Time Frame – 40 minutes

Materials

• Diagram of distribution of foraminifera during Miocene time (Figure 1)
• Sample locality map San Joaquin Basin, California (Figure 2)
• Illustration of various foraminifera found in Miocene rocks samples (Figure 3)

Advanced Preparation

In order to complete this activity, the student should have a concept of the foraminifera and how different benthic foraminifera prefer a particular environment, which may be associated with water depth.  The student should also have had an introduction to the concept of geologic time and feel comfortable working with a map.  Familiarity with the concepts of petroleum reservoir rock and source rock from studies of earth sciences is useful.  Introduce the concept that benthic foraminifera live in a preferred environment; but after their death their shells may be transported to a different environment.  Transportation occurs downslope due to gravity processes.  Therefore, in a sample the students will find a mixture of foraminifera, specifically a death assemblage, representing foraminifera living at one time in that environment and foraminifera transported from shallower water depths into that environment.  Guide the students to initiate the mental leap of understanding that in order to interpret the water depth in which the foraminifera were deposited and later buried, the deepest dwelling forms represent the true water depth.  Specifically, at a present water depth of 1,000m, foraminifera will be found there which preferred to live in that environment as well as foraminifera that were brought in from shallower water by downslope processes (i.e. the Amazon Fan moves sediment, including foraminifera, in a downslope direction off of Brazil).  Foraminifera preferring parameters characteristic of water depths greater than 1,000m would not be found at this sight.  The students should interpret past water depth based on the deepest dwelling form found in the sample.

Procedure

1. Examine Figure 1 to learn about the habitats of benthic foraminifera during Miocene time (about 6 Ma for this example).  For this exercise, the Miocene ocean is divided into four zones based on water depth: 1) 0-50m; 2) 50-150m; 3) 150-500m; 4) 500-1500m.  The names of the foraminifera that prefer each environment are listed, and a schematic drawing for each species is shown.  As a research scientist for Earthquest Inc., you have been assigned to work in a field area in the southeastern part of the San Joaquin Basin of California. Earthquest Inc. is interested in the Miocene rocks of this area because in other parts of the basin, age equivalent rocks contain petroleum reserves. Your job is to collect samples and discover what environments were present here approximately 6 Ma. Specifically, beach sands have been found to be good reservoir rocks for containing oil.  Silt and clay sediments with abundant organic material deposited in water depths of greater than 1,000m have been found to be good source rocks for oil. Your job is to evaluate the potential for both good reservoir rock and source rock in the study area.

2. Figure 2 is a map of the different localities you chose to sample in the study area. Notice that the Sierra Nevada Range borders the basin you are studying to the east. There are 10 samples collected from Miocene rocks.  You disaggregate the sample material to obtain any microfossil material, specifically foraminifera, from the rock. Next, you examine the fossil material and sediment under a microscope. The contents of the various samples are shown in Figure 3.

Formative Assessment

Make an interpretation for water depth for the various samples you have analyzed.  Notice that each box displays the various foraminiferal species found in the sample.  Compare each sample in Figure 3 with the key to Miocene environments in Figure 1. In the blank for paleo-water depth interpretation under each sample, put a water depth range inferred from your analysis.  Notice that Sample 2 has been interpreted for you. What would cause the lack of foraminifera in Sample 9?  Look at the sample’s position to other samples on the map in Figure 2.

After you have made an interpretation for each sample, mark these paleo-water depth numbers on your map (Figure 2). Notice that the interpretation for Sample 2 has been done for you. Look at the distribution of water depths on your map. Based on this information could you give an estimate of where the beach was located during Miocene time in the study area? Using a colored pencil, highlight and label the potential trend of the beach (shoreline) and its location. Do you see potential for source rocks in the study area during this time? If you do, highlight and label the potential source rock area with a different colored pencil. State your recommendation to Earthquest Inc. as to the description of the general environment of this area during Miocene time and the potential for reservoir and source rock in the area. Make a specific recommendation to your company as to whether they should continue looking in this general area for hydrocarbon prospects based on your research.

Summative Assessment

Instructor will lead students through the visualization module on the Ice Age. The CD is included at the back of the workbook. This CD-ROM visualization module was developed at the University of Minnesota, by the Department of Geology and Geophysics. The module was funded in part by the National Science Foundation. This material requires the installation of Quicktime version 3.0 or greater. To install this go into the folder called Quicktime on the CD and click on the installer icon. Once Quicktime is installed, begin the module by clicking on the TheIceAge.htm file. (If the movies are not working correctly, try reinstalling the Quicktime on this CD.)