Tuesday, January 5, 2010

Fort Bragg: Glass Beach Marine Terrace Sorting

Fort Bragg, CA exists on a marine terrace.  Its tectonic genesis I'll leave for another day.  The wave cut cliffs exposing the sediments of these terraces vary.  Here in March of 2009, I found part of a terrace eroding towards the city just south of Glass Beach, revealing conglomerate.  A quarter was used for scale (thankfully!).  Remember, you can always click on the image to view it (middle click to view in a new tab in Firefox or right click or command click to see a menu where you can open the photo in a new window or tab).

In these particular photos, there appears to be conglomerate in normal grading (as opposed to reverse grading).  It goes from very large (often discoid) pebbles to small pebbles to granules.   This graded bedding appears to be largely clast-supported (orthoconglomerate).

Why might this graded bedding be occurring along the coast within a terrace?  Given that the pebbles are very well rounded, they were likely deposited by a stream or were weathered away regularly by wave action prior to uplift.

 If this feature were in fact caused by fluvial processes, the larger clasts would be indicative of very rapid flows which were able to move and deposit large pebbles (perhaps by saltation or rolling). Most of the coarse to medium grain sands AND smaller pebbles would have been entrained within the flow.   These clasts probably washed out into the sea.  Eventually the stream velocity decreased, dropping out smaller pebbles and coarser sand.  Then the whole process repeated itself.

One should also note that at the bottom of this graded bedding there is a jumble of large and small pebbles with coarse sand; perhaps this indicates rapid stream velocity changes at the start of a flooding event?

Finally, if this were a feature created by wave action, it might be said that material may have been deposited into the ocean by mass wasting.  Providing that the ocean at this location was shallow enough, wave action could slowly wear down the larger clasts by abrasion.  Storms, perhaps, cause this mass wasting.  The exact mechanism for this kind of sorting under these conditions is a little iffy to me.  But perhaps I'll revisit this later when I've learned more.  However, I believe that fluvial conditions are more apt for this situation as water intensely sheds westward off the Coastal Ranges all along the north coast of California.  Since this is in a terrace, perhaps it represents an uplifted portion of an old stream channel (note that Pudding Creek is a several blocks north of here)?  Sea levels, of course, have not always been where they are now; so it is reasonable that the ultimate baselevel for this stream could have been further out (a shoreline shift).

Perhaps I'll use this opportunity to disclaim any accuracy in my interpretations of photographs I've taken over the last ten years.  First of all, I am a very fresh geology student and so I will be wrong and miss important indicators regularly.  Second of all, I'm relying on only the details in the photographs and my spotty memory.  However, do feel free to elucidate what I might have missed and/or explain pertinent processes.

1 comment:

  1. Perhaps this environment may have been a braided or bedload stream? I Guess the only way to find for certain is to go back. Drat.