3 August 2015
As part of my work on the GEODE project, I’m always looking for good imagery to teach key concepts in geoscience.
One important concept that I’ve been thinking about lately is the principle of relative dating on the basis of inclusions.
Just as you can’t bake a loaf of raisin bread without already having raisins in your kitchen, rock units that are included in another rock unit must be older than the rock that includes them. Chunks of sandstone included as xenoliths in a granite imply an older sandstone that had chunks broken off, and then later a granite formed, including some of those chunks among its magmatic crystals.
One local rock unit that illustrates this process well is the Sykesville Formation, a metasedimentary unit of unknown depositional age, that was metamorphosed during the late Ordovician Taconian Orogeny. It looks a lot like our other Piedmont metagraywacke units such as the Laurel Formation and the Mather Gorge Formation, but it’s distinctive because of its profusion of inclusions — so called “olistoliths.”
Here are six new GigaPans I shot last Friday morning to highlight some of the features of the Sykesville, as exposed near the Potomac River at “Chain Bridge Flats,” the very un-flat section of bedrock terrace / floodplain immediately upstream of Chain Bridge, in Washington, DC’s westernmost corner.
One possibility is that these rocks represent an accretionary wedge complex, and so they have been “tectonically mixed” and jumbled up different sized particles (from different original depositional settings) into what you see here, prior to being metamorphosed. Another possibility is that these rocks’ textures reflect their original depositional character (matrix-supported diamictite), and therefore imply perhaps submarine landslides or glacial dropstones. Some of the clasts are quite angular whereas others appear to be well-rounded. Of particular note is the fact that some of the inclusions are amphibolites and gneisses with internal foliations that differ from the Taconian foliation that is so prominent in the matrix. This implies a previous generation of tectonism in the clasts’ source area, and suggests to me that that source area was continental crust (as opposed to, say, the Taconian volcanic island arc).
This is a great spot to introduce students to key concepts in relative dating, explore the differential weathering of clasts of different compositions, and delve into some fascinating questions of the interpretation of enigmatic rocks.