# Practice Interlude: Stratigraphic Correlation

You have been hired to draw a cross-section from four borehole logs provided by an exploration company. The cores were drilled in a very level landscape so the top of each represents the same elevation. On this page you will find a summary of the types of data the company has provided, and instructions on how to use the data.

# Borehole Log Data

Borehole logs can be found on the pages that follow. The logs contain the following information:

### Depth

For each borehole you are given the depth of each sedimentary facies contact below the ground surface.

### Facies Description

The company has also provided some preliminary information on the sedimentary facies. For each facies they may have identified the depositional environment, provided a description of the rock, or collected a rock sample for you to identify.

### Facies Labels

Each facies is labelled using two identifiers. The first identifier is a number that indicates which core the facies is in. The second identifier is a letter that gives the locationrelative to the bottom of the core.

Example 1: Facies 1-A is in Core 1. It is at the bottom of the core.

Example 2: Facies 1-B is in Core 1. It is the unit immediately above Facies 1-A

# What to Do With the Borehole Log Data

### Worksheets

Start by downloading the field notes and correlation worksheets. Even if you don’t have a printer, you will need these for reference. Note: Field Notes worksheets contain the same borehole log information as the pages in this manual except for sample images.

Lab 4 Practice Field Notes.docx

Lab 4 Practice Field Notes.pdf

Practice Correlation Worksheet.pdf

If you don’t have access to a printer you can draw out your own version of the worksheet. The key thing to keep in mind is that you make yourself a vertical scale. The scale on the worksheet is 25 m increments spaced 1 cm apart. If you use letter-sized paper in a portrait orientation, you can squeeze in a scale of 20 m increments spaced 1 cm apart. Start 1 cm from the top. (Letter-sized paper is 27.9 cm long, and this scale will take up 25 cm.) You can tape a number of pages together if you need more room to work.

### Analysis

a. For each stratigraphic log, mark the depth at which each facies occurs. For example, Facies 1-F extends from the ground surface to a depth of 125m. Draw a line in Core 1 at 125m, and above that write “1-F” to identify the facies. Facies 1-E occurs below Facies 1-F, ending at 250m. This is labelled on the worksheet. Draw a line at 250m in Core 1 to indicate the bottom of 1-E. Continue until all the facies layers are shown in the stratigraphic logs.

b. Where a facies has already been interpreted: write the name of the facies name on the stratigraphic log beside the unit.  In Core 1, the facies for the 120m – 250m depth interval has been identified for you as “1-E Marine Shelf.” Use the same format to add facies to the diagram.

c. Describe the sedimentary rock samples collected. Space is provided in the field note section for your rock description (Descr.).

d. For the facies that have not already been identified: use the provided written descriptions and/or your rock identifications to determine the depositional environment for each facies. Write these on the stratigraphic log worksheet as you did in b. NOTE: At this point, you may not have enough information to determine the depositional environment for all facies. See (e) below.

e. Draw lithostratigraphic correlation lines to connect equivalent rock units. Use solid lines for those correlations that you are confident about, and dashed lines for more uncertain relationships. Using what you have learned about which depositional environments occur beside each other, identify where there may be lateral facies changes or where deposits pinch out. Use this to help you determine the depositional environments for any unidentified facies.

f. Identify and label possible unconformities. There are disconformities or nonconformities only.

This same cross section will be used again in Lab 9. During lab 9, we will examine the fossil content of these units to determine time-stratigraphic correlations between the drill cores. This additional stratigraphic information will help you to complete the lithostratigraphic correlations you started in d and e above, as well as adding time-stratigraphic correlations.