Name: Answers Reg. lab day: Tu W Th Geology 1023 Stratigraphic Correlations (Lab #4, Winter 2010) Introduction Stratigraphic correlation is the process of comparing rocks at one locality with related rocks at another. It is a basic and powerful tool in understanding the geological history of an area. Most of the principles are covered in your course notes and Chapter 17 (pp. 438 449 and 459 462) of Monroe and Wicander. This lab will give you some hands-on experience in making correlations in preparation for interpreting geological history. Contacts Contacts are surfaces or narrow zones across which there is a change in lithology (e.g., texture, composition, etc.). Contacts can be conformable or unconformable. Conformable contacts indicate more or less continuous deposition but some sort of change in the environment of deposition as suggested by the lithologic change (e.g., change in depth, oxidation, salinity, velocity of flow, etc.). A short pause in deposition may have occurred, but there is no significant time lost. Conformable contacts may be either abrupt or gradational. Unconformities, which are always abrupt, indicate that a significant amount of time elapsed between the deposition of the units above and below the surface. Most unconformities are erosional surfaces. 1. Name the type of unconformity represented by the heavy lines on each of the sketches shown in Figure 1. Ang. unconf. Non-conform. Disconformity
Correlations Winter 2010 Page 2 of 11 Stratigraphic columns Diagrams showing the sequence of sedimentary rocks at an outcrop are columns drawn to scale. In addition, standardised patterns and symbols are used to represent different lithologies, textures, sedimentary structures, fossils, etc. Two kinds of graphic stratigraphic column are illustrated in Figure 2. It is usual to show the "erosional profile" perspective given at the right, which extends coarser, more massive, and usually more resistant layers to the right and indents the less resistant layers to the left. 2. A stratigraphic section is simulated in the lab by rock samples cemented to plywood. a) Examine the samples and complete the following table by adding in the features of the different rock types in the central column. This may include any structural, textural, or chemical features such as laminations, fossils, reaction with HCl, etc. Rock description Structure/features/fossils Thickness (m) Sandstone Slabby 14 Shale Raindrop imprints 1 Limestone Fossils, fizzes 6.5 Shale Fossils, fizzes 0.5 Limestone Fossils, fizzes 4 Shale fizzes 1 Limestone Fossils, fizzes 4 Shale Brown, oxidized surface 6 Sandstone Cross laminated 2 Conglomerate Quartz pebbles 10 Sandy siltstone Laminated 2 Shale Unfossiliferous 2
Correlations Winter 2010 Page 3 of 11 b) Plot the simulated section on the grid provided. Use a vertical scale of 1 line = 1 m. Start your diagram at the bottom a couple of columns in from the left. Use standard symbols for the lithologies as shown in the columns on Figure 2 and use the following widths for each lithology: conglomerate, 4 columns; sandstone, 3.5 columns; limestone, 3 columns; calcareous shale and siltstone, 2.5 columns; shale, 2 columns. c) You will see that your stratigraphic column is broadly divisible into 3 parts (upper, middle, and lower). In a real section these parts would be formations. Draw heavy horizontal lines dividing the column into three "formations" and name them using the usual twofold convention. Assume that the bottom rocks are present at Huggins, the middle portion at Horton and the upper portion at Patterson. If the unit is uniform in composition you can name it using the lithologic term (e.g., "Wolfville Sandstone"), otherwise use the term "formation" (e.g., "Horton Bluff Formation"). Physical Correlation Sedimentary rock formations can be locally correlated with some degree of confidence by comparing their physical and chemical characteristics even when there are no fossils. As distances between outcrops increase correlations become increasingly inaccurate and require as much evidence as possible (e.g., fossils). Physical correlation can be done by a) "walking out" the outcrop, b) lithologic similarity, c) position in a sequence, d) relationships to unconformities or structures, and e) geophysical logs for wells. "Walking Out" an Outcrop If a rock unit is well exposed, it may be possible to correlate the rock layers from one area to another simply by "walking" along the outcrop, or from outcrop to outcrop. An aerial photograph that permits you to "walk" the outcrop is shown in Figure 3. 3. Figure 3 is a portion of an aerial photo showing a well defined fold at Liscomb Mills near Ecum Secum, Nova Scotia. a) The way the sunlight falls on the beds suggests that the beds just south of the lake at letter "c" dip to the south. What type of fold is it? Plunging syncline b) Correlate the numbered positions (1, 2, 3, and 4) with the equivalent lettered positions (a, b, c, and d). 1 = d 2 = c 3 = b 4 = a c) Indicate the age sequence of positions 1 to 4. youngest = oldest = 1 4 2 3
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Correlations Winter 2010 Page 6 of 11 Lithologic Similarity Correlation can also be made by looking for similar compositions, similar variations in composition, colour, grain sizes (largest, smallest, average, sorting, etc.), grain characteristics (roundness, sphericity, frosting, etc.), and/or sedimentary structures. Some layers may be thin, sharply defined, laterally persistent, and of markedly different composition. For example, local storm deposits or volcanic ash deposits may stand out and are excellent time markers. Such units are called key beds or marker beds. 4. Correlate the units in the two columns in Figure 4 on the basis of lithologic similarity (standard symbols are used). Draw straight lines (in pencil) between levels of possible time equivalence, such as formation contacts, etc.
