Wednesday, September 16, 2009

Summer Adventures Part 2: Clastic Dike

Late during the summer I revisited a classic and bizarre geologic site - the Santa Cruz Mudstone west of Santa Cruz, California. This spot in particular is called Shark's Cove, and is near Bonny Doon State Beach. This place looks like it is right out of a pirate story. Robert Louis Stevenson could have based a whole pirate story after this place.

Yar, here there be clastic dikes.

The Late Miocene (7-9 Ma) Santa Cruz Mudstone overlies the early Late Miocene Santa Margarita Sandstone. The Santa Margarita Sandstone is famous for large scale cross-strata formed by huge mega-bedforms (i.e. eolian dune size), as well as one of the most well preserved (and still under-studied, aside from sea cows and walruses) Tortonian/Serravalian marine vertebrate assemblages on the planet. The Santa Cruz Mudstone is known for a few marine verts (Parabalaenoptera baulinensis, from further north) but most famously for two things: methane cold seep-related carbonate pipes (a later post) and (arguably more famous) for its extensive clastic injections, dikes, and sills.A neat sea cave! But what else is it?

Fluidized sand (the Santa Margarita Sandstone is barely cemented; it looks like a holocene beach deposit, and is very easily sifted through for finding shark teeth) was injected into the overlying siliceous mud of the future Santa Cruz Mudstone (after a huge local transgression), typically as sills (parallel with bedding) and vertical dikes, as well as blobs. Above is a photo of me in a natural arch, with the dike above. The below photo shows the Santa Margarita Ss. derived dike (Tsm) and the Santa Cruz Mudstone (Tsc).
More of the dike, and idiots in the background who almost got stranded here at high tide.

EDIT: I failed to mention that this is the largest clastic dike system on the planet. For more pictures of weird fluidized clastic injections, visit the Injected Sands Group (University of Aberdeen), and read:

Scott et al, 2009. The Process of Sand Injection: Internal Structures and Relationships with Host Strata (Yellowbank Creek Injectite Complex, California, U.S.A.). Journal of Sedimentary Research 79:568-583

Boehm and Moore, 2002.
Fluidized sandstone intrusions as an indicator of Paleostress orientation, Santa Cruz, California. Geofluids 2:147-161


Antagonist Jason said...

This made me excited to be alive. Thank you.

Apparently southeastern Washington is known for its unusual clastic dikes. I need to go there.

Robert Boessenecker said...

I'm glad that this excited you so much, and sincerely appreciate very dry humor of this caliber.

Must be clastic dikes in terrestrial rocks; I've not heard of these (not that I'm some big dike expert or anything).

Antagonist Jason said...

I wasn't joking! About either! Although in hindsight, it is actually funny. :)

I looked up clastic dikes on Wikipedia (I hadn't heard of them before) and read this:

"Unusual clastic dikes in southeastern Washington state appear to be related to loading by outburst floods.[4] These dikes formed when the flood deposited Touchet Formation dried, leaving deep cracks which subsequently filled with debris."

Anonymous said...

There are photos of clastic dikes on my blog, Northwest Geology Field Trips
They are preserved in unlithified Pleistocene deposits on Whidbey Island in Puget Sound. Go to:

and scroll to the bottom. There are links to two field trips, each have photos. Click on the photos to enlarge them.
Dave Tucker
Research Associate
Western Washington University

Anonymous said...

Have you observed any sheeting in the sandstone dikes? Photos?

Robert Boessenecker said...

Not that I've been able to tell - then again, there are dikes along several miles of coastline, so I'm sure somebody who could do a little more exploring could look for some.