August 20. Appalachian folding

Map of fold belt in central Pennsylvania from Pennsylvania Geological Survey

We talked about the Appalachian-Ouachita orogeny a lot last month, as Gondwana collided with what is now eastern North America in one of the main acts of the assembly of Pangaea. Collision was certainly underway during the Pennsylvanian Period, last month, but it also certainly continued into the Permian. Even after the land masses were attached, compression continued. Consider India and Eurasia – they’ve been colliding for at least the past 30 million years, and the consequences of the ongoing collision are seen in earthquakes throughout the Himalayan region today, and well beyond. The Appalachian orogeny was a similar event. 

It seems that especially the deformation, folding and faulting, that extended well into the continent and beyond the zone of active collision took place in the early to middle Permian Period. Some of the broad folds that dominate central and western Pennsylvania today were probably formed during the Permian. They still control the topography, 275 million years later, as erosion preferentially focuses on less resistant rock layers, leaving the more resistant beds as long high-standing ridges. You can get such mountain ridges even in a syncline, a down-folded zone in the rock, because of alternating high- and low-resistant beds.

The geologic map of Pennsylvania shows a beautiful zig-sag pattern in the rock units, because the folds, anticlines and synclines, have been tilted. Visualize a sheet of paper, squeezed so that it forms a scoop-like bend. Then point the end away from you down. Then, cut the paper off along a line parallel to the floor. The edge of the paper will make a broad U or V shape, depending on how tightly you bent the paper. That’s the zig-sag effect of folded rocks that have been tilted so that they plunge down into the earth.

I think the consensus is that most of the deformation in the Appalachian-Ouachita belt of North America was pretty much over by late Permian time. At that point, Pangaea with a major mountain range running through part of it was pretty much just sitting there. I’m not saying there were no earthquakes – I bet there were plenty. But the intense folding and faulting that resulted from two continents impinging on each other was pretty much done. And actually, as we’ll hear in a few days, it’s possible that the supercontinent was already beginning to break apart as early as the late Permian. Nothing lasts forever!

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Today’s birthday is Eduard Suess. His name came up a couple weeks ago as the Austrian geologist who used the widespread nature of glossopteris fossils to suggest the existence of the supercontinent of Gondwana. He was born August 20, 1831.

—Richard I. Gibson

Appalachian folding 

Map of fold belt in central Pennsylvania from Pennsylvania Geological Survey

July 5. The Alleghenian Orogeny

I’ve been saying that Gondwana was coming for months, for millions of years. It’s finally arriving. The earlier tectonic collisions in southeastern and east-central North America were between the North American continent and various stringers – island arcs, microcontinents, branches of Baltica, and further north, in Newfoundland and Labrador and Greenland, there was a true continent-continent collision between North America and Baltica.

But the really big crunch was between what was now a combined supercontinent – Laurasia or Laurussia, the assembled North America and Europe or Baltica – and the really big supercontinent of Gondwana, which comprised most of the rest of the world’s large continental blocks.

What is now northwest Africa and northern South America, plus a triangular zone between them, was approaching the southeastern margin of North America, what is now more or less New Jersey to the Carolinas and west to Arkansas and Oklahoma. Gondwana began to arrive about 325 million years ago, six or seven million years before the end of the Mississippian, and the crunching really got underway during the Pennsylvanian.

This mountain-building event is often called the Alleghenian Orogeny, named for the Allegheny Mountains in West Virginia and Pennsylvania where the effects of the orogeny are preserved. And the rocks there were more specifically deformed at the time we’re talking about now, the very late Mississippian and the Pennsylvanian. The term Appalachian Orogeny can be used to refer to the entire spectrum and time span of the events that created the diverse ranges of the Appalachian Mountains, and that would reach from late Ordovician at least into the Permian, 200 million years or more. So Alleghenian Orogeny is a better, more specific term.

How can we distinguish the related, episodic events from each other? The simple way is to look at which rocks are deformed. If Ordovician rocks are folded and faulted, but Devonian rocks are not, then the deformation must have occurred after the Ordovician rocks were formed but before the Devonian layers were laid down. It’s the old law of superposition – deposits or events that come earlier cannot lie over, or affect, rocks that come later. It can get pretty complicated of course, and unraveling the sequences of events is sometimes pretty challenging. It keeps geologists busy!

