correlation of sedimentary stratacollege.cengage.com/geology/chernicoff_fox_tanner/earth/...however,...

Correlation ofSedimentary Strata

Goals___________________________________________

1. introduce geologic time classification units: time units, time-rock units, and physical rock units2. illustrate an example of physical correlation using the basal Cambrian sequence of rock units in the western United States3. demonstrate that the succession of similar depositional environments that existed at different

times during the Cambrian indicate a transgressive sequence4. show the utility of using trilobites as index fossils for time correlation of Cambrian strata, and for locating successive positions of the ancient Cambrian shoreline5. develop a trilobite zonation for the Cambrian strata of the western United States

Introduction_______________________________________

Three distinct classification schemes are combined to interpret geologic time. The first involves timeunits. The largest time unit is the eon. Eons are subdivided into eras. Eras are subdivided into periods,and periods into epochs. The period is the unit central to construction of the geologic time scale. Eachperiod is defined by the fossils found in an interval of strata exposed at a particular geographic location(see Table 1).

Table 1. The geologic time scale

PERIOD AUTHORÕS ORIGINALLOCATION

AUTHOR(FIRST DESCRIBEDBY)

DATE MODERNDESIGNATEDLOCATION

Quaternary Northern France Desnoyers 1829 FranceTertiary Base of Italian Alps Arduino 1759 FranceCretaceous France, Belgium DÕHalloy 1822 EnglandJurassic Jura Mountains, France Von Humboldt 1795 EnglandTriassic Germany Von Alberti 1834 AlpsPermian Town of Perm, Russia Murchison 1841 Urals, RussiaCarboniferous

(Pennsylvanian)

North-central England

(Pennsylvania, USA)

Conybeare/Phillips

(Williams)

1822

(1891)

Britain

(Pennsylvania,USA)

Carboniferous

(Mississippian)

North-central England

(upper Mississippi RiverValley, USA)

Conybeare/Phillips

(Winchell)

1822

(1870)

Britain

(Missouri, USA)

Devonian Southern England Sedgwick/Murchison 1839 EnglandSilurian Wales, Great Britain Murchison 1835 WalesOrdovician Wales, Great Britain Lapworth 1879 WalesCambrian Wales, Great Britain Sedgewick 1835 Wales

For example, the base of the Cambrian Period is defined as the lowest (oldest) layer ofsedimentary rock containing a distinctive extinct fossil marine arthropod called a trilobite. The top of theCambrian Period is defined as the highest (youngest) sedimentary rock layer containing a particulartrilobite, such as Saukia, which is not found above that layer. The strata that occur above the last Saukia-bearing layer are assigned to the younger (next) Ordovician Period. The totality of rocks that formed during each of the time periods are organized into time-rock units,each corresponding to its particular assigned time unit. The largest time-rock unit is the system. Systemsare subdivided into series, and series are subdivided into stages. For example, all rocks of Cambrian agebelong to the Cambrian System. All Late Cambrian rocks belong to the Late Cambrian Series , andall Early Late Cambrian rocks belong to the Dresbachian Stage. Table 2 illustrates the organization ofall rocks formed during the Cambrian Period.

Table 2. The Cambrian System (modified from Levin 1999, Appendix B, p. A8)

SERIES(North America)

STAGES(North America)

ROCK UNITS(Grand Canyon, USA)

STAGES(Europe)

CROIXIAN

(Late Cambrian)

Trempealeauan___________________Franconian___________________Dresbachian(Early Late Cambrian)

Rock units deposited,then later eroded away

Dogellian

___________________Festiniogian___________________

Maentwrogian

ALBERTAN

(Medial Cambrian)

Stages not designated inNorth America

Muav LimestoneFormation___________________Bright Angel ShaleFormation

Menevian___________________Solven

WAUCOBAN

(Early Cambrian)

Stages not designated inNorth America

Tapeats SandstoneFormation-----------------------------Land undergoingerosion at this time,rock units neverdeposited

Lenan___________________Atdabanian___________________Tommotian

Physical rock units are organized into formations, which are defined as rock bodies with similarlithological characteristics and identifiable contacts (upper and lower boundaries of the rock formation)with overlying and underlying lithologically distinct strata. Formations are recognizable intervals of stratathat can be mapped between geographically separated exposures. Table 2 shows the time-rock placement forCambrian formations exposed in the Grand Canyon. Formations can be subdivided into smaller, distinctive intervals of strata called members, orgrouped into larger intervals containing several similar formations called groups. (The Tapeats, BrightAngel, and Muav Formations were originally placed together in the Tonto Group).

