February 28. Cambrian-Ordovician extinction

The beginnings and endings of the subdivisions of geologic time are usually well recorded in the rocks. Many of them are major changes in the life of the time, as indicated by fossils. And many of those changes are mass extinction events.

Extinctions occur probably almost continuously, but there’s clear evidence in the fossil record for relatively short time spans when the rate of extinctions ramped up dramatically, killing many more species than usual. These mass extinctions punctuate the geologic record. You’re probably familiar with the mass extinction at the end of the Cretaceous period, 65 million years ago, when most of the dinosaurs died, and the even greater event at the end of the Permian period when about 96% of all marine species vanished.

But there were two or maybe four mass extinctions during the Cambrian period that were probably worse than any later events except the Permian one. The last of these events was about 488 million years ago, and is taken to mark the end of the Cambrian.

Many of the marine animals that we described in the Cambrian explosion of life died. Brachiopods and trilobites especially saw a serious reduction in the number of species, and this is clearly recorded in the fossil record.

What caused it? Until quite recently we haven’t been able to point to smoking guns, explicit causes for mass extinctions. You’re undoubtedly familiar with the idea of an asteroid impact causing the end Cretaceous extinction. Other extinctions are not so clear cut.

There is evidence for increased glaciation at about the start of Ordovician time, and that’s been cited as a possible cause or factor in the mass extinction. Besides colder temperatures, glaciation lowers sea level by locking water up in ice, so there would have been fewer of the popular shallow water niches for trilobites and such to live in. Cooler water is also less able to hold oxygen, so oxygen depletion is also cited as a possible factor in the end-Cambrian extinction.

Bottom line: we have some reasonable well thought-out ideas for causes of the Cambrian mass extinctions. But we really don’t know. UPDATE: New dating evidence ties one of the middle to late Cambrian mass extinctions, the one at 510-511 million years ago, to volcanic eruptions in Australia. Here's the link.

* * *

Today, February 28, 1743, is the birth date of René-Just Haüy, at St-Just in Picardy, France. Haüy was a mineralogist, often called the Father of Crystallography. He studied the regular way minerals break apart, a property called the mineral’s cleavage, and applied mathematical approaches to crystal forms, anticipating the much later understanding of molecular crystal structure. He was imprisoned during the French Revolution, but survived, and under Napoleon became a professor of mineralogy. He died in 1822. 

—Richard I. Gibson

Image from Wikipedia under GNU free documentation license

February 27. The Great Cambrian Conflict

Adam Sedgwick

Adam Sedgwick was born in Yorkshire, England, son of a not-so-well-to-do preacher. He was an unruly student, but made it to Cambridge at age 20. With poor-man’s clothes and a hinterland accent, he didn’t fit in too well with his wealthy classmates, but he was near the head of his class until he was felled by a bout of typhoid, which would leave him sickly for years. Of necessity – it was required by the Church, which controlled Cambridge University – he studied theology, which he detested so much that he applied for a professorship in geology, about which he knew nothing. But he was elected to the job, which paid a measly hundred pounds a term.

As an infant science, geology left him plenty of room for invention, and maybe for objectivity as well. His self-taught geology and enjoyment of a free life led him eventually to ramble around Wales, where he described the lowest, oldest series of sedimentary rocks he could find, and called them Cambrian.

Roderick Murchison

Sedgwick of course encountered other geologists, including Roderick Murchison, whose birthday we celebrated a few days ago. They became friends, though they were from different walks of life. Murchison was rich, a son of landed gentry in Scotland. In contrast to Sedgwick’s solitary camping expeditions into the wilds of North Wales, Murchison took with him his “wife and maid, two good gray nags and a little carriage, saddles being strapped on behind for occasional equestrian use.” He smoked expensive cigars with colleagues in a salon-like atmosphere even if it was in a carriage.

Where Sedgwick focused on the physical nature of the rocks – we’d call that petrology and lithology today – Murchison focused more on the fossils in them. Sedgwick defined the Cambrian from its position low in the section and from its rock types, while Murchison defined strata in South Wales based on their fossils. He called that package of rock the Silurian, for an ancient Celtic tribe who lived in South Wales, the Silures. All well and good.

Murchison and Sedgwick teamed up to work in Devonshire and Cornwall, jointly announcing the Devonian Period in 1839. This was a controversy of its own, which we’ll talk about at the appropriate time… but as the friends continued to extend their work on their other units, the Cambrian and Silurian, problems developed. Sedgwick was increasingly plagued by health problems while Murchison actively extended his Silurian System.

It became evident to Murchison that some of Sedgwick’s Cambrian rocks actually contained fossils that should be classified as Silurian, so he extended his Silurian formation lower and lower in the section, taking up more and more of the Cambrian. This upset Sedgwick, although he had tacitly—or, he said later, inadvertently—approved the extension, and sometimes he denied the whole thing in harsh terms. The friendship was at an end, and the controversy pervaded British geology for the next 40 years. Everyone chose one side or the other, but on the whole Murchison’s later career was far more successful than Sedgwick’s. Murchison was knighted, and he became director of the British Geological Survey. Sedgwick, in declining health, kept a professorship, but seems to have been relegated to a by-way in British geology.

Both Sedgwick and Murchison died before their controversy was settled. It fell to English geologist Charles Lapworth to study the Cambrian and Silurian strata and to propose that it was necessary to include another time period there, embracing parts of Sedwick’s Cambrian and parts of Murchison’s Silurian. He called it the Ordovician, and we’ll be there in a couple days.

—Richard I. Gibson

Images are public domain.

February 26. Sponge spicules

Perhaps you recall on February 4 we talked about archaeocyathids, animals that most scientists believe were an early type of sponge. By the Late Cambrian, the archaeocyathids were extinct, and that might be because of the increase in numbers and diversity of more modern sponges.

There were probably sponges in late Precambrian time. The Ediacara fauna includes probable sponges. But these primitive animals took part in the Cambrian explosion, specifically the explosion in development of hard parts – the same kinds of hard parts that sponges have today.

Most modern kitchen sponges are plastics made from oil and natural gas, but natural sponges are still harvested for household sponges. Their roundish or cylindrical bodies consist of spongin, a protein similar to the collagen in humans that makes up things like tendons and skin. And sponges are full of holes – pores, which gives the name to their phylum, Porifera. Those holes are vital to their simple lives, necessary for circulating water to bring in nutrients and wash out wastes. Sponges don’t have nervous systems or circulatory systems. The water they live in does it all for them.