Because this collision really was the big one, it caused considerably greater deformation of the rocks than the earlier events had. The layers of rock in the Appalachian Basin, in western Pennsylvania and West Virginia, were squeezed into a series of elongate folds. Further south, in Virginia, the Carolinas, and Tennessee, the push was strong enough to break the rocks, and there are some really big faults there that were formed at this time.

There are lots of consequences to the collision between Gondwana and Laurasia, which we’ll touch on as we get further into the Pennsylvanian Period this month. The whole process will continue well into the Permian, a total of at least 60 million years just for this culminating collision. For comparison, India began to collide with Asia about 40 million years ago – and it’s definitely not done causing deformation, which we see today as earthquakes scattered through the region.

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John Ewing was born July 5, 1924, in Lockney, Texas. He had a long career with the Woods Hole Oceanographic Institution, Lamont-Doherty Geological Observatory, and Columbia University. His work focused on marine geophysics, and he’s considered to be the inventor of the air gun for marine seismology. He used geophysical data to unravel the structure of oceanic crust and the transitions between oceanic and continental crust.

—Richard I. Gibson

Illustration from USGS

June 23. Appalachian in Iowa

The effects of continent-continent collisions can be felt far into the continents that are involved. Today’s collision between India and Eurasia, which has been going on for 30 or 40 million years, has helped to produce extension 2500 miles, 3700 kilometers away from the collision, at Lake Baikal in Siberia. 

The Appalachian-Ouachita Orogeny had impacts as far away as Iowa quite early in the collision. Early Mississippian marine carbonate rocks were tilted and folded and uplifted and eroded before Pennsylvanian coal-bearing strata were deposited. There’s a pretty good unconformity between those sets of rocks.

—Richard I. Gibson

Reference: Coal Deposits of Iowa, by C.R. Keyes, Iowa Geological Survey, 1894.

March 23. Appalachian basin

When blocks of the earth’s crust collide, several different things can happen. When good dense oceanic crust impinges on relatively light continental crust, the denser one, oceanic crust, usually goes down under the lighter one. This process is called subduction, and it’s going on all over the world today. The continental crust above isn’t immune to effects – it can get uplifted, depressed, scrunched and broken, and volcanoes can and do pop up through the continental crust. Probably the best example of this today is the Andes Mountains along the west coast of South America, where the oceanic plate underlying the Pacific Ocean is diving down beneath the South American continental plate.


If two relatively low density blocks – two continents, or a continent and something like an island arc – collide, then neither is likely to really descend beneath the other. This is a true head-on collision, and it can make some of the highest mountain ranges. This is what’s happening to day where the Indian continental plate collides with the Eurasian Plate. The Himalayas form.

An obvious consequence of uplifted mountains is erosion – it starts as soon as rocks are above sea level. The Queenston Delta that we talked about the other day is the evidence of that kind of erosion from an uplifting mountain range. Sometimes there is so much erosion that the weight of the sediment is enough to bow down the crust itself, starting a trough-like depression along the mountain front. It can become a self-perpetuating thing, a depression, a basin, into which more and more sediment pours, and all that sediment keeps pushing the crust further and further down…. And so on. The weight of the stuff that’s colliding and being pushed up over the edge of the continent helps, too – all adding up to a physically low area to receive sediments. It’s called a foreland basin, because it’s in the foreland, adjacent to a rising mountain uplift.

That’s what happened in eastern North America, where the Appalachian Mountains are today. We’ll be talking about the Appalachian Mountains for months – millions of years – as various things happen over time to contribute to their formation. But that’s getting started now, toward the end of the Ordovician, as the Taconic Orogeny gets started.

During the Cambrian and early Ordovician, most of eastern North America was pretty stable, accumulating relatively thin uniform packages of sediment over pretty large areas. In the late Ordovician, as collisions began to bow down the crust, and lift up mountains to be eroded, changes began. Packages of sediment thicken noticeably to the east or southeast (see cross section above - the colored part is the Upper Ordovician), closer to the source area. More sediment is dumped close to the mountains than is carried hundreds of miles away from the mountain front, even though those far-flung sediments are part of the process as well. The mountains may have been in New England, extending down to Tennessee and Georgia, but the eroded mud made its way as far northwest as Ohio and beyond.

The foreland basin that began during late Ordovician time is called the Appalachian Basin, and like I said, it will be months before we’ve heard the end of it.

—Richard I. Gibson

See also this excellent SmartFigure by Callan Bentley. One of the best visualizations of the tectonic development of the Appalachians that I've seen.

Cross section from Harris & Milici, 1977, USGS Prof. Paper 1018.
Map from USGS