Geology 2 Spring 1997
Name:
Lab day & time:
PRELAB #2. SEDIMENTARY ROCKS, STRUCTURES
AND ENVIRONMENTS
This prelab assignment is due at the beginning of your lab session. You will not be allowed to participate in the lab if you have not completed this prelab exercise.
Review Chapters 5 & 6 of Chernicoff and the attachments to this pre-lab. Answer the following.
1. Describe and contrast mechanical weathering
(p.91) and chemical weathering (include information about source materials, processes, and materials produced).
2. Describe and contrast detrital and chemical sedimentation.
3. The naming of detrital sedimentary rocks is based upon their grain size. List the following rocks in the order of descending grain size, from coarsest to finest: Siltstone, Sandstone, Mudstone, Conglomerate
4. We distinguish three kinds of sandstone. Name and tell the composition and appearance of each.
1.
2.
3.
5. What is the primary mineral (or elements) in each of the following chemical sedimentary rocks?
limestone
coal
evaporite
chert
6. List and/or sketch some sedimentary characteristics and structures that are commonly observed in sediments deposited in river and river flood plain environments ("fluvial" environments). Briefly tell (or sketch) why you would expect those characteristics to be formed in this environment.
The attached information and figs 6-3 to 6-7 in Chernicoff will help you.
TIPS FOR CLASSIFYING SEDIMENTARY ROCKS
Begin by determining whether or not the rock has a detrital or clastic texture. If it does (and the particles are not shell fragments) estimate the size of the particles and refer to Table 1 Part A. The distinction between clay- and silt-sized particles is that individual silt-sized particles can barely be seen and individual clay-sized cannot be seen with the naked eye.
For non-clastic rocks, the best way to start is to test for a reaction with hydrochloric acid (HCl). If the rock reacts then it is either limestone (calcium carbonate), or a non-carbonate rock that has been cemented with calcite. If the rock reacts with the HCl only when finely powdered, then it is dolomite. If the rock does not react to HCl (even in powdered form), check the hardness and other properties to determine mineral composition (e.g. chert, gypsum, or halite). If no mineral characterization fits and the rock is black and dense then it is probably coal. Once you have determined the mineral composition, refer to Table 1 part B for a rock name.
TABLE 1: CLASSIFICATION OF SEDIMENTARY ROCKS
Part A: Clastic Sedimentary Rocks
| Grain Size | Characteristics | Rock Name |
|---|---|---|
| > 2 mm | poorly sorted rounded fragments | CONGLOMERATE |
| > 2 mm | poorly sorted angular fragments | BRECCIA |
| 1/16 - 2 mm | quartz ± feldspar ± other minerals ±
rock fragments, moderate to well sorted |
SANDSTONE |
| 1/256 - 1/16 mm | particles barely visible, very well sorted | SILTSTONE |
| < 1/256 mm | particles barely visible, very well sorted | SHALE |
Part B: Chemical Sedimentary Rocks
| Grain Size | Characteristics | Rock Name |
|---|---|---|
| Calcium
carbonate CaCO3 |
may be crystalline or contain fossils, reacts with HCl | LIMESTONE |
| Calcium
carbonate CaCO3 |
poorly cemented aggregate of calcareous shells, reacts with HCl | COQUINA |
| Calcium
carbonate CaCO3 |
all microfossils, fine-grained, reacts with HCl | CHALK |
| Dolomite (Ca, Mg)CO3 |
commonly altered from limestone. Only reacts with HCl when powdered | DOLOMITE/DOLOSTONE |
| SiO2 | hard (scratches glass), conchoidal fracture | CHERT |
| Halite (NaCl) | salty taste | ROCK SALT |
| Gypsum (CaSO4) | soft, scratches with fingernail | ROCK GYPSUM |
| Organic matter | black, very low density | COAL |
Many other structures, characteristic of the environment in which the sedimentary rock was formed and the processes occuring in this environment, are preserved in the sedimentary rock record. These structures can be caused by inorganic and organic processes.
Inorganic structures include:
Graded bedding is when the particle size changes gradually in a vertical direction (normally grains become finer (or smaller) from bottom to top). Graded bedding results when a sediment-laden current containing particles of a variety of sizes loses energy and slows down. The largest particles settle out first followed by successively smaller particles.
Ripple marks are wavy undulations on a sediment (generally sand) surface formed as water or wind moves across it. Ripple marks produced by a current will be asymmetrical, with a steep slope downcurrent (see Figure 6-6 in your textbook). Ripple markes produced by the back and forth movement of waves will be symmetrical.
Cross-bedding is secondary layering which occurs at an angle to the primary bedding orientation (see Figure 6-5 in your textbook). Cross beds are the internal structure produced by ripples or dunes.
Parallel laminations are thin layers parallel to the bedding which are formed by the settling of fine-grained sediment.
Channels are concave-upward structures that truncate layers at their margins (see Figure 6-3 in your textbook). The channels are generally filled with coarser particles than the surrounding material. They are erosional features formed by scouring currents. Channels form in a high energy environment when only the coarse material can settle out.
Mudcracks are polygonal patterns of cracks that form on the surface of wet mud when it dries out and shrinks (see Figure 6-7 in your textbook). Mudcracks are associated with regions of periodic wetting and drying such as shallow lakes and desert basins.
Organic Structures include:
Tracks are footprints observed on bedding produced by organisms such as dinosaurs or birds.
Trails are straight or curved grooves on bedding produced by crawling organisms such as worms.
Burrows are tube-like excavations made by organisms in soft sediment. These are generally formed in shallow (0-100 meter) water.
SEDIMENTARY ENVIRONMENTS
Sedimentary rocks reveal information about the environment in which they formed.
One branch of geology, called sedimentology, specializes in discovering and understanding these environments. Understanding the depositional environment of a package of sedimentary rocks permits geologists to make large scale geographic interpretations for areas in the geologic past. This is important for several reasons, including understanding earth surface processes, drawing inferences about regional tectonic events, and identifying areas where oil, gas, and other economically important materials will be found.
In order to reconstruct the depositional environment of a given set of rocks, geologists use a variety of clues found in the rocks. These include rock textures (grain sizes, sorting, and rounding), sedimentary structures, sedimentary beds and bed geometries. Each depositional environment produces a typical association of these characteristics. By comparing the features found in modern sediments (where depositional processes can be observed in action) with the features in sedimentary rocks, geologists can identify the environment in which the rocks were formed. Below are some important environments of deposition and the characteristics of the rocks formed in them.
| Sedimentary Environment | Sedimentary Rock Characteristics |
|
Beach |
sand sized grains well-sorted by grain size with occasional larger particle well-rounded grains parallel lamination or low-angle cross-bedding |
|
Shallow Marine |
may be bedded or massive sands, silts or limestones tracks and burrows are common marine fossils (shells) symmetrical ripples limestones common |
|
Deep Marine |
silt-clay sized grains usually well-bedded some parallel laminations may be evident; other structures are absent limestone and chert are common large fossils are absent often black or dark-colored |
|
Fluvial (river flood plain) |
wide range in grain-size and sorting angular grains common graded bedding common (fining upward sequences) beds are laterally inconsistent--they may pinch out asymmetrical ripples common large-scale cross beds common channels common mudcracks, organic matter, and plant fossils common red beds common arkose sandstones common in mountainous regions |
|
Aeolian (sand dunes) |
sand-sized grains well-sorted and well-rounded grains large scale cross-beds common may have ripples, tracks, and trails |