Corona Heights Fault

Emily found this fault by searching the internet for local slickenline photos and landing at Andrew's site, geology.about.com. What did we do before the internet? The fault outcrops in the Castro Area (Corona Heights) where a post-1906 aggregate quarry has been repurposed for a playground and pet cemetery. The relatively recent exposure of the fault allows excellent preservation of the slip surface itself.

The fault is a thin breccia zone (less than a meter) with an anastomosing network of highly polished, slickenlined surfaces within the breccia zone.

Nils admiring his reflection in the fault surface. Seriously it is so so shiny. I've never seen anything like it. Twice now I have gone out to the Marin Headlands Terrane (same cherts) to look for fault surfaces and see if I can find anything similar. Can't. Went the second time because it was too foggy the first time and I worried I might have missed it.
Emily and Amir worked on a big normal fault in southern Oregon which had a similar fault rock structure - thin, superfine-grained polished slicked core, thin breccia zone with pinch-swell structures, rapidly dropping off to undeformed bedrock on either side of the fault. That thin breccia zone probably takes up a lot of deformation and accomodates the roughness on the polished slip surface. (See Sagy, Brodsky and Axen (2007) in Geology; it's available here.)

Here's Emily getting a grip on "Layer II", that granular layer which must break or flow in order for slip on these uneven, anastomozing polished surfaces to occur. Her hand is on that ~ 40cm layer of breccia in between two slip surfaces.

The texture of the polished surface is truly remarkable. You can see some rounded fragments of chert - which are green, while the local rock is all red chert. They are embedded in a translucent super fine-grained silica matrix. You can see through it. It is positively glassy (in a descriptive sense; I have not examined it for molecular structure). It has beautiful tensile cracks which are generally perpendicular to local slickenlines - and rotate as the slicks rotate and the fault surface undulates. Truly amazing.

OK what you can almost see in this photo are tiny round white spots in the superfine silica layer - they are lensoid fractures in the silica. I have some theories about what this stuff is and what the tiny fractures mean. But I have apparently been too far out on a limb lately (at least for some reviewers) so I will just ask you, on the off chance that anybody is still reading this blog...
1. What is the shiny, transluscent silica and how did it form?
2. Why does it have tiny lens-shaped fractures inside?

Probably the way I worded those questions leads toward my ideas about the answers... but ... anyway...

Flying over Greenland

On the plane from Heathrow to San Francisco, Nils and I sat across the aisle from an Icelandic glacial geologist on his way to vacation to Hawaii. When we flew over the east coast of Greenland I had to wake Nils up and make him look out the window! So amazing.

Medial moraines where two glaciers flow together:


A beautiful long fjord:

Tidewater glaciers calving

Field of bergs:

Winners

The Art of Subduction took 3rd place at the annual Geological Society of South Africa Western Cape Branch Quiz night.

Nice job team.

Fun with Folds

BrianR at Clastic Detritus posted a great photo of some disharmonic folds in bedded strata. Here are some more!


I particularly like the chocolate-tablet layers visible on the top of this outcrop.

Rock of the Week #10

1. What are the sedimentary structures in this rock?
2. What can they tell you about the depositional environment?
3. Why is this rock reddish in colour?




Solution to Rock of the Week #9:

This is a SKARN – that is, a contact metamorphic rock produced when magma intruded low-grade country rock. This sample comes from the Birch Creek Pluton in the White-Inyo Mountains of California. The older rock here was a Cambrian (530Ma) carbonate and shale marine platform sequence. The intrusion of granitic plutons (80Ma) transferred heat and Si-rich fluids into the sedimentary rocks, causing metasomatism and metamorphism. This was a moderate-sized pluton and the zone of contact metamorphism around it is about 600m thick. This sample was taken ~200m from the pluton where temperature reached about 500°C. It contains garnet, dolomite, calcite, fluorite, epidote, and quartz and is classified as a Ca-Fe-Si skarn.


1. (1 pt) 2 minerals?
: 1 point if 2 of the above listed minerals are given.

2. (1 pt) Rock name?
: 1 point for skarn (only).

3. (2 pt) How did it form?
: 1 point for contact metamorphism or metasomatism.

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....

On the topic of scientific dissent

There is a buzz going around the blogosphere in the wake of press releases on a new paper coming out which challenges the asteroid impact correlation with the K-T extinction event.

This seems like an opportunity to point out that the whole search for a "smoking gun" in extinction events is probably off-target: All the largest extinction events in earth's history seem to be a result of positive interference between multiple factors which change the habitability of one or more environments on earth. In the case of the K-T event, the longer-term carbon cycle perturbation associated with the flood basalt eruption of the Deccan Traps in central India (68-60Ma) coincided with the Chixulub impact in Yucatan, Mexico (64.5Ma) as well as the increasing domination of the plant world by angiosperms (flowering plants) which may have been less edible to the terrestrial herbavores. Several other events, both fast and slow, have been suggested to correlate temporally with the end-Cretaceous extinctions.

WHY DO WE NEED A SMOKING GUN? Clearly this is a multiple-whammy event. Every K-T paper seems to have to take a side. From my reading it appears to me that a long (multiple-million-years) decline in biodiversity (caused by plate tectonics and climate effects such as volcanism and plate reorganization in latitude and coast-line length) was punctuated and probably accentuated by spikes in volcanism rates (CO₂ increases) and at least one big asteroid impact. Without the combined effects of ALL of these driving factors, the extinction event would have been less pronounced, or the biota would have had more leeway to recover. Maybe the "everybody's right" approach isn't as headline grabbing.

Anyway, the more interesting discussion growing out of all this is the topic of scientific mavericy: the role of a scientist or group of scientists who argue against the generally preferred interpretations.

The Lab Lemming and my perennial favorite, Female Science Professor have each written superior posts on the topic. FSP in particular highlights the intrinsic value of minority opinions to the overall debate, while acknowledging the erroneous effects of the popular media's tendancy to highlight "both sides" of a "debate" by digging out a quote from some dinosaur who thinks the world is flat.

There are ways for non-experts, including science journalists to tell the difference! And we ought to hold them accountable for this!

The paper which triggered these discussions (Press Release here) reports some new stratigraphic work in the Yucatan area which IS INCONSISTENT with one of the key attributes of the hypothesis that the Chixulub Impact ALONE was responsible for the mass extinctions of the K-T boundary. The stratigraphy records both the extinction event and the impact event and they are NOT PRESENT IN THE SAME STRATIGRAPHIC LEVEL, suggesting that they did not occur at the same time.

This is a key piece of data, which is important to the ongoing conversation on the relative role of different factors which caused the extinction. The community of scientists who promote the idea that this impact directly led to the extinctions will have to somehow explain or discredit this data or their case for their model will be weakened. Although there will be moments of drama (and for us, a "dramatic moment" involves somebody standing up in a dark conference room and telling somebody else they are wrong.... ooooo drama), people are right now in their offices reviewing their own data sets and hypothesizing new explanations or new narratives that incorporate the new data along with all the existing data. Science will go on. So will the debates about the dinosaurs' extinction.

This does not make the authors of this study some kind of gadfly harbingers of doom, shooting poison darts into the currently dominant hypothesis. I beg of you, journalists and general public -- do not mistake the give-and-take of ideas, data and hypotheses in the scientific sphere for intellectual cage wrestling. Disagreeing does not have to lead to drama between scientists... it's part of our daily lives and we enjoy the process of working out the details to find concensus or narrow the points of contention. If you are married to or dating a scientist, you may experience the great joy of this process as an integral part of your personal life.