Northern Virginia Community College Lithostratigraphic Correlation Questions

Directions

The lithofacies maps from the previous exercise give us a view into the environment at one point in time in Earth’s history. Analysis of strata across a geographic extent will give us a much more in-depth picture of how the environment has changed over geologic time. Exposure of strata across a wide geographic area, like the Grand Canyon, is relatively rare. Most locations on Earth only provide a small subset of the entire stratigraphy that exists beneath our feet at any given location. Places with little elevation change and a flat, featureless landscape may provide no evidence at all of what rocks lie below. In Virginia, for example, we see strata exposed in road cuts, quarries, and river channels but we never get a good look at the entire stack of strata as it may exist at any location within the state. For areas without good rock exposure (outcrop), we must use drill core and well data.

When geologists do see rock exposed at the surface, their first instinct is to note all of its characteristics and relationships that may exist, many of these you have already learned about in previous modules. Regardless of whether the rock is igneous, metamorphic or sedimentary rock, geologists record as many defining details as possible so that they may associate one particular rock outcrop with any other known rock exposures in the vicinity. This is the beginning of the process of lithostratigraphic correlation that we will look at in this lab exercise. When we correlate, we are trying to determine how the rock at our location fits into the known stratigraphy for the area.

Igneous and metamorphic rock primarily occur as the foundation of the continental crust which geologists call “basement rock.” Basement rock gets covered by sedimentary layers over geologic time. This is particularly true in tectonically quiescent areas where no active mountain building is taking place. This would include the vast areas of passive continental margins (like the current east and Gulf coasts of North America) and epicontinental seas. Basement rock can be revealed again in areas of mountain building where the basement rock gets shoved up above the surface by tectonic processes or plowed over cleanly by advancing glaciers.

We have learned for the reading that the basic lithostratigraphic unit is called a formation. In order to be given this designation, a layer of rock must be laterally continuous and mappable across an area. It must bear very similar characteristics when seen at locations across that area, and have similar, distinct boundaries, or contacts, with layers above and below. A formation is distinguishable from location to location.

Let’s take a look at roadcut in West Virginia along US Hwy 48 (Corridor H) in the gigapixel image below. The first thing that you probably notice is that the layers are tilted to the right. The photos are from the Valley and Ridge Province of West Virginia where the rock layers have been folded during the formation of the Appalachian Mountains. We are looking at one limb of an anticline/syncline pair. We can see a distinction between two different rock formations by the difference in color. The lower (older) rock unit is named the Brallier Formation (abbreviated, Fm) and is largely composed of black and grey shale. The younger, upper rock unit is the Chemung Formation which is composed of light colored sandstone. If you zoom in just a bit you can see several thin beds of tan sandstone within the grey/black Braillier Fm. There is a relatively distinct contact between the two formations – black/grey shale changes rather abruptly into tan sandstone. ***NOTE*** This is a facies change. The environment was changing between one of offshore mud and clay to nearshore silt and sand. Geologists say that “it’s a good day when you can put your finger on a contact,” especially one as well defined as this. This facies change also represents a marine regression (see figure below image). Shale overlain by sandstone is an indication that sea level was going down and the shoreline was moving, in this case, to the west. These formations are from the Devonian Period of the Paleozoic Era. At this point in geologic history the Appalachian Mountains were being built to the east. As the mountains were building in the east, they pushed the shoreline to the west and new mountains meant an influx of siliciclastic sediment. The offshore environment was displaced by a nearshore environment resulting in the facies change.

***NOTE*** If you are have problems seeing the Gigamacro images, switch to another browser. Firefox tends to work the best.

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View the image below. Here is another location along the same highway where the Brallier Fm is exposed. The distance between these two roadcuts is several miles however we can identify and correlate these two exposures as being from the same formation by comparing the rock. One thing to note – the beds in this roadcut are dipping in the opposite direction than those seen in the previous exposure. We’re on a different limb of the anticline/syncline pair.

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Lithostratigraphic Correlation Example Problems

Let’s take a look at an example of how geologists perform lithostratigraphic correlation. In this problem (see images below), we are presented with localities 1, 2 & 3 where sedimentary rock layers are exposed at the surface (1 & 2) and a third locality where the sedimentary strata is known from well drilling information. It is an interesting note that whenever a well is drilled, whether for water, fossil fuels or any other exploration, there will be a geologist or technician onsite that will provide detailed descriptions of the rock units brought up to the surface. This person is often referred to as the “well logger.”

The localities in the diagram may be separated by many miles. A few different types of rock units are exposed at localities 1 & 2 and a complete stratigraphic column is found at locality 3. The three steps between diagrams will show how the correlation process proceeds. In Step 1, we complete a detailed investigation of the characteristics of each rock unit. In Step 2 we correlate those rock units that are the same. In Step 3, we draw a complete geologic cross-section diagram of the subsurface rocks.

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Problems for you to complete

Problem #1

The diagram below provides stratigraphic columns across western Montana. This is the same location from which the data was taken to build your Lithofacies map from the Cambrian Period of geologic time. The red line that runs through the bottom of the sections represents the appearance of a trilobite fossil from the Cambrian. You can compare this data with your lithofacies map. Save this diagram to your computer to complete the lithostratigraphic correlation using straight lines as shown in the practice problems. Do not attempt to freehand the correlation. The lines must be drawn straight. Simple drawing tools are available using many different applications. Google Drawing is a free drawing tool that is available from Google.

The different lithologies are as follows from oldest to youngest:

• (Youngest) Red Lion Fm = Limestone which can have a small amount of shale at its base
• Hasmark Fm = Dolostone
• Park Shale = Shale
• Meagher Limestone = Limestone
• Wolsey Shale = Shale
• (Oldest) Flathead Quartzite = slightly metamorphosed Sandstone

Steps to complete:

1. Save this diagram to your computer.
2. Use a drawing program to complete the correlation. Google Drawing is a free application but you can use any application that you’re familiar with to draw straight lines.
3. You can also download and complete this by hand but you must use a ruler.
4. Important Note: ***Stratigraphic Correlation is completed using straight lines between sections. Do not try and draw this on a trackpad or freehand on your computer. Your lines must be straight or you will receive reduced credit.***
5. Save your completed correlated diagram to your computer as a PDF. If you completed the correlation by hand, take a picture of your completed diagram, upload it to your computer and save it as a PDF.
6. Upload the PDF using the “Submit” button found at the top, right of the assignment directions.
7. A correctly correlated submission will be worth 15 points toward your grade on this assignment

Picture 6 I will post later

Problem #2

This following series of stratigraphic columns were taken across a border section of Pennsylvania and New York. The age of these rocks is Devonian. These rock formations are very similar to the rock formations that we looked at in the beginning of this lab discussion from the Valley and Ridge of West Virginia. Note: ***The intervals of relative time when sediments were being deposited at each location are indicated by the small letters.

Steps:

1. Save this diagram to your computer.
2. Use a drawing program to complete the correlation. Google Drawing is a free application but you can use any application that you’re familiar with to draw straight lines.
3. You can also download and complete this by hand but you must use a ruler.
4. Important Note: ***Stratigraphic Correlation is completed using straight lines between sections. Do not try and draw this on a trackpad or freehand on your computer. Your lines must be straight or you will be told to do it over correctly.***
5. Include your name on the correlation problem.
6. Save the completed correlation problem to your computer as a PDF. If you completed the correlation by hand, take a picture of your completed diagram, upload it to your computer and save it as a PDF.
7. Upload the PDF using the “Submit” button found at the top, right of the assignment directions.
8. A correctly correlated submission will be worth 15 points toward your grade on this assignment.

Picture 7 later