The Williston Basin



The Williston Basin contained some of the largest oil deposits in North America.  Consequently, it has been explored extensively by oil geologists.  Exploration is typically done by drilling, and retrieving Ņdrill cores,Ó some 8 inches in diameter, extending from the surface to the depth of the exploratory drilling.  By use of micro-fossils contained in the drill core, oil geologists can assign different sections of the core to different geological eras.  Because some of the rock layers, or Ņrock unitsÓ are exposed on the surface in various locations in Montana, the Dakotas, and Saskatchewan, it is possible to identify particular layers that have been named.


Cores from North Dakota contain the entire geological column, from the present to the Precambrian.  At least, this is the interpretation that geologists have used, based on the fossils.  One of the things we really want to know is the actual ages of these rocks.


Igneous rocks like granite and basalt are particularly good for age determination.  Any radioactive potassium (K40) that they contain will decay at the standard half-life of this radioactive element.  K40 decays to non-radioactive Ar40.  Argon is a gas, and escapes into the atmosphere as soon as it is formed, unless it is trapped in solid rock.  Igneous rock formed from magma; argon escapes from magma. Therefore, any Ar40 trapped in such rocks has accumulated since the rock solidified.  By careful measurement of the amounts of K40 and Ar40 in igneous rock, therefore, it is possible to determine how long it has been since that rock formed.


The particular core that we will examine is from a drill hole of about 10,000 feet.  The core, if reassembled into a single unit, would look something like this:



 - 10,000 feet


Different types of rocks have different appearances, in terms of color, texture, chemical composition, etc.  These different characteristics allow us to identify specific rock units.  The alignment of the core with known rock units is shown here:



The arrows indicate the locations of samples that were taken for analysis.  Below is a table that provides the raw data (the ratio of argon to potassium) and the age that has been calculated from this ratio (feel free to check the accuracy of the calculations).  Measurement of several samples allows the estimation of the error of measurement.  All of the ages are presented in MYA, Ņmillions of years ago.Ó 


Rock Unit

Ratio of Ar40/K40

Calculated Age (MYA)

Deadwood Formation


533 ± 50

Flaxville Formation


11 ± 0.5

Hell Creek formation


63 ± 2

Junction of Lobo Shale and Tullock Members of Fort Union Formation


59 ± 3

Junction of the Mowry Shale and the Newcastle Sandstone


97 ± 3

Junction of the Winnipegosis Formation and the Ashern Formation


395 ± 5

Minnelusa Formation


287 ± 9

Nesson Formation


169 ± 5

Niobrara Formation


83 ± 3

Stony Mountain Formation


452 ± 38

Top of the Opeche Formation


241 ± 8


These are the data.  What should we make of them?