Archive for August 7th, 2019

Last fieldwork of the summer: A local section with a mystery limestone

August 7th, 2019

It was a delightful August day in northeastern Ohio with pleasant temperatures and thunderstorms that obligingly went around us. Nick Wiesenberg, our geology technician, and I were invited by Dr. Nigel Brush (Ashland University) and friends to examine an outcrop of the Cuyahoga Formation (Lower Carboniferous) near Jeromesville, about a 25 minute drive west of Wooster. Nigel has been investigating a strange limestone in a thick shale sequence that doesn’t seem to fit into our known stratigraphy for the region. I’m always up for a new limestone to look at! As a bonus, the shales here have excellent fossils, especially in concretionary beds. The outcrop, with Nick for scale, is about ten meters high. It is on land owned by the Bricker family, who are geology enthusiasts with beautiful rock and fossil collections.

The shale cliff is steep and has been cut rapidly by the stream (Quaker Springs Run). assisted by occasional dredging of the channel for gravel. The limestone in question is about a meter over our heads here. That’s me, Nigel Brush, and Jeff Dilyard. Photo by Nick.

The shale beds contain siderite concretionary beds loaded with fossils. The most common are spiriferid and spiriferinid brachiopods preserved as internal and external molds and occasionally their original calcite. The taxonomy of these brachiopods is well known, but their paleoecology is still to be determined. (Future Independent Study projects!)

Here is a piece of that limestone eroded from the cliff into the stream. You can immediately see how different it is from the surrounding muddy shales. It is cross-bedded, coarse-grained, and loaded with shale intraclasts (the gray chips), which are mud flakes that were incorporated into the rock while it was being deposited. It looks very much like a storm deposit, possibly associated with a sudden drop in sea level (and rise again).

We took a piece of the above limestone and cut it with a rock saw to begin assessing its composition and fabric. The round grains you see are almost entirely crinoid fragments, mostly stem and arm fragments. The gray chips are the shale intraclasts. Clearly this rock has a story to tell! We will now make acetate peels of this limestone for microscopic study.

One primary question we must address is just where this unit fits in the local stratigraphy. We know it is the Cuyahoga Formation, but which member? It could be the Wooster Shale, Armstrong, or Meadville Shale. One clue would be the conodonts present in the rock. Conodonts are phosphatic dental elements of long-extinct swimming animals. They have considerable biostratigraphic value and so can be used to place units in a stratigraphic column. Since they are phosphatic, they can be collected from limestones by dissolving the carbonate matrix with formic acid and sifting through the remaining insoluble residue. I did this for my dissertation many years ago, and most recently Dean Thomas (’17) dissolved Ordovician limestones for these little critters. I started the process this evening in the paleo lab. Above you see two basins which contain limestone fragments in a 10% solution of formic acid. They are bubbling furiously as the carbonate dissolves. In a few days I’ll sieve the residues and see if we have any conodonts. You’ll be the first to know.

Thank you, Nigel Brush, for the invitation and all your expert fieldwork. It was a fun day as the summer wanes and our first classes of the fall rush towards us.


Shamrock Glacier, Neacola Mountains, Alaska

August 7th, 2019

While in the Neacola Mountains of Alaska last month, we flew over Shamrock Glacier. This first image is from the head of the glacier, where crevasses have been filled in with snow during the accumulation season.

Farther down the north-flowing glacier, we see the merging of the east and west branches and a fine example of a medial moraine.  However, also note on the far lower-right the recently exposed rock. This part of the glacier is shrinking in size, becoming a narrower ice stream.

The toe of Shamrock Glacier is just plain beautiful, ending at a small lake that is dammed from two larger lakes by a ring of moraines. Estimates in a blog post by Mauri Pelto are that the glacier extended all the way out to that moraine as recently as 1950.

In fact, back in 2015, Mauri showed a Landsat satellite image showing the retreat of Shamrock Glacier away from its moraine from 1987 to 2014.  Updating this with the most recent July 2019 imagery, you can see the continued retreat of Shamrock Glacier just in the past four years.  A big section on the left has calved off, and the glacier has slipped off a rise on the right.

Finally, taking a view from the ground, we can better see how the glacier has not only retreated back, but also thinned and narrowed over the past few years. (The 2015 line is based on a photo from Jerry Pillarelli It’s still a pretty glacier, and the sound of calving icebergs while eating lunch  is always welcome.  However, it won’t be long before it retreats upslope sufficiently to no longer calve off into the lake.