Straight nautiloid fossil

It seems I forgot to show this fossil when I found it in February of last year with my MSSE advisor John Graves. We were out in the Needmore Formation of the Fort Valley then. The Needmore is a formation I visited again yesterday with some colleagues in other outcrops further to the west.

In shade (penny for scale):

In sideways-angled sunlight (thumb for scale):

I’ll debrief yesterday’s field trip when I get some more time… for now, let me just toss this little fellow out there for your enjoyment.

If anyone can I.D. it based on these two images, please leave your assessment in the comments section.

Hackles, ribs, plumes

Today, you get a photo from GMU structure student Nik D. This is a small exposure in the Hampshire Formation (Devonian) on New Route 55 in West Virginia. It shows a fine example of plumose structure with the not-often-seen concentric ribs running perpendicular to the ‘plumes.’ At the edge of the joint, you can see the flaring fringe of hackles. Top edge of a Rite-In-The-Rain field notebook for scale:

Where shall we zoom in? How about these two boxes?

Close-up of the concentric ribs:

Close-up of the hackle fringe on the edge of the joint:

…Even the hackles have hackles!

Good stuff: I love me some fine plumose structure!

Flames and pillows, Route 55

I took a look at some interesting blobby structures in the Swift Run Formation last week, and walked readers through my logic in tentatively concluding that they were ball & pillow structures (soft sediment deformation), though overprinted by a pervasive (Alleghanian) cleavage. As we move west in the Appalachian mountain belt, the rocks are less cleaved: the strain is instead taken up in large anticlines and synclines with a few thrust faults thrown in. Though arched and fractured, the rocks’ fabrics remain pretty close to what they were at the time of deposition.

Fortunately, in the Valley & Ridge province along New Route 55 in West Virginia, you can see some sweet examples of (undeformed) ball & pillow in the Hampshire Formation (Devonian; part of the Acadian clastic wedge). Here’s a view looking up at the bottoms of some of these sandy sags (quarter for scale):

I like that partial weathering-out of the features into the third dimension…

To refresh your memory, ball & pillow forms when a heavy load of sand gets dumped (underwater) on top of a soft, squishy deposit of mud. The sand sags downward in broad “balls” (if you have a point locus of sinking) or “pillows” (if the sags have a linear axis to them). In between, the low-viscosity mud squirts up in cuspate “flame” structures. Check out this fine example (quarter for scale), found by GMU structure students Joe M. and Justin O. on the northern side of the road:

…And here we have the same photo, with the sand, mud, flames, and ball & pillow all labeled for you. The white arrows represent the downward sagging of the sand; the red arrows represent the upward squirting of the mud.

In some places, the pillows have weathered out. Here’s one (quarter for scale) that is now upside down on the grassy knoll beneath its source outcrop:

Note the thin laminae of mud clinging to its exterior, like a coat of paint!

Here’s a second sandstone pillow that has weathered out of the cliff and popped down onto the grassy slopes below. Obviously, the surface being photographed is a cross-section through the saggy pillow:

The “upper left” of this sample would have been the lowpoint of the sag if it were in situ on the outcrop. You can see the smooth margin on the left side, and the rougher zone on the right where it detached from the overlying part of the sandy layer. Let’s now zoom in on this box to see something really cool:

If we go deep here, we can see that the laminations of sand within the pillow show small Z-folds (and S-folds on the other side, though they’re not as obvious in the photograph) that are “parasitic” on the main fold. Here they are, highlighted (white arrows) and traced out (black lines):

The axes of these small-scale structures verge on the axis of the main fold, meaning that their axial planes (blue traces) “tip over” with increasing deformation towards parallelism with the main fold’s axial plane. We’ve seen similar things before. The axial planes play the same game as the cleavage plane. This is the first time I have ever observed such a structure associated with soft-sediment deformation, however.

Has anyone else ever seen S- or Z-folds associated with soft sediment deformation? It makes total sense to me that they would be there based on simple physics, but this was the first time I had ever seen it myself. I’d be curious to get a sense of how common or rare this might be.

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 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.

Transect Trip 14: ripple marks

Where are some ripple marks in the Hampshire Formation. Cell phone service is a lot more localized here in West Virginia, so we’ll see how many posts I manage today…