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{\*\listoverridetable{\listoverride\listid1138494988\listoverridecount0\ls1}{\listoverride\listid265769797\listoverridecount0\ls2}{\listoverride\listid1082288984\listoverridecount0\ls3}}{\info{\title 186-233A Earth & Life History}{\author GRAEME WALLACE}
{\operator Earth & Planetary Sciences}{\creatim\yr2001\mo10\dy22\hr13\min3}{\revtim\yr2001\mo10\dy22\hr13\min4}{\version3}{\edmins1}{\nofpages4}{\nofwords1470}{\nofchars8381}{\*\company  }{\nofcharsws10292}{\vern113}}
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\widctlpar\adjustright \fs20\cgrid {\f1\fs24 186-233A Earth & Life History
\par 
\par }{\i\f1\fs24 Mid-term examination given on October 17, 2001.
\par }{\f1\fs24 
\par 1. Match each statement to an appropriate rock type:
\par 
\par {\pntext\pard\plain\f1\cgrid \hich\af1\dbch\af0\loch\f1 a)\tab}}\pard \fi-360\li360\widctlpar\jclisttab\tx360{\*\pn \pnlvlbody\ilvl0\ls1\pnrnot0\pnlcltr\pnstart1\pnindent360\pnhang{\pntxta )}}\ls1\adjustright {\f1\fs24 Banded iron formations (BIFs)\'85
appeared during the Archean and early Proterozoic eons. Its presence suggests that the Earth's early atmosphere contained hardly any oxygen while parts of the oceans may have been oxygenated.
\par {\pntext\pard\plain\f1\cgrid \hich\af1\dbch\af0\loch\f1 b)\tab}}\pard \fi-360\li360\widctlpar\jclisttab\tx360{\*\pn \pnlvlbody\ilvl0\ls1\pnrnot0\pnlcltr\pnstart1\pnindent360\pnhang{\pntxta )}}\ls1\adjustright {\f1\fs24 Uraninite-bearing conglomerates\'85
 limited to the Archean, and could not have formed if rivers carrying such pebbles had been in contact with an oxygenated atmosphere.
\par {\pntext\pard\plain\f1\cgrid \hich\af1\dbch\af0\loch\f1 c)\tab}}\pard \fi-360\li360\widctlpar\jclisttab\tx360{\*\pn \pnlvlbody\ilvl0\ls1\pnrnot0\pnlcltr\pnstart1\pnindent360\pnhang{\pntxta )}}\ls1\adjustright {\f1\fs24 red beds\'85
 become abundant during the early to middle part of the Proterozoic eon and it contains iron in a form which suggests that the level of atmospheric oxygen had risen significantly since Archean time.
\par {\pntext\pard\plain\f1\cgrid \hich\af1\dbch\af0\loch\f1 d)\tab}}\pard \fi-360\li360\widctlpar\jclisttab\tx360{\*\pn \pnlvlbody\ilvl0\ls1\pnrnot0\pnlcltr\pnstart1\pnindent360\pnhang{\pntxta )}}\ls1\adjustright {\f1\fs24 Trypania-bearing shales\'85
 typically form in calm offshore waters but this particular example contains a euharyotic fossil of Precambrian age. (}{\i\f1\fs24 Stromatolitic limestones contain layered trace fossils and cellular 
filaments from cyanobacterial mats, which are prokaryotic organisms}{\f1\fs24 .)
\par {\pntext\pard\plain\f1\cgrid \hich\af1\dbch\af0\loch\f1 e)\tab}}\pard \fi-360\li360\widctlpar\jclisttab\tx360{\*\pn \pnlvlbody\ilvl0\ls1\pnrnot0\pnlcltr\pnstart1\pnindent360\pnhang{\pntxta )}}\ls1\adjustright {\f1\fs24 Tillites\'85
occur sporadically throughout Earth's history and formed when glaciers carried rocks from continents an dropped them into deep waters.
\par {\pntext\pard\plain\f1\cgrid \hich\af1\dbch\af0\loch\f1 f)\tab}}\pard \fi-360\li360\widctlpar\jclisttab\tx360{\*\pn \pnlvlbody\ilvl0\ls1\pnrnot0\pnlcltr\pnstart1\pnindent360\pnhang{\pntxta )}}\ls1\adjustright {\f1\fs24 Stromatolitic limestone (nearshore
 sandstones was also accepted) contains trace fossils, and is re more likely to be found in a Precambrian thrust-and-fold belt than in an Archean greenstone belt.
