#unconformity

Crealock37

Things to Know

Unconformity (2022)

Unconformity is a word used quite often by geologists. This is in reference to a contact between rocks that are unconformable or that were not deposited in sequence. There are several types of unconformities. Take the one above that I took at John Day Fossil Beds. The pale strata at the bottom of the hills are the fossil bearing John Day Formation from the Eocene/Oligocene (56-23.03 Ma). This is primarily welded tuff and tuffaceous siltstones. The dark strata above that is the Miocene Columbia River Basalts.

There is a distinct erosional surface between the two layers. This is called a disconformity. A disconformity is  an unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition.

In the Wind River Gorge in Wyoming, there is another kind of unconformity called a nonconformity. A nonconformity occurs when sedimentary rocks are deposited on top of igneous or metamorphic rocks that have been eroded.  In the above photograph, Cambrian Flathead Sandstone was deposited on top of eroded Archean granite.

Perhaps you have heard of the Great Unconformity in the Grand Canyon? It's pretty famous even by non-geologists. It is the final kind of unconformity, the angular unconformity. An angular unconformity is a gap in the geologic record where tilted, eroded rock layers are overlain by younger, horizontal layers of sedimentary rock.

In review:

Tune in tomorrow for a look at the father of paleontology and make sure you remember today's terminology. It will be important for next Tuesday! Fossilize you later!