Microscopic sponge spicules

So what happened to sponges during the Cambrian explosion? They developed things called spicules – pointed structures, sometimes microscopic and sometimes macroscopic, that they used to help support their spongy bodies and that may have provided at least a bit of defense against predation.

Although sponge spicules can be made of hardened spongin or calcite like most shells, many are siliceous – SiO₂, the same as the mineral quartz. That’s the most common mineral in the earth’s crust and it’s the most common constituent of sand. Because silica is resistant, sometimes sponge spicules are all that survives in the fossil record from what may have been a great abundance of sponges.

In addition to providing a support structure for sponges, spicules might have served as little fiber optic bars focusing light into a sponge. This might have helped attract algae or other organisms that sponges had symbiotic relationships with, but study of this aspect of sponges is pretty new. It does have implications for the fiber optic industry, because the cold-temperature secretion of silica by sponges would probably be cheaper than the high temperatures used in the industry today, and it might allow for more efficient introduction of impurities to improve optical characteristics. A common example of such impurities being used is the photo-sensitive chemicals introduced into eyeglasses to make them darken in ultraviolet light. Another is adding tiny amounts of the rare-earth element lanthanum to improve the refractive properties of camera lenses. I discuss some of these applications in my other book, What Things Are Made Of and if you’re interested, you can find information about it here.

Today is Joseph Le Conte’s birthday. He was born in 1833 on Woodmanston Plantation, Georgia, and he became a prominent physician and geologist, the first professor of geology at the University of California at Berkeley. He was a friend of John Muir and was a founding member of the Sierra Club.

—Richard I. Gibson

Photo by NOAA (public domain).

February 25. Viburnum Trend

When we talk about mineral deposits, we often don’t know accurately the geologic time when the minerals came in – it might be much, much later than the rocks in which the deposits are found. That’s changing, as we get better and better at dating techniques, but for most of these podcasts dealing with mineral deposits, we’ll probably focus on the age of the host rocks and talk about the time the minerals came in more speculatively.

The lead belt of southeastern Missouri is concentrated in Cambrian rocks, especially the Bonneterre formation, which is mostly dolomite, calcium magnesium carbonate. It’s much like limestone, calcium carbonate, but the magnesium in there skews the crystal structure, so dolomite crystals contain more intermolecular space than calcite. That makes them good candidates for oil reservoirs, or as hosts for mineral deposits.

The Bonneterre rocks originated in a warm, shallow sea during Late Cambrian time, about 495 million years ago. They might have been limestone originally, converted to dolomite by magnesium-rich water percolating through the rock at a later time. The whole process of dolomitization is complex and not thoroughly understood, at least not by me – I’d like to find someone who knows more about it to talk with, as a future podcast.

Galena from Sweetwater Mine, Viburnum Trend District,
Reynolds County, Missouri, USA.
Photo by Rob Lavinsky, CC-by-SA.

The rocks sat there for a long time – probably at least a hundred million years – until the Devonian, about 385 million years ago, or maybe until the Pennsylvanian, 280 million years ago. The jury is still out, as far as I can tell, on when the minerals were deposited in the rock. My eyes tend to glaze over when I read the phrase “hot mineral rich waters came in” – because that’s often a cop-out meaning, we don’t really know. But, research continues, and this kind of thing is getting to be more and more pinned down as more information comes in.


Waters that were heated by mountain-building activity, volcanism and the physical collision of plates, must have collected a lot of lead. Those waters were most likely driven into the Bonneterre formation from the south, possibly from as much as several hundred miles away, until they found a suitable rock in which to crystallize. There’s more galena, lead sulfide, in southeast Missouri than anywhere else in the world. The ores in Missouri are part of a class of mineral deposits called Mississippi Valley Type, which occur in sedimentary rocks.

Southeastern Missouri has produced lead since about 1721, when early French explorers began mining. They produced as much as 1,500 pounds of lead ore per day, which was shipped down the Mississippi and on to France. Production has been pretty much continuous since about 1802, when Moses Austin began smelting ore, a year before the territory became part of the United States in the Louisiana Purchase.

Historic mine and mill buildings at the Federal Mine and Mill #3,
now a part of Missouri Mines State Historic Site.
Note the tailings dam in the background.

There are three major sub-districts within the lead belt. The newest is called the Viburnum Trend, a long string of mines that began lead production in 1960. Missouri produces a lot of zinc, which commonly goes along with lead, and fair amounts of other metals including silver. Missouri produces about 70% of the lead in the United States, with Alaska the second-leading producer. Idaho is third, and that’s it – that’s all the lead production in the U.S. 

Until 2011 the nearly 400 tons of lead that came from US mines was enough to make the United States a net exporter of lead, but in 2011 and 2012 the U.S. imported 2 to 4% of its lead needs. Eighty-six percent of U.S. lead consumption goes to make lead-acid batteries for cars and trucks, and thanks to recycling, we get about three times as much lead from old batteries as we do from mines in Missouri, Alaska, and Idaho.

China, the world’s leader in lead production with nearly eight times US production, and half of all the lead mined in the world, is also one of the growing consumers as their auto and battery-powered bicycle market soars.

—Richard I. Gibson

Technical paper on timing of mineral development
Galena specimen photo by Rob Lavinsky, under CC-by-SA-3.0
Mine photo by John Weber, USFWS.

February 24. Cambrian Jellyfish

Jellyfish may not have changed a lot in hundreds of millions of years. I guess that’s one measure of success, or at least an ability to survive environmental changes. There are in fact plenty of varieties of jellyfish today, but we don’t know that much about their evolutionary history because as soft-bodied animals, they don’t leave much in the way of fossils.

Middle Cambrian cnidarian jellyfish. Black bar is 5 mm (© 2007 Cartwright et al.; under Creative Commons license)

In 2007 scientists from the University of Kansas, the University of Utah, the Smithsonian, and the University of Sao Paulo described some Cambrian jellyfish fossils from Utah that are remarkable in their preservation. The authors were able to identify such fragile structures as tentacles and organs, suggesting that modern aspects of jellyfish were developed within a few million years of the Cambrian explosion. For most other phyla, especially the chordates, which include us, evolving modern characteristics was a long process. Here's the paper. (also source of photo, used under Creative Commons license)

The rocks that hold these fossils are about 505 million years old, which puts them in the Middle Cambrian, just before the start of the Late Cambrian.

—Richard I. Gibson

February 23. Trilobite poop

With over 4,000 mineral species, you could overflow this calendar with beautiful pictures and words about minerals, but most minerals don’t have a lot of specific connection to particular time periods in earth history. Some mineral deposits do, and we’ll talk about them. Today’s mineral, glauconite, does have a connection to the Cambrian, at least to some degree.

