Yee Haw! Finally a weekend outside. Soooo beautiful near Worcester, just on the eastern edge of the Syntaxis of the Cape Fold and Thrust Belt. Thirteen students, myself, and two post-grads, sunshine, birds, bugs, fynbos in bloom, a cozy wine farm, and good driving directions. Field trip bliss.
(Manditory photo of living thing)Check out this beautiful Aloe! My friend Nicci painted this one. I think it's the same one as Laingsburg. Thanks to Nicci I noticed subtle changes both in the spatial distribution, frequency, and morphology/colour of aloe as I walked up and down the hills. The fynbos is so highly derived some of those plants have a 20m ecotome. Not even exaggerating!
Ah, but what is that lovely outcrop in the background, you ask? Well dear friends, it is the leading edge of the Peninsula Formation (Table Mt Group) thrust over the Malmesbury Group. Now, if you were in a hurry you might just think that the Peninsula always unconformably overlies the Malmesbury so what's the deal with calling it the leading edge? But that would mean you had missed this:
The basal cataclasite. Gritty as can be and with a suspiciously fractal-looking grain size distribution. OK so I didn't measure any grains but it sure as hell isn't sandstone. Nor is it limestone in spite of some opinions I heard in the field. Not everybody shares my enthusiasm for the oral textural analysis (sorry Bill) but it sure can tell you the difference between calcite (mohs hardness 3) and quartz (mohs hardness 7). Someday I'll post a tutorial. That will have to be for Heather's weekly gross-out.
Before I get too far along, must show you a map:
Each map area is about 3.5 km2, each student had 2 days to map one area. I mapped both (but didn't walk the whole place) and in the process reassured myself that I am up to the job. Thank goodness for Hilde and Poleta. In fact, thank goodness for "the Scissors" because it helped me out I think. I won't post my map here because I need secrets for next year. In fact I am having doubts about putting my interpreted photographs. Maybe if I remove all potential search terms no students will find it?
We'll start with some basics. The Malmesbury Group is a big pile o' turbidites, but pretty much the least deformed accretionary wedge I've ever read about. In fact I live on MG (covered by colluvium) but it never outcrops. In the mapping area it is low greenschist facies, phyllite, with a nice sheen. Bedding is cryptic most of the time, unless you find one of these kick ass pebble beds. Here's one for anybody who needs to see what S0 - S1 intersection is all about! Stereonet shows bedding/cleavage intersection.
The MG is tightly folded - S1 is the axial planar cleavage (according to urban legend, I have yet to see a fold axis, why does this remind me suddenly of Cummington MA and H. Bob Burger and Lena reaching into the isoclinally folded turbidite outcrop and coming out with a pissing fistful of garter snakes? Because nobody had seen a fold axis there either. But I digress.) So here we have established the angle between bedding and cleavage is about 30 degrees. This held true everywhere I measured it (again, no fold axes!!) So maybe it's just as likely tied to imbrication and underplating. Who's to say. Either way it is early - right?
The same outcrop from another angle. Streched Pebbles! "Cigars" as we call them. Strain ellipse proxies extraordinaire! Funny thing - the e1 axis lies in the S0 plane and not in the S1 plane. What does that mean I wonder?
Complicating the matter - here are some relatively late porphyroblasts - I think they are chloritoid - they are also stretched, but in S1. I don't know how I forgot to measure the trend and plunge of these but oh well, something for next year I guess.
But what about the faults christie! Well you'll have to wait for the next post. Hooo boy do I have some faults to show you. In the meantime, here's my intentionally obtuse schematic to tide you over.
off to Laingsburg on Saturday morning - expect 8 days radio silence...