Formation names are usually derived from local geographic landforms located near the studiedoutcrop, such as Bright Angel Creek for the Bright Angel Shale Formation, and also in consideration ofthe dominant rock type found in the formationÐÐin this example, shale. Geologists also often abbreviatethe name of a formation by omitting the word formationÐÐfor example, Bright Angel Shale.

The lower and upper boundaries of rock formations do not usually correspond to the boundaries ofseries or stages for which they are assigned. This is because rock formations represent environments ofsediment deposition, which persisted independently of the boundaries defined by the appearances andextinctions of organisms.

To summarize the relationship between time units, time-rock units, and physical rock units,we describe the Tapeats Sandstone as being of Cambrian age and belonging to the Cambrian Systemand the Waucoban Series (see Table 2). Correlation of sedimentary strata is essential to synthesize the enormous number of sedimentary rockformations of a given geologic age for a continent such as North America into a smaller number of

changing depositional environments through that geologic age. The distribution of depositionalenvironments on the continent during a particular geologic age provides information essential to theconstruction of paleogeographic maps (maps depicting the prehistoric world) of the region for the entiregeologic period, and eventually for the entire world for that same geologic period.

For example, all rock formations of Cambrian age that contain marine fossils pinpoint thelocation of marine depositional environments. These rocks document Cambrian seas and oceans thatexisted at some point in time during the approximately 40-million-year interval of the Cambrian Period. Formations possess distinctive rock types and rock structures, but because each rock body represents acertain depositional environment, the overall age of a widespread rock formation may be different at widelyseparated geographic locations. Nearly every group of Cambrian rock formations (called a sequence) in thewestern United States from southern California to the Black Hills of Wyoming begins with a basal rockformation containing sands (later cemented to form sandstone) deposited along the margins of the ancientCordilleran Seaway. The modern shoreline environment is relatively narrow (less than 1 mile) from wavesto beach to dunes, and most dunes are only a few tens of feet above sea level. Since a vast tidal flat couldnot physically extend between California and Wyoming, the basal Cambrian sandstone located in southernCalifornia must represent a shoreline of a different Cambrian age from the basal Cambrian sandstonelocated in Wyoming. Since depositional environments migrate with changing conditions, such as rising sea level (due tomelting glaciers), it is the widely distributed marine life preserved as fossils that will provide the timelines needed for correlation of widely separated rock formations of suspected similar age. In addition to thenarrow shoreline environment represented only by sandstone, the depositional environments occupied byshallow marine bottom-feeding organisms such as trilobites also included vast, shallow platforms (todaycalled the continental shelves) in which sedimentary rock deposits of siltstone, shale, and limestonecontain their fossilized remains. Trilobite larvae were widely distributed by ocean currents, makingtrilobites universally abundant Cambrian marine fossils. Careful collection of trilobite fossils found at successively higher (younger) levels throughout onesequence of Cambrian sedimentary rock formations from the western United States will help to establishdifferent ages for each formation, which then can be compared with trilobite fossils found in another nearbyor even widely separated Cambrian sequence from the same region. Time correlation is accomplished bymatching the two rock formation sequences with time lines that connect levels both containing the sametrilobite genus. A complete succession of trilobite zones for the Cambrian System was constructed byLochman-Balk (1971), by using several Cambrian rock sequences located in the western United States thatrepresent the entire span of Cambrian time. Transgression (advance) of the Cambrian seas from southernCalifornia in the Early Cambrian to eastern Wyoming in the Late Cambrian is clearly documented by theoccurrence of trilobite fossils in Cambrian sedimentary rock sequences of the western United States.

Exercises________________________________________

Physical Correlation of Rock Units

Physical correlation of rock formations is accomplished by matching the same key rock layers andcontacts from two formations in geographically separate locations. Where the two exposures are within ashort distance, this correlation can be made by walking along the key layer or contact from its outcrop atone location through its continuation to the next location. However, where terrain is rough or impassable,as in the Grand Canyon, this direct approach may be impractical or impossible.

