EXAMPLE!!!!!

Name:_____________________________ March 25, 1999

 

EPS101, SECTION 005, SECOND HOURLY EXAMINATION

 

I hope you had a good spring break.  Please note that the format of this exam is somewhat different from the first hour examination.  There are four (4) parts to this examination, which is printed on both sides.  The total number of points for each section is given in parentheses.  Should you require clarification on a particular question, please do not hesitate to ask!  In Part IV, there are several extra credit questions at the end. The exam will be returned next Tuesday.

 

Part I.  Multiple choice. (There is only one correct answer; 30 points.)

 

1.         The two features in a sediment or sedimentary rock most responsible for layering or stratification are

a. color and organic material abundance variations.

b. grain size (sorting) and grain composition variations.

c. shape and color variations.

d. fossil content and color variations.

 

2.  Marbles are metamorphic rocks consisting essentially of carbonate minerals.  What is the most logical protolith (parent rock)?

a. rhyolite

b. halite (rock salt)

c.  limestone

d.  basalt

e.  peridotite

 

3.         You are mapping a large area in the western United States and discover laterally continuous (traced over considerable distances) , nearly constant thickness ( of several meters) layers of pure quartz sandstone alternating with layers of shale .  What kind of depositional environment does this setting most likely represent?  

a.    alluvial fan

b.    continental dune

c.    glacial lake

d.    beach/nearshore marine, moderate latitudes

e.    none of the above

 

4. The geologic eras of the Phanerozoic eon are, from oldest to youngest,

a. Paleozoic, Cenozoic, Mesozoic

b. Paleozoic, Mesozoic, Cenozoic

c. Cenozoic, Mesozoic, Paleozoic

d. Proterozoic, Mesozoic, Cenozoic

 

5. What is the generally accepted age for the Precambrian/Cambrian boundary?

a. 65 million years

b. less than 6,600 YEARS

c. about 535 to 545 million years  (+/- a few million years)

d. over 3.4 billion years

 

6. How do the probable depositional environments in which a  lithic-arenite and a fine-grained, quartz-rich sandstone compare?

a    the source of the lithic arenite was a coarse-grained intrusive igneous rock, like a granite from a  batholith in an eroded magmatic arc.

b.   the quartz-rich sandstone was derived from a rugged, actively eroding terrane whose bedrock was  entirely amphibolite.

c.   the lithic arenite was deposited close to a magmatic arc, where fine-grained volcanic rocks were   actively being eroded and relatively rapidly  transported. 

d.    the quartz-rich sandstone was deposited in an alluvial fan environment,  at the front of an actively  eroding mountain range in a very arid climate.

 

7.         A disconformity is identified when there is a significant break in time between layers of parallel strata.  Which of the following probably occurred to produce a  disconformity.

a. strata were exposed for a period of time, without disruption, prior to deposition of younger strata. 

b. strata were tipped upside down prior to deposition of younger strata. 

c. the provenance of the sedimentary rock below the unconformity must have been proximal or else grains in that rock would not be angular

d. pre-existing strata must have been deeply incised prior to deposition of younger strata. 

e. the beds below the disconformity were deformed prior to deposition of overlying strata. 

 

8.   The impressive/damaging flooding of the Mississippi River several years ago represented major transportation of sediment!  If you were to go back to the majority of the flooded areas that are now drained, what kind of sediment would you most likely find? 

a.         cobbles and pebbles

b.         quartz sand

c.         mud and silt, deposited in what are referred to as “overbank” areas.

d.         carbonate mud

e.         pyroxene sand

 

9. In comparison to sedimentary rocks, igneous and metamorphic rocks are referred to as "crystalline" rocks because

a.       they are all Precambrian in age and have thus  been metamorphosed, to some degree.

b.       unlike sedimentary rocks, igneous and metamorphic rocks consist of three-dimensionally interlocking mineral grains, this feature serves to hold the rock together.

c.       igneous and metamorphic rocks contain crystals.

d.       all of the above.

 

10.       Diagenesis

a.         involves the sum of processes by which sediment is converted or lithified to sedimentary rock at relatively low temperatures and pressures near the surface of our planet.

b.         is the process where sediment grains are cemented to form a detrital sedimentary rock.

c.         refers to the breakdown of organic material in sediments, beginning at temperatures greater than 300oC.

d.         refers to the formation of micas from clay minerals in mudstones at temperatures exceeding 300oC.

e.         none of the above.

 

11.       Primary sedimentary "structures" are important in determining the "up direction" in sequences of sedimentary rocks.  Which of the following is correct?

a.         In a shale-sandstone-limestone sequence, the limestone bed is always the oldest.

b.         Crossbeds in sandstones deposited in any environment are convex upwards.

c.         Burrowing organisms enter the moist, juicy, organic rich sediment from the bottom of the beds and work their way to the top.

d.         In graded beds, the coarsest sediment is usually at the base of the bed, resulting in a fining upward sequence.

e.      The law of superposition states that older strata are deposited on younger strata

f. All other factors remaining constant, the greater sediment is transported from the source, the more angular the grains.

