Welcome to Pismo Beach! Find parking on Wilmar Avenue, and head southwest to the staircase at the end of the street. Upon descending the staircase, head southeast on the beach to the coordinates. Looking up at the cliff in front of you, you will find the Wilmar Avenue Fault.
The Wimar Avenue Fault is relatively young, defined as late Quaternary. Quaternary is the geologic time period of 2.58 million years ago up to the present. This fault exists at the southeastern base of the San Luis Range. The fault displacement has been measured at a vertical separation of 6.4 meters. This results in an estimation that the fault had a net slip rate of about 0.04-0.08 mm/year.
Facing the fault from the beach, the northeastern side of the fault has some interesting characteristics (the taller, sloped side of the fault closer to the staircase). This section has older strata (layers) of Miocene rocks placed above younger Pliocene strata. The Miocene-epoch rocks are from the Rincon and Obispo formations, while the Pliocene-epoch rocks are from the Squire Member sub-unit of the Pismo Formation. The Pliocene strata also appear to have been overturned. The Squire Member beds are folded into an overturned syncline.
To log this Earthcache, please send me a note with the following:
1. Name of this Earthcache and umber of people in your group
2. Can you see the split between the two layers where older, Miocene rocks are above younger, Pliocene rocks? Estimate how high up these layers meet from where you stand.
3. Explain why it makes sense that the layers of Pliocene rock are overturned.
Sources:
-Lutz, A. T., Abramsonward, H., Lettis, W. R., Thompson, S., and Page, W. D., “The Pismo Formation and evidence for Pliocene tectonic evolution of the San Luis Range, central coast California”, vol. 2011, 2011.
-Nitchman, Steve P. and D. Burton Slemmons. “The Wilmar Avenue fault: A late Quaternary reverse fault near Pismo Beach, California.” (1994).
-https://pubs.geoscienceworld.org/books/book/434/chapter-abstract/3798739/The-Wilmar-Avenue-fault-A-late-Quaternary-reverse?redirectedFrom=fulltext
-https://www.britannica.com/science/Quaternary
-https://www.pge.com/includes/docs/pdfs/shared/edusafety/systemworks/dcpp/SSHAC/workshops/source_characterization/SSC_0305_Hall_LTSP_Trench_Data.pdf
-https://en.wikipedia.org/wiki/Pismo_Formation
-https://www.mindat.org/paleo_strat.php?id=13322
-https://www.merriam-webster.com/dictionary/syncline
-https://www.jf2.com/geowalk/geowalk.html