Chapter 5—The Sedimentary Archives

CHAPTER OVERVIEW

The title of this chapter emphasizes the historical significance of sedimentary rocks as indicators of past environments. When interpreting sedimentary rocks, it is important to consider the tectonic elements of a continent as well as particular areas such as cratons, orogenic belts, shields, and platforms. These tectonic settings determine the type of depositional environments, for example, marine, continental, or transitional. These listed environments of deposition are associated with specific types of sandstones, shales, limestones, and other sedimentary rocks. Some positive general indicators relate to color, grain size, sorting, textural maturity, and sedimentary structure. The field of stratigraphy is introduced using examples of how it is applied to the geologic column and world wide correlations. This chapter concludes with a description of the different types of maps used by geologists and the economic resources found in sedimentary rocks.

LEARNING OBJECTIVES

By reading and completing information within this chapter, you should gain an understanding of the following concepts:

  • Explain how the tectonic setting is reflective of the different kinds of sedimentary rocks.
  • Describe the different kinds of tectonic settings and how each is reflective of the size and shape of clastic particles.
  • Define the principle elements of a continent, including cratons, shields, platforms and mobile belts.
  • Describe the different kinds of environments of deposition (marine, continental, and transitional) and characters of each.
  • Discuss how sedimentary rock color is indicative of some of the different environments.
  • Discuss the types of sedimentary rocks and how each is interpretative of different environments: sandstones, shales, and carbonates.
  • Discuss how the sedimentary rock record is interpretative through rock units, facies, sea level changes, correlation, and unconformities.
  • Describe the different types of mapping used in environmental interpretation including geologic maps, paleogeographic maps, isopoch maps, and lithofacies maps.

CHAPTER OUTLINE

  1. Tectonic Setting is the Biggest Factor in Sediment Deposition
  2. Depositional Basins
  3. Cratons, Shields, Platforms, and Orogenic Belts
  1. Environments Where Deposition Occurs
  2. Marine Depositional Environments
  3. Shallow Marine (Continental Shelf) Environment
  4. Continental Slope Environment
  5. Deep Marine Environment
  6. Continental Depositional Environments
  7. Stream Deposits
  8. Alluvial Fans
  9. LakeDeposits (Lacustrine)
  10. Glacial Deposits
  11. Wind Deposits
  12. Transitional Depositional Environments
  13. Deltas
  14. BarrierIslands, Lagoons, Tidal Flats, and Estuaries
  1. What Rock Color Tells Us
  2. Shades of Black
  3. Hues of Red and Other Colors
  1. What Rock Texture Tells Us
  2. Grain Size and Sorting
  3. The Shape of Grains
  4. Orientation of Grains
  1. What Sedimentary Structures Tell Us
  1. What Four Sandstone Types Reveal About Tectonic Settings
  2. Maturity
  3. Quartz Sandstone
  4. Arkose
  5. Graywacke
  6. Lithic Sandstone
  1. Limestones and How They Form
  1. Organizing Strata to Solve Geologic Problems
  2. Rock Units (Formally Called Lithostratigraphic Units)
  3. How We Name Rock Units: Geography + Rock Type
  4. What Are Facies
  1. Sea-Level Change Means Dramatic Environmental Change
  1. Stratigraphy and the Correlating of Rock Bodies
  2. Correlating Rock Bodies
  3. Correlation Methods
  4. Using Lithostratigraphic Correlation
  5. Using Chronostratigraphic Correlation
  1. Unconformities: Something is Missing
  1. Depicting the Past
  2. Geologic Columns and Cross-Sections
  3. Geologic Maps
  4. Paleogeographic Maps
  5. Lithofacies Maps

Key Terms (pages are given in parentheses)

angular unconformity (110): An unconformity below which older strata dip at a different angle (usually steeper) than the overlying strata.

alluvial fan (86): The result of abrupt deposition when stream-transported materials accumulate quickly where a rapidly flowing river emerges from a mountainous area onto a flat plain.

arkose (99): Sandstones containing 25% or more of feldspar (derived from erosion of a granitic source area).

barrier island (89): A transitional (between marine and non-marine) environment builds islands parallel to shorelines by wave and current action.

biofacies (104): When a facies can be distinguished because of biologic (rather than lithologic) differences, it can be designated a biofacies. Fossil assemblages differ allowing for the recognition of different environments.