Geologists construct a stratigraphic column to represent the bottom-to-top sequence of rocklayers at a particular location. The rock type; thickness of each layer; sedimentary structures such as ripplemarks, mudcracks, tracks, trails, and burrows; contained fossils; and the nature of the contacts betweenunderlying and overlying rock formations are all recorded and their positions in the sequence carefullyplotted. This column can then be compared with columns from other locations for purposes of correlationbetween other localities. Where the identity of individual layers cannot be traced between stratigraphic columns, the overallchange in rock type through the succession of the formation can be used to correlate rock formations ofthe two columns. It is important to remember that physical correlation matches rock formationsrepresenting the same depositional environment, unbounded by time planes. The ash layer left by the May18, 1980, eruption of Mount St. Helens truly represents a time plane. The basal Cambrian sandstonerepresents a narrow shoreline environment that migrated eastward as sea level rose throughout the CambrianPeriod. The basal Cambrian sandstone in California is much older than its counterpart in Utah, eventhough they may look identical in terms of rock type, rock thickness, and contacts with underlying andoverlying rock formations. As the distance between two rock column locations increases, so does thelikelihood of their disparate ages. Contacts between rock formations that are unconformable (that is, thatshow a gap in the geologic time scale) represent larger-scale events such as regional uplift and erosion.Such events were of greater duration and as such can be traced over great distances.

The Cambrian Transgressive Sequence

The basal Cambrian sequence of sedimentary strata in western North America (see Figure 1) begins atnearly every location with a several-hundred-meter-thick sequence of cross-bedded sandstone overlying aneroded older crystalline bedrock. The sandstone contains current ripples, burrows, and trilobite tracks. Thedepositional environment of this interval is probably coastal plain to beach. The trilobite fossils found inthis interval can be used to date the position for the Cambrian shoreline at that location. By connectingtwo such locations with sandstones of the same age, the approximate position of the shoreline during thattrilobite zone age can be illustrated for successive paleogeographic maps of North America during thestages of the Cambrian Period.

The basal interval is overlain by a thick sequence of finer sandstone, siltstone, and shalecontaining trilobites, brachiopods, and trace fossils. The depositional environment indicated by thisinterval is a shallow nearshore marine shelf. Overlying the finer clastics (sedimentary rock) is a thick tovery thick succession of thick-bedded limestone and dolomite containing trilobites, brachiopods, and algalstromatolites. This depositional environment was a shallow marine offshore platform (Beus 1990). The change in depositional environments through time from coastal plain through shallow nearshore tooffshore shelf (see Table 3) indicates a rising sea level or a transgressive sequence. Coarse clasticsediments characterize the shoreline, fine clastics the nearshore shelf, and limestones the outer shelf, oroffshore platform. Throughout Cambrian time, the seas rose across the lowlands to reach a maximumflooding of most of the North American continent in latest Cambrian to early Ordovician time. This isindicated by the progressively younger age of the basal Cambrian sandstone (as indicated by trilobitefossils) from southern California to Wyoming and New Mexico (west to east). In sections 15 and 16(Figure 11), the upper portions of the transgressive sequence dolomite and limestone rock formations)contain Early Ordovician fossils.

Table 3. Depositional environments of the basal Cambrian sequence in the western United States

ROCK TYPE OTHER FEATURES ENVIRONMENTREPRESENTED

EXAMPLE IN THISEXERCISE

Sandstone Current ripples,oscillation ripples,burrows, trilobite tracks

Coastal plain to beach Tapeats Sandstone

Siltstone Trilobites, brachiopods,trace fossils representingtracks and burrows

Lagoons, bays, estuaries Bright Angel Shale(lower part)

Shale Trilobites, brachiopods,trace fossils

Shallow nearshoremarine shelf

Bright Angel Shale(upper part)

Limestone Trilobites, brachiopods,algal stromatolites

Shallow marine,offshore platform

Muav Limestone

Figure 1. The basal Cambrian transgressive sequence

Trilobites as Index Fossils

Trilobites (see Figure 2) are an extinct group of arthropods that were mobile bottom feeders, probably asscavengers. Because they shed their tough exoskeleton many times as they grew, their remains are verycommon Cambrian fossils.

