Mark Wilson August 19th, 2016
Last month we featured a fossil slab kindly donated by Dale Chadwick of Lancaster, Pennsylvania. Dale is an enthusiastic fossil collector with a very useful website for his favorite sites and specimens. I promised to show the other side of this rock, and here it is.
Again, this is a fine sandstone from the famous Calvert Formation (lower to middle Miocene) exposed at the Calvert Cliffs, Plum Point, Calvert County, Maryland, in the stratigraphic Shattuck Zone 10. Some horizons are especially fossiliferous with large numbers of gastropods and bivalves. This is what we refer to us a death assemblage, meaning these shells are not preserved in their life positions but how they accumulated just before final burial. These rocks and their fossils were the initial basis of Susan Kidwell’s important work on taphonomic feedback, or how shell accumulations affect the succeeding living communities.
So what are the prominent fossils in this slab? Dale has the answers on his website. I’ve annotated the image and made a list below:
Thank you again to Dale Chadwick for this gift! I will use it in my paleontology course this very month.
Kelley, P.H., 1983, Evolutionary patterns of eight Chesapeake Group molluscs: Evidence for the model of punctuated equilibria: Journal of Paleontology 57: 581–598.
Kelley, P.H. 1988. Predation by Miocene gastropods of the Chesapeake Group: stereotyped and predictable. Palaios 3: 436-448.
Kidwell, S.M. 1986. Taphonomic feedback in Miocene assemblages: Testing the role of dead hardparts in benthic communities: Palaios 1: 239–255.
Kidwell, S.M., Powars, D.S., Edwards, L.E. and Vogt, P.R. 2015. Miocene stratigraphy and paleoenvironments of the Calvert Cliffs, Maryland, in Brezinski, D.K., Halka, J.P. and Ortt, R.A., Jr., eds., Tripping from the Fall Line: Field Excursions for the GSA Annual Meeting, Baltimore, 2015: Geological Society of America Field Guide 40, p. 231–279.
Wooster’s Fossils of the Week: Trepostome bryozoans, burrow systems, and bedding features in an Upper Ordovician limestone from southeastern Minnesota
Mark Wilson August 12th, 2016
One of the little mysteries on the recent Minnesota research trip by Wooster students, faculty and staff is the origin of thin limestone beds in the middle of the thick shales of the Decorah Formation (Upper Ordovician). How did such accumulations of almost pure carbonate develop on such a muddy seafloor? Are they storm beds? Some sort of diagenetic feature? The records of brief sealevel changes? Brief interruptions in the supply of silicate sediments to the basin? Turbidites of carbonate material swept into a deeper basin? Above is a view of the top surface of such a limestone bed, this one found in the middle of the Decorah in Shop Quarry (N 43.97232°, W 92.38332°). The light-colored twiggy objects are broken colonies of trepostome bryozoans; the network of holes are burrows of a trace fossil called Chondrites.
An outcrop view of one of these carbonate beds in the Decorah Formation, this one at Wangs Corner (N 44.41047°, W 92.98338°). These units are only a few centimeters thick, and have a variety of petrographic fabrics. This one appears to be an almost pure biosparite with Thalassinoides burrows penetrating from above carrying down a light brown sediment.
Sawing a rock and then polishing a cut surface is always fun and profitable! This is a cross-section through the Shop Quarry slab, oriented with the top upwards. A little bit of iron oxide diffused through the sediments provides the touches of red in the fabric of the limestone.
This closer view of the cut surface shows the exquisite bedding features, along with the bryozoans (B) and trace fossils (T) in cross-section. The burrows pass through the bedding and pie down into the rock a brownish sediment from above. These burrows were made by some sort of deposit-feeding organism that was mining the sediment for organic material. The bedded sediment may be slightly graded in grain size, meaning the many beds may consist of thin fining-upwards sequences. Note how the beds are contorted around the bryozoans as if they were dropped into the sediment while it was still accumulating.
This slab of bryozoans, trace fossils and contorted laminae looks to me like a storm bed formed quickly during and after the seafloor was significantly disturbed by currents. When conditions returned to normal some worm-like deposit-feeders in the fine sediment above sent their mining tunnels down deep into the carbonate looking for food. We have a hypothesis to test!