biostratigraphic correlation (108): The correlation of rock units by the use of its fossil content.

cement (in clastic sedimentary rock) (91): A chemical precipitate that crystallizes in the voids between grains following deposition. Some cements include silica (SiO2), and calcium carbonates (CaCO3).

chronostratigraphic correlation (108): Correlation that links units by age equivalences as determined by fossils or radioactive dating methods.

clast (82): An individual grain, fragment, or constituent of detrital sediment or sedimentary rock produced by physical breakdown of a larger mass.

continental shelf (84): Oceanic topographic regions included in the shallow marine realm. They are nearly flat, smooth surfaces that fringe the continents in widths that range from only a few kilometers to about 300 km at depths that ranges from low tide to about 100 meters. They resemble continents in their structure and composition.

continental slope (85): Those areas of the ocean floor that extend from the seaward edge of the continental shelves down to the ocean depths. The surface is inclined from 3 to 6 degrees.

correlation (107):Determination of the equivalence of rock bodies in different locations.

craton (82): The long-stable region of a continent, commonly with Precambrian rock, either at the surface or only thinly covered with younger sedimentary rocks.

cross-bedding (94, 96): An arrangement of laminae or thin beds traverse to the planes of stratification. The inclined laminae are usually at inclinations of less than 30 degrees and may be either straight or concave.

delta (82): Transitional environments (between marine and non-marine) form deltas when streams enter bodies of standing water, undergo an abrupt loss of velocity, and drop their load of sediment.

discharge (of stream) (86): The rate of stream flow at a given instant expressed in terms of volume per unit of time.

disconformity (110): A variety of unconformity in which bedding planes above and below the plane of erosion or non-deposition are parallel.

eolian environment (88): Environment in which wind is an important agent of sediment transport and deposition.

epicontinental sea (107): In the geologic past, there have been repeated advances of the sea into low-lying regions of the interiors of continents. The resulting inland sea is termed epeiric, meaning “a sea over a continent.”

estuary (89): The seaward mouth of rivers that are engulfed by the sea. Freshwater from streams and saltwater from the ocean are brought together. Mixing may occur, and typically, the lighter freshwater will flow seaward above the denser saltwater.

facies (104): A particular aspect of sedimentary rocks that is a direct consequence of sedimentation in a particular depositional environment. It allows identification of the environment of deposition.

ferric (91): The combination of two iron atoms with three oxygen atoms. Ferric minerals include hematite and tend to color the rock red, brown, or purple.

ferrous (91): Occurs when oxygen is in short supply such as stagnant lake or ocean where the iron atoms have lost only two electrons.

formation (103): A mapable, lithologically distinct body of rock having recognizable contacts with adjacent rock units.

geologic map (103): Map showing the distribution of rocks of different kinds and ages that lie directly beneath the loose rocks and soil covering most areas of the surface.

graded bedding (95): Consists of repeated beds, each of which has the coarsest grains at the base and successively finer grains nearer the top.

graywacke (99): Immature sandstone consisting of significant quantities of dark, very fine-grained material (usually clay, chlorite, micas, and silt). There is little or no cement, and the sand-sized grains are not in close contact because they are separated by the finer matrix particles. The matrix constitutes about 30% of the rock, with the remaining coarser grains consisting of quartz, feldspar, and rock particles.

group (104): The combination of formations into larger units.

hematite (90): Ferric mineral that tends to color the rock red, brown, or purple.

isopach map (113): A map depicting the thickness of a sedimentary unit.

lacustrine deposit (86): Pertaining to lakes, as in lacustrine sediments (lake sediments). Sediments that are deposited in lakes.

lagoon (89): Water-filled area found between the mainland and a barrier island. Represents a transitional environment (between marine and non-marine).

lithic sandstone (99): A rock that has a more transitional composition and texture is termed a lithic sandstone. Feldspars are relatively scarce, whereas quartz, muscovite, chert, and rock fragments are abundant. Fine-grained detrital matrix does not exceed 15% and the remaining voids are filled with mineral cement or clay. Quartz grains are better rounded and more abundant, the sorting is better, and the quantity of matrix is lower than in greywacke.

lithofacies (104): When the differentiating characteristics are lithologic (rather than biologic), facies can be designated as lithofacies.

lithostratigrapic correlation (115): Bodies of rock identified by their distinctive lithologic and structural features without regard to time boundaries. Features used include texture, grain size, clastic or crystalline, color, composition, thickness, type of bedding, nature of organic remains, and appearance of the unit in surface exposure.

matrix (in clastic sedimentary rock) (98): A category of bonding material that consists of finer clastic particles (often clay) that were deposited at the same time as the larger grains and that fill the crevices between them.

maturity (of sandstone) (98): Aggregates of well-rounded, well-sorted, highly stable minerals (such as quartz).

member (104): Distinctive smaller units within formations.

mud cracks (94, 97): Indicate drying after deposition. Developed by shrinkage of mud or clay on drying and are most abundant in the subaerial environment.

nonconformity (111): An unconformity developed between sedimentary rocks and older plutons or massive metamorphic rocks that had been exposed to erosion before the overlying sedimentary rocks were deposited.

offlap sequence (104): A sequence of sediments resulting from a marine regression and characterized by an upward progression from offshore marine sediments (often limestone) to shales and finally sandstones (above which will follow an unconformity).

onlap sequence (104): A sequence of sediments resulting from a marine transgression. Normally, the sequence begins with a conglomerate or sandstone deposited over an erosional unconformity and followed upward in the vertical section by progressively more offshore sediments.

orogenic belt (83): Great linear tracts of deformed rocks, primarily developed near continental margins by compressional forces accompanying mountain building.

paleogeographic map (113): A map showing the geography of a region or area at some specific time in the geologic past.

paraconformity (111):Recognized as a type of unconformity only by missing fossil zones.

platform (83): That part of a craton covered thinly by layered sedimentary rocks and characterized by relatively stable tectonic conditions.

quartz sandstone (98): Characterized by the dominance of quartz with little or no feldspar, mica, or fine clastic matrix. Quartz grains are well sorted and well rounded. Commonly held together by such cements as calcite and silica.

red beds (90): Prevailing red, usually clastic sedimentary deposits.

ripple marks (95): Commonly seen sedimentary features that develop along the surface of bedding planes. Symmetric ripple marks are formed by the oscillatory motion of water beneath waves. Asymmetric ripple marks are formed by air or water currents and are useful in indicating the direction of movement of currents.

rock unit (= lithostratigraphic unit) (103): Bodies of rock identified by their distinctive lithologic and structural features without regard to time boundaries. Features used include texture, grain size, clastic or crystalline, color, composition, thickness, type of bedding, nature of organic remains, and appearance of the unit in surface exposure.

rounding (of sedimentary particles) (93): A particle becomes rounded by having sharp corners and edges removed by impact with other particles. The roundness can be used to infer the history of abrasion. It reflects the distance the particles had traveled, the transporting medium, and the rigor of transport. Wind is the best transport medium.

scour marks (97): A mark produced by the cutting or scouring action of a current flowing over the bottom of a river or body of water.

shield (82): Large areas of exposed ancient crystalline rocks; a component of cratons.

sorting (91): A measure of the uniformity of the sizes of particles in a sediment or sedimentary rock.

sphericity (of sedimentary particles) (93): A descriptive term regarding shape of a particle and how closely the grain approaches the shape of a sphere.

stratigraphy (107): The science of rock strata; concerned with all characters and attributes of rock as strata, and their interpretation in terms of mode of origin and geologic history.

stream deposits (86): Sand, silt, and clay along the banks, bars, and floodplains of streams. Discharge will alter water velocity and type of sediment being transported.

structural cross-section (113): A cross-section showing the features transected by a vertical plane. It is a level line that is parallel to sea level.

tectonics (82): The structural behavior of a region of the Earth’s crust.

tidal flat (89): Represents a transitional environment (between marine and non-marine). Mud-covered flats that are alternately inundated and drained of water by tides.

turbidite (86): Sediment deposited from a turbidity current and characterized by graded bedding and moderate to poor sorting.

turbidity current (86): A mass of moving water that is denser than surrounding water because of its content of suspended sediment and that flows along slopes of the sea floor as a result of that higher density.

unconformity (110): An erosional gap or hiatus in a rock record. A surface separating an overlying younger rock formation from underlying formations and representing an episode of erosion or nondeposition.

Walther’s Principle (106): Walther recognized onlap and offlap patterns of sedimentations. He observed that the succession of facies occurring laterally is also seen in vertical succession of facies. This relationship, in which the vertical succession of facies corresponds to the lateral succession, is known as Walther’s Principle.

