5/30/2009

A paradigm switch on the simple seds


Sedimentology, when I was an undergraduate student, was kind of seen as the easy course. Structural geology was the bottleneck course that you'd have to be insane to take before your junior year (See exhibit 1: insane undergraduate seen here in final throws of thesis preparation at which point I was sleeping under the poster printer in the computer lab to the chagrin of the academic staff who found this horribly embarrassing.)

Don't get me wrong, Sedimentology was a lot of work and rigorous work at that. In particular, I credit Sed with really helping me develop my field observation and writing skills. But I think we thought it was easy because it was accessible - it's geology's first line in a lot of ways. You can see an outcrop and look around you and see the depositional setting at the same time. I was reminded of this when my brother visited last year and we were on a beach watching each wave lay down a thin, well-sorted layer of course white sand. All around the beach were beautiful outcrops of Ordovician Peninsula Formation - the fluvial/marginal marine pile of ... massive and laminated well-sorted coarse white sand. I couldn't resist pointing this out and I saw the Principal of Uniformitarianism spontaneously pop into my brother's pointy social scientist head. Sedimentology, more than some other fields, includes opportunities for accessible discoveries like this for introductory geology students and anybody else who's not old and jaded about little obvious things like the Age of the Earth. In other words, seeing seds in the field can give people an easy entree into the paradigms of geology... and everything that logically follows from something as profound as Uniformitarianism. These moments do not require someone to be able to conceptualize Deep Time or visualize things way beyond human scales in order to see Geology in Action. Teaching structure and tectonics involves a lot of convincing students to suspend the tangible world for a minute and imagine things NOBODY HAS EVER SEEN and convincing them that some gnarly rotten outcrop of blueschist 1000's km from Cape Town is enough evidence to suggest that these things are REAL and HAPPENING RIGHT NOW far below the earth's surface on continents they have never visited.

So... Let's just say that for years I labored under the impression that field sedimentology was somehow light weight science. I see it now in some of my students as well. Most of the textbooks make it clear that if you can just identify some sedimentary structures that the depositional environment will be uniquely determined and then you just put together some transgression/regression model and you're done. There are obvious complications (e.g. the afore mentioned Peninsula Formation - if the structures are fluvial, why does it have the grain composition and size distribution characteristic of aeolian sandstones?) but it's always possible to work up a story of reworking during environmental changes to get around some of that.

So it turns out of course that South African geology has given me yet another paradigmal smack down (is that a word?). In yet another installment of the IBD but not really chronicles, there are mystery environments recorded in the Cape Supergroup rocks that surround us here in the Cape for which there is no conveniently adjacent depositional model with which to compare.

Take the Peninsula Formation for example. It's nearly all quartz. Like upwards of 98% SiO2. It is bedded on a scale of on order 1-10m. It is regionally extensive over 100s of km (1000s if you include comparative formations in South America, which we should). It is incredibly well sorted with regards to grain size and rounding. The crossbedding shows bars and meanders and shoals and maybe some lagoons. It's been through the Permo-Triassic Cape Orogeny which involved a lot of (now extremely hard to detect) layer-parallel thrusting so aside from the section 400m from my house on Table Mountain, nobody can really be sure how thick it is - but it's really thick. (Exhibit 2: Sandstones so thick you have to take a cable car from the base of the section to the top).

So... moving on to the "easy" sedimentology part... How do you make a huge sand sea (or erg) of perfectly sorted and rounded quartz sand? On today's earth: wind. Dry, hot or cold, and plantless ergs occur on every continent in specific climate zones. So we have analogs - but they are not the only possibilities. Here is an exception to Uniformitarianism - in the Ordovician there were no land plants anywhere. So environments existed then for which we have no analogs on earth - places where sediment was unconstrained by vegetation which could have occurred everywhere and not just in places of extreme climate.

Issue 2: the sedimentary structures. We can imagine a case where a wind-sorted erg was re-sorted by rivers and shallow seas by sea level rise or continental subsidence. The Ordovician southern world was a very cold place, and Africa resided at more southerly latitudes than it does today. Or - a situation where a windy sandy desert provided sediment to a subsiding margin where the sand was sorted by rivers and surf as it accumulated along the coastline. Maybe the overlying units can provide more of a clue.

To be continued....

5 comments:

BrianR said...

"Most of the textbooks make it clear that if you can just identify some sedimentary structures that the depositional environment will be uniquely determined..."

Like other subdisciplines, textbooks oversimplify to the point of absurdity.

Very few sed structures are diagnostic of a single dep environment (e.g., trough cross beds can form almost anywhere).

"...and then you just put together some transgression/regression model and you're done."

Again, this is a decent start for undergrads to grasp what stratigraphy is and how Earth history is recorded in rocks.

Since you are a structural geologist, look at the cartoon block diagrams of fault types ... that's simple, what's so complicated about structural geology? :)

Great post ... I like reading these kind of posts, especially from educators.

Fault Rocks said...

@ Brian - Exactly!

BrianR said...

I know very little of the rocks you mention here ... but there are Paleozoic successions around the globe that are enigmatic when it comes to dep setting. Still more work to be done ... let us know what you find out!

Sandro said...

Absolutely, Nature is much more complex than books explain. Yet good books are to be read and re-read many times.

Nice posting and insight.

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