Friday fold: Siccar Point, Scotland

As with last week, I’m going to show you someone else’s fold today. This one should have strong resonance with most geologists, because it’s a fold in the tilted (and contorted) older strata exposed below the famous unconformity at Siccar Point, Scotland:

I found this image on the British Geological Survey’s online repository of images, which are available for public use with attribution. I found out about the BGS photo repository via a post on StructuralGeology.org.

The photo was taken by T.S. Bain in 1979. Rock hammer (lower left) for scale.

The specific rock type here is shale, and their age is Silurian. Note the thinning of the limbs of the fold, and the relatively thick hinge area.

Happy Friday – may your workday rapidly thin (like the limbs of this “similar” fold), and your weekend be as thick as this fold hinge!

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 of Appalachian mountain building were relatively provincial affairs, this Alleghanian phase was a full-on continent-on-continent smackdown. The Himalaya (India colliding with Eurasia) would be a good modern analogue for the Pennsylvanian and Mississippian Appalachians.

When I was live-blogging the trip, I posted this photo of Judy Gap:

It was a bit hard to get it all into one measly iPhone frame (hence the tilted angle: those trees are in fact vertical!), but what you’re looking at here is the erosion-resistant Tuscarora Sandstone (Silurian in age; quartz-rich beach deposits) that outcrop as a ridge. However, here at Judy Gap, there are two ridges. What gives? This is where I was introduced to a new term that is apparently becoming a common phrase in the structural geology literature: contraction fault.

The story most Physical Geology students get about fault types is that tectonic extension causes normal faults, while tectonic compression causes reverse faults. Contraction faults are faults that display an apparent “normal” sense of motion, but were caused by a compressional tectonic regime. How the heck does that work, you may ask? Consider the following diagram:

So the deal with contraction folds is that they might start out “reverse” but are then rotated and tipped over as deformation proceeds. The former footwall becomes the new “hanging wall,” and the sense of motion is obscured by this new orientation. This means that they do represent contractional strain, but a freshman geology student is unlikely to spot it at first glance.

The Germany Valley to the east of Judy Gap is a big breached plunging anticline, as I attempted to show with this iPhone photo from the Germany Valley Overlook along Route 33:

It’s a bit easier to see if you jump up in the air 10 kilometers or so. Fortunately, that’s precisely why God created Google Earth:

The valley is hemmed in by a big V-shaped fence of mountains, all held up by the Tuscarora. It’s tough stuff. During Alleghanian folding, the crest of the anticline was breached, and water was able to get inside and gut the weaker rocks. The quarry annotated in the photo is mining the same Cambrian and Ordovician carbonates seen in the Shenandoah Valley back in Virginia (Lincolnshire and Edinburg Formation equivalents). A pattern geologists have noted with eroded anticlines is that older rocks are exposed in the middle of the structure, with younger rocks flanking them along the sides.

So that’s a glimpse of the big picture of deformation in the Valley & Ridge, but we can also see cool deformation at smaller scales… Stay tuned…

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 A of the image above shows the overall stratigraphic sequence for the Blue Ridge and the Valley & Ridge provinces in Virginia and West Virginia. You’ll notice that the small, detailed stratigraphic column I used to start the last two posts covers just the bottom 6 layers in this stack. Zoomed out to the bigger picture, we see ~40 layers overall. Lynn Fichter of James Madison University, one of the leaders of the Transect Trip, has published an excellent information-dense guide to the mid-Atlantic column. It’s a terrific reference for anyone looking to learn more about these rocks and the story they tell.

Part B of the image above shows the tectonic interpretation of these different packages of rock — some represent rifting, some represent passive margin sedimentation, some represent clastic influence from various orogenies occurring to the east (Taconian and Acadian).

The cartoon cross-section below, modified from an original by Steve Marshak in his excellent introductory textbook Earth: Portrait of a Planet, shows the tectonic evolution of the east coast over the past ~1 billion years of geologic time. It is reprinted here with Steve’s permission.

The story begins with the Grenville Orogeny, an episode of mountain building that completes the assembly of the Rodinian supercontinent. This is followed by Iapetan rifting, followed by three pulses of Appalachian mountain-building: the Taconian (“Taconic“) Orogeny, the Acadian Orogeny, and the culminating event of Pangean supercontinental assembly, the Alleghanian (“Alleghenian”) Orogeny. Finally, Pangea breaks up in the Mesozoic, an event also known as Atlantic rifting. Two complete Wilson Cycles are preserved by the Appalachian mountain belt!

The Valley & Ridge province received sediment courtesy of the Taconian and Acadian Orogenies, but wasn’t directly involved with the tectonic collision in any deformational way. Notice how west of both those orogenies in the Marshak diagram you see a fresh layer of sediment being deposited atop the North American craton.

During the field trip, I posted some iPhone photos of the sedimentary strata that accumulated in the Valley & Ridge during the mid-Paleozoic, shed off from the orogenic activity to the east. For example, the Brallier Formation’s turbidites record a time when sea was west and mountains were east. Or the Juniata Formation’s red beds speak of a time in the late Ordovician when an advancing clastic wedge had piled sediment up above sea level. This shot of some of those red beds preserves some beautiful depositional relationships from ~440 million year old river systems.

Let’s annotate that, shall we?

Even in the Ordovician, rivers did what they do today, spilling over their bansk and building up natural levees. Same as it ever was, people.

That “sediment only; no deformation” regime for the Valley & Ridge changed with the Alleghanian Orogeny. That’s when deformation propagated to the west, encompassing the flat-lying Valley & Ridge strata into a proper fold-&-thrust belt. Later, differential erosion of these folded and faulted layers would etch the landscape into a series of valleys and ridges… hence the province name. More on that deformation in the next post.

Transect Trip 18: Judy Gap

Fault-duplicated double section of the (Silurian aged) erosion-resistant Tuscarora Sandstone: