Tim N7KOMOct 6, 2024
After #pota at Lighthouse Field State Beach (US-3461). The #UCSC #EPS Reunion crew did a walking #geology tour along Santa Cruz's Westcliff Drive. Connecting terraces with sea level fluctuations and uplift.
Show threadTim N7KOM

We started at the lighthouse, standing on the Westcliff marine terrace. Here we discussed the Loma Prieta earthquake (1989) and if it is a "representative #earthquake" that explains the uplift of the Santa Cruz Mountains.

Loma Prieta took place on a left-step in the San Andreas #Fault. Left-steps in Right Lateral Faults lead to compression and a thrust component. It is one way to build mountains. #LomaPrieta #SanAndresFault #SAF

Oct 6, 2024 at 10:05pmWeb
Show threadTim N7KOMOct 6, 2024
Fun fact: the original location of the lighthouse is now in the ocean, where the small cove at #SteamersLane is. The cove was not there when the lighthouse was built. 10 years after construction the lighthouse was moved 500 feet inland.
The lighthouse that current stands is actually the third iteration of the lighthouse. It was moved a third time, closer to the coast at the behest of a historical society. Of course, if you don't learn from history you are bound to repeat it. There is actually a sea-cave currently undercutting the lighthouse. It will need to be moved yet again at some point in the future.
Show threadTim N7KOMOct 6, 2024
As we walked along the terraces we had the opportunity to look back and make observations that the terraces are tilted slightly. Looking south to Santa Cruz the terraces get lower due to folding. #geology
Show threadTim N7KOMOct 6, 2024
There is another terrace level at sea level. There is debate about if this is a different terrace forming event or just a harder rock layer under a softer rock layer that makes up the west cliff terrace.
One thing to consider is that we are currently in an interglacial period. There is a terrace that has been mapped under the sea and correlates with another interglacial period (~7000-10000 years ago). This also means that most of the archeological sites from this time are under water.
We have human footprints that were dated 20,000 years ago and human remains from 9000 (IIRC).
Show threadTim N7KOMOct 6, 2024
FYI, I'll need to review my notes to confirm those dates. I think those are ballpark though.
Show threadTim N7KOMOct 6, 2024
Our walk took us to Swift Street and we got lucky with the low tide. We caught up with a UCSC Intro to Structural Geology class on a field trip.
Show threadTim N7KOMOct 6, 2024
This is really a fantastic outcrop. Here there are cold-seep fluid escape structures. Biogenic methane intruded into the existing mudstone, these columns deformed the mudstone and eventually calcified. The structures exposed now are commonly referred to as "toilet bowls" due to the ring shape. #geology
Show threadTim N7KOMOct 6, 2024
Here's a vertical part of the outcrop. The methane seep structures penetrated the mudstone and sometimes stick out in a blobby way. Pop quiz: Why do you think these blobs stick out from the mudstone?
Show threadTim N7KOMOct 6, 2024

How do we know that the mudstone was already lithified (a rock) when the methane seeps intruded them?

The answer is in the fractures we observe. The mudstone is highly fractured, but these same fractures do not cross into the blobs. The methane blobs deformed and fractured the mudstone when they intruded.

Show threadTim N7KOMOct 6, 2024
Here's another blob with a deposit of unfractured (relatively) sandstone nearby.
Show threadTim N7KOMOct 6, 2024
The students got to practice their geologic sketching skills. Across the beach from the outcrop are two zones of fractured rock separated by a zone of sandy rock. Where they are separated is a pile of boulders for erosion control. Why might there be two distinct spots where erosion needs to be controlled?
There are two faults cutting the cliff and they are weaker rock. Thus they erode easier.