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9 May 2014
Friday fold: knuckling under in the Mesilla Valley shale
Here’s a fold I saw in Texas, in the Mesilla Valley shale, close to the contact with the Muleros Andesite at Cristo Rey: This is a pretty wild looking fold. Let’s zoom in on the most deformed portion: Annotation: white is top of the distinctive, blocky, buckled bed, and black is its bottom side. Red shows brittle fractures in that same bed: Looks as if it rolled over on itself …
7 May 2014
Bedding / cleavage relationships in the Edinburg Formation
Here’s a little scene along Route 340 / 522, north of Front Royal and south of Double Tollgate, Virginia, in the Shenandoah Valley: The rock here is limestone and shale of the Edinburg Formation, a late Ordovician unit that records the transition from passive margin sedimentation to the increasingly ‘dirty’ clastic influence of the Taconian Orogeny. Have a look: I hope you’ll notice there are layers of two distinct lithologies …
29 April 2014
A second look at the mass transport deposit on Corridor H
Remember this past winter when Alan Pitts and I found what we interpreted to be a mass transport deposit (a submarine landslide/slump) along the new section of Corridor H leading up the Allegheny front? Well, I was back out there yesterday, with Dan Doctor (USGS Reston) and Jay Kaufman (University of Maryland). One new thing we found was lots of weathered-out “ploudins” (pillows/boudins), many of which had a “sleigh” shape …
8 April 2014
Deformation associated with the intrusion of the Muleros Andesite
Yesterday, I showed off a few views of the contact between the Cretaceous aged Mesilla Valley Formation shale and the hypabyssal Muleros Andesite which intruded into it during the Eocene at Mt. Cristo Rey (on the US/Mexico border where Texas meets New Mexico). Today, I’d like to look at some of the structure associated with the contact zone. First off, take a look at this image, which is looking orthogonal …
7 April 2014
Contact between Muleros Andesite and Mesilla Valley Formation shale at Mt. Cristo Rey
There are two rock units in this photo. One is igneous, one is sedimentary. Can you find the contact between them? It’s somewhere along this dashed line… The Mesilla Valley Formation is Cretaceous shale with some sandstone. The Muleros Andesite (pretty much identical to the Campus Andesite you find at UTEP) is Eocene. Here’s a closer, more precisely-constrained, look at it: …but that one is in the shade. It’s bolder …
4 April 2014
Friday fold: ‘Ben Folds One’
We have a guest Friday fold today, from reader Ben Mackay-Scollay of the Monash University School of Geosciences in Melbourne: Ben writes: Hey there Professor Bentley, been a fan of your blog for a while and I thought you might be able to use this for your Friday Fold series. It’s an upright fold at Bermagui in New South Wales, where I visited recently as part of my post-grad coursework …
17 March 2014
Angle of repose in Devonian shale, Corridor H
Where the Needmore Formation is falling apart on Corridor H, sloughed-off shale is piling up in a tiny talus slope: You could slap a protractor on that photo and get a pretty good measurement of the angle of repose of this chippy, flaky granular material. Eyeballing it, that looks like about 45° to me.
6 March 2014
Puzzling over the Wallbridge Unconformity along Corridor H
The Wallbridge Unconformity is a surface of stratigraphic hiatus or erosion between the depositional influence of the Tippecanoe and Kaskaskia epeiric seas. After Alan Pitts and I located ourselves in the Oriskany Sandstone (terminal Tippecanoe stratum), we looked stratigraphically above the quartz sandstone for the overlying unit, which should be the Needmore Formation shale (beginning of the Kaskaskia sequence). Indeed, the quartz sandstone was overlain by a black shale at …
17 February 2014
A marine incursion in the Hampshire Formation?
I went out last Tuesday to Corridor H, the exemplary new highway cutting through the Valley and Ridge province of eastern West Virginia. Joining me was former student Alan Pitts, a devotee of Corridor H from way back in the early days when we just called it “New Route 55.” The boondoggle highway is now open all the way west to the Allegheny Front, practically into the Canaan Valley. On Tuesday, …
1 October 2013
New macro GigaPans of sedimentary rocks from the Massanutten Synclinorium
I have two new GigaPans of hand samples to share with you this morning… The Edinburg Formation graptolites from Mint Spring, Virginia, that I featured here back in May, can now be explored in GIGAmacro hand sample: link Students: are these colonial or solitary organisms? Benthic, nektonic, or planktonic? Does this relate to their usefulness as index fossils? And here’s a graded bed from the Martinsburg Formation (Ordovician turbidites) exposed …
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.