If youre ever in the vicinity of Albany, New York, be sure to visit John Boyd Thacher State Park. The park is situated atop the cliffs of the Helderberg Escarpment, with a commanding view of the Mohawk Valley and mountains beyond. Even better, the limestones and sandstones are quite fossiliferous, making Thacher State Park is one of the best-known Devonian paleontological localities in New York. (But be aware: fossil collecting is not allowed in New York state parks!) New York State Museum geologist Chuck Ver Straeten led a guided field trip of several localities in the park today, including the Carrick Road Quarry, Minelot Creek, and the Indian Ladder Trail. Your roaming correspondent was on hand to capture the geologizing for posterity. After an introductory lecture at the Emma Treadwell Thacher Nature Center, the group set out for Carrick Road Quarry. The quarry region is a somewhat newer addition to the park, and is not located within the main park area. Map: https://google/maps/@42.6772185,-74.0470529,255m/data=!3m1!1e3 See those green lines in the satellite image? These are joints in the rock which have become vegetated. The bedding plane also shows evidence of Quaternary glacial striations, though they are faint and thus cannot be seen in the Google Maps. While the joints trend in a northeast-southwest direction, the glacial groves are more north-south. The rock itself is brown Lower Devonian Oriskany Sandstone, a quartz sandstone linked to the Acadian Orogeny. This topmost bedding surface is thoroughly bioturbated with Zoophycos burrows---you could not walk without stepping on Zoophycos. No other macrofossils were visible in this layer. However, other layers were exposed in blocks and the edges of the quarry. The bulk of the Oriskany Sandstone and the underlying Becraft Limestone were more fossiliferous, with large (~2-3cm) brachiopods, bivalves, gastropods, and crinoid fragments. The group then returned to the main part of the park, stopping at upper Minelot Creek near the Paint Mine picnic area. There we saw pieces of Esopus Shale and New Scotland Limestone, with more brachiopods and bryozoans (both fenestrate and ramose). The brachiopod Leptaena was quite common here---they sure get around, dont they? Both sedimentary and biomorphological evidence pointed toward a deeper water environment than at the first stop, probably deep subtidal or offshore. Finally, the field trip set off down Indian Ladder Trail, descending through the Lower Devonian Coeymans and Manlius Formations before reaching the Upper Silurian Rondout Formation below Minelot Falls. As we went down, we saw stromatoporoid sponge reefs, small tidal channels, and likely desiccation crack structures. We saw tentaculitids and ostracods near the bottom of the Manlius, perhaps indicative of hypersaline, restricted conditions. In general, the section deepens upward. (I have to wonder if these stromatoporoid reefs are analogous to the transgressive stromatoporoid reefs in the Lexington Limestone and Cincinnatian of Kentucky...) And though we didnt get close to it on the field trip, below the Rondout is a thick exposure of Upper Ordovician grey flysch from the Taconic Orogeny. The literature is a bit conflicted on the affinity of these layers: Ive seen this exposure referred to as either Schenectady Formation or the Indian Ladder beds of the Franklin Formation. Revision is likely needed, but these fine siliciclastics are probably Cincinnatian in age (Edenian?)
Posted on: Mon, 30 Jun 2014 04:55:37 +0000
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