You are browsing the archive for primary structures Archives - AGU Blogosphere.
29 September 2017
The Friday folds are small soft-sediment deformational features within a dismembered, folded sandstone (a “ploudin”) from a mass transport deposit from the latest Devonian of West Virginia.
22 September 2017
It’s the First Friday of Fall! Here’s a sort of fold to help you celebrate: a section through a ∧ shaped bend in a vesicular basalt flow from the eastern flanks of Mount Etna in Sicily. It’s due to volcanic lava flowing rather than ductile deformation of a pre-existing solid rock (our usual habit with this feature), but I think we can appreciate it regardless:
9 August 2017
A showcase of five new 3D digital models of awesome rock samples and outcrops, produced using Agisoft Photoscan.
15 July 2017
I collected only a single rock on my summer travels in France and Italy. (Those of you who know me will realize how extraordinary this low number is!) It’s a flow-banded rhyolite from Vulcano, in the Aeolian Islands north of Sicily a few weeks ago. It contains porphyritic vesicular basalt xenoliths. I featured a similar sample on Twitter yesterday on the occasion (supposedly) of “International Rock Day”: For #InternationalRockDay , …
6 June 2017
Can soft sediment deformation “loading structures” (ball & pillow) occur when the two strata are identical in composition? Grok on these field photos and chime in with your best hypothesis.
5 June 2017
I’ve been busy making 3D models lately. Here are three ones united by a theme of being sand that was deposited relative to mud. In one case we have scouring to make flutes, in another case we have have localized sagging to make “ball & pillow” structures, and in the third case we have an extraordinary submarine landslide deposit. For two of them, the shale has been preferentially etched away, …
25 May 2017
Deep in the Archean, things suddenly got violent in the deep water of the Mapepe Formation’s oceanic setting. What was a quiet precipitation of chert suddenly was torn apart and tumbled downslope. Was an earthquake to blame? A bolide impact? This is the result.
23 May 2017
Journey to the Silurian period in what is today the Valley & Ridge province of eastern West Virginia to see some exquisite sedimentary rocks that represent deposition in a very arid, very shallow setting.
9 May 2017
The Foreknobs Formation is a Devonian unit in the Valley & Ridge province of the Mid-Atlantic Region. It was deposited in relatively shallow near-shore conditions during the Acadian Orogeny. On a field trip to Corridor H, a new highway transecting the West Virginian Valley & Ridge province on Monday, I stopped to document a couple of beds showing very nice ripple marks. These ones are symmetrical, and thus likely represent …
1 May 2017
Volcanic “hailstones” called accretionary lapilli rained down on South Africa 3 billion years ago, and have survived to be seen in the present day, along the R40 road through Barberton Mountain Land, near the Bulembu border crossing into Swaziland.
13 January 2017
On the western coast of Islay, Saligo Bay showcases turbidites of the Neoproterozoic Colonsay Group. The Smaull Graywacke shows Caledonian (late Ordovician) folding and cleavage superimposed on world-class graded bedding. There’s also a nice dolerite dike to examine.
21 December 2016
Remember the diamictite I featured here a few weeks ago, from Islay? It was the one that might be a Snowball Earth diamictite. Well, if you follow Snowball Earth science at all, you’ll doubtless be aware that the glaciogenic sediments are characteristically overlain by “cap” carbonates. There’s a stratigraphic successor to the Port Askaig Tillite, too – it’s called the Bonahaven Dolomite. Unlike what you might expect for a cap …
26 October 2016
Archean meteorite impact evidence from the Fig Tree Group in Barite Valley, Barberton Greenstone Belt, South Africa
What does the sedimentary record of a giant meteorite impact look like? Turns out it is full of tsunami breccia, fine ash, hail-like spherules, and a barium-rich ocean’s crystal precipitates. Join us in the Barberton Greenstone Belt of South Africa to learn more.
19 October 2016
At the Volcano Museum in Stykkishólmur, I learned that Iceland has fossils. Specifically, they have a display of bivalve (clam) fossils there, and when I asked where to find them, I was directed to a point further east on the Snæfellsnes Peninsula. The next day, I set off to find them. Here was what I saw on a cliff high above me, at the spot nearest to where I thought …
21 September 2016
Based on this photo, what do you think Stađarbjargavík might have to offer? If you guessed columnar jointing in basalt, you’d be right! Looking down the fjord, south of Hofsós (in Iceland): The place is basically a series of miniature Giant’s Causeways, full of unpopulated exemplars of cooling columns! Little coves separate the small peninsulas, each filled with rounded column bits: Here’s a spot where one more is about to …
15 September 2016
Since I showed off some Icelandic pillow basalts yesterday, today I thought I would showcase a new 3D model of big pillows in Columbia River basalt of eastern Washington, taken from a photo set I made when I was out there in May:
14 September 2016
Time is short these days, but I know you hanker for amazing geology. How about some pillow basalts from the Snæfellsnes* Peninsula, far western Iceland? Note the cm-demarcated pencil for scale. See if you can find it in the GigaPan version below: [gigapan src=”http://gigapan.org/gigapans/188175/options/nosnapshots,hidetitle/iframe/flash.html” height=”250″ scrolling=”no” width=”100%”] Link Handheld GigaPan by Callan Bentley, stitched with Microsoft ICE _________________________ * “Snay full snooze”
1 September 2016
The news yesterday of 3.7 Ga stromatolites in Greenland prompts a closer look at 3.22 Ga microbially-induced sedimentary structures in the Barberton Greenstone Belt’s Moodies group sandstones.
31 August 2016
In Cape Town for the International Geological Congress, Callan hikes up Table Mountain and finds some superb primary sedimentary structures in sands and shales of the Graafwater Formation.
2 August 2016
This is the Split Rock at Clachtoll, on the shore of the North-West Highlands of Scotland. You’re looking out to sea, over the Minch. It’s the site that graces the cover of the excellent book A Geological Excursion Guide to the North-West Highlands of Scotland, by Kathryn Goodenough and Marten Krabbendam. “Clach toll” apparently means “Split rock” — Go figure. The Split Rock is an easy landmark to steer toward …