Sedimentary Rocks: Depositional Environments and Structures

Purpose

The purposes of this lab are (1) to review the processes that form sedimentary rocks and to observe the products of these processes and (2) to extend your examination of sedimentary rocks to understanding the geological history that they record. Much of the text of this lab is background information to either help you with the context or to help you with the required work. Please read through the entire lab before starting.

Sediments are the end product of the destruction of preexisting rocks (igneous, metamorphic or sedimentary) on the earth's surface. The two major processes leading to decomposition of rocks are (1) mechanical breakage and (2) chemical dissolution. Mechanical breakage increases the available surface area for chemical attack, and chemical alteration weakens rocks and makes them more susceptible to breaking. Accordingly, the two processes work hand-in-hand to weather rocks.

Composition, texture and color are important means of identifying sedimentary rock types, and the observation of sediment types in modern depositional settings is a key factor in identifying means of distinguishing ancient environments based on sedimentary rock types and facies associations. The process of interpreting ancient environments is somewhat of a sleuthing job. We apply the principle of uniformity, first described by the famous geologist James Hutton in 1785. That is to say that we assume that there is uniformity in natural law through time and space. Understanding the natural processes operating on earth today provides us with a guide to past Earth events. The principle of uniformity does not mean that today's world is an exact replica of the past or future. It simply means that we can work to understand today's world and apply our understanding to interpreting the past and to predicting the future.

Part 1: Observations of Feldspars' Weathering Products

Examine the set of weathering samples which were selected to demonstrate the results of weathering and soil formation. Sample 1 is a small box containing a variety of feldspar samples. Feldspar is the most abundant mineral in the continental crust, and the general varieties are plagioclase with compositions ranging CaAl2Si2O8 - NaAlSi3O3, and K-feldspar, KAlSi3O8. The surfaces of these samples are clean, and the characteristic luster of feldspar is apparent. Sample 2 is a pegmatite that contains feldspar crystals with smoky quartz crystals. The chalky appearance of the feldspar surface is residual clay, likely kaolinite. Kaolinte has a compositions of Al2Si2O5(OH)4, and is a product of chemical alteration of the feldspar. Compare the chemical formulae of feldspars with kaolinite. Sample 3 is ordinary soil. This soil still contains many "nutrients" and is good for growing plants and altering minerals. Sample 4 is the aluminous clay mineral kaolinite. Kaolinite is also a product of intense weathering, and it in turn is broken down to release silicon to solution and to produce aluminum oxides and hydroxides. Kaolinite is mined for making fine porcelain and other ceramic products.

Part 2: Classification of Sedimentary Rocks

Sediments can be broadly divided into two groups: (1) clastic - the residues of weathering and (2) chemical/biochemical precipitates - products of crystallization of ions that were dissolved during weathering.

The dominant constituents of sedimentary rocks are:

quartz- (SiO2) one of the most abundant minerals in the exposed continental crust, and an extremely hard, resistant and chemically stable mineral.
calcite- (CaCO3) a major constituent of limestone (a very common sedimentary rock type) and a common cementing agent in shale and sandstone (clastic rocks).
clay- residual product developed from weathering of silicate minerals.
rock fragments- relatively unweathered remnants of weathering.

Other important constituents of sedimentary rocks are:

dolomite- ((Ca,Mg)CO3) may replace calcite in limestone
feldspar and mica - relatively unweathered residues
halite and gypsum - (NaCl, CaSO4*2H2O) evaporites
organic mater - e.g. coal

Sedimentary rocks are characterized by layering. Layering is produced by physical or chemical changes that occur in their environment of deposition. Layers can be found in sedimentary rocks in a wide variety of scales, from much less than a millimeter to many meters. Layering and other structures within sedimentary rocks provide important clues as to their origin. Many of the specimens cannot be seen to have obvious layering due to the small size of the samples relative to the layering. In your description of the rocks, make a note of any evidence for layering.

Texture is an important aspect of identifying and describing sedimentary rocks. The important textures you will be examining today are:

clastic - made up of grains
crystalline - interlocking crystals resulting from crystallization
skeletal - composed of fossil shells

Sedimentary Structures

In addition to texture, sedimentary rocks often have structures that provide further information about their depositional environments. For example, tectonic processes may overturn sedimentary rocks. The preservation of sedimentary structures allows geologists to determine which way was up in the original sequence and therefore, which samples were deposited first. Sedimentary Structures include:

Cross bedding - Bedding planes form at an angle to the horizontal. These are common in dunes and ripples. Detailed study of cross beds can reveal the flow direction of the water that deposited the sediment.