The occurrence of trilobite fossils in sandstone, siltstone, shale, and limestone rocks illustratestheir mobility, which allowed them to inhabit many different depositional environments at the samegeologic time. For this reason trilobites are exceptionally good index fossils, or indicators of specific, verynarrow ranges of geologic time.

The presence of trilobites in sandstones can give us an approximate age for the position of theCambrian shoreline at a particular location. The tremendous thickness of sediments deposited in theshallow Cordilleran Seaway during the Cambrian Period has preserved a nearly complete record of thetrilobite animals that lived there.By comparing the trilobite genera found in a number of Cambrian rock formation sequences in the westernUnited States, a complete succession of trilobite zones was established by Lochman-Balk (1971). Bytaking a collective inventory of the trilobite occurrences in (sixteen) different Cambrian rock formationsequences (sixteen stratigraphic columns) in the following activities, you will be able to locate all of thesetrilobite zones, and to designate their relative positions within the Cambrian System.

Figure 2. Early Cambrian (Olenellus), (Middle) Cambrian (Bathyuriscus),and Late Cambrian (Saukia) trilobites (after Harrington et al. 1959)

Lab 21: Correlation of Sedimentary Strata

Name: _______________________ Lab Section: _______ Date: __________

Activity 1: Physical Correlation of Adjacent Cambrian Rock Formation Sequences

1. Figure 3 illustrates two stratigraphic columns of the basal Cambrian sequence in the GrandCanyon region located approximately 10 miles apart. Answer the following questions.

Figure 3 (Basal Cambrian sequences in the Grand Canyon, Arizona (after Lochman-Balk, 1971) a. Color each of the columns in Figure 3 according to the following scheme: sandstoneÐÐyellow;

siltstoneÐÐorange; shaleÐÐgreen; mudstone---purple limestone/dolomiteÐÐblue. Using Figure 1 (BassTrail Section) (it is now)as a guide, designate the contacts (boundaries) between the Tapeats Sandstoneand the Bright Angel Shale, and the Muav Limestone in the Little Colorado section.

b. Connect the contacts between the two columns to indicate your correlation based on physical

stratigraphy. What features did you consider most important in making your decision?

Bass Trail Section(after Lochman-Balk, 1971)

Little Colorado Section(after Lochman-Balk, 1971)

2. Figures 4 through 7 illustrate sixteen stratigraphic columns of basal Cambrian sequences

(containing sandstone, siltstone, shale, limestone, dolomite, and interbedded mudstoneformations) in the western United States. Refer to these figures in answering the followingquestions.

a. In Figure 4, color each stratigraphic column according to the following scheme:sandstoneÐÐyellow; siltstoneÐÐorange; shaleÐÐgreen; mudstone Ðpurple,limestone/dolomiteÐÐblue. Draw in the contacts between the three formations (Tapeats Sandstoneand the Bright Angel Shale, and the Muav Limestone) in each section.

b. Connect the contacts between the four columns in Figure 4 to indicate your correlation basedon physical stratigraphy. What features did you consider most important in making yourdecisions?

c. In Figure 5,color each stratigraphic column according to the following scheme:sandstoneÐÐyellow; siltstoneÐÐorange; shaleÐÐgreen; mudstone Ð purple ; limestone/dolomiteÐÐblue. Draw in the contacts between the three formations (Tapeats Sandstoneand the Bright Angel Shale, and the Muav Limestone) in each section.

d. Connect the contacts between the four columns in Figure 5 to indicate your correlation based on physical stratigraphy. What features did you consider most important in making your decisions?

e. In Figure 6, color each stratographic column according to the following scheme:sandstoneÐÐyellow; siltstoneÐÐorange; shaleÐÐgreen;mudstone Ðpurple;limestone/dolomiteÐÐblue. Draw in the contacts between the three formations (Tapeats Sandstoneand the Bright Angel Shale, and the Muav Limestone) in each section.

f. Connect the contacts between the four columns in Figure 6 to indicate your correlation based on physical stratigraphy. What features did you consider most important in making your decisions?

g. In Figure 7, color each stratigraphic column according to the following scheme:sandstoneÐÐyellow; siltstoneÐÐorange; shaleÐÐgreen;mudstone Ð purple;limestone/dolomiteÐÐblue. Draw in the contacts between the three formations (Tapeats Sandstoneand the Bright Angel Shale, and the Muav Limestone) in each section.

h. Connect the contacts between the four columns in Figure 7 to indicate your correlation based on physical stratigraphy. What features did you consider most important in making your decisions?

