24 January 2023
A coastal cliff collapse at Blacks Beach, San Diego, California
Posted by Dave Petley
A coastal cliff collapse at Blacks Beach, San Diego, California
On 20 January 2023 a large section of coastal cliff failed at Blacks Beach in San Diego, California. The landslide was captured on a video that has been posted to Youtube by Kent Ameyaro. It is well worth viewing:-
There is another video of a part of the landslide on the KTVB Youtube channel:-
And there is a nice view of the aftermath of the failure on the CBS 8 San Diego Youtube channel:-
The landslide at Blacks Beach is located at 32.885, -117.253. I’ve found a nice description of the geology of this area, including this image of the sedimentary sequence at Black Beach. The Ardath Shale is described as:
“…a mix of shale, silty shale, with some beds of sandstone and conglomerate that are similar in physical appearance to some beds in the other Eocene-age rock formations. However, the Ardath Shale is dominated by steep-slope-forming shale (not the nearly vertical cliffs of the sandstone and conglomerate beds). The soft shale beds erode easier forming alcoves beneath waterfalls and is susceptible of forming slumps.”
The landslide itself is complex. The principal mechanism of failure is a deep-seated rotational failure, which goes through its main (most rapid) movement phase from about 3:00 for about 45 seconds. During this time the uplift of a shale bed at the toe develops beautifully, and the slumping at the crown of the landslide is also clear (and is evidenced by the exposure of less weathered material at the base of the cliff at the top of the slump block in the image below):-
In the three minutes leading up to this point multiple smaller failures and collapses can be seen. This is probably an indication that movement was occurring on the rotational shear surface, causing extensive internal deformation to the slump block, driving these smaller failures.
After the main phase of movement the landslide became more stable, but smaller collapses continued to occur. This is common in such circumstances.
Whilst the landslide occurred on a sunny day, California has endured a succession of atmospheric river events that have been associated with heavy rainfall and large waves. Thus, the landslide might be the result of higher than normal pore water pressures and/or toe erosion.
Many thanks to loyal reader Jonas Van Rutte for highlighting this landslide to me.
Whooops! I ahould have looked at your blog before emailing you about this. For a non-geologist, I didn’t do too bad a job of guessing what I was looking at: I thought that was shale, and I figured out something rotational was going on, but I couldn’t understand how a landslide could be creating uplift. “Deep-seated failure” is the key: I take it the sandstone blocks falling down from above are not the cause, but more effect, and the real failure is at the level of the shale but out of sight, rotating down as that chunk rotates up.
I’m still a little puzzled at the mechanism, but this helps.
Bit by bit, reading various AGU blogs over the years, I am slowly getting a bachelors level education in geology? Your blog has been one of the most educational… thank you!
“During this time the uplift of a shale bed at the toe develops beautifully”
At the end of the film someone puts their hand on this new exposure. I assume there’s been a huge amount of friction, so are the rocks warm to the touch?
Isn´t this more a case of competent rock on an incompetent base? Isn´t the (black) shale bed at the toe squeezed out by the weight of the competent block causing the tower- or slab shaped competent blocks to topple?
[That is possible, although previous studies of failures on this section of cliff indicate that failures occur through rupture within the clay strata. See for example: https://digitallibrary.sdsu.edu/islandora/object/sdsu%3A30251 D.]
Later vids show most of the exposed toe eroded by the next high tide:
It was a textbook example of a slide daylighting in flat ground out in front of the face. There may be recurrent movement as the surf planes off that bit of resisting mass.
great video. The start is all superficial small to medium to large rock falls and then movement on the deeper failure plane reveals itself. Thanks for posting.
Thanks for up-dating “Slope movements by blocks sliding on horizontal bedding surfaces” by F.O’Brien,D.Cruden&T,H.Kim(Proceedings 5th Canadian Conference on Geotechnique & Natural Hazards,Kelowna 2011).The rock falls & topples are from the passive block of the January 29,1982 compound slide as it continues its slow journey towards the beach.
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Remarkably event captured with video is just another occurrence on the eroding southern California coast. I was privileged to work as a graduate student intern with Gerald Kuhn and Professor Francis P. Shepard at Scripps conducting coastal erosion – during the 1977-78 exceptional wet season that followed a 40-year period of benign weather. We observed numerous coastal bluff failures, from Pt. Loma to Oceanside, which ranged from similar deep-seated rotational failures, to block falls (110-ft section of bluff at Encinitas which took out the beach stairway, and erosion of soft sediment cliffs at La Jolla Shores just down the beach from Scripps. These features were documented in the reports for the Sea Grant project as well as a book written by Kuhn and Shepard. Two decades later, Gerry continued observing and mapping geologic activity along the coast in May, after another El Nino winter, when three or more large bluff failures occurred at San Onofre State Beach. In that area, the more rigid Monterey formation slide out over a weaker Miocene mudstone, which produced an elevated toe in the surf zone to the benefit of surfers and fishermen. The Echo Arch campground and roads were completed disrupted by the northern most slide, which continued to move for years afterward — eventually covering up the exposure of the Cristianitos fault at the south end of the San Onofre Nuclear Generating Station. For my interests in strike-slip faulting, with transpression and transpression, the lateral fault on the side of one of the landslides provided an outstanding geologic-scale “sand-box” model of Wrench Fault Tectonics. Vertical cut in the offset beach sands showed a set of inward (toward the slide) dipping reverse-separation faults whereas the surface exposure showed left-stepping echelong Riedel shears – classic dextral oblique-reverse fault character. Unfortunately, high surf the following night eroded the beach deposits. Gerry completed a report for the CA State Parks describing these landslides, and historical slides that we investigated in 1977-78 with aerial photos and field studies. Interesting feature for these was that sumps formed inland from the sea cliff which eventually became the head scarps for the large translational slides. Further studies by Scripps scientists have been published in the area in the past several years.
Thank you so much for this fascinating information! I was near this collapse when it happened and had no idea (we were at Dike Rock tidepools.) I have been trying to find a list of past Torrey Pines State Beach bluff collapses and landslides. So far, I can only find information as far back as 2008. Could you give me a hint of where I might find more information? I am not a geologist, but I am trying my hardest. Thank you!