Glauconite is a complex potassium-iron alumino-silicate, K₂(Mg,Fe)₂Al₆(Si₄O₁₀)₃(OH)₁₂. It can be found in many kinds of sedimentary rocks, and in many ages right up to the present, but it’s pretty common in the Cambrian. It occurs as little green pellets, often intermixed with good quartz sand, or interbedded with limestone. What made these pellets?

Cambrian Lion Mountain Sandstone
(green in lower portion from abundant glauconite), central Texas.

To put it bluntly, glauconite pellets are trilobite poop. OK, not just trilobites, and that’s not the only way glauconite forms. But the little round grains in marine rocks are thought to be an alteration from the original fecal pellets excreted by marine organisms. It can also precipitate directly, and it can form when some iron-bearing minerals are weathered, but the pellets in sandstones are generally accepted to represent fecal material.

Some rocks contain enough glauconite to be called greensands, but more often, the sand-sized glauconite grains are scattered through the rock and aren’t obvious until you look at it under magnification. Then they practically pop out at you. The Lion Mountain Sandstone, in the Llano Region of central Texas, is a Cambrian formation rich in glauconite – and no real surprise, some parts of the rock are mostly broken up trilobite skeletons. It’s a cool rock, and it was probably laid down in a wide sandy tidal flat. With trilobites crawling all over the place and pooping left and right. Occasional storms must have broken up the trilobite shells and dumped them into the piles in which they are found today.

Glauconite is green because the iron in it is in its reduced state, rather than oxidized which would lead to a rusty red color. That means relatively anoxic, low oxygen, conditions, such as might be found on a sea floor below wave base or in a stagnant mud, or the gut of a trilobite. The presence of glauconite pellets is taken to mean that the rock they are in was formed in marine conditions, and that’s a useful conclusion to draw from the presence of little green grains in a rock.

—Richard I. Gibson

Cambrian Lion Mountain Sandstone (green in lower portion from abundant glauconite), central Texas. Photo by Erimus via Wikipedia, public domain. 

February 22. Cambrian Stratigraphy of western Montana

When I was a student at Indiana University’s geology field course, out here in Montana, we learned the stratigraphic section.

The Cambrian part is Flathead-Wolsey-Meagher-Park-Pilgrim. The Flathead is the oldest layer of the Cambrian out here, and I hope you aren’t surprised to learn that it’s a clean quartz sandstone like the Tapeats in the Grand Canyon and the Posdam back east. Like them, the Flathead sandstone sits above a profound unconformity, a break in the rock record, and the rocks below it are Precambrian in age, hundreds of millions of years older than the Flathead. It’s pinkish, like the Potsdam, because of some iron oxide cement, and it has little round green grains in it in places – we’ll talk about them tomorrow – but mostly, it’s just nice sandstone.

Trilobite Bathyuriscus formosis, Cambrian Meagher formation, Montana.
Photo by Stephen W. Henderson, used by permission.

The stratigraphic section here in Montana is a lot like the Cambrian section in the Grand Canyon. Above the Tapeats sandstone in the Grand Canyon we have the Bright Angel Shale, followed by the Muav limestone. Here in Montana, the Flathead sandstone is followed by the Wolsey Shale, then the Meagher Limestone. Then the Park Shale, and then the Pilgrim formation, limestones and dolomites.

The seas came in, the seas came out…. Alternating shale and limestone might mean that, but there are other ways to make it happen. I’m planning to have a conversation with an expert on Cambrian stratigraphy in a week or so – we might be in the Ordovician by then, but if we are we’ll just think back on the Cambrian when that conversation happens.

From the point of view of someone mapping geologic layers, the importance of the sequence – Flathead, Wolsey, Meagher, Park, Pilgrim – is that it’s really the best way, sometimes the only way, to be sure which rock or rocks you might be looking at. In western Montana, there’s another pinkish quartz sandstone called the Quadrant – a chunk of it looks an awful lot like a chunk of the Flathead, to the point that it’s virtually impossible to tell them apart in the field. But the Quadrant is Pennsylvanian in age, around 280 million years old, rather than around 500 million years for the Flathead.  If you look at the rocks below the Quadrant, you won’t find the Precambrian unless there’s some complicated structural thing going on, like faulting. And if you look above, you won’t find the precise sequence of the Wolsey, a specific kind of shale, the Meagher, a limestone with distinctive characteristics, the Park shale, and then the Pilgrim formation. It’s that sequence that’s like a fingerprint that tells you you’re in the Cambrian, even if the individual chunks of rock can’t tell you that for sure.

—Richard I. Gibson

Trilobite Bathyuriscus formosis, Cambrian Meagher formation, Montana. Photo by Stephen W. Henderson, used by permission.

February 21. Ohio oil fields

You don’t expect much oil and gas in the Cambrian. Partly that’s because it’s so old and deep, the rocks that might hold oil or gas may have been buried so deeply that the hydrocarbons, the oil and gas, may have volatilized, turned to gas and seeped out. Or the pressure could have reduced the porosity to not much. And since oil and gas come mostly from decaying plants, you have to wonder if there was enough life around to accumulate to be cooked into oil. But we do find some oil in Cambrian rocks.

Oil wells in back yards in Cardington, Ohio, 1964. Photo from Ohio Geological Survey.

In Morrow County, Ohio, just north of Columbus, in the 1960s there was quite an oil boom for a while. It discovered a bunch of little fields in a special kind of oil trap. The upper part of the Cambrian in Ohio includes carbonates – limestones and dolomites, the kinds of rocks that flowing water can dissolve to make caves. At some point not too long after they were deposited, parts of those layers were eroded into hills, and the hills, standing above water line were dissolved – not really into caves, but little dissolution cavities developed in the rocks, excellent spaces to have an oil reservoir.

Ordovician rocks – we’ll talk about the Ordovician next month – were deposited over those rocky hills and served as a tight, impermeable seal to keep fluids from escaping. Oil migrated into those little hills, and stayed trapped until the 1960s, when thousands of wells tapped hundreds of fields. They have produced about 38 million barrels of oil and 35 billion cubic feet of natural gas over time – not that much in the grand scheme of things, but not too shabby, either. To put it in perspective, the United States today consumes almost 20 million barrels of oil every day, so all of the 38 million barrels produced by those wells in the 50 years since the 1960s amounts to about 2 days’ consumption.