Figure 4. Selected sequences (1Ð4) of basal Cam

brian rock formations in the w

estern United States

Activity 2: Correlation of Basal Cambrian Sequences in the Western UnitedStates Based on Trilobite Fossils

1. Figures 8 through 11 illustrate the distribution of trilobite fossils in the sixteen stratigraphic columns of basal Cambrian sequences (containing sandstone, shale, and limestone) in the western United States. Refer to these figures in answering the following questions.

a. In Figure 8, connect the rock intervals in the four columns that contain the same trilobites toindicate your correlation based on the similarities in trilobite distribution between each section.How do these strata of equal age differ from those based on similar rock types that were drawn forFigure 4?

b. In Figure 9, connect the rock intervals in the four columns that contain the same trilobites toindicate your correlation based on the similarities in trilobite distribution between each section.How do these strata of equal age differ from those based on similar rock types that were drawn forFigure 5?

c. In Figure 10, connect the rock intervals in the four columns that contain the same trilobites toindicate your correlation based on the similarities in trilobite distribution between each section. How do these strata of equal age differ from those based on similar rock types that were drawn for Figure 6?

d. In Figure 11, connect the rock intervals in the four columns that contain the same trilobites toindicate your correlation based on the similarities in trilobite distribution between each section. How do these strata of equal age differ from those based on similar rock types that were drawn for Figure 7?

Activity 3: Constructing a Trilobite Zonation Chart for the CambrianSequences of the Western United States

1. Figure 12 shows the trilobite distribution for three important Cambrian stratigraphic columnsand one drill core section in the western United States. On the basis of the sequentialdistribution of the trilobite index fossils throughout the different levels of the fourcolumns, complete Table 4 with the names and pictures (see Figures 13Ð15) of eachtrilobite index fossil.

a. There are five Early Cambrian trilobite index genera (see Figure 13). List and paste themin their correct sequence (oldest at the bottom) on Table 4.

b. There are five Medial Cambrian trilobite index genera (see Figure14) List and paste them in their correct sequence on Table 4.

Figure 8. Trilobites in sequences (1Ð4) of basal C

ambrian rock form

ations in the western U

nited States


ambrian rock form


nited States

* stratigraphic occurrence

Figure 12. Cambrian stratigrahic sections of the western United States.

Figure 12 (continued). Cambrian stratigrahic column (drill core) from the western United States.

17. Shell Southwest Richey #32-33B Well; McCone County, Montana (after Lochman 1964)

c. There are four Early Late Cambrian (Dresbachian) trilobite index genera (see Figure 15). List and paste them in their correct sequence in Table 4.

d. There are three Medial Upper Cambrian (Franconian) trilobite index genera and one brachiopod genus (see Figure15). List them in their correct sequence on Table 4.

e. There is a single Late Upper Cambrian (Trempealeauan) trilobite index genus (see Figure 15). List and paste it on Table 4.

Activity 4: Constructing a Correlation Chart for the Basal CambrianSandstone in the Western United States

1. Use Figures 8 through 12 to complete Table 5, which illustrates the age of the basal Cambriansandstone in each sequence of rock formations from the western United States.

a. The index trilobite fossils that occur in the basal sandstone layer at localities 2, 5, 9, 3, 4,14, 15, 17, and 12 are shown on Table 5 by the symbol ÒSSÓ in each corresponding localitycolumn. Complete Table 5 by locating the trilobite found in the basal sandstone at localities1, 6, 10, 11, 7, 8, 13, and 16. Consult with Table 4 to be certain of the appropriate age asindicated by each trilobite genus. At that correct level, write in the symbol ÒSSÓ to indicatethe time-rock (stage) position for each shoreline represented by the enclosing sandstone strata.

2. The position of ÒSSÓ (the shoreline sandstone depositional environment) in successive columns of Table 5 appears to move upward from left to right. The geographic localities are listed from westernmost at the far left to easternmost at the far right. What does this indicate about the position of successively (younger) later shorelines in the western United States during the Cambrian Period?

