Administrative duties and exhibits have kept me away from the blog for the last few weeks. I somehow found the time to take an actual non-working (mostly) vacation this week (my first one in years), and that’s given me a chance to catch my breath and focus on the blog a bit.
Just before Christmas I made a trip to Indiana to pick up a specimen (more on that in a future post), and while I was there Andy Moore took us on a visit to Oakes Quarry Park near Dayton, Ohio. Ohio has several fossiliferous quarries that have been turned into educational parks, which I think is an excellent practice that other states should consider emulating.
The Oakes Quarry Park exposes about 8 meters of the the Silurian Brassfield Formation, a fossil-rich limestone and dolostone. The quarry floor is a dolostone that is absolutely riddled with burrows:
The wall of the quarry exposes several other units, including a crinoidal limestone made up mostly of broken crinoid fragments:
There are finer-grained units that include a variety of fossils, including gastropods and nautiloid cephalopods:
There are also limestone beds that are filled with rugose and tabulate corals:
As you may have expected, the different units are not distributed at random through the quarry, but occur in a particular sequence. In the marked up image below, the blue represents the burrowed dolostone (not visible in this image), the red is the crinoidal limestone, the green are the fine-grained beds that include nautiloids, and the yellow are the coral-rich limestones:
This seems to represent a transgressive sequence, in which the water was getting deeper over time. The burrowed dolostone is probably an intertidal deposit which was actually exposed (or nearly so) at low tide. As the water got somewhat deeper, this spot became the surf zone right along the beach. The high wave energy carried broken fragments of various organisms, especially crinoids. Crinoids are delicate animals that have skeletons made up of hundreds of individual pieces that will quickly disassociate under high-energy conditions.
Above the crinoidal limestones are slightly deeper-water conditions. There is some variation here; the finer-grained beds are probably the deepest water, and may have been below normal wave base, while the coral-rich units were almost certainly above wave base (but still subtidal). The alternating occurrence of these units probably represents slight changes in local sea level. This didn’t have to be due to actual regional changes in sea level; it could easily be the result of the migration of coral reefs as they grew across the sea floor, with the water getting shallower when the reef is present because of the thickness of the reef.
There’s one more interesting piece of this story. At the top of the quarry the corals are all beveled off:
During the Pleistocene, glaciers covered this area, stripping away the younger sediments and grinding away the top of the coral-rich limestone. So, on a single surface, we have evidence for a tropical coral reef and a continental glacier, separated by over 430 million years.