Where did the oil come from? Good question. It’s in Cambrian rocks now, but did it start there? Oil reservoirs are not usually where the oil originates. It starts in a rock with lots of organic material, a source rock. Heat, from burial, cooks that solid organic matter over sometimes millions of years, and oil is generated. Then it migrates until it reaches a suitable place to accumulate, a reservoir. The oil in Cambrian reservoirs in Ohio is probably from organic-rich black shales of Ordovician age – younger than the reservoir. How do you push light oil DOWN into older formations? Well, you don’t, with some unusual exceptions. The deep Appalachian basin where the Ordovician shales were heated up – oil people call it the oil kitchen – was deeper than the reservoirs up in central Ohio. The oil did migrate up, as it pretty much must – but it got into older rocks that were physically above the younger source beds. Seems counterintuitive, but it happens more often than you might think.

—Richard I. Gibson

Photo: Oil wells in back yards in Cardington, Ohio, 1964. Photo from Ohio Geological Survey.
http://pubs.er.usgs.gov/publication/70020638

February 20. Calcareous algae

We’ve talked about the Cambrian explosion and some of the cool critters that evolved during it, like trilobites and brachiopods. But the less obvious life was still around, if not thriving. That included blue-green algae, or cyanobacteria. Those little guys way back in the Precambrian were the primary builders of our oxygen-rich atmosphere. But during the Cambrian, calcareous algae, that is those that could create fine layers of calcite, calcium carbonate, were still abundant enough to make small reefs in the Cambrian rocks near Saratoga, New York, at Jackson, Wyoming, and elsewhere.

Cambrian stromatolites near Saratoga Springs, NY.
Photo by Michael C. Rygel, via Wikipedia
under Creative Commons Attribution Share Alike unported license

It’s fair to call these things stromatolites, just as we did during the Precambrian. Stromatolites have survived to the present, but they have declined from their peak in the Precambrian. Some scientists think they suffered from the Cambrian proliferation of new grazing animals like trilobites, which might have roamed the surfaces of stromatolites, scraping the living algae off as food. This seems reasonable, and there is also a well-documented example from the Ordovician of stromatolites increasing in abundance during extinction events that killed off marine animals. Conversely, stromatolites decreased as animal life recovered from the extinctions.

Should we care about ancient algae? Well, ancient algae and other plants are the biggest sources of organic matter that becomes oil and natural gas. You decide whether or not to care about them.


Two noteworthy geologists were born on this day. Nathaniel Southgate Shaler was born February 20, 1841, in Newport, Kentucky. He became a fixture in the paleontology and geology departments at Harvard University. Ray C. Moore was also born today, in 1892, in Roslyn, Washington. He worked for the US Geological Survey and the University of Kansas, and he initiated the massive Treatise on Invertebrate Paleontology, 50 volumes, still in progress, and the definitive encyclopedia on invertebrate fossils. And on this day in 1962, John Glenn orbited planet earth.

—Richard I. Gibson

Photo by Michael C. Rygel, via Wikipedia under Creative Commons Attribution Share Alike unported license.

February 19. Potsdam Sandstone

Potsdam, New York, is about 1800 miles from the Grand Canyon, but because of the extent of the Cambrian transgression by the sea, the lowest, oldest Cambrian rocks in both places are quite similar. Both the Potsdam in New York and the Tapeats in the Grand Canyon are sandstones, lithified from sediments laid down in a near-shore marine environment. The Potsdam sand was probably eroded off the high-standing Adirondacks, which may have almost been an island in the Cambrian sea. And both the Potsdam and the Tapeats lie above a profound unconformity, above Precambrian rocks that are hundreds of millions of years older.

The Potsdam in New York is mostly a clean quartz sandstone like you might expect from a beach, with some hematite (iron oxide) cement that makes it pinkish. It makes a good building stone and Canada’s House of Parliament, in Ottawa, is made from it.

One difference between the Potsdam and the Tapeats in the Grand Canyon is that the Potsdam is younger – probably late Cambrian in age, even though it’s the oldest Cambrian layer present, rather than early to middle Cambrian for the Tapeats. That reflects the millions of years that it took for the sea to encroach, to transgress, across much of North America. There’s a lot of that sand though. The Potsdam is as thick as 1,500 feet around Lake Champlain.

Potsdam near Chippewa Bay, New York, above the unconformity (Precambrian below). Photo by Michael C. Rygel via Wikimedia Commons, under Creative Commons Share Alike Unported license.

We used to give similar Cambrian sandstones in Ontario, Michigan, Indiana, Virginia, and as far west as Iowa and Wisconsin and even Wyoming the same name, Potsdam – but while the origin is practically the same, and the sandstones may be stratigraphically equivalent, it’s probably not correct to think of the sand as a continuous sheet of sand, at least not at the same time. The sea in which the sand was laid down varied in space as well as time. So these sandstones have different names today.

Today, February 19, in 1792, was the birth date of Roderick Impey Murchison, in Tarradale, Scotland. Together with Adam Sedgwick, Murchison became one of the great early British geologists who helped define many of the Paleozoic time intervals. In a few days, we’ll talk about the feud between Sedgwick and Murchison over the position of the top of the Cambrian in Britain.

Also on this day, February 19, 1600, the volcano Huaynaputina erupted in southern Peru. It was the largest volcanic eruption in South America in historic times. The years 1600-1602 were the coldest in at least 600 years in Russia, and many people starved. The wine harvest in France and Germany was negatively impacted, and climatic effects were noted in Japan and China as well. Ten villages were buried under ash in Peru, where at least 1500 died.

—Richard I. Gibson

Photo by Michael C. Rygel via Wikimedia Commons, under Creative Commons Share Alike Unported license.  

February 18. The Southern Oklahoma Rift

You remember Rodinia, the supercontinent that came together during the Proterozoic, and then rifted apart toward the end of the Proterozoic and in the early Cambrian? Today I want to talk about one chapter in that rifting apart that failed.

Oklahoma’s Wichita Mountains are all that’s left of that failed rift, called the Southern Oklahoma Rift (called an aulacogen on the map). In early Cambrian time, about 540 to 525 million years ago, this area was much like the Red Sea, or East Africa. The continent was pulling apart, extending, and the cracks allowed diverse kinds of magmas, from granitic to basaltic, to ascend from deeper in the earth. It pulled apart enough that there was even a general rise in the earth’s mantle below what is now southern Oklahoma. The break was trying to turn into a new oceanic basin, but although the mantle was involved, and lavas flowed out, it never completely rifted. It’s kinda like the Mid-Continent Rift of Kansas, Iowa, and Minnesota that we talked about a half billion years ago, although this break is almost perpendicular in orientation to that older one.

The crust subsided along two major west-northwest trending fault zones, and lavas as well as thick sediments went into the basin that formed. These rocks are exposed today in the Wichita and Arbuckle Mountains.

But wait, you say – that was half a billion years ago. I thought erosion would have leveled the mountains by now. Well, you’re right. It would, and it did. But the break in the crust created a weak zone, still weak millions of years later. Fast forward to this coming July and August, when we’ll talk about the Pennsylvanian time period when Africa was colliding with eastern North America. Those forces were great enough to affect what is now Oklahoma too, and the formerly downdropped zone became active again – and this time, because of the squeeze play driven by Africa’s collision, things popped up. We call this a rejuvenation of the old fault zone, and it was in the opposite sense to the original rift structure. So that’s 250 million years ago or so – and the Wichita and Arbuckle Mountains are the low, eroded remnants of that uplift. But the surface rocks, granites and their volcanic equivalents, rhyolite, and other igneous rocks, date back to the original rift in the Early Cambrian.

This zone probably extended west of Oklahoma, across what is now the Texas Panhandle (which has some complex geology beneath its flat surface), and into northeastern New Mexico, southwestern Colorado, and even into present-day Utah and maybe beyond. It has been rejuvenated in various ways over time. It was (and is) a big-time weak zone in the North American crust. It’s also been studied a lot, because there’s oil and gas trapped in some of these rocks and structures.

At about the same time, but perpendicular to the Southern Oklahoma Rift, and a few hundred miles to the northeast, another rift was trying to break the continent apart. This is called the Reelfoot Rift or Mississippi Embayment. It runs from northeastern Arkansas and western Tennessee up the Mississippi River to southwestern Indiana. This zone, old as it is, is still active. The famous New Madrid earthquakes of 1811 and 1812, some of the most powerful earthquakes known in North America, reflect the presence of this ancient rift. We’ll talk more about those earthquakes another day.

—Richard I. Gibson

Map from Van Schmus, W. R., Bickford, M. E., and Turek, A., 1996, Proterozoic geology of the east-central mid-continent basement; in, Basement and Basins of Eastern North America, B. A. van der Pluijm, and P. A. Catacosinos, eds.: Geological Society of America, Special Paper 308, p.
7-32.


Technical Links:
https://gsa.confex.com/gsa/2001NC/finalprogram/abstract_5657.htm
http://www.ogs.ou.edu/MEETINGS/Presentations/OilGasMar2012/Keller_Southern_OK.pdf

February 17. What's a Brachiopod?

Today, let’s talk about brachiopods. Possibly you’ve never heard about brachiopods – they are not widely distributed today, and when you do see their shells you might dismiss them as just another odd bivalve, or two-shelled mollusk like a clam. But brachiopods are not clams. They are their own phylum, not closely related to mollusks at all.

The main visible difference may seem subtle – brachiopods are symmetrical through a plane that divides the two shells vertically, while mollusks, if they have any symmetry at all, are symmetrical between the shells. There are other differences as well, especially in internal organs. Brachiopods have things called lophophores, tentacles that they extend and wave around to create a current from which they can filter food particles out of the ocean water where they live.


Brachiopods first appeared in the early Cambrian, part of the Cambrian explosion. They were incredibly prolific and successful during much of the Paleozoic Era, and in fact there are 12,000 fossil species known, in 5,000 genera, that is, groups of related species, in contrast to only 100 genera known today. Brachiopods suffered a lot in the Permian-Triassic extinction, at the end of the Paleozoic Era 250 million years ago, and their decreasing diversity after that time may also reflect the growing success and diversity of the bivalves that occupy some of the same ecological niches as brachs.

Although a few species reached nearly eight inches across, most brachiopods are an inch or two across, a perfect size for preservation and easy for collectors to find. There are two main types, articulate and inarticulate. Articulated brachs have two shells that are attached to each other along a hinge line, so the critter opens in a way similar to a clam. Inarticulate varieties have two shells that were not attached, but were held together by the animal’s muscular system. One of the most famous inarticulate brachs is one called Lingula – also called a living fossil, because the types that exist today are hardly changed from Lingulas that lived in the Cambrian period. They are also interesting because their shells are not made of calcium carbonate, like most clams, scallops, snails, and other shelly creatures, but they’re made of calcium phosphate, the mineral apatite, the same mineral that makes your bones and teeth.

I’ve never seen a modern brachiopod, alive or dead. But they were so prolific in Paleozoic seas that most any fossil collector will have some in his or her collection. We’ll talk about some of them later in the Paleozoic.

—Richard I. Gibson

Images from USGS.

February 16. Cambrian rocks of the Grand Canyon

White line marks Great Unconformity,
with Tapeats Sandstone above.

The Grand Canyon of the Colorado River is a geologist’s dream. The rocks scream out their relationships, and as you descend into the canyon, the rocks are older and older. In the inner gorge, the dark-colored rocks are Precambrian in age. They are metamorphic rocks, altered during their long lives by heat and pressure. And the top of the Precambrian rocks is a surface called an unconformity. That means a break in the rock record – a gap in time when sediments were not laid down, or they were eroded away, or sometimes a combination of both. The unconformity in the Grand Canyon is called an angular unconformity, because the layers below it are at an angle to the layers above it – a clear violation of the rule of original horizontality that we talked about a few days ago. Not only were the lower rocks cooked and changed, they were tilted – all before they were eroded off to create that unconformity surface.

The Great Unconformity in the Grand Canyon is part of a nearly continent-wide break. The amount of time it represents varies, even within the Grand Canyon area, from as little as 175 million years to possibly as much as a billion years or more, depending on the age of the rocks beneath the erosion surface.

In the Tapeats Sandstone

But it’s February, and we’re in the Cambrian now. Let’s talk about the rocks above the unconformity – the Cambrian strata. There are three distinct packages of rocks, called formations, in the Cambrian of the Grand Canyon. The lowest, the oldest, is called the Tapeats sandstone. When you look into the Canyon, if you can see the inner gorge, the Tapeats is the relatively thin, resistant lip on the rim of the gorge. It’s probably around 525 million years old, which puts it in the Middle Cambrian, and it averages something like 200 feet thick, pretty thin for the Grand Canyon.

Above, and younger than the Tapeats we find the Bright Angel Shale. Shale is a fine-grained rock that solidified from mud, and it often has really thin beds, sometimes microscopic. All of that adds up to a rock unit that may be a lot less resistant to erosion than something like sandstone, and that’s the case in the Grand Canyon. Consequently, the top of the Tapeats Sandstone is marked by a wide, flattish expanse called the Tonto Platform. It’s the place where the Bright Angel Shale would have been but it’s been eroded away – at least eroded back, pretty far from the rim of the inner gorge. When it’s still present, it tends to form slopes rather than cliffs because it’s more easily eroded. The Bright Angel is reddish and greenish in color because of variable iron content, and it contributes to the beautiful colors deep in the canyon. It’s around 500 feet thick, which gives plenty of room for lots of erosional variety and interesting landforms.

The upper, youngest part of the Cambrian in the Grand Canyon is the Muav Formation. It’s a multi-colored limestone interbedded with mudstone and some other rocks. It’s as much as 600 feet thick, and it’s a resistant cliff-former, making some of the first steep cliffs above the inner gorge and the Tapeats Sandstone.

Traditionally, geologists interpreted a change in rock type from sandstone that might have been deposited on a beach, to shale, which would be the finer sediment carried out into deeper water, to limestone, which could form in very deep water – all that would have been seen as evidence of the Cambrian Transgression that we talked about on February 5, with the seas encroaching and getting deeper and deeper across North America. That’s generally the way it worked, but it’s also possible for things like limestone to form in fairly shallow water – think of the calcareous white sand beaches on the west coast of Florida – so don’t look at it as entirely smooth and continuous. Stuff happened.

Geologists name rock formations, like they name periods of geologic time, to make it easier to refer to them, but it’s not arbitrary – there are distinct characteristics in each formation that make each one relatively easy to identify. Names come from a lot of sources, but all the Cambrian formations, Tapeats, Bright Angel, and Muav, were named for creeks and canyons in the Grand Canyon area.

—Richard I. Gibson

February 15. Plate Tectonics 1

Nerds in a Bar, volume 3. Colleen Elliott and Dick Gibson discuss the evolution of thought on continental drift. This episode isn’t specific to the Cambrian, but it will help provide general background for future presentations.

Image from USGS (public domain).

February 14. Fossils and Preservation

Nerds in a Bar, volume 2. Colleen Elliott and Dick Gibson discuss what a fossil is, and how fossils are preserved.

Cambrian trace fossils, public domain photo.

February 13. The Burgess Shale, again

Today’s podcast is a brief report on my personal favorite Burgess Shale animals: Anomalocaris and Opabinia.

Anomalocaris - up to 6 feet long

Opabinia - less than 3 inches long

And here is a link to the February 2014 news report mentioned in the podcast (thanks to Colleen Elliott for finding this). And another link, with photos.

UPDATE: from predator to filter feeder

Artist’s reconstructions both by Nobu Tamura, via Wikipedia under GNU Free Documentation License: Anomalocaris  •  Opabinia.

February 12. The Burgess Shale and Charles Walcott

So much has been written about the Burgess Shale I’m not sure there’s much I can add, given how accessible that information is today. I’ll just say that the soft-bodied fossils found in the Burgess Shale, in the Canadian Rockies near the town of Field, British Columbia, were some of the most weird and wonderful fossils ever found. They are the subject of Stephen Jay Gould’s 1989 book, Wonderful Life, which I recommend highly. It’s a wonderful book, and a great starting point for exploring the Cambrian explosion through the explosion in scientific investigation that has taken place in the past 25 years.

Walcott and his son and daughter working in the Burgess quarry, c. 1913.

But maybe I can talk about Charles Doolittle Walcott, the man who discovered the Burgess fauna. Walcott was born in New York in 1850. He never finished High School, but he became a knowledgeable expert on New York’s fossils, and when he was 29, he joined the new United States Geological Survey as a geological assistant. 15 years later, he was the director of the Geological Survey.

His middle name, Doolittle, certainly seems like a misnomer, because he was always a doer, and he did a lot.

He became the Secretary, which is to say the head, of the Smithsonian Institution in 1907 and continued in that job until he died in 1927. In those days, the head of the Smithsonian was anything but a desk bound bureaucrat, and it was in that job that he led fossil explorations to the Canadian Rockies, where he discovered the Burgess fauna in 1909. Over the next 15 years, his digs uncovered more than 65,000 specimens and brought them back to the Smithsonian.

Today, February 12, is the birthday of Charles Darwin in 1809, the same day Abraham Lincoln was born. Charles Walcott received an honorary doctorate from the University of Cambridge in 1909 as part of the centennial celebration of Darwin’s birth. February 12, 1813, was the birth date of James Dwight Dana. He devised the system of mineralogy, and wrote the textbooks still in use with revisions, today, that have educated tens of thousands of geology students over the past 150 years or more. Yet another prominent geologist was born on this day, in 1850. William Morris Davis came up with some of the earliest theories of landscape formation and erosion. Many of those ideas have been superseded, but Morris laid the groundwork for the modern science of geomorphology.    

—Richard I. Gibson

Photo (public domain) from Smithsonian via Wikipedia.

February 11. The concept of stratigraphy

Today I thought we’d talk a bit about the concept of stratigraphy, some ideas that will help with understanding of the geologic events we’re talking about.

Steno

Stratigraphy is the study of strata, or layers within the earth. Stratum, the singular of strata, comes from Latin for a bed, and ultimately from a word meaning to spread out—and that's what geological strata do: they spread out over wide areas. The science focuses mostly on sedimentary layers, beds of sandstone, shale, limestone and so on. One key aspect of stratigraphy is the law of superposition – an fancy way of saying that lower layers are older than higher layers. This may seem obvious – if it doesn’t, think about throwing some red sand into a pail on Wednesday, then on Thursday come back and throw in some lime. The sand is older than the lime. It was not obvious to early scientists, and it was Nicholas Steno, a Danish Catholic Bishop, who lived in the 1600s and pioneered and promoted this and other basic aspects of geology. He also conceived the principle of original horizontality, which says that layers of sediment – sand, silt, mud – were laid down in horizontal layers under the action of gravity. There are some obvious exceptions to this, such as deposits on mountain or undersea slopes, but it’s a general principle that matters greatly when we look at rocks that have been deformed by faulting or folding.

Smith

If you’re interested in Steno’s story, I can recommend a book by Alan Cutler called The Seashell on the Mountaintop.