Activity 5: Constructing the Positions for the Basal Cambrian Shorelinesin the Western United States

1. Figure 16 shows trilobite localities 1, 2, 5, 9, 6, 10, and 11, containing Early Cambriantrilobites in the basal sandstone. Construct an Early Cambrian shoreline on this map by joininglocalities characterized by the index trilobite Olenellus found in the basal sandstone.

2. Figure 17 shows trilobite localities 3, 4, 7, 8, and 13, containing Middle Cambrian trilobites in the basal sandstone. Construct a shoreline from the Poliella-Albertella zone by joining the localities on this map characterized by these index trilobites found in the basal sandstone.

3. Figure 18 shows trilobite localities 12, 14, 15, 16, and 17, containing Late Cambrian trilobitesin the basal sandstone. Construct a shoreline from the Dunderburgia-Elvinia zone by joininglocalities on this map characterized by that index trilobite found in the basal sandstone.

4. What has happened to the relative position of the shorelines that you have drawn on Figures16, 17, and 18? What factors could have caused this phenomenon to happen, given that no icecapsexisted at this time?

Figure 13. Lower Cambrina index trilobites of the western United States( after Harring ton et al. 1959)

Figure 14. Middle Cambrian index trilobites of the western United States(after Harrington et al. 1959)

Figure 15. Upper Cambrian index trilobites and brachiopod of the western United States(after Harrington et al. 1959)

Table 4a. Early Cambrian trilobite index zones (data from Palmer 1971)



TRILOBITE INDEX ZONES

WAUCOBAN

_______________________________________

_______________________________________

_______________________________________

_______________________________________

Table 4b. (continued). Medial Cambrian trilobite index zones (after Lochman-Balk1971)




CROIXIAN

Trempealeauan

________________

Franconian

________________

Dresbachian

_______________________________________

_______________________________________

_______________________________________

_______________________________________

_______________________________________

Table 4c (continued). Late Cambrian trilobite index zones (after Lochman-Balk 1971)




ALBERTAN

_______________________________________

_______________________________________

_______________________________________

_______________________________________

Figure 16. Location of selected basal Early Cambrian sequences in the westernUnited States

Figure 17. Location of selected basal Middle Cambrian sequences in the westernUnited States.

Figure 18. Location of selected basal Late Cambrian sequesnces in the westernUnited States

REFERENCES

Beus, Stanley S., and Michael Morales, eds. 1990. Grand Canyon Geology. New York: Oxford UniversityPress.

Harrington, Hilary J(Horacio J. is correct)., et al. 1959. ÒTrilobite Systematic Descriptions.Ó In Treatise onInvertebrate Paleontology, ed. Raymond C. Moore, Part O, Arthropoda 1, pp. O170ÐO560. Lawrence,Kansas: Geological Society of America and University of Kansas Press.

Hayes, Phillip T., and George C. Cone.(Figures 7 and 11 on Chapter 21 Landscape File) 1975. Cambrianand Ordovician Rocks of Southern Arizona and New Mexico and Westernmost Texas. United StatesGeological Survey Professional Paper 873.

Hintze, Lehi F., and Richard A. Robison. 1987. ÒThe House Range, Utah: Cambrian Mecca.Ó InGeological Society of America Centennial Field Guide, Rocky Mountain Section, pp. 257Ð260. Boulder.

Levin, Harold H. 1999. The Earth Through Time, 6th ed. Orlando: Saunders College Publishing.

Lochman, Christina, 1964. ÒUpper Cambrian Faunas from the Subsurface Deadwood Formation, WillistonBasin, Montana.Ó Journal of Paleontology 38: (33-60)

Lochman-Balk, Christina, 1971. ÒThe Cambrian of the Craton of the United States.Ó In Cambrian of theNew World, ed. C. H. Holland, pp. 79Ð167. New York: Wiley-Interscience.

Palmer, Allison R. 1971. ÒThe Cambrian of the Great Basin and Adjacent Areas, Western United States. InCambrian of the New World, ed. C. H. Holland, pp. 1Ð78. New York: Wiley-Interscience.

Stewart, (That is fine) John H. 1970. Upper Precambrian and Lower Cambrian Strata of the SouthernGreat Basin, California and Nevada. United States Geological Survey Professional Paper 620.

correlation of sedimentary stratacollege.cengage.com/geology/chernicoff_fox_tanner/earth/...however,...

Documents