Correlations Winter 2010 Page 7 of 11 Position in Sequence In contrast to marker beds as described above, units commonly "pinch out" between locations or they may "grade" into each other. Such "laterally equivalent" units can be correlated if they have readily identifiable units below and above them. 5. Correlate the units in Figure 5 on the basis of lithologic similarity and position in sequence. Note that this forces you to correlate two units of dissimilar lithology. This indicates different environments of deposition at the two locations at the time when the original sediments were being deposited (shallow water at one location, deeper water at another).
Correlations Winter 2010 Page 8 of 11 6. Figure 6 shows three stratigraphic columns A, B, and C. a) Correlate the columns on the basis of lithologic similarity and position in sequence. d b) Which lettered unit pinches out laterally? c) Fossil evidence indicates that the line drawn between positions 7 and 8 is a time line (i.e., same age everywhere). We will assume that surfaces parallel to a time line are also time lines. Are lines 1-2 and 3-4 time lines? no d) What does the above imply about unit d? Longer period of deposition of d at location C than at location B
Correlations Winter 2010 Page 9 of 11 Relationships to Unconformities and Structures It is generally possible to correlate lithic units on the basis of unconformities. Typically one correlates units that lie directly above an unconformity. Lithic units which have structures (such as mud cracks and rain prints) not found in the rest of the section may also be correlated. 7. Figure 7 shows two stratigraphic columns with an unconformity. a) Correlate the two columns. Note that the cherty limestone is thicker in column B. b) Which column experienced the most erosion? c) Which column might have had more uplift during erosion? d) Which column might have experienced a longer period of erosion? A A A
Correlations Winter 2010 Page 10 of 11 Geophysical Logs Not all petroleum exploration wells are cored because of the cost. Instead, petroleum geologists measure various geophysical properties of the rock units penetrated during drilling. Typical geophysical "logs" show various electrical properties such as self-potential (SP) and resistivity, as well as radioactivity, sonic properties, and others. Geophysical properties are related to various properties of the rock such as hardness, porosity, permeability, pore fluids, rock compositions, etc. Variation in lithology commonly corresponds to variation in geophysical properties. Once measured, the different geophysical properties are plotted on paper strips showing the variation of each with depth. A core will be taken in at least one well to allow correlation of the geophysical properties with the associated lithologies. 8. Figure 8 shows three wells 1, 2, and 3. Both lithologic and geophysical data are given for well 1, but geophysical data only for wells 2 and 3. The geophysical properties shown are self-potential (SP) and resistivity. Self-potential (or spontaneous potential) arises because the presence of fluid (electrolyte) and variations in composition lead to small voltages (self or spontaneous potentials). Resistivity is the resistance to the flow of electrical current. a) Complete the lithological patterns for wells 2 and 3 by correlating the S.P. and resistivity curves on the geophysical well logs. b) Note that there are 3 lithologies, sandstone, shale, and limestone. Assuming that the diagram represents a west (well 1) to east (well 3) transect, what is the overall direction of clastic sediment transport? From west to east. c) Is the overall trend from bottom to top of the sequence transgressive or regressive? d) Note that sandstone/shale unit (G) is missing in wells 2 and 3? Why might a unit be missing? (Think of last week s lab). There are two possibilities. e) Which of the two is more likely in this case? Regressive. Not deposited / a facies variation / pinching out Unconformity /erosion in the wells 2 & 3. Not deposited / a facies variation / pinching out
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