A Devonian reef

When I visited Delaware Water Gap National Recreation Area and the surrounding area back in 2017, one of the places I stopped was at a Lower Devonian sponge-coral bioherm. A bioherm is a geological term for a mound-like feature made up of fossils. The useful thing about "bioherm" is that it just refers to the shape, rather than making any conclusions about how that structure came about. (You see, there are reefs, and then there are things that are like reefs but aren't reefs, and there are also just plain old accumulations of skeletal fragments, and there's actually a lot of nuance involved that can tell you things about depositional environment and so forth.)

Looks like circular stromatoporoid colonies with tabulates between them—or are there also some corallites in the circles? Things get complicated in a reef, especially 400 million years and some weathering later.

Anyway, this particular bioherm is one of a group identified as "patch reefs", which is more or less what it sounds like: a relatively small discrete reef. There's a handful of these scattered throughout the area, found in the Shawnee Island Member of the Coeymans Formation. They are described as including a central core up to 160 x 70 m (525 x 230 ft) in area and 15 m (15 ft) thick, surrounded by flank beds of skeletal debris. They are primarily formed of stromatoporoid sponges and favositid tabulate corals, and grew on a marine carbonate shelf (Monteverde 2001; Precht 1988). The rugged modern topography was still in the future; the Acadian Orogeny hadn't even kicked in yet and the previous topography generated by the Taconic Orogeny had been well eroded by the early Devonian.

This one's a bit easier to tell as a Favosites chunk (even if the photo isn't that sharp).

There hasn't been much occasion to talk about stromatoporoids previously. The first thing to know about stromatoporoids is that despite the confusingly similar name, they have nothing to do with stromatolites. Stromatolites, like Cryptozoon rosemontensis from back in 2014, are layered sedimentary structures left by microbial colonies. Stromatoporoids also show a layered appearance in cross-section, but that's because of layered body tissues: they were sponges (even if this is not immediately apparent), and produced body fossils, not trace fossils. The body, mineralized as calcium carbonate, consists of horizontal laminae supported by vertical pillars. A basic stromatoporoid plan has been adopted several times, including by some modern sponges, but classic Stromatoporoidea had its heyday in the middle Paleozoic as a reef-forming group. I don't think anyone has attempted to identify the Coeymans Formation stromatoporoids to genus or species since White (1882) invoked Stromatopora, but I might have missed that reference.

Not sure what's going on here; maybe a branching-type stromatoporoid (e.g., Amphipora) has gotten into the act as well.

On the other hand, the common Coeymans patch reef tabulate has been identified to the genus level as Favosites (Weller 1903; Willard et al. 1939; Epstein et al. 1967). Swartz and Swartz (1941) put it in F. helderbergiae, which is appropriate for the time and place, but you wouldn't be able to tell just walking by (unless you carry your own thin-sectioning equipment and microscope with you when you're out for a walk). Favosites, the "honeycomb coral", is rather more impressive than most of the tabulates in the Ordovician of Minnesota, rare Foerstephyllum colonies excepted. The reason for "honeycomb coral" isn't obvious from these photos, but if you did have a loose colony sitting in front of you, you'd be able to see the logic: it's a coral colony that resembles a chunk of honeycomb. Each "cell" of the "honeycomb" is a corallite that held a polyp, much smaller than the polyps of horn corals and other rugose corals but still much larger than the tiny animals of a bryozoan colony.

Not dinosaur skin, just weathering across a favositid that's been more effective on the corallite walls than the fill, producing a pebbly-looking "negative".

This photo shows a clear demarcation between favositid on the left and not-favositid on the right.

References

Epstein, A. G., J. B. Epstein, W. J. Spink, and D. S. Jennings. 1967. Upper Silurian and Lower Devonian stratigraphy of northeastern Pennsylvania, New Jersey, and southeasternmost New York. U.S. Geological Survey, Washington, D.C. Bulletin 1243.

Monteverde, D. H., leader. 2001. Road log and stop descriptions; Day 1, Stop 5: Montague mini-mall fossil site; flank of a coralline bioherm in the Coeymans Formation. Pages 191–198 in J. D. Inners and G. M. Fleeger, editors. 2001: a Delaware River odyssey. Field Conference of Pennsylvania Geologists, Harrisburg, Pennsylvania. Guidebook for the Annual Field Conference of Pennsylvania Geologists 66.

Precht, W. F. 1988. Lower Devonian reefs of the Coeymans Formation in the northern Appalachian Basin. Pages 514–519 in H. H. J. Geldsetzer, N. P. James, and G. E. Tebbutt, editors. Reefs; Canada and adjacent areas. Canadian Society of Petroleum Geologists, Calgary, Alberta. Memoir 13.

Swartz, C. K., and F. M. Swartz. 1941. Early Devonian and Late Silurian formations of southeastern Pennsylvania. Bulletin of the Geological Society of America 52:1129–1192.

Weller, S. 1903. The Paleozoic faunas. New Jersey Geological Survey, Trenton, New Jersey. Report on Paleontology 3.

White, I. C. 1882. The geology of Pike and Monroe Counties. Geological Survey of Pennsylvania, Harrisburg, Pennsylvania. Report of Progress 9(6).

Willard, B., F. M. Swartz, and A. B. Cleaves. 1939. The Devonian of Pennsylvania. Pennsylvania Geological Survey, Harrisburg, Pennsylvania, 4th series. General Geology Report 19.