June 7. Lands and Seas

The Mississippian Period was much like the earlier part of the Paleozoic Era in North America – much of the continent was covered by warm shallow seas in which abundant life contributed to the thick sheets of limestone that were laid down. Eastern North America was still a highland, the result of the ongoing collisions with Europe and the microcontinents that were rifting away from Gondwana, and the ocean between North America and Gondwana was getting narrower and narrower.  

In western North America, the Antler uplift that we discussed at the end of May was still there, and it was shedding sediment into the western seaway. The combined weight of those sediments and the piles of thrusted rocks pushed over the western edge of North America by the Antler Orogeny pushed the earth’s crust downward, creating what’s called a foreland basin. The Antler foreland basin extended through what is now eastern Nevada, western Utah, and into parts of Idaho and California. It was a relatively deep trough, and the mud that found its way into it contained a lot of organic material, washing in from both the west and the east. The resulting rock is called the Chainman Shale – a rock we mentioned in the Devonian, in May, as the source rock for the oil fields of Nevada. The Chainman is an excellent source rock, as much as 8% total organic carbon in some places.

There was some tectonic activity in what is now Alaska’s North Slope and across the Arctic islands of northern Canada – it’s called the Innuitan or Ellsmerian Orogeny and it was mostly taking place toward the end of the Devonian and into the Mississippian Period.

In Europe, Gondwana was pretty much encroaching on the southern margin of Baltica, but it was a complex interaction with lots of smaller blocks colliding. The seaway between Gondwana and Europe, called the Tethys Sea, is actually still there. We call it the Mediterranean Sea today. 

—Richard I. Gibson

Links to Paleogeographic maps:
North America
Western North America
Europe

Article:

Innuitan Orogeny

June 6. Echinoids

Echinoids are another class of echinoderms. Echinoderms today are probably most familiar as starfish, but they also include sand dollars and sea urchins, and those groups are part of the class Echinoidea. Fossil echinoids are common in carbonate rocks from the Mississippian Period. The oldest known echinoids are from the late Ordovician. Most of the classes of echinoderms appear to have become established during the Ordovician, and many, including echinoids, have survived to the present.

photo by Debivort via Wikipedia, under GFDL

Modern sea urchins have a globular body with five-fold symmetry, typical of all echinoderms, and a forest of spines encrusting the body. Fossil echinoids usually show only the body, often with a distinct 5-point star design on top. Ancient echinoids probably had spines as well, but they are not usually preserved intact with the body. 

Echinoids began to decline during the Pennsylvanian or late Carboniferous period, which we’ll cover next month, and by the Permian extinction there were only six species that we know about, and only two survived into the Triassic. But that was enough to give rise to the modern varieties of echinoids, which total about 950 species today.

* * *

On June 6, 1912 and for several days thereafter, Mt. Katmai in the Alaska Peninsula erupted. The series of eruptions devastated the area and created the landscape known today as the Valley of 10,000 Smokes, for the many steaming and smoking fumaroles and vents that were formed, including one large volcano called Novarupta. The summit of Mt. Katmai collapsed to make a caldera more than two miles across. The eruption was related to the subduction of the oceanic Pacific Plate beneath Alaska, and it was probably the largest volume of material erupted in the 20th century, at about 11 to 13 cubic kilometers, or about 3 cubic miles of ash and lava. That volume is about 30 times the volume erupted by Mt. St. Helens in 1980. The only other 20th century eruption that comes close was that of Mt. Pinatubo in the Philippines in 1991, which is also estimated at about 11 cubic kilometers of ejected material. Today, the Valley of 10,000 Smokes is part of the Katmai National Park and Preserve.

—Richard I. Gibson

photo by Debivort via Wikipedia, under GFDL

June 5. Blastoids

First, an update. Some new work seems to have pinned down the cause of one of the multiple mass extinctions during the middle and late Cambrian period. The extinction at 510 to 511 million years ago correlates well with voluminous volcanism in Australia. I’ve put a link on the blog episode for February 28 to an article about this.

Now, back to the Mississippian. It’s June 5, and today’s topic is blastoids. Blastoids are a class of echinoderms, quite similar to crinoids. Crinoids have decreased tremendously from their peak during the Paleozoic, but they’re still with us today. Blastoids however are extinct – they didn’t make it past the Great Dying at the end of the Permian Period. But since they began during the Ordovician, or possibly in the late Cambrian, blastoids had a run of more than 230 million years.

Photo by DanielCD via Wikimedia Commons under GFDL.

Like crinoids, blastoids were animals that had a plant-like stalk and a system of holdfasts, root-like structures that held them to the sea floor. Unlike crinoids, blastoids’ skeletons were held together by solid interlocking calcareous plates. In most crinoids, their bodies were held together by muscular tissues, so after death, crinoids tended to fall apart. Blastoids are often found intact. It also seems that blastoids and crinoids may have had different systems for moving water through their bodies to provide oxygen to the animal.

The pentagonal symmetry typical of echinoderms is usually well displayed by blastoids, and the body fossils often look like a flower bud or some kind of nut. There’s a wide range in size, of course, but some of the most common genus, Pentremites, are on the order of a half inch to an inch long.

Blastoids reached their peak of diversity and numbers during the Mississippian, and in some places, such as the fossil locality known as Pentremites Hollow, near Bloomington, Indiana, they are exceedingly abundant.

—Richard I. Gibson

Photo by DanielCD via Wikimedia Commons under GFDL.