Course project 1:

Virtual field trip to the Jackfork Group, Arkansas

Goals of the project

Every good research project must begin with a question. In this case, the question is, " how were the rocks of the Jackfork Group deposited and in what type of environment were they originally formed?"

This project is designed to get you thinking about some of the basic properties of sedimentary rocks that can be used to interpret their environment of deposition. In particular, we will focus on characterizing lithology (rock type), bedding style, and grain size for rock units from the Pennsylvanian age Jackfork Group. I built this website to provide you with some background information for the project, but a great deal of the background and techniques you'll need to do the project will be gained in class.

NOTE: You will need Adobe Acrobat Viewer to see some of pdf files that are links below.

Background

The Jackfork Group (JFG) outcrops in two general belts in the Ouachita Mountains of Arkansas and Oklahoma. The northern, or "outer" belt and the southern, or "inner" belt.

To see an outcrop map of the JFG, click here.

Outcrops are typically found in roadcuts, quarries, and dam spillways, where excavation has exposed particularly thick sections of the JFG units. In this project, you will be examining outcrops exposed at the DeGray Dam spillway, a classic outcrop that has been investigated by sedimentologists for the better part of the last 30 years. You'll notice that the DeGray spillway is located in the "inner" belt, near Arkadelphia, AR. In general, when you begin a project such as this, it's a good idea to get a sense of the overall stratigraphy of the region.

To see a generalized stratigraphic column for the Ouachita units, click here.

This stratigraphic column is very generalized. It shows the entire Jackfork Group as being comprised by sandstone, which is definitely not the case. It's just that, when compared to underlying units, it is much more sand-rich.

Part 1: Lithologies of the Jackfork Group

You will see examples of the most common rocks of the JFG in class. One of the rock types, mud clast breccias, are fairly difficult to show in hand specimen, mainly because they tend to fall apart when you try to sample or handle them. I have provided two outcrop photos of these units, to give you a flavor of what they look like: Mud clast breccia 1 and Mud clast breccia 2.

Mud-rich sandstones of the JFG are typically found in beds like these. Notice in this photo that the lower bed is very red in color; this is the mud-rich sandstone unit. This bed is separated from the mud-poor sandstone bed above it by a thin mud-clast breccia and an even thinner (< 1 cm) shale.

In thin section, you can see pronounced differences between mud-rich and mud-poor sandstones.

 

Part 2: Bedding style of the Jackfork Group

Morris (1977) measured the first, reasonably detailed section of the Degray Spillway. His section is helpful in terms of understanding the style of bedding in the JFG and how the different lithologies are related to one another, through time. Click here to see the stratigraphic column for the DeGray Spillway section measured by Morris (1977) You might find that this section is more easily viewed if you print it.

The Lower DeGray Spillway section, corresponding to the lower third of the section of Morris (1977), and has a different style of bedding than the Upper DeGray Spillway section. You can also check out the middle portion of the section by clicking here.

I measured a very detailed section from the Lower DeGray spillway that roughly corresponds to the outcrop photo shown above. This section is from the east wall of the spillway, but there is also a west wall!! I measured a comparably detailed section their and was able to create a correlation diagram between the east and west walls; these sections are about 100 m apart.

 

Part 3: Grain size trends in Jackfork Group sandstones

Grain size data are quite useful for interpreting the depositional history of sedimentary rocks. You will be working with grain size data that I collected using a petrographic microscope (you'll get some practice doing this in class, just to see how it's done and to get you familiar with the data you'll be working with). In the field, I collected hand specimens from the outcrop through a number of sandstone beds--sampling them every 10-20 cm or so upward through the bed--and made thin sections of them. Each sample has a unique code, labelled as YEAR-LOCATION-SAMPLE#. For instance, if I collected a sample in the 1996 field season from the DeGray spillway location and it was the 51st sample I collected, it would be numbered coded as "96-DG-51". Every one of these samples was keyed to a specific height in my detailed measured sections and in the section of Morris (1977). For this project, you will have access to several parts of this dataset:

 

References

Morris, R. C., 1977, Flysch facies of the Ouachita trough; with examples from the spillway at DeGray Dam, Arkansas, in Symposium on the geology of the Ouachita Mountains, Volume I, Stratigraphy, sedimentology, petrography, tectonics, and paleontology, Little Rock, Arkansas, p. 158-168.