Mark Wilson August 5th, 2016
This week’s specimen comes from one of my favorite fossiliferous units: the Matmor Formation (Middle Jurassic, Callovian) of Makhtesh Gadol in southern Israel. I’ve been delighted by the fossils and lithologies of the Matmor since 2003. This particular fossil is exposed in a bedding plane of the very rich subunit 65, which I’ve mentioned before in this blog. It is a mytilid bivalve identified as Mytilus (Falcimytilus) jurensis It has the classic wing shape of its order.
M. jurensis is very common in the Matmor Formation, especially in the upper third where it can be seen protruding from limestones at a variety of angles. The species was widespread throughout the Tethys Ocean, now recorded by sediments in the Middle East and Mediterranean regions.
Mytilid bivalves are very common today as well, and they have the same life mode as they did at least 150 million years ago. They attach to hard substrates in shallow waters with strong fibers they secrete called byssal threads. Above we see our M. jurensis shell with several others clustered around a gastropod shell to which they were attached. The organic byssal threads are long gone, of course, but the shells remain in their living positions.
I like to use these Fossils of the Week to explore their taxonomic histories. The specimens, after all, are usually not exceptionally well preserved or rare, but they all have stories. Mytilus (Falcimytilus) jurensis proved to be a challenge when it came to identifying the author of the species.
First I went to the online catalogue of the Muséum National D’Histoire Naturelle in Paris — an excellent resource. There I found the above image and information. Someone named Roemer named the species in 1836. So who was this Roemer and what was the publication?
After considerable searching, I learned our taxonomist was Friedrich Adolph Roemer (1809-1869), a German geologist born in Hildesheim, part of the Kingdom of Westphalia. He had a younger brother, Carl Ferdinand von Roemer, who was also a geologist, creating some confusion.
Friedrich Roemer has an 1836 book (above) that roughly translates as The Fossils of the North German Oolitic Mountains, “oolitic” referring to a kind of limestone common in the European Jurassic; for awhile it was essentially synonymous with “Jurassic”.
On Plate IV, fig. 10, of this 1836 book is a pair of drawings of Mytilus jurensis. So far all is on track for sorting out the taxonomic history of the species.
Surprise! When we look at the description in the text on page 89, we see that Roemer gives an undated credit for the species to “Merain”. Who is Merain?
I thought I’d never find the identity of this “Merain”, but through the extraordinary resource of Google Books, I uncovered the earliest record of Mytilus jurensis. It is on page 13 of Thurmann (1833). Note that following the species (fourth line above) is “Mèr.” and then “n. sp.”, meaning “new species”. (I have no idea what the intervening “M. Bas.” indicates. [Update: See comment by Christopher Taylor below.]) There is no description of the species, and no illustration, but there’s the first mention of it.
So is “Mèr.” short for Roemer’s “Merain”? Turns out Roemer misspelled the last three letters — it is “Merian”.
Peter Merian (1795-1883) was a Swiss geologist and paleontologist who was born in Basel. He studied scientific topics at the University of Basel, the Academy of Geneva, and the University of Gottingen. After two years in Paris, Merian returned to Baasel and began to specialize in the geology and fossils of the Jura Mountains. He was appointed a professor of physics and chemistry at the University of Basel, and later an honorary professor of geology and paleontology. He was also Director of the Natural History Museum in Basel. Along with his work on Triassic and Jurassic fossils, he also made contributions to glaciology and meteorology. Peter Merian died in Basel in 1883 after a long, notable career. He certainly looked the part of a dashing 19th Century Swiss geologist. Kevin McNally could play him in the movie! And now we know that he was the man who named Mytilus jurensis in 1833. Roemer (1836) was probably credited with the species at one point because he had the first description and figures. Merian, apparently, just provided the name in someone else’s book.
Here is an 1829 geological map by Peter Merian of a portion of the Jura Mountains, one of the first of the region.
Cox, L.R. 1937. Notes on Jurassic Lamelibranchia V. On a new subgenus of Mytilus and a new Mytilus-like genus. Journal of Molluscan Studies 22: 339-348.
Freneix, S. 1965 – Les Bivalves du Jurassique moyen et supérieur du Sahara tunisien (Arcacea, Pteriacea, Pectinacea, Ostreacea, Mytilacea). Annales de Paléontologie, t. 51, vol. 1, p. 51-113.
Liu, C. 1995. Jurassic bivalve palaeobiogeography of the Proto-Atlantic and application of multivariate analysis method to palaeobiogeography. Beringeria 16: 31123.
Liu, C., Heinze, M. and Fürsich, F.T. 1998. Bivalve provinces in the Proto-Atlantic and along the southern margin of the Tethys in the Jurassic. Palaeogeography, Palaeoclimatology, Palaeoecology 137: 127-151.
Merian, P. 1829. Geognostischer Durchschnitt durch das Jura-Gebirge von Basel bis Kestenholz bey Aarwangen, mit Bemerkungen über den Schichtenbau des Jura im Allgemeinen. Zürich.
Roemer, F.A. 1836. Die Versteinerungen des Nordeutschen Oolithen-Gebirges. Hahn. 218 pages.
Thurmann, J. 1833. Essai sur les soulèvemens Jurassiques du Porrentruy, avec une description géognostique des terrains secondaires de ce pays, et des considérations générales sur les chaines du Jura. Mém. Soc. Hist. Nat. Strasbourg 1: l-84.
Mark Wilson August 4th, 2016
Four students, Geological Technician Nick Wiesenberg and Professor Mark Wilson (me!) have just completed eight days of geological fieldwork in the Upper Ordovician of southeastern Minnesota as part of Wooster’s Senior Independent Study program. The students are, from the left, Etienne Fang, Nikki Bell, Rachel Wetzel, (me), and Dean Thomas. We have blog posts for each day of the expedition, which have now appeared after we arrived home. Because the entries did not come out daily, you can read them by using the blog calendar or this list:
July 26: Team Minnesota Assembles!
July 27: Team Minnesota visits the Upper Ordovician of Iowa
July 28: Wooster Geologists tramping through the brush of southern Minnesota
July 29: The work begins
July 30: Paleontological fieldwork in southeastern Minnesota
July 31: Team Minnesota finishes its work (in Iowa, funny enough)
August 1: A wet cave and the Mighty Mississippi River
August 2: A visit to the Dalles of St. Croix in Wisconsin
Thank you again to Andrew Retzler (Wooster Geology ’11), a Paleozoic Geologist at the Minnesota Geological Survey, for suggesting this trip and for being such a superb guide to the stratigraphy of southeastern Minnesota. We could not have done it without him! The same is true of Nick Wiesenberg, who was an essential member of the team, being absolutely critical when it came to logistics and planning.
Now Dr. Shelley Judge and I begin advising the laboratory portion of the student projects. More posts from Team Minnesota will appear soon!
Minnesota 2016 Localities —
C/W-730 Shop Quarry: N 43.97232°, W 92.38332°
C/W-731 Market: N 43.97233°, W 92.38215°
C/W-732 Golden Hill: N 43.98788°, W 92.47962°
C/W-733 Wangs Corner: N 44.41047°, W 92.98338°
C/W-734 Turkey Run: N 44.38441°, W 92.91199°
C/W-735 Decorah-Bruening Quarry: N 43.29036°, W 91.76558°
Mark Wilson August 2nd, 2016
Rochester, Minnesota — On its last day in the field, Team Minnesota had a geological trip to the Dalles of St. Croix in Interstate State Park, Wisconsin and Minnesota. It was beautiful, and we practically had the place to ourselves, not counting several million mosquitoes.
The Dalles are where the St. Croix River cuts through a series of Middle Proterozoic (1.1 billion year old) basalts. (Dalles is a French word for a narrow river gorge with rocky sides.) The basalts are of the Chengwatana Volcanic Group and represent ten thick flows of lava. They are much more resistant than the surrounding sedimentary rocks, so the river was forced into a narrow, deep and fast channel. In the top image Rachel Wetzel is looking across the river to the Minnesota side.