While in many ways Steno was the father of stratigraphy, the one who really implemented stratigraphic ideas in a modern way was a British surveyor, William Smith. To this day I still think of him as William “Strata” Smith, as he was called when I first took physical and historical geology classes back in the 1960s. He recognized that different layers or strata of rocks had distinct fossil assemblages, and that he could recognize those characteristic fossils to help him identify the rock packages elsewhere, even if they were distant and disconnected from the original rocks. And even if the kind of rock changed. That meant that the same kinds of fossils, in a sandstone here, but in a limestone there, meant those diverse rocks were of the same age.

Smith made the first geologic map of England and Wales, published in 1815. That was The Map that Changed the World, in the title of the book by Simon Winchester that recounts Smith’s story.

—Richard I. Gibson

The "map that changed the world"

February 10. Baikalian orogenies

Parts of Asia and Europe were also growing during the early Cambrian, if not quite on the scale of Gondwana that we talked about yesterday. The Baltic Craton, also known as the Russian or East European Platform, added a triangular block called the Timan-Pechora terrane, in what is now northwestern Russia and the adjacent Arctic Ocean. It was probably added in very late Precambrian time, possibly overlapping into the early Cambrian. The map shows this new addition outlined in green, and the red line with the cross marks is the zone where the two continents came together. This would have been a mountain range during the Cambrian, and even today, thanks to some rejuvenation, it is a range of hills.

At around the same time, very late Precambrian, the Baikalian Orogeny (named for Lake Baikal, in southern Siberia) added some small continental blocks and island arc terranes to the southern margin of Siberia – which was not at the time connected to the Baltic Shield and Europe. That’s a much later assembly, marked by the Ural Mountaina.

Most of the Baikalian “events” spanned at least 150 million years, and were largely accomplished by the time the Cambrian opened. They set the scene, provide the geography, for not just the Cambrian, but for a good bit of Paleozoic time that we’ll be discussing over the coming months.

—Richard I. Gibson

February 9. Pan-African Orogeny

You remember Rodinia, the supercontinent that assembled around a billion years ago and started to split apart again around 750 million years ago? Well, it’s time to put it back together again. At least some big pieces of it.

Yellow = West Gondwana, Lilac = East Gondwana

Near the end of the Proterozoic and into early Cambrian time, most of what we know as Africa today came together, along with some other important continental blocks. It wasn’t one big collision, but several collisions, which brought what is today central Africa, the Sahara, Congo, and Cape (or Kalahari) Cratons, together with east Africa, Arabia, Madagascar, India, and eventually Antarctica. That happened along a zone called the Mozambique Belt today.

At the same general time, South America and the West African craton were added on the other side of the Sahara-Congo-Cape continent. The final amalgamation, called the all-Africa, or Pan-African Orogeny, resulted in a supercontinent – not one involving all the continents, but pretty super nonetheless – that stayed pretty much intact for the next 350 million years. It’s named Gondwana.

You might have heard this called Gondwanaland – that’s how I learned it back in college – but Gondwana means “forest land of the Gonds,” so Gondwanaland is redundant. Who were the Gonds? They were – and still are – a native people of central India. Their homeland contains rocks that helped us understand the assembly and breakup of the supercontinent that now has the name Gondwana.

Don’t forget that while various continental pieces were coming together, in other parts of the world extension and pull-apart were happening. North America, for example, was pretty much going its own way during the Cambrian.

—Richard I. Gibson

Image from Wikipedia, public domain.


Further reading:
http://www.utdallas.edu/~rjstern/pdfs/PanAfricanOrogeny.pdf
Map

February 8. The Chengjiang fauna

Chengjiang, in Yunnan, southwest China, is one of the most celebrated and important fossil localities in the world. When well-preserved soft-bodied animals were found there in the 1980s, they were immediately compared to the famous Burgess Shale fossils that we’ll talk about in a few days. And at about 525 million years old, they were found to be about 10 million years older than the Burgess fauna.

Maotianshania cylindrica, a nematode worm.

This was a big deal – because the Burgess shale animals were practically unique in the world. With the discovery of Chengjiang, the record of early Cambrian life forms expanded in time as well as space. Many of the fossils in China are the same types as those in the Burgess Shale, and these critters are critical to our understanding of the Cambrian Explosion.

The life that lived in the Cambrian of China included trilobites, as well as sponges, jellyfish, lots of kinds of worms, and importantly, the oldest probable chordates. Chordates have notochords, a linear arrangement of nerves that in vertebrates like us has evolved into our backbone and spinal chord. This means that the ancestors of modern birds, reptiles, fish, amphibians, and mammals – and us – are pretty ancient, at least 525 million years old.

I would encourage listeners to check the links below to sources for more information and photos of the Chengjiang fauna.

Photo by SNP under GFDL. 

Update (April 2014): New arthropod with cardiovascular system reported
http://en.wikipedia.org/wiki/Maotianshan_Shales
http://www.fossilmuseum.net/Fossil_Sites/Chengjiang.htm

February 7. Trilobites

Let’s talk today about some of the most common life forms that lived in Cambrian seas. Trilobites were arthropods, invertebrates with segmented bodies, jointed legs, and relatively hard exoskeletons on the outside of their bodies. The group includes insects, spiders, centipedes, and crustaceans like shrimp, lobsters, and crabs. And trilobites.

Trilobite is a simple name meaning three lobes, which reflects their basic body plan – a central lobe flanked by two more that cover the legs. They appeared during the Cambrian explosion, around 540 to 521 million years ago, and were incredibly diverse and successful – all told, there are more than 17,000 species of trilobites, and they survived for about 270 million years.

Trilobites are a kind of holy grail for geology students, at least those of us who lived in places where they were not common. Because of their many segments and legs and other parts, they tended to break apart when they died, so they are fragile and can present a challenge to collectors to find and retrieve, but when you do, it’s a treasure. Finding your first trilobite was a big deal – at least for me. I even have a trilobite etched on my mug down at Quarry Brewing.

They range in size from tiny – around a millimeter – to huge, at 70 centimeters or 28 inches long. Their exoskeletons were not hard calcium carbonate, like so many shells are, but were a material called chitin, more like humans’ fingernails, and containing calcium, phosphorous, and organic material. This adaptation certainly offered some protection compared to soft-bodied animals like jellyfish, and allowed them to be mobile as well.

Specific subdivisions of Cambrian time around the world are usually related to the kinds of trilobites found in a particular time interval. So you have the Olenellus Zone, or the Bathyuriscus Zone, or whatever. Trilobites are the yardsticks of Cambrian time.

