1 mm) and coarse sand (1 to 2 mm). (c) The term clay can refer either to a range of grain size (< 1/256 mm) or to a family of sheet silicate minerals known as clay minerals.

Name of particle Range limits of Names of loose Name of diameter (mm) sediment consolidated rock Boulder > 256 boulder gravel boulder conglomerate Cobble 64 to 256 cobble gravel cobble conglomerate Pebble 2 to 64 pebble gravel pebble conglomerate Sand 1/16 to 2 sand sandstone Silt 1/256 to 1/16 silt siltstone Clay < 1/256 clay mudstone and shale

Guide to grain sorting. Note that the degree of sorting is independent of the absolute sizes of the grains involved.

Clastic sedimentary rock classification key.

Texture Grain Size Composition Other Rock Name Texture Gravel-sized clasts Rounded mostly rock Conglomerate grains Gravel (particles fragments larger than 2 mm) Gravel-sized clasts Angular mostly rock Breccia grains fragments Commonly quartz, feldspar, rock Sandstone fragments Sand (particles Predominately Quartz

visible, but less Quartz sandstone than 2 mm) Predominately Arkosic

Clastic feldspar sandstone Predominately Lithic

Lithic sandstone Silt (particles not visible, feels gritty Most often quartz, and cannot be Siltstone some feldspar scratched by fingernail) Clay (particles not visible, feels Clay minerals and Mudstone or smooth and is quartz shale easily scratched by fingernail)

Chemical Sedimentary Rocks

Chemical sedimentary rocks are formed by the precipitation of compounds from aqueous solutions. For example, limestone forms from the precipitation of calcium carbonate (calcite) from seawater. Often, biology plays a key role in the formation of limestones as the calcite comes from the shells of sea creatures. Another example of a chemical sedimentary rock is an evaporite, a rock that forms when water is evaporated from closed basins in arid climates. As evaporation progresses, the remaining water can become highly saline and eventually will become supersaturated with respect to a variety of dissolved constituents, leading to their precipitation from solution. Common evaporite minerals include gypsum and halite. Silica is undersaturated in sea water so one would not expect to find it as a direct precipitate from sea water. However, small siliceous organisms such as diatoms, radiolarians, and some sponges are highly efficient in removing silica from sea water to form their skeletons. After these organisms die they sink and accumulate on the sea floor. Many cherts are formed by lithification and recrystallization of such deposits.

Organic Sedimentary Rocks

An unusual sedimentary rock is coal, a carbon-rich rock that forms when organic matter (trees and other plant matter) is buried and compressed in an oxygen-poor environment so that decomposition does not proceed. This is common in swampy settings.

Chemical and organic sedimentary rock classification key.

Texture Composition Other Properties Rock Name Microcrystalline quartz Scratches glass Chert Three perfect cleavages at Halite Rock Salt 90°, tastes salty Softer than fingernail, Rock Gypsum cleavages not at 90° Gypsum Chemical Readily reacts with dilute (crystalline) Calcite Limestone hydrochloric acid Powdered rock reacts with dilute hydrochloric acid Dolomite Dolostone (much less reactive than calcite) Brown to black, low Organic Plant material (carbon) Coal specific gravity Fossiferous Calcite Fossils and fine grains Limestone Entirely composed of shell Calcite Coquina fragments Biochemical Oolitic Calcite Ooids (layered spheres) Limestone Diatoms, very white color, Quartz often low density and Diatomite friable

Station 1

a. Describe samples 1a and 1b.

Structure Grain size Sorting Rounding Flow Depositional direction (e. Environment g. R to L) 1a

b. Examine samples 1a and 1b.

What sedimentary structure is common to both samples?

What is the flow direction in 1a? (R to L, L to R, or indeterminate?)

Which side is the top of sample 1b (A or B)?

Station 2

a. What sedimentary structure is present in samples 2a and 2b?

b. Which side is the top (A or B) and how is this shown?

2a______

2b______

c. Specimen 2b contains another prominent sedimentary structure. What is this sedimentary structure and how did it form?

Station 3

Describe samples 3a and 3b.

Grain size Sorting Rounding Structures

Sample 3a formed in a different type of environment than 3b. Was the environment for 3a much higher energy, much lower energy, or did the two environments have a similar energy? What indicates this?

Station 4

Describe sample 4a.

Grain size Sorting Rounding Structure Depositional Environment 4a

Find the burrow in 4a. Would one expect to find other sedimentary structures? Why or why not?

Station 5

Examine samples 5a and 5b.

What sedimentary structure is shared by these rocks?

How did it form?

Identify samples 5a and 5b.

5a______

5b______

Clastic or Representative Sorting Roundness and Other (e.g. Sample Composition Rock Name Chemical Grain Size Sphericity fossil content)