Geology 101 Name(s):

Lab 5: Sedimentary rocks Objective: To gain a familiarity with sedimentary processes and the classification of sedimentary rocks. To be able to infer, at least broadly, the depositional environment from sedimentary rocks and structures. Needed: Weathering samples W1 through W6 (Tub 18), sedimentary rock samples M15, 16, 17 (Tub 19), R18 – R27 (Tubs 20 – 29), R33 (Tub 35) and S1 (Tub 36) Sedimentary rocks are those that are formed either by direct deposition of material by crystallization of minerals in aqueous solution or by organisms, or through sediment settling under gravity. The settled sediment or biological material will lithify, in which it will be compacted and then chemically cemented into a rock. To make sediment, a rock of any type must be weathered (either physically or chemically) on the Earth’s surface, then erosion will transport the sediment to where the sedimentary rock forms. Weathering 1. Samples W1 and W2 are igneous rocks; you should verify that W1 is granite and W2 is gabbro. Sample W3 is a partially weathered igneous rock. Sample W4 is beach sediment. a. Identify the minerals in the clasts breaking off of W3. b. Did sample W3 start off as W1 or W2? c. Look at sample W4 with a hand lens. Did this sediment come from W1 or W2? d. When sample W4 lithifies, what will its basic rock name be? Hint: Would it still be an igneous rock? e. What would a beach composed of sediment from the weathering of a pluton made of W2 look like? (Hint: think of Hawaiian beaches)

Physical weathering 2. a. Imagine freezing a sealed bottle full of water. What will happen to the bottle? What type of physical weathering is this? b. Examine the section of four-inch diameter concrete drain pipe from my backyard. What type of physical weathering caused the holes in the pipe? Chemical weathering 3. Examine the penny. What the green material on it? How did it form? If you were to remove all of the green material, would the penny weigh the same as it did before or less? Soils and soil classification Soils comprise the weathered bedrock (regolith) and organic material at the surface of the Earth. Most soil investigations begin with an analysis of the particle (grain) size distribution, a measurement of the percentage of each category of primary particles by weight. Most scales used to measure particle sizes (including the one used here) identify four categories — gravel (cobble, pebble or larger), sand, silt and clay. A cross-section of a soil will typically divide the soil into layers called horizons. There are usually four horizons within a soil: the uppermost O horizon, which is the layer with organic-rich decomposing material; the A horizon, which is mostly inorganic particles with air in between the grains; the B horizon, which is nearly all inorganic particles with water in between the grains; and the bottom C horizon, which is the regolith with no organic materials, with bedrock below that. If you classify soils by their grain size distribution, then the triangular soil texture diagram (on the next page) can be used. This kind of classification, as shown in the diagram, relies only on the percentages of sand, silt and clay. Note that this information will be useful for some endeavors (construction) but less useful for others (agriculture).

Fertility, drainage and stability of the soil are some very important soil properties that are assumed to be correlated with the particle size distribution, or texture, of the soil. 4. Use the triangular soil texture diagram provided to classify the following soils (note: these are not the soil samples on the side display cabinet):

Soil % sand % silt % clay Texture

A

15

51

34

B

72

14

14

C

47

32

21

Although precise measurement of the particle size distribution is an involved laboratory procedure, a good first approximation can be made on site with no more equipment that your hand and a little water. Instead of determining exact percentages, this field test (called the three-test technique) determines the textural class of the soil. The textural classes each have specified ranges of sand, silt and clay percentages (they are the thick-outlined areas on the triangular soil texture diagram). The boundaries between two classes of soil are not as distinct as shown; one soil class grades into the adjacent classes.

5. Examine each of the soil samples on the side display cabinet and determine its textural class using the three-test technique flow chart shown on the next page. Use the Munsell soil color chart to determine the soil color; use the color notation given in the chart.