Trilobites had compound eyes, much like many insects, with lenses made of calcite, calcium carbonate.   As we discussed yesterday, the development of eyes and light sensitivity may have created evolutionary pressure that drove, or even initiated, the Cambrian explosion. Trilobites seem to have hung out in mud a lot, and that’s good news, because the really fine grained sediment in mud lends itself to preserving details in fossils, even the eyes. And it might help that muddy sea floors might tend to be anoxic, stagnant areas, with less oxygen to attack and decompose the animals when they died.

We’ll talk about trilobites from time to time again over the next few months, to point out a particularly weird or interesting variety.

The Wikipedia page provides a reasonable overview about trilobites. If you’re geeky enough to like cool names for trilobite parts like the cephalon and pygidium, start there and move on to the Trilobites.info site. If not, just go with head and tail.

When Trilobites Ruled the World - NY Times, March 2014

Blog extra: Feb. 7, 1812, was the date of the fourth and strongest in the series of earthquakes that shook southern Missouri around the town of New Madrid. The Feb. 7 quake damaged buildings in St. Louis and rerouted the Mississippi River, creating Reelfoot Lake in what is now northwest Tennessee.

—Richard I. Gibson

February 6. In the blink of an eye

Today is a follow up to the discussion Colleen Elliot and I had about the Cambrian Explosion a few days ago. In 2004, A book by Oxford zoologist Andrew Parker was published entitled In the Blink of an Eye – how vision sparked the big bang of evolution. It offers arguments for the idea that the development of eyes led to increases in predatory behavior, and was the ultimate cause for the Cambrian explosion.

Trilobite eye (NASA photo)

First, Parker defines the explosion not as the proliferation of animals, but as the proliferation of calcareous and phosphatic shells and exoskeletons in animals. He makes the argument that with new discoveries, we see that most or all of the existing phyla of animals had already been present for some millions of years when the explosion happened. So what we see as an explosion is really just those organisms figuring out a new way of living, inside various types of armor.

The focus of In the Blink of an Eye is why – why did this change happen? Parker objects to most of the possible causes that Colleen and I discussed the other day because they would probably have generated a gradual change, not an abrupt one as is observed. Or they were too far removed in time, as we mentioned regarding the end of Snowball Earth. So he makes the case for the development of eyes as the diving force behind the change.

It’s not hard to imagine that if, suddenly, some animals became able to sense movement, or to recognize patterns that indicated other animals – FOOD! – that the “arms race” Colleen and I mentioned in passing could have developed. There is pretty good evidence that animals DID develop eyes at the time of the Cambrian diversification, about 542 to 530 million years ago. It’s reasonable, but in my opinion not absolutely conclusive that it might have driven the increase in diversity that we see happening then.

For this to happen across species, in fact across phyla, at about the same time, there must have been some common reason. Maybe the development of eyes drove the Cambrian explosion, but why did eyes develop in so many kinds of animals at the same time?

Parker analyzes this question carefully, and ultimately suggests that there could have been a dramatic, and sudden increase in the amount of light reaching those animals, so that sensitivity to light became a useful survival mechanism. What could cause a sudden increase in light on earth?

Well, the sun could have suddenly become more luminous, or the atmosphere and oceans could have become more transparent. While those might seem far-fetched, there’s actually good reason to think they might have been possible, especially the changes on earth. There’s some evidence that the atmosphere, even while it was becoming increasingly oxygen-rich, might have been fog-like during the Precambrian. If something happened – a threshold was reached, or conceivably, a nearby supernova event drenched the earth in enough radiation to change it – it might have become more transparent. Or the water could have become more transparent because of changes in mineral content. Such changes certainly DID occur – but were they enough to suddenly change the amount of light reaching primitive animals, enough to make them all, suddenly, develop sensitivity that led to arms and armor as the world became an eat or be eaten kind of place?

I don’t know. The Blink of an Eye is a fascinating idea, very much worth considering, and the book is a good read filled with lots of earth history. Personally I think the jury is still out on why the Cambrian Explosion happened, and it remains perhaps the most intriguing and poorly understood event in the history of life since it originated on earth.

—Richard I. Gibson

 Further reading:

In the Blink of an Eye (Amazon)
In the Blink of an Eye (B&N) 
Calcite in the oceans (NASA - source of trilobite eye photo)
Trilobite eyes

February 5. The Cambrian transgression

During the early Cambrian, sea levels were rising. This produced a near-global transgression – which is not a sin, but rather it just means that the seas were advancing, covering more land area than they had previously. The opposite is a regression, when the seas become relatively smaller.

So how does that happen? With minor exceptions, the volume of water on earth has been more or less constant, at least since pretty early in the earth’s history, back in early January sometime. But the volume of the ocean basins that hold the water can change and does.  One way that can happen is by rifting apart continents, as Rodinia was splitting into several smaller continents in late Proterozoic and early Cambrian time. The volume change comes about because of the mid-ocean ridges, the point where oceanic crust is pulling apart. Today, the oceanic ridge system is the longest mountain range on earth, and added together, it takes up a pretty notable volume of oceanic water. Enough that if there are a lot more oceanic ridges, it can result in sea level rise. Likewise, if there were a lot more oceanic trenches, very deep water, that could also accommodate at least a little more water than a flat ocean floor.

We know from concerns today about sea-level rise that melting and freezing ice caps can contribute to sea level changes. And a small effect might even come about because of water temperature. Warmer water expands, if only a bit, but when the entire ocean expands, it can make a difference in sea level, especially on very low, flat shores.

Paleogeographic map by Ron Blakey via Wikipedia under CC-BY-SA & GFDL

In North America during the Cambrian, shallow seas covered a vast amount of the continent, all except high area on the Superior Craton and a long narrow peninsula called the Transcontinental Arch, extending from Minnesota southwest to what is now Colorado and New Mexico. Abbreviations in the middle Cambrian (about 500 million years ago) paleogeographic map above: NA-North America. B-Baltica (Europe). S-Siberia. SA-South America. AF-Africa. AUS-Australia. CH-China. ANT-Antarctica. Here is another version of a map showing the arch, and another one.  

The sea took millions of years to transgress across North America during the Cambrian. As it progressed further and further, the shoreline beach also changed position. Consequently there was a lot of sand – sandstone today – that marks the base of the Cambrian across much of North America, but don’t think of it as one big beach – think of it more like a continuously migrating beach that, over millions of years, ended up depositing sand across many hundreds or even thousands of square miles of the continent. We’ll talk about some of those sandstones as we work our way through February.

—Richard I. Gibson

Paleogeographic map by Ron Blakey via Wikipedia under CC-BY-SA & GFDL.