EES 115 Section Measuring
OBJECTIVES
To measure a part of the Lehigh Gap section and to make and record detailed sedimentologic observations. MATERIALS
Brunton compass, notebook, pencil/pen, colored pencil, Jacob staff, measuring tape, 30-cm ruler, hand lens, grain size chart, HCL INTRODUCTION
The previous lab afforded you the opportunity to learn about the Paleozoic section in eastern Pennsylvania and choose a small section of rock to measure. This lab asks you to put your noses on that section and to begin making a detailed description and stratigraphic stratigraphic column. A section is measured with three main purposes in mind: (1) to make an accurate description d escription of the rock units representative of a particular area, (2) to record full sequential descriptions of the rocks within larger units; these descriptions will for the basis for interpretations of depositional environments, tectonic setting, etc., and (3) to record the exact stratigraphic position of biostratigraphic and lithostratigraphic data. As the rocks you are measuring are not rich in fossils, we cannot stress stress biostratigraphic criteria. Rather, your collection of hand samples last week for thin section work will help you collect petrographic data that will be used to understand the origin and tectonic significance of the rocks. Compton (1985) outlines five preliminary steps that you must take prior to measuring the section. These are: • • • • •
Researching the rock units Selecting localities Reconnaissance Precision Preparations for the field
You have not done extensive research of the rock units, but we have spoken about them for some time, including the strat section abstracts. abstracts. Last week, we did the appropriate selection of localities and reconnaissance. And you more or less have the proper equipment preparing you for field work. A discussion of precision precision is necessary before you are turned loose on the outcrop. First to consider is mechanical precision of the surveying methods you will use to measure rock thickness. As we have discussed, there are several ways to do this, this, and they are not trivial given the obvious (local) structural structural deformation in the outcrop. A careful accounting of the strike strike and dip of the beds using a brunton compass allows you to simply stretch a tape on the outcrop, and
calculate true thickness using trigonometry (Figure 1). This works reasonably well, especially in places were the outcrop is orthogonal, rather than oblique to the strike.
Figure 1. Sketch of a right triangle illustrating how rock thickness can be obtained from a tape and brunton measurement. For places where the outcrop is oblique to the strike, or when the outcrop is essentially the ground surface, the preferred method of distance measurement is the Jacob Staff (Figure 2). We will not demonstrate all of the aspects of Jacob Staff use in the field for this exercise (this is something that you will learn at field camp). But, the Jacob Staff can be used as a simple meterstick. You place it on the outcrop, and measure rock thickness orthogonal to the direction of dip. Changes in strike and dip attributed to structural complications can typically be accounted for easily by carefully maintaining the staff orthogonal to dip, and rotated 90 degrees from strike.
Figure 2. Sketch of a Jacob Staff and its use with a (a) Brunton Compass, and (b) as a meter stick. The second consideration of precision will establish if you are a “lumper” or a “splitter”. That is, how finely-divided should you make your measured section? Lumpers tend to generalize features across beds, resulting in a section with few units. Splitters tend to find variations within a given bed resulting in very detailed, but potentially cumbersome sections. I want you to focus on describing the rocks at the level of individual beds to groups of similar beds. You have only 20 m of section to describe, so this level of detail will not be overly ambitious.
GETTING STARTED WITH MEASURING THE SECTION
You approach measuring the section as you would undertake the climbing of a mountain. You would not just rush onto a trail without first knowing the scale of the climb, how much distance is going to be covered, how much elevation is to be gained or lost. To help you answer these questions, you refer to a topographic map. In the same way, you need to create a “map” of the section as a guide to clue you into how you will approach the measuring process. So find your 20-m long portion of the section, view it from a distance where you can see its details, without looking at the entire outcrop, sit down and get comfortable, and DRAW IT. Make sure the drawing is to scale both in the horizontal and vertical, and you indicate the direction you are facing. The drawing should be annotated to identify any major lithostratigraphic features such as a proposed formation or member break, color changes, gross textural changes, etc. Try to identify at this stage, “units” or packages of rock that you will need to collect detailed descriptions on. Use this time to confer with your partner and pick your contacts. Measure the section from the base to the top (not in the reverse order). Your first description should be keyed as “Unit 1”. Some geologists follow the convention of making only word descriptions of their section (Figure 3).
Figure 3. Example of written descriptions of rock units.
I prefer you to make a graphical log of your section in addition to the written description (Figure 4). This graphical log should contain symbols that illustrate sedimentary structures, presence of fossils or trace fossils, etc. Textural grain size criteria is best recorded in the width that you draw the column. For coarse grain sizes, the column should be wide, for fine grain sizes, the column should be narrow. Graduate the page(s) in your notebook where you will be recording the graphical column following the example below in Figure 4. This technique of keying graphical column width into grain size has the added advantage of conveying information about which units were relatively durable, and protruded from the outcrop, and which ones were relatively erodible and were recessed in the outcrop. It works well if you use the left page of your notebook for the graphic log, and the right page for the description.
Figure 4. A graphical log, with written description. Note the careful organization of unit and thickness data into a column format. Consistency of order in your written descriptions is very important because you will want to compare your units to the units being described by others in the class. You must follow this order in your description:
• • • • • • •
Color Grain size and sorting Bedding characteristics (thickness, nature of contacts, presence of grade d bedding) Sedimentary structures (if any, includes trace fossils, bioturbation) Compositional data (mineralogy) and/or classification Cementing agent and degree of induration Notes on any samples collected and/or presence of fossils
Structural complications and covered portions of your section should be noted as such directly in the column, allotting for the true thickness of these features. Given that you have chosen sections of rock more or less devoid of these complications, you should not have to record them in your graphical log.
Figure 5. Thickness classes for bedding (left) and bedding geometries (right).
Figure 6. Various types of cross-stratification and the related bedforms. a) Formation of crossstratification, b) dependence of flow velocity and grainsize on the type of cross-stratification, c) ripples, d) trough cross-beds, e) tabular cross-beds, f) hummocky cross-beds, g) cross-bed terminologies.
Figure 7. An example of sedimentary form – the lateral accretion surface.
EXAMPLE DESCRIPTIONS
Attached is a sample of a detailed stratigraphic section measured though clastic sedimentary rocks exposed in the Rio Grande rift of New Mexico. Use it as a reference in putting your section together. REFERENCES
Compton, R. R., 1983, Geology in the Field: New York, John Wiley and Sons, 398 p. Connell, S. D., Koning, D. J., and Cather, S. M., 1999, Revisions to the stratigraphic nomenclature of the Santa Fe Group, northwestern Albuquerque basin, New Mexico: New Mexico Geological Society Guidebook 50, p. 337-353.