Tavşanlı Zone field trip, part 3

Picking up where we left off last time, we were in some partly-serpentenized peridotite, part of the Burham Ophiolite in Turkey’s Tavşanlı Zone, an ancient tectonic suture.

Our next stop on the field trip allowed us to visit some diabase dikes:

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Here’s a close-up of the right contact of the dike with the host peridotite:

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The field notebook’s long edge is ~18 cm. And here it is again, annotated:

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Near the village of Oranheli, we stopped to examine a jadeite meta-granitoid, a rock only a metamorphic petrologist could love. There were, however, a lot of metamorphic petrologists on the trip, and they were very keen on checking it out. This was the first of many occasions when random Turkish citizens would stroll up to our odd group to find out just what the hell we were doing:

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Further along, we saw a meta-basite (meta-basalt) within the meta-granitoid, and there I got a refreshing whiff of structure. Here’s a random isoclinal fold of a meta-granitoid dike cross-cutting the meta-basite, with a Turkish 1-lira coin (about the same size as a U.S. quarter) for scale:

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Next up were some very cool rocks: marbles with extremely elongated calcite crystals.

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These needle-like crystals are interpreted as being pseudomorphs of aragonite, the form of CaCO3 which is stable at high pressures and low temperatures.

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A bit further on, we return to metamorphosed shale and graywacke (now schist and “grayfels”), sheared out and pervasively deformed at blueschist conditions. I took a few photos of charismatic folds in the unit:

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Annotated, roughly showing the trace of foliation:

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Sandy layer folded over into a recumbent position, set in a sheared mass of meta-shale:

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Thicker sandy layer, in a recumbent isoclinal fold (white pen, 14 cm long, for scale):

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Zooming in on the above photo, to show the lovely, smaller wavelength parasitic folds which decorate the snout of the big fold:

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Extensional fractures along an isoclinally-folded, recumbent sandy layer:

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Small S-folds in the sheared shale (just above hammer):

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Coming down onto this roadside outcrop of sheared shale and graywacke were cobbles and boulders of float from somewhere up above. They were of a quartz-pebble conglomerate that showed a stretching lineation. Check out these two faces of typical samples:

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Now, here they are again, with the X, Y, and Z axes of the strain ellipsoid (longest, intermediate, and shortest, respectively) labeled for your benefit.

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This conglomerate has been sheared into a lovely L-S tectonite, with X>Y~Z. In other words, it’s mostly lineated, with only a weakly-defined foliation, indicating the stress field was mostly constrictional. (I collected a muddy sample of this stretched-pebble meta-conglomerate, and when I washed it off in the hotel shower the next morning, I was delighted what a cool sample I had selected. It has some awesome structural features; I’ll show it to you some other time…)

Our final stop of Day 1 of the trip was this spectacular overview of the Kocasu Gorge, a canyon which cuts across the structural trend of the area at approximately a right angle. (The canyon cuts north-south; the strike of the folded & thrusted rock units runs approximately east-west.)

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As the sun set, Aral showed us where we were, and the overall synclinal structure of the area.

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I recorded it in my field notebook like this:

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With this context established, we loaded back on the bus and drove for a couple of hours to get to a town with a decent hotel. We dined and slept, and the next morning got up ready for more suture-zone rocks.

Transect debrief 4: transgression, passive margin

…So where were we? Ahh, yes: an orogeny, and then some rifting. What happened next to Virginia and West Virginia? Let’s consult the column…

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After the rifting event opened up the Iapetus Ocean, seafloor spreading took place and tacked fresh oceanic crust onto the margin of the ancestral North American continent. As North America (“Laurentia”) moved away from other continental fragments (Congo craton, Amazonia craton), it got a little bit calmer ’round these parts. From the continent’s perspective, the spreading center moving further and further offshore.

This shift of the tectonic locus out to the middle of an ocean basin means that the edge of the ancestral North American continent could finally relax a bit. The magmatic intrusions became a distant memory, and the crust cooled, contracted a bit, and sank. This subsidence allowed seawater to lap up onto the edge of the continent, and with the seawater came sediments. Rivers draining the exposed North American continent brought sediments to the sea, and dumped them. We geologists call this “passive margin sedimentation,” and it results in relatively “mature” sediments: those that have been well-worked over, typically rich in quartz and well sorted and with more rounded component grains.

As time went by, the edge of the continent subsided more and more, and any given spot in the modern-day Blue Ridge transitioned from streams to beach to continental shelf. The sedimentary stack reflects this increasing distance from the shoreline: a transgressive sequence.

It starts at the bottom with Weverton Formation: conglomerates and sandstones (and as I discovered on the Transect Trip, siltstones too). Here’s a piece of the Weverton from a GSW trip several springs ago:

The Weverton is overlain by muddy deposits of the Harpers Formation, which can also include coarse sandy units, as I learned on the Transect Trip. Here’s a shot of the Harpers Formation at Harpers Ferry, West Virginia, the type locality. This was taken five years ago, back when I had just gotten out of grad school, and spent a year teaching at George Mason University (pre-NOVA). [The student pictured is Steve Elmore, who just earned his master's from GMU, working with Bob Hazen. Congratulations, Steve!]

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The Harpers is really important, because it contains some Olenellus trilobite fossils, which constrain its age to be Cambrian.

The Harpers is overlain by another sandstone: a clean, pure quartz package named the Antietam Formation. For me, the Antietam is a favorite local rock, because it is studded with Cambrian-aged Skolithos trace fossils. On the trip, I used the iPhone to upload a few photos of these, but here’s a higher-resolution image to savor:

You’re looking at the bedding plane of the Antietam in the above image, with your sight-line parallel to the paleo-vertical orientation of the tubes. Wow. Beyond all reason or deeper interest, I just love Skolithos tubes. I look at this outcrop, and I wonder: is this a palimpsest? or a small wormy Manhattan? In other words: was this multitude of burrows generated by a small population that dug in the same area over a long period of time, or by a huge population living cheek-to-jowl over a relatively brief moment?

