March 21. Queenston delta

Red rocks of the Queenston Formation
in Ontario

You recall from our discussion of the Bighorn Dolomite a few days ago that much of western North America was a tropical shallow sea into the Late Ordovician. To the east, in Illinois, Michigan, Indiana, Ohio, and Kentucky, things were changing because of the onset of a mountain-building event, the Taconic Orogeny, even further east. A string of volcanic islands – an island arc, probably with other sorts of things in it, something like Indonesia today – began to collide with the ancestral core of North America, and a mountain range formed in what is now central New England and points to the north and south.

What happens as soon as you lift up a mountain range? It starts to erode. The stuff eroded off this mountain range was carried by large river systems to the west, especially into what is now eastern Ontario and western New York and Pennsylvania. The sediments were dumped into the shallow sea that was there, forming a huge delta, probably quite a bit larger in area than today’s Mississippi Delta. The pile of sediment is called a clastic wedge, because it contains clastics – broken pieces of rock, worn down to gravel, sand, silt and mud, and it’s a wedge because it’s shaped like a doorstop – thickest toward the mountains, and thinning off to the far west. It’s called the Queenston Delta and the rocks in it are called the Queenston Formation.

Yellow=sandy sediments of Queenston Delta.
Mud across Ohio is part of the system, too.,

The Queenston Formation – it’s called the Juniata Formation and other names in some places –  was laid down over a period of several million years, about 451 to 446 million years ago. In places it’s close to 1,000 feet, 300 meters, thick. A lot of it contains iron oxide cement, making the rock reddish in places, and suggesting the deltaic nature of the sediments – sometimes underwater, sometimes exposed to air.

The Queenston Delta system was so vast, and contained so much sediment, that it may have contributed to a reduction in atmospheric CO2 which could have reacted with all that exposed sediment. Maybe even enough to reduce the ongoing Ordovician greenhouse situation and to contribute to the glaciation that’s coming at the end of the period.

—Richard I. Gibson

Photo by Ian Muttoo licensed under the terms of the cc-by-sa-2.0   

Map from Ohio Geological Survey, Ohio Geology, Fall 1997 (PDF)