Multiple-Choice Questions

1.The term used for the central, topographically subdued region of a continent is

a.orogenic syncline.c.craton.

b.crustal trench.d.tectonic subduction zone.

2.What is the proper order of these clastic rocks from coarsest to finest texture?

a.sandstone, shale, siltstone, claystone

b.sandstone, siltstone, shale, claystone

c.shale, sandstone, siltstone, claystone

d.siltstone, claystone, sandstone, shale

3.What is the smallest particle size found in sedimentary rocks?

a.siltc.fine sand

b.clayd.granules

4.Which of the following sedimentary rock types does not fit within the clastic grouping?

a.shalec.limestone

b.sandstoned.conglomerate

5.Which of the following is not a primary sedimentary structure?

a.mud cracksc.folds

b.cross beddingd.ripple marks

6.The three major types of environments of deposition are

a.continental slopes, rises, shelves.

b.continental slopes, marine, nonmarine.

c.transitional, marine, nonmarine.

d.continental slopes, marine, lagoonal.

7.The characteristic appearance or aspect of a rock from which its environment of deposition can be determined is

a.biolithic zone.c.facies.

b.isopach zone.d.paleoclimatic zone.

8.Transitional sandstone in which feldspars are relatively scarce and quartz muscovite and chert are abundant; the quartz grains that are more rounded and abundant are called

a.arkoses.c.graywackes.

b.orthoquartzites.d.subgraywackes (lithic sandstone).

9.What are immature sandstones consisting of significant quantities of dark, very fine-grained materials with a fine matrix that consists of clay, chorite, micas and silt?

a.arkosesc.graywackes

b.orthoquartzitesd.subgraywackes

10.A type of sandstone that may develop as a basal sandstone derived from the erosion of granitic coastal area experiencing an advance of the sea or in adjacent areas adjacent to granite mountains is

a.arkoses.c.orthoquartzite.

b.subgraywacke.d.graywacke.

11.A map showing the geography of a region or area at some specific time in the geologic past is

a.a paleogeographic map.c.an isopach map.

b.a geologic interval map.d.a lithofacies map.

12.Lithofacies maps are useful in determining changes mostly in which of the following areas?

a.sea floor spreading

b.geosyncline formation

c.zones of tectonic instability

d.aerial variation in the overall lithologic character of a given stratigraphic unit

13.A map prepared by geologists to illustrate changes in the thickness of a formation or time rock unit is a (an)

a.paleoclimatic map.c.lithofacies map.

b.paleogeographic map.d.isopach map.

14.This method of correlation links rock units by age equivalences, determined by fossils or radioactive dating.

a.lithostratigraphicc.chronostratigraphic

b.biostratigraphicd.paleostratigraphic

15.Spherical grains of concentric laminated calcium carbonate which are formed in shallow, saline marine conditions are called

a.crinoids.c.blastoids.

b.ooid.d.septarian nodules.

16.This type of unconformity is characterized by stratified sedimentary rocks resting on older igneous or metamorphic rocks.

a.angular unconformityc.disconformity

b.nonconformityd.paraconformity

17.A ferric mineral that tends to color rock red, brown, or purple and are dubbed “red beds.”

a.hematitec.graywacke

b.micrited.orthoclase feldspar

18.Lithostratigraphic units are bodies of rock having distinctive features without regard to time boundaries.

a.time-rock unitc.rock units

b.formationd.lithofacies unit

19.Four sedimentary structures are formed during or after deposition, but before lithification. Which one of the following would not be included?

a.mud cracksc.graded-bedding

b.cross-beddingd.scour marks

20.A body of rock that can be recognized wherever it occurs by the characteristics of composition, texture, or color is

a.time rock.c.formation.

b.rock interval.d. rock unit.

Fill in the Blank

1.The study of the deformation or structural behavior of a large area of the Earth’s crust over a long period of time is .

2.Hues of brown, red, and green often occur in sedimentary rocks as a result of their ______content.

3.Geologists designate strata colored in shades of red, brown, or purple by ferric iron as _____ .

4.The size, shape, and arrangement of mineral and rock grains in a rock constitute its .

  1. One category of bonding material that consists of finer clastic particles (often clay) that were deposited at the same time as the larger grains and that fill the crevices between them is called .

6.An expression of the range of particle sizes deviating from the average size is .