Review Questions and Answers; Sedimentary Rocks

In Earth's crust, igneous rocks exceed sedimentary rocks in volume. Neither rock type is evenly distributed. In the interiors of continents, sedimentary rocks occur as thin veneers covering over much larger volumes of igneous and metamorphic rocks deeper in the crust. Ocean basin rocks are mainly igneous with very thin covers of sediments. Sedimentary strata many kilometers in thickness accumulate only in relatively restricted basins along the edges of continents or in deep rift basins where continental blocks are splitting apart.

2. What minerals are most common in detrital sedimentary rocks? Why are these minerals so abundant?

Detrital refers to mineral grains and rock fragments, such as sand grains or pebbles, that are produced during the weathering process and transported to the site of deposition as particles.

The most abundant detrital minerals in sediments are quartz and clays. Quartz is an abundant mineral in many rocks. It resists cracking and mechanical weathering and is resistant to solution and decomposition from chemical weathering. Thus quartz can be thought of as a residual mineral, left over and concentrated in detrital sediments after the other rock-forming minerals, such as feldspars and ferromagnesians, have been decomposed.

Clays are major components of shales and mudstones but are rarely abundant in other common rocks. Clays are the weathering products of feldspars and aluminum-bearing, ferromagnesian minerals like hornblende and biotite. Being stable in the weathering environment, clays are major soil components; thus they are readily eroded, transported, and deposited as detrital sediments.

3. What is the primary basis for distinguishing among various detrital sedimentary rocks?

This is done on the basis of clast size, as shown in the following table.

4. The term clay can be used in two different ways. Describe the two meanings.

Clay is the name for a group of sheet-structured, aluminum silicate minerals (the clay minerals). The term also denotes the very fine-sized (< 1/256 mm in diameter) grain fraction of detrital sediments; these tiny particles may or may not be clay minerals.

5. Why does shale usually crumble quite easily?

Shales are usually fissile with numerous, small cracks that allow water ready access during weathering, and they contain clay minerals as dominant components. Many clay minerals strongly adsorb water and swell, a process that greatly lowers the mechanical strength of the shale and generates internal stresses to push open new cracks and extend old ones deeper into the unweathered portions of the rock. Thus deeply weathered shales typically crumble; fresh, unweathered shales are brittle and splintery, and the broken fragments can be very hard and tough. Black shales typically contain disseminated pyrite; this mineral readily decomposes in the presence of water and oxygen, generating acidic waters that hasten disintegration of the rock.

6. How are the degree of sorting and the amount of rounding related to the transportation of sand grains?

A sand grain, whatever its initial shape, is gradually rounded during transport. Sharp corners and edges are preferentially abraded, resulting in rounded but not necessarily spherical grains.

Well-sorted sands are deposited in aqueous and terrestrial environments characterized by vigorous current activity. Fine-sized particles are winnowed out and carried elsewhere; coarser, gravel-sized clasts are absent, left behind or abraded and broken into smaller particles. The currents are highly selective; although finer particles may be available, only sand grains of equivalent size are deposited, such as in an sand dune or on a beach. Generally, well-sorted sands are also well rounded, the grains having been extensively transported prior to deposition. However, this may not always be the case. Consider a sand bar deposited along the shoreline of a pluvial lake such as Lake Bonneville, The sand may be well sorted, but poorly rounded, the grains having traveled only a short distance from a glacial outwash or delta deposit.

7. Distinguish between conglomerate and breccia.

Conglomerates are detrital sedimentary rocks dominated by pebbles or larger clasts. The clasts are rounded, mainly by stream transport, and many conglomerates exhibit good stratification. Breccias are rocks composed of similarly-sized, but angular, mineral and rock fragments. They can form by explosive fragmentation during volcanism, by fracturing along faults and as products of mass wasting. Breccias of sedimentary origin are closely related to mass-wasting processes, and the detrital clasts have moved only a short distance from their bedrock source.

8. Distinguish between the two categories of chemical sedimentary rocks.

The dominant constituents of chemical sedimentary rocks were transported to the site of deposition in solution. Two, different categories are defined on the basis of the precipitation mechanism. If evaporation caused precipitation, the rocks are evaporites; if living organisms were involved, such as with the precipitation of calcium carbonate by algae or invertebrate animals, the rocks are biogenic (biochemical) in origin.

9. What are evaporite deposits? Name a rock that is an evaporite.

Evaporites are chemical sedimentary rocks, such as bedded salts, precipitated from isolated bodies of seawater or saline lakes undergoing intense evaporation. Gypsum, anhydrite (calcium sulfates), and halite (sodium chloride) are evaporites.

