21 March 2011
The Shenandoah Valley of western Virginia records the switch in late Ordovician time from passive margin sedimentation associated with the Sauk and Tippecanoe epeiric seas, to active margin sedimentation associated with the onset of the Taconian Orogeny to the east. Higher up in the stack, a similar pattern is seen: a return to passive margin sedimentation with the deposition of the Helderberg Group of limestones, and then more active margin sedimentation as the Acadian Orogeny sloughs off huge quantities of siliciclastic sediment.
Here’s a cartoon version of the stratigraphic column for the Shenandoah Valley and the strata exposed in Massanutten Mountain in its middle (the whole package is folded into the Massanutten Synclinorium):
Notice how there are two yellow layers in there. These are distinctive packages of bentonite, weathered (devitrified) volcanic ash. Some of these bentonites make for superb regional “time-stamps” that aid in the correlation of strata at widely-dispersed locations. The Tioga Bentonite is an example of this — it’s found throughout the Appalachian Basin and was deposited when an Acadian volcano erupted in massive style. Deeper in the stack is the bentonite I would like to focus on today — the one found in the Edinburg Formation. Known as the Millbrig Bed in North America, this thick layer of (formerly) volcanic ash outcrops as a distinctive yellowish-brown layer that weathers away much faster than the silty limestone above and below it. Here’s a shot of the thickest bentonite (what I guess to be the Millbrig Bed) on the classic “Tumbling Run” outcrop south of Strasburg, Virginia:
The interesting thing about the Millbrig Bed is that it has been correlated with another bentonite layer, on another continent. The Kinnekulle bed, also known as the “Big Bentonite,” is a distinctive stratigraphic marker in Scandinavia. Because they were erupted at about the same time (~454 Ma), stratigraphers have correlated the two, saying they are the same ash bed, deposited from a truly enormous volcanic eruption – presumably from a volcano somewhere out in the Iapetus Ocean.
(source: Ron Blakey, Northern Arizona University, then modified by me)
A new paper in Geology by Bryan Sell and Scott Samson of Syracuse University examines whether the European and North American bentonites really match up, however. They find that, on a geochemical basis, they are unlikely to be from the same eruption.
Sell and Samson examined trace elements in crystals of apatite which grew as phenocrysts in the source magma, and were entrained in the ash cloud as it erupted. The trace elements in these crystals can vary widely from eruption to eruption, and so they serve as a good geochemical “fingerprint” that allows us to source the eruption to a specific volcano. They sampled the Millbrig bed in southern Virginia (near the town of Hagan), and used “Big Bentonite” samples from the island of Bornholm, Denmark.
Here’s the key figure in their new paper, (modified and colored by me):
“MK-b” is the North American bentonite; KK-b is the Kinnekulle bentonite of Scandinavia. The key thing to note is the two distinct clusters of data: One in red shows analyses from the Millbrig bed; the yellow, blue, and green show analyses from the “Big Bentonite” in Scandinavia. Based on iron, magnesium, and manganese concentrations, these are two different geochemical fingerprints. Though Scandinavia and Virginia both swallowed a lot of volcanic ash in the late Ordovician, the authors conclude that:
The differences between the MK-b and KK-b in apatite trace element concentrations are so great that they cannot be considered the same bed. This is also true of the intrabed samples; there are no subsections in the MK-b that are similar to any of the vertical subsections in the KK-b. Thus we conclude that no portions of the two beds correlate.
And there you have it. The Big Bentonite isn’t so big after all. It’s two bentonites from two eruptions.
It will be interesting to see how this affects tectonic reconstructions of the position of North America and Baltica during the late Ordovician.
Sell, B., & Samson, S. (2011). Apatite phenocryst compositions demonstrate a miscorrelation between the Millbrig and Kinnekulle K-bentonites of North America and Scandinavia Geology, 39 (4), 303-306 DOI: 10.1130/G31425.1