 

12.  Regional metamorphism occurs

a.  when rocks are heated due to the local effects of magma injection

b. when rocks are buried to sufficiently great depths, usually in response to tensional stresses.

c.  when rocks are buried to sufficiently great depths, typically in association with mountain building and contractional processes, which facilitate crustal thickening.

d.  when rocks are buried to temperature and pressure conditions above the liquidus.

 

13.       What would you find deposited in a continental shelf environment at low latitudes?

a. inorganic and organic limestones

b. arkoses

c. coal

d. chert, consisting of shell fragments of radiolaria

e.  glacial varves

f.        none of the above.

 

14. A principle assumption in isotopic age determinations is:

a. the rate of decay of an unstable parent isotope remains constant over time.

b. the generation of daughter products begins as soon as the mineral is formed.

c. stable daughter products diffuse readily out of the crystals holding unstable parents.

d. atoms of unstable isotopes are characterized by an unequal number of electrons and protons.

e. virtually all minerals are capable of containing unstable isotopes of particular atoms.

 

15.       When solid rock is exposed at the earth's surface, it is subjected to which of the following processes which will result in sedimentary material.

a.  metamorphism

b.  lithification

c.  subduction

d.  weathering

e.  cementation

Part II.  Relative and Absolute Geologic Time

The sketch on the ATTACHED colored sheet is a vertical cross-section through a part of Earth's continental crust with a rather interesting (depends on your point of view) geologic history.  All important geologic materials, or sequences of rock types are labeled.  Below, please place in succinct outline form the events producing this section of crust.  NOTE: I=intrusive igneous rocks; V = volcanic rocks; M=metamorphic rocks; S=sedimentary rocks; U = unconformities!!! Also, answer the questions on the next page related to the materials in the cross section.

Chronology, Youngest Event    (15 points)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Oldest Event

PLEASE NOTE THAT II GIVE PARTIAL CREDIT (LOTS) FOR THIS QUESTION, SO IF YOU CANNOT DETERMINE THE RELATIVE HISTORY OF ALL MATERIALS, AT LEAST DO SO FOR AS MANY AS YOU ARE CONFIDENT!
Questions:  (20 points)

 

1. Rock M1 is a metamorphosed granite (now a gneiss) and has been dated using U-Pb zircon methods as 1400 +/-22.2 million years; what geologic eon does it belong to?

__________________________________________________________

 

2. Intrusion I3 has been dated using the K-Ar method as 135 +/-5.5 million years; what geologic era does it belong to?

__________________________________________________________

 

3. Rocks S1,S2,S3, and S4 are respectively conglomerate, coarse-grained arkose, thin shales, fine-grained quartz-rich sandstone, additional thin shales, and limestone. From a standpoint of depositional environments, what took place in this area during the time of deposition of these rocks ?___________________________ _______________________  __________________ ____________________________________________________________________________How might this have occurred?   _______________________________________________.

 

4. In the right part of your cross-section, sedimentary rocks S15 to S19 have been tilted and are now overlain by sedimentary rocks S1 to S4.  What kind of unconformity is this?  __________ _______________________________ Why must sedimentary rocks S1 to S4 be younger than sedimentary rocks S15 to S19? ________________________________________________.

 

5.  Volcanic rock V4 has been dated using the U-Pb zircon method as 55+/- 4 million years.  This determination essentially dates the time the rock cooled below about 800oC.  To what geologic eon does the rock belong?  ______________________ Biotite phenocrysts from the rock give a K-Ar age date of about 14 million years.  What geologic era is represented by this date?  _____________________.  Why is there a  discrepancy in age determinations? ______________________________________________________________.

 

8.      Metamorphic rock M3 is a quartzite.  What is one of the most abundant minerals in the rock?  ______________ _____    What is a logical protolith (parent) for the rock and where would it have formed?   _______________________________________________________.

 

9.      Sedimentary rock S1 is a conglomerate, and contains cobbles of slate, phyllite, garnet schist, and mica-bearing gneisses.  What kinds of rocks are these cobbles? _______ ____________ ____________ ___________________________________________.  How would they have originated ? ______________ __________________________________.

