Sunday, January 24, 2010

Bioturbation and ash beds in the Purisima Formation

This is really just a collection of annotated photos I took over winter break, and are sedimentological and ichnological in nature. These are all photos taken of Purisima Formation exposures in San Mateo county. During the winter, more intense storm activity cleans off the coastal cliffs and makes examining trace fossils, sedimentary structures, and bed geometry an easier task. In some cases, the most beautiful trace fossils and sedimentary structures are associated with ash beds. Above is part of a very thin ash bed (8-30cm thick - the ligher colored sediment) that has almost been destroyed by bioturbators (by the way, bioturbation is the disruption of primary sedimentary fabric by burrowing organisms/infauna). In fact, for most of its exposure it has been pierced by burrows (where the ash has been displaced into the burrow fill) that it looks like a dotted line. Above shows a U-shaped burrow; I can't remember what taxon this is - because some of them (i.e. Diplocraterion?) are defined based on spreiten (laminae within the burrow) inside the 'U'.

Here yu can see how little of the original bed is left; much of the ash has been introduced as burrow fill, and mixed and diluted with regular sediment (fine-very fine silty sandstone in this case), hence the blotches looking slightly less 'pure' than the primary ash.

A couple hundred feet away this bed thickens and isn't completely chewed up by bioturbation, enough so that you can see original sedimentary fabric within. In this case, it is a mix of swaley cross stratification (a small-scale version of hummocky cross stratification) and climbing ripples. Both of these sedimentary structures indicate uni- or bi-directional flow with a relatively high rate of sedimentation, i.e. sediment is just dumping out of suspension. This can happen during hyperpycnal flow - often occuring as a dense plume of sediment rich water introduced into the ocean from a river mouth, say after a big storm.

This ash bed, on the other hand, is huge. There's a reason I don't have a scale bar; this is about a 5-10 meter thick ash bed. This has a completely different set of weird features - some pretty incredible soft sediment deformation. These look like giant scale ball and pillow or loading structures.
This cliff right here is about 200 feet high, and we're looking at about 1/3 of it or so. These S.S.D. features continue for the entire outcrop length of this bed. The thinner ash bed featured above is visible in the very bottom of this photograph. The intense loading features here could, of course, be caused by a relatively rapid influx of a LOT of ash to the seafloor, which could lead to liquefaction of more typical sediment at the former (i.e. pre eruption) sediment-water interface (i.e. sea bottom), and leading to the big pillow-like lobes of ash, and the long upward pointing 'fingers' of sand.

No comments: