Where This Rock Is From (place, type of locality, etc.)
Santa Cruz, California - Panther Beach
What This Rock Is
Coarse sand (from the Margarita sandstone) injected up section into the Santa Cruz mudstone, a local member of the Monterey Formation. About 8 Ma. The sand has "bubbles" of oil-sand floating in it, and flowing wavy laminations and compaction bands give it a complicated and beautiful texture.
See Sherry et al. (2012), available on the author's website (http://eps.mcgill.ca/~tsherry/) for more detail.
What I Like About This Rock
It's really beautiful and records a really interesting and detailed geological history. Don't underestimate sandstone! It isn't always simple. In this case, the sand was injected from below, possibly during an earthquake or other triggering event, and flowed upward through cracks in the mudstone. When it reached its final position, the pore water escaped from the compacting sands, creating a series of dewatering structures. Some of these are crosscut by basically coeval, but slightly slower rising of oil-sand. The whole thing was later cemented by limonite from fresh groundwater, emphasizing the microstructure with orange and yellow bands. It's striking and looks like a lava lamp.
- Don't damage the outcrop! It's weakly consolidated.