Sunday, May 13, 2012

Green River Formation

Students in 8th Grade Earth Science at Delano Middle School were recently working with samples of the Green River Formation from Southwestern Wyoming.  The formation consists of sandstones, mudstones, siltstones, shales and other sedimentary rock types that were deposited in several freshwater lakes.  The Green River Formation is Eocene in age (48 million years old) and has several layers that are very fossiliferous.  Fossil Butte National Monument (http://www.nps.gov/fobu/index.htm) is located within the formation and is well known for the preservation of fish fossils found there.



The sediments of the Green River Formation are deposited in layers, a dark layer representing the summer months and a lighter layer representing deposition in the winter months.  A dark layer and light layer taken together represent one years worth of sediment deposition.  These pairs of layers are called varves, each varve equals one year.  The presence of varves allows students to determine how long the lake was in existence by counting varves, in a way, very similar to counting tree rings.



The varved shale was obtained from the Ulrich's Fossil Fish Gallery and Preparatory (http://www.ulrichsfossilgallery.com/) as approximately 8 inch by 10 inch sheets, about one centimeter in total thickness.



This larger piece of varved shale was then cut down into individual shale billets for student use.






Using various methods of magnification, students were then able to count the number of varves in each shale billet and determine the number of varves per millimeter.  Each block of students came up with a different class average of varves per millimeter, but our averages ranged from 7.3 - 8.1 varves per millimeter.  Stating this another way, it took between 7.3 - 8.1 years to deposit one millimeter of sediment.

Students were then able to calculate how long it would take to deposit a meter's worth of sediment (7,300 - 8,100 years) as well as the entire 260 meter thickness of the Green River Formation (1,898,000 - 2,106,000 years).  Though it may not exactly match the accepted ages for the formation, the numbers obtained by our students are close to the accepted ages.