\par {\pntext\pard\plain\f1\cgrid \hich\af1\dbch\af0\loch\f1 g)\tab}}\pard \fi-360\li360\widctlpar\jclisttab\tx360{\*\pn \pnlvlbody\ilvl0\ls1\pnrnot0\pnlcltr\pnstart1\pnindent360\pnhang{\pntxta )}}\ls1\adjustright {\f1\fs24 Volcanic ash beds\'85
 likely to include minerals that contain radioactive elements and their daughter elements which will yield a reliable absolute age. }{\i\f1\fs24 (Only partial marks were given for uraninite-bearing conglomerates\'85
 Although uraninite contains uranium and, therefore, can be dated, the uraninite is eroded from sources that could be of several
 different ages. Dating uraninite pebbles gives the age of their igneous source(s), rather than the age of its deposition as part of the conglomerate.)}{\f1\fs24 
\par {\pntext\pard\plain\f1\cgrid \hich\af1\dbch\af0\loch\f1 h)\tab}}\pard \fi-360\li360\widctlpar\jclisttab\tx360{\*\pn \pnlvlbody\ilvl0\ls1\pnrnot0\pnlcltr\pnstart1\pnindent360\pnhang{\pntxta )}}\ls1\adjustright {\f1\fs24 Nearshore sandstone\'85
 is common in the stratigraphic record after Archean time, but it rarely contains well-preserved fossils.
\par }\pard \widctlpar\adjustright {\f1\fs24 
\par 2. Supercontinents existed at various times throughout Earth's history.
\par {\pntext\pard\plain\f1\cgrid \hich\af1\dbch\af0\loch\f1 a)\tab}}\pard \fi-360\li360\widctlpar\jclisttab\tx360{\*\pn \pnlvlbody\ilvl0\ls2\pnrnot0\pnlcltr\pnstart1\pnindent360\pnhang{\pntxta )}}\ls2\adjustright {\f1\fs24 
Full marks given for a total of any three of the following lines of evidence used by earth scientists to reconstruct their position, shape and size:
\par {\pntext\pard\plain\cgrid \hich\af0\dbch\af0\loch\f0 -\tab}}\pard \fi-360\li360\widctlpar\jclisttab\tx360{\*\pn \pnlvlblt\ilvl0\ls3\pnrnot0\pnstart1\pnindent360\pnhang{\pntxtb -}}\ls3\adjustright {\f1\fs24 t
he paleomagnetism preserved in certain igneous and sedimentary rocks provides information about the latitudes at which they acquired their magnetism in the past
\par {\pntext\pard\plain\cgrid \hich\af0\dbch\af0\loch\f0 -\tab}}\pard \fi-360\li360\widctlpar\jclisttab\tx360{\*\pn \pnlvlblt\ilvl0\ls3\pnrnot0\pnstart1\pnindent360\pnhang{\pntxtb -}}\ls3\adjustright {\f1\fs24 
chains of mountain building zones (orogenies) of similar age found on continents that are now separated may indicate that they were once sutured into a single larger continent.
\par {\pntext\pard\plain\cgrid \hich\af0\dbch\af0\loch\f0 -\tab}}\pard \fi-360\li360\widctlpar\jclisttab\tx360{\*\pn \pnlvlblt\ilvl0\ls3\pnrnot0\pnstart1\pnindent360\pnhang{\pntxtb -}}\ls3\adjustright {\f1\fs24 
similarities in the types of fossils on continents that are now separated
\par {\pntext\pard\plain\cgrid \hich\af0\dbch\af0\loch\f0 -\tab}}\pard \fi-360\li360\widctlpar\jclisttab\tx360{\*\pn \pnlvlblt\ilvl0\ls3\pnrnot0\pnstart1\pnindent360\pnhang{\pntxtb -}}\ls3\adjustright {\f1\fs24 
similarities in the sequences of sedimentary rocks found near the edge of ancient cratons
\par {\pntext\pard\plain\cgrid \hich\af0\dbch\af0\loch\f0 -\tab}}\pard \fi-360\li360\widctlpar\jclisttab\tx360{\*\pn \pnlvlblt\ilvl0\ls3\pnrnot0\pnstart1\pnindent360\pnhang{\pntxtb -}}\ls3\adjustright {\f1\fs24 
 radiometric dating of igneous rocks (volcanic lavas or ash beds) found below and above sedimentary rocks, can prove the similar age of sedimentary rocks that show similar sequences on separate continents (and might have been derived from a mountain belt