Regardless, the sand-then-mud-then-sand-again picture painted by the succession of Weverton-Harpers-Antietam isn’t a “textbook” transgressive sequence, but it might make more sense if you consider the Antietam sands as barrier island deposits, with the Harpers being deposited in a Pamlico-Sound-type setting.

Finally, the transgression is complete when we get to the top of the Blue Ridge sequence and see the Tomstown Formation, a carbonate unit:

IMAGE CREDIT: USGS

The Tomstown tells of a time when sea level had gotten so high locally that the shoreline was way, way, way far away. There were no clastic sediments making it out to this location, and all that was available to precipitate were the ions dissolved in the seawater. No sand, no pebbles, no mud: only carbonate.

The sequence of sedimentary strata continues, but to follow its succession upwards, you’ll have to travel across the Blue Ridge Thrust Fault to the west, into the Valley & Ridge province. More on that in the next post. For the moment, let me share a cartoon sequence of images by Tom Gathright (1976), showing the overall stratigraphic evolution of the Blue Ridge province*:

*Note that Gathright used the outmoded names “Hampton” instead of Harpers, and “Erwin” instead of Antietam. Please forgive him and move on.

That last panel, showing Alleghanian deformation, is something we will attack in a future post. For now, I’m satisfied to have finally climbed to the top of the Blue Ridge stratigraphic stack.

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Gathright, Thomas M., 1976. Geology of the Shenandoah National Park, Virginia. Virginia Division of Mineral Resources Bulletin 86, Charlottesville, VA. [buy it from SNPA Bookstore]

Transect debrief 1: starting in the basement

It is time to debrief the post-NE/SE-GSA field trip that I went on, affectionately dubbed the “Transect Trip” for the past 27 iPhone-uploaded “live”-geoblogged posts.

First off, I’d have to say that I enjoyed the live-field-blogging experiment overall, though I’ve got some critiques of the process and products. I think it’s amazing that I can upload photos and short blog posts from my iPhone to this site with a minimum of hassle. However, I can’t do much more than that. It’s not as easy to tag the posts or geotag the photos. I can’t compose annotations. In fact, I can’t even be sure the photos will be in focus, since the iPhone camera is a static lens. And there’s no macro feature on the iPhone camera, a source of some frustration for a guy like me that likes to photograph small things. Further, typing with my thumbs is laborious, keeping the live-geoblogged posts on the terse side.

So, when I asked what readers thought of the whole enterprise, I wasn’t surprised to get feedback that it would be nice to put things in a bit more context. I aim to start that process today, with the first rock we encountered, a charnockite (orthopyroxene-bearing granitoid). The rock type is named for Job Charnock, founder of Calcutta, India, whose tombstone is made of charnockite:

Charnockites are common rocks in the core of Virginia’s Blue Ridge “anticlinorium.” Here’s a nice photo of a fresh sample, showing the rusty/clayey weathering “rind” on the sample:

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Compare that image with this version, the original that I uploaded from the field trip via my iPhone:

Pretty profound difference in quality, eh?

So, here’s the deal with these charnockites. Volumetrically, they are a big part of the “basement complex” that cores the Blue Ridge. There are also a bunch of other flavors of granitoid down there; about fifteen discernible rock units in all. Our understanding of the basement complex has gotten a thorough re-working in recent years thanks to the coordinated efforts of many geologists who have focused on reexamining the Blue Ridge. Chief among these scientists in Scott Southworth of the USGS in Reston, who led an effort to remap the area in and around Shenandoah National Park. Dick Tollo (GWU), Bill Burton (USGS), Joe Smoot (USGS), Chuck Bailey (W&M), and John Aleinikoff (USGS) were part of the effort, too. The rocks were found to be more diverse than previously thought, and thus “complex.” Aleinikoff was responsible for a suite of new dates on the granitoids and their metamorphic successors in the basement complex. They have crystallization ages ranging from 1,183 Ma (±11 Ma) to 1,028 Ma (± 9 Ma): all Mesoproterozoic in age, and thought to be related to the Grenville Orogeny.

Some of these granitoids were deformed during Grenvillian mountain-building and attained a foliation which strikes northwest, in contrast to the later (Paleozoic) Appalachian foliation, which strikes northeast.

The plutonic rocks of the Blue Ridge province’s basement complex are the oldest rocks in Virginia, and they were the first ones we encountered on this field trip. All through that first day, we climbed upward through the stratigraphic column, meeting younger and younger rocks.

Transect Trip 27: fluvial overbank deposits

Over on the far right by Chuck Bailey (yellow shirt) you can see the crescent-shaped profile of a river channel (gray color). To the left of that, you can see levee deposits, and beyond that (to the left) crevasse splay deposits and the floodplain (dark red mudstones). This is in the Hampshire Formation, part of the Acadian “clastic wedge.”

Transect Trip 26: Bouma sequence

Here we are in the Brallier Formation, a Devonian turbidite sequence. Prominent in the middle of this photo is the Bouma “C” horizon with the cross- bedding.

Transect Trip 25: sheared microbrachiopods

This one is also from the Millboro Formation: itty bitty brachs (not too much oxygen in those depths for building up big body sizes) that have been sheared during Alleghanian deformation.

Transect Trip 24: soft sediment deformation

This photo is from the Millboro Formation: mostly deep water black shale, but with the occasional heavy turbidite coursing in and settling its bulk down on the squishy mud beneath. Some folks on our trip suggested these might be seismites: soft sediment deformation resulting from earthquake-induced vibration.

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