The Matagamon Sandstone

I recently had occasion to go to north-central Maine for work. If you've never been there, the geology is the tectonic equivalent of taking a bunch of little leftover bits of colorful modeling clay and smooshing them together: the area was on the margin of the North American craton during the Paleozoic and thus was the recipient of a conveyor belt of crustal fragments. Of course, when this happens, you get all kinds of interesting structural features and metamorphism, which does unfortunately tend to obscure the original geology. Tack on the Pleistocene glaciations and subsequent dumping of drift, followed by the growth of forests, and you can see how things can get complicated and confusing to follow.

One of the geologic units I observed in this region is the Lower Devonian Matagamon Sandstone. The Matagamon has been interpreted as part of a deltaic system that advanced to the northwest during the Acadian Orogeny (Hall et al. 1976; Pollock et al. 1988). We've got a pretty good idea of when its deposition ended because it transitions upward into the Traveler Rhyolite (Rankin 1965), the explosive component of a supervolcano that erupted approximately 407 million years ago (Seaman et al. 2019). Curious about the guts of that volcano? Look no further than Katahdin.

Maine's tallest mountain? The crystallized heart of one of North America's largest volcanoes? Why not both?
(Incidentally, they told us this was the best time of year to do geologic work in north-central Maine. They were right!)

Anyway, the Matagamon is a fossiliferous unit, with an assemblage dominated by brachiopods. Clarke (1909) described a few assemblages from this formation, which was then identified as the Moose River Sandstone (it did not receive its present name until Rankin 1965). The fauna includes plant fragments, corals, brachiopods, monoplacophorans, bivalves, nautiloids, gastropods, tentaculitids, trilobites, crinoids, and invertebrate trace fossils. Brachiopods certainly seemed to be the most abundant fossils in the outcrops I saw.

A fairly large brachiopod with Leptaena-type ridges.

A bulbous shell on the left and a cylindrical object of unknown origin on the right.

A shell bed.

Fossils tended to be abundant in localized areas, mostly preserved as molds, external casts, and steinkerns, with occasional shell material in the brachiopods. (Overall, the rocks, the fossils, and their preservation rather reminded me of the somewhat younger Mahantango Formation from the Delaware River valley.) In some cases, the fossils had been stained bright orange, very appropriate for autumn and Halloween.

Slightly orange small flat ribbed brachiopods, resembling potato chips.

Two strongly orange brachiopods: a small shell with few but heavy ribs on the left, and a much larger brach with many fine ribs in the center.

Some calcitic material remains with these shells.

References

Hall, B. A., S. G. Pollock, and K. M. Dolan. 1976. Lower Devonian Seboomook Formation and Matagamon Sandstone, northern Maine: a flysch basin-margin delta complex. Pages 57–63 in L. R. Page, editor. Contributions to the stratigraphy of New England. Geological Society of America, Boulder, Colorado. Memoir 148.

Pollock, S. G., A. J. Boucot, and B. A. Hall. 1988. Lower Devonian deltaic sedimentary environments and ecology: examples from the Matagamon Sandstone, northern Maine. Pages 81–99 in R. D. Tucker and R. G. Marvinney, editors. Structure and stratigraphy. Maine Geological Survey, Augusta, Maine. Studies in Maine geology: papers to commemorate the 150th anniversary of C. T. Jackson’s reports on the geology of Maine. Volume 1.

Rankin, D. W. 1965. The Matagamon Sandstone–a new Devonian formation in north-central Maine. U.S. Geological Survey, Washington, D.C. Bulletin 1194-F.

Seaman, S. J., R. Hon, M. Whitman, R. A. Wobus, J. P. Hogan, M. Chapman, G. C. Koteas, D. Rankin, A. Piñán-Llamas, and J. C. Hepburn. 2019. Late Paleozoic supervolcano-scale eruptions in Maine, USA. GSA Bulletin 131(11–12):1995–2010.