The Tioga Bentonites are a series of ash bed layers occurring in three Sedimentary basins in the eastern and midwestern United States. The primary basin they are found in is the Appalachian Basin, as well as the Illinois Basin and the Michigan Basin. Due to an unconformity these ash beds are not present in the southern Appalachians.[1]
| Tioga Ash Beds | |
|---|---|
| Stratigraphic range: Lochkovian-Eifelian ~ | |
| Type | Geological formation |
| Unit of | Helderberg Group, Onondaga Limestone and Marcellus Shale |
| Sub-units | Bald Hill, Sprout Brook, Tioga Middle Course Zone, and Tioga A-G |
| Lithology | |
| Primary | Tuff |
| Location | |
| Region |  Indiana,  Illinois,  Maryland,  Michigan,  New York,  Ohio,  Pennsylvania,  Tennessee,  Virginia and  West Virginia |
| Country |  United States |
| Extent | Appalachian Basin, Illinois Basin and Michigan Basin of eastern North America |
| Type section | |
| Named for | Tioga Gas Fields |
| Named by | Ebright, Fettke, and Ingham |
Description
editThere are 7–15 layers of ash depending on the location covering ~30 million years. These numerous layer are broken down into 4 clusters. The Bald Hill K-bentonites are located in the Kalkberg Formation or New Scotland Formation were deposited 417.6 million years ago. The Sprout Brook K-bentonites are located in the Esopus Formation or Needmore Shale deposited 408.3 million years ago. One cluster called the Tioga Middle Coarse Zone (MCZ) is located in the Onondaga Group deposited 391.4 million years ago. Finally Tioga A-G K-bentonites are located in the Onodaga Group and the Union Springs Member of the Marcellus Formation deposited 390.0 million years ago.[2]
The volcanic ash was deposited in sea water.[1] After sedimentation covered these beds the ash was slowly converted to a meta-Bentonite called Illite.[3] These layers are important because they allow very precise dating of the ash bed with K–Ar dating. Potassium 40 decays at a known rate into Argon 40. Argon is an inert gas. It is released from the rock once it is molten so once the rock or ash solidifies the clock is reset. So when the ratio between K40 and Ar40 is measured the more Ar40 trapped in the rock compared to K40 the older the rock is.[4]
References
edit- ^ a b Dennison, J. M. (November 1, 1986). "Tioga Bentonite in the Appalachian basin: Final report". OSTI 7057656.
{{cite journal}}: Cite journal requires|journal=(help) - ^ Ver Straeten, Charles. "DEVONIAN STRATIGRAPHY AND K-BENTONITES IN THE CHERRY VALLEY – SCHOHARIE VALLEY REGION". Research Gate.
- ^ Keller, W.D.; Reynolds, R.C. (January 1986). "Morphology of clay minerals in the smectite-to-illite conversion series by scanning electron microscopy". Research Gate.
- ^ "Potassium-Argon Dating". August 10, 2010. Archived from the original on August 10, 2010. Retrieved February 22, 2023.