 formed when these continents were sutured into a supercontinent)
\par }\pard \widctlpar\adjustright {\f1\fs24 
\par {\pntext\pard\plain\f1\cgrid \hich\af1\dbch\af0\loch\f1 b)\tab}}\pard \fi-360\li360\widctlpar\jclisttab\tx360{\*\pn \pnlvlbody\ilvl0\ls2\pnrnot0\pnlcltr\pnstart1\pnindent360\pnhang{\pntxta )}}\ls2\adjustright {\f1\fs24 
Any two of the three following factors:
\par {\pntext\pard\plain\cgrid \hich\af0\dbch\af0\loch\f0 -\tab}}\pard \fi-360\li360\widctlpar\jclisttab\tx360{\*\pn \pnlvlblt\ilvl0\ls3\pnrnot0\pnstart1\pnindent360\pnhang{\pntxtb -}}\ls3\adjustright {\f1\fs24 
reverse greenhouse effect caused by a drop in the level of atmospheric CO2 might drop down. A suggested cause is an increased productivity of 
organic matter coinciding with its fast burial rate in sediments (preventing the decomposition of dead organic matter to form atmospheric CO2);
\par {\pntext\pard\plain\cgrid \hich\af0\dbch\af0\loch\f0 -\tab}}\pard \fi-360\li360\widctlpar\jclisttab\tx360{\*\pn \pnlvlblt\ilvl0\ls3\pnrnot0\pnstart1\pnindent360\pnhang{\pntxtb -}}\ls3\adjustright {\f1\fs24 
movement of large landmasses towards higher latitudes, closer to either the North or South pole\'85 this tends to promote the accumulation of snow on land which reflects back solar radiation and further cools the climate;
\par {\pntext\pard\plain\cgrid \hich\af0\dbch\af0\loch\f0 -\tab}}\pard \fi-360\li360\widctlpar\jclisttab\tx360{\*\pn \pnlvlblt\ilvl0\ls3\pnrnot0\pnstart1\pnindent360\pnhang{\pntxtb -}}\ls3\adjustright {\f1\fs24 
cycles in the intensity of solar radiation reaching the Earth.
\par }\pard \widctlpar\adjustright {\f1\fs24 
\par {\pntext\pard\plain\i\f1\cgrid \hich\af1\dbch\af0\loch\f1 c)\tab}}\pard \fi-360\li360\widctlpar\jclisttab\tx360{\*\pn \pnlvlbody\ilvl0\ls2\pnrnot0\pnlcltr\pnstart1\pnindent360\pnhang{\pntxta )}}\ls2\adjustright {\i\f1\fs24 Dating }{\f1\fs24 
 evidence (such as tillites or striations on continental rocks) is not a }{\i\f1\fs24 confirmation}{\f1\fs24 
 that an ice age took place. To properly answer the question, one had to identify a type of isotopic signal which is directly influenced by processes taking place during an ice age.  The best answer was the proportions of }{\i\f1\fs24 stable}{\f1\fs24 
 isotopes of }{\i\f1\fs24 oxygen}{\f1\fs24  (O}{\f1\fs24\super 18}{\f1\fs24  /O}{\f1\fs24\super 16}{\f1\fs24 ) which can be analyzed from }{\i\f1\fs24 fossil marine CaCO}{\i\f1\fs24\sub 3}{\f1\fs24\sub  }{\i\f1\fs24 shells}{\f1\fs24 
 (such as brachiopods). The reason? Marine shells record proportions of oxygen isotopes left in the sea during the growth of ice caps.  Ice caps tend to draw O}{\f1\fs24\super 16}{\f1\fs24  in greater proportion
s than its average concentration in seawater because this lighter isotope evaporates faster from tropical waters than O}{\f1\fs24\super 18}{\f1\fs24  and doesn't condense (and rain down) as fast as O}{\f1\fs24\super 18}{\f1\fs24 
. Marks were also given for answering  the stable isotopes of carbon, i.e. C }{\f1\fs24\super 13}{\f1\fs24 /C }{\f1\fs24\super 12}{\f1\fs24  from marine CaCO}{\f1\fs24\sub 3}{\f1\fs24 
 shells  These  isotopes record a possible side effect of an ice age, i.e. the increased weathering of organic matter (which has lower C }{\f1\fs24\super 13}{\f1\fs24 /C }{\f1\fs24\super 12 }{\f1\fs24 proportions than average 
C) that was buried in sediments earlier, but is exposed to weathering and decomposition because sea level dropped while ice caps grew.