Soil sample

Ribbon test

Sandi-ness

Smooth-ness/

stickiness

Soil texture Color (using the Munsell

chart notation)

1

2

3

4

THREE-TEST TECHNIQUE FOR SOIL TEXTURE ANALYSIS This field technique comprises three tests determining the relative amounts of clay, sand and silt in a soil, and classifying the soil TEXTURE accordingly. Ribbon test — gather a small handful of the soil and moisten (not saturate) it. Clench that handful of moist soil and see if a continuous ribbon of wet soil is extruded from between your thumb and bent forefinger. No ribbon/poor ribbon — go to A Fair ribbon (short stretches with many breaks) — go to B Good continuous ribbon — go to C A. Next, perform the sandiness test — moisten the soil (if necessary) and roll some between your thumb and forefingers; if the soil feels coarse (lots of graininess) then this is a positive result. Positive result (sandy) — go to D Negative result (not sandy) — go to E B. Next, perform the sandiness test — moisten the soil (if necessary) and roll some between your thumb and forefingers; if the soil feels coarse (lots of graininess) then this is a positive result. Positive result (sandy) — soil is SANDY CLAY LOAM Negative result (not sandy) — go to F C. Next, perform the sandiness test — moisten the soil (if necessary) and roll some between your thumb and forefingers; if the soil feels coarse (lots of graininess) then this is a positive result. Positive result (sandy) — soil is SANDY CLAY Negative result (not sandy) — go to G

D. Finally, perform the aggregate test — in a dry sample of the soil, look for aggregates (dirt clods). No aggregates — soil is SAND Some aggregates — soil is SANDY LOAM (or LOAMY SAND) E. Finally, perform the smoothness test — moisten the soil (if necessary) and roll some between your thumb and forefingers.

Soil feels smooth (no graininess) — soil is SILT (or SILTY LOAM) Soil feels even slightly coarse — soil is LOAM F. Finally, perform the stickiness test — moisten the soil (if necessary) and roll some between your thumb and forefingers.

Soil feels sticky but not slick — soil is CLAY LOAM Soil feels sticky and slick — soil is SILTY CLAY LOAM G. Finally, perform the stickiness test — moisten the soil (if necessary) and roll some between your thumb and forefingers.

Soil feels sticky but not slick — soil is CLAY Soil feels sticky and slick — soil is SILTY CLAY 6. Which of the samples (#1, 2, 3 or 4) would have the greatest permeability? Why? 7. Which of the samples (#1, 2, 3 or 4) would most likely be the best soil to grow plants in? Why isn’t this the same answer as question 6? Making sedimentary rock 8. Lithification (making a sedimentary rock) involves both compaction and cementation. W5 and W6 are glacially deposited sedimentary rocks of the same composition. Which has undergone more compaction? How can you tell? 9. a. Sedimentary rocks are held together by cement, a non-mineral chemical compound, which forms bonds (though not as strong as chemical bonds) between

mineral grains. The three common cements are silica (SiO2), calcite or iron oxide (rust). How would you identify each cement (think of a test for each)? Iron oxide — Calcite — Silica — b. Look at rock samples R18 and R19. What is the cement that holds each together? R18 _________________ R19 _________________ 10. How does the cement get into these rocks? 11. Of course, like all rocks, sedimentary rocks are made of minerals. The most common minerals were given on the second page of Lab 3. Using the Mineral ID sheets from the lab manual, identify the following minerals: Mineral # Distinguishing features (color, cleavage,

hardness, magnetism, density, etc.) Mineral name

M-15

M-16

M-17

12. Now consider the actual mineral grains in rock samples R18, R19, R24 and R25. Because sand-sized dark minerals are very hard to identify, sedimentary petrologists (much to the horror of igneous petrologists) use the term “dark lithic fragments” to categorize lots of little dark minerals.

Sample Hint to ID the mineral Most common mineral in the rock

R18

Use glass plate

R19

Well, look at it

R24

Acid bottle?

R25

Carefully, get it moist and see what happens to its size

Identifying sedimentary rocks Sedimentary rocks can be classified in a number of ways. For our purposes, the first division to be made is between clastic sedimentary rocks (those that are made of weathered and eroded grains) and non-clastic or other sedimentary rocks (these include sedimentary rocks of biological and chemical origin). Flow chart for identifying sedimentary rocks — If the rock is made of grains or other materials which have been deposited by wind, water or ice, or else was generated by biological or surface chemical activity, it's a sedimentary rock. First step. If the rock is made of broken up bits of rock (including extremely fine grains) → GO TO Second step alternative A. (Clastic rocks)

Else → GO TO Second step alternative B. (Chemical or biological rocks)

Second step alternative A. Consider the most common grain size in the rock from the following list. cobble or

pebble > 2 mm easily visible to naked eye; "grains" may

contain identifiable minerals sand 0.062 — 2 mm visible to naked eye silt 0.005 — 0.062

mm not visible but can be felt between fingers or across teeth

clay < 0.005 mm not visible; cannot be felt between fingers or across teeth

If the most common grain size is cobble or pebble → conglomerate If the most common grain size is sand → sandstone (arenite) If the most common mineral is quartz → quartz arenite If the rocks is medium gray to red and well-sorted → arkose

If the rock is dark-colored and has much fine grain-size material in its matrix → greywacke

If the most common grain size is silt or clay or a combination of both:

If the rock splits into irregular or regular layers → shale If the rock is massive (no layering) → mudstone The terms siltstone and claystone are also used on occasion.

