Whoa, back from the dead!
The main reason I can think of that I haven’t been blogging is that it’s summer in PA and I’ve been taking advantage of the weather while it’s still here, and thus spending my free time playing outside instead of blogging. Also, nothing all that interesting has been happening in terms of grad school (other than my advisor returning from his year-long sabbatical, which has definitely correlated with an uptick in my productivity).
Anyway, right now I am researching something that is interesting enough to me that I want to share it with someone, but I don’t think I can be concise enough for a tweet or a Facebook status update, so I figured I would blog about it. But first, check out this awesomely retro geologic map:
(I’ve been reading a lot of papers from the 1960s this morning as part of this thing I’m trying to figure out, and it’s kind of cool. I especially like that the sampling location I circled in red looks like it’s in pretty much the exact same place as one of my samples… meaning some dudes were out at that same roadcut in the ’60s taking samples to get a crystallization age, and then I was out there last summer in the exact same place getting samples for a different study. I don’t know why but I love stuff like that!)
Anyway, that rock I was sampling at that red location is from the Shasta Bally batholith, a big northwest-trending elongate blob of granitic rock. On its southern end, there are some marine sedimentary rocks that are part of the Great Valley Group (the part I scribbled on in green… how nice that I have Photoshop and it still looks like I’m using MS Paint because I’m just that janky.) The important thing is that as far as I can tell from reading a whole bunch of papers (and many of them cite the same 1960s map by Murphy et al that I haven’t been able to get my hands on) it’s a depositional contact, which just means the sedimentary rocks were deposited directly on the granitic Shasta Bally rocks, no faulting or anything crazy, and therefore the Shasta Bally batholith had to be there first. Which is weird because the batholith was emplaced something like 7 km underground, crystallized to solid rock about 136 million years ago, and then it had to get brought to the surface so these sediments could be deposited on top of it… but from what I can tell (and this is where it gets confusing, because different people say different things and the geologic time scale is basically defined by fossils which then have to be correlated with radiometric ages blah blah blah…) the overlying Great Valley Group sediments here are at least Hauterivian if not mid-Valanginian, which corresponds to them being ~130-135 million years old. What?!? I just feel like that’s crazy fast for a batholith to be exhumed and then all of a sudden have an ocean on top of it. (I know one or two million years sounds like a lot, but in geologic time… it’s not really that long.)
So I guess the conclusion of this story is that there is no conclusion, yet… but basically I’m fascinated by the fact that this batholith is seriously under-investigated (is that even a word? I was going to say “understudied” but that made it sound like I was talking about a ballet performance). Like I can’t even find any studies of its petrology, but people have studied the shit out of a lot of the adjacent plutons that I’ve dated. It’s just interesting and it’s refreshing to be interested in my thesis research again :)