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Conglomeratic submarine canyon fill, Point Lobos, California

October 18, 2011

Wow, this is one of the best normally graded turbidites I have ever seen!  I saw this at the Point Lobos State Natural Preserve near Monterey, CA on a recent field trip with SPODDS (my former research group) .  These rocks exposed along the Pacific coast have been uplifted by San Andreas transpression, and they are part of a Paleocene submarine canyon fill, where the erosional contact with the granite is exposed very well and mapped by Ed Clifton.  Here is my attempt to photograph that from afar (cliff about 12m high):

Within the canyon fill there are Volkswagen-sized chunks of granodiorite that have likely been sourced locally, spalled off of the canyon walls.  In addition to the granodiorite clasts are mass transport deposits (MTDs)consisting of mudstone and interbedded thin sandstone (the photo below is a slump/slide as the parent rock hasnt been sheared that much).  Regional dip here is horizontal, and the MTD has been rotated and syndepositionally faulted a bit.  Note the topography created on the top of the mass transport deposit (MTD) is infilled with mud-clast rich conglomerate that was probably ripped up off the top of the mudstone (camera lens cap for scale):

Where the mudstone is in place and not slumped off of the canyon walls or nearby continental slope, it is packed with very interesting bioturbation.  I am no ichnologist, but I believe that the burrows below were created by a Paleocene shrimp-like creature of some kind – these types of burrows usually get assigned to the Ophiomorpha ichnofacies.  This is a bedding plane exposure, and the direction of burrowing is from left to right, as evidenced by the concave structures within the burrow.  [[Update – see this link for a paper on the burrows here – http://t.co/aT03wEh9 ]]

Finally, a grat modern analog for the Point Lobos submarine canyon fill is the modern Monterey canyon, which is only about 40 miles to the north.  The Monterey is also eroded into granodiorite as well as Franciscan accretionary wedge material.  The thalweg of the canyon is filled with conglomerates, as recently published by Charlie Paull. Here is an image of the canyon from GeoMapApp:

Whew!  OK, well if you are ever in the Monterey, CA area, make sure and go down to Point Lobos – if not for the geology, go for the sea lions and otters and kelp forests!  And don’t forget to go to the Monterey Bay Aquarium while you are there…

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5 Comments leave one →
  1. Bryan O'Sullivan permalink
    October 18, 2011 22:42

    The Monterey Bay canyon is especially trippy because of the lack of any modern feature on dry land to explain why it’s so deeply incised. I’m amazed people don’t play that observation up more for visitors.

    But I’d no idea there was granodiorite in the bedrock around there! Lovely.

  2. October 18, 2011 23:27

    Great conglomerated and field location! And those MTDs are pretty awesome too. Its really interesting to see in outcrop the source/shape relationship between the less-sorted mud-clast rich conglomerate and the underlying slump/slide deposits.

    • October 19, 2011 09:31

      Hi Alan, I finally got my conglomerate post up, after you insipred me with yours! I have many other turbidite conglomerate locales to come – that was what got me interested in turbidites, that you could get those coarse grains that far offshore!

      • October 19, 2011 12:29

        oops. Stupid typo, I didn’t mean to use conglomerate as a verb there. Yeah I agree, coarse grains in offshore settings are really fascinating.
        I have a few other marine and fan conglomerates in my photography stockpile from Ireland and Italy with curious depositional settings. After reading your latest conglomerate post, I’m thinking I will post some conglomerates soon too.

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