Friday, March 15, 2013

Repost: Blackhawk landslide, California - from Pathological Geomorphology

This article was originally posted at the soon-to-be-defunct site, Pathological Geomorphology, and was transferred here (thanks Kyle!) to the active Pathological Geomorphology (Tumblr). My original post was a response and addition to Callan's Blackhawk landslide post on Posterous, which is now here on the Tumblr site. I've transferred the original photos, added captions, and added a Google Maps location link at the bottom. I've added the original comments (with soon-to-be-dead links to Posterous) at the bottom. Also see this related post here on LFD.

I'm posting a few additional views of the Blackhawk Landslide in California, to emphasize the truly pathological geomorphology of the area. In the views showing slide toe and slide source, I've moved the source area farther into the mountain from the original given by San Diego State [dead (kmz) link originally sourced here]. I've done this based on prior knowledge and published maps. Basically, the entire steep northern face of Blackhawk Mountain is the slide source area.
Google Earth view showing the slide source and slide toe.
Google Earth view looking from above the toe toward the source.
Google Earth view looking from above the source toward the toe.
Additionally, there are other older landslides in the area, most notably the one sometimes called the Silver Reef landslide, which is just east of the Blackhawk. Also, many additional breccias and possible landslide deposits have been mapped in the area. The geologic maps (Big Bear City quad and Cougar Buttes quad) are really awesome, especially if you know how complex the geology really is. There are numerous thrusts from the south, placing brecciated rocks of multiple ages on usually older rocks. In at least one place, bedrock has been pushed over one of the older breccias. Or is it really bedrock?

Back in the 1980's, I mapped a large area of Blackhawk Mountain and surrounding areas at 1inch = 500 feet. I found, and so have others working in the nearby northern slopes of the San Bernardino Mountains, that if you start at the top, clearly in bedrock, sometimes brecciated, and map down, then you will be tempted to map bedrock until you are clearly in landslide debris. If you start from the top and map up, the opposite is likely to happen.
Google Earth aerial view.
Aerial view with annotated geology.
The reason for northward thrusting is the bend in the San Andreas fault, causing the westward, southward side to be pushed northward, breaking over the steepened San Bernardino Mountains in low-angle faults. The thrusts cause brecciation, and the steepening, brecciation, and low-angle faulting predisposes the area to massive sliding. Stratigraphy is somewhat retained in the slides, and gold has been mined from nicely pre-broken landslid rock of the Blackhawk Slide. Silver occurs in somewhat disturbed veins in the Silver Reef Slide.

In the annotated view, the Younger LS deposits (yellow) are Holocene or late Pleistocene; the Older LS deposists (turquoise and pink) are middle or early Pleistocene; the Moderately Old LS deposits (purple and orange) are middle or early Pleistocene; the very old debris flow fan deposit (purple) is middle or early Pleistocene; and the QT Breccias (blue and red) are Pleistocene or Pliocene.

Lockwood DeWitt responded:
Lockwood DeWitt
Nice! The annotated map/image at the end really help clarify the messyness of interpretation here.
Silver Fox responded:
Silver Fox
And it doesn't even show the really screwed up bedrock!
Katharine North responded:
Great images, Silver Fox! I'm going back and forth between the annotated image and the non-annotated ones, looking at the landscape differently each time, picking out the details. I like your note about the effect of where you start mapping - good to remember!

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