Neoarchean to Mesoarchean Ga

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1 Name: Answers Geology 1023 Lab #7, Winter 2014 Shields & Precambrian life forms Lab day: Tu W Th Shields are the oldest (and most stable) parts of the continental masses. They consist of rocks of Precambrian age. These are overlain by flat-lying (or gently dipping) sediments of the platform (next lab). Despite the fact that the Precambrian amounts to 7/9 th of geologic time, and that, in Canada, rocks of Precambrian age supply a large part of our mineral wealth, they have not been studied as extensively as younger rocks. The main reason is the virtual absence of fossil material. What fossils are present have only recently been recognised and are of little stratigraphic value. Detailed study of the shield has only become possible with the advent of reliable radiometric dating (mostly in the past 20 years). This lab focuses on shield rocks, some of the methods used to study them, and a short study of the sort of organisms that were present in Precambrian times. 1. Use Map 2543 (Bedrock Geology of Ontario, East-Central Sheet), Map 2586 (Total Magnetic Field of Ontario, East-Central Sheet), and Map 900A (Principle Mineral Areas of Canada) to do the following exercises. NOTE: The Total Magnetic Field (TMF) is mainly an indication of the magnetite content of the underlying rocks. The TMF includes both the remnant magnetisation and the magnetisation due to the present Earth's magnetic field. In most continental rocks, the latter is much stronger than the former. Hence we can ignore the effects of remnant magnetisation. a) The oldest rocks on this map sheet (units 4 to 9) are a typical greenstone belt succession and form the Abitibi Greenstone Belt and other smaller greenstone belts. What age are these rocks? Neoarchean to Mesoarchean Ga b) The greenstone belts are closely associated with intrusive rocks of similar age. Name three rocks that are abundant in these intrusions. 3 of: tonalite, granodiorite, granite, diorite, monzonite c) Most of the map area is part of the Superior and Southern provinces of the Canadian Shield. However, the southeastern corner of the map around Lake Nipissing is part of the Grenville Province. What ages of rocks are characteristic of the Grenville Province? Mesoproterozoic ( Ga) d) Name three rocks that are abundant in the Grenville Province. Migmatite, gneiss, felsic igneous rocks e) Swarms of mafic dykes (called diabase on this map) occur in this area. They also show

2 Shields & Precambrian life-forms Winter 2014 Page 2 of 5 up clearly on the magnetic map (Map 2586), because they contain quite a lot of magnetite. Are the most abundant diabase dykes older or younger than the greenstone belts? younger f) How can you tell (without looking at the map legend!)? Cross-cutting relationships g) The youngest rocks in this area are Paleozoic and Mesozoic rocks of the James Bay area. How can you tell that the Precambrian rocks extend under these Phanerozoic rocks? (Hint: look at Map 2586.) Magnetic pattern visible through younger rocks h) What types of geological contacts occur between the Precambrian rocks and the Phanerozoic rocks? Non-conformities and faults i) This map area includes many of Canada's most important mining areas. Use the Principal Mineral areas of Canada Map (900A) to list the most important metals obtained from each of the following areas: (1) Kirkland Lake (ON) (2) Timmins (ON) (3) Flin Flon (MB) (4) Fermont (QC) Au, Ag 4 of: Au, Ag, Ni, Cu, Zn, Ag, Se, Te, In, Cd, Cu, Zn, Au, Ag Fe (5) Sudbury (ON) 4 of: Ni, Cu, Co, PGM, Au, Ag, Se, Te. Life in the Precambrian Life in the Precambrian was primitive (by modern standards). Organisms in the Precambrian did not possess hard parts and thus preservation is sporadic and very poor. With the exception of stromatolites (see below) Precambrian fossils are rare. But the apparent explosion of life in the Paleozoic with the appearance of organisms with hard parts obscures the fact that some very important biological events occurred in the Precambrian. The evidence for what occurred is very fragmentary but modern molecular biology has supported many of the inferences made from this fragmentary evidence. The landmark events are: the development of life, the prokaryotic cell, symbiosis and the development of the eukaryotic cell, colonial life (unicellular), multicellular life.

