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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
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 …
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: …
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 …
3 July 2015
Friday fold: Opal Range, Alberta
Howard Allen is the Friday folder who keeps on giving… Here’s his latest: Multiple folds at top of ridge, Opal Range, Alberta. Photographed from Kananaskis Highway 40, looking SE. Beds are Carboniferous carbonates, probably Mount Head and/or Etherington formations. There are lots of great folds to be seen along that road. The Kananaskis Trail is in the “tourist shadow” of the nearby Trans-Canada Highway, which means it’s well worth your …
26 June 2015
Friday fold: Subvertical carbonates
Another Friday, another Friday fold from Howard Allen: Folds in near-vertical beds, north side of Grizzly Creek, Opal Range, Alberta. Beds are Carboniferous carbonates, Mount Head and Etherington formations. Enjoy your day!
22 May 2015
Friday fold: Welsh carbonates
Kate Littler sent in this Friday fold, via Twitter: lovely parasitic folds in the Carb limestone, West Angle Bay south Wales. Awesome. Looks like a great place. Thanks, Kate! Happy Friday!
30 April 2015
Early Paleozoic fossils in Nealmont Formation, Germany Valley, West Virginia
Looking at Ordovician carbonates in Germany Valley, West Virginia, a few weeks ago on Rick Diecchio’s GMU sedimentology and stratigraphy course field trip: Lots and lots of brachiopods… Crinoid columnals mized with brachiopods: A set of coarsely-infilled trace fossils: Crinoid stem: Nice strophomenid brachiopod: Bryozoan? Receptaculid?
17 April 2015
Fault breccia in Beekmantown limestone
Here’s another sight at the Eocene dikes site in Bluegrass Valley, Virginia, mentioned yesterday: That’s a gorgeous fault breccia, emplaced parallel to bedding, and parallel to the felsic dike (which can be found a few feet to the west / right of these photos): It was very poorly lithified, shockingly crumbly to the touch, considering the big slab of rock downhill (to the right) of it. Here’s a link to …
16 April 2015
Eocene dike and sill in Ordovician limestone
A virtual field trip to a quarry in far western Virginia, showing anomalous igneous intrusions (a dike and a sill) of Eocene age cross-cutting early Paleozoic carbonates.
Callan Bentley is Associate Professor of Geology at Piedmont Virginia Community College in Charlottesville, Virginia. He is a Fellow of the Geological Society of America. For his work on this blog, the National Association of Geoscience Teachers recognized him with the James Shea Award. He has also won the Outstanding Faculty Award from the State Council on Higher Education in Virginia, and the Biggs Award for Excellence in Geoscience Teaching from the Geoscience Education Division of the Geological Society of America. In previous years, Callan served as a contributing editor at EARTH magazine, President of the Geological Society of Washington and President the Geo2YC division of NAGT.