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Exercise 4 Facies relationships

An objective of many geologic studies is to reconstruct the paleogeography

of a given area for a particular instant in geologic time. In other words, for a

specified date, say, 100 m.y. ago, where was the shoreline and what was the

approximate areal distribution of various continental, nearshore marine and

offshore marine depositional environments?

An approximation of paleogeography can be achieved by constructing and

interpreting maps that show the thicknesses and kinds of sediments that

were being deposited during a particular time interval. Thickness maps are

also known as isopach maps, and they utilize contour lines to connect points

of equal sedimentary rock thickness across an area. Facies maps plot the

areal distribution of different sedimentary rock types across an area for a

given interval of time.

Part 1 Examine the map and legend below (Fig. 1), which shows the generalized

distribution of Late Cretaceous sedimentary rock types in the western U.S.

Rocks of Late Cretaceous age are know to occur in the shaded area. This

area is bounded on both the east and west by “zero lines.” Late Cretaceous

rocks are not preserved in the unshaded areas to the west and east of the

“zero lines.”

Figure 1—Generalized Late Cretaceous facies map of western U.S. (from Brice et al., 2001)

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a. On Figure 1, draw a line showing the approximate western edge of the sea

that covered much of the western U.S. during Late Cretaceous time. In

other words, draw a line separating nonmarine (terrestrial) and marine

facies.

b. The “zero lines” on the map mark the lateral extent of Late Cretaceous

rocks. The absence of Late Cretaceous rocks to the west and east of the

shaded area may be the result of erosion, in which case the original extent

of Late Cretaceous rocks would have been greater than shown, or the “zero

lines” may indicate the actual limits of the original basin of deposition. What

lithologic characteristics of the rocks adjacent to the “zero line” might one

look for as an indication that the “zero line” represents the true basin edge?

c. On the basis of the rock type near the western “zero line,” what

inference can be made about the probable topography of western Idaho,

Nevada, and western Arizona during Late Creatceous time?

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Figure 2—Detailed facies map showing distribution of rock type in western U.S. Abbreviations: congl. = conglomerate; ss = sandstone; sh = shale; m = marl or chalk (from Petersen and Rigby. 1999).

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d. The map in Figure 2 is similar to that in Figure 1, but it contains more

detail and the actual data points for reconstructing the distribution of

facies in the western U.S. late Cretaceous seaway. Complete the map by

drawing lines separating the various rock types that can be recognized.

e. Do the “facies belts” parallel the zero line?

f. What was the most likely direction of transport of the siliciclastic

sediments? Where is the probable source area for the coarse sediments

along the western border of the map?

g. Were the Colorado Rocky Mountains present during late Cretaceous time

when these sediments were being deposited? Explain your reasoning.

h. Where would you expect the greatest thickness of late Cretaceous

sediments to have accumulated? Explain your reasoning.

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Figure 3—Isopach map of upper Cretaceous rocks in western U.S. Contour interval = 2000 ft (from Petersen and Rigby, 1999).

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i. Figure 3 is an isopach map for upper Cretaceous rocks in the same general

area as in Figures 1 and 2. Contour lines connect points of equal rock

thickness. In the space provided below, construct a thickness profile along

the line A–A’. The top of your profile should correspond with the top of the

upper Cretaceous interval and it should be flat. The base of your profile will

vary depending on rock thickness.

A A’

0’ ______________________________________________

2000’ _____________________________________________

4000’ _____________________________________________

6000’ _____________________________________________

8000’ _____________________________________________

10000’ _____________________________________________

12000’ _____________________________________________

14000’ _____________________________________________

16000’ _____________________________________________

Part 2 The map in Figure 4 shows both rock types and thicknesses for lower

Silurian rocks in the eastern U.S., including the eastern part of Iowa. This is

a combination facies and isopach map.

a. Where is the most likely source area of lower Silurian siliciclastic rocks

shown in Figure 4? What kind of topography probably existed in this source

area?

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Figure 4—Facies and isopach map of lower Silurian rocks in eastern U.S. Isopach contours represent rock thickness, which reaches

400 ft in Pennsylvania, and ~1000 ft in a few places within the 400 ft contour (from Brice et al. 2001).

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b. Give two possible explanations for the absence of lower Silurian rocks in

the elongate area that extends across Tennessee, Kentucky, and into

southern Indiana and Ohio. Which of the two possibilities is more likely.

Explain your reasoning.

