GEOL 02 Lab 9 Field Trip III Centerville Stratigraphic Section Name: Date:

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1 Name: Date: Team Name: Team Members: Our goal today is to describe the sedimentary lithostratigraphic section of exposed bedrock along the road near Centerville Beach, interpret the depositional environment, and use this interpretation to fit the mapped lithostratigraphic units into the published regional geologic context. When taking notes of our observations, we want to take the notes in the same order each time, as usual. We will work as a two groups. We will learn: How to construct a stratigraphic column How to measure sedimentologic and geologic unit thickness How to interpret geologic observations within a regional geologic context We will first measure and describe the sediment stratigraphic section along the road near Centerville Beach, CA. Our note books will be arranged in a similar manner as for the Mad River Slough lab, with a stratigraphic column on the left and the description on the right. We will break the stratigraphic section into a small number of geologic units (3 5) based upon the big picture (be a lumper today). Then, describe the sedimentological variation of sedimentologic sub units within each major geologic unit. Along with describing the rocks, we will measure the thickness of the different sedimentary units. The geologic units that we are mapping are part of the Miocene to Pleistocene Wildcat formation and the Quaternary Hookton formation (Ogle, 1953). Ogle (1953) is posted on the lab website. At the end of this handout is a two page summary of the Wildcat and Hookton formations from Aalto (2006 and references therein), published in the 2006 Pacific Cell Friends of the Pleistocene Field Trip Guidebook. Part I. Geologic Description We will use our field notebooks to take notes about the sediments. We will interpret different sedimentary units, what are included in each sedimentary unit, and the boundaries between these units. For each sedimentary unit, we will describe the following: color, texture, structure, and notes. For each contact, we will note the depth and sharpness of the contact. We will also want to make a sketch of our field site (i.e. make a sketch of the road outcrop; include the location of where you collected strike and dip data). 1 P age

2 Part II. Stratigraphic Section Measurement The goal of measuring a stratigraphic column is to accurately characterize the thickness of different rock types. These data can be used to interpret depositional environments, variations in sediment type, and changes in sedimentation rate in space and time, etc. Most detailed interpretations require an accurate measure of how much of each rock type is present, bed thicknesses, etc. It is easy to measure the thickness of flat lying beds; you can put a ruler next to them and measure from bottom to top. For beds that have been tilted or folded, more care is needed. Measurements need to be made perpendicular to the bedding. Otherwise, results will depend on the amount of folding and the direction you are looking rather than the depositional processes that formed the rock. Geologists often use a Jacob s Staff to measure bedding thicknesses. A Jacob s Staff is a pole that is marked off in suitable units, such as decimeters. A Brunton is used as a clinometer to measure the angle of the pole from vertical and helps align the Jacob s Staff perpendicular to bedding for accurate measurements. To measure bed thickness, place the Jacob s Staff on the bedding plane at the base of the beds you want to measure. Next align the staff at right angles to bedding and sight downdip, perpendicular to strike, to the beds. The distance from the base of the staff to the sight point on the Brunton is equal to the thickness of strata between the base of the staff and the point sighted. There are a number of steps for doing this measurement accurately: Figure 1. A. Holding a Brunton compass against the Jacob staff in sighting position. B. Measuring the stratigraphic thickness between two bedding surfaces with a Jacob staff and clinometer (here, a Brunton; Figure from Compton, 1985; p. 230). 2 P age

3 1. Measure the strike and dip of bedding where you intend to measure the section; record the data and set the clinometer on the Brunton to the angle of dip. This could be recorded on your outcrop sketch. 2. Place the staff at the base of the unit to be measured and tilt it downdip (exactly perpendicular to strike) until the clinometer bubble in the Brunton is centered. 3. Study the point sighted on the ground and decide if the staff can be placed on it for your next measurement; if so, note the point carefully by eye or place an object at that point. You have measured an amount of stratigraphic section based upon where you placed the Brunton on the Jacob s staff. 5. If the base of the Jacob s Staff cannot be placed on the point you sighted for your next measurement, move the base of the staff along the lower bedding surface until a suitable point can be sighted. 6. Draw your stratigraphic column, describing the rocks in this unit. Measure the positions of beds within this stratigraphic interval using the Jacob s Staff. 7. Move the base of the Jacob s Staff to the sited point, and make your next measurement. Proceed similarly to the top of the unit. Using a Jacob s Staff and Brunton requires sighting through a small hole, and it may be tempting to save time by estimating the alignment of the staff rather than using the clinometer. Moderate errors in alignment, however, can cause large errors in measurements (Figure 2). In addition, when sighting up or down a slope, one tends to tilt the staff so that it is perpendicular to the ground surface. This gives errors that tend to accumulate through a series of measurements, giving a systematic over or under estimate of the true stratigraphic thickness. When the staff is correctly oriented with the clinometer, the error should be no more than a few centimeters per measure and will tend to average out in successive Figure 2: Errors in measurement (Figure from Compton, 1985; p. 231). 3 P age

