It is always exciting to a geologist when thin-sections of curious rocks are completed and ready for view. A thin-section is a wafer of rock (30 microns thick) glues to a glass slide and examined by transmitted light through a petrographic microscope. They provide fantastic views of the mineralogy, paleontology, and structure of a rock in exquisite detail. Thin-sections are also full of mysteries since we have essentially two-dimensional slices through three-dimensional materials.
Thin-sections from the Decorah Formation samples collecting by Team Minnesota this past summer were finally available this week for study. I took the first look at slides of limestones containing ferruginous (iron-rich) ooids we gathered as part of Etienne Fang’s (’17) Independent Study. The first structures I saw were the crazy dark outlines above. What sort of fossils are these, I wondered. Could they be sponges? Odd bryozoans? Borings through the rock fabric? I was ready to post them here as mystery fossils to solicit your opinions. Now, though, they instead make a cautionary tale.
There are many of these features in a single slide from the Decorah Formation exposed at the Golden Hill outcrop near Rochester, Minnesota. Some are astonishingly complex. It then began to occur to me that these structures were too convoluted and unpredictable to actually be fossils. It also bothered me that to focus on them required to put the rest of the field out of focus. That only made sense if these oddities were in the epoxy, not the rock itself.
Etienne showed me how to demonstrate that these funny loops were not in the rock with this view: You can just make out the greenish lines overlapping the cut surface of this ferruginous ooid. Turns out I was excited about air bubbles in the cementing epoxy. They have nothing to do with the rock. I nearly posted my own pseudofossils.
I held out hope, though, that these odd white objects in another thin-section of ooid-rich limestone. They appear to be ghostly outlines of ooids with a finely-textured object inside. They look like seeds with embryos within. Several are scattered through the thin-section. Another mystery fossil!
Strange also how once again the details of the internal object can only be seen with the rest of the slide out of focus. Yes, another artifact in the epoxy. This time we may be looking at holes left by ferruginous ooids plucked from the rock through the grinding process, pulling a patch of epoxy with them. Somehow this happened when the now-missing ooid was wedged against another. Nothing to see here, folks.
At least I can take the opportunity to show how cool Etienne’s ferruginous ooids are. Note that this one has a greenish layer midway through the cortex. It looks like the mineral chamosite to me. Spectacular detail in the lamellae, but only visible if the image is over-exposed.
There are plenty of real fossils in these thin-sections, of course. My favorites are these bifoliate bryozoans (lower right half) with their zooecia filled with ferruginous material. Note that the ooid above has had some of its lamellae dissolved away, probably because of some mineral diversity. Also note in the upper right another one of those crazy air bubbles.
The lesson I learn over and over: think, but then think again.