Geology 1023 Lab #2, Winter Structural Geology & Interpreting Sedimentary Rocks

Transcription

1 Geology 1023 Lab #2, Winter 2004 Name: Answers Lab day: Tu W Th F Structural Geology & Interpreting Sedimentary Rocks I. Folds a review Using the appropriate terms (anticline, syncline, upright, overturned, recumbent, plunging) label the folds below in the spaces provided. See pages of your text for additional help. Plunging syncline Inclined (overturned?) anticline Recumbent syncline

2 Structure & Sedimentary Rocks Winter 2004 Page 2 of 9 II. Faults a review In the following diagrams label the hanging wall and foot wall (A or B), if appropriate, and the type of fault shown (normal, reverse, thrust, oblique-slip, or strike-slip). See page 281 of your text for additional help. A B Normal fault B A Reverse fault A B Oblique fault B A Thrust fault n/a n/a Strike-slip fault

3 Structure & Sedimentary Rocks Winter 2004 Page 3 of 9 III. Strike and dip review. The strike of an inclined plane is the compass direction of a horizontal line on that plane. The strike can have any value from 0º (usually written 000º) to 360º (which are equivalent). Note that from now on we will follow the Right Hand Rule. That is, we measure the strike so that the plane we are measuring dips to the right of the direction of strike. See the PowerPoint tutorial on the Compass, Drawing a Strike and Dip, and the Right Hand Rule (online). The dip of an inclined plane is the amount of inclination of that plane from the horizontal. Dip varies from horizontal (00º) to vertical (90º). As a precaution (to ensure that the Right Hand Rule has been followed) we also indicate the general direction of dip. A general compass direction will serve here, e.g., N, SW, etc. 1. a Assuming that the vertical edge (right or left hand) of the page points to North, draw a strike and dip symbol for each of the following readings. Be sure to use the correct symbol. bedding 090/45S cleavage bedding bedding 135/30SW 225/10NW 045/27SE cleavage 150/76W b Give strike and dip readings for the following bedding symbols. 045/16 SE 270/72 N 260/31 NW 135/89 SW 060/15 SE

4 Structure & Sedimentary Rocks Winter 2004 Page 4 of 9 2. Draw a cross section A-B for figure (a) and name the structure. Draw a topographic profile and geological cross-section for A-B in figure (b). Complete the block diagrams (c) and (d) and name the structures.

5 Structure & Sedimentary Rocks Winter 2004 Page 5 of 9 IV. Sedimentary rocks some review and some new material The two major types of sedimentary rocks are: Clastic (or detrital) Made up of fragments of pre-existing rocks deposited by a flowing medium. The character of clastic rocks is influenced mostly by physical factors, which include: distance from the source area; nature of the transport medium (ice, air, or water); speed of the medium. Clastic sedimentary rocks are composed of: Grains Discrete particles deposited by sedimentary processes. Matrix Variable amounts of finer grained material (usually mud) between the grains. Cement Chemically precipitated material that binds the grains and matrix together. Pores Any remaining open spaces. Non-clastic (or chemical, or biochemical) Made up of chemically or biologically derived material. Rocks made from fragments of biologically derived material are sometimes called bioclastic. 3. Indicate which of the Group 1 samples are clastic or non-clastic Clastic AC, AD Non-clastic AA, AB Four major properties are used in classifying and interpreting sedimentary rocks. These properties are composition, texture, sedimentary structures, and colour. Composition Composition refers to the mineral and chemical make-up of the rock (including the composition of the grains, matrix, and cement in clastic rocks). Clastic rocks are composed dominantly of silicate minerals and are commonly referred to as siliciclastic. Carbonates are dominantly composed of calcite and dolomite. Evaporites are dominantly composed of sulphates and halides (chlorides). Chert is the most important non-clastic silicic rock and is often referred to as a chemical sedimentary rock along with evaporites and chemically precipitated dolostones and limestones. The term biochemical is used for peat, coal, and most limestone. 4. Indicate which of the Group 2 samples are carbonates, evaporites, biochemical, or chemical sedimentary rocks. Note that some samples will be in several categories. Carbonate: Evaporite: Biochemical: Chemical: AE, AF, AG AH, AI AE, AF, AJ AG?, AH, AI, AK