3 Shields & Precambrian life-forms Winter 2014 Page 3 of 5 First, a little revision. 2. Name the 7 major categories of the Linnaean classification system (in the correct order). kingdom phylum class order family genus species Kingdom Monera The Monera are essentially single celled organisms with no cell nucleus. The most abundant fossil representatives of the Kingdom Monera are stromatolites. They are the only fossils present in the Archean, and Paleoproterozoic. Stromatolites are formed by the sediment-binding and cementing action of cyanobacteria (photosynthetic bacteria). See Fig , p. 493 (Fig , p. 514, 5ed) of Monroe & Wicander. The earliest stromatolites are about 3.5 Ga and there are stromatolites living today. These organisms have been around for 3,500,000,000 years! 3. Study the stromatolite specimens provided (264A and 264B at the back of the lab). Draw a portion of one, with a scale. Label the laminations. Ediacaran fauna Though stromatolites are colonial (i.e., they consist of multiple individuals) each of the individuals is a single identical cell. True multicellular (metazoan) organisms developed in the Proterozoic. Metazoans (both individual and colonial) contain a range of cell types. In addition, with true metazoans, different cells are organised into different tissues (muscles, nerves, digestive tissue, etc.). True metazoan (multicellular) animals arose in the Neoproterozoic and there are a number of localities worldwide that preserve animals from the latest Neoproterozoic. These are commonly referred to as Ediacaran faunas (after the first discovery in Ediacara, Australia) and date from about 570 Ma. Three important groups of organisms with modern representatives, which appear to have arisen in the latest Proterozoic are: Porifera (sponges), Cnidaria (a large phylum which includes jelly fish, sea anemones, and corals) and Annelida (segmented worms).

4 Shields & Precambrian life-forms Winter 2014 Page 4 of 5 Sponges (Phylum Porifera; Late Precambrian to Recent) are very primitive organisms. They are not really metazoans. They contain a few different types of cell but these aren t organised into tissues and there is no central nervous system. Sponges are often simple bag or bowl shaped structures of varying sizes. The organism inhales water through a porous structure, extracts food and then exhales the water. There are/were many variations on this theme. Many sponges had skeletal (reinforcing) elements composed of either calcareous or siliceous needle-like microscopic structures called spicules, some have organic fibres called spongin as a skeleton (classic bath sponge). Some fossil sponges, however, had massive vesicular calcareous skeletons, such as the stromatoporoids, which often formed reefs (e.g., Devonian in Alberta). Note that the forms you are examining here are not Precambrian in age. Precambrian Porifera all lacked skeletons (some modern sponges also have only jelly-like support structures). 4. Examine the fossil sponges at the back of the lab. Specimens 258A and 258B are Girtyocoelia (Carboniferous-Permian). It is a fossil calcareous sponge with a fused skeleton, which preserved inhalant pores and exhalent osculum. Draw a diagram with scale.use the morphology handout to help you label the inhalant pores and the osculum. Phylum Cnidaria (=Coelenterata) (Late Precambrian to Recent) includes corals, sea anemones, and jellyfish. Only the corals produce a skeleton today and there are/were solitary and colonial forms. Three types of corals were important fossils and contributed to reef formation. The living coral animal is called a polyp and it inhabits a cavity called a corallite. Note that the forms you are examining here are not Precambrian in age. Precambrian Cnidaria all lacked skeletons (as do modern jellyfish and sea anemones). There are three important groups of corals rugose, tabulate and scleractinian corals. Rugose (wrinkled) corals were very important in the Paleozoic. These consist of single or colonial cups each of which had a radiating array of walls called septa and basal floors tabulae on which the organism sat (see diagram in handout). As the individual organism grew the cup became bigger (giving solitary rugose corals their distinctive horn shape). Corallites in colonial forms are separated from one another by walls. The solitary forms (often called horn corals ) show the external rugae (wrinkles) more clearly. Tabulate corals were also important in the Paleozoic. All tabulate corals were colonial and the individual corallites were smaller than the rugose corals. Corallites looked like tubes partitioned by prominent tabulae (horizontal partitions). Septae were small or absent.

5 Shields & Precambrian life-forms Winter 2014 Page 5 of 5 Modern (scleractinian) corals are similar in some respects to the rugose corals. They have prominent septa with a radiating pattern. Modern colonial corals differ from the colonial tabulate and rugose corals in that they commonly do not have separating walls between corallites. 5. Examine the fossil corals at the back of the lab. The fossil set in the drawers has four genera of coral. a) Which ones are they? b) Identify two using the fossil identification chart and give the age (or age range) for each. Any 2 of: Favosites (Sil-Penn); Eridophyllum (L-M Dev); Name: Age: Lophophyllidium (Penn-Perm); Caninia (Miss-Penn); Heliophyllum Name: (Dev); Zaphrentis Age: (Dev) c) Make a labelled drawing of one solitary coral and of one colonial coral from the display at the back of the lab. Use the morphology handout to help you label each appropriately. Labels should include one or more of septum (pl: septa), calice, tabula (pl: tabulae), ruga (pl: rugae). Make sure you have a scale. Solitary form: F10, F11, F12, F13 Colonial form: or