Part 3

Sea level has not remained constant throughout Earth’s history. Fluctuations

in sea level occur in response to the cyclical waxing and waning of continental

ice sheets. During glacial maxima, significant volumes of water are “locked

up” in the form of continental glaciers, and sea level is relatively low. When

continental glaciers recede, significant volumes of meltwater return to the

ocean and sea level rises. Fluctuations in sea level also can occur in response

to changes in the volume of mid-ocean ridges. During episodes of active plate

tectonics, rates of sea-floor spreading increase, which in turn causes mid-

ocean ridges to become thermally buoyant and rise. This effectively reduces

the volume of the ocean basin and forces sea level to rise and flood coastal

plains. During episodes of less active plate tectonics, mid-ocean ridges may

cool and sink, which effectively increases the volume of the ocean basin and

causes a drop in sea level.

Changes in sea level have a profound influence on the areal distribution of

sedimentary environments. When sea level rises, the shoreline and adjacent

sedimentary environments migrate in a landward direction (transgression).

When sea level falls, the shoreline and adjacent sedimentary environments

migrate in a seaward direction (regression).

The lateral migration of sedimentary environments ultimately results in an

orderly vertical succession of sedimentary rocks. For example, during a

transgression, nearshore marine environments migrate landward across the

former coastal plain. A core through a transgressive sequence would consist

of coastal plain deposits overlain vertically by nearshore marine deposits. Conversely, during a regression, coastal plain sedimentary environments

migrate seaward across the former nearshore marine environments. A core

through a regressive sequence, then, would consist of nearshore marine

deposits overlain vertically by coastal plain deposits.

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The relationship between laterally migrating sedimentary environments and

vertical associations of rock facies is expressed in Walther’s Law: “In a depositionally continuous vertical succession of sedimentary rocks, vertically adjacent lithofacies must at one time have been laterally adjacent sedimentary facies.”

Figure 5—Cross sections W, X, and Y depict the preserved geologic record of migrating sedimentary environments (from time A to time C; older to younger) (from Brice et al., 2001).

a. Examine the diagrams in Figure 5, paying attention to the relative

direction in which sedimentary environments migrated through time. Does

this sequence of diagrams depict a transgression or a regression? Explain

your reasoning.

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Figure 6—Cross section depicting the migration of sedimentary environments (from Brice et al., 2001).

b. Examine the cross section in Figure 6. Does the vertical succession of

lithofacies at point A represent a transgression or a regression? Explain

your reasoning.

Part 4 Examine the two lithofacies maps in Figure 7, which show the areal

distribution of sedimentary rock types of middle Cambrian (upper map) and

late Cambrian (lower map) age.

a. What type of sediment, in terms of texture and composition, was

deposited along the shore of the Cambrian seas?

b. What type of sediment was deposited farther out to sea?

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Figure 7—Lithofacies maps for the middle Cambrian (A) and late Cambrian (B) in

north-central U.S. and southern Canada (from Brice et al., 2001).

c. During the interval of time between the middle Cambrian and the late

Cambrian, did the sea transgress or regress? Explain your reasoning.

d. When did the sea cover western Colorado: middle Cambrian or late

Cambrian?

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e. What evidence in Figure 7B suggests the presence of islands in the late

Cambrian sea?

Part 5 Figure 8 (two pages) consists of fifteen stratigraphic sections measured

through Devonian rocks in New York and Pennsylvania. The locations of

measured sections are shown in the inset map. Rock thickness at each of the

sections is drawn at the same scale. Time lines, determined by the use of

fossils, are shown by dotted lines in each column and marked by small

letters. For example, all rocks at the level of the dotted line marked by “a”

in each column were deposited at the same time. Assume that the

conglomerates were deposited as alluvial fan sediments, that the sandstones

were deposited as nearshore marine sediments, and that the shales were

depostied as slightly more offshore marine sediments.

a. Detach both pages containing the measured sections, place them side-by-

side so that the sections are in correct order, and then tape the pages

together.

b. Construct a lithologic cross section by drawing lines from column to

column connecting equivalent lithofacies. [Do not draw lines to connect time

lines, but use the time lines to help guide your lithologic correlations from

column to column.] Once your cross section is complete, answer the following

questions:

c. Are the conglomerates the same age at each of the different localities

where they occur?

d. If not, does the age of the conglomerates vary in a consistent pattern

from east to west? How?

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e. What trend is visible in the sandstone beds as they are traced from east

to west?

f. The shale beds become thinner as they are traced from west to east. How

can you explain this pattern?

g. Where and what is the most likely source of the siliciclastic sediments

that are now preserved as rock? What was the direction of transport?

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Figure 8 (page 1 of 2)

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Figure 8 (cont.)