Fusilinids experience pressure solution
Today, I would like to share with you some spectacular images that I took using a new toy, a Nikon binocular dissecting microscope with digital camera mount. These photos show a limestone in which you can find a large number of the single-celled foraminifera called "fusilinids." These benthic forams are about the size and shape of grains of rice, and here you will be looking at them in cross-section, seeing the spiral shape with numerous internal chambers that helps support their cytoplasmic bulk. Remember that these are macroscopic, not microscopic: each fusilinid is a single cell the size of a rice grain!
Now, you might be wondering why I'm so keen on showing off fossils. After all, this isn't a paleo blog... But there's more than just fossilization going on here. These fusilinids have also been squeezed. The weight of overlying sedimentary strata has compressed this rock perpendicular to the bedding plane, (top to bottom, in all these photos) and some of the fusilinids got crammed against their neighbors. Now, fusilinids make their skeletal material from the mineral calcite, and calcite can go into solution when the pressure is high enough. In places, you can see where one fusilinid has penetrated into its neighbor, dissolving the neighbor away as it intrudes. The following two images are close ups of the upper image. Photo #2 is from the lower left of the first image; Photo #3 is from the upper right of the first image:
In both, you can see where the edge of one of these internally-spiraled, ellipsoid-shaped fusilinids has dissolved its way into a neighboring fusilinid, disrupting the neighbor's internal architecture and symmetry. Insoluble minerals like dark-colored clays build up along this dissolution horizon. Here's one more photo for good measure:
Pretty cool, eh? The fossils serve as strain markers, hinting to us about how much of the rock's calcitic volume has been lost.
I would like to thank the Nikon representative who demonstrated the camera for me, Stanley M., for taking the time to show how the device works, and for allowing me to make some images before I had officially bought the thing.
Now, you might be wondering why I'm so keen on showing off fossils. After all, this isn't a paleo blog... But there's more than just fossilization going on here. These fusilinids have also been squeezed. The weight of overlying sedimentary strata has compressed this rock perpendicular to the bedding plane, (top to bottom, in all these photos) and some of the fusilinids got crammed against their neighbors. Now, fusilinids make their skeletal material from the mineral calcite, and calcite can go into solution when the pressure is high enough. In places, you can see where one fusilinid has penetrated into its neighbor, dissolving the neighbor away as it intrudes. The following two images are close ups of the upper image. Photo #2 is from the lower left of the first image; Photo #3 is from the upper right of the first image:
In both, you can see where the edge of one of these internally-spiraled, ellipsoid-shaped fusilinids has dissolved its way into a neighboring fusilinid, disrupting the neighbor's internal architecture and symmetry. Insoluble minerals like dark-colored clays build up along this dissolution horizon. Here's one more photo for good measure:
Pretty cool, eh? The fossils serve as strain markers, hinting to us about how much of the rock's calcitic volume has been lost.
I would like to thank the Nikon representative who demonstrated the camera for me, Stanley M., for taking the time to show how the device works, and for allowing me to make some images before I had officially bought the thing.
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