The basalts have many vesicles (gas bubble holes) that later filled with minerals, producing a structure called an amygdale. (Thanks, Dr. Pollock!) The lava flows vary in the number and size of their vesicles, and the mineralogy of the amygdales. The round white features in this basalt are quartz amygdales. There are also some brownish feldspar phenocrysts (large crystals in the basalt matrix).
This is what a weathered surface of the amygdaloidal basalt looks like. I was at first fooled into thinking it was a sandstone with quartz pebbles! (Such a unit exists unconformably above the basalts.)
There are enormous glacial potholes excavated into the basalt at the Dalles of St. Croix, some as high as 30 meters above the present river. They were formed about 10,000 years ago as glacial meltwater poured across this basalt in volumes many times higher than the river today. Stones would become trapped in eddies and whirlpools, spinning around and grinding their way into the basalt below them. These may be the largest glacial potholes in the world. Etienne Fang shows the size of one. She is sitting on debris, so the holes goes considerably deeper.
Thus Team Minnesota 2016 completed its expedition! Tomorrow the students fly out of the Minneapolis-St. Paul Airport, and Nick and I drive 12 hours or so back to Wooster with our samples and equipment. Our next posts will be about our observations and ideas from labwork back in Wooster.
Mark Wilson August 1st, 2016
Rochester, Minnesota — Since Team Minnesota efficiently finished its fieldwork yesterday, we have two days before the students fly out of the Minneapolis-St. Paul airport. The good weather has given us a gift of time, so we’re using it like enthusiastic geologists.
This morning we drove down to Harmony, Minnesota, near the Iowa border to visit Niagara Cave. This cave is unusual because it has a stream with a 60-foot waterfall and (a new term to me) active vadose canyons. The cave is entered down a sinkhole into the Dubuque Formation, and then it descends through the Stewartville Formation and reaches its lowest level in the Prosser Formation. These are Ordovician units above the Platteville-Decorah-Cummingsville sequence we did our work with. The cave passages follow joint patterns inn these limestones. It is plenty wet down there.
Plenty of fossils are exposed in the cave. These are sections of gastropods in the ceiling of one section. They were high up so I have little idea of their sizes. I’m just amazed the photo worked out.
In the afternoon we drove northeast to Great River Bluffs State Park on the Mississippi. We had spectacular views of the river and its opposite bank in Wisconsin. Above is a view eastward of Queen’s Bluff overlooking the river.
The main navigation channel of the Mississippi is in the foreground, with the Black River Delta in the middle ground. In the distance are the bluffs on the Wisconsin side, about 4-5 miles away.
Mark Wilson July 31st, 2016
Rochester, Minnesota — We returned to Decorah, Iowa, today to measure and sample the Decorah Shale (Upper Ordovician) in its type locality. It was much drier here than on our last attempt! Above is the gray Decorah Shale topped by the brown Cummingsville Formation.
We started below the quarry exposure along the road to get the top of the Platteville Formation. This is the main locality for Dean Thomas (above) who is sampling these three units for conodonts and their associated carbonate petrographic facies. You may see tiny red ribbons on the outcrop that mark where we removed small blocks of limestone for lab analysis back in Wooster.
While Dean measured section and took samples, Etienne Fang (pictured), Rachel Wetzel and Nikki Bell worked through the Decorah Shale collecting fossils, with special concentration on the beautiful “gumdrop” bryozoan Prasopora.
Team Minnesota has now completed its fieldwork! From the left, Etienne Fang (Bethesda, Maryland), Nikki Bell (Santa Monica, California), Rachel Wetzel (Sewickley, Pennsylvania) and Dean Thomas (Montpelier, Virginia). These students did fine work, setting themselves up for several months of lab analyses back in Wooster. I’m proud of this group!
We still have a couple of days in Minnesota before the students fly to their homes and Nick Wiesenberg and I make the long drive back to Wooster with our samples and equipment. We plan to see the geological and historical sights within range of Rochester, which will, of course, be duly reported here.
Mark Wilson July 30th, 2016
Rochester, Minnesota — It was a good day for fossil collecting on the Minnesota prairie. Above you see a handful of articulated orthid brachiopods collected by Nikki Bell at the Shop Quarry exposure near Rochester.