\par }\pard \widctlpar\adjustright {\i\f1\fs24 
\par }{\f1\fs24 3.a)  Characteristics of greenstone belts in Abitibi:
\par {\pntext\pard\plain\cgrid \hich\af0\dbch\af0\loch\f0 -\tab}}\pard \fi-360\li360\widctlpar\jclisttab\tx360{\*\pn \pnlvlblt\ilvl0\ls3\pnrnot0\pnstart1\pnindent360\pnhang{\pntxtb -}}\ls3\adjustright {\f1\fs24 i) graywackes and shales
\par {\pntext\pard\plain\cgrid \hich\af0\dbch\af0\loch\f0 -\tab}}\pard \fi-360\li360\widctlpar\jclisttab\tx360{\*\pn \pnlvlblt\ilvl0\ls3\pnrnot0\pnstart1\pnindent360\pnhang{\pntxtb -}}\ls3\adjustright {\f1\fs24 iii) belts of relatively small width
\par {\pntext\pard\plain\cgrid \hich\af0\dbch\af0\loch\f0 -\tab}}\pard \fi-360\li360\widctlpar\jclisttab\tx360{\*\pn \pnlvlblt\ilvl0\ls3\pnrnot0\pnstart1\pnindent360\pnhang{\pntxtb -}}\ls3\adjustright {\f1\fs24 
iv) altered volcanic rocks that were originally ultramafic in composition
\par {\pntext\pard\plain\cgrid \hich\af0\dbch\af0\loch\f0 -\tab}}\pard \fi-360\li360\widctlpar\jclisttab\tx360{\*\pn \pnlvlblt\ilvl0\ls3\pnrnot0\pnstart1\pnindent360\pnhang{\pntxtb -}}\ls3\adjustright {\f1\fs24 
vi) association with gold and other metallic ore deposits that are mined today
\par {\pntext\pard\plain\cgrid \hich\af0\dbch\af0\loch\f0 -\tab}}\pard \fi-360\li360\widctlpar\jclisttab\tx360{\*\pn \pnlvlblt\ilvl0\ls3\pnrnot0\pnstart1\pnindent360\pnhang{\pntxtb -}}\ls3\adjustright {\f1\fs24 
vii) found along the margin of small felsic cratons
\par }\pard \widctlpar{\*\pn \pnlvlcont\ilvl0\ls0\pnrnot0\pndec }\adjustright {\f1\fs24 
\par }\pard \li360\widctlpar{\*\pn \pnlvlcont\ilvl0\ls0\pnrnot0\pndec }\adjustright {\f1\fs24 Characteristics of thrust-and-fold belts:
\par {\pntext\pard\plain\cgrid \hich\af0\dbch\af0\loch\f0 -\tab}}\pard \fi-360\li360\widctlpar\jclisttab\tx360{\*\pn \pnlvlblt\ilvl0\ls3\pnrnot0\pnstart1\pnindent360\pnhang{\pntxtb -}}\ls3\adjustright {\f1\fs24 
ii) marine sedimentary rocks deposited in shallow-water environments
\par {\pntext\pard\plain\cgrid \hich\af0\dbch\af0\loch\f0 -\tab}}\pard \fi-360\li360\widctlpar\jclisttab\tx360{\*\pn \pnlvlblt\ilvl0\ls3\pnrnot0\pnstart1\pnindent360\pnhang{\pntxtb -}}\ls3\adjustright {\f1\fs24 
v) wide igneous belt bordered by a metamorphic zone
\par {\pntext\pard\plain\cgrid \hich\af0\dbch\af0\loch\f0 -\tab}}\pard \fi-360\li360\widctlpar\jclisttab\tx360{\*\pn \pnlvlblt\ilvl0\ls3\pnrnot0\pnstart1\pnindent360\pnhang{\pntxtb -}}\ls3\adjustright {\f1\fs24 
vii) broad sheets of sedimentary rocks thrust over large cratons
\par }\pard \widctlpar\adjustright {\f1\fs24 
\par b) The high heat flow 
during the Archean eon kept the crust relatively thin and mostly mafic, and convection during the mantle was more intense than later on.  As a result, cratons were kept small because they rifted (i.e. broke up) more often. The subduction of oceanic crust 
l
ed to collisions of small cratons between which were trapped narrow belts of ultramafic volcanic rocks and sedimentary rocks. Sedimentary rocks like graywackes and shales accumulated because material eroded from continental rocks was not subjected to the 
intense wave action. In contrast, thrust-and fold belts contain thick sheets of sandstones and limestones belts which formed on broad shallow-water continental platforms but were thrust and folded when they were pushed on the edge of larger cratons.