Graded Bedding - A sample has variable grain sizes that are organized such that the size of the largest clasts increases in some direction in the rock. The sequence “fines upwards” meaning that the finer grains were originally deposited after the coarse ones and therefore should lie on top of them in a stratigraphic column. Think back to the Hjulstrom Curve!

Other structures you should look out for today are ripple marks (which can be symmetric or asymmetric), rain drop impressions and mudcracks.

Refer to this site for pictures: http://www.gpc.edu/~pgore/geology/historical_lab/sedstructureslab.php

Identifying Clastic Sedimentary Rocks

The basic criterion for classifying clastic textures is grain size, with subordinate subdivisions made on the basis of rounding, sorting and cementation. Particles are referred to as coarse grained (over 2mm in diameter), medium grained (1/16 to 2 mm in diameter), fine grained (less than 1/16 mm diameter). Conglomerates contain abundant coarse clasts, sandstones are composed of medium grained clasts, and siltstones and shales are composed of fine grained clasts.

Identifying Chemical and Biochemical Sedimentary Rocks

Chemical (or biochemical) sediments have a variety of textures that are also subdivided based on size of the crystals or fragments contained in the sediment. Minerals precipitated from sea water or lakes develop a network of interlocking crystals, or crystalline texture, similar to those found in igneous rocks but generally consisting of one dominant mineral. The individual crystals are generally about the same size and interlock to form a dense rock. Skeletal textures and oolitic texture (special variety of grain) are frequently found in limestone formations. Crystalline and skeletal textures are subdivided as with clastic textures into coarse (greater than 2 mm), medium (1/16 to 2 mm) and fine (less than 1/16 mm).

Part 3: Sedimentary Environments of Deposition

Following is a brief guide to the properties of sedimentary rocks deposited within various sedimentary environments. For photographs and sketches, refer to your text book or the web.

  1. Continental Environments- predominantly siliciclastic sediments (conglomerate, sandstone, siltstone, etc.) characterized by scarce fossils and no marine fossils.
    1. Fluvial (rivers)

      Alluvial Fans - deposits that form at the base of mountains where rapidly flowing streams suddenly emerge from a narrow valley, spread our, slow down, and dump the larger particles in their sediment load. They are poorly sorted and clasts are frequently angular. The composition of the fragments is similar to the rocks exposed in the nearby mountains. Sedimentary structures are not well developed and fossils are very rare.

      Braided Rivers - characterized by many channels separated by bars or small islands. Braiding results from rapid, large fluctuations in the volume of river water, and an abundance of coarse sediment. There are two main types of braided river facies: 1) rippled, cross-stratified gravels and coarse sandstones (bars) and 2) horizontally stratified, fine to coarse sands (channels). In a vertical section through an ancient braided river these will tend to alternate.

      Meandering Rivers - confined to one, highly sinuous channel, and contain finer sediment load than braided rivers. Meandering rivers also form bars, but they are formed on the inside bend of meander loops. As a result these bars build outward, the streams become more and more sinuous and migrate across the river basin. There are two main types of meandering river facies: 1) rippled, cross-bedded, fining-upward sequences of gravel and sand (bars) and 2) fine-grained sediments, such as silt and clay, containing burrows and plant debris (overbank or flood deposits). In a vertical section through an ancient meandering system, these will tend to alternate.