10. When a body of seawater evaporates, minerals precipitate in a certain order. What determines this order?

The minerals precipitate in the order of their relative solubilities. Calcium carbonate (calcite and aragonite) and dolomite are the least soluble and precipitate first. However, much of the dolomite associated with evaporite sections is known to originate secondarily by reaction of primary calcium carbonate and very late stage, Mg-rich brines. The calcium sulfates (anhydrite and gypsum) reach saturation next. Extensive, additional evaporation is required for sodium chloride to reach saturation, and the highly soluble salts such as KCl (sylvite) reach saturation only after most of the sodium chloride has been removed from the brine by precipitation of halite.

11. Each of the following statements describes one or more characteristics of a particular sedimentary rock. For each statement, name the sedimentary rock that is being described.

(a) An evaporite used to make plaster. - Gypsum (calcium sulfate) is a major ingredient of plasters.

(b) A fine-grained detrital rock that exhibits fissility. - Shale typically breaks into thin plates or pen-shaped fragments, as contrasted to mudstone which typically breaks into more equidimensional chunks or blocks.

(c) Dark-colored sandstone containing angular rock particles as well as clay, quartz, and feldspar. - Graywacke; the original sediment is transported by turbidity currents and deposited well below the influence of wave action. The lithified rock consists of sand grains embedded in a clayey/chloritic matrix.

(d) The most abundant chemical sedimentary rock. - The chemical sedimentary rock limestone is composed of calcium carbonate (CaCO3).

(e) A dark-colored, hard rock made of microcrystalline quartz. - Chert is a bedded deposit of very fine-grained quartz (silica).

(f) A variety of limestone composed of small spherical grains. - The spherical grains are calcite oolites; the rock is an oolitic limestone.

12. How is coal different from other biochemical sedimentary rocks?

In most biochemical rocks, the rock is composed of inorganic mineral matter (calcite, silica, etc.) precipitated directly or indirectly by once living organisms. Coal, in contrast, is composed of the compacted, macerated, original remains of plants. Coal is mostly carbon but contains small percentages of minerals such as quartz, clays, and pyrite.

13. Compaction is an important lithification process with which sediment size?

Compaction is very important for the finer-grained sediments (silts, muds, and clays) and less so for the coarser detrital sediments. During compaction, water is squeezed from the sediment, and the individual particles are forced into close, mutual contact and bonded tightly.

14. List three common cements for sedimentary rocks. How might each be identified?

The three, most common, chemical cements in sedimentary rocks such as sandstone are silica (quartz), calcium carbonate (calcite), and the iron oxides. Calcite-cemented sandstones are typically light colored, and the calcite reacts vigorously with dilute HCl or other acids. Quartz-cemented sandstones are also light colored, but they do not visibly react with acids and the cementing material is quite hard (7 on the Mohs scale). Sandstones with iron oxide cements are easy to recognize by their red, yellow, or brown colors.

15. What is the primary basis for distinguishing among different chemical sedimentary rocks?

Their mineralogy! Limestones are calcite and dolostones are dolomite, a calcium magnesium carbonate mineral. Evaporites include bedded halite (NaCl) and gypsum or anhydrite (calcium sulfates). Hardness, taste, luster, and response to acids help in mineral identification; microscopic and x-ray studies can be employed if necessary.

16. Distinguish between clastic and nonclastic textures. What type of texture is common to all detrital sedimentary rocks?

Clastic means fragmental or particulate, and texture describes the shapes, sizes and mutual packing arrangements of the mineral grains and/or detrital particles and cements. Being deposits of transported mineral grains and/or rock fragments, all detrital sediments and sedimentary rocks have clastic textures.

Nonclastic textures include the crystalline textures of chemical rocks like chert and evaporites. Limestones may have crystalline or clastic textures, depending on the nature of the original sediment and on the subsequent geologic history of calcite solution and crystallization in the rock.

17. Some nonclastic sedimentary rocks closely resemble igneous rocks. How might the two be distinguished easily?

The similarities arise because many igneous and chemical sedimentary rocks have crystalline textures, in which the mineral grains are tightly interlocked and bonded together. Most igneous rocks consist of two or more, dominant silicate minerals. However, don’t forget the carbonatites! All sedimentary rocks may show stratification, and those with crystalline textures are typically composed of only one dominant mineral, such as calcite, dolomite, or silica. The minerals (calcite, dolomite, and evaporite phases) of nonclastic sedimentary rocks are much softer than the silicate minerals of igneous rocks. Carbonate minerals react with strong acids evolving CO2, and the common evaporite minerals such as halite are easily soluble and can be identified by taste. Fine-grained to microcrystalline, siliceous, sedimentary rocks such as chert might be confused with aphanitic igneous rocks. However, "bedded" chert is monomineralic and is commonly stratified.

