section measuring

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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 column. A section is

measured with three main purposes in mind: (1) to make an accurate description of the rock unitsrepresentative 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 andlithostratigraphic data. As the rocks you are measuring are not rich in fossils, we cannot 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 thesection. 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 sometime, including the strat section abstracts. Last week, we did the appropriate selection oflocalities and reconnaissance. And you more or less have the proper equipment preparing you

for field work. A discussion of 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 rockthickness. As we have discussed, there are several ways to do this, and they are not trivial given

the obvious (local) structural deformation in the outcrop. A careful accounting of the strike and

dip of the beds using a brunton compass allows you to simply stretch a tape on the outcrop, and


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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). Wewill 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 meter-

stick. 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 withina 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.


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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 thesection 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 thedrawing 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 toidentify 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 worddescriptions of their section (Figure 3).

Figure 3. Example of written descriptions of rock units.


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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 offossils 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 graphicalcolumn 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 yournotebook 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 andthickness data into a column format.

Consistency of order in your written descriptions is very important because you will want tocompare your units to the units being described by others in the class. You must follow this

order in your description:


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Color

Grain size and sorting

Bedding characteristics (thickness, nature of contacts, presence of graded 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 inthe 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).


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Figure 6. Various types of cross-stratification and the related bedforms. a) Formation of cross-

stratification, 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.


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Figure 7. An example of sedimentary form the lateral accretion surface.


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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.


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