    2. Lacustrine (lakes) - difficult to characterize. They may contain numerous sedimentary structures, including cross-bedding, ripples, graded beds, footprints, mudcracks, and raindrop impressions. Fossils may be common. Plant fossils and freshwater bivalves and gastropods are particularly abundant.
    3. Paludal (swamps and marshes) - organic-rich shale and sandstone or coal deposits with thin stringers of silstone and shale. Plant fossils are common in all stages of preservation.
    4. Eolian (deserts and near beaches) - recognized by dune deposits, although the dominant sedimentary layering that is preserved is horizontal.
    5. Glacial - range in size from small bodies deposited by valley glaciers (alpine glaciers) to large sheets dumped from continental glaciers. Characterized by a variety of facies, but the most unique is diamictites, or pebbly mudstones.
  2. Transitional to Shallow Marine Environments
    1. Deltas - form where rivers enter a standing body of water, slow down, and deposit more sediment than can be removed by waves and currents. Although deltas also from in lakes, the largest deltas occur in the oceans. Deltas are composed of several sub-environments, from the fluvial delta-top to the submarine base of the delta. Accordingly there are numerous types of fossils and sedimentary structures possible. Ancient delta deposits are most easily recognized by the larger package. Because of their formation process, there is a lateral gradation in particle size (and sedimentary rock type) along the delta, from sand near the river outlet to submarine clay deposits at the edges. As more sediment is added, the delta builds out into the standing body of water, with coarser sediments migrating across the clays that used to be at the delta edges. This results in a coarsening upward (regressive) sequence.
    2. Beaches and Barrier Islands - long, narrow accumulations of sand parallel to the shoreline. Barrier islands are separated from land by a shallow lagoon or marsh. Beach facies are composed primarily of fine- to medium-grained, well-sorted sand that displays subhorizontal parallel laminations and low-angle, seaward-, landward- and alongshore-dipping crossbeds. The variously dipping crossbeds are a result of the back-and-forth action of tides and longshore currents. Burrows are common in sediments of the transition zone between the beach and open shelf.
    3. Clastic shelf - bounded by coastal environments on the landward side and by the continental slope on the seaward side. Sediments consist mainly of sand and mud, and nearshore sands commonly grade seaward through a transition zone of mixed sand and mud to deeper-water muds. Cross bedding is common in the sands and bioturbation is common in the muds.
    4. Carbonate shelves and platforms - located primarily at low latitudes in clear, shallow, tropical seas where little continental, clastic sediment is introduced.
  3. Deep Marine
    1. Pelagic - fine-grained sediments deposited far from land influence by slowly settling particles suspended in the water column.
      1. carbonate ooze - carbonate shells of tiny planktonic organisms (foraminifera, coccolithoforids)
      2. silica ooze - silica shells of tiny planktonic organisms (radiolarian, diatoms)
      3. red clay - clay-sized particles of continental origin (mostly transported by wind). Very high Fe and Mn contents produce the coloration, and frequently Mn pavements, crusts and nodules are found in this environment.
    2. Turbidites - fining-upward deposits that were transported seaward in deep-sea channels and canyons by high-density, sediment laden currents. In map view, turbidites form fans that spread outward on the sea floor from the mouths of the canyons.

Part 4: Changing depositional environments/The stratigraphic column

In the homework, you saw that the depth to the basement of a basin deepens as the basin is filled with sediment. As a result, basins often have sediment sequences thicker than their original basin depth. Often geologists reconstruct a vertical column through the sediment to examine its stratigraphy, i.e. the changing rock types or fossil content within the depositional sequence. A stratigraphic column, like that shown below from Glen Canyon, AZ, is drawn to represent the sequence within the basin. Many of the rock layers shown here are exposed in the Grand Canyon as well. This sequence is largely a change from sanstone (Ss) deposition to shale (Sh) deposition. Other formations (Fm), which have a mix of rock types, are also present, but we will not consider these here. Note the curves within layers that represents cross bedded structures.

Glen Canyon, AZ stratigraphic column

Lab Report

  1. Discuss weathering products in the context of weathering reactions and the components of sedimentary rocks. Use your class notes to help you with this. Address the following questions in your discussion.
    • What is the most abundant mineral in upper continental crust?
    • What are the solid products of weathering?
    • What is dissolved?
    • What are the sedimentary rock types that form from different weathering products?
  2. As in our previous rock/mineral lab, you will be describing different samples.
    1. Describe the clastic samples provided as conglomerate, sandstone, siltstone or shale. Include in your description at least two other distinguishing characteristics such as grain types, rounding, sorting, or structures present.
    2. Describe the chemical and biochemical samples in terms of texture. As with the clastic samples, include in your description two other observations such as color, grain types, or others. If fossils are present, are they terrestrial or marine?
  3. What are the general differences in appearance that distinguish clastic from chemical and biochemical samples?
  4. Revisit you lecture notes on different depositional environments. (The pictures should be helpful!) Considering the inherent energy of each environment, list the rock types you have examined in this lab that could reasonably be products of each of the depositional environments described above. Include a reason. **Please organize this question by depositional environment, not by sample number! Also, note that it may be possible, even likely, that a specimen could be logically thought to have originated from more than one environment and that not all of the described environments are represented by the rocks in this lab.**
  5. Walther's law states that vertical successions of conformable (that is no significant time gaps) sedimentary rocks reflect adjacent sedimentary environments from the past. The vertical changes result from systematically changing sedimentary environments, typically a product of changes in "base level" or in other words changes in the relative level of standing water (ocean or lake) and the river or stream entering that body. Examine the stratigraphic column. Note that sandstones are abbreviated Ss and shales Sh. Which rocks are the oldest? Describe the changes in sedimentary environments that may have produced the transition from sandstones to shale.

Updated February 28, 2008
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