Shop Quarry (N 43.97232°, W 92.38332°) has long been abandoned and is quickly reverting back to woodlands except for the steepest parts of the old walls. The grey streak visible through the trees is our site where the Decorah Shale is exposed.
The shale has thin limestone beds penetrated thoroughly by the trace fossils Chondrites (the small holes visible in the slab above). This particular rock shows hummocky cross-stratification on its top surface (the dips and ridges), almost certainly an indication it was deposited during an Ordovician storm.
Rachel Wetzel (Pennsylvania) is working with the fossils from this quarry and our other Decorah outcrops to put together a general paleoecological reconstruction and then explore in detail the bryozoans. She is particularly adept at effortlessly clinging to steep slopes!
Dean Thomas (Virginia) will be assessing the carbonate petrology and conodonts of the Decorah Shale, along with the underlying Platteville and overlying Cummingsville Formations. His main outcrop will be in Iowa, but he’s also collecting limestone specimens within the Decorah at other locations for biostratigraphic correlation. All four student projects are integrated, with the whole team collecting data and samples for each individual project.
A typical lunch in the woods for us. We’ve had surprisingly few mosquitoes, and very patchy interactions with parasites. Dean is a tick magnet, and I’ve been badly chiggered!
Our last stop of the day was the annoying Golden Hill roadcut (N 43.98788°, W 92.47962°). It is next to an off-ramp of US 52/63 so we have to reach it by descending through a bouldery, overgrown culvert from above. The students did their jobs, though, and did good sampling.
This spindly green bush is wild parsnip (Pastinaca sativa), a plant we’re careful to avoid despite its abundance in some very inconvenient places. It is often referred to as poison parsnip for good reasons. These plants contain “furocoumarins, chemicals that when combined with skin and ultraviolet light can cause what’s known as a phytophotodermatitis”. You have to contact the sap of the plant and then be exposed to the sun, so brushing against it is not a problem. We worry about grabbing a stalk and breaking it in our hands while clinging to slippery slopes.
Of course, most plants here are benign and often beautiful. These yellow flowers were abundant today at Shop Quarry. I suspect this is Lotus corniculatus (Birds-foot Trefoil). Unfortunately the description of this flower says, “This is a terrible pest plant …”
It was a successful day for Team Minnesota, especially since we were able to end our work just before a bout of thunderstorms. Tomorrow we return to Iowa, which we hope will be much drier than it was yesterday.
Mark Wilson July 29th, 2016
Rochester, Minnesota — Today we started collecting specimens and data for the Team Minnesota student Independent Study projects. We began with a long drive south to Decorah, Iowa, to measure a thick section of our Upper Ordovician target units at the Decorah-Bruening Quarry (N 43.29036°, W 91.76558°), but a patch of persistent and heavy rain lingered over the area all morning. We gave up and headed back north to the Rochester, Minnesota, region, where it was dry and sunny. Our first stop was at Wangs Corner (N 44.41047°, W 92.98338°) to collect fossils from the Decorah Formation for a taxonomic and paleoecological assessment. Rachel Wetzel and Nikki Bell are the Team Paleontologists for this work.
Wangs is a little crossroads in this part of the Minnesota prairie. (Photo by Nick Wiesenberg.)
Dean Thomas has a role at this outcrop as well. He will be doing a conodont biostratigraphy and paleoenvironmental study of the Decorah and units above and below. At Wang’s Corner he found a thin biosparite bed in the midst of the calcareous shale that he can use to help stratigraphically position this section of the Decorah, which has no visible upper or lower contacts.
This bed is a beautiful nearly pure, coarse, well-cemented biosparite/grainstone in contrast to the argillaceous beds above and below. The orange patches in the top of the rock are burrows (likely Thalassinoides) filled with sediment from above. The traditional interpretation of these units is that they were formed by storms. Why they are so clay-poor is a mystery.
Our last stop was the Turkey Run locality (N 44.38441°, W 92.91199°). Here the Decorah Formation is just barely exposed through the weeds. The students gamely collected fossils as the bright sun made us forget the disappointing morning rain.