4 measurements. Thus, it is worth learning to measure accurately now; sighting becomes easier with practice. Sighting with a Jacob s Staff becomes increasingly awkward as dips become steeper. Lines of sight typically become shorter, however, so accuracy is maintained (Figure 3A). For dips greater than 70, the geologist can kneel and look along strike, viewing the clinometer face on and making the projection to the ground by estimation (Figure 3B). If some beds stick up significantly higher than others, it may be necessary to measure the thicknesses of the low lying beds and weathering resistant beds separately, using modified techniques. One can place the Brunton anywhere along the staff to measure an arbitrary thickness. Be careful to accurately record the thickness represented by each measurement, especially if they are different for each measurement. Figure 3: Jacob s Staff orientations depend on the relative dip of bedding and slope (Figure from Compton, 1985; p. 231). Figure 4. Sighting along strike with a Jacob staff. A. Holding the Brunton compass against the staff and sighting along edge of lid. B. Position for taking a measure. 4 P age

5 Steep walls of valleys eroded across strike often have the best exposures, and they must be measured by views parallel to strike. In such cases, the clinometer is set to the dip as usual but the lid of the compass is opened so as to make an angle of goo with the compass face (Figure 4A). The observer holds the compass against the staff and stands facing the compass and outcrop, looking exactly along strike (Figure 4B). The outcrop is sighted along the upper edge of the lid, and the staff is held so that the side facing the observer is vertical. Part III. Report We will want to communicate what we mapped today and place our observations within local geologic context (ogle, 1953; Aalto, 2006). Prepare a report that includes the standard sections (intro, methods, results, discussion, and conclusion). This report is due two weeks from the day that we collect these data. In your report, describe how we mapped the geologic units (descriptions in our field notebooks), measured stratigraphic section (Jacob s Staff), and which geologic units mapped by Ogle (1953) are probably the geologic units that we investigated for this lab. We will include a Stratigraphic Column. Compton (1989) describes how to present stratigraphic information in a report in chapter 11 (this is posted on the lab web site). In preparation for this report, there are several questions that you will want to answer. Please list these questions and the answers as an appendix to your report (part of the same electronic document). Consider the assignment and write a purpose statement. What is the purpose of the project? Consider both information you will analyze and learning objectives. Use field data and topographic map to determine the general location and site(s) latitude/longitude for the project (The Ogle (1953) map is posted on the lab web site). List the Formation Names, general rock types, and rock ages present in the study area. Use your data table to make a list the rock types present at the location. List the orientation of the bedding (strike and dip) at the locations where you measured them. In your report, include a location map, a table of geologic units, a stratigraphic column, and a sketch of the outcrop (feel free to take a photo, but if you include the photo in the report, you will need to label the geologic units and their contacts, on the photo). Below is a suggestion about some of the information that might be included in your report, and where it should go. 5 P age

6 Introduction: GEOL 02 Lab 9 The location of the study (latitude/longitude and a map), the purpose of the study (to describe the geologic setting), the general rock type mapped in the region, and the age of the rocks mapped in the area. Methods: Field methods include equipment and procedures used. Results: Describe the thickness of major stratigraphic units, rock types, fossils, and bedding. This should include references to your stratigraphic columns, data table, and sketches (if present). Discussion: This results section should provide the basis for an interpretation of the geologic history of this rock exposure. The history should be discussed from oldest event to the youngest (describe the column from bottom to top). a) Depositional environment represented by each unit (base this on grain size being a measure of energy, fossils, color, etc.). b) Geologic transitions represented by changes within or between units (environmental changes through time). c) Post depositional processes (weathering, cementation, leaching, uplift and tilting of the beds, etc.). d) Combine a c to construct a geologic history of this location. e) Discuss how this project has influenced your view of the physical world. Consider the implications for our (humans) role in the world and/or the nature of geologic change, and/or what you have learned. Conclusion: A summary of your report. 6 P age

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