- Units A-M, Turtle Cove
Member, John Day Formation
- Scientific fossil collecting
involves recording precisely what layer each specimen is found
in. This requires detailed work in stratigraphy, the measurement
and description of rock units within a geologic formation. Field
notes taken by paleontologists working at John Day Fossil Beds
indicate which rock unit each fossil is collected from.
- Fossils are almost always found in sedimentary rocks which form gradually over time as sediments accumulate and become cemented. Changes in sediment or patterns of deposition create natural boundaries between rock units.
- GLOSSARY
- sanidine - a type of feldspar
- biotite - a type of mica
- Ma - "Megannum", millions of years
- tuff - a rock formed from the consolidation of volcanic ash
- Dateable Layers
- Volcanic layers form in a geologic instant, creating new mineral crystals with each volcanic event. The age of these layers can be determined by measuring the radioactive decay of potassium to argon in potassium-rich minerals such as sanidine or biotite. These layers provide the most accurate data about when a particular species lived. Volcanic layers usually do not contain fossils, but are used as age brackets for other layers.
- Volcanic layers that have been dated in the
- Turtle Cove section include:
- Volcanic layers form in a geologic instant, creating new mineral crystals with each volcanic event. The age of these layers can be determined by measuring the radioactive decay of potassium to argon in potassium-rich minerals such as sanidine or biotite. These layers provide the most accurate data about when a particular species lived. Volcanic layers usually do not contain fossils, but are used as age brackets for other layers.
- 22.6 Ma Across the River Tuff
- 25.3 Ma Tin Roof Tuff
- 27.2 Ma Biotite Tuff
- 27.5 Ma Deep Creek Tuff
- 28.9 Ma Picture Gorge Ignimbrite
- 28.7 Ma Blue Basin Tuff
- 29.8 Ma A-B Tuff