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You are browsing the archive for July 2015 - Mountain Beltway.

31 July 2015

Friday fold: a return to the duplex structure in the Gastropod Limestone

Some time ago, I featured as Friday fold the extraordinarily complex duplex structure to be seen in the Cretaceous “gastropod limestone” member of the Kootenai Formation at Sandy Hollow, Montana. Today, let’s take a deeper look through a couple of hand-shot GigaPan images: Here’s the bigger of the two: link Here’s one with students for scale: link

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30 July 2015

Stromatolites of the Helena Formation, Grinnell Glacier Cirque, Montana

My favorite place to have lunch in Montana is at the Grinnell Glacier cirque in Glacier National Park. This is the dining room table: You’re looking at a bedding-plane-parallel exposure of Mesoproterozoic stromatolites here. Every few years, I’m lucky enough to hike up there with motivated students and share food atop this unparalleled view into the shallow seas of more than a billion years ago. Stromatolites are sedimentary structures that …

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29 July 2015

Positively-weathering volcanic dike near Granby, Colorado

My friend Barbara am Ende sent along this lovely image of a dike in Colorado: Here’s the site. You can see the dike in Google Earth. Dikes are fractures, filled with molten rock, which then cools and solidifies, sealing the crack shut. In this case, once it got uplifted to Earth’s surface and exposed, the dike rock is tougher (more resistant to weathering) that the older rock it cut across. …

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28 July 2015

Millboro Formation shale in outcrop and in hand sample

Another site from the GMU sedimentology field trip in April: An outcrop on Route 33 in Brandywine, West Virginia, showing the Millboro Formation. It’s mostly shale, with some intriguing sandstones, too. There are fossils and diagenetic carbonate nodules (concretions). Here’s the outcrop, the largest GigaPan I’ve taken so far (7.9 billion pixels): link The shale itself looks… like shale. It’s fine-grained, and dark (high carbon content, suggesting low oxygen levels …

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27 July 2015

Conglomeratic ne plus ultra

There’s something about conglomerates that just draws me in. Here’s a lovely example — you might even say it’s an exemplar — from Sandy Hollow in Montana: That’s the basal conglomerate of the Cretaceous Kootenai Formation, one of the mappable units in this mappable region. Feast your eyes on those well-rounded pebbles!

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25 July 2015

More books

I am way, way, way behind in reporting on the books I’ve read. As time goes by, the list gets longer, and the “book report” more daunting… So I’m going to do a brief book report in hopes of clearing out the backlog: What if? by Randall Munroe Really entertaining scientific answers to ridiculous questions, by the cartoonist author of XKCD. Highly recommended. Unique. The Owl Who Liked Sitting on …

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24 July 2015

Friday folds: Fox’s and Foch’s

A final Friday fold (for now) from Howard Allen: This is : A view south across Kananaskis Lakes, Peter Lougheed Provincial Park, Alberta, through mist/low-hanging clouds. Snow highlights the Sarrail Creek Syncline and Warspite Anticline on the north faces of mounts Fox (left/east) and Foch (right/west). Rocks are Lower Carboniferous carbonates of the Banff, Livingstone and Mount Head formations. Happy Friday all – and thanks for sharing all these great …

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23 July 2015

Purcell Sill, layer-jumping

After reading the post last weekend about the discordant offshoot of the Purcell Sill, Rich Gashnig (post-doc at Georgia Tech) sent me a few photos he shot at Piegan Pass (at the head of a side-canyon adjacent to the one containing Grinnell Glacier). They show the Purcell Sill leaving one stratigraphic horizon and jumping to another, with the intermediate zone of Helena Formation limestone bending to accommodate the different positions: …

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22 July 2015

Oolitic soft sediment deformation in Helena Fm. limestone

Another gem from the Grinnell Glacier cirque: Zooming in on the contact, showing the concentrically-zoned ooids: Near the tip of the flame structure (?), I noted alignment of longer platy / flaky components within the oolitic layer: This looks like a loading structure – soft sediment deformation due to a density inversion – perhaps when some high-energy event (a storm?) dumped a bunch of relatively coarse ooids atop some squishy …

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18 July 2015

Purcell Sill → Dike, Grinnell Glacier Cirque, Montana

The Neoproterozoic Purcell Sill is a stark, obvious black stripe in the strata of Glacier National Park. Here it is emerging from behind “The Salamander” glacier, above Grinnell Glacier Cirque: Zooming in, you can see the “baked” (bleached) zones above and below this concordant intrusion. But this time, during my visit to this special place, I noticed a discordant offshoot from the main sill: See it? Up there at the …

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