EAS 423: Sedimentology and Stratigraphy

Measuring Sedimentary Structures for Paleocurrent Reconstruction

Objectives: Today’s exercise will incorporate several field data collection skills with some common

analytical tools to help us develop our abilities to gain valuable information from sediment structures

commonly found in our region. During this exercise, which will admittedly take more than just today,

we will complete the following tasks:

1. Use compasses in the field to measure sedimentary bedding and structures;

2. Practice observing and sketching features at various scales;

3. Take detailed, accurate field notes to document our data and experiences, including grain

size and rock descriptions, stratigraphic relationships, and sedimentary structures;

4. Compile directional (orientation) data using a Rose diagram;

5. Use field measurements and rose diagram results for paleocurrent reconstruction.

Background: As most of you know, the Allegheny Plateau and the Appalachian Basin are underlain by

the Catskill Delta, which contains a diverse suite of sedimentary rocks, indicative of their depositional

environment (Figure 1). During the late Silurian through the Devonian, the Acadian Orogeny built up an

early stage of the Appalachian Mountains. Sediments were shed off the western slopes of this mountain

belt into an inland sea, creating thick sequences of shales, limestones, and sandstones. Later, during the

Pennsylvanian Period, these sediments were

buried even deeper as Pangea was uplifting what

we recognize as the modern Appalachian Mts.

during the Alleghenian Orogeny. That is a lot of

exciting geology! But how do we know all this?

We’ve discussed basic sedimentology and

stratigraphy at length, and the role that

transgressions and regressions play in

sedimentary environments. But there is more to

the story which allows us to develop more

detailed pictures of these past settings. Several of

the rock units in our region preserve

“paleocurrent” indicators. These are structures

within the bedding of larger sedimentary

packages that give clues to the energy and direction of depositional mechanisms (wind, water, ice, and

mass movements). One such unit is the Pottsville Sandstone (Pennsylvanian) which, fortunately for us,

outcrops in several convenient locations in Ohiopyle State Park!

Because the Pottsville Sandstone represents a changing near-shore environment, we must first identify

the sedimentary structures and what they imply. You should look for (and be able to define) the

following:

Laminar bedding

Graded bedding

Ripple marks (various types)

Cross-bedding

Figure 1: Catskill Delta formation during Devonian Period. From SEPM STRATA, Society for Sedimentary Geology;

Illustration by Lynn S. Fichter of the Department of Geology James Madison University, Harrisonburg, Virginia.

In the Field: Previously, we’ve worked with Brunton and Silva compasses to measure strike and dip of

units. We will be using these functions for this exercise to measure the general orientation of the

Pottsville Formation within Ohiopyle State Park. However, we will also use our compasses to measure

the orientation of the aforementioned structural features in the field.

We will begin our day at Cucumber Falls in Ohiopyle State Park. Review the map of the park, provided

as Figure 2 at the end of this packet. Our goal is to make 3-4 stops, of at least 1-2 hours. At each stop,

you are to complete the following in your field notebook:

1. Note and describe your map location;

2. Measure and describe the rock units in general;

3. Sketch the outcrop, paying particular attention to those units that have evident sedimentary

structures;

4. Measure and describe present sedimentary structures, keying them to your sketches;

Work in groups of 2-3 in completing this exercise. Each student should be completing steps 1-3 on their

own. However the measurements should be done as a group with one or two people alternating the

measurement and the other student writing down the results.

In the Lab: Now that you’ve got your measurements, we’ll compile them into a Rose diagram (Figure 3).

This is simply a circular bar graph (histogram) that allows us to display orientation data in a way that

matches that data with its compass direction. Rose diagrams offer a quick, clear visual aid to draw

conclusions about trends in our data.

Ask your instructor for as many Rose diagram blank sheets as you’ll need, based on the number of rock

units you’ll be analyzing. We’ll go through this simple procedure as a group. Once you’ve completed

the diagrams, choose the three which give the clearest indication of trends. Using these three “best”

examples, complete the following tasks and write up your results as a historical reconstruction of the

time/space of the Pottsville Sandstone on which we focused. Your write-up should be 2-3 pages, and

should be heavy on connections between your field observations and your paleocurrent and

paleogeographic interpretations.

Your write-up should include:

1. A BRIEF introduction explaining the project and general geology of our site(s);

2. Explanation of your site selection and Justification for these diagrams as the “best;”

3. Results – an explanation of the data including (not in the page count) any tabular data as well as

the rose diagrams themselves;

4. Analysis of results, including the interpretation and significance of the findings;

5. Conclusion that includes the implications of this site evaluation in the historical context of the

region.

Write-ups will be graded on a 25-point scale, using the following criteria:

1. Neatness and Clarity: 5 points

2. Completeness: 5 points

3. Geological Reasoning/Accuracy: 15 points

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Figure 3: Rose Diagram