Second step alternative B. Identify the most common mineral in the specimen (use mineral ID chart if necessary). If the most common mineral is quartz → chert If the most common mineral is halite → rock salt

If the most common mineral is gypsum → rock gypsum If it is black-colored, not very dense and flaky → coal

(also look for plant fibers)

If it fizzes, the most common substance is calcium carbonate, usually in the form of the mineral calcite (be careful you are not fizzing the cement)

If the rock is not very dense and pure white → chalk If the rock is made of broken-up shells → coquina If the rock is dense and white, gray or black → limestone One other thing: fossils (Latin for “dug up”) are the remains of living organisms. If the fossil is literally the body of the organism (or parts such as skeleton or shell), it is called a hard parts fossil; if the fossil merely records the shape of an organism (like a leaf impression in silt) or the passage of an organism (like preserved footprints), then it is called a trace fossil. For any sedimentary rock, if it contains any fossils, use the adjective fossiliferous in front of the rock name. Needed: sedimentary rock samples R18 – R27 (Tubs 20 – 29), R33 (Tub 35) and S1 (Tub 36) 13. Fill in the following table for clastic sedimentary rocks. Begin by determining the average grain size of the clasts in the rock (use the grain size terms in the flow chart), then the grain sorting (the choices are: well-sorted, moderately sorted, poorly sorted and unsorted) and the grain roundness (the choices are; well-rounded, sub-rounded, sub-angular and angular). See the diagrams to determine which type of rounding and sorting the grains have. Under fossils, your choices are none, hard parts or trace fossils. Finally, identify the rock, using the flow chart. Grain sorting: Grain rounding:

Clastic sedimentary rocks Sample # Grain

size Grain

sorting Grain

roundness Fossils Rock name

R18

R19

R20

R21

R22

14. Fill in the following table for “other” sedimentary rocks. Begin by determining the rock’s mineral composition. Then add any other details that help identify it. Under fossils, your choices are none, hard parts or trace fossils. Finally, identify the rock, using the flow chart. “Other” (chemical and biological origin) sedimentary rocks Sample # Mineral

composition Other

defining details

Fossils Rock name

R23

R24

R25

R26

R27

Sedimentary rock properties and depositional environments 15. Return to R18 and R19 and circle the correct answers: a. Which rock contains the most stable mineral clasts? R18 R19 (at the Earth's surface) b. Which rock is composed of rounder grains? R18 R19 c. Which rock is more well-sorted? R18 R19 d. Based on a-c, which sample was deposited furthest from its source (and thus is called mature)? R18 R19 16. The energy of the system (how much force is behind the medium of transport (air or water)) can be characterized by the size of the particles the system can carry. For instance, high-energy systems can carry large grains; low-energy systems can carry small grains. Examine and rank rocks R18, R21 and R20 in order from highest energy to lowest energy depositional system. 17. Some limestones (R33) are dense, fine-grained and black. So is basalt (R5). What test can you perform to tell them apart, and how does each behave in the test? 18. Which rock (R33 or R5) has fossils? By the way, in general, why didn’t you worry about fossils in igneous rocks? The place in which the sediment is deposited or the organisms lived is called the depositional environment. Examples of depositional environments include terrestrial environments (like lakes, deserts and rivers), transitional environments (like beaches and tidal flats) and marine environments (like continental shelves and the abyss). Note that, over time, a beach area may be uplifted by plate tectonics so that you will find a transitional depositional environment quartz-rich sandstone deep in a mountain range!

19. In what depositional environment did rock R25 form (see diagram above)? Hint: these kinds of rocks are called evaporites. Explain how they form. 20. Look at sedimentary structure S1, which is an example of preserved ripple marks. Are they symmetrical or asymmetrical? Based on that answer and on the wavelength of the ripples, is it more likely that these ripples were originally deposited in a desert, a river, or a tidal flat? How are they preserved so that you can see them today?