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28 March 2010

Transect debrief 8: late brittle deformation

The final chapter in our Transect saga is now here. In some ways, it’s the least thrilling of the bunch. On the other hand, when I see a nice example of this structure, it makes me squeal like a little girl. I refer, of course, to plumose structure, the small-scale architecture of a joint surface. We saw multiple great examples on the trip, but my favorites came with the first …

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27 March 2010

Transect debrief 7: Brittle-ductile deformation

On the transect trip, I also saw some nice meso-scale “minor” structures that probably formed during Alleghanian deformation. Prominent among the ones that really impressed me were these en echelon tension gash arrays, deforming the Antietam Formation quartz sandstone and well exposed in blocks used to construct the wall along Skyline Drive and the Sandy Bottom Overlook in Shenandoah National Park: Good Lord! Ain’t those things beautiful? They also give …

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26 March 2010

Transect debrief 6: folding and faulting

Transect debrief 6: folding and faulting

Okay; we are nearing the end of our Transect saga. During the late Paleozoic, mountain building began anew, and deformed all the rocks we’ve mentioned so far. This final phase of Appalachian mountain-building is the Alleghanian Orogeny. It was caused by the collision of ancestral North America with the leading edge of Gondwana. At the latitude of Virginia, that means northwestern Africa (Morocco and/or Mauritania). Whereas the first two pulses …

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25 March 2010

Transect debrief 5: sedimentation continues

We just looked at the Chilhowee Group, a package of sediments that records the transition for the North American mid-Atlantic from Iapetan rifting through to passive margin sedimentation associated with the Sauk Sea transgression. Well, if we journey a bit further west, we see the sedimentary stack isn’t done telling its story. The saga continues through another two pulses of mountain building. Consider this “unfolded, unfaulted” east-west cross-section cartoon: Part …

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24 March 2010

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… 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 …

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22 March 2010

Transect debrief 3: Rodinian rifting

The Grenville Orogeny, responsible for Virginia’s basement complex, was one mountain-building event among many that helped put together a Mesoproterozoic supercontinent called Rodinia. But Rodinia didn’t last: it broke apart during the Neoproterozoic to form the Iapetus Ocean basin. This rifting event is recorded in Virginia’s Blue Ridge province in the Swift Run Formation and the Catoctin lava flows. It’s probably about time to start putting some of these rock …

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21 March 2010

Transect debrief 2: weathering the Grenvillian landscape

From the basement complex, the next unit up in the Blue Ridge province’s stratigraphic sequence is the Swift Run Formation. It rests atop an erosional unconformity. After the Grenville Orogeny (~1.1 Ga) added a swath of new crust along the margin of the North American continent, the landscape began to weather and erode. Eventually, an episode of rifting broke open rift valleys and a new ocean basin, the Iapetus. The …

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20 March 2010

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 …

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