 

 

Part III.  Matching.  (20 points total)

 

 1.        sorting _______                      a.         annual lacustrine deposition

 2.        varves_______                       b.         detritus in a super-arid environment

 3.        compaction______                 c.         decay of unstable nucleii in atoms

4.         porosity     _______                d.         younger material deposited on older material

5.        relative age_______                 e.         limestone spheres, pptd in shallow water

 6.        radioactivity______                 f.          rock salt, future home of low level radioactive waste

 7.        continental shelf_______        g.        most voluminous sedimentary rock

 8.        halite    _______                     h.         inert gas, able to move through a crystal at high T

 9.        natural gas  _______              i.          build up of detritus, mountain front

10.       shale/mudstone _____           j.          amount of void space

11.       superposition  _______          k.         order of geologic events and rock formation

12.       alluvial fan            _______     l.          zone of leaching

13.       paleontology_______              m.        site of major carbonate rock formation

14.       clay           _______                 n.         any sediment rich in organic material

15.       Triassic_____                        o.         another word for a sandstone

16.       source rock     ________        p.         period in the Mesozoic

17.       argon_______                         q.         methane

18.       round feldspars ______          r.          study of ancient life and its evolution

19.       arenite______                         s.         relatively uniform sediment grain size

20.       ooid    ______                         t.          decrease in void space, decrease in water

 

 

Part IV.  (Relatively) short answer questions. (35 points, therefore each question is worth two or three points!).

 

 1.        What is the term given to particle sizes of about a mm to at most a few mm for detrital sedimentary material?_________________________________________ _____________________________ . 

 

2.         You find a fine-grained sedimentary rock consisting of extremely fine (i.e. mud size) “grains” of calcium carbonate, and nothing else.  What is the rock?    Where would it most likely have formed and why?  _____________ ___________________________________________ ______________________ _____________________________________ ________________  ____________________________________________________________________________

 

3.         Name three kinds of sandstone _________________________________

 

4. What is the geologic time scale? _______________________________________ ____________________________________________________________________________________________________________________________.

 

 

5.        Why is hydrocarbon formation considered part of sedimentary diagenesis? ___________________________________________________________________________________ _________________________________________________________________ ______________________________________________________________

 

 6.        Many fine grained muds are deposited at a rate of about 1 centimeter per 1000 years.  At this rate, how long would it take to accumulate a stratigraphic sequence that is half a kilometer thick.  (One kilometer equals a thousand meters.  There are 100 centimeters in a meter.) ________________________________________________________________

 

7.         How do clastic (detrital) sedimentary rocks differ from chemical and biochemical sedimentary rocks? __________________________________________________________ ________________________________________________________________________.

 

8.         Sketch a pressure vs. temperature graph and indicate where the realm of sedimentation and diagenesis "fits" with   respect to metamorphism.  Also, show examples of  melting reaction for dry earth materials and wet earth materials.

 

 

 

 

 

 

 

 

 

 

 

 

 

9.         What is the most important kind of metamorphic reaction?  ______________________________________.  Of what importance is this kind of reaction in influencing the melting behavior of rocks? _________________ ___________________________________________________________________________?

10.      The rock quartzite is the metamorphic equivalent of a sandstone.   Suppose you start with a sandstone formed in an environment where chemical weathering has been intense.  What will the mineralogy of the sandstone be?  _______________________________.  What do you (in an educated fashion) guess will happen to this rock as progressive metamorphism occurs and why? _______________________________ ___________________  ____________ _______   ____________________  _________________________ __________________________________________________.

 

11.       In recent years, geoscientists have been exceedingly interested in learning about past changes in Earth's climate, for a range of reasons.  To investigate such changes in climate, what kind of sedimentary rocks might you choose to study, and why? ___________________________________ _________ _________________________ ________________________________________________________________________.

 

12.     Sketch the likely distribution of sediment  types surrounding a relatively small, circular to sub-circular rapidly uplifting mountainous region consisting principally of granites and  gneisses. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

How would this differ if the “source” region consisted only of limestones? __________________________________________________________________________ _______________________________________________________________________.

 

 

 

13. How can cross-cutting age relations be used to deduce or understand relative geologic ages?__________________________________________________________________________ ____________________.

 

 

 

 

 

 

 

Extra Credit Questions: (five points each) (note that you are not penalized for wrong answers)

14. In the context of geologic time, why is the principle of uniformitarianism important to geologists?________________________ __________________________________

________________________________________________________________.

 

 

 

15.   What is the difference between regional metamorphism and contact metamorphism?  _______________ __________________________________________________.

 

 

 

 

 

16   In isotopic age determinations, what fundamental process is utilized to quantitatively estimate the age of a  particular mineral in which the decay of an unstable isotope of some element, like potassium-40, over time has occurred? __________________________________________________________________________________ ______________________________________________________________. 

 

 

 

 

 

 

 

17. Draw a cross section of an angular unconformity and label the unconformity and the different kinds of rocks involved. 

 

 

 

 

 

 

 

 

 

 

 

18. At the crest of the Sandia Mountains, upper Paleozoic sedimentary rocks rest on top of the Precambrian Sandia Granite.  The age of intrusion of the granite is about 1.4 billion years.  What is this relationship called? 

______________ ____________________________________ __________________Explain, succinctly, how it may have developed___________________________ ____ _____________