6 Structure & Sedimentary Rocks Winter 2004 Page 6 of 9 Texture The term texture refers to the nature of the constituent particles (crystals or grains), the size of the grains, and, in clastic rocks, the relationship between the grains, matrix, cement, and pores. Clastic rocks show the greatest amount of textural variation and are described in terms of four variables: grain size, roundness, sphericity, and sorting. Grain size has 3 major divisions. The following table gives the grain size, the name for the unconsolidated sediment and the equivalent sedimentary rock. Grain size Sediment Rock > 2 mm (c.g.) gravel conglomerate, breccia 1/16-2 mm (m.g.) sand sandstone < 1/16 mm (f.g.) mud (silt and clay) mudrock (siltstone mudstone, shale) 5. Circle the rocks or sediments that present among the samples in Group 3. gravel sand mud (silt/clay) conglomerate/breccia sandstone mudrock Roundness refers to the degree to which sharp edges have been removed from the particles during transportation. There is a continuum from angular to fully rounded. In most cases particles are relatively well rounded. Even small amounts of transportation tend to knock off sharp edges. Because finer grains tend to be transported farthest, roundness is only an issue with the coarse-grained rocks. Conglomerate contains rounded fragments. Breccia has angular ones. True sedimentary breccias are relatively rare and were deposited very close to the source. Sphericity refers to the degree of equidimensionality of the particles. The more equidimensional the particles are, the greater the sphericity. This means that a cubic particle has high sphericity whereas a rod-shaped or pancake-shaped particle has lower sphericity. Do not confuse roundness with sphericity. Note that a cigar and a ball bearing are both well rounded but differ markedly in sphericity. However, a dice and a ball-bearing differ markedly in the degree of roundness but have high sphericity. 6. a Rank the samples in Group 4 according to roundness. Least AS AR AQ/AP AP/AQ b Rank the samples according to sphericity from least (left) to most (right). Least AP AR/AS AS/AR AQ Sorting refers to the degree of variability in the particle size and composition. A rock/sediment with particles of the same size/mineral is well sorted. Rock/sediment with a wide range of grain

7 Structure & Sedimentary Rocks Winter 2004 Page 7 of 9 size/composition are poorly sorted. Sorting by size and composition are described separately, but, as a rule, sorting in one way is accompanied by sorting in the other. 7. a Rank the samples in Group 5 according to sorting by size. Least AU AT AV/AW AW/AV b Rank the samples in Group 5 according to sorting by composition. Least AU/AV AV/AU AT AW Sedimentary structures Sedimentary structures in a rock result from processes that occurred during deposition or shortly thereafter. These processes may be primary (either inorganic or organic) or secondary (physical, or chemical). Primary structures form during sedimentation. Secondary structures form after deposition but prior to (significant) lithification. Primary sedimentary structures Inorganic Bedding at all scales (thick bedded to laminar) Ripples (current and wave) Cross-laminations or cross-bedding Graded bedding Sole markings (flutes, grooves) Cracks Raindrop prints Evaporite mineral pseudomorphs Organic Tracks and trails Burrows (visible discrete structures) Bioturbation (bedding disturbed by biological activity) Stromatolites (small mound structures up to 30cm across, algal mats) Reefs (large mound structures from meters to kilometres across/long) Secondary Physical Fissility in shales (dewatering?) Cone in cone (dewatering) Load structures Chemical Nodules and concretions Dendrites

8 Structure & Sedimentary Rocks Winter 2004 Page 8 of 9 8. Look at the display of structures at the back of the lab then examine the samples in Group 6. Indicate which of them show the following structures. Some samples may show more than one structure and some of the listed structures will not be represented at all. bedding AX, AY, AZ, BA, BC, BD burrows AZ concretions BA cracks cross-lam. or bedding BC dendrites BB graded bedding BC load structures AX raindrop prints AY ripples (current) ripples (wave) stromatolites sole markings tracks/trails Colour The colour of a rock is the result of many of the above features. Particularly in fine-grained rocks it may be useful in determining composition and the environment of deposition. Common colours in clastic rocks include grey to black, nearly white, green, red, and various shades of brown (yellow to reddish shades). The following are simplifications will help you make your first interpretations of environment of deposition. Red (hematite-bearing) subaerial, non-marine fluvial environments. Dark grey to black (usually shales) strongly reducing, often deep water environments. Plant fossils are indicative of non-marine environments. Pale shades (white, pale green, yellow) oxidising to mildly reducing, shallow to relatively deep water environments. 9. Using colour (and any other features you think appropriate) indicate which of the samples in Group 7 were deposited in oxidizing, reducing, marine, or non-marine environments. Oxidizing BF (BG?) Reducing BE, BH, BI Marine BG (BH?) Non-marine BE, BF, BI Maturity Mature sediments or rocks contain well-rounded and highly spherical grains of chemically stable minerals (usually quartz). They are well sorted, and contain little or no matrix (mud). 10. Rank the samples in Group 8 according to maturity. BL BK BJ Least

9 Structure & Sedimentary Rocks Winter 2004 Page 9 of Describe the rocks R1 to R5 from the Rock and Fossil drawers at the back of the lab. Clastic / Nonclastic Composition Grain size (c.g./m.g./f.g.) Rounding (poor/mod/well) Sphericity (low/med/high) Sorting size (poor/mod/well) Sorting comp. (poor/mod/well) Structures Colour Name Environment 1023-R1 clastic Silt f.g. Not vis. Not vis. Not vis. Not vis. Bedding Raindrip imprints Reddish Mudrock (siltstone) Non-mar. Oxidiz R2 Clastic Sand m.g. Mod. Mod. Well Mod/poor Bedding Pale grey Sandstone (arenite) Mar? Oxidiz R3 Non-clast. SiO2 Cavities Greyish Chert Any 1023-R4 Clastic Sand m.g. Mod. Mod. Mod/well Poor/mod Porous Pink/red Arkose Non-mar. oxidiz R5 Non-clast Calc (shel) Fossils Brownish Limestone Mar. oxidiz. Texture (where applicable)