Name:
Lab day:
Answers
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Geology 1023 – Lab #8, Winter 2014
Platforms and Paleozoic life-forms
1. Schematic geologic cross-sections A-B and C-D (shown below) cross the edge of the
continental platform in western and eastern North America, respectively (Fig. 1).
a) Why do Cambrian sedimentary rocks get thicker to the southwest and southeast,
respectively, on these sections?
They are “basins” at the edge of the craton, sediment
accumulated in the basin and then transgressed.
b) Do these sections indicate transgression or regression during the Cambrian? Give the
evidence for your answer.
Transgression.
Fining upward sequences/younger rocks onlap.
c) Why are there no Cambrian sedimentary rocks on the Canadian Shield?
Shield was above sea-level during the Cambrian and
provided the sediment for the basins.
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Figure 1. Map of North America showing positions of cross sections A-B and C-D, the edge of
the Canadian Shield and the locations of the Grand Canyon and East Central Ontario map areas.
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2. Use the “Geological Map of the Grand Canyon National Park, Arizona” to answer the
following questions:
a) Other than age, what is the major difference between Older Precambrian rocks and
Younger Precambrian rocks?
Grade of metamorphism
b) What is the approximate total thickness of Cambrian rocks in the Grand Canyon area?
(Hint: look at the cross-sections.)
~ 1000’ (305 m)
c) What type of contact is present between the Cambrian Muav Limestone and the
Devonian Temple Butte Limestone?
A disconformity
d) Why are no Ordovician or Silurian rocks present in this area?
If deposited they were eroded away, probably never
deposited.
e) Based on cross-section D-D', what type of fault is the Butte Fault?
Reverse dip-slip (not thrust, too steep).
f) The Colorado River began to erode the Grand Canyon only 7 million years ago.
Calculate the average rate of erosion to form the canyon on cross-section A-A'.
5000’/7,000,000 yr = 0.0007ft/yr (Accept 6.5–7.5x10-4 ft/yr;
7.8–9.0 x 10-3 in/yr; 2.0–2.3 x 10-2cm/yr)
Paleozoic Life
Animals first evolved the capacity to secrete a hard skeleton (or carapace or shell) at the end of
the Precambrian. The possession of “hard parts” increases the likelihood of preservation by many
orders of magnitude and therefore Phanerozoic fossils are far more abundant than Precambrian
ones. This phenomenon gives the impression of an “explosion” of life at the beginning of the
Cambrian. The most important phyla for the Paleozoic include Arthropoda (trilobites),
Hemichordata (graptolites), Brachiopoda, Porifera (sponges), and Cnidaria (corals). You’ve
already seen some Paleozoic sponges and corals in the last lab.
Phylum Arthropoda (Precambrian to Recent): Arthropods are invertebrates characterised by a
segmented body (often with a 3-fold division), with paired and jointed appendages. Most forms
shed (moult) the exoskeleton during development. The exoskeleton is calcareous or chitinous
Platforms and Paleozoic life-forms — Winter 2014
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(organic). Modern arthropods include crabs, lobsters, shrimps, insects, spiders, ticks, barnacles
and scorpions. Insects and spiders are rare as fossils because they do not live in a depositional
environment and do not have a mineralised skeleton.
Trilobites are an extinct group of arthropods that were widespread in the Paleozoic (Cambrian –
Permian). They dominated the Early Paleozoic seas. Their skeletons are segmented, and divided
into three regions from front to back and from side-to-side. Trilobites are important index fossils
in the lower Paleozoic, particularly the Cambrian.
3. Examine the trilobite specimens and models at the back of the lab. Use the handout on the
morphology of trilobites and sketch one of the trilobite specimens provided. Label at least
two of the following parts on your drawing (where present): cephalon, thorax, pygidium,
axial lobe, pleural (lateral or side) lobes, eye, spines, thoracic segments, and glabella.
Phylum Brachiopoda (Cambrian to Recent): Brachiopods are solitary, benthic, and exclusively
marine bivalved invertebrates that are not related to clams though they resemble them
superficially. Brachiopods are readily distinguished from “clams” (Phylum Mollusca) by having
paired shells (called valves) whose plane of symmetry runs across the shells. Clams have a plane
of symmetry between the shells or no symmetry at all (e.g., oysters). Most brachiopods are
attached to the seafloor by a fleshy stalk (pedicle) that protrudes through a hole (pedicle opening)
in one of the valves (the pedicle valve) at the hinge (posterior) region of the shell.
4. Your fossil set (in drawer) contains 6 brachiopod specimens. Which ones are they?
F1
F2
F3
F4
F5
F6
5. Identify and give the age (age range) of two of them (using the identification charts).
Two
of: Athyris (Dev-Tr); Mucrospirifer
(M. Dev);
Name:
Age:
Chonetes
(Sil-Perm); Stegerhynchus
(M. Sil);
Name:
Age:
Hustedia (Miss-Perm); Composita (U. Dev – Perm)
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6. Examine the specimens of brachiopods at the back of the lab. Using the diagrams in the
morphology handout, make a drawing of a brachiopod, with scale. Label at least two of the
following features (where present): hinge line, pedicle opening, growth lines, costae, and
sulcus.
7. Look at the following pair of sketches.
a) Which one is the brachiopod (write “brachiopod” in the appropriate slot)?
brachiopod
b) Why? (What is/are the distinguishing feature(s)?)
Plane of symmetry cuts across the shell
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Phylum Hemichordata (Late Cambrian to Recent): The hemichordates are a group of solitary
and colonial, non-segmented, marine benthic and planktic organisms. Their distinguishing
characteristic is a primitive central nerve called a notochord. The graptolites (Class
Graptolithina) are a very important fossil group and are worldwide index fossils for both the
Ordovician and Silurian.
Class Graptolithina (Late Cambrian to Early Carboniferous): Graptolites are colonial
microscopic organisms. The colony occupied an organic skeleton composed of thin strips of
small cups in which the microscopic individuals lived. They are usually preserved as carbonised
remains on bedding surfaces of deep-water black shale. Graptolites resemble pencil markings
(hence their name) and the cups on the skeleton show as small serrations (like a fine saw blade).
8. Examine the graptolite specimens at the back of the lab. Draw one of the graptolite
specimens with a scale and, using the morphology handout, label a stipe and a theca.
9. How are graptolites preserved?
Carbon film (carbonisation)
10. Why are graptolites important fossils?
Short time range – excellent index fossil
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11. Compare the following sketches with your identification charts.
a) Identify the following genera (which we’ll call “Assemblage A”) and give their
individual age ranges in the spaces provided.
Pliomerops
Orthambonites
L. Ord – M. Ord
Ord
b) What is the age of “Assemblage A”?
L. Ord – M. Ord
c) “Assemblage B” has F2 and F22 (from drawer). Identify and give the age of each.
F2
Name:
Mucrospirifer
Age:
F22
Name:
Calymene
Age:
M. Dev
Sil – M. Dev
d) What is the age of “Assemblage B”?
M. Dev
e) Assume that Assemblage “A” is found in layer 1 (see below) and assemblage “B” is
found in layer 3. What is the potential age of layer 2?
Rock layer
Assemblage
3
B
2
1
A
Age
M. Dev
L. Ord – M. Dev
L. Ord – M. Ord