Shield was above sea-level during the Cambrian and provided the sediment for the basins.

Transcription

1 Name: Answers Geology 1023 Lab #8, Winter 2014 Platforms and Paleozoic life-forms Lab day: Tu W Th 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.

2 Platforms and Paleozoic life-forms Winter 2014 Page 2 of 7 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.

3 Platforms and Paleozoic life-forms Winter 2014 Page 3 of 7 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' /7,000,000 yr = ft/yr (Accept x10-4 ft/yr; x 10-3 in/yr; x 10-2 cm/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

4 Platforms and Paleozoic life-forms Winter 2014 Page 4 of 7 (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 Name: of: Athyris (Dev-Tr); Mucrospirifer Age: (M. Dev); Chonetes Name: (Sil-Perm); Stegerhynchus Age: (M. Sil); Hustedia (Miss-Perm); Composita (U. Dev Perm)

5 Platforms and Paleozoic life-forms Winter 2014 Page 5 of 7 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

6 Platforms and Paleozoic life-forms Winter 2014 Page 6 of 7 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

7 Platforms and Paleozoic life-forms Winter 2014 Page 7 of 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. Orthambonites Pliomerops 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: Age: F22 Name: Age: 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 Age 3 B 2 1 A Mucrospirifer Calymene M. Dev L. Ord M. Dev L. Ord M. Ord M. Dev Sil M. Dev