\par 
\par 4. a) Ediacaran (same as Precambrian Vendian fauna) fauna shows:
\par {\pntext\pard\plain\f1\cgrid \hich\af1\dbch\af0\loch\f1 i)\tab}}\pard \fi-360\li360\widctlpar\jclisttab\tx360{\*\pn \pnlvlbody\ilvl0\ls1\pnrnot0\pnlcltr\pnstart1\pnindent360\pnhang{\pntxta )}}\ls1\adjustright {\f1\fs24 absence of mineralized hard parts

\par }\pard \widctlpar\adjustright {\f1\fs24 ii) preservation in sandstone
\par 
\par The earliest Paleozoic fauna (small shelly fossils, Tommotian and Burgess Shale faunas) show:
\par iii) body plans including limbs and mouths (not found in Ediacaran fossils)
\par iv) evidence of complex burrowing behaviour (Ediacaran-age sediments show far less burrows and they tend to be simple resting traces rather than complex feeding traces)
\par v) preservation in limestone and shales
\par vi) scarcity of stromatolitic limestone (presumably because grazers can now feed on cyanobacterial mats)
\par vii) diverse ecological roles
\par viii) mineralized hard parts that covered most of the body
\par 
\par b) You were expected to justify your answer by using what you had just sorted out in two categories\'85
 You could illustrate progress from simpler to more complex forms of life by pointing out the decline of fossils left by prokaryotes (vi) , the increasingly complex behaviour of animals larger than unicellular eukaryotes s
uggested by (iv) and (vii), the evolution of complex body plans including mouth and limbs (iii) and mineralized hard parts (viii).  The only element suggesting that the Ediacaran fauna produced innovations that left no descendants is the strange preservat
ion of Ediacaran fossils in sandstones rather than in limestone or shale. This was interpreted by some as a sign that Ediacaran animals were covered by a tough membrane unlike the soft-bodied organisms preserved in the middle Cambrian Burgess Shale.

\par 
\par 5. a)  an unconformity is evidence of erosion or non-deposition between two sedimentary formations. Evidence of an unconformity therefore appears at  the }{\i\f1\fs24 contact between formations.}{\f1\fs24 
 An unconformity could be detected within a sedimentary rock formation if t
he formation clearly contained an incomplete succession of fossils from its bottom to its top. The best answer was "between I and II in the first section and/or "between I and IV" in the second section.  The most convincing evidence was the change in the 
o
rientation of layering (not horizontal in formation I, but horizontal in formation IV) which suggested that formation I was deposited, then tilted (by movement along a fault during extension or compression of the crust) before sedimentation started again.
 
\par b) Formation VI crosscuts formations I to VI because it is an igneous intrusion (you are also told that it is datable and unfossiliferous, and the symbol does not correspond to any of the sedimentary rock types given in the legend). An intrusion can only 
penetrate through formations that are already laid down, so its age is }{\i\f1\fs24 younger}{\f1\fs24  than any of the formations that are crosscut.
\par 
\par c) Formations II and IV both contain a datable formation, labelled III at both sites because its age is identical at both location
s.  Therefore, formations II and IV could be about the same age (at least right below and above the marker bed III). They consist of different rock types, however, which indicate different environment of depositions. Sandstone generally accumulates in nea
rshore marine environments with energetic wave action while shale accumulated in calmer, deeper water. Their different fossils probably also have different environmental requirements during life.
\par 
\par {\pntext\pard\plain\f1\cgrid \hich\af1\dbch\af0\loch\f1 d)\tab}}\pard \fi-360\li360\widctlpar\jclisttab\tx360{\*\pn \pnlvlbody\ilvl0\ls2\pnrnot0\pnlcltr\pnstart1\pnindent360\pnhang{\pntxta )}}\ls2\adjustright {\f1\fs24 
A correlation is an interpretation of some kind of equivalence established between rocks that are geographically separated. }{\i\f1\fs24 Lithologic correlation}{\f1\fs24 
 could be done here on the basis of similar rock type (limestone); this reflects an interpretation that these two limestones, now found at widely separated locations, accumulated under similar environmental conditions. }{\i\f1\fs24 
Stratigraphic or temporal correlation}{\f1\fs24 , done on the basis of similar index fossils,}{\i\f1\fs24  }{\f1\fs24  is an interpretation that both limestones were deposited during the same interval of time.   Nautiloids, being swimming
 predators rather than bottom-dwelling filter-feeders, are likely to be good index fossils, i.e. they consist of genera and species that appeared for relatively short intervals in the fossil record.
\par }\pard \widctlpar\adjustright {\f1\fs24 
\par }}