18. Why are seafloor sediments useful in studying climates of the past? (Box 6.2)

Skeletal remains of foraminifera and other tiny, marine organisms are very common in many marine sediments. Various species are known to have lived for only short periods of time, geologically speaking, or to have occupied distinctive habitats. Some species lived in cold waters; others lived in warm waters. Thus some species are excellent environmental indicators; others are good index fossils, providing precise ages for sediments. Calcium carbonate skeletal remains are used to determine paleotemperatures; the isotopic composition of oxygen in the carbonate is measured and compared to the isotopic compositions of calcite equilibrated with seawater at known temperatures. Thus age, environmental, and paleotemperature data derived from seafloor sediments are used to reconstruct past climates and climatic variations.

19. What is probably the single most characteristic feature of sedimentary rocks?

Most sediments and sedimentary rocks show an original layering (stratification) because they were deposited in nearly horizontal sheets or lenses. Numerous, thin strata in shales and some sandstones are easily visible. In other rocks such as graywacke and reef-deposited limestone, the deposit is a single, massive bed or lens; internal stratification may not be so evident.

20. Distinguish between cross-bedding and graded bedding.

Both are characteristic of sedimentary rocks, but they originate in quite different environments. Graded bedding (Fig. 6.21) indicates that bottom currents were absent from the depositional environment. The sediment was not reworked following deposition. A sediment-laden turbidity current, initiated by slumping of sediments in shallower waters, loses energy and slows down as it moves along the bottom into deeper waters. Unaffected by bottom currents, the particles settle out in the order of their grain sizes, the coarser ones first and the fine silts and clays last. Thus the deposit exhibits an internal, vertical grading in particle sizes but lacks internal stratification (bedding) surfaces.

Cross-bedding (Fig. 6.20) describes the multiple sets of typically thin, non-parallel strata that develop internally as sand dunes, ripples, and some fluvial beds accumulate. Some sets of strata are laid down parallel to subhorizontal, gently inclined, transport surfaces. Others are inclined at angles up to 35 degrees, the angle of repose for sand. These inclined strata are deposited on the steeper, leeward, downcurrent-facing slopes (usually a slip face) of moundlike, sand accumulations such as dunes, ripples, or sand waves in a stream channel. By definition, deposits with graded bedding are poorly sorted; cross-bedded sands are commonly well sorted.

21. How do current ripple marks differ from oscillation ripple marks?

Current ripples (Fig. 6.23 A) result from unidirectional flowing currents; they can be considered as small-scale dunes that migrate in the flow direction, upcurrent in high velocity regimes and downcurrent in moderate to low velocity regimes. The ripples are asymmetric with a more gentle slope facing upcurrent and the steeper slip face inclined downstream. The different slope inclinations can be used to infer paleocurrent directions and the "dunes" are usually crossbedded, allowing the original stratigraphic top direction to be determined in most cases. Current ripples are common features in fluvial and eolian sandstones and they can also form in marine environments, such as in a tidal channel.

Oscillation ripples form in response to the back and forth sloshing of bottom waters as shallow-water waves pass overhead. The ripples are symmetrical with relatively narrow, peaked crests and broad, shallow depressions. Slightly coarser "lag deposit" grains typically occupy the centers of the depressions. These ripples form in shallow water environments subjected to a steady supply of incoming waves that pass by in more or less a consistent direction. Because they require steady, consistent, incoming waves to develop, symmetrical ripples are formed mainly in coastal marine environments.

22. List two conditions that favor the preservation of organisms as fossils.

Hard parts (shell material, bone, teeth) and rapid burial favor fossil preservation. The hard materials may be preserved directly; more commonly, impressions of the hard part surfaces are preserved in shales and mudstones. To be preserved as fossils, soft bodied organisms must be buried before they decompose; thus rapid burial is essential for their preservation.

23. Which type of fossilization is indicated by each of the following statements? Which one is an example of indirect evidence? (Box 6.3)

(a) A leaf preserved as a thin carbon film. - Carbonization; as the leaf decomposes, the more volatile components are lost, leaving behind a carbon-coated impression. These fossils are commonly preserved in shale, mudstone, tuff, and other very fine grained sedimentary rocks.

(b) Small internal cavities and pores of a log are filled with mineral matter.

This process describes petrifaction. The wood is buried and petrified by infilling of open space with silica deposited from groundwater. Removal of woody cell tissue and simultaneous deposition of silica constitute further petrifaction by replacement.

(c) Fossil dung. - Fossilized dung can be preserved by vastly different processes such as freezing and petrifaction. The fossils, known as coprolites, are indirect evidence for once-living organisms.

(d) This is created when a mold is filled with mineral matter. - The statement describes a fossil cast. Casts form when a void, opened by dissolution of a fossil hard part (typically shell material) is filled by mineral matter deposited from circulating waters. If the interior volume of the shell was empty (not filled by mud or mineral precipitates), the cast will exhibit only a replica of the outer surface of the shell. If the original shell was filled with mud or other fine-grained sediment, the cast may show